EP2456229A1

EP2456229A1 - Loudspeaker system and control method

Info

Publication number: EP2456229A1
Application number: EP10191594A
Authority: EP
Inventors: Temujin Gautama
Original assignee: Knowles Electronics Asia Pte Ltd
Current assignee: Knowles Electronics Asia Pte Ltd
Priority date: 2010-11-17
Filing date: 2010-11-17
Publication date: 2012-05-23
Also published as: WO2012066029A1

Abstract

A loudspeaker system combines a loudspeaker, a sealed loudspeaker enclosure and a pressure sensor in the sealed enclosure, preferably implemented as a microphone. The signal registered by the microphone within the loudspeaker enclosure is a signal that is directly related to the loudspeaker diaphragm displacement.

Description

This invention relates to a loudspeaker system.

It is well known that the output of a loudspeaker should be controlled in such a way that it is not simply driven by an input signal. For example, an important cause of loudspeaker failures is a mechanical defect that arises when the loudspeaker diaphragm is displaced beyond a certain limit, which is usually supplied by the manufacturer. Going beyond this displacement limit either damages the loudspeaker immediately, or can considerably reduce its expected life-time.
There exist several methods to limit the displacement of the diaphragm of a loudspeaker, for example by processing the input signal with variable cutoff filters (high-pass or other), a gain stage, or a dynamic range compression module, the characteristics of which are controlled via a control feedback loop.
The measured control signal is referred to as the displacement predictor and it conveys information on how close the loudspeaker is driven to the displacement limit by the input signal. The control method requires modelling of the loudspeaker characteristics so that the displacement can be predicted in response to a given input signal. The model predicts the diaphragm displacement, also referred to as cone excursion, and it can be linear or non-linear.
The control system can be used for loudspeaker protection as mentioned above and also linearisation of the loudspeaker output. The input signal is typically pre-processed in such a way that the predicted displacement stays below the limit.
The parameters of the loudspeaker model, which is used for the prediction of the displacement, are estimated on the basis of measurements of the voltage across and the current flowing into the loudspeaker voice coil. To identify the parameter values of a loudspeaker model, the voice coil voltage and current are thus required. To sense the current flowing into a loudspeaker voice coil, dedicated hardware is required, for example a resistor in series with the voice coil. This affects the electrical characteristics of the loudspeaker circuit or may be cumbersome to implement in such a way that the measurements are for example independent of temperature changes.
It is possible to determine the voice coil displacement mechanically, for example using an optical system which physically (rather than electrically) detects the voice coil position. However, this requires complex additional hardware associated with the loudspeaker.
It would therefore be desirable to determine the voice coil displacement in a more cost effective manner and without requiring electrical measurements of the voice coil current or voltage.
According to the invention, there is provided a loudspeaker system, comprising:

a loudspeaker in a sealed enclosure;
a pressure sensor in the sealed enclosure; and
a processor adapted to derive an estimate of the loudspeaker membrane displacement from the sensed pressure and for controlling the audio processing for the loudspeaker in dependence on the estimated loudspeaker membrane displacement.

The pressure sensor is used to record the pressure in the sealed enclosure, and this is used to characterise the loudspeaker membrane displacement.
The invention is based on the recognition that a pressure sensor registers a signal that can be used as a measure of the diaphragm displacement.
Using the invention, a loudspeaker model can be estimated without a means for sensing the current. Instead, a pressure sensor signal can be used, which only requires standard hardware. Since the pressure sensor signal is directly related to the diaphragm displacement, it yields more accurate information than the voice coil current signal with respect to the loudspeaker non-linearities. Therefore, non-linear models can be more easily and more accurately estimated.
Information regarding the diaphragm displacement can also be obtained using an accelerometer that is mounted onto the loudspeaker diaphragm. However, for small loudspeakers, such as micro-speakers, the mounting of the accelerometer would have a considerable effect on the properties of the loudspeaker.
The pressure sensor can comprise a microphone and the sealed enclosure can comprise the loudspeaker back volume.
The processor is preferably adapted to implement loudspeaker protection on the basis of the estimated loudspeaker membrane displacement. This estimated loudspeaker membrane displacement can thus be derived without voice coil current or voltage monitoring.
The invention also provides a method of controlling a loudspeaker output, comprising:

detecting a pressure in a sealed enclosure in which the loudspeaker is housed;
estimating the loudspeaker diaphragm displacement from the detected pressure; and
controlling audio processing for the loudspeaker in dependence on the estimated loudspeaker membrane displacement.

An example of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a loudspeaker enclosure of the invention;
Figure 2 shows how the pressure sensor signal can be used to estimate diaphragm displacement;
Figure 3 shows a loudspeaker system of the invention; and
Figure 4 shows a loudspeaker control method of the invention.

The invention provides a system which combines a loudspeaker, a sealed loudspeaker enclosure and a pressure sensor in the sealed enclosure, preferably implemented as a microphone. The signal registered by the microphone within the loudspeaker enclosure is a signal that is directly related to the loudspeaker diaphragm displacement.
Figure 1 shows in schematic form the system of the invention.
The system comprises a loudspeaker 10 mounted in a sealed enclosure 12, thus corresponding to a closed-box configuration.
A microphone 14 is mounted within in the sealed enclosure 12.
The microphone can be a completely separate off-the-shelf component that is simply mounted in the enclosure
Under closed-box assumptions, the acoustic pressure, p(t), is constant throughout the enclosure, and it is determined by the changes in the volume of the enclosure, ΔV(t): $p (t) = \frac{- Δ V (t)}{V} {ρ c}^{2},$

where V₀ is the volume when the diaphragm is in its rest position, ρ is the density of air and c is the speed of sound. The volume change is caused by a displacement, x(t), with respect to the diaphragm rest position (an outward displacement corresponds to a positive displacement): $Δ V (t) = x (t) S_{d},$

where S_d is the effective diaphragm radiating area. Therefore, $p (t) = \frac{- x (t) S_{d}}{V} {ρ c}^{2},$
When the loudspeaker diaphragm moves, the volume of the enclosure is changed, due to which the pressure within the enclosure changes. The microphone that is mounted within the enclosure registers the pressure, and therefore, the microphone signal is related to the diaphragm displacement.
The microphone signal can be used to estimate the loudspeaker model parameters. In traditional (linear) approaches, the electrical impedance, which is the frequency transfer function between voice coil voltage and current, is measured, after which the model parameters are obtained by minimising the discrepancy between the measured impedance and the impedance predicted by the model with respect to the model parameters. Similarly, model parameters can be obtained by minimising the discrepancy between the voltage-to-excursion transfer function that is measured and that predicted by the model with respect to the model parameters. In this way, the expected displacement for a given input signal can be predicted before the signal is sent to the loudspeaker.
The microphone signal can instead be used directly as a measure of the diaphragm displacement (without the need for a model). In this case, the displacement for a given input signal is estimated after the signal is sent to the loudspeaker.
To demonstrate that a pressure sensor signal can be used to determine voice coil displacement, the diaphragm displacement has been measured for one speaker design using a laser displacement meter, and the signal has been compared to the signal recorded by a microphone that is mounted within the same sealed loudspeaker enclosure. The frequency transfer functions from source signal to the laser signal and to the microphone signal are computed and shown in Figure 2.
The transfer functions have been normalised such that the peak amplitudes correspond to unity. It can be visually observed that there is a close match between the laser displacement (plot 20) and the microphone signal (plot 22). The correspondence degrades for lower frequencies, in this case below 200 Hz.
The invention can be used in systems that are aimed at loudspeaker modelling, protection and linearisation. The signal registered by the microphone can also be used as a reference signal for acoustic echo cancellation (AEC), which offers the advantage that the microphone signal contains the nonlinearities that are due to the non-linear behaviour of the loudspeaker.
Figure 3 shows a loudspeaker system of the invention. A digital-to-analog converter 30 prepares the analog loudspeaker signal, which is amplified by amplifier 32.
The microphone (or other pressure sensor) 14 is used to estimate the loudspeaker displacement, and the displacement estimator is provided to a processor 34, which implements a control algorithm to control the audio processing. This implements loudspeaker protection and/or acoustic signal processing (such as flattening, or frequency selective filtering).
The way the displacement signal is used is the same as the way the known displacement estimate derived from electrical analysis or from optical analysis is used. Thus, the acoustic signal processing is not described in detail in this application.
Figure 4 shows the control method of the invention.
Step 40 comprises the pressure detection in the sealed enclosure in which the loudspeaker is housed.
Step 42 comprises estimating the loudspeaker diaphragm displacement from the detected pressure. As shown in Figure 2, the pressure signal is linearly proportional to the displacement.
Depending on the control mechanism, it may be necessary to calibrate the microphone sensitivity in such a way that the relationship to the diaphragm displacement is known on an absolute scale. This can be done in a calibration step, where for a given input test signal, the diaphragm displacement is measured (or known) and is related to the microphone signal.
Step 44 comprises the known control of the audio processing for the loudspeaker in dependence on the loudspeaker membrane displacement.
The use of a microphone has been described above, but there are other pressure sensors that can be mounted in the speaker enclosure, such as piezoresistive pressure sensors.
No detailed designs for the microphone, loudspeaker and enclosure have been presented as these are entirely conventional. The invention is based on the recognition that pressure sensing within the loudspeaker enclosure can be used to estimate the loudspeaker diaphragm displacement.
The invention can be used in miniature loudspeakers, such as used in portable battery operated devices, such as mobile phones, but it may equally be used in larger devices.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

A loudspeaker system, comprising:
a loudspeaker (10) in a sealed enclosure (12);

a pressure sensor (14) in the sealed enclosure (12); and

a processor adapted to derive an estimate of the loudspeaker membrane displacement from the sensed pressure and for controlling the audio processing for the loudspeaker in dependence on the estimated loudspeaker membrane displacement.
A system as claimed in claim 1, wherein the sealed enclosure comprises the loudspeaker back volume.
A system as claimed in any preceding claim, wherein the pressure sensor comprises a microphone (14).
A system as claimed in any preceding claim, wherein the processor is adapted to implement loudspeaker protection on the basis of the estimated loudspeaker membrane displacement.
A system as claimed in any preceding claim, wherein the estimated loudspeaker membrane displacement is derived without voice coil current or voltage monitoring.
A method of controlling a loudspeaker output, comprising:
(40) detecting a pressure in a sealed enclosure in which the loudspeaker is housed;

(42) estimating the loudspeaker diaphragm displacement from the detected pressure; and

(44) controlling audio processing for the loudspeaker in dependence on the estimated loudspeaker membrane displacement.
A method as claimed in claim 6, wherein detecting a pressure comprises collecting a microphone signal.
A method as claimed in claim 6 or 7, comprising implementing loudspeaker protection on the basis of the estimated loudspeaker membrane displacement.
A method as claimed in any one of claims 6 to 8, comprising deriving an estimated loudspeaker membrane displacement without voice coil current or voltage monitoring.