Abstract:
A system and method to perform signal processing using a loudspeaker output are described. The system includes an enclosure configured to define a back cavity of the loudspeaker and components to obtain a representation of the loudspeaker acoustic output at the back cavity. The system also includes a processor to process the representation to perform the signal processing.

Description:
BACKGROUND 
       [0001]    Loudspeaker diaphragm motion generates acoustic energy in front of and behind the loudspeaker. The acoustic energy in front provides the expected loudspeaker acoustic output. The acoustic energy in the back is usually confined so that it does not interfere with the loudspeaker acoustic output in the front, but can provide a measure of the loudspeaker acoustic output. In handheld devices, such as smart phones and cell phones, for example, loudspeakers are usually implemented with a sealed back cavity design. That is, the acoustic energy generated in the back of the loudspeaker is confined within a sealed cavity. In this case, an acoustic pressure measurement in the back cavity serves as a measure of the loudspeaker acoustic output. The loudspeaker acoustic output serves as a reference for many purposes. The loudspeaker acoustic output is used as a reference in digital signal processing (DSP) algorithms such as echo cancellation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]    For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
           [0003]      FIG. 1  is a block diagram of a physical system for obtaining a representation of loudspeaker acoustic output according to an embodiment of the invention; 
           [0004]      FIG. 2  is an acoustic circuit model of the system shown in  FIG. 1 ; 
           [0005]      FIG. 3  is a block diagram of another system for obtaining a representation of loudspeaker acoustic output according to another embodiment of the invention; 
           [0006]      FIG. 4  is a block diagram of another system for obtaining a representation of loudspeaker acoustic output according to another embodiment of the invention; 
           [0007]      FIG. 5  is a process flow of a method of performing signal processing using loudspeaker output according to embodiments of the invention; and 
           [0008]      FIG. 6  is an exemplary system to perform signal processing using loudspeaker output according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    As noted above, the loudspeaker acoustic output is a measure used for many purposes including, for example, as a reference in echo cancellation. As also noted above, the acoustic pressure in a back cavity of the loudspeaker is a measure of the loudspeaker acoustic output in most handheld devices. The back cavity pressure measurement may be a more accurate measure for echo cancellation than the traditional voltage applied to the loudspeaker, especially in handheld devices. This is because loudspeakers, used in handheld devices, often display nonlinear distortion that makes the echo path nonlinear. Using the back cavity pressure measurement as the reference signal gives the echo cancellation algorithm a more accurate measure of the true acoustic signal to cancel. However, the microphones used in handheld devices cannot handle the high sound pressure levels (SPL) in the back cavity of the loudspeaker so that obtaining the back cavity pressure is not possible with typical handheld device microphones. For example, lumped element analysis provides an SPL simulation that indicates SPLs in the back cavity are on the order of 55 decibel Pascal (dBPa) in handheld devices. However, typical microphones used in smart phones can deal with 15 to 25 dBPa, and even higher performance microphones deal with only 35 to 40 dBPa. As a result, applications that require loudspeaker acoustic output have used other references for signal processing. In echo cancellation, for example, the voltage applied to the loudspeaker to produce the audio output has been used as a reference. However, because the loudspeaker changes that input prior to outputting the audio signal (the nonlinearity), the voltage reference does not result in accurate echo cancellation. As to another alternative measurement, measuring the acoustic pressure in front of the loudspeaker (rather than in the back cavity) results in an unreliable signal, because acoustic coupling changes based on how a handheld device is held and also because the signal is contaminated with the addition of sound sources (e.g., room noise, handheld device user&#39;s voice). Embodiments of the system and method described herein relate to obtaining an attenuated measure of the back cavity pressure as a representation of loudspeaker acoustic output. 
         [0010]    It should be understood at the outset that although illustrative implementations of one or more embodiments of the present disclosure are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
         [0011]      FIG. 1  is a block diagram of a physical system for obtaining a representation of loudspeaker acoustic output according to an embodiment of the invention. A transparent box is shown as a representation of the device  100  that comprises the system for obtaining the representation of loudspeaker acoustic output. The device  100  may be a smart phone, a cell phone, or another handheld device, for example. The loudspeaker  110  of the device  100  is shown with a sealed back cavity  120 . While the back cavity  120  is shown in the shape of a cube, alternate shapes are contemplated for both the loudspeaker  110  and the back cavity. An in port  130  (e.g., hole) in the back cavity  120  is shown. A microphone  140  is disposed at the opening or in port  130  in the back cavity  120 . An optional out port  150  (e.g., tube) is arranged at the microphone  140  and extends to the interior of the device  100 . The ports (in port  130  and out port  150 ) may both be holes (in the back cavity  120  and in the microphone  140 ) or one or both may be a tube or have a non-circular cross section. Exemplary dimensions for the in port  130  implemented as a tube may be on the order of 0.3 millimeters (mm) in length with a circular cross section and a diameter on the order of 0.1 mm. Exemplary dimensions for the out port  150  implemented as a tube may be on the order of 1 mm for the length and 1 mm for the diameter. 
         [0012]      FIG. 2  is an acoustic circuit model of the system shown in  FIG. 1 . The system comprises a filter implementation to attenuate the sound from the back cavity  120  to the microphone  140  so that a standard microphone  140  in a device  100  (e.g., handheld) can pick up the sound. The filter implementation includes the in port  130  which may be a hole, for example. As the acoustic circuit model of  FIG. 2  indicates, the in port  130  leads directly to a microphone  140 . The out port  150 , which may be another hole or a tube from the front of the microphone  140  leads to the interior of the device  100  (e.g., smart phone). The microphone  140  acoustic impedance, which is largely capacitive, is part of the filter implementation that attenuates the sound from the back cavity  120 . With the filter implementation, even an SPL on the order of 63 dBPa in the back cavity  120  only exposes the microphone  140  to approximately 17 dBPa according to exemplary simulations. Simulations further indicate that the filter implementation (in port  130  and microphone  140  acoustic impedance) and microphone  140  itself do not affect the output of the loudspeaker  110  or the SPL in the back cavity  120 . 
         [0013]      FIG. 3  is a block diagram of another system for obtaining a representation of loudspeaker acoustic output according to another embodiment of the invention. As in  FIG. 1 , the loudspeaker  110  and back cavity  120  are shown in a device  100  illustrated as a transparent box. As noted above, the back cavity  120  may have a different shape than the cube shown in  FIG. 3 . In this embodiment, the SPL in the back cavity  120  is attenuated by a diaphragm  350  (e.g., metal disk). The diaphragm  350  is formed inside the back cavity  120  at an opening  330  (hole) in the back cavity  120 . A microphone  140  on the other side of the opening  330  receives an attenuated acoustic pressure based on the diaphragm  350 . The pressure in the back cavity  120  distends the diaphragm  350 . As the thickness of the diaphragm  350  increases, the pressure decreases. Thus, the amount of attenuation of the SPL at the microphone  140  can be controlled by controlling the thickness of the diaphragm  350 . 
         [0014]      FIG. 4  is a block diagram of another system for obtaining a representation of loudspeaker acoustic output according to another embodiment of the invention. As in  FIGS. 1 and 3 , the loudspeaker  110  and back cavity  120  are shown in a device  100  illustrated as a transparent box. The device  100  and the back cavity  120  may have different shapes than shown in  FIG. 4 . The accelerometer  410  may be mounted to one of the walls  125  of the back cavity  120 , as shown in  FIG. 4 . The wall  125  flexes under the load of the SPL in the back cavity  120 . This flexing by the wall  125  is sensed by the accelerometer such that the accelerometer output is an attenuated representation of loudspeaker acoustic output. The amplitude of the flexing can be adjusted by changing the shape and thickness of the wall  125  (changing the spring constant). In this embodiment, the wall  125  acts as a diaphragm and the accelerometer  410  may be thought of as a contact microphone indicating the pressure proportional to SPL in the back cavity  120 . 
         [0015]      FIG. 5  is a process flow of a method of performing signal processing using loudspeaker  110  output according to embodiments of the invention. At block  510 , arranging back cavity attenuation is according to one of the embodiments discussed above. The arranging may include disposing an in port  130  and out port  150  at a wall of the back cavity  120  with a microphone  140  therebetween, as discussed with reference to  FIGS. 1 and 2 . The arranging may also include disposing a diaphragm  350  inside an opening  330  of the back cavity  120  with a microphone  140  on the other side of the opening  330 , as discussed with reference to  FIG. 3 . The arranging may instead include disposing an accelerometer  410  on a wall  125  of the back cavity  120 , as discussed with reference to  FIG. 4 . At block  520 , obtaining a representation of loudspeaker acoustic output in the back cavity  120  is done by measuring acoustic pressure in the back cavity  120 . According to the embodiments described herein, obtaining the representation includes obtaining the microphone  140  output or the signal from the accelerometer  410  based on the embodiment being implemented. Performing signal processing (e.g., echo cancellation) based on the loudspeaker  110  output at block  530  includes using the loudspeaker acoustic output representation from the back cavity  120  such that the nonlinear component (echo) is included in the calculation. 
         [0016]      FIG. 6  is an exemplary system to perform signal processing using loudspeaker  110  output according to embodiments of the invention. The device  100  may be a handheld device such as a smart phone, for example, and may include a display  601  and input interface  602  (e.g., keyboard). A representation  620  of loudspeaker acoustic output in the back cavity  120  of the loudspeaker  110  is provided to a processing system  610  of the device. The components that provide the representation  620  of loudspeaker acoustic output include the in port  130 , out port  150 , and the microphone  140  according to one embodiment, a diaphragm  350  and microphone  140  according to another embodiment, and an accelerometer  410  according to yet another embodiment. The processing system  610  includes one or more processors, one or more memory devices, an input interface and an output interface and may be part of the digital signal processing system of the device  100 . The representation  620  may be provided to the processing system  610  according to one of the embodiments discussed above. For example, the representation  620  may be microphone  140  output obtained following attenuation of the SPL in the back cavity  120  according to the embodiment discussed with reference to  FIGS. 1 and 2 . 
         [0017]    While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
         [0018]    Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.