DE102023107434B3 - Calibrating a loudspeaker system - Google Patents

Abstract

The invention relates to a method for calibrating a loudspeaker system in an acoustic environment. The method comprises steps of creating a local frequency response based on a recorded test signal and creating a target frequency response. Based on a difference between the local frequency response and the target frequency response, a difference frequency response is created. A list of exclusion frequency ranges associated with minima of the local frequency response is created and one or more filter frequency ranges that do not overlap with the exclusion frequency ranges are identified. A target filter frequency is selected from the identified filter frequency ranges and a calibration filter related to the target filter frequency is implemented in an equalizer of the loudspeaker system. The invention further relates to a loudspeaker system configured to carry out the above method for calibrating a loudspeaker system.

Description

Field of the invention

The present invention relates to a method for calibrating a loudspeaker system. The present invention further relates to a loudspeaker system.

Background of the invention

Loudspeaker systems are used in a variety of different acoustic setups, ranging from small speaker systems such as a stereo system in the living room to larger systems for use in large venues such as concerts. Methods for calibrating loudspeaker systems are known from EN 101 05 184 A1 , the EP 1 401 243 A1 and the EN 10 2018 120 229 A1 known.

The frequency response of a loudspeaker in a room is typically affected by a number of unwanted interference effects, such as constructive and destructive interference effects. Compensation for such interference can be achieved by equalizing the frequency response using filters. However, for some types of interference, the use of filters may not be particularly suitable and, in the worst case, could lead to overloading of loudspeakers in the loudspeaker system.

Summary of the invention

The inventors of the present invention have recognized the above-mentioned challenge regarding the compensation of interference effects and provide the method below for calibrating a loudspeaker system to obtain a desired frequency response without risking overloading of loudspeakers of the loudspeaker system.

• Anlegen eines Audiotestsignals an das Lautsprechersystem, um einen Audiotestton in der akustischen Umgebung zu erzeugen, Aufnehmen des Audiotesttons an einer Hörposition in der akustischen Umgebung, um ein aufgenommenes Testsignal zu erhalten, und Bereitstellen eines lokalen Frequenzgangs basierend auf dem aufgenommenen Testsignal;
• Bereitstellen eines Zielfrequenzganges für den mindestens einen Lautsprecher in der akustischen Umgebung;
• Erstellen eines Differenzfrequenzgangs basierend auf einer Differenz zwischen dem Zielfrequenzgang und dem lokalen Frequenzgang;
• Erzeugen einer Liste von Ausschlussfrequenzbereichen, die Minima des lokalen Frequenzgangs zugeordnet sind, basierend auf einem Ausschlusskriterium;
• Identifizieren eines oder mehrerer Filterfrequenzbereiche, die Minima und/oder Maxima des Differenzfrequenzgangs zugeordnet sind, wobei sich die Filterfrequenzbereiche nicht mit den Ausschlussfrequenzbereichen überlappen;
• Auswählen einer Zielfilterfrequenz, ausgewählt aus den identifizierten Filterfrequenzbereichen; und
• Implementieren eines mit der Zielfilterfrequenz in Beziehung stehenden Kalibrierungsfilters in dem Equalizer, um einen gefilterten Frequenzgang bereitzustellen, wobei das Kalibrierungsfilter angeordnet ist, eine Differenz zwischen dem gefilterten Frequenzgang und dem Zielfrequenzgang zu reduzieren.

One aspect of the invention relates to a method for calibrating a loudspeaker system in an acoustic environment;
wherein the speaker system comprises at least one speaker, an audio amplifier, an equalizer and an audio signal processor;
the method comprising the following steps:

• Applying an audio test signal to the loudspeaker system to generate an audio test tone in the acoustic environment, recording the audio test tone at a listening position in the acoustic environment to obtain a recorded test signal, and providing a local frequency response based on the recorded test signal;
• Providing a target frequency response for the at least one loudspeaker in the acoustic environment;
• Creating a difference frequency response based on a difference between the target frequency response and the local frequency response;
• Generating a list of exclusion frequency ranges associated with minima of the local frequency response based on an exclusion criterion;
• Identifying one or more filter frequency ranges associated with minima and/or maxima of the difference frequency response, wherein the filter frequency ranges do not overlap with the cutoff frequency ranges;
• Selecting a target filter frequency selected from the identified filter frequency ranges; and
• Implementing a calibration filter related to the target filter frequency in the equalizer to provide a filtered frequency response, the calibration filter arranged to reduce a difference between the filtered frequency response and the target frequency response.

A loudspeaker is a device designed to convert an audio signal into sound in the form of acoustic waves, i.e. sound waves. Typically, a loudspeaker comprises a speaker diaphragm that moves back and forth in response to the audio signal to generate pressure waves of air, i.e. sound waves.

The intensity of a sound wave can be quantified by the sound pressure level. The sound pressure level can be measured and expressed in decibel units relative to a reference level.

For example, the sound pressure level L _p (in dB units) of a sound wave can be calculated using the following equation, where p is the sound pressure of the sound wave and p ₀ is the reference sound pressure: $$Lp=20lO{G}_{10}\left(\frac{p}{p_0}\right)dB$$

A commonly used reference level for sound is the perception threshold of an average person, i.e. the lowest sound pressure that a person can hear. For example, the reference sound pressure can be on the order of 20 micropascals.

If you compare two sound pressure levels of two different sound waves, the difference between the two sound pressure levels can also be called the sound pressure level.

A speaker system is understood to mean a system comprising one or more speakers, for example comprising a single speaker operating as a stand-alone device or, for example, two speakers working together to provide a stereo reproduction of an audio signal. The speaker system may further comprise one or more speaker driver units, such as amplifiers, that provide an amplified audio signal to the one or more speakers. A speaker that receives an amplified audio signal from an external driver unit may also be referred to as a passive speaker.

In one embodiment of the invention, the loudspeaker system comprises at least one loudspeaker drive unit, such as at least one amplifier, which is a separate device from the one or more loudspeakers of the loudspeaker system. In an alternative embodiment of the invention, the one or more loudspeakers of the loudspeaker system comprises one or more loudspeaker drive units, such as one or more amplifiers. A loudspeaker comprising its own drive unit(s) may also be referred to as an active loudspeaker.

An audio amplifier can also be understood as an audio power amplifier. The audio amplifier is designed to amplify an electronic audio signal. Typically, an audio signal fed to a loudspeaker is an audio signal that has been amplified by an audio amplifier. Therefore, a loudspeaker system typically includes an audio amplifier.

The loudspeaker system may also include an equalizer. An equalizer may be understood to mean a device that is designed to create a balance between frequency components of an electronic signal. An equalizer may thus be used to amplify and/or attenuate individual frequency bands or frequency ranges. Some types of equalizers work by implementing frequency filters, e.g. calibration filters, at separate filter frequencies. The equalizer may be a separate device of the loudspeaker system or it may be implemented as a module in other components of the loudspeaker system, e.g. implemented as software. The equalizer may be a parametric equalizer, i.e. a variable multiband equalizer.

The loudspeaker system may comprise an audio signal processor. An audio signal processor may be understood to mean a device designed to process an audio signal in either a digital or an analog format, wherein the processing may take place in any domain, for example in the time domain or in the frequency domain. An audio signal processor may therefore be able to generate, for example, a representation of the frequency composition of an audio signal. The audio signal processor may further comprise the equalizer, for example the equalizer may be an audio processing module of the audio signal processor, such as a software-implemented audio processing module.

An acoustic environment can be understood as any environment that affects the propagation of acoustic sound from the loudspeaker system, for example a room, a hall or even an open music concert area in front of a stage. A room in particular can have a strong influence on the acoustic sound heard by a person in that room. The sound waves can, for example, reflect off walls and obstacles in the room, thereby creating sound wave interference in the room. Interference of waves can be understood as a wave phenomenon in which several waves overlap and create constructive and/or destructive interference. In areas of constructive interference, the waves add up so that the combined amplitude is greater than the amplitude of a single wave. For example, if the peak of one sound wave meets the peak of another sound wave of the same frequency at the same point, the amplitude is the sum of the individual amplitudes. In contrast, in destructive interference, a peak of one sound wave can meet a trough of another sound wave with the same frequency, and the amplitude is then equal to the difference between the individual amplitudes. Furthermore, when a large number of loudspeakers generate sound waves simultaneously, interference can also occur as a result of interfering sound waves from different loudspeakers. The interference of waves that affect the acoustic sound reproduction in a room can also contain so-called room modes, i.e. frequencies at which standing waves are likely to occur due to the physical dimensions in the room, e.g. distance between solid walls, and which create spatially defined nodes and antinodes at which the sound is perceived as particularly attenuated or amplified.

An interference pattern of a loudspeaker system is typically dependent on the frequency of the emitted sound wave. One frequency may exhibit one interference pattern, while another frequency may exhibit a different frequency pattern. At a certain point in the room, a certain sound frequency from a loudspeaker system may interfere constructively and another sound frequency may interfere destructively.

Interference effects in an acoustic environment, such as a room, are typically most pronounced at frequencies corresponding to wavelengths comparable in size to the characteristic dimensions of the acoustic environment, such as the size of the room, e.g. distances between walls. Such pronounced frequencies may be in the range of 20 Hz to 150 Hz, but in principle interference effects are not limited to any particular range of frequencies.

An objective of a loudspeaker system is to reproduce the audio signal in such a way that the acoustic sound produced is perceived by a listener as intended, i.e. the acoustic sound produced resembles the audio signal as closely as possible. However, at certain locations within the acoustic environment, for example at certain listening positions within the acoustic environment, interference effects may distort the sound emitted by the loudspeaker system. The influence of the interference effects on the reproduction of the audio signal can be described by a frequency response. A frequency response can thus describe a change in the sound pressure level, i.e. a gain, over a range of frequencies, e.g. due to the acoustic environment. An example of an ideal frequency response can be a frequency response characterized by a gain of 0 dB over the entire frequency range, i.e. a flat frequency response; however, most acoustic environments are susceptible to interference effects as mentioned above and therefore the frequency response of the acoustic environment may deviate from this ideal frequency response. For example, in a room, an audio test signal may be applied to a loudspeaker system to produce an audio test tone. At a location in the room, such as a listening position, a recording of the audio test tone may be made to provide a recorded test signal. The frequency composition of the recorded test signal will most likely differ from the frequency composition of the audio test signal, i.e. some frequencies in the recorded test signal may be less or more pronounced than the corresponding frequencies in the audio test signal.

A local frequency response can be determined based on a difference between the recorded test signal and the audio test signal. The composition or shape of the local frequency response can be strongly influenced by interference effects. For example, a sound pressure level maximum of the local frequency response can be present at the frequency of the sound pressure level maximum due to constructive interference, and a sound pressure level minimum of the local frequency response can be present at the frequency of the sound pressure level minimum due to destructive interference. The local frequency response can thus be understood as a representation of the influence of interference effects on sound from a loudspeaker system at a specific position within an acoustic environment.

To a person listening to sound from a loudspeaker system, interference effects due to the acoustic environment may be perceived as distortion of the desired sound. Such distortion may be corrected through the use of one or more calibration filters. For example, if a local frequency response includes a sound pressure level maximum, a calibration filter may be implemented in an equalizer such that an audio signal applied to the loudspeaker system is attenuated at frequencies of the audio signal at the sound pressure level maximum of the local frequency response. And similarly, if a local frequency response includes a sound pressure level minimum, a calibration filter may be implemented in the equalizer such that an audio signal applied to the loudspeaker system is amplified at frequencies of the audio signal at the sound pressure level maximum of the local frequency response.

An acoustic environment may exhibit one or more particularly pronounced destructive interference effects at certain frequencies. Such destructive interference may be referred to below as an acoustic null. An acoustic null may, for example, be characterized by a sound pressure level that is reduced by 9 dB compared to an average sound pressure level of a local frequency response.

In general, one purpose of an equalizer is to modify the frequency response of a loudspeaker system using one or more filters. For example, an equalizer can be used to smooth the frequency response of a loudspeaker system by applying filters, such as first-order and second-order filters, to an audio signal, thereby compensating or correcting peaks and/or dips in the local frequency response. However, there may be problems associated with such corrections. problems, and these are problems particularly related to the acoustic nulls mentioned above. An implementation of a filter that amplifies frequencies of an audio signal at an acoustic null point may require excessively large amounts of energy to amplify, which may simply not be reproduced by the amplifier, or which may result in damage to the loudspeaker system due to overloading of the one or more loudspeakers. Alternatively, an equalizer may be arranged to ignore minima of the local frequency response to circumvent the above-mentioned problem of overloading, while only using one or more filters to correct maxima in the local frequency response.

The present invention relates to a method for performing a calibration of a loudspeaker system by implementing one or more filters for correcting a local frequency response. According to embodiments of the invention, the method may be programmed into a loudspeaker system, and one or more steps of the method may be controlled at least partially by external inputs from a control device, such as a smartphone, computer or tablet.

The method according to the invention is briefly summarized in this section and described in more detail below. The method is based on measuring a local frequency response within an acoustic environment that can be compared to a target frequency response to produce a difference frequency response. These frequency responses are analyzed to create a list of cutoff frequency ranges, which are frequency ranges where acoustic nulls are expected. Remaining frequency ranges are then analyzed to determine a target filter frequency, i.e. a frequency at which a filter can be used. When the filter is implemented, a filtered frequency response is provided that can be calculated based on the local frequency response in combination with the parameters of the implemented filter. Typically, the parameters of the implemented calibration filter can be chosen to reduce a difference between the filtered frequency response and the target filter frequency. After implementing a filter, the procedure can be repeated to implement any number of filters so that the difference between the filtered frequency response and the target frequency response is further reduced, accounting for acoustic nulls through cutoff frequency ranges.

The method according to the invention will now be described in more detail.

In a preferred embodiment of the invention, a local frequency response is generated by applying an audio test signal to the loudspeaker system to generate an audio test tone that is recorded at a listening position, thereby providing a recorded test signal. The listening position may be any position within the acoustic environment where correction of the local frequency response is desired. Based on the recorded test signal and the audio test signal, a local frequency response may be generated, e.g. based on a ratio or difference between the recorded test signal and the audio test signal. According to an embodiment of the invention, the step of providing a local frequency response may comprise performing a plurality of recordings of an audio test signal at a corresponding plurality of listening positions. The local frequency response may be determined based on an averaging of the sound pressure levels of the plurality of recordings.

The method further comprises a step of providing a target frequency response for the at least one loudspeaker in the acoustic environment. The target frequency response may be understood as a frequency response desired by a listener of the loudspeaker system at a particular location within the acoustic environment, i.e. at a listening position within the acoustic environment. As an example, the target frequency response may be a flat frequency response or a non-flat frequency response based, e.g., on the preferences of the listener of the loudspeaker system and/or a genre or type of audio content to be reproduced by the loudspeaker system. As an example, a desired frequency response may be preset for amplifying low frequency sounds for reproducing certain music genres such as rap or hip hop. Alternatively, the desired frequency response, i.e., the target frequency response, may be preset for amplifying mid-frequency sounds, such as frequencies in the range of 300 Hz to 3 kHz, to reproduce audio comprising mainly human voices, as mainly found in e.g., radio or television broadcasts.

According to one embodiment of the invention, the target frequency response is pre-programmed into the loudspeaker system or externally supplied to the loudspeaker system from a control device, such as a smartphone, computer or tablet. As an example, a user or listener of the loudspeaker system can configure a desired frequency response on the control device and select the desired frequency response to the loudspeaker system as a target frequency response. Alternatively, the user or listener can select a pre-programmed frequency response using the control unit, for example by entering a selection of a pre-programmed frequency response from a list of pre-programmed frequency responses. After selecting or configuring the desired frequency response, the desired frequency response is transmitted, e.g. wirelessly, to the loudspeaker system and used by the loudspeaker system as the target frequency response.

According to an embodiment of the invention, the target frequency response may be determined based on the local frequency response, but the target frequency response may be different from the local frequency response. As an example, the target frequency response may represent a flat frequency response based on an average of the sound pressure level of the local frequency response.

Based on a difference between the target frequency response and the local frequency response, a difference frequency response is generated. The difference frequency response can be generated by subtracting the local frequency response from the target frequency response or alternatively (depending on a particular implementation of the method) by subtracting the target frequency response from the local frequency response. In this sense, the difference frequency response can be considered as a representation of a difference between a measured frequency response and a desired frequency response, i.e. a target frequency response.

If the local frequency response is identical to the target frequency response, the difference frequency response is represented by a flat frequency response at 0 dB, meaning that there is no difference between the local frequency response and the target frequency response. However, a difference frequency response may typically have a number of differences between the two due to interference effects in the acoustic environment, and these differences may be represented by a series of minima/valleys below 0 dB and/or maxima/peaks above 0 dB in the difference frequency response. As an example, a minimum of -10 dB at a particular frequency in the difference frequency response may indicate that the local frequency response has a sound pressure level at that particular frequency that is 10 dB below the sound pressure level in the target frequency response. According to embodiments of the invention, a minimum that is below -9 dB in the difference frequency response is referred to as an acoustic null.

The method is based on the use of an exclusion criterion. The exclusion criterion is used to determine whether a frequency range of the difference frequency response can be considered as a frequency range comprising an acoustic zero. In a preferred embodiment, the exclusion criterion is a sound pressure level threshold. This threshold is used to exclude frequency ranges containing certain minima/valleys of the difference frequency response from further processing.

According to an embodiment of the invention, the sound pressure level threshold is in the range from -20 dB to -2 dB, for example in the range from -15 dB to -5 dB, for example -9 dB. A frequency range of the difference frequency response with a minimum/valley whose sound pressure level exceeds negative -9 dB is included in a list of exclusion frequency ranges. Frequency ranges listed in this list cannot be used in the implementation of filters according to embodiments of the invention.

The extent/width of such an exclusion frequency range can be determined, for example, by zero crossings of the sound pressure level of the difference frequency response. A zero crossing is a frequency at which the sound pressure level is approximately 0 dB. For example, an exclusion frequency range can extend from a first frequency at which the sound pressure level of the difference frequency response is approximately 0 dB to a second frequency at which the sound pressure level of the difference frequency response is approximately 0 dB, not including additional zero crossings, and includes a sound pressure level minimum below a sound pressure level threshold, for example below -9 dB. Other methods for limiting the exclusion frequency ranges can be used, e.g. by fixed frequency bands, by steep slopes, by user-defined ranges, etc.

The method further comprises identifying one or more filter frequency ranges. In a preferred embodiment of the invention, the method comprises sorting all frequency ranges between adjacent zero crossings of the sound pressure level of the difference frequency response into filter frequency ranges and exclusion frequency ranges such that each frequency range associated with a sound pressure level of the difference frequency response below a sound pressure level threshold is added to a list of exclusion frequency ranges, while the remaining frequency ranges are added to a list of filter frequency ranges. The frequency ranges appearing in the list of filter frequency ranges represent frequency ranges on which the application of filters can be based. The steps for identifying filter frequency ranges The calibration of the exclusion and exclusion frequency ranges may preferably be carried out within a predefined frequency interval, e.g. the typical audio range from 20 Hz to 20 kHz or a narrower range of particular interest, e.g. to improve speech intelligibility in the range from 300 Hz to 3 kHz, or e.g. a range where the benefits of calibration are particularly pronounced, e.g. from 10 Hz to 300 Hz, for example from 20 Hz to 200 Hz, for example within a predefined frequency interval from 20 Hz to 150 Hz. The lower and upper limits of these frequency intervals may be referred to as the frequency limits of the calibration procedure.

The next step of the method according to the invention is to select a target filter frequency at which a calibration filter is to be implemented in the equalizer. A calibration can also be called a parametric equalizer filter. A calibration filter can preferably be a digital biquad filter, but is not necessarily limited to this example, and other types of filters known to those skilled in the art can also be implemented. A digital biquad filter can be understood as a type of second-order recursive linear filter that has a transfer function in a frequency range that is a ratio of two quadratic functions. A digital biquad filter is a type of infinite impulse response filter. It is typically characterized by parameters such as gain, filter frequency and filter quality factor. The gain can describe the change in sound pressure level that the filter exerts at the filter frequency. A gain can be either positive, e.g. 5 dB, or negative, e.g. -5 dB, and a calibration filter can therefore be used to cover a range of frequencies, e.g. B. a filter frequency range, to either amplify or attenuate. The filter quality factor can describe the width of the frequency interval around the filter frequency that is affected by the implementation of the filter. So one calibration filter can be narrow and only affect a relatively narrow range of frequencies, while another calibration filter can be wide and affect a relatively wide range of frequencies, which is parameterized by the filter quality factor. The filter quality factor is therefore related to the bandwidth of the filter.

The target filter frequency is selected from a specific filter frequency range. In a preferred embodiment, a target filter frequency is selected based on the sound pressure level, i.e. the frequency within the filter frequency ranges at which the greatest absolute sound pressure level of the difference frequency response is present is selected as the target filter frequency. In other embodiments, the selection of a target filter frequency may be based on an integration of the sound pressure level within the various filter frequency ranges, e.g. a target filter frequency is selected from the filter frequency range having the greatest absolute integrated sound pressure level.

According to the present invention, the calibration filter can then be implemented at the target filter frequency, i.e. the filter frequency of the calibration filter can be the target filter frequency.

The implementation of a calibration filter provides a filtered frequency response. The filtered frequency response is thus the resulting frequency response after the implementation of one or more filters. The parameters of the implemented filters are chosen to reduce a difference between the filtered frequency response and the target frequency response. For example, in some preferred embodiments, the gain is chosen such that the filtered frequency response is approximately equal to the target frequency response at the filter frequency.

In one embodiment of the invention, the filter quality factor is selected such that the difference between the filtered frequency response and the target frequency response is minimized within the filter frequency range associated with the target filter frequency. This selection may be made based on an adaptation method. In other embodiments of the invention, a filter quality factor may be selected based on an integrated sound pressure level within the filter frequency range associated with the target filter frequency. In still further embodiments of the invention, a filter quality factor may be selected based on a full width at half maximum (FWHM) of the difference frequency response at the target filter frequency. The selection of the filter quality factor based on a FWHM value (Full Width at Half Maximum) of the difference frequency response at the target filter frequency may be advantageous in that the FWHM value is a representative measure of the width of the peak/valley at the target filter frequency and thus a suitable measure for determining the width of the filter to be applied, i.e. the filter quality factor.

When filter parameters are selected, the filter can be implemented in the equalizer of the loudspeaker system. The loudspeaker system can then be ready to receive an input audio signal, which can be filtered by the implemented calibration filter to provide a filtered audio signal. The filtered audio signal can then be output from the loudspeaker system as sound, and in areas of acoustic In this environment, the sound may be less distorted than if no filter had been implemented.

Instead of implementing only one filter according to the present inventive method, all steps for implementing a filter can be repeated any number of times to implement any number of filters. There are in principle no restrictions on which steps of the method have to be repeated. For example, in some embodiments, after obtaining a filtered frequency response, this filtered frequency response can be used as a new difference frequency response, and based on this new difference frequency response, a new target filter frequency can be selected at which a new filter can be implemented. In some of these embodiments, the list of exclusion frequency ranges can be updated after implementing a calibration filter, while in other embodiments the list of exclusion frequency ranges is not updated.

Embodiments of the invention may comprise the implementation of any number of filters, for example between 5 and 20 filters, or even more filters, for example between 5 and 100 filters, for example 25 filters or 75 filters. The number of filters that can be applied depends on the number of available bands in the equalizer and the audio signal to be filtered. Each time a filter is implemented, the filtered frequency response may approach the target filter frequency further, explicitly disregarding acoustic nulls as these are related to cutoff frequency ranges. Ideally, the method according to the invention is then able to compensate for e.g. interference effects in an acoustic environment without damaging a loudspeaker, e.g. overdriving a loudspeaker or wasting energy through a large pressure compensation at the frequency of an acoustic null.

According to the invention, a method is provided for calibrating a loudspeaker system in an acoustic environment. By performing a calibration according to the present invention, a frequency response of the acoustic environment is realized which is as close as possible to a desired target frequency response. Furthermore, by excluding certain frequency ranges which have minima with sound pressure levels below a threshold sound pressure level, it is avoided that filters are used to try to compensate for these minima. This is particularly advantageous because these minima, or acoustic nulls, can be difficult to compensate for without overloading the loudspeaker of the loudspeaker system, and/or the energy used for an unsuccessful compensation attempt would simply be wasted. In this way, a method is obtained for compensating for interference effects, while at the same time protecting the loudspeaker(s) of the loudspeaker system from overloading. A further effect is that the power consumption of the loudspeaker system can be reduced, since no electrical energy is wasted in attempts to compensate for acoustic nulls that cannot be meaningfully compensated anyway.

According to an embodiment of the invention, the step of recording the audio test sound at a listening position in the acoustic environment comprises recording the audio test sound using an electronic processing device comprising a microphone.

An electronic processing device comprising a microphone is understood to mean any electronic device that is configured to record acoustic sound, for example an audio test signal, and to perform some type of signal processing on the recorded acoustic sound, for example the recorded audio test signal. The electronic processing device can be a smartphone, a tablet, a laptop or any other suitable portable electronic device comprising a microphone and a processor. In one embodiment of the invention, the electronic processing device comprises the audio signal processor.

According to one embodiment of the invention, the target frequency response is based on the local frequency response.

According to one embodiment of the invention, the target frequency response is an average of the local frequency response.

The target frequency response may be provided based on the local frequency response. As an example, the target frequency response in a frequency interval may be the average sound pressure level of the local frequency response in that frequency interval, such that the target frequency response is a constant independent of the frequencies within the frequency interval.

According to one embodiment of the invention, the target frequency response is based on a predetermined frequency response.

The target frequency response may be based on a predetermined frequency response stored in a memory communicatively coupled to the audio signal processor.

Embodiments of the invention may have several predetermined frequency responses stored. This may be for different calibration purposes, for example for different types of acoustic environments, for example a kitchen, a living room, a hallway or a bathroom. Alternatively, it may be for different types of calibrations, for example calibrations that focus on certain frequency intervals, for example calibrations that may enhance bass frequencies, midrange frequencies or treble frequencies.

According to one embodiment of the invention, the target frequency response is defined by a user of the loudspeaker system.

In some embodiments of the invention, a user may set the target frequency response, e.g. directly via the speaker system or via a device such as a smartphone, computer or tablet. It may be advantageous to allow the user to adjust the target frequency response to ensure an adaptive calibration process with a high degree of control.

The target frequency response does not have to be constant over the entire frequency interval. For example, it may be determined based on a calculation of the average sound pressure level of the local frequency response in combination with a pre-programmed frequency response. In other words, the target frequency response may represent a pre-programmed frequency response modulated by the calculated average sound pressure level of the local frequency response. A pre-programmed frequency response, e.g. a preset equalizer setting, may include any equalizer setting from the following: Acoustic, Bass Booster, Bass Reducer, Classical, Dance, Electronic, Hip-Hop, Jazz, Piano, Pop, R&B, Rock and Vocal Booster. The above list of equalizer settings is not exhaustive, and other settings suitable for other types of audio content are conceivable.

In one embodiment of the invention, the target frequency response is provided by first applying a preset equalizer setting characterized by its own pre-programmed frequency response and then modulating this frequency response based on the local frequency response. As an example, the pre-programmed frequency response may be shifted in sound pressure level according to an average sound pressure level of the local frequency response. In another embodiment of the invention, a preliminary target frequency response is generated based on the local frequency response, for example an average of the local frequency response. This preliminary target frequency response is then modulated by the pre-programmed frequency response, such as that explained in the list of preset equalizer settings above. In another embodiment of the invention, the step of providing a target frequency response is performed in a combined calculation step taking into account both a local frequency response and a pre-programmed frequency response.

It may be advantageous to determine the average sound pressure level of a target frequency response based on the average sound pressure level of a local frequency response. The average sound pressure level of a local frequency response in a large room may differ from the average sound pressure level of a local frequency response in a small room. To account for such differences, the local frequency response and/or the target frequency response can be adjusted in terms of absolute level so that subtracting one frequency response from the other results in a meaningful difference frequency response.

According to one embodiment of the invention, the target frequency response is based on a recording of an auxiliary audio test tone.

An auxiliary audio test tone can be understood as any acoustic sound generated by the loudspeaker system.

The recording can be made at any recording position within the acoustic environment. The recording can be used to determine a frequency response of the recording position. The recording positions can be located, for example, in front of at least one loudspeaker or far away from the at least one loudspeaker, e.g. at any location within the acoustic environment. In one embodiment of the invention, a plurality of recordings are made at a plurality of recording positions within the acoustic environment.

The target frequency response may be based on an average of sound pressure levels of recordings of the auxiliary audio test tone. For a single recording, the target frequency response may be based on an average of the sound pressure levels of the single recording. For more than one recording of the auxiliary audio test tone, the target frequency response may be based on a plurality of averages of sound pressure levels, each sound pressure level average associated with a particular recording of the auxiliary audio test tone at a particular recording position.

Based on recordings of an audio test tone, the target frequency response can be generated. The local frequency response can further be used in generating the target frequency response, e.g. to match the average sound pressure level of the target frequency response to the sound pressure levels of the local frequency response.

According to one embodiment of the invention, the recording of an auxiliary audio test tone is a close-range measurement. Thus, the target frequency response can be determined by a close-range measurement, i.e. a measurement in the immediate vicinity of the at least one loudspeaker. A close-range measurement can be carried out by placing the loudspeaker far from acoustic obstacles in the acoustic environment, e.g. walls of a room, and placing the microphone in the immediate vicinity of the loudspeaker or in particular in the immediate vicinity of a woofer of the loudspeaker. Close proximity can be understood to mean a distance of 0 to 50 centimeters, for example from 1 to 30 centimeters, for example from 5 to 20 centimeters, for example from 5 to 15 centimeters. In this way, the direct sound wave coming from the loudspeaker, e.g. the woofer of the loudspeaker, always has a much higher sound pressure level than sound waves coming to the microphone from reflections in the acoustic environment, e.g. reflections from walls of a room. The recording obtained in this way is representative of a reflection-free measurement of the loudspeaker. This close-range measurement can therefore serve directly as the target frequency response. In addition, the auxiliary audio test tone can also be the audio test tone.

A target frequency response based on at least one recording of an auxiliary audio test tone in front of a loudspeaker may be advantageous because it isolates a frequency response of a loudspeaker to some extent. A target frequency response may then include a frequency response of a loudspeaker and consequently the method will perform calibration primarily to correct distortions of the acoustic environment, not distortions of the loudspeaker.

According to embodiments of the invention, a short-range measurement is carried out to generate a preliminary target frequency response. A local frequency response is then created and an offset between the preliminary target frequency response and the local frequency response is determined. Finally, the target frequency response is generated based on the predetermined frequency response and the offset. This ensures that the local frequency response and the target frequency response are at similar general sound pressure levels and thus a meaningful difference frequency response can be obtained by subtracting the two.

According to one embodiment of the invention, the target frequency response is based on a plurality of recordings of the auxiliary audio test tone.

The target frequency response may be based on a plurality of recordings of an auxiliary audio test tone generated by the loudspeaker system. The plurality of recordings may be performed at a plurality of recording positions within the acoustic environment. One purpose of performing a plurality of recordings of an auxiliary audio test tone may be to gain a general impression of the acoustics of the acoustic environment. For example, an acoustic environment may have one or more acoustic nulls, and by recording an auxiliary audio test tone at a plurality of recording positions within the acoustic environment, it may be prevented that a target frequency response is set based on an acoustic null, whereby the target frequency response would contain sound pressure levels that are too low. By recording at multiple recording positions, pronounced fluctuations in the frequency response of the acoustic environment, e.g. due to acoustic nulls, can thus be effectively smoothed out. The recordings of the auxiliary audio test tone may be performed by the electronic processing device comprising a microphone. Thus, during calibration of the loudspeaker system using the electronic processing device, a user of the loudspeaker system may perform both auxiliary audio test tone recordings and audio test tone recordings. This recording process may require the user to position themselves at various recording positions within the acoustic environment. It may further be advantageous to establish the target frequency response based on multiple recordings taken at various recording positions within the acoustic environment, as this may reduce the impact of noisy measurements, e.g. measurements where other noise from unwanted sound sources is present.

The target frequency response may also be created by a combination of a pre-programmed/determined frequency response as described above and one or more recordings of an auxiliary audio test tone, for example a plurality of recordings of an auxiliary audio test tone. In this sense, a pre-programmed frequency response can be effectively modulated in sound pressure level by using recordings of a test tone recorded at one or more recording positions within the acoustic environment, i.e. the pre-programmed Frequency response is modulated to the acoustics of the specific acoustic environment.

According to one embodiment of the invention, the auxiliary audio test tone is the audio test tone.

The auxiliary audio test tone and the audio test tone can be the same tone. This can improve the quality of the difference frequency response based on the difference between the local frequency response and the target frequency response.

According to one embodiment of the invention, the exclusion criterion comprises a sound pressure level threshold.

The method according to the invention is based on the creation of a list of exclusion frequency ranges based on an exclusion criterion. The list of exclusion frequency ranges should preferably include frequency ranges in which acoustic zeros are present. The exclusion criterion determines whether a frequency range is added to the list of exclusion frequency ranges. The use of a sound pressure level threshold as an exclusion criterion is advantageous in that frequency ranges can be excluded from the filtering depending on a sound pressure level of the difference frequency response within the frequency range. This can make it possible to exclude frequency ranges that have an acoustic zero from the filtering.

According to an embodiment of the invention, a frequency range associated with a minimum of the local frequency response is assigned to the list of exclusion frequency ranges if an absolute value of a sound pressure level of the difference frequency response within the frequency range exceeds the sound pressure level threshold.

By identifying sound pressure level minimums/troughs in the difference frequency response and comparing them to the sound pressure level threshold, it can be determined whether a frequency range associated with that minimum should be excluded from filtering, e.g. from implementing a calibration filter at a frequency close to or equal to a central frequency of that minimum. This ensures that a calibration filter is not implemented at a frequency that corresponds to a frequency for which there is an acoustic null at the listening position in the acoustic environment.

According to one embodiment of the invention, the sound pressure level threshold is a threshold value in the range of 2 dB to 20 dB, for example in the range of 5 dB to 15 dB, for example 9 dB.

An acoustic zero is associated with a minimum in the local frequency response, however, since there are a variety of minima that are not associated with an acoustic zero, there must be a way to distinguish an ordinary minimum from an acoustic zero minimum. This step of distinguishing between the two types of minima can be done by comparing a sound pressure level of the minimum in the difference frequency response with the sound pressure level threshold. As an example, if a sound pressure level threshold is set to 9 dB, this means that if a minimum of the difference frequency response exceeds a value of negative 9 dB, i.e. -9 dB, this minimum is treated as an acoustic zero and the frequency range associated with this minimum is added to the list of exclusion frequency ranges.

According to an embodiment of the invention, a frequency range associated with a minimum of the local frequency response is assigned to the list of exclusion frequency ranges if a frequency width associated with the minimum exceeds a frequency width threshold.

According to an embodiment of the invention, a frequency range associated with a minimum of the local frequency response is assigned to the list of exclusion frequency ranges if an integrated sound pressure level associated with the minimum exceeds an integrated sound pressure level threshold.

It is not only the extreme value of the sound pressure level of the local frequency response that can determine the sound distortion and the character of an acoustic null, but also the width associated with the extreme value. Thus, in some embodiments, it may not only be an extreme value of the sound pressure level that determines whether a frequency range is added to the list of exclusion frequencies. For example, a width associated with a minimum of the local frequency response may be taken into account. Alternatively, an integrated sound pressure level associated with a minimum of the local frequency response may be taken into account. This may be advantageous for an exclusion criterion to ensure an optimal selection of frequency ranges for the list of exclusion frequency ranges.

According to one embodiment of the invention, the exclusion criterion comprises a frequency interval.

It may be preferable for the calibration procedure to operate only in one frequency interval. For example, there may be no reason to apply a calibration procedure at frequencies that are are not audible to humans. In addition, there are frequency intervals that are typically more susceptible to interference effects. Limiting the calibration method to a relevant frequency interval can significantly reduce the time required to perform the procedure, as opposed to performing the procedure over a wider frequency interval, for example an interval from 20 Hz to 20 kHz.

According to embodiments of the invention, only frequency ranges contained in the frequency interval can be added to the list of exclusion frequency ranges.

According to one embodiment of the invention, the frequency interval comprises frequencies in the range from 10Hz to 500Hz, for example in the range from 20Hz to 200Hz.

Typically, an acoustic environment mainly affects a frequency interval, i.e. spurious effects such as acoustic nulls are most pronounced in a certain frequency interval. Therefore, it may be advantageous to target this interval when applying the method for calibrating a loudspeaker system in order to reduce the time required to perform the method. The frequency interval may include the frequency range from 20Hz to 20kHz, for example from 20Hz to 2kHz, for example from 20Hz to 200Hz, for example from 20Hz to 80Hz, but is not limited to these examples.

According to one embodiment of the invention, the endpoints of the exclusion frequency ranges are based on zero crossings of the difference frequency response.

According to one embodiment of the invention, the end points of the filter frequency ranges are based on zero crossings of the difference frequency response.

In one embodiment of the invention, the calibration method identifies all frequency ranges between adjacent zero crossings of the sound pressure level of the difference frequency response within the frequency interval. The boundaries of the frequency interval may also be the boundaries of the outermost frequency ranges. Each frequency range is sorted to be either a filter frequency range or an exclusion frequency range, such that any frequency range associated with a sound pressure level of the difference frequency response below a sound pressure level threshold, for example below -9 dB, is added to a list of exclusion frequency ranges, while the remaining frequency ranges are added to a list of filter frequency ranges.

According to one embodiment of the invention, the target filter frequency is within a selected filter frequency range of the one or more filter frequency ranges and is a central frequency of a maximum or minimum of the difference frequency response within the selected filter frequency range.

After identifying one or more filter frequency ranges, a target filter frequency is selected. The target filter frequency can be a frequency at which a calibration filter is to be implemented. In this sense, the target filter frequency can be a central frequency of the calibration filter, i.e. a frequency at which the calibration filter has the greatest influence on the local frequency response.

In a preferred embodiment of the invention, the difference frequency response is analyzed to locate the largest absolute sound pressure level within the filter frequency ranges, and the target filter frequency is the frequency of this largest absolute sound pressure level.

In other embodiments, a selected frequency interval is determined based on an integration of the absolute sound pressure levels of the individual filter frequency ranges. The frequency interval that yields the largest value upon integration may be the selected filter frequency range from which a target filter frequency is selected. The target filter frequency may then be determined as a maximum of the absolute sound pressure level, as a mean frequency, as a weighted mean frequency using the sound pressure level as a weight, or by an adjustment.

According to one embodiment of the invention, the calibration filter is an infinite impulse response filter.

A calibration filter can be an infinite impulse response filter, preferably a digital biquad filter but not limited to this example. Infinite impulse response filters are advantageous because they can apply to a relatively narrow range of frequencies.

According to one embodiment of the invention, the calibration filter is a biquad filter, for example a digital biquad filter.

A biquad filter, such as a digital biquad filter, can be characterized as a type of second-order filter with infinite impulse response. In a frequency representation, its transfer function is a ratio of two quadratic functions.

According to one embodiment of the invention, the calibration filter comprises a filter gain.

According to one embodiment of the invention, the filter gain is based on a sound pressure level of the difference frequency response at the target filter frequency.

A filter gain can be understood as the change in sound pressure level exerted by the filter at a target filter frequency or a range of frequencies in close proximity to the target filter frequency, and can be either positive or negative.

Within the scope of the calibration method of the invention, it is advantageous that the gain of a calibration filter can be selected so that a filtered frequency response can approach the target frequency response as closely as possible.

According to embodiments of the invention, the filter gain can be selected so that the sound pressure level of the filtered frequency response at the target filter frequency is as close as possible to the sound pressure level of the target frequency response at the same target filter frequency, for example is identical or approximately the same sound pressure level.

According to one embodiment of the invention, the calibration filter comprises a filter quality factor.

A filter quality factor or Q factor is a factor related to an equalizer. It is understood as the ratio of a center frequency to the bandwidth, that is, the ratio between the target filter frequency and a frequency range surrounding the target filter frequency. For a fixed target filter frequency, the bandwidth is inversely proportional to the Q factor, meaning that as Q is increased, the bandwidth becomes narrower. The quality factor is a useful tool of a parametric equalizer because it allows a signal to be attenuated or boosted within a very narrow or wide range of frequencies within each equalizer band. Wide and narrow bandwidths (low and high Q, respectively) can be used in conjunction with each other to achieve the desired effect.

According to one embodiment of the invention, the filter quality factor is selected based on minimizing a difference between the target frequency response and the filtered frequency response.

According to one embodiment of the invention, the minimization is performed by a search algorithm.

The filter quality factor may be chosen to minimize the difference between the target frequency response and the filtered frequency response. This selection may be performed by a matching algorithm or a search algorithm. For example, a list of possible filter quality factors is provided and a binary search algorithm performs a search through this list to find the filter quality factor that gives a minimal difference between the target frequency response and the filtered frequency response. As an example, a quality factor or Q value that gives a minimal difference between the target frequency response and the filtered frequency response at the target filter frequency may advantageously be found from a reduced range of Q values. The difference between the target frequency response and the filtered frequency response has been shown to have some dependence on the Q value. Observations show that the difference has a second order dependence on the Q value and that this dependence has only a single extremum at which the difference is smallest. This observation can be specifically taken into account by the search algorithm when searching for the best possible figure of merit. The search algorithm can start by determining the difference for a given starting Q value and then determine the difference for neighboring Q values, i.e. both smaller and larger Q values. Based on these Q values, it can be determined whether the minimum difference is achieved by a Q value that is smaller or larger than the given starting Q value. Due to the dependency described above, a large number of Q values can be ignored and an optimal Q value can be found more quickly than if all Q values had to be evaluated. After determining that a more suitable Q value lies at higher or lower values than the initial Q value, the search algorithm may proceed to evaluate the difference at a Q value that lies in the middle of an interval of Q values that begins with the initial Q value and ends in an upper or lower limit of the applicable Q values of the equalizer. The search algorithm may perform this process in several steps to determine an optimal Q value for the filter to be applied at the target filter frequency.

According to one embodiment of the invention, the minimization is performed within a filter frequency range comprising the target filter frequency.

The minimization of a difference between the target frequency response and the filtered frequency response can be performed within the filter frequency range in which the target filter frequency range lies. In one embodiment of the Invention, the difference between the target frequency response and the filtered frequency response is calculated, this difference is squared and this squared difference is integrated within the frequency range including the target filter frequency. The filter quality factor is then chosen to minimize this integrated squared sum.

According to an embodiment of the invention, the method further comprises a step of selecting an auxiliary target filter frequency within a selected cutoff frequency range, wherein the auxiliary target filter frequency is a central frequency of a minima of the local frequency response within the selected cutoff frequency range, and wherein the method further comprises a step of implementing an auxiliary calibration filter related to the auxiliary target filter frequency in the equalizer.

Providing full compensation of an acoustic null may be problematic, but performing partial compensation may be advantageous. Therefore, in some embodiments of the invention, an auxiliary calibration filter may be implemented at an auxiliary target filter frequency that is within a cutoff frequency range. The auxiliary calibration filter may be a filter that is a different type of filter than a calibration filter, in the sense that its gain may be limited. For example, an auxiliary calibration filter may have an auxiliary filter gain, and the magnitude of this auxiliary filter gain may be limited to, e.g., 9 dB. The implementation of an auxiliary calibration filter is not limited to taking place at a particular time during the steps of the calibration method. An auxiliary calibration filter may be implemented after the implementation of a calibration filter, before the implementation of a calibration filter, or between two implementations of calibration filters.

According to an embodiment of the invention, the method further comprises a step of providing an input audio signal and filtering the input audio signal using the calibration filter to provide a filtered audio signal to be reproduced by the loudspeaker system.

When one or more filters have been implemented into the equalizer, the speaker system can be considered to be calibrated to the acoustic environment with respect to a particular recording position. The speaker system can then receive any input audio signal and apply any implemented filters to that input audio signal to produce a filtered audio signal that can be output to the acoustic environment by one or more speakers of the speaker system. The input audio signal can be any type of audio, for example, music or audio synchronized to a movie, but is not limited to these examples.

According to an embodiment of the invention, one or more steps selected from the steps of applying an audio test signal, recording the audio test tone, providing a local frequency response, providing a target frequency response, determining a difference frequency response, creating a list of exclusion frequency ranges, identifying one or more filter frequency ranges, selecting a target filter frequency, and implementing a calibration filter are performed a plurality of times.

In some embodiments of the invention, the calibration method of the invention is an iterative calibration method in which one or more of the method steps are repeated one or more times, e.g. a plurality of times. In this context, an iteration may be understood as a repetition of one or more steps of the method, which may be based on inputs from a previous iteration. In an iterative calibration method according to these embodiments, the difference frequency response may be updated based on the implementation of a calibration filter, and an additional calibration filter may be implemented based on the updated difference frequency response.

In some embodiments of the invention, the difference frequency response may be updated based on the filtered frequency response, i.e. the filtered frequency response is used as an updated difference frequency response. This difference frequency response may then serve as a basis for an additional target filter frequency, wherein an additional calibration filter may be implemented in the equalizer.

In these embodiments, the list of exclusion frequency ranges and the filter frequency ranges may be updated based on the updated difference frequency response.

In other embodiments of the invention, the audio test signal is fed to the equalizer to generate a filtered audio test signal based on the calibration filters implemented in the equalizer. The filtered audio test signal can then be used by one or more loudspeakers of the loudspeaker system to generate a filtered audio test tone in the acoustic environment, which in turn can be recorded to provide a filtered recorded test signal that is used as the basis for an updated local frequency response. Based on an updated local frequency response, an additional calibration filter can be implemented.

Iterations of the calibration procedure can be repeated any number of times, e.g. until a target frequency response criterion is met or a predefined number of times, for example 5 times or 20 times. A target frequency response criterion can be understood as a criterion that evaluates the filtered frequency response to determine whether it sufficiently resembles the target frequency response, e.g. this evaluation could determine whether the sound pressure level difference between the filtered frequency response and the target frequency response exceeds 3 dB within any filter frequency range, and if the difference exceeds 3 dB, an additional calibration filter can be applied. A target frequency response criterion is not limited to this example.

By using an iterative calibration procedure in which multiple iterations are performed, it is possible to reduce distortion in an acoustic environment to a greater extent than if only a single filter is used.

• Mindestens einen Lautsprecher, einen Audioverstärker, einen Equalizer und einen Audiosignalprozessor, wobei das Lautsprechersystem zur Durchführung des Verfahrens nach einem der vorhergehenden Ansprüche eingerichtet ist.

One aspect of the invention relates to a loudspeaker system comprising:

• At least one loudspeaker, an audio amplifier, an equalizer and an audio signal processor, wherein the loudspeaker system is arranged to carry out the method according to one of the preceding claims.

A loudspeaker system can be understood as a system comprising one or more loudspeakers and one or more loudspeaker driver units. According to one embodiment of the invention, the loudspeaker system is a single, stand-alone device, for example an active loudspeaker. In another embodiment of the invention, the loudspeaker system is a distributed system comprising a plurality of electrically connected devices, for example two or more passive loudspeakers that are electrically connected to a loudspeaker driver unit, e.g. an amplifier. The equalizer can be an integral part of the amplifier or a dedicated device.

According to embodiments of the inventions, the loudspeaker system may be arranged to enable a calibration method according to the method of the invention. This may require an equalizer in which calibration filters may be implemented so that each input audio signal supplied to the loudspeaker system may be filtered by calibration filters.

A loudspeaker system according to the invention should preferably comprise an audio signal processor. The audio signal processor may comprise the equalizer.

In some embodiments of the invention, audio amplifiers and equalizers are included in a speaker system controller, which may also include an audio signal processor and means for receiving an input audio signal.

An audio signal processor may be an internal audio signal processor and it may be an external audio signal processor. An external audio signal processor may be understood as an audio signal processor that is integrated into an external device and not into a speaker system controller. An external audio signal processor may be characterized in that a speaker system controller may receive an input audio signal to be output as sound from one or more speakers of the speaker system independently of the external audio signal processor. In contrast, an internal audio processor may be integrated into a speaker system controller. Embodiments of the invention may include both an internal audio signal processor and an external audio signal processor.

An external audio signal processor can be located in an external device, i.e. an electronic processing device, for example a smartphone, laptop or tablet. An external device can communicate with any other part of the speaker system, such as the equalizer or an internal audio signal processor, through any connection means. Examples of connection means are wired connections, for example a cabled connection, and wireless connections, for example a Bluetooth connection, for example Bluetooth A2DP or Bluetooth aptX, or a Wi-Fi connection.

The method according to the invention comprises steps of applying an audio test signal, recording an audio test tone, providing a local frequency response, providing a target frequency response, generating a list of exclusion frequency ranges, identifying filter frequency ranges, selecting a target filter frequency and implementing a calibration filter. Any steps of the method of the invention can be performed on an internal audio signal processor, and Any steps of the process can be performed on an external audio signal processor.

For example, in some embodiments of the invention, an external audio signal processor may perform the step of recording an audio test tone which is then provided to an internal audio signal processor configured to perform other steps of the method. In other embodiments of the invention, an external audio signal processor may provide a local frequency response, provide a target frequency response, create a difference frequency response, create a list of exclusion frequency ranges, identify filter frequency ranges, and select a target filter frequency.

The drawings

• 1 eine akustische Umgebung illustriert, in der ein Lautsprechersystem gemäß einer Ausführungsform der Erfindung kalibriert werden kann,
• 2a-2b Schritte zur Bereitstellung eines Zielfrequenzganges gemäß einer Ausführungsform der Erfindung illustrieren,
• 3a-3d Schritte zur Implementierung von Kalibrierungsfiltern basierend auf Analyse von Frequenzgängen gemäß bevorzugten Ausführungsformen der Erfindung illustrieren,
• 4 ein Flussdiagramm, das das Kalibrierungsverfahren gemäß Ausführungsformen der Erfindung beschreibt, illustriert, und
• 5a-5b verschiedene Konfigurationen eines Lautsprechersystems gemäß Ausführungsformen der Erfindung illustrieren.

Various embodiments of the invention are described below with reference to the drawings, in which

• 1 illustrates an acoustic environment in which a loudspeaker system can be calibrated according to an embodiment of the invention,
• 2a-2b illustrate steps for providing a target frequency response according to an embodiment of the invention,
• 3a-3d illustrate steps for implementing calibration filters based on analysis of frequency responses according to preferred embodiments of the invention,
• 4 illustrates a flow chart describing the calibration method according to embodiments of the invention, and
• 5a-5b illustrate various configurations of a loudspeaker system according to embodiments of the invention.

Detailed description

1 illustrates an acoustic environment 50, e.g. a room in which loudspeakers 11 of a loudspeaker system are arranged to radiate acoustic sound. Due to the configuration of the room, e.g. the arrangement of walls, floor, ceiling and possible furniture, the sound radiated by the loudspeakers 11 may not be perceived as intended by a listener/user in the room. In other words, the acoustics of the room influence the perception of the sound in the room, or in other words, the frequency response of the room is strongly influenced by the configuration of the room.

In the following detailed description, reference is made to a local frequency response and a target frequency response. The local frequency response is the frequency response of the loudspeaker system as complemented/influenced by the configuration of the acoustic environment, and the target frequency response is a frequency response desired by the listener in the room. Steps of the method according to the invention include providing a local frequency response and a target frequency response, with the intention of implementing at least one calibration filter so that a filtered frequency response can approach the target frequency response compared to the local frequency response. The listener can decide to calibrate the loudspeaker system so that the local frequency response at a listening position 52 is as close as possible to the intended target frequency response.

In the 1 In the embodiment of the invention shown, the target frequency response is based on recordings of an audio test tone, and these recordings are made at different recording positions 51 within the acoustic environment. For example, an audio test tone is recorded at each of the three recording positions 51 shown to provide three recorded test signals. Each recorded test signal can then be used to generate a recording position frequency response 25, so that a total of three recording position frequency responses are provided. These three recording position frequency responses can then be averaged to provide a target frequency response on which a difference frequency response is based. In other embodiments of the invention, the three recorded test signals can be averaged, and a target frequency response can be generated based on the averaged recorded test signal. A local frequency response can then be generated based on a recording of an audio test signal at a listening position 52.

2a-2b illustrate an embodiment of the invention in which a target frequency response is created based on recordings of an audio test tone from three recording positions 51.

Each recording made at a recording position 51 is used as a basis for providing a recording position frequency response. In 2a Three recording position frequency responses 25 are shown, which were obtained from recordings at the three 1 shown recording positions 51, but the recording positions could be located anywhere in space, and in other embodiments of the invention the number of recordings is not limited to 3, but can be any number of recordings As in 2a As shown, the frequency responses 25 of the recording positions differ from each other, illustrating that the influence of the configuration of the acoustic environment 50 on the frequency response is different for different recording positions 51. The recording position frequency response 25 shows the sound pressure level (in dB units) as a function of frequency. In this embodiment, the frequencies of the recording position frequency responses 25 are in the range of 10 Hz to just over 200 Hz, but in other embodiments of the invention the interval of frequencies may be any other interval.

2b shows an average 26 of the recording position frequency responses, which is an average of the three recording position frequency responses 25, as in 2a In this embodiment of the invention, the target frequency response is chosen as the mean of the average 26 of the pickup position frequency responses, and this mean 27 of the pickup position frequency responses is calculated within a lower and upper calibration method frequency limit 33, which in this embodiment is 20 Hz and 200 Hz, but in other embodiments of the invention these limits 33 may assume other frequency values.

In other embodiments of the invention, an average 26 of the recording position frequency responses can even be used directly as a target frequency response. Alternatively, in some embodiments, an average of the recording position frequency responses 27 can be used as a basis for a target frequency response, wherein an average can be understood as an average over an interval of frequencies, so that the average of the recording position frequency responses 27 is constant, as in 2b shown.

3a-3d illustrate steps for implementing one or more calibration filters according to embodiments of the invention.

3a shows a non-averaged local frequency response 20 which represents a frequency response as measured at the listening position 52. The measured non-averaged local frequency response 20 of this embodiment is smoothed to obtain a local frequency response 21 which is shown as a smooth curve in the diagram of 3a can be seen.

For example, the local frequency response 21 is a running average of the non-averaged local frequency response 20, or the local frequency response 21 is obtained by applying a noise filter to the non-averaged frequency response 20. The frequency response diagram also shows a target frequency response 22, which is a desired frequency response. Finally, the frequency response diagram also shows a filtered frequency response 28, which is obtained after applying two calibration filters in an equalizer 14 (not shown in the figure) of the loudspeaker system. The following section describes in detail how these two calibration filters are determined.

The goal of the calibration procedure is to implement one or more calibration filters such that a filtered frequency response 28 is as close as possible to the target frequency response 22 within the calibration procedure frequency limits 33, without compensating for acoustic nulls that can be seen to manifest themselves as large minima/valleys in the local frequency response 21.

To carry out the method, a difference frequency response 23 is provided based on a difference between the local frequency response 21 and the target frequency response 22. 3b shows a frequency response diagram including the difference frequency response 23. In this example, the difference frequency response 23 is determined by subtracting the target frequency response 22 from the local frequency response 21. Next, a number of frequency ranges are identified within the calibration method frequency boundaries 33, and in this embodiment of the invention these are identified based on zero crossings of the difference frequency response 23, ie based on frequencies at which the difference frequency response 23 is at 0 dB. In other embodiments of the invention, the frequency ranges are identified in different ways.

The frequency ranges include exclusion frequency ranges 31 associated with large valleys/minima in the difference frequency response 23. These large valleys represent acoustic nulls caused by destructive interference of sound radiated by the loudspeakers 11 of the loudspeaker system. The exclusion frequency ranges are selected based on an exclusion criterion. In this embodiment, the exclusion criterion is a criterion based on a sound pressure level threshold 32 which is -9 dB, but in other embodiments of the invention, other exclusion criteria may be used. If a valley in the sound pressure level of the difference frequency response 23 is below -9 dB, the corresponding frequency ranges 31 are added to a list of exclusion frequency ranges. This list of exclusion frequency ranges specifies which frequency ranges do not need to be compensated, i.e. where no calibration filter should be used. The frequency ranges further include filter frequency ranges 30, which are frequency ranges relating to maxima and minima in the difference frequency response 23, but which not affect the largest minima, ie the excluded frequency ranges 31. In this embodiment of the invention, the exclusion filter frequency ranges 31 and the filter frequency ranges 30 are shown as non-overlapping or non-intersecting.

3c shows a next step of the process in which a target filter frequency 41 is selected. The diagram is based on the same diagram as in 2b The target filter frequency 41 is selected based on the largest absolute sound pressure level of the difference frequency response 23 within the calibration method frequency limits 33. As can be seen, this target filter frequency does not belong to an exclusion filter frequency range 31 as in 3b identified. The target filter frequency specifies a central frequency for a calibration filter 40 to be implemented - in this example, the filter to be implemented is a digital biquad filter. Typically, such a calibration filter is characterized by a central filter frequency, or target filter frequency 41, a filter gain, and a filter quality factor. The target filter frequency 41 is used as the frequency at which a calibration filter is to be implemented.

The effect of the calibration filter 40 is to reduce the difference frequency response 23 at the target filter frequency 41 as much as possible, i.e. to ensure that the difference frequency response 23 is as close to 0 dB as possible. The calibration filter 40 is characterized by a filter gain 42 which in this embodiment of the invention is set to a magnitude corresponding to the magnitude of the maxima to be filtered. Note that the filter gain 42 is opposite to the magnitude of the maxima in the difference frequency response 23 to be corrected. The calibration filter 40 is further characterized by comprising a quality factor which is a ratio of the filter gain 42 to a bandwidth of the filter. In this embodiment, a calibration filter 40 with a high quality factor corresponds to a narrow filter in frequency, and a calibration filter with a low quality factor corresponds to a wide filter in frequency. The filter gain 42 and filter quality factor are chosen to minimize a difference between the filtered frequency response 24 and the target frequency response 22.

3c shows the effect of implementing the calibration filter 40 at the target filter frequency 41. A filtered difference frequency response 24 is shown as a result of implementing the calibration filter 40. The figure shows that the filter has actually reduced the difference frequency response 23 locally at the target filter frequency 41 and at frequencies in close proximity to the target filter frequency 41.

The calibration method thus finds a preferred filter gain 42 and filter quality factor based on a minimization procedure that seeks a minimum in the difference between a resulting filtered frequency response and the target frequency response, and implements a corresponding calibration filter 40 in an equalizer of the loudspeaker system. A frequency representation of the implemented calibration filter 40 is shown in the figure. By subtracting the frequency representation of the calibration filter 40 from the difference frequency response 23, a difference frequency response based on a filtered frequency response 24 is obtained, which is closer to a sound pressure level of zero in the vicinity of the target filter frequency.

In a preferred embodiment of the invention, at least one additional calibration filter 40 is implemented. In 3d shows how such an additional calibration filter 40 is implemented at a new target filter frequency 41 just below 50 Hz. Since this target filter frequency 41 is associated with a minimum of the filter frequency range 30 associated with the difference frequency response 23, and this minimum has a sound pressure level that is not below the sound pressure level threshold 32, ie is not associated with an exclusion frequency range, a calibration filter 40 can be applied.

The calibration filter 40 from 3d is determined in a similar way to the 3c shown calibration filter, however, this calibration filter 40 uses a smaller filter gain 42 because the minimum to be corrected in the difference frequency response 23 has a smaller amplitude than the amplitude of the maximum compensated with the previous calibration filter. In addition, the filter is arranged with a positive gain because the peak to be corrected is a minimum and not a maximum as in the previous filter.

After the two calibration filters 40 have been implemented, a filtered frequency response 28 is obtained (see 3a) . As in 3a As can be seen, the filtered frequency response 28 is closer to the target frequency response 22 than the local frequency response 21 at the target filter frequencies used for the two calibration filters. In other embodiments of the invention, additional calibration filters are implemented so that the filtered frequency response 28 can come even closer to the target frequency response 22.

In other embodiments of the invention, in which a plurality of calibration filters 40 are implemented in the equalizer of the loudspeaker system, a filtered frequency response 28 obtained after implementation after a first calibration filter 40 can be used as a new local frequency response, and from this new local frequency response a new difference frequency response is calculated by subtraction with the same target frequency response. The newly obtained difference frequency response is then analyzed in a similar manner as described above, a new calibration filter is implemented in the equalizer and a new filtered frequency response is obtained. The method can thus be viewed as a recursive method in which a filtered frequency response is used as input (as a local frequency response) and an improved filtered frequency response closer to the target filter frequency response is obtained.

4 illustrates a flowchart describing a calibration method according to embodiments of the invention.

First, an audio test signal S1 is supplied to the loudspeakers 11 of the loudspeaker system located in an acoustic environment 50. In a next step S2, the loudspeakers 11 emit a corresponding audio test tone, which is picked up by a microphone 12 of an electronic processing device to provide a recorded test signal (step S3). The electronic processing device in this embodiment may be a smartphone, but other electronic processing devices such as tablets or computers such as laptops may be used in other embodiments of the invention. Based on the audio test signal (step S1) and the recorded test signal (step S3), a local frequency response S4 is provided. In addition, a target frequency response is provided in a step S5. The target frequency response in this example is based on additional recordings of the audio test signal that were performed at different recording positions 51 within the acoustic environment 50. In other embodiments of the invention, the target frequency response 22 is provided based on an auxiliary audio test tone, which may be a tone emitted by one or more loudspeakers 11 and different from the audio test tone. In still other embodiments of the invention, the target frequency response 22 is provided based on the local frequency response 21 and/or a pre-programmed frequency response.

Next, in a step S6, a difference frequency response 23 is determined based on a difference between the local frequency response 21 provided in step S4 and the target frequency response 22 provided in step S5. Based on the difference frequency response 23, the filter frequency ranges 30 are determined in a step S7 and the exclusion frequency ranges 31 are determined in a step S8.

Next, in a step S9, a target filter frequency 41 is selected within one of the identified filter frequency ranges 30. Thereafter, in a step S10, a calibration filter 40 is configured at the target filter frequency 41. The step of configuring the calibration filter 40 includes selecting an appropriate filter gain 42 and a quality factor so that the calibration filter can provide a filtered frequency response 28 that is closest to the target frequency response 22 at the target filter frequency 41. This step of configuring the calibration filter 40 by selecting an appropriate quality factor is performed based on a minimization procedure that searches for the quality factor for which the difference between the resulting filtered frequency response 28 and the target frequency response 22 is minimized as much as possible. The minimization procedure is performed by a search algorithm that searches for an optimal quality factor or Q value. In this example, the calibration filter 40 is based on a digital biquad filter. Thus, in step S11, a filtered frequency response based on the configured calibration filter 40 is provided.

Subsequently, the configured calibration filter 40 is implemented in an equalizer of the loudspeaker system in a step S12.

Some embodiments of the invention are based on iterations of some of the aforementioned steps, and these embodiments relate to the implementation of a plurality of calibration filters 40. In some of these embodiments, after a filtered frequency response has been provided (step S11), the difference frequency response S6 is updated by basing the difference frequency response provided in step S6 on a difference between the target frequency response provided in step S5 and the filtered frequency response provided in step S11, rather than on a difference between the target frequency response provided in step S5 and the local frequency response provided in step S4. Based on this, a new set of filter parameters can be found, an additional calibration filter can be implemented in the equalizer, and the filtered frequency response can be updated. This process can be repeated iteratively as many times as desired.

If one or more calibration filters have been implemented in the equalizer, the sound speaker system is prepared to receive, in a step S13, an input audio signal 60 which is filtered according to the one or more calibration filters 40 implemented in the equalizer of the speaker system to provide a filtered audio signal emitted as sound by one or more speakers 11. This achieves a reproduction of an input audio signal 60 which takes into account the acoustics of the acoustic environment 50 without overdriving the speakers 11 to compensate for acoustic nulls in the acoustic environment 50.

5a -b illustrate embodiments of the invention in which a user 70 of the loudspeaker system may use an electronic processing device, e.g. a smartphone device 18, to control the calibration process as described in detail above.

As in 5a As can be seen, the loudspeaker system comprises a loudspeaker system controller 19 which comprises amplifiers 13, an equalizer 14 and an internal audio signal processor 15. The loudspeaker controller 19 is arranged to receive an input audio signal 60 to be filtered in the equalizer 14, amplified in the amplifiers 13 and output from the loudspeakers 11.

In this embodiment of the invention, the calibration method according to the invention is controlled by a user 70 via a smartphone device 18. The user 70 of the loudspeaker system initializes the calibration method using a smartphone device 18, e.g. by pressing a button on a screen of the smartphone device 18. The instruction to initialize the calibration method can be received from a communication interface 17 of the loudspeaker system controller 19, which enables wireless communication with the smartphone device 18. The smartphone device 18 can prompt the user 70 to position himself at one or more recording positions 51 (not shown) within the acoustic environment 50 to create a target frequency response 22 by determining one or more recording position frequency responses 25 by recording an audio test tone using the microphone 12 of the smartphone device 18.

The smartphone device 18 then prompts the user 70 to position themselves at a listening position 52 that is representative of a position where the user is typically positioned when listening to sound, e.g., music, from the speaker system. Based on a recording of an audio test signal by the smartphone device 18, a local frequency response 21 is generated and the method proceeds with implementation of a calibration filter 40 into the equalizer 14 of the speaker system, as described above.

In other embodiments of the invention, the audio test signal or auxiliary audio test signal used to determine pickup position frequency responses 25 to determine a target frequency response 22 is different from the audio test tone used to obtain the local frequency response 21.

In other embodiments of the invention, the target frequency response is provided in a different manner, such as based on a pre-programmed frequency response that the user 70 can select on the smartphone device 18. For example, the user can select a pre-programmed frequency response on the smartphone device that is suitable for a particular type of music, and this pre-programmed frequency response can then be adjusted to a target frequency response based on recordings.

According to this embodiment of the invention, the smartphone device 18 comprises an external audio signal processor 16 (external to the speaker system) arranged to perform the processing of sound picked up by the microphone 12. The steps of the method as described above can be performed in any manner between the external audio signal processor 16 and the internal audio signal processor 15. In embodiments using multiple processors as in 5a As shown, the processors are communicatively connected, e.g., both the smartphone device 18 and the speaker system controller 19 include a communication interface 17 that enables wireless communication between the devices.

The method step that creates a difference frequency response 21 may be performed with the smartphone device 18 in some embodiments and with the speaker system controller 19 in some other embodiments.

5b shows a further embodiment of the invention, in which the loudspeaker system comprises a single active loudspeaker 80. The active loudspeaker 80 comprises an audio amplifier 13, an equalizer 14, an internal audio signal processor 15 and a converter unit 81, which is the component of the active loudspeaker that converts an audio signal into acoustic sound, e.g. by using a loudspeaker membrane actuated by a voice coil. It further comprises a communication interface 17 that enables communication with an electronic processing device, e.g. a smartphone device 18, operated by a user 70 of the loudspeaker. The active loudspeaker can thus function in a similar way to the one in the embodiment of 5a described loudspeaker system, and the method for implementing calibration filters 40 in the equalizer 14 is identical.

In the 5b In the embodiment shown, the steps of the method of the invention can be distributed in any way between an external audio signal processor 16 of the smartphone device 18 and an internal audio signal processor 15 of the active loudspeaker 80. For example, the calibration can be initiated by the user and the audio test tone can be picked up by a microphone 12 of the smartphone device 18 to generate a picked up test signal that is transmitted to the internal audio signal processor 15 of the active loudspeaker 80, in which the remaining steps of the method are performed to finally implement at least one calibration filter in the equalizer 14.

In some iterative embodiments of the invention, an audio test signal is applied to the loudspeaker system prior to each implementation of a calibration filter.

List of reference symbols:

11

Speakers

12

microphone

13

Audio amplifier

14

Equalizer

15

internal audio signal processor

16

external audio signal processor

17

Communication interface

18

Smartphone device

19

Speaker system controller

20

non-averaged local frequency response

21

local frequency response

22

Target frequency response

23

Difference frequency response

24

filtered difference frequency response

25

Recording position frequency response

26

Average of the recording position frequency responses

27

Average of the recording position frequency responses

28

filtered frequency response

30

Filter frequency range

31

Cutoff frequency range

32

Sound pressure level threshold

33

Calibration procedure frequency limit

40

Calibration filter

41

Target filter frequency

42

Amplitude of the filter gain

50

acoustic environment

51

Recording position

52

Listening position

60

Input audio signal

70

user

80

active speaker

81

Converter unit

S1-S13

Steps of the calibration procedure in the flow chart

Claims (30)

Method for calibrating a loudspeaker system in an acoustic environment (50); wherein the loudspeaker system comprises at least one loudspeaker (11), an audio amplifier (13), an equalizer (14) and an audio signal processor (15, 16); wherein the method comprises the following steps: S2: applying an audio test signal to the loudspeaker system to generate an audio test tone in the acoustic environment (50), S3: recording the audio test tone at a listening position (52) in the acoustic environment (50) to obtain a recorded test signal, and S4: providing a local frequency response (21) based on the recorded test signal; S5: providing a target frequency response (22) for the at least one loudspeaker (11) in the acoustic environment (50); S6: creating a difference frequency response (23) based on a difference between the target frequency response (22) and the local frequency response (21); S7: generating a list of exclusion frequency ranges associated with minima of the local frequency response (21) based on an exclusion criterion; S8: identifying one or more filter frequency ranges associated with minima and/or maxima of the difference frequency response (23), wherein the filter frequency ranges do not overlap with the exclusion frequency ranges; S9: selecting a target filter frequency (41) selected from the identified filter frequency ranges and S10: implementing a calibration filter (40) related to the target filter frequency (41) in the equalizer (14) to provide a filtered frequency response, the calibration filter (40) being arranged to reduce a difference between the filtered frequency response and the target frequency response (22). Procedure according to Claim 1 wherein the step of recording the audio test tone at a listening position (52) in the acoustic environment (50) comprises recording the audio test tone using an electronic processing device (18) comprising a microphone (12). Procedure according to Claim 1 or Claim 2 , wherein the target frequency response (22) is based on a predetermined frequency response. Method according to one of the preceding claims, wherein the target frequency response (22) is defined by a user (70) of the loudspeaker system. Method according to one of the preceding claims, wherein the target frequency response (22) is based on a recording of an auxiliary audio test tone. Procedure according to Claim 5 , where the recording of an auxiliary audio test tone is a distance measurement. Procedure according to Claim 5 , wherein the target frequency response (22) is based on a plurality of recordings of the auxiliary audio test tone. Procedure according to Claim 5 , where the auxiliary audio test tone is the audio test tone. Method according to one of the preceding claims, wherein the exclusion criterion comprises a sound pressure level threshold (32). Procedure according to Claim 9 , wherein a frequency range associated with a minimum of the local frequency response (21) is assigned to the list of exclusion frequency ranges if an absolute value of a sound pressure level of the difference frequency response (23) exceeds the sound pressure level threshold (32) within the frequency range. Procedure according to Claim 10 , wherein the sound pressure level threshold (32) is a threshold value in the range of 2 dB to 20 dB. Method according to one of the preceding claims, wherein a frequency range associated with a minimum of the local frequency response (21) is assigned to the list of exclusion frequency ranges if a frequency width associated with the minimum exceeds a frequency width threshold. Method according to one of the preceding claims, wherein a frequency range associated with a minimum of the local frequency response (21) is assigned to the list of exclusion frequency ranges if an integrated sound pressure level associated with the minimum exceeds a threshold value for the integrated sound pressure level. Method according to one of the preceding claims, wherein the exclusion criterion comprises a frequency interval. Procedure according to Claim 14 , where the frequency interval covers frequencies in the range of 10 Hz to 500 Hz. Method according to one of the preceding claims, wherein the end points of the exclusion frequency ranges are based on zero crossings of the difference frequency response (23). Method according to one of the preceding claims, wherein the end points of the filter frequency ranges are based on zero crossings of the difference frequency response (23). A method according to any one of the preceding claims, wherein the target filter frequency (41) is within a selected filter frequency range of the one or more filter frequency ranges and is a central frequency of a maximum or minimum of the difference frequency response (23) within the selected filter frequency range. A method according to any preceding claim, wherein the calibration filter (40) is an infinite impulse response filter. Method according to one of the preceding claims, wherein the calibration filter (40) is a biquad filter. Method according to one of the preceding claims, wherein the calibration filter (40) has a filter gain. Procedure according to Claim 21 , wherein the filter gain is based on a sound pressure level of the difference frequency response (23) at the target filter frequency (41). A method according to any preceding claim, wherein the calibration filter (40) comprises a filter quality factor. A method according to any preceding claim, wherein the filter quality factor is selected based on minimizing a difference between the target frequency response (22) and the filtered frequency response. Procedure according to Claim 24 , wherein the minimization is carried out within a filter frequency range (30) which includes the target filter frequency (41). Procedure according to Claim 25 , where the minimization is performed by a search algorithm. A method according to any preceding claim, wherein the method further comprises a step of selecting an auxiliary target filter frequency within a selected cutoff frequency range, and wherein the auxiliary target filter frequency is a central frequency of a minima of the local frequency response (21) within the selected cutoff frequency range, and wherein the method further comprises a step of implementing an auxiliary calibration filter related to the auxiliary target filter frequency in the equalizer (14). A method according to any preceding claim, wherein the method further comprises a step of providing an input audio signal (60) and filtering the input audio signal (60) using the calibration filter (40) to provide a filtered audio signal to be reproduced by the speaker system. Method according to one of the preceding claims, wherein one or more steps selected from the steps of applying (S2) an audio test signal, recording (S3) the audio test signal, providing (S4) a local frequency response (21), providing (S5) a target frequency response (22), creating (S6) a difference frequency response (23), generating (S7) a list of exclusion frequency ranges, identifying (S8) one or more filter frequency ranges, selecting (S9) a target filter frequency (41) and implementing (S10) a calibration filter (40) are carried out several times. Loudspeaker system comprising: at least one loudspeaker (11), an audio amplifier (13), an equalizer (14) and an audio signal processor (15, 16), wherein the loudspeaker system is configured to carry out the method according to one of the preceding claims.

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