CN117956385A - Calibration of loudspeaker systems - Google Patents

Abstract

The present invention relates to a method for calibrating a loudspeaker system in an acoustic environment. The method comprises the steps of providing a local frequency response and providing a target frequency response based on the recorded test signal. A difference frequency response is established based on a difference between the local frequency response and the target frequency response. A list of rejection frequency ranges associated with a minimum of the local frequency response is generated, and one or more filtering frequency ranges that do not overlap with the rejection frequency ranges are identified. A target filter frequency is selected from the identified filter frequency range, and calibration filtering related to the target filter frequency is implemented in an equalizer of the loudspeaker system. The invention also relates to a loudspeaker system arranged to carry out the above-mentioned method for calibrating a loudspeaker system.

Description

Calibration of loudspeaker systems

Technical Field

The invention relates to a method of calibrating a loudspeaker system. The invention also relates to a loudspeaker system.

Background

Loudspeaker systems are used in a variety of different acoustic environments, ranging from small loudspeaker systems such as stereophonic sounds in living rooms to larger systems used in large places such as concerts.

The frequency response of loudspeakers in a room is often affected by several undesirable interference effects, such as constructive and destructive interference effects. Compensation for such interference may be performed by equalizing the frequency response using a filter, however for some types of interference, filtering may not be particularly suitable and may in the worst case result in overdriving of the loudspeaker in the loudspeaker system.

Disclosure of Invention

The inventors of the present invention have recognized the challenges mentioned above in relation to the compensation of interference effects and in the following a method of calibrating a loudspeaker system to obtain a desired frequency response without the risk of overdriving any loudspeaker of the loudspeaker system is provided.

An aspect of the invention relates to a method for calibrating a loudspeaker system in an acoustic environment;

wherein the loudspeaker system comprises at least one loudspeaker, an audio amplifier, an equalizer, and an audio signal processor;

Wherein the method comprises the steps of:

Applying an audio test signal to the loudspeaker system to generate an audio test sound in the acoustic environment, recording the audio test sound 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 of the at least one loudspeaker in the acoustic environment;

Establishing a difference frequency response based on a difference between the target frequency response and the local frequency response;

generating a list of rejection frequency ranges associated with a minimum of the local frequency response based on rejection criteria;

identifying one or more filtered frequency ranges associated with a minimum and/or maximum of the difference frequency response, wherein the filtered frequency ranges do not overlap with the rejection frequency ranges;

selecting a target filtering frequency selected from the identified filtering frequency range; and

A calibration filter associated with the target filtered frequency is implemented in the equalizer to provide a filtered frequency response, wherein the calibration filter is arranged to reduce a difference between the filtered frequency response and the target frequency response.

A loudspeaker is a device arranged to convert an audio signal into sound in the form of sound waves (i.e. sound waves). Typically, a loudspeaker includes a loudspeaker diaphragm that reciprocates in accordance with an audio signal to produce pressure waves of air, i.e., acoustic waves.

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

As an example, the sound pressure level L _p (in dB) of the 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:

the reference level for sound that is often used is the perception threshold of a standing person, i.e. the lowest sound pressure that a person may hear. As an example, the reference sound pressure may be of the order of 20 micropas.

When comparing two sound pressure levels of two different sound waves, the difference between the two sound pressure levels may also be referred to as a sound pressure level.

A loudspeaker system is understood to be a system comprising one or more loudspeakers, e.g. comprising a single loudspeaker operating as a separate device or two loudspeakers operating e.g. in association to enable a stereo reproduction of an audio signal. The loudspeaker system may further comprise one or more loudspeaker drive units, e.g. amplifiers providing amplified audio signals to the one or more loudspeakers. A loudspeaker that receives an amplified audio signal from an external drive unit may also be referred to as a passive loudspeaker.

In an embodiment of the invention the loudspeaker system comprises at least one loudspeaker drive unit, e.g. 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 comprise one or more loudspeaker drive units, e.g. one or more amplifiers. A loudspeaker comprising its own drive unit may also be referred to as an active loudspeaker.

An audio amplifier is also understood to be an audio power amplifier. The audio amplifier is arranged to amplify the electronic audio signal. Typically, the audio signal supplied to the loudspeaker is an audio signal that has been amplified by an audio amplifier. Thus, loudspeaker systems typically include an audio amplifier.

The loudspeaker system may comprise an equalizer. An equalizer may be understood as a device arranged to adjust the balance between frequency components of an electronic signal. The equalizer may thus be used to amplify and/or attenuate individual bands or frequency ranges. Some types of equalizers work by implementing frequency filtering, such as calibration filtering, at separate filtering 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. in software. The equalizer may be a parametric equalizer, i.e. a multi-band variable equalizer.

The loudspeaker system may comprise an audio signal processor. An audio signal processor may be understood as a device arranged to process an audio signal in a digital or analog format, and the processing may take place in any domain, for example in the time domain or the frequency domain. The audio signal processor may thus be capable of generating, for example, a representation of a frequency composite of the audio signal. The audio signal processor may further comprise said equalizer, which may be, for example, an audio processing module of the audio signal processor, such as a software implemented audio processing module.

An acoustic environment may be understood as any environment that affects the propagation of acoustic sound from a loudspeaker system, for example, a room, a hall or even an open concert area in front of a stage. In particular, a room may exhibit a strong influence on the acoustic sound that a person in the room will hear. The sound waves may be reflected by, for example, walls and obstacles in the room, and thereby create acoustic wave interference within the room. Wave interference can be understood as the phenomenon of waves in which multiple waves overlap to produce constructive and/or destructive interference. In the region of constructive interference, the waves add up such 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, the peaks of an acoustic wave may meet the troughs of another acoustic wave of the same frequency, and the amplitude is equal to the difference of the individual amplitudes. Furthermore, when multiple loudspeakers produce concurrent sound waves, interference may also occur due to interfering sound waves from different loudspeakers. The interference of waves affecting the reproduction of acoustic sound in a room may also comprise so-called room modes, i.e. standing waves may occur due to physical dimensions in the room (e.g. the distance between hard walls) and create frequencies of spatially fixed nodes and antinodes where the sound is considered to be particularly attenuated or intensified.

The interference pattern from a speaker system is typically dependent on the frequency of the emitted sound waves. Thus, one frequency may have one interference pattern while another frequency has another frequency pattern. At a given location within the room, a given frequency of sound from the loudspeaker system may thus interfere constructively, and another frequency of sound may interfere destructively.

Interference effects in an acoustic environment, such as a room, are typically most pronounced at frequencies corresponding to wavelengths commensurate with the size of the characteristic dimensions of the acoustic environment, such as the size of the room, e.g., the spacing between walls. The frequency of such protrusions may be in the range from 20Hz to 150Hz, but in principle the interference effect is not limited to any particular frequency range.

The aim of a loudspeaker system is to reproduce an audio signal such that the generated acoustic sound is perceived by a listener as desired, i.e. the generated acoustic sound resembles the audio signal as much as possible. However, at particular locations within the acoustic environment, such as at particular listening locations within the acoustic environment, interference effects may distort the sound emitted by the loudspeaker system. The effect of interference effects on the reproduction of an audio signal can be described by a frequency response. The frequency response may thus describe, for example, a change in sound pressure level across a frequency range, i.e., gain, due to an acoustic environment. An example of an ideal frequency response may be a frequency response characterized by a 0dB gain across the entire frequency range, i.e. a flat frequency response, however, most acoustic environments are prone to interference effects as mentioned above, and thus the frequency response of an 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 audio test sounds. A location within the room, such as a listening location, may perform recording of audio test sounds to provide a recorded test signal. The frequency composite of the recorded test signal will likely be different from the frequency composite of the audio test signal, i.e. some frequencies in the recorded test signal may be less obvious or more obvious than corresponding frequencies in the audio test signal.

The local frequency response may be obtained based on a difference between the recorded test signal and the audio test signal. The composition or shape of the local frequency response may be greatly affected by interference effects. The sound pressure level maximum of the local frequency response may be, for example, due to constructive interference at the frequency of the sound pressure level maximum, and the sound pressure level minimum of the local frequency response may be, for example, due to destructive interference at the frequency of the sound pressure level minimum. The local frequency response may thus be understood as a representation of the effect of interference effects on sound from a loudspeaker system at a specific location within the acoustic environment.

For a person hearing sound from a loudspeaker system, the interference effect due to the acoustic environment can be seen as a distortion of the desired sound. This distortion may be corrected by implementing one or more calibration filters. For example, if the local frequency response includes a sound pressure level maximum, calibration filtering may be implemented in the equalizer such that the audio signal applied to the loudspeaker system is attenuated at the frequency of the audio signal at the sound pressure level maximum of the local frequency response. And similarly, if the local frequency response includes a sound pressure level minimum, calibration filtering may be implemented in the equalizer such that the audio signal applied to the loudspeaker system is amplified at the frequency of the audio signal at the sound pressure level maximum of the local frequency response.

The acoustic environment may provide one or more particularly pronounced destructive interference effects at a particular frequency. This destructive interference may be referred to as an acoustic null (acoustic null), as used hereinafter. The acoustic null may be characterized by, for example, a 9dB reduction in sound pressure level compared to the average sound pressure level of the local frequency response.

In general, the purpose of an equalizer is to change the frequency response of a loudspeaker system using one or more filters. The equalizer may for example be used to smooth the frequency response of the loudspeaker system by applying filters, such as a first order filter and a second order filter, to the audio signal, whereby peaks and/or dips in the local frequency response are compensated or corrected. However, there may be problems associated with such corrections, and these are particularly problems related to the acoustic nulls described above. Implementations of filters that amplify the frequency of the audio signal at an acoustic null may require an excessive amount of energy to amplify, which may simply not be reproducible by the amplifier or may result in damage to the loudspeaker system due to overdrive of the one or more loudspeakers. Alternatively, the equalizer may be arranged to ignore the minimum value of the local frequency response to avoid the problem of overdrive described above, while implementing only one or more filters to correct the maximum value in the local frequency response.

The present invention relates to a method for performing calibration of a loudspeaker system by implementing one or more filters to correct a local frequency response. According to an embodiment 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 partly using external input from a control device, such as a smart phone, a computer or a tablet computer.

The process of the present invention is outlined in this paragraph and described in more detail later. The method relies on measuring a local frequency response within the acoustic environment, which can be compared to a target frequency response to produce a difference frequency response. These frequency responses are analyzed to produce a list of rejection frequency ranges, which are frequency ranges in which the presence of acoustic nulls is expected. The remaining frequency range is then analyzed to identify the target filtering frequency, i.e., the frequency at which the filter can be implemented. When a filter is implemented, a filtered frequency response is provided, which may be calculated based on the local frequency response in combination with parameters of the implemented filter. In general, the parameters of the calibration filtering implemented may be selected to reduce the difference between the filtered frequency response and the target filtered frequency. After implementation of the filters, the procedure may be repeated to implement any number of filters such that the difference between the filtered frequency response and the target frequency response is further reduced while taking into account the acoustic nulls by rejecting the frequency range.

The process of the invention will now be summarised in more detail.

In a preferred embodiment of the invention, the local frequency response is generated by applying an audio test signal to the loudspeaker system to generate an audio test sound, which is recorded at the listening position such that a recorded test signal is provided. 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 sound, a local frequency response may be generated, for example, based on a ratio or difference of the recorded test signal to 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 audio test sounds at a corresponding plurality of listening positions. The local frequency response may be provided based on an averaging of the plurality of recorded sound pressure levels.

The method further comprises the step of providing a target frequency response of the at least one loudspeaker in the acoustic environment. The target frequency response may be understood as the frequency response expected by a listener for a loudspeaker system at a particular location within the acoustic environment, i.e. at a listening location within the acoustic environment. As examples, the target frequency response may be a flat frequency response, or a non-flat frequency response based on, for example, a listener's preference for a loudspeaker system and/or a style or type of audio content to be reproduced by the loudspeaker system. As an example, the desired frequency response may be biased towards amplification of low frequency sounds for playback of some music genres, such as tongue-in-tongue (rap) or hip-hop (hip-pop). Alternatively, the desired frequency response, i.e. the target frequency response, may be biased towards amplification of mid-range frequencies, e.g. frequencies in the range from 300Hz to 3kHz, for playback of audio mainly comprising human speech mainly present in e.g. radio or television broadcasts.

According to an embodiment of the invention, the target frequency response is preprogrammed into the loudspeaker system or externally supplied to the loudspeaker system from a control device, such as a smart phone, computer or tablet computer. As an example, a user or listener of the loudspeaker system may configure a desired frequency response on the control device and supply the desired frequency response to the loudspeaker system as the provided target frequency response. Alternatively, the user or listener may use the control means to select an already preprogrammed frequency response, for example by entering a selection of a preprogrammed frequency response from a list of preprogrammed frequency responses. After selection or configuration of the desired frequency response, the desired frequency response is, for example, wirelessly transmitted to the loudspeaker system and used by the loudspeaker system as a target frequency response.

According to an embodiment of the present invention, the target frequency response may be determined based on the local frequency response, however the target frequency response may be different from the local frequency response. As an example, the target frequency response may be represented as a flat frequency response, which is based on an average of sound pressure levels of the local frequency response.

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

If the local frequency response is the same as the target frequency response, the difference frequency response is represented by a flat frequency response at 0dB, indicating no difference between the local frequency response and the target frequency response. However, the difference frequency response may often bring about several differences between the two due to interference effects in the acoustic environment, and these differences may be represented by several minima/dips below 0dB and/or maxima/peaks above 0dB 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 that is 10dB below the sound pressure level in the target frequency response at that particular frequency. According to an embodiment of the invention, the minimum value below-9 dB in the difference frequency response is referred to as the acoustic null.

The method is based on the use of exclusion criteria. The rejection criteria is used to determine whether the frequency region of the difference frequency response can be considered to be a frequency region that includes acoustic nulls. In a preferred embodiment, the rejection criteria is a sound pressure level threshold. This threshold is used to exclude frequency ranges containing some minima/dips 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. The frequency range of the difference frequency response, including the minimum/dip with a sound pressure level exceeding-9 dB in the negative direction, is added to the list of rejection frequency ranges. The frequency ranges appearing on this list may not be used in the implementation of the filter according to the embodiments of the invention.

The extent/width of this rejection frequency range may be determined, for example, by the zero crossings of the sound pressure level of the difference frequency response. Zero crossing is understood to be a frequency at which the sound pressure level is approximately 0 dB. For example, the rejection frequency range may extend from a first frequency with a sound pressure level of approximately 0dB for the difference frequency response to a second frequency with a sound pressure level of approximately 0dB for the difference frequency response, including no additional zero crossings, while including a sound pressure level minimum below a sound pressure level threshold, such as below-9 dB. Other ways of defining the rejection frequency range may be applied, e.g. by a fixed frequency band, by a steep slope, by a user-defined range, etc.

The method of the present invention further includes identifying one or more filtered 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 a filtered frequency range and a rejection frequency range, such that any frequency range associated with the sound pressure level of the difference frequency response below the sound pressure level threshold is added to the list of rejection frequency ranges and the remaining frequency ranges are added to the list of filtered frequency ranges. The frequency ranges appearing on the list of filtered frequency ranges represent the frequency ranges over which the application of the filter may be based. The steps of identifying the filtering frequency range and the rejection frequency range may preferably be performed within a predefined frequency interval, e.g. a typical audio range of 20Hz to 20kHz, or a narrower range of particular interest, e.g. to improve speech intelligibility in the range of 300Hz to 3kHz, or a range where the benefits of e.g. calibration are particularly pronounced, e.g. within a predefined frequency interval from 20Hz to 150Hz, e.g. from 10Hz to 300Hz, e.g. from 20Hz to 200Hz. The lower and upper limits of these frequency bins may be referred to as calibration method frequency limits.

The next step of the method according to the invention is to select the target filter frequency at which the calibration filtering is to be performed in the equalizer. Calibration may also be referred to as a parametric equalizer filter. The calibration filtering may preferably be with a digital biquad filter, but is not necessarily limited to this example, and other types of filters known to the skilled person may also be implemented. A digital biquad filter can be understood as a second order recursive linear filter with a transfer function in the frequency domain as a ratio of two quadratic functions. The digital biquad filter is an infinite impulse response filter. Which is generally characterized by parameters that may be gain, filter frequency, and filter quality factor. The gain may describe the change in sound pressure level at which the filtering is applied at the filtering frequency. The gain may be positive, e.g. 5dB, or negative, e.g. -5dB, and the calibration filtering may thus be used to amplify or attenuate a frequency range, e.g. a filtered frequency range. The filter quality factor may describe the width of the interval of frequencies around the filter frequency that are affected by the implementation of the filter. One calibration filter may thus be narrow and only affect a relatively narrow frequency range, while the other calibration filter may be wide and affect a relatively wide frequency range, which is parameterized by a filter quality factor. The filter quality factor is thus related to the bandwidth of the filter.

The target filtering frequency is selected from the identified filtering frequency range. In a preferred embodiment, the target filter frequency is selected based on sound pressure level, for example, the frequency of the maximum absolute sound pressure level at which there is a difference frequency response in the filter frequency range is selected as the target filter frequency. In other embodiments, the selection of the target filtering frequency may depend on the integration of sound pressure levels within different filtering frequency ranges, e.g. selecting the target filtering frequency from the filtering frequency range having the largest absolute integrated sound pressure level.

According to the invention, the calibration filtering may then be performed at the target filtering frequency, i.e. the filter frequency of the calibration filtering may be the target filtering frequency.

An implementation of calibration filtering provides a filtered frequency response. The filtered frequency response is thus the resulting frequency response after implementation of the one or more filters. The parameters of the implemented filter are selected to reduce the difference between the filtered frequency response and the target frequency response. For example, in some preferred embodiments, the gain is selected 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 over a filter frequency range associated with the target filter frequency. This selection may be performed based on a fitting procedure. In other embodiments of the invention, the filter quality factor may be selected based on an integrated sound pressure level within a filter frequency range associated with the target filter frequency. In still other embodiments of the invention, the filter quality factor may be selected based on the Full Width Half Maximum (FWHM) of the difference frequency response at the target filter frequency. Selecting the filter quality factor based on the full width half maximum value of the difference frequency response at the target filter frequency may be advantageous because the FWHM value is a representative measure of the width of the peak/dip at the target filter frequency and is thus a suitable measure for determining the width of the filter to be applied, i.e. the filter quality factor.

When the filter parameters are selected, the filter may be implemented in an equalizer of the loudspeaker system. The loudspeaker system may then be ready to receive an input audio signal, which may be filtered by the implemented calibration filtering to provide a filtered audio signal. The filtered audio signal may then be emitted by the loudspeaker system as sound, and within a region of the acoustic environment, the sound may be less distorted than if the filter was not implemented.

Rather than implementing only one filter in accordance with the current method of the present invention, any steps associated with the implementation of a filter may be repeated any number of times to implement any number of filters. There is in principle no restriction as to which steps of the method need to be repeated. For example, in some embodiments, after obtaining a filtered frequency response, this filtered frequency response may be used as a new difference frequency response, and a new target filtering frequency, at which a new filter may be implemented, is selected based on this new difference frequency response. In some of these embodiments, the list of rejection frequency ranges may be updated after implementation of the calibration filtering, while in other embodiments, the list of rejection frequency ranges is not updated.

Embodiments of the invention may include implementing any number of filters, such as between 5 and 20 filters, or even more filters, such as between 5 and 100 filters, such as 25 filters or 75 filters. The number of filters that can be applied depends on the number of available frequency bands in the equalizer and the audio signal to be filtered. Each time the filter is implemented, the filtered frequency response may be further approximated to the target filtered frequency while the acoustic nulls are not specifically considered, as these are associated with the rejection frequency range. Ideally, the method according to the invention may then be able to compensate for interference effects in e.g. an acoustic environment, without damaging the loudspeaker, e.g. overdriving the loudspeaker, or wasting energy due to large pressure compensation at frequencies of acoustic nulls.

According to the present invention, a method for calibrating a loudspeaker system in an acoustic environment is provided. By performing the calibration according to the invention, a frequency response of the acoustic environment is achieved which can be as close as possible to the desired target frequency response. Furthermore, by excluding certain frequency ranges that include minima of sound pressure levels below the threshold sound pressure level, the application of filters to attempt to compensate for these minima is avoided. This is particularly advantageous because these minimums or acoustic nulls may be difficult to compensate but do not drive the loudspeakers of the loudspeaker system, and/or energy for unsuccessful attempts at compensation would be wasted. Thus, a method of compensating for interference effects and at the same time protecting the loudspeaker of the loudspeaker system from overdrive is achieved. Yet another effect is that the power consumption of the loudspeaker system can be reduced, since no power is wasted when trying to compensate for acoustic nulls that cannot be reasonably compensated anyway.

According to an embodiment of the present invention, the step of recording the audio test sound at a listening position in the acoustic environment comprises: the audio test sound is recorded using an electronic processing device that includes a microphone.

An electronic processing device comprising a microphone is understood to be any electronic device arranged to record acoustic sound, such as an audio test signal, and to perform any type of signal processing on the recorded acoustic sound, such as a recorded audio test signal. The electronic processing device may be a smart phone, tablet computer, laptop computer, or any other suitable handheld electronic device including a microphone and a processor. In one embodiment of the invention, the electronic processing device comprises the audio signal processor.

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

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

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 an average sound pressure level of the local frequency response in the frequency interval such that the target frequency response is a constant independent of frequencies within the frequency interval.

According to an 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 associated with the audio signal processor.

Embodiments of the present invention may have several stored predetermined frequency responses. This may for example be used for different calibration purposes, for example for different types of acoustic environments, such as a kitchen, living room, lobby or washroom. Alternatively, it may be used for different types of calibration, e.g. calibration focusing on certain frequency intervals, e.g. calibration of bass frequencies, mid-range frequencies or treble frequencies may be improved.

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

In some embodiments of the invention, the user may define the target frequency response, for example, directly via a speaker system or by a device such as a smart phone, computer, or tablet computer. It may be advantageous to allow a user to tailor the target frequency response to ensure an adaptive calibration method with a high degree of control.

The target frequency response may not be constant across the frequency interval. For example, it may be determined based on the average sound pressure level of the local frequency response in combination with the calculation of the preprogrammed frequency response. In other words, the target frequency response may represent a preprogrammed frequency response of the calculated average modulation of the sound pressure level by the local frequency response. The preprogrammed frequency response, such as a preset equalizer setting, may include any equalizer setting selected from the group consisting of: acoustic, subwoofer, classical, dance music, electronic, hip-hop, jazz, piano, pop, rhythmic brus, rock, and vocal enhancers. The list of equalizer settings described above is non-exhaustive and other settings suitable for other types of audio content are contemplated.

In one embodiment of the invention, the target frequency response is provided by first applying a preset equalizer setting characterized by its own preprogrammed frequency response, and then modulating the frequency response based on the local frequency response. As an example, the sound pressure level of the preprogrammed frequency response may be shifted according to the average sound pressure level of the local frequency response. In another embodiment of the invention, the preliminary target frequency response is generated based on a local frequency response, such as an average of the local frequency responses. This preliminary target frequency response is then modulated by a preprogrammed frequency response, such as the preprogrammed frequency response illustrated in the list of preset equalizer settings described above. In a further embodiment of the invention, the step of providing the target frequency response is performed in a combined calculation step, wherein the local frequency response and the preprogrammed frequency response are taken into account.

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

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

The auxiliary audio test sound may be understood as any acoustic sound produced by the loudspeaker system.

Recording may occur at any recording location within an acoustic environment. The recording may be used to provide a recording position frequency response. The recording location may be, for example, in front of the at least one loudspeaker, or remote from the at least one loudspeaker, for example, anywhere within the acoustic environment. In one embodiment of the invention, the plurality of recordings are performed at a plurality of recording locations within the acoustic environment.

The target frequency response may be based on an average of recorded sound pressure levels of the auxiliary audio test sound. For a single recording, the target frequency response may be based on an average of sound pressure levels of the single recording. For more than one recording of the secondary audio test sound, the target frequency response may be based on a plurality of averages of sound pressure levels; each average sound pressure level is associated with a unique recording of the auxiliary audio test sound at a unique recording location.

Based on the recording of the audio test sound, a target frequency response may be generated. The local frequency response may also be used to generate a target frequency response, for example, matching an average sound pressure level of the target frequency response to a sound pressure level of the local frequency response.

According to an embodiment of the invention, the recording of the auxiliary audio test recording is a near-measured value. Thus, the target frequency response may be obtained by taking a proximity measurement, i.e. a measurement very close to the at least one loudspeaker. The proximity measurement may be performed by placing the loudspeaker away from an acoustic obstacle of the acoustic environment, such as a wall of a room, and placing the microphone very close to the loudspeaker, or in particular very close to the woofer of the loudspeaker. A close proximity is understood to mean a distance of from 0 cm to 50 cm, for example from 1 cm to 30 cm, for example from 5 cm to 20 cm, for example from 5 cm to 15 cm. In this way, the direct sound wave coming out of the loudspeaker (e.g., the woofer of the loudspeaker) is always much larger than the sound pressure level of the sound wave reaching the microphone from reflection in the acoustic environment (e.g., reflection from a wall in the room). The recordings obtained in this way thus represent anechoic measurements of the loudspeaker. This proximity measurement can thus directly serve as the target frequency response. Further, the auxiliary audio test sound may be an audio test sound.

The target frequency response based on at least one recording of the auxiliary audio test sound in front of the loudspeaker may be advantageous because it may isolate the frequency response of the loudspeaker to some extent. The target frequency response may then comprise the frequency response of the loudspeaker, and thus the method will mainly perform calibration to correct for distortions of the acoustic environment, rather than of the loudspeaker.

According to an embodiment of the invention, a proximity measurement is performed to generate a preliminary target frequency response. Next, a local frequency response is provided and an offset between the preliminary target frequency response and the local frequency response is determined. Finally, a target frequency response is generated based on the predetermined frequency response and the offset. Hereby it is achieved that the local frequency response and the target frequency response are at similar general sound pressure levels and that a meaningful difference frequency response can thus be obtained by a subtraction between the two.

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

The target frequency response may be based on a plurality of recordings of auxiliary audio test sounds produced by the loudspeaker system. The plurality of recordings may be performed at a plurality of recording locations within the acoustic environment. The purpose of performing multiple recordings of auxiliary test sounds may be to establish a general impression of the acoustics of the acoustic environment. For example, the acoustic environment may include one or more acoustic nulls, and by recording auxiliary audio test sounds in a plurality of recording locations within the acoustic environment, a target frequency response may be prevented from being established based on the acoustic nulls, whereby the target frequency response would contain an excessively low sound pressure level. By recording in a plurality of recording positions, it is thus possible to effectively smooth out a different change in the frequency response of the acoustic environment due to, for example, an acoustic null. Recording of the secondary audio test sound may be performed by said electronic processing means comprising a microphone. Thus, a user of the loudspeaker system may perform the recording of the auxiliary audio test sound and/or the recording of the audio test sound using the electronic processing device during calibration of the loudspeaker system. This recording procedure may require the user to locate himself/herself at various recording locations within the acoustic environment. Generating the target frequency response based on multiple recordings recorded at various recording locations within the acoustic environment may be further advantageous because this may reduce the impact of noisy measurements, such as the presence of other measurements of sound from unwanted sound sources.

The target frequency response may also be provided by a combination of a preprogrammed/determined frequency response as described above and one or more recordings of auxiliary audio test sounds, e.g. a plurality of recordings of auxiliary audio test sounds. In this sense, the sound pressure level of the preprogrammed frequency response can be effectively modulated by using recordings of test sounds recorded at one or more recording locations within the acoustic environment, i.e. the preprogrammed frequency response is modulated in accordance with the acoustics of a particular acoustic environment.

According to an embodiment of the invention, the auxiliary audio test sound is the audio test sound.

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

According to an embodiment of the invention, the rejection criteria comprises a sound pressure level threshold.

The method according to the invention relies on generating a list of rejection frequency ranges based on rejection criteria. The list of repulsive frequency ranges should preferably comprise frequency ranges in which acoustic nulls are present. The rejection criteria decides whether to add a frequency range to the list of rejection frequency ranges. The use of a sound pressure level threshold as rejection criterion is advantageous in that a sound pressure level that depends on the difference frequency response in the frequency range may exclude the frequency range from filtering. It may furthermore be possible to exclude that the frequency range comprising the acoustic nulls is not filtered.

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

By identifying the minimum/dip in sound pressure level in the difference frequency response and comparing these to a sound pressure level threshold, it can be determined whether the frequency range associated with the minimum should be excluded from filtering, for example from implementation of calibration filtering at frequencies very close to or equal to the center frequency of the minimum. Thereby ensuring that calibration filtering is not implemented at frequencies corresponding to frequencies where acoustic nulls exist at listening positions within the acoustic environment.

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

The acoustic nulls are associated with minima in the local frequency response, however, since multiple minima exist that are not associated with acoustic nulls, there must be a way to distinguish common minima from acoustic nulls minima. This step of distinguishing between the two types of minima may be accomplished by comparing the sound pressure level of the minimum in the difference frequency response to a sound pressure level threshold. As an example, if the sound pressure level threshold is set to 9dB, this means that if the minimum value of the difference frequency response exceeds a negative 9dB value, i.e. -9dB, then the minimum value is considered an acoustic null and the frequency range associated with the minimum value is added to the list of rejection frequency ranges.

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

According to an embodiment of the invention, the frequency range associated with the minimum value of the local frequency response is assigned into the list of rejection frequency ranges when the integrated sound pressure level associated with the minimum value exceeds an integrated sound pressure level threshold.

Not only the extreme values of the sound pressure level of the local frequency response may determine the characteristics of the sound distortion and the acoustic nulls, but also the width associated with said extreme values. Thus, in some embodiments, not only the extremum of the sound pressure level determines whether to add the frequency range to the list of rejection frequencies. For example, a width associated with a minimum of the local frequency response may be considered. Alternatively, an integrated sound pressure level associated with the minimum of the local frequency response may be considered. This may be advantageous for the rejection criteria to ensure an optimal selection of frequency ranges for the list of rejection frequency ranges.

According to an embodiment of the invention, the rejection criteria comprises a frequency interval.

It may be preferable that the calibration method only works in the frequency interval. There is no reason to apply the calibration method at frequencies that are inaudible to humans, for example. Furthermore, there are frequency bins that are generally more sensitive to interference effects. Limiting the calibration method to the relevant frequency interval may significantly reduce the time required to perform the method, as opposed to performing the method over a frequency interval extending, for example, from the interval of 20Hz to 20kHz, etc.

According to an embodiment of the present invention, only the frequency ranges contained in the frequency interval may be added to the list of rejection frequency ranges.

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

In general, the acoustic environment mainly affects the frequency interval, i.e. interference effects such as acoustic nulls are prominent in a certain frequency interval. It may therefore be advantageous to target this interval when applying the method of calibrating a loudspeaker system in order to reduce the time required to carry out the method. The frequency interval may include a range of frequencies from 20Hz to 20kHz, such as from 20Hz to 2kHz, such as from 20Hz to 200Hz, such as from 20Hz to 80Hz, but is not limited to these examples.

According to an embodiment of the invention, the end points of the rejection frequency range are zero crossings based on the difference frequency response.

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

In one embodiment of the invention, the calibration method identifies the entire frequency range between adjacent zero crossings of the sound pressure level of the difference frequency response within the frequency interval. Here, the limitation of the frequency range may be a limitation of the outermost frequency range. Each frequency range is sorted into a filtering frequency range or a rejection frequency range such that any frequency range associated with a sound pressure level below a sound pressure level threshold, e.g., below a difference frequency response of-9 dB, is added to the list of rejection frequency ranges while the remaining frequency ranges are added to the list of filtering frequency ranges.

According to an 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 center frequency of a maximum or minimum value 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 filtering frequency may be a frequency at which calibration filtering is to be performed. In this sense, the target filter frequency may be the center frequency of the calibration filter, i.e. the 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 a maximum absolute sound pressure level within the filtered frequency range, and the target filtered frequency is the frequency of this maximum absolute sound pressure level.

In other embodiments, the selected frequency interval is found based on an integration of the absolute sound pressure level for each filtered frequency range. The frequency interval from which the maximum value is provided from the integration may be a selected filter frequency range from which the target filter frequency is selected. The target filtered frequency may then be found as the maximum value of the absolute sound pressure level, as the average frequency, as a weighted average frequency using the sound pressure level as a weight, or obtained from a fit.

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

The calibration filter may be an infinite impulse response filter, preferably a digital biquad filter, but is not limited to this example. Infinite impulse response filters are advantageous because they can be applied to a relatively narrow frequency range.

According to an embodiment of the invention, the calibration filter is a biquad filter, such as a digital biquad filter.

Biquad filters, such as digital biquad filters, may be characterized as a class of second-order infinite impulse response filters. In the frequency representation, its transfer function is the ratio of the two quadratic functions.

According to an embodiment of the invention, the calibration filtering comprises a filtering gain.

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

The filter gain may be understood as the change in sound pressure level applied by the filter at or very close to the target filter frequency and may be positive or negative.

Within the calibration method of the present invention, it is advantageous that the gain of the calibration filter can be selected such that the filtered frequency response can be as close to the target frequency response as possible.

According to an embodiment of the invention, the filter gain may be selected such 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, e.g. the same or approximately the same sound pressure level.

According to an embodiment of the invention, the calibration filtering comprises a filtering quality factor.

The filter quality factor or Q factor is an equalizer dependent factor. It is understood as the ratio of the center frequency to the bandwidth, i.e. the ratio between the target filter frequency and the frequency width around the target filter frequency. For a fixed target filtering frequency, the bandwidth is inversely proportional to the Q factor, which means that as Q increases, the bandwidth narrows. The quality factor is a useful tool for parametric equalizers because it allows the signal to be attenuated or enhanced over a very narrow or very wide frequency range within each equalizer band. Wide and narrow bandwidths (low and high Q, respectively) may be used in combination with each other to achieve the desired effect.

According to an embodiment of the invention, the filtering quality factor is selected based on a minimization of a difference between the target frequency response and the filtered frequency response.

According to an embodiment of the invention, the minimization is carried out by a search algorithm.

The filter quality factor may be selected to minimize the difference between the target frequency response and the filtered frequency response. This selection may be performed by a fitting algorithm or a search algorithm. For example, a list of possible filter quality factors is provided, and a binary search algorithm performs a search of this list to find the filter quality factor that provides the smallest difference between the target frequency response and the filtered frequency response. As an example, a quality factor or Q value that provides a minimum difference between the target frequency response and the filtered frequency response at the target filter frequency may advantageously be found from a reduced Q value range. The difference between the target frequency response and the filtered frequency response has been shown to exhibit some dependence on the Q value. The observations show that the difference exhibits a second-order dependence on the Q value, and that this dependence exhibits only a single extreme where the difference is minimal. The search algorithm may specifically consider this observation when looking for the best possible quality factor. The search algorithm may begin by establishing the difference at a given starting Q value and then establishing the difference for adjacent Q values, i.e., smaller and larger Q values. From these Q values it can be determined whether the minimum difference is obtained by a Q value that is less than or greater than the given starting Q value. Thus, due to the above-mentioned dependencies, a large number of Q values can be ignored, and the optimal Q value can be found faster than if all Q values had to be evaluated. After determining that the more appropriate Q value stays at a higher or lower value than the starting Q value, the search algorithm may continue to evaluate the difference at the Q value that stays in the middle of the Q value interval, starting from the starting Q value and ending at the upper or lower limit of the applicable Q value of the equalizer. The search algorithm may perform this method in multiple steps in order to establish an optimal Q value for the filter to be applied at the target filtering frequency.

According to an embodiment of the invention, the minimizing is performed within a filter frequency range including the target filter frequency.

The minimization of the difference between the target frequency response and the filtered frequency response may be performed within a filter frequency range in which the target filter frequency range is located. In one embodiment of the present invention, the difference between the target frequency response and the filtered frequency response is calculated, this difference is squared, and the difference of this square is integrated over a frequency range that includes the target filtered frequency. The filter quality factor is then selected to minimize the sum of squares of this integration.

According to an embodiment of the invention, the method further comprises the step of selecting an auxiliary target filtering frequency within a selected rejection frequency range, and wherein the auxiliary target filtering frequency is a center frequency of a minimum value of the local frequency response within the selected rejection frequency range, and wherein the method further comprises the step of implementing auxiliary calibration filtering in relation to the auxiliary target filtering frequency in the equalizer.

Providing complete compensation for acoustic nulls may be problematic, however, it may be advantageous to perform partial compensation. Thus, in some embodiments of the invention, the auxiliary calibration filtering may be implemented at an auxiliary target filtering frequency that is within the rejection frequency range. The auxiliary calibration filter may be a different type of filter than the calibration filter, which may be limited in gain. For example, the auxiliary calibration filtering may include a filter gain auxiliary filter gain, and the magnitude of this filter gain auxiliary filter gain may be limited to, for example, 9dB. The implementation of the auxiliary calibration filtering is not limited to any particular time occurring during the steps of the calibration method. The auxiliary calibration filter may be implemented after the implementation of the calibration filter, before the implementation of the calibration filter, or between two implementations of the calibration filter.

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

When one or more filters have been implemented in the equalizer, the loudspeaker system may be considered to be calibrated to the acoustic environment with respect to a certain recording location. The loudspeaker system may then receive any input audio signal and apply any implemented filters to this input audio signal to provide a filtered audio signal that may be emitted into the acoustic environment by one or more loudspeakers of the loudspeaker system. The input audio signal may be, for example, any type of audio, such as 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 following steps are performed a plurality of times: applying an audio test signal, recording the audio test sound, providing a local frequency response, providing a target frequency response, establishing a difference frequency response, generating a list of rejection frequency ranges, identifying one or more filtering frequency ranges, selecting a target filtering frequency, and performing calibration filtering.

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

In some embodiments of the invention, the difference frequency response may be updated based on the filtered frequency response, e.g., the filtered frequency response is applied as an updated difference frequency response. This difference frequency response may then serve as a basis for additional target filtering frequencies, where additional calibration filtering may be implemented in the equalizer.

In these embodiments, the list of rejection frequency ranges and the filtered frequency range may be updated based on the updated difference frequency response.

In other embodiments of the present invention, the audio test signal is supplied to the equalizer based on any implemented calibration filtering in the equalizer to produce a filtered audio test signal. The filtered audio test signal may then be used by one or more loudspeakers of a loudspeaker system to produce filtered audio test sounds in an acoustic environment, which in turn may be recorded to provide a filtered recorded test signal that serves as a basis for an updated local frequency response. Based on the updated local frequency response, additional calibration filtering may be implemented.

The iteration of the calibration method may be repeated any number of times, for example, until the target frequency response criterion is met or a predefined number of times, for example, 5 times or 20 times. The target frequency response criterion may be understood as a criterion of evaluating the filtered frequency response to determine whether it is sufficiently similar to the target frequency response, e.g., such evaluation may determine whether the sound pressure level difference between the filtered frequency response and the target frequency response in any of the filtered frequency ranges exceeds 3dB, and if the difference exceeds 3dB, additional calibration filtering may be implemented. The target frequency response criteria is not limited to this example.

By utilizing an iterative calibration method in which a plurality of iterations are performed, it is possible to reduce distortion in an acoustic environment to a higher degree than in the case where only a single filter is implemented.

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 any of the preceding claims.

A loudspeaker system is understood to be a system comprising one or more loudspeakers and one or more loudspeaker drive units. According to an embodiment of the invention, the loudspeaker system is a single separate device, e.g. an active loudspeaker. In another embodiment of the invention, the loudspeaker system is a distributed system comprising a plurality of electrical connection means, e.g. two or more passive loudspeakers electrically connected to a loudspeaker drive unit, e.g. an amplifier. The equalizer may be an integral component of the amplifier or may be a dedicated device.

According to an embodiment of the invention, the loudspeaker system may be arranged to facilitate the calibration method according to the method of the invention. This may require an equalizer, wherein calibration filtering may be implemented such that any input audio signal provided to the loudspeaker system may be filtered by the calibration filtering.

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

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

The audio signal processor may be an internal audio signal processor and may be an external audio signal processor. An external audio signal processor may be understood as an audio signal processor integrated into an external device and not integrated into the loudspeaker system controller. The external audio signal processor may be characterized in that the loudspeaker system controller may receive, independently of the external audio signal processor, an input audio signal to be emitted as sound by one or more loudspeakers of the loudspeaker system. Instead, the internal audio processor may be integrated in the loudspeaker system controller. Embodiments of the present invention may include an internal audio signal processor and an external audio signal processor.

The external audio signal processor may be in an external device, i.e. an electronic processing device, such as a smart phone, a laptop computer or a tablet computer. The external device may communicate with any other part of the loudspeaker system, such as an equalizer or an internal audio signal processor, through any connection means. Examples of connection elements are wired connections such as cable connections and wireless connections such as bluetooth connections (e.g. bluetooth A2DP or bluetooth aptX), or Wi-Fi connections.

The method according to the invention comprises the following steps: applying an audio test signal, recording an audio test sound, providing a local frequency response, providing a target frequency response, generating a list of rejection frequency ranges, identifying a filter frequency range, selecting a target filter frequency, and performing calibration filtering. Any of the steps of the method of the present invention may be performed on an internal audio signal processor and any of the steps of the method may be performed on an external audio signal processor.

For example, in some embodiments of the invention, the external audio signal processor may perform the step of recording audio test sounds which are then provided to the internal audio signal processor arranged to perform other steps of the method. In other embodiments of the invention, the external audio signal processor may provide a local frequency response, provide a target frequency response, establish a difference frequency response, generate a list of rejection frequency ranges, identify a filtering frequency range, and select a target filtering frequency.

Drawings

Various embodiments of the invention will be described hereinafter with reference to the accompanying drawings, in which

Fig. 1 shows an acoustic environment of a loudspeaker system that may be calibrated,

Fig. 2a-2b illustrate steps of providing a target frequency response,

FIGS. 3a-3d show steps of performing calibration filtering based on analysis of frequency response according to a preferred embodiment of the invention, FIG. 4 shows a flow chart describing a calibration method according to an embodiment of the invention, and

Fig. 5a-5b show different arrangements of a loudspeaker system according to an embodiment of the invention.

Detailed Description

Fig. 1 shows an acoustic environment 50, such as a room, in which loudspeakers 11 of a loudspeaker system are arranged to emit acoustic sound. Due to the arrangement of the room, i.e. the manner in which the walls, floor, ceiling and possibly furniture are arranged, it may be that the sound emitted by the loudspeakers 11 is not perceived as desired by a listener/user in the room. In other words, the acoustic effect of a room affects the perception of sound in the room, or in other words, the arrangement of the room has a great influence on the frequency response of the room.

The local frequency response and the target frequency response will be mentioned in the detailed description that follows. The local frequency response is the frequency response of the loudspeaker system enhanced/affected by the arrangement of the acoustic environment, and the target frequency response is the frequency response desired by a listener in the room. The steps of the method according to the invention comprise providing a local frequency response and a target frequency response, with the intention that at least one calibration filtering is performed such that the filtered frequency response may be close to the target frequency response compared to the local frequency response. The listener may decide to calibrate the loudspeaker system so that the local frequency response at the listening position 52 becomes as close as possible to the intended target frequency response.

In the embodiment of the invention shown in fig. 1, the target frequency response is based on recordings of audio test sounds, and these recordings are performed at different recording positions 51 within the acoustic environment. For example, recording of audio test sounds is performed at each of the three illustrated recording locations 51 to provide three recorded test signals. Each recorded test signal may then be used to generate a recording position frequency response 25 such that a total of three recording position frequency responses are provided. The three recorded position frequency responses may then be averaged to provide a target frequency response based on which the difference frequency response is based. In other embodiments of the invention, three recorded test signals may be averaged and a target frequency response may be generated based on the averaged recorded test signals. The local frequency response may then be generated based on the recording of the audio test sound at the listening position 52.

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

Each recording performed at the recording position 51 serves as a basis for providing a recording position frequency response. In fig. 2a, three recording position frequency responses 25 are shown and these are obtained from performing recordings at three recording positions 51 as shown in fig. 1, however the recording positions may be anywhere in the room and in other embodiments of the invention the number of recordings is not limited to 3, but may be any number of recordings. As shown in fig. 2a, the recording position frequency responses 25 are different from each other, which illustrates the effect of the arrangement of the acoustic environment 50 on the frequency response, being different for different recording positions 51. The recording position frequency response 25 shows the sound pressure level (in dB) as a function of frequency. In this embodiment the frequency of the recording position frequency response 25 is in the range from 10Hz to slightly more than 200Hz, however in other embodiments of the invention the frequency interval may be any other interval.

Fig. 2b shows the average of the recording position frequency response at 26, which is the average of the three recording position frequency responses 25 as shown in fig. 2 a. In this embodiment of the invention the target frequency response is chosen as the mean of the mean 26 of the recording position frequency response, at 27, and the mean 27 of the recording position frequency response is calculated within the upper and lower calibration method frequency limits 33, which in this embodiment are 20Hz and 200Hz, although in other embodiments of the invention these limits 33 may take on other frequency values.

In other embodiments of the invention, the average 26 of the recorded position frequency response may even be directly regarded as the target frequency response. Alternatively, in some embodiments, the average 27 of the recording position frequency response may be regarded as the basis of the target frequency response, wherein the average may be understood as the average across the frequency bins such that the average 27 of the recording position frequency response is constant as shown in fig. 2 b.

Figures 3a-3d illustrate steps for implementing one or more calibration filters, according to an embodiment of the invention.

Fig. 3a shows a non-averaged local frequency response 20, which represents the frequency response measured at the listening position 52. The measured non-averaged local frequency response 20 of this embodiment is smoothed to provide a local frequency response 21, which can be seen as a smoothed curve on the graph of fig. 3 a.

For example, the local frequency response 21 is an 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 the target frequency response 22 as the desired frequency response. Finally, the frequency response diagram also shows a filtered frequency response 28, which is obtained after two calibration filters are used in the equalizer 14 (not shown) of the loudspeaker system. How to determine these two calibration filters will be described in detail below.

The goal of the calibration method is to implement one or more calibration filters such that the filtered frequency response 28 is within the frequency limits 33 of the calibration method, as close as possible to the target frequency response 22, without compensating for acoustic nulls (acoustic nulls), which may be considered as large minima/dips (LARGE MINIMA/dips) of the local frequency response 21.

To carry out the method, a difference frequency response 23 is also provided, based on the difference between the local frequency response 21 and the target frequency response 22. Fig. 3b shows a frequency response diagram containing a difference frequency response 23. In this example, the difference frequency response 23 is obtained by subtracting the target frequency response 22 from the local frequency response 21. Next, within the calibration method frequency limits 33, several frequency ranges are identified, and in an embodiment of the invention, these are identified based on zero crossings of the difference frequency response 23, i.e. based on the difference frequency response 23 being at a frequency of 0 dB. In other embodiments of the invention, the frequency ranges are identified in different ways.

The frequency range includes a rejection frequency range 31 that is associated with a large dip/minimum in the difference frequency response 23. These large dips represent acoustic nulls due to destructive interference of the sound emitted by the loudspeaker 11 of the loudspeaker system. The rejection frequency range is selected based on rejection criteria. In this embodiment, the rejection criteria is a criterion based on a sound pressure level threshold 32 of-9 dB, but other rejection criteria may be used in other embodiments of the invention. When the dip in the sound pressure level of the difference frequency response 23 is below-9 dB, the corresponding frequency range 31 is added to the list of rejection frequency ranges. This list of rejection frequency ranges specifies which frequency ranges do not need compensation, i.e. frequency ranges in which calibration filtering should not be performed. The frequency ranges further comprise a filtered frequency range 30, which is a frequency range related to the maximum and minimum values in the difference frequency response 23, however these do not relate to the maximum minimum value, i.e. the rejection frequency range 31. In this embodiment of the invention, rejection filter frequency range 31 and filter frequency range 30 are shown as non-overlapping or disjoint.

Fig. 3c shows the next step of the method in which the target filtering frequency 41 is selected. The drawing is based on the same drawing as shown in fig. 2 b. The target filter frequency 41 is selected based on the maximum absolute sound pressure level of the difference frequency response 23 within the calibration method frequency limit 33. As can be seen, this target filtering frequency does not belong to the rejection filtering frequency range 31 as identified in fig. 3 b. The target filter frequency sets the center frequency of the calibration filter 40 to be implemented, which in this example is a digital biquad filter. Typically, this calibration filtering is characterized by a center filter frequency, or target filter frequency 41, filter gain, and filter quality factor. The target filter frequency 41 is used as the frequency at which the 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 0dB 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 equivalent to the maximum value to be filtered. It should be noted that the filter gain 42 is opposite in magnitude to the maximum value in the difference frequency response 23 to be corrected. Another feature of the calibration filter 40 is to include a quality factor, which is the ratio of the filter gain 42 to the bandwidth of the filter. In this embodiment, the calibration filter 40 with a high quality factor corresponds to a filter with a narrow frequency, and the calibration filter with a low quality factor corresponds to a filter with a wide frequency. The filter gain 42 and the filter quality factor are selected to minimize the difference between the filtered frequency response 24 and the target frequency response 22.

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

The calibration method is thus based on searching for the minimum value of the difference between the resulting filtered frequency response and the target frequency response, finding the preferred filter gain 42 and filter quality factor with a minimization procedure, and implementing the corresponding calibration filter 40 in the equalizer of the loudspeaker system. Represented by the frequency of the implemented calibration filter 40. By subtracting the frequency representation of the calibration filter 40 from the difference frequency response 23, a difference frequency response based on the filtered frequency response 24 is obtained that is closer to zero sound pressure level around the target filter frequency.

In a preferred embodiment of the present invention, at least one additional calibration filter 40 is implemented. In fig. 3d it is shown how this additional calibration filtering 40 is implemented at a new target filtering frequency 41 slightly below 50 Hz. Since this target filtering frequency 41 is associated with the filtering frequency range 30 associated with the minimum value of the difference frequency response 23, and the minimum value has a sound pressure level not below the sound pressure level threshold 32, i.e. it is not associated with the rejection frequency range, the calibration filtering 40 may be applied.

The calibration filter 40 of fig. 3d is determined in a similar way as the calibration filter shown in fig. 3c, however this calibration filter 40 uses a smaller filter gain 42, since the amplitude of the minimum value in the difference frequency response 23 to be corrected is smaller than the amplitude of the maximum value compensated for by the previous calibration filter. Furthermore, the filtering is designed to have a positive gain, since the peak to be corrected is the minimum value and not the maximum value as the previous filtering.

After performing the calibration filtering 40 twice, a filtered frequency response 28 is obtained (see fig. 3 a). As seen on fig. 3a, the filtered frequency response 28 is closer to the target frequency response 22 than the local frequency response 21 at the target filter frequency for the two calibration filters. In other embodiments of the present invention, additional calibration filtering is implemented such that the filtered frequency response 28 may become or even be closer to the target frequency response 22.

In other embodiments of the invention, wherein a plurality of calibration filters 40 are implemented in an equalizer of a loudspeaker system, the filtered frequency response 28 obtained after the first calibration filter 40 is implemented may be used as a new local frequency response, and the new local frequency response is subtracted from the same target frequency response to calculate a new difference frequency response. The new difference frequency response is then obtained by analysis in a similar manner as described above, new calibration filtering is implemented in the equalizer, and a new filtered frequency response is obtained. The method can thus be regarded as a recursive method in which the filtered frequency response is applied as an input (as a local frequency response) and a filtered frequency response is obtained that is closer to the improved target filtered frequency response.

Fig. 4 shows a flow chart describing a calibration method according to an embodiment of the invention.

Initially, the audio test signal S1 is provided to the loudspeaker 11 of the loudspeaker system present in the acoustic environment 50. Next step S2, the loudspeaker 11 emits a corresponding audio test sound, which is recorded by the microphone 12 of the electronic processing device to provide a recorded test signal (step S3). The electronic processing device in this embodiment may be a smart phone, however other electronic processing devices such as a tablet computer or notebook computer such as a laptop computer may be used in other embodiments of the invention. Based on the step S1 audio test signal and the step S3 recording test signal, a local frequency response is provided (step S4). In addition, a target frequency response is provided in step S5. The target frequency response in this example is based on performing additional recordings of audio test sounds at various recording locations 51 within the acoustic environment 50. In other embodiments of the invention, the target frequency response 22 is provided based on auxiliary audio test sounds, which may be sounds emitted by one or more loudspeakers 11 and which are different from the audio test sounds. In still other embodiments of the present invention, the target frequency response 22 is provided based on the local frequency response 21 and/or the preprogrammed frequency response.

Next, in step S6, a difference frequency response 23 is established based on the difference between the local frequency response 21 provided in step S4 and the target frequency response 22 provided in step S5 (step S6). Based on the difference frequency response 23, a filtered frequency range 30 is identified in step S7 and a rejection frequency range 31 is identified in step S8.

Next, in step S9, a target filter frequency 41 is selected within one of the identified filter frequency ranges 30. Next in step S10, the 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 quality factor such that the calibration filter can achieve 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 based on a minimization procedure that searches for a quality factor that minimizes the difference between the resulting filtered frequency response 28 and the target frequency response 22 as much as possible. The minimization procedure is performed by a search algorithm that searches for the best 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 is provided based on the configured calibration filter 40.

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

Some embodiments of the present invention are based on iterations of some of the steps previously mentioned, and these embodiments relate to implementation of a plurality of calibration filters 40. In some examples of these embodiments, after the filtered frequency response has been provided (step S11), the difference frequency response provided in step S6 is updated, which is based on the difference between the target frequency response provided in step S5 and the filtered frequency response provided in step S11, instead of the 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, additional calibration filtering can be implemented in the equalizer, and the filtered frequency response can be updated. This procedure may be repeated any number of times repeatedly.

When one or more calibration filters have been implemented in the equalizer, the loudspeaker system is ready to receive an input audio signal 60 in step S13, which is filtered according to the one or more calibration filters 40 implemented in the equalizer of the loudspeaker system to provide a filtered audio signal that is emitted as sound by the one or more loudspeakers 11. In turn, a reproduction of the input audio signal 60 is obtained which takes into account the acoustics of the acoustic environment 50 without overdriving any loudspeaker 11 to compensate for the acoustic nulls in the acoustic environment 50.

Fig. 5a-b show embodiments of the invention in which a user 70 of a loudspeaker system may use an electronic processing device, such as a smart phone device 18, to control the calibration method as described in detail above.

As seen in fig. 5a, the loudspeaker system comprises a loudspeaker system controller 19 comprising an amplifier 13, an equalizer 14 and an internal audio signal processor 15. The loudspeaker controller 19 is arranged to receive an input audio signal 60 which is to be filtered in the equalizer 14, amplified in the amplifier 13 and emitted by the loudspeaker 11.

In this embodiment of the invention, the calibration method of the invention is controlled by the user 70 using the smart phone device 18. The user 70 of the loudspeaker system initiates the calibration method by using the smart phone device 18, for example by pressing a screen button of the smart phone device 18. The instructions to initiate the calibration method may be received by the communication interface 17 of the loudspeaker system controller 19, which enables wireless communication with the smart phone device 18. The smartphone device 18 may prompt the user 70 to locate himself/herself at one or more recording locations 51 (not shown here) within the acoustic environment 50 to establish the target frequency response 22 by recording audio test sounds using the microphone 12 of the smartphone device 18 to obtain one or more recording location frequency responses 25.

The smartphone device 18 then prompts the user 70 to position himself/herself at the listening position 52, which represents the position the user is typically located at when hearing sound (e.g., music) from the loudspeaker system. Based on the recording of the audio test signal by the smart phone device 18, a local frequency response 21 is generated and the method continues with implementation of the calibration filter 40 into the equalizer 14 of the loudspeaker system, as detailed above.

In other embodiments of the invention, the audio test signal or auxiliary audio test signal used to obtain the recording position frequency response 25 for obtaining the target frequency response 22 is different from the audio test sound used to obtain the local frequency response 21.

In other embodiments of the present invention, the target frequency response is provided in a different manner, for example also based on a preprogrammed frequency response that the user 70 can select on the smartphone device 18. For example, a user may select a preprogrammed frequency response on the smartphone device 18 that is appropriate for a certain type of music, and this preprogrammed frequency response may then be appropriate based on the recorded target frequency response.

According to this embodiment of the invention, the smart phone device 18 comprises an external audio signal processor 16 (external to the loudspeaker system) arranged to perform processing of the audio recorded by the microphone 12. The steps of the method as described above may be carried out in any way between the external audio signal processor 16 and the internal audio signal processor 15. In an embodiment utilizing several processors as shown in fig. 5a, the processors are communicatively associated, e.g., the smart phone device 18 and the loudspeaker system controller 19 each include a communication interface 17 enabling wireless communication between the devices.

The method steps of establishing the difference frequency response 21 may be performed using the smart phone device 18 in some embodiments and may be performed using the loudspeaker system controller 19 in some other embodiments.

Fig. 5b shows another embodiment of the invention, wherein 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 transducer unit 81, which is a component of the active loudspeaker, which converts the audio signal into acoustic sound by using a loudspeaker diaphragm, e.g. actuated by a voice coil. It furthermore comprises a communication interface 17 controlled by a user 70 of the loudspeaker system, enabling communication with an electronic processing device, such as a smart phone device 18. The active loudspeaker may thus function in a similar manner to the loudspeaker system described in the embodiment of fig. 5a, and the method of implementing the calibration filter 40 in the equalizer 14 is the same.

In the embodiment shown in fig. 5b, the steps of the inventive method may be distributed in any way between the external audio signal processor 16 of the smart phone device 18 and the internal audio signal processor 15 of the active loudspeaker 80. For example, the user may initiate a calibration and the microphone 12 of the smart phone device 18 may record audio test sounds to generate a recorded test signal that is communicated to the internal audio signal processor 15 of the active loudspeaker 80, with the remaining steps of the method being performed to ultimately implement at least one calibration filter in the equalizer 14.

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

List of reference numerals:

11. Megaphone

12. Microphone

13. Audio amplifier

14. Equalizer

15. Internal audio signal processor

16. External audio signal processor

17. Communication interface

18. Smart phone device

19. Loudspeaker 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. Averaging of recorded position frequency response

27. Recording the mean of the position frequency response

28. Filtered frequency response

30. Filter frequency range

31. Rejection frequency range

32. Threshold sound pressure level

33. Calibration method frequency limitation

40. Calibration filtering

41. Target filtering frequency

42. Amplitude of filter gain

50. Acoustic environment

51. Recording position

52. Listening position

60. Input audio signal

70. User' s

80. Active loudspeaker

81. Transducer unit

S1-S13 calibration method flow chart steps

Claims (30)

1. A 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);

Wherein the method comprises the steps of:

Applying an audio test signal to the loudspeaker system to generate an audio test sound in the acoustic environment (50), recording the audio test sound at a listening position (52) in the acoustic environment (50) to obtain a recorded test signal, and providing a local frequency response (21) based on the recorded test signal;

-providing a target frequency response (22) of the at least one loudspeaker (11) in the acoustic environment (50);

-establishing a difference frequency response (23) based on a difference between the target frequency (22) response and the local frequency response (21);

Generating a list of rejection frequency ranges associated with a minimum of the local frequency response (21) based on rejection criteria;

Identifying one or more filtered frequency ranges (30) associated with a minimum and/or maximum of the difference frequency response (21), wherein the filtered frequency ranges (30) do not overlap with the rejection frequency ranges (31);

-selecting a target filtering frequency (41) selected from the identified filtering frequency range (30); and

-Implementing a calibration filter (40) related to the target filter frequency (41) in the equalizer (14) to provide a filtered frequency response (28), wherein the calibration filter (40) is arranged to reduce a difference between the filtered frequency response (28) and the target frequency response (22).

2. The method of claim 1, wherein the step of recording the audio test sound at a listening position (52) in the acoustic environment (50) comprises: the audio test sound is recorded using an electronic processing device (18) including a microphone (12).

3. The method of the preceding claim 1, wherein the target frequency response (22) is based on a predetermined frequency response.

4. The method of the preceding claim 1, wherein the target frequency response (22) is defined by a user of the loudspeaker system.

5. The method of the preceding claim 1, wherein the target frequency response (22) is based on a recording of auxiliary audio test sounds.

6. The method of claim 5, wherein the recording of secondary audio test sounds is a proximity measurement.

7. The method of claim 5, wherein the target frequency response (22) is based on a plurality of recordings of the auxiliary audio test sound.

8. The method of claim 5, wherein the auxiliary audio test sound is the audio test sound.

9. The method of the preceding claim 1, wherein the rejection criteria comprises a sound pressure level threshold (32).

10. The method of claim 9, wherein the frequency range (31) is assigned into the list of rejection frequency ranges when an absolute value of a sound pressure level of the difference frequency response (21) exceeds the sound pressure level threshold (32) within a frequency range (31) associated with a minimum value of the local frequency response (21).

11. The method of claim 10, wherein the sound pressure level threshold (32) is a threshold in a range from 2dB to 20 dB.

12. The method according to the preceding claim 1, wherein when a bandwidth associated with a minimum value of the local frequency response (21) exceeds a bandwidth threshold, a frequency range (31) associated with the minimum value is assigned into the list of excluded frequency ranges.

13. The method of the preceding claim 1, wherein when an integrated sound pressure level associated with a minimum value of the local frequency response (21) exceeds an integrated sound pressure level threshold value, a frequency range (31) associated with the minimum value is assigned into the list of rejection frequency ranges.

14. The method of claim 1, wherein the rejection criteria comprises a frequency interval.

15. The method of claim 14, wherein the frequency interval is included in a range from 10Hz to 500 Hz.

16. The method according to claim 1, wherein the end points of the rejection frequency range (31) are based on zero crossings of the difference frequency response (21).

17. The method according to claim 1, wherein the end points of the filter frequency range (30) are based on zero crossings of the difference frequency response (21).

18. The method according to claim 1, wherein the target filter frequency (22) is within a selected filter frequency range (30) of the one or more filter frequency ranges (30) and is a center frequency of a maximum or minimum value of the difference frequency response (21) within the selected filter frequency range (30).

19. The method according to claim 1, wherein the calibration filter (40) is an infinite impulse response filter.

20. The method according to claim 1, wherein the calibration filtering (40) is with a biquad filter.

21. The method according to claim 1, wherein the calibration filtering (40) comprises a filtering gain.

22. The method of claim 21, wherein the filter gain is based on a sound pressure level of the difference frequency response (21) at the target filter frequency (41).

23. The method according to claim 1, wherein the calibration filtering (40) comprises a filtering quality factor.

24. The method of the preceding claim 1, wherein the filter quality factor is selected based on a minimization of a difference between the target frequency response (22) and the filtered frequency response (28).

25. The method of claim 24, wherein the minimizing is performed within a filter frequency range (30) comprising the target filter frequency (41).

26. The method of claim 25, wherein the minimizing is performed by a search algorithm.

27. The method according to the preceding claim 1, wherein the method further comprises the step of selecting an auxiliary target filtering frequency within a selected rejection frequency range (31), and wherein the auxiliary target filtering frequency is a center frequency of a minimum value of the local frequency response (21) within the selected rejection frequency range (31), and wherein the method further comprises the step of implementing an auxiliary calibration filtering in relation to the auxiliary target filtering frequency in the equalizer (14).

28. The method according to the preceding claim 1, wherein the method further comprises the step of providing an input audio signal (60) and filtering, using the calibration filter (40), what is the input audio signal (60) to provide a filtered audio signal to be reproduced by the loudspeaker system.

29. The method according to claim 1, wherein one or more steps selected from the group consisting of: -applying an audio test signal, -recording the audio test sound, -providing a local frequency response (21), -providing a target frequency response (22), -establishing a difference frequency response (23), -generating a list of rejection frequency ranges (31), -identifying one or more filter frequency ranges (30), -selecting a target filter frequency (41), and-applying a calibration filter (40).

30. A loudspeaker system, comprising:

at least one loudspeaker (11), an audio amplifier (13), an equalizer (14) and an audio signal processor (15), wherein the loudspeaker system is arranged to carry out the method according to any of the preceding claims.