Patent Publication Number: US-10779086-B2

Title: Audio processor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. § 119 of European Patent application no. 18214191.1, filed on 19 Dec. 2018, the contents of which are incorporated by reference herein. 
     FIELD 
     This disclosure relates to an audio processor for acoustic transducers having more than one voice coil. 
     BACKGROUND 
     Dual voice coil loudspeakers or speakers typically have two identical voice coils driving a single loudspeaker rather than a single voice coil. The two voice coils may be connected in series or in parallel to alter the loudspeaker impedance when driven by a single amplifier. Alternatively, each of the two voice coils can be driven independently by left and right audio channels of a stereo audio signal, which allows a single speaker to be used to output stereo signals. Dual voice coil loudspeakers may be used in audio systems such as for example vehicle infotainment systems and mobile devices. 
     SUMMARY 
     Various aspects of the disclosure are defined in the accompanying claims. In a first aspect there is provided an audio processor for a multi-voice-coil acoustic transducer, the audio processor comprising: at least one phase shifter; and a plurality of audio outputs, each output being configured to be coupled to a respective coil of the multi-coil acoustic transducer: wherein the audio processor is configured to adjust the phase difference between an audio signal supplied to each of the voice coils to attenuate the acoustic output due to the audio signal. 
     In one or more embodiments, the audio processor may comprise an audio input configured to receive an audio signal; wherein the plurality of audio outputs comprises a first audio output and a second audio output; wherein the first audio output is coupled to the audio input; and the phase shifter comprises a phase inverter having a phase inverter input coupled to the audio input and a phase inverter output coupled to the second audio output: wherein the first audio input is configured to be coupled to the first coil of a dual voice coil acoustic transducer and the second audio output is configured to be coupled to the second coil of the dual voice coil acoustic transducer. 
     In one or more embodiments, the audio processor may comprise a further audio input configured to receive a further audio signal and a mixer having a first mixer input coupled to the audio input, a second mixer input coupled to the further audio input and a mixer output configured to be coupled to the first audio output. 
     The second audio output may be further configured to be coupled to a single voice-coil acoustic transducer. 
     In one or more embodiments, the audio processor may comprise a further mixer having a first further mixer input coupled to the further audio input, a second further mixer input coupled to the phase inverter output, and a further mixer output coupled to the second audio output. 
     The first audio output may be coupled to an input of a further phase inverter, wherein the further phase inverter output is coupled to a third audio output, wherein the second audio output is further configured to be coupled to the first coil of a further dual voice coil acoustic transducer and the third audio output is configured to be coupled to the second coil of the further dual voice coil acoustic transducer. 
     In one or more embodiments, the audio processor may comprise a reference signal generator coupled to the audio input. The reference signal generator may be configured to generate a signal at an audible frequency. The reference signal generator may be configured to generate a signal at an inaudible or ultrasound frequency. 
     In one or more embodiments, the audio processor may comprise a current sensor having an input configured to be coupled to the first voice coil of a dual voice coil acoustic transducer and the second voice coil of the dual voice coil acoustic transducer and an output and a controller having a first controller input coupled to a current sensor and a second controller input coupled to the reference signal generator. 
     In one or more embodiments, the controller may be configured to determine an acoustic transducer characteristic from a comparison of the reference signal and the detected current signal. 
     In one or more embodiments, the controller may be configured to determine a difference in a characteristic of the first coil of the dual coil acoustic transducer and the second coil of the dual coil acoustic transducer from a comparison of a detected current signal from the first coil and a detected current signal from the second coil. 
     In one or more embodiments, the audio processor may comprise an audio compensator arranged between the further audio input and the first further mixer input, wherein the controller has an output coupled to the compensator and wherein the compensator is configured to adapt an audio signal received on the further audio input dependent on the determined difference. 
     In one or more embodiments, the audio processor may comprise a scaler including the phase inverter, wherein the scaler is adapted to cross-mix the audio input signal and the phase inverted audio signal dependent on a volume control input signal and to output the cross-mixed signal to the second audio output. 
     Embodiments of the audio processor may be comprised in an audio system comprising a dual voice coil acoustic transducer having a first voice coil coupled to the first audio output and a second voice coil coupled to the second audio output. 
     Embodiments of the audio processor may be comprised in an audio system comprising a dual voice coil acoustic transducer having a first voice coil coupled to the first audio output and a second voice coil coupled to the second audio output and a further dual voice coil acoustic transducer having a first voice coil coupled to the second audio output and a second voice coil coupled to the third audio output. 
     In a second aspect, there is provided a method of audio processing for a multi voice coil acoustic transducer, the method comprising: receiving an audio signal: adjusting the phase difference between the audio signal supplied to each of the voice coils to attenuate the acoustic output due to the audio signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures and description like reference numerals refer to like features. Embodiments of are now described in detail, by way of example only, illustrated by the accompanying drawings in which: 
         FIG. 1  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 2  illustrates an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 3  shows an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 4  illustrates an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 5  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 6  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 7  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 8  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 9  shows an audio system including an audio processor for a dual-voice coil loudspeaker according to an embodiment. 
         FIG. 10  illustrates a method of driving a multi voice-coil acoustic transducer according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an audio system  150  including an audio processor  100  for a dual voice coil acoustic transducer according to an embodiment. The audio system  150  may further include an amplifier system  108  and a dual voice coil loudspeaker or speaker  120 . The audio processor  100  may have an audio input  102 . The audio processor  100  may include a phase shifter  110 . The input of the phase shifter  110  may be connected to the audio input  102 . An output of the phase shifter  110  may be connected to a first audio processor output  104 . The audio input  102  may be connected to a second audio processor output  106 . The audio input  102  be connected directly to the second audio processor output  106  or be connected via additional audio processing modules (not shown) included in the audio processor  100 . 
     The audio processor first output  104  may be connected to an input of a first audio amplifier  122  of the audio amplifier system  108  which may be a class-D amplifier. The output  114  of the first amplifier  122  may be connected to a first voice coil L of a dual voice coil loudspeaker  120 . The audio processor second output  106  may be connected to an input of a second audio amplifier  124  of the audio amplifier system  108  which may be a class-D amplifier. The output  112  of the second amplifier  124  may be connected to a second voice coil L 2  of a dual voice coil loudspeaker  120 . As shown in  FIG. 1 , the amplifier system  108  is connected to the dual voice coil loudspeaker in a single ended configuration with one terminal of L 1  and L 2  connected to the respective amplifier output  114 ,  112  and the other terminal connected to ground  116 . It will be appreciated that in other examples, the amplifier system  108  may have differential outputs. 
     In operation, the audio processor  100  may output an audio signal on the second audio processor output  106  and a phase shifted version of the audio signal on the first audio processor output  104 . The phase shift applied is chosen such that the audio signal and the phase shifted version of the audio signal will destructively interfere mechanically in the respective voice coils of the dual voice coil loudspeaker  120 . For the dual-voice coil loudspeaker  120 , this phase shift may be a phase inversion. This destructive interference results in either significantly attenuated or no acoustic output from the dual voice coil loudspeaker  120  due to the audio signal while the current flowing in coils L 1 , L 2  results in power still being dissipated. The inventor of the present disclosure has appreciated that this effect, which may conventionally be considered undesirable, may be used in various audio processing applications as further described herein. In some examples, the audio signal on the audio input  102  may be generated internally in the audio processor  100 . In other examples, the audio signal may be received from an external audio source. 
     The audio processor  100  may be implemented in hardware or a combination of hardware and software, for example software running on a digital signal processor or other microprocessor. The audio processor  100  and the amplifier system  108  may be implemented as separate devices or integrated together as a smart audio amplifier on a single device. In some examples, the phase shifter  110  may be selectively disabled or bypassed dependent on the audio signal and the desired operating mode of the audio system  150 . 
     In some examples other dual voice coil acoustic transducers may also be driven by the audio processor  100 . For example, a dual voice coil haptic actuator such as a linear resonant actuator, or more generally electric motors which may be rotating or linear. It will be appreciated that in other examples, acoustic transducers with more than two voice coils may be similarly driven with audio signals being phase shifted to destructively mechanically interfere resulting in reduced or no acoustic output due to those audio signals. The phase shift required for the illustrated audio system  150  may be a phase inversion for a dual voice-coil acoustic transducer. For other multi-coil acoustic transducers, the phase shift may be different. For example, for an even number of identical voice coils where X voice coils are driven with an in-phase audio signal and X voice coils driven with a phase inverted or 180-degree phase shifted audio signal, then cancellation will result. In another example an acoustic transducer using a tri-phase rotating motor required three audio signals with relative phase shifts or phase differences of 120 degrees. In some examples multiple phase shifters may be required. 
       FIG. 2  shows an audio processor  200  for a dual-coil acoustic transducer according to an embodiment. The audio processor  200  may have an audio input  202  connected to a first input of a first mixer  230 . The audio input  202  may be connected to a first input of a second mixer  240 . The output  208  of a phase inverter  210  may be connected to a second input of the first mixer  230 . The output of the first mixer  230  may be connected to a first audio output  204 . The output of the second mixer may be connected to a second audio output  206 . The phase inverter  210  may apply a phase shift of 180 degrees to a received signal. 
     A second input of the second mixer  240  may be connected to an output  212  of a reference signal generator  220 . The reference signal generator output  212  may be considered as a further audio input source for the audio processor  200 . Consequently, the second input of the second mixer  240  may be considered as an audio input. The reference signal generator output  212  may be connected to an input of the phase inverter  210 . 
     In operation the audio processor  200  is included in an audio system using a dual-voice coil acoustic transducer (not shown). The first audio output  204  may be connected to one of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier (not shown). The second audio output  206  may be connected to the other of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier. 
     The reference signal generator  220  may generate a signal in the audible frequency range. A further audio signal containing for example speech or music may be received on the audio input  202 . The first mixer  230  may mix the further audio signal with a phase inverted version of the reference signal and output the first mixed audio signal on the first audio output  204 . The second mixer  240  may mix the further audio signal with the reference signal and output the second mixed audio signal on the second audio output  206 . 
     The reference signal and the phase inverted reference signal destructively interfere mechanically, and the dual voice coil acoustic transducer will not produce any audible sound due to the reference signal. The reference signal may be generated with a relatively large amplitude and still be inaudible even when is generated at an audible frequency. However, the further audio signal will be clearly audible as it is output with the same phase on both voice coils. Although the reference signal will not be audible, a current flow due to the reference signal flows in each of the voice coils. The current flow due to the reference signal and phase-inverted reference signal dissipates power in the voice coils of the dual voice coil acoustic transducer. While this may conventionally be considered undesirable, the inventor of the present disclosure has appreciated that, for example, dual voice coil car speakers which may be subject to low temperature may be self-heated by generating a reference signal of sufficient amplitude which dissipates power in the voice coils. Self-heating the loudspeakers in this way may allow the speakers to perform at an optimal level until the vehicle cabin temperature is at a suitable level. In some examples, the reference signal may be generated at ultrasonic or inaudible frequencies. Because of the destructive interference resulting in no acoustic output due to the reference signal, there may be no interference with other apparatus which uses ultrasound. In addition, because no acoustic output is generated, this may avoid the possibility of an adverse reaction by some animals to an ultrasound signal. 
       FIG. 3  shows an audio processor  300  for a dual-coil acoustic transducer according to an embodiment. The audio processor  300  may have an audio input  302  connected to a first input of a first mixer  330 . The audio input  302  may be connected to a first input of a second mixer  340 . The output  308  of a phase inverter  310  may be connected to a second input of the first mixer  330 . The output of the first mixer  330  may be connected to a first audio output  304 . The output of the second mixer  340  may be connected to a second audio output  306 . 
     A second input of the second mixer  340  may be connected to an output  312  of a reference signal generator  320 . The reference signal generator output  312  may be considered as a further audio input source. The reference signal generator output  312  may be connected to an input of the phase inverter  310 . The reference signal output  312  may be connected to a first input of a controller  350 . A second input of the controller  350  may be connected to an output  314  of a current sensor  360 . The current sensor  360  may have a current sensor input  316 . 
     In operation the audio processor  300  is included in an audio system using a dual-voice coil acoustic transducer (not shown). The first audio output  304  may be connected to one of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier. The second audio output  306  may be connected to the other of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier. The current sensor input  316  may be connected to each of the voice coils. 
     The reference signal generator  320  may generate a signal in the audible frequency range. A further audio signal containing for example speech or music may be received on the audio input  302 . The first mixer  330  may mix the further audio signal with a phase inverted version of the reference signal and output the first mixed audio signal on the first audio output  304 . The second mixer  340  may mix the further audio signal with the reference signal and output the second mixed audio signal on the second audio output  306 . 
     The reference signal and the phase inverted reference signal destructively interfere mechanically, and the dual voice coil acoustic transducer will not produce any audible sound due to the reference signal. The reference signal may be generated with a relatively large amplitude and still be inaudible even when it is generated at an audible frequency. However, the further audio signal will be clearly audible as it is output with the same phase on both voice coils. Although the reference signal will not be audible, a current flow due to the reference signal flows in each of the voice coils. This current flow may be detected by the current sensor  360 . The controller  350  may compare the amplitude of the detected current with the generated reference signal and determine an acoustic transducer characteristic value from the comparison. 
     This characteristic may be for example an impedance value or DC resistance for each of the coils. The DC resistance may be determined from an AC reference signal by determining current flow through one of the voice coils. Typically, the voltage across the dual voice-coil speaker  120  is known and in the ideal case where both voice coils are identical impedance measured will only consist of the DC component. This is because mechanical and inductive parts of the impedance have been cancelled out. The controller  350  may output the characteristic value on the controller output  318 . This controller output  318  be connected to a further audio processing module (not shown) which may for example adapt the further audio signal dependent on the measured characteristic. By measuring a characteristic using a reference signal in the audible frequency range and with relatively large amplitude compared to the maximum audio amplitude that the speaker may play, the determination of the characteristic may be more accurate. The amplitude of the reference signal may for example be in a range up to 20% of maximum amplitude of the audio signal for the loudspeaker. In other examples, the amplitude of the reference signal may be greater than 20% of the maximum amplitude of the audio signal for the loudspeaker. 
       FIG. 4  shows an audio processor  400  for a dual-coil acoustic transducer according to an embodiment. The audio processor  40 X) may have an audio input  402  connected to an input of compensator  470 . An output  422  of the compensator  470  may be connected to a first input of a first mixer  430 . The audio input  402  may be connected to a first input of a second mixer  440 . The output  408  of a phase inverter  410  may be connected to a second input of the first mixer  430 . The output of the first mixer  430  may be connected to a first audio output  404 . The output of the second mixer  440  may be connected to a second audio output  406 . 
     A second input of the second mixer  440  may be connected to an output  412  of a reference signal generator  420 . The reference signal generator output  412  may be considered as a further audio input source. The reference signal generator output  412  may be connected to an input of the phase inverter  410 . The reference signal output  412  may be connected to a first input of a controller  450 . A second input of the controller  450  may be connected to an output  414  of a current sensor  460 . The current sensor  460  may have a current sensor input  416 . A controller output  418  may be connected to the compensator  470 . 
     In operation the audio processor  400  is included in an audio system using a dual-voice coil acoustic transducer (not shown). The first audio output  404  may be connected to one of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier. The second audio output  406  may be connected to the other of the voice coils of a dual-voice coil acoustic transducer via an audio amplifier. The current sensor input  416  may be connected to each of the voice coils. 
     The reference signal generator  420  may generate a signal in the audible frequency range. A further audio signal containing for example speech or music may be received on the audio input  402 . The compensator  470  may apply a compensation factor to the further audio signal. This may include equalisation, dynamic range control, or other filtering. The compensator  470  outputs a compensated further audio signal to the first mixer  430 . The first mixer  430  may mix the compensated further audio signal with a phase inverted version of the reference signal and output the first mixed audio signal on the first audio output  404 . The second mixer  440  may mix the further audio signal with the reference signal and output the second mixed audio signal on the second audio output  406 . 
     The reference signal and the phase inverted reference signal destructively interfere mechanically, and the dual voice coil acoustic transducer will not produce any audible sound due to the reference signal. The reference signal may be generated with a relatively large amplitude and still be inaudible even when it is generated at an audible frequency. However, the further audio signal will be clearly audible as it is output with the same phase on both voice coils. Although the reference signal will not be audible, a current flow due to the reference signal flows in each of the voice coils. This current flow may be detected by the current sensor  460 . The controller  450  may compare the amplitude of the detected current with the generated reference signal and determine an acoustic transducer characteristic value from the comparison. This characteristic may be for example an impedance value or DC resistance for each of the coils. The DC resistance may be determined from an AC reference signal by determining current flow through one of the voice coils. Typically, the voltage across the dual voice-coil speaker  120  is known and in the ideal case where both voice coils are identical impedance measured will only consist of the DC component. This is because mechanical and inductive part of the impedance have been cancelled out. However, this is only the case when both voice coils are identical. If the dual voice coils are not perfectly identical, also the difference of mechanical and inductive part will be available in the impedance measured. 
     The controller  450  may then determine a difference in the characteristic for each of the voice coils from the impedance measurement. The controller  450  may output an error signal corresponding to the difference in the characteristics to the compensator  470 . The compensator  470  may adjust or compensate the further audio signal output on the first audio output  404  dependent on this error signal. 
     The inventor of the present disclosure has further appreciated that although typically the coils of a dual voice coil acoustic transducer are designed to be identical, in practice due to manufacturing variations this is not the case. By measuring the currents due to the reference signal from each coil and determining the difference, the compensator  470  may adapt the further audio signal to account for the difference. In another example, the phase inverter may have a variable gain and the controller may adapt the gain of the phase inverter to compensate the phase-inverted reference signal to improve the destructive cancellation. 
       FIG. 5  shows an audio system  550  including an audio processor according to an embodiment  500 . Audio system  550  may include an amplifier system  508 , a dual coil loudspeaker  120  and a haptic motor  530 . Audio processor  500  has a first audio input  502  connected to a first input of first mixer  520  and a second audio input  518  connected to a second input of first mixer  520  and an input of a phase inverter  510 . An output of the mixer  520  is connected to a first audio output  504  An output of the phase inverter  510  is connected to a second audio output  506 . 
     The audio processor first output  504  may be connected to an input of a first audio amplifier  522  of the audio amplifier system  508  which may be a class-D amplifier. The differential outputs  514 ,  514 ′ of the first amplifier  522  may be connected to a first voice coil L 1  of a dual voice coil loudspeaker  120 . The audio processor second output  506  may be connected to an input of a second audio amplifier  524  of the audio amplifier system  508  which may be a class-D amplifier. The differential outputs  512 , 512 ′ of the second amplifier  524  may be connected to a second voice coil L 2  of a dual voice coil loudspeaker  120 . The second amplifier differential outputs  512 , 512 ′ may be connected to a haptic motor  530  such as a linear resonant actuator. In other examples other acoustic transducers may be connected instead of the haptic motor  530  such as single voice coil speakers, piezo transducers. 
     In operation, a first audio signal received on first audio input  502  is mixed with a second audio signal received on the second audio input  518 , The mixed audio signal is output on the first audio output  504 . The second audio signal is phase inverted by the phase inverter  510  and the phase inverted second audio is output on the second audio output  506 . 
     The second audio signal and the phase inverted version of the second audio signal will destructively interfere mechanically in the respective voice coils of the dual voice coil loudspeaker  120 . This destructive interference results in either significantly reduced or no acoustic output from the dual voice coil loudspeaker  120  due to the second audio signal while the current flowing in coils L 1 , L 2  results in power still being dissipated. The second phase-inverted audio signal is received by the haptic motor  530  which may result in an audible output as there is no destructive interference. In some examples the haptic motor  530  may be replaced by a loudspeaker or other acoustic transducer. The audio processor  500  allows amplifier system  508  to be shared between two acoustic transducers if one is a dual voice coil acoustic transducer. 
       FIG. 6  shows an audio system  650  including an audio processor  600  according to an embodiment. Audio system  650  may also include an amplifier system  608 , a first dual coil loudspeaker  120  and a further dual coil loudspeaker  120 ′. Audio processor  600  has a first audio input  602  connected to a first input of first mixer  620  and a second audio input  618  connected to a second input of first mixer  620  and an input of a first phase inverter  610 . An output of the first mixer  620  is connected to a first audio output  604  An output of the first phase inverter  610  is connected to a first input of a second mixer  640 . A second input of second mixer is connected to the first audio input  602 . The output of the second mixer is connected to the second audio output  606 . The output of the first mixer  620  is connected to an input of a second phase inverter  630 . An output of the second phase inverter is connected to a third audio output  634 . 
     The audio processor first output  604  may be connected to an input of a first audio amplifier  622  of the audio amplifier system  608  which may be a class-D amplifier. The differential outputs  614 ,  614 ′ of the first amplifier  622  may be connected to a first voice coil L 1  of a dual voice coil loudspeaker  120 . The audio processor second output  606  may be connected to an input of a second audio amplifier  624  of the audio amplifier system  608  which may be a class-D amplifier. The differential outputs  612 , 612 ′ of the second amplifier  624  may be connected to a second voice coil L 2  of a dual voice coil loudspeaker  120 . The second amplifier differential outputs  612 , 612 ′ may be connected to a first voice coil L 1 ′ of a second dual voice coil loudspeaker  120 ′. The third audio output  634  may be connected to a third amplifier  626  in the amplifier system  608 . The differential outputs  632 ,  632 ′ may be connected to a second voice coil L 2 ′ of the second dual voice coil loudspeaker  120 ′ 
     In operation, a first audio signal received on first audio input  602  is mixed with a second audio signal received on the second audio input  618 , The mixed audio signal is output on the first audio output  604 . The second audio signal is phase inverted by the phase inverter  610 . The inverted second audio signal is mixed with the first audio signal and the mixed signal is output on the second audio output  606 . The inverted first audio signal is output on the third audio output  634 . In this configuration the first audio signal is played through the first dual voice-coil speaker  120  and the second audio signal is played through the second dual voice-coil speaker  120 ′. 
     The second audio signal and the phase inverted version of the second audio signal will destructively interfere mechanically in the respective voice coils of the dual voice coil loudspeaker  120 . This destructive interference results in either significantly reduced or no acoustic output from the dual voice coil loudspeaker  120  due to the second audio signal while the current flowing in coils L 1 , L 2  results in power still being dissipated. The first audio signal and the phase inverted version of the first audio signal will destructively interfere mechanically in the respective voice coils of the second dual voice coil loudspeaker  120 ′. This destructive interference results in either significantly reduced or no acoustic output from the second dual voice coil loudspeaker  120 ′ due to the second audio signal while the current flowing in coils L 1 ′, L 2 ′ results in power still being dissipated. The audio system  650  may allow stereo play back or be used for active cross-over filters if for example the first dual voice coil loudspeaker  120  is a woofer and the second dual voice coil loudspeaker  120 ′ is a tweeter. 
       FIG. 7  shows an audio system  750  including an audio processor  700  according to an embodiment. Audio system  750  may include an amplifier system  708 , a first dual coil loudspeaker  120  and a further dual coil loudspeaker  120 ′. Audio processor  700  has a first audio input  702  connected to a first input of first mixer  720  and a second audio input  718  connected to a second input of first mixer  720  and an input of a first phase inverter  710 . An output of the first mixer  720  is connected to a first audio output  704 . An output  732  of the first phase inverter  710  is connected to a first input of a second mixer  740 . A second input of second mixer  740  is connected to the first audio input  702 . The output of the second mixer is connected to the second audio output  706 . 
     The audio processor first output  704  may be connected to an input of a first audio amplifier  722  of the audio amplifier system  708  which may be a class-D amplifier. The differential outputs  714 ,  714 ′ of the first amplifier  722  may be connected to a first voice coil L 1  of a dual voice coil loudspeaker  120 . The differential outputs  714 ,  714 ′ of the first amplifier  722  may be connected to a second voice coil L 2 ′ of the second dual voice coil loudspeaker  120 ′ with opposite polarity to the connections to the first voice coil L 1  of the dual voice coil loudspeaker  120  and so acts as a second phase inverter  730 . 
     The audio processor second output  706  may be connected to an input of a second audio amplifier  724  of the audio amplifier system  708  which may be a class-D amplifier. The differential outputs  712 , 712 ′ of the second amplifier  724  may be connected to a second voice coil L 2  of a dual voice coil loudspeaker  120 . The second amplifier differential outputs  712 ,  712 ′ may be connected to a first voice coil L 1 ′ of a second dual voice coil loudspeaker  120 ′. 
     In operation, a first audio signal received on first audio input  702  is mixed with a second audio signal received on the second audio input  718 , The mixed audio signal is output on the first audio output  704 . The second audio signal is phase inverted by the phase inverter  710 . The inverted second audio signal is mixed with the first audio signal and the mixed signal is output on the second audio output  706 . In this configuration, similar to the audio system  650 , the first audio signal is played through the first dual voice-coil speaker  120  and the second audio signal is played through the second dual voice-coil speaker  120 ′. In this case only two audio amplifiers  722 ,  724  are needed to drive two dual-voice coil speakers. 
     The second audio signal and the phase inverted version of the second audio signal will destructively interfere mechanically in the respective voice coils of the dual voice coil loudspeaker  120 . This destructive interference results in either significantly reduced or no acoustic output from the dual voice coil loudspeaker  120  due to the second audio signal while the current flowing in coils L 1 , L 2  results in power still being dissipated. The first audio signal and the phase inverted version of the first audio signal will destructively interfere mechanically in the respective voice coils of the second dual voice coil loudspeaker  120 ′. This destructive interference results in either significantly reduced or no acoustic output from the second dual voice coil loudspeaker  120 ′ due to the second audio signal while the current flowing in coils L 1 ′, L 2 ′ results in power still being dissipated. The audio system  750  may allow stereo play back or be used for active cross-over filters if for example the first dual voice coil loudspeaker  120  is a woofer and the second dual voice coil loudspeaker  120 ′ is a tweeter. 
       FIG. 8  shows an audio system  850  including an audio processor  800  for a dual voice coil acoustic transducer according to an embodiment. The audio processor  800  may have an audio input  802 . The audio input  802  may be connected directly to the first audio processor output  804  or be connected via additional audio processing modules (not shown) included in the audio processor  800 . The input of the phase shift scaler  810  may be connected to the audio input  802 . An output of the phase shift scaler  810  may be connected to a second audio processor output  806 . The phase shift scaler  810  may have a volume control input  820 . 
     The audio processor first output  804  may be connected to an input of a first audio amplifier  122  of the audio amplifier system  108  which may be a class-D amplifier. The output of the first amplifier  122  may be connected to a first voice coil L 1  of a dual voice coil loudspeaker  120 . The audio processor second output  806  may be connected to an input of a second audio amplifier  124  of the audio amplifier system  108  which may be a class-D amplifier. The output  112  of the second amplifier  122  may be connected to a second voice coil L 2  of a dual voice coil loudspeaker  120 . The amplifier system  108  is connected to the dual voice coil loudspeaker  120  in a single ended configuration with one terminal of L 1  and L 2  connected to the respective amplifier output  112 ,  114  and the other terminal connected to ground  116 . 
     In operation, the audio processor  800  may output an audio signal on the first audio processor output  804  and a scaled and phase adjusted version of the audio signal on the second audio processor output  806  controlled by the volume control input  820 . The effect of this is that dependent on the phase and amplitude of the scaled audio signal, the degree of destructive interference in the respective voice coils of the dual voice coil loudspeaker  120  varies. This destructive interference results in either significantly reduced or no acoustic output from the dual voice coil loudspeaker  120  while the current flowing in coils L 1 , L 2  results in power still being dissipated. When the scaler is set to −1.0 there will be silence, because the audio signal and the scaled phased adjusted signal are destructively interfering. When the scaler set to 1.0 there is maximum output, because the scaled phased adjusted signal and the audio signal are constructively interfering. The audio processor  800  may provide a volume control for the audio system  850 . The overall dissipated power in the voice coils varies by a factor of two which may be used for heating the speaker  120 . The inventor of the present disclosure has appreciated that this effect may be used to self-heat the dual voice coil loudspeaker  120  even at low volumes. 
     An example implementation of the phase shift scaler  810  is shown in  FIG. 9  including a phase inverter  830  and a mixer  840 . The audio input  802  is connected to the input of the phase inverter  830  and a first input of the mixer  840 . An output  832  of the phase inverter  830  is connected to a second input of the mixer  840 . An output of the mixer  840  is connected to the second audio processor output  806 . The output of the phase shift scaler is determined by cross-mixing the in-phase and phase inverted signal determined by the volume control input given by the expression
 
Output=inPhase*volCtrl+phaseInv*(1−volCtrl)
 
     Hence, 
     when volCtrl=0, mixer output is the phase inverted signal 
     when volCtrl=0.5, mixer output is 0 
     when volCtrl=1.0, mixer output is in-phase signal 
     This way there is a translation between a normal volume scaler range [0.0 1.0] and the scaler required for the dual coil phase inversion. In other examples, the scaler may apply a scale factor between −1 and +1 to the output signal. In this case a value of −1 results in a phase-inverted signal, a value of 0 results in zero output and a value of 1 means the output is in phase. 
       FIG. 10  shows a method of driving a multi-voice coil acoustic transducer  900 . In step  902  an audio signal is received which may be speech, music or a reference signal. In step  904  the phase difference between the audio signal supplied to each of the voice coils of a multi-coil acoustic transducer may be adjusted in order to attenuate the acoustic output due to the audio signal 
     The phase difference between the audio signal applied to each voice coil of the multi voice coil acoustic transducer may be chosen so that the audio signals destructively interfere mechanically, and the multi voice coil acoustic transducer will not produce any audible sound due to the audio signal. The audio signal may be generated with a relatively large amplitude and may still be inaudible even though it is generated at an audible frequency. The method  900  may be used to dissipate power in the voice coil to heat a speaker without generating audible output. Alternatively, or in addition, the current flowing through each voice coil can be measured to monitor some loudspeaker characteristics like the DC resistance. 
     For example, for a dual voice-coil acoustic transducer, the audio signal which may be a reference signal is fed into the speaker and phase inverted for one coil. The reference signal can then be correlated with the measured current signal to obtain the voltage current relation or impedance. The phase inverted playback will significantly reduce the audibility of the reference signal. The reduced audibility allows a higher-level reference signal to be used which may improve the signal-to-noise ratio of the measurement. 
     An audio processor for a multi voice coil acoustic transducer is described. The audio processor may receive or generate an audio signal. The audio signal may have one or more phase shift applied. The audio signal may be used to drive a first coil of a dual voice coil acoustic transducer. The phase-shifted audio signals may drive the other coils of a multi voice-coil acoustic transducer. The phase shift is selected so that the phase difference between the audio signal driving each voice coil may result in destructive interference in the multi voice-coil loudspeaker resulting in reduced or no acoustic output due to the audio signal. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
     In some example embodiments the set of instructions/method steps described above are implemented as functional and software instructions embodied as a set of executable instructions stored on a non-transitory, tangible computer readable storage medium which are effected on a computer or machine which is programmed with and controlled by said executable instructions. Such instructions are loaded for execution on a processor (such as one or more CPUs). The term processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor can refer to a single component or to plural components. 
     Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
     The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
     For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.