Patent Publication Number: US-7214179-B2

Title: Low acceleration sensitivity microphone

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 119 to: U.S. Provisional Application No. 60/558,693 entitled: “Low Acceleration Sensitivity Microphone,” having a filling date of Apr. 1, 2004; and U.S. Provisional Application No. 60/643,074 entitled: “Active Vibration Attenuation for Implantable Microphone,” having a filing date of Jan. 11, 2005; and claims priority under 35 U.S.C. 120 as a continuation-in-part to U.S. patent application Ser. No. 10/982,639 entitled “Active Vibration Attenuation for Implantable Microphone,” having a filing date of Nov. 5, 2004; the contents of each of which are incorporated herein as if set forth in full. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to implanted microphone assemblies, e.g. as employed in hearing aid instruments, and more particularly, to implanted microphone assemblies having reduced sensitivity to acceleration. 
     BACKGROUND OF THE INVENTION 
     Until recently, a large number of people affected by sensorineural hearing loss of 55 dB or more have been unable to receive adequate therapeutic benefit from any available technology. This problem has been alleviated to some extent by the development of a class of hearing aids generally referred to as implantable hearing instruments, which include, for example, middle ear implants and cochlear implants. Generally, such implantable hearing instruments utilize an implanted transducer to stimulate a component of the patient&#39;s auditory system (e.g., tympanic membrane, ossicles and/or cochlea). By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g. U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate causing stimulation of the cochlea through its natural input, the so-called oval window. 
     Amongst users of implantable hearing instruments, there is a strong desire for a small, fully implantable system. In such hearing instruments, the entirety of the instrument&#39;s of various hearing augmentation components, including a microphone assembly, is positioned subcutaneously on or within a patient&#39;s skull, typically at locations proximate the mastoid process. 
     As may be appreciated, implantable hearing instruments that utilize an implanted microphone require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, such implantable microphones may be typically positioned in a surgical procedure between a patient&#39;s skull and skin, at a location rearward and upward of a patient&#39;s ear (e.g., in the mastoid region). Accordingly, the hearing instrument must overcome the difficulty of detecting external sounds (i.e., acoustic sounds) after attenuation by a layer of skin. In this regard, a subcutaneously located microphone must provide adequate acoustic sensitivity while being covered by a layer of skin between about 3 mm and 12 mm thick. 
     Further, a subcutaneously located microphone must also be able to discriminate between acoustic sounds and unwanted vibrations. That is, acceleration within patient tissue (e.g., caused by tissue-borne vibration) may cause pressure fluctuations that are commingled with pressure fluctuations caused by acoustic sounds impinging on tissue overlying an implanted microphone. This undesirable commingling of ambient acoustic signals and tissue-borne acceleration signals is at the root of several problems facing the designers of implantable hearing systems. 
     One particular problem relates to vibrations caused by the implant wearer&#39;s voice, chewing or vibration caused by the hearing instrument itself (e.g., an electromechanical transducer) may generate distortion of wearer&#39;s own voice, feedback and/or reduce acoustic sensitivity. For example, sound emanating from the vocal chords of a person wearing an implantable hearing instrument passes through the bony structure of the head (i.e., as a vibration) and reaches the implanted microphone of the implantable middle ear hearing system or fully implantable cochlear implant. The vibration reaches the microphone and may induce pressure fluctuations within the skin due to acceleration. Accordingly, such pressure fluctuations may be amplified just as a pressure fluctuation caused by the deflection of the skin&#39;s surface by an acoustic sound. In this regard, the implanted microphone detects the combination of these two sources as a single varying pressure. Further, in systems employing a middle ear stimulation transducer, the microphone may produce feedback by picking up and amplifying vibration caused by the stimulation transducer. As such, the bone-borne vibration undesirably limits the maximum achievable gain of the implantable hearing instrument. 
     In order to achieve a nearly natural quality of the implant wearer&#39;s voice and detect acoustic signals with sufficient sensitivity, an implanted microphone needs to compensate for acceleration pressures and/or feedback. The aim of the present invention is to design an implantable microphone that achieves these goals. 
     SUMMARY OF THE INVENTION 
     Although all microphones possess some degree of acceleration sensitivity, unwanted responses from acceleration is not a significantly limiting to the performance of acoustic microphones, that is, microphones designed to operate in air. The inventors of the present invention have recognized that the same is not true, however, for subdermal/implanted microphones as acceleration within tissue arising from tissue-borne vibration (e.g., from talking or chewing) causes pressure fluctuations that are combined/commingled by the implanted microphone with pressure fluctuations caused by external/ambient sounds. In this regard, pressure fluctuations in tissue (e.g., overlying an implanted microphone) may arise from external pressures such as ambient acoustic signals (i.e., sound) impinging on the skin as well as from acceleration within the tissue caused by vibration. Accordingly, a method and system for distinguishing or isolating an acoustic signal component from a commingled signal is desirable. 
     In appreciating the nature of this problem, it is important to distinguish between two mechanisms that cause a microphone to be sensitive to vibration: acceleration sensitivity of the microphone&#39;s internal microphone element (e.g., the element that translates pneumatic pressures into an electrical output) and acceleration pressure sensitivity, which acts on the entire microphone including the microphone diaphragm. Typically, a microphone element may be selected with sufficiently low acceleration sensitivity (e.g., 60 dB SPL/g) such that the inherent vibration sensitivity of the microphone is dominated by the acceleration pressure sensitivity. In this regard, acceleration pressure is a limiting factor in implantable microphone design. In any case, some degree of vibration sensitivity and attendant acceleration sensitivity is inherent in any microphone, despite design measures to minimize it. 
     It is possible to reduce vibration sensitivity of a microphone assembly through vibration isolation. In its simplest form, a vibration isolator is a spring (i.e., compliant member) that suspends the assembly to be isolated. The spring rate of the isolator is chosen so that the resonate frequency of the suspended microphone is much lower than the lowest frequency to be isolated, typically by a factor of five or more. While reducing the effects of vibration directly on the microphone, it will be appreciated that vibration may still cause acceleration induced pressure variations within the tissue overlying the microphone and microphone diaphragm. Therefore, such acceleration must still be accounted for to better isolate ambient acoustic signals from a commingled signal. In this regard, the inventors have further recognized the desirability of actively accounting for the effects of acceleration and have produced a microphone that exhibits low sensitivity to acceleration-induced pressures. 
     More specifically, the present invention utilizes an output that is indicative of acceleration acting on the microphone (e.g., an acceleration signal) to counteract and/or cancel the effects of acceleration-induced pressures in an output signal of a microphone diaphragm. Generally, the acceleration signal and the acceleration induced pressures effects in the output signal will correspond to a common source (e.g., a common vibration). Accordingly, the magnitude an/or phase of the acceleration signal acceleration induced pressure effects may be mathematically related. Counteracting and/or canceling the effect of acceleration may be done in a variety of ways, including but not limited to, pneumatically, mechanically, electrical analog, or digitally, or combinations thereof. In order to better account for the effect of acceleration on the microphone, the acceleration signal, microphone diaphragm output signal, or both may be filtered and adjusted for gain (again, including but not limited to pneumatically, mechanically, electrical analog or digitally or any combination thereof) before utilizing the acceleration signal for counteraction/cancellation. This filtering may have the effect of more closely matching the acceleration signal to the acceleration responses in the microphone output signal and thereby substantially reduce the effects of acceleration on the implanted microphone. The filter or filters may be adjustable and/or adaptive in order to allow optimal rejection of vibration for a given patient under a variety of changing conditions. 
     According to one aspect of the invention, a method for isolating an acoustic signal from a commingled signal in an implantable microphone is provided. The method includes providing a first output that corresponds to a transcutaneously received pressure signal, where the first output has an acoustic component and an acceleration component. A second output is supplied that corresponds to an acceleration force acting on the implantable microphone. At least a portion of the first and second outputs are used to generate a combined output, wherein an acceleration component of the combined output is less than the acceleration component of the first output. 
     The first output may correspond to an output of a first microphone diaphragm that is positioned to receive pressure variations through overlying tissue. As noted above, such pressure variations may include commingled acoustic and acceleration pressures. Generally, the second output is substantially isolated from acoustic forces acting on the implanted microphone assembly. Stated otherwise, the second output of the cancellation surface may be a response having an acceleration component that is significantly larger than an acoustic component or that is substantially free of an acoustic component. 
     The outputs may be combined in any appropriate manner including pneumatically and electrically. Generally, the acceleration component of the second output is removed from the acceleration component of the first output to at least partially isolate the acoustic component. This isolated acoustic component may then be utilized to actuate an actuator of a hearing instrument. This may entail additional processing of the isolated acoustic component. 
     How the first and second outputs are used to generate a combined output may correspond to how the first and second outputs are obtained. For instance, if the first and second outputs are obtained by deflecting first and second diaphragms in response to applied forces, the volumetric deflection of these outputs may be pneumatically combined. Alternatively, each volumetric deflection may be converted into an electrical signal and electrically combined. As a further alternative, the second output may originally include an electrical output. For instance, where the second output is generated by an accelerometer the output of such an accelerometer may be an electric signal indicative of, for example, the phase and/or magnitude of the movement of the accelerometer. Furthermore, each output may also be filtered to better match the second output to the acceleration component of the first output. 
     According to another aspect of the invention, a method for use in an implantable microphone is provided. The method includes locating a first diaphragm to receive ambient acoustic signals (e.g., through overlying tissue) and positioning a cancellation surface to be isolated from such ambient acoustic signals, wherein at least a portion of the cancellation surface is operable to respond to acceleration forces. A first output is provided by the first diaphragm in response to an applied acceleration. This output includes a first acceleration component and an acoustic component. A second output is supplied by the cancellation surface in response to the applied acceleration. The second output includes a second acceleration component. 
     Portions of the first and second outputs may be used to generate a third output. Preferably, a third acceleration component of the third output will be less than the first acceleration component of the first output for at least a desired frequency range. In this regard, at least a portion of the first acceleration component is removed form the acoustic component of the first diaphragm. The using step may include subtracting said second output from said first output. 
     Generally, the first diaphragm is located on the surface of a microphone housing such that it may receive incident ambient acoustic signal through overlying tissue. The cancellation surface may be positioned within an interior of such a housing to isolate the cancellation surface from incident ambient acoustic signals. The cancellation surface may be rigidly connected to such a housing to allow the cancellation surface to respond to acceleration forces acting on the microphone housing. Alternatively, the cancellation surface may be co-located with and/or affixed to the first diaphragm such that these elements experience similar acceleration. 
     According to another aspect of the present invention, an implantable microphone is provided that includes a housing having an internal chamber with an aperture thereto and a first diaphragm sealably positioned across the aperture. The first diaphragm is operative to move in response to acoustic forces and acceleration forces present in media (e.g. tissue) overlying the first diaphragm. That is, the first diaphragm receives a combined pressure signal that includes an acoustic pressure component and an acceleration pressure component corresponding to tissue-borne vibrations. The microphone also includes a reference or cancellation surface at least a portion of which moves relative to the housing in response to the acceleration forces acting on the housing. A sensor is utilized to generate an output that is indicative of relative movement between the first diaphragm and the cancellation surface. 
     Typically, the relative movement between the first diaphragm and the cancellation surface is dominated by the pressure component(s) associated with acoustic forces received by the first diaphragm and which may be isolated from the cancellation surface. In this regard, the first diaphragm moves in relation to the acoustic forces while the cancellation surface remains substantially stationary (i.e., in relation to the acoustic forces). Both the first diaphragm and the cancellation surface move in relation to the acceleration forces. Accordingly, an output signal indicative of a change in the relative positions of the microphone diaphragm and the cancellation surface generally corresponds to the acoustic forces (e.g., an ambient acoustic signal). 
     That is, the output signal of the sensor may correspond to the acoustic forces acting on the microphone diaphragm. Accordingly, this output signal may be further processed and/or utilized for stimulating a component of the patient&#39;s auditory system (e.g., tympanic membrane, ossicies and/or cochlea). Stated otherwise, the microphone utilizes a diaphragm and a cancellation surface to distinguish between pressure fluctuations in tissue overlying the microphone caused by ambient acoustic sounds and acceleration in patient tissue. 
     While the output signal may be indicative of a relative movement of the first diaphragm relative to the cancellation surface, this relative movement may be monitored/measured in a number of different ways. For instance, physical changes between the microphone diaphragm and the cancellation surface may be measured. For example, a pressure of an enclosed space defined between the first diaphragm and the cancellation surface may be monitored to identify such relative movement. Alternatively, a distance between pre-selected regions on each of the first diaphragm and cancellation surface may be monitored to identify changes in relative position. 
     In other arrangements, relative movement may be monitored electrically. For instance, a change in voltage between pre-selected regions on the first diaphragm and the cancellation surface may be monitored. In this regard, a first electrode may be interconnected to the first diaphragm and a second electrode may be interconnected to the cancellation surface in order to measure a voltage across an electrically active element disposed therebetween (e.g., a compressible electret or a piezo-electric member). Alternatively, the first diaphragm and/or the cancellation surface may be made of a conductive material such that those elements themselves form electrodes and/or capacitor plates. In this regard, changes in a capacitance may be monitored. In further arrangements, coils and or magnets may be interconnected to the diaphragm and/or cancellation surface such that voltages and or inductances may be generated in corresponding relation to the relative change. 
     Alternatively, force may be utilized to identify relative movement between the first diaphragm and cancellation surface. In this regard, force-measuring devices (e.g., piezo-electric members) may be physically disposed between surfaces of the first diaphragm cancellation surface. Alternatively, strain gages may be utilized to monitor changes within each of the first diaphragm and the cancellation surface. 
     In all cases, it may be desirable that the magnitude of the response of the cancellation surface to acceleration be chosen to substantially match the response of the first diaphragm to acceleration, and the phases should be substantially matched as well in order to achieve enhanced cancellation. It may be preferred that such magnitude and phase matching occur in a frequency range of interest (e.g., and acoustic hearing range). This may require that the resonant frequency of each the first diaphragm and cancellation surface be less than about 2000 Hz and more preferably less than about 200 Hz. These resonant frequencies are typically below an acoustic hearing frequency range. Further, it may be desirable that the first diaphragm and cancellation surface have substantially equal resonant frequencies and/or equal damping factors. 
     In another arrangement, the resonant frequency of the first diaphragm and/or cancellation surface may be greater than most or all of an acoustic hearing frequency range. In such an arrangement, the response of the first diaphragm and/or cancellation surface may be flatter over a greater frequency range. This may permit more easily matching outputs from these elements. 
     The cancellation surface may be any surface that is operative to move in response to acceleration forces applied to the microphone. In this regard, the cancellation surface may be considered an accelerometer or more generally a motion sensor. The physical configuration of this accelerometer/motion sensor (hereafter accelerometer) and the output of the accelerometer may vary. For instance, the accelerometer may include a compliantly supported mass (e.g., a proof or seismic mass). Inertial movement of the proof mass in response to acceleration forces may physically counteract the movement of the microphone diaphragm in response to acceleration. 
     In another arrangement, the cancellation surface may generate an electrical output (acceleration signal), which may subsequently be combined (e.g., subtracted) with an electrical microphone output signal of the first diaphragm. Use of such electrical signals may facilitate filtering of the acceleration signal of the accelerometer and/or the microphone output signal to better match, for example, the phase and/or magnitude of these signals. 
     In further arrangement, the output of the cancellation surface may pneumatically counteract the acceleration response of the first diaphragm. In such an arrangement, portions of the cancellation surface and first diaphragm may each move relative to an enclosed space. Accordingly, if the first diaphragm and cancellation surface displace substantially equal and opposite volumes relative to the enclosed space (i.e., in response to acceleration), the change in the volume of the enclosed space will primarily represent the acoustic forces on the first diaphragm. In one particular arrangement, the cancellation surface is a mass loaded second diaphragm (e.g., cancellation diaphragm), which may be disposed within the housing of the microphone such that it is substantially isolated from acoustic forces. To permit the second diaphragm to respond similarly to the first diaphragm, the mass loading of the second diaphragm may approximate the mass loading of the first diaphragm by overlying media. 
     In another arrangement, where the cancellation surface is a mass loaded second diaphragm, the first and second diaphragms may move relative to first and second enclosures, respectively. Accordingly, first and second microphone elements may generate first and second electrical output signals, which may subsequently be combined to produce a microphone output signal having a reduced acceleration response. 
     According to another aspect of the present invention, an implantable microphone is provided that allows for pneumatically extracting an acoustic signal from a transcutaneously received pressure signal. More particularly, the microphone includes housing having an internal chamber and an aperture thereto and a first diaphragm sealably positioned across the aperture. The microphone further includes a second diaphragm at least partially disposed within the housing and in a spaced relationship with the first diaphragm, wherein the first and second diaphragms define a trapped volume. A proof mass is attached to the second diaphragm to deflect the second diaphragm in response to acceleration. A sensor is operative to sense pressure changes within the trapped volume and generate an output signal indicative thereof. 
     The first diaphragm is operative to transcutaneously receive a combined pressure signal that includes an acoustic pressure component corresponding with an acoustic signal (i.e., from acoustic pressure impinging on the external) and an acceleration component corresponding with acceleration in tissue overlying the first diaphragm. The second diaphragm is operative to respond to an acceleration signal while being substantially isolated from the acoustic signal. The movement of the first and second diaphragm relative to the trapped volume constitutes a summation process, which cancels the acceleration component of the first diaphragm using the acceleration response of the second diaphragm. Accordingly, the output of the sensor corresponds to the acoustic signal. 
     To match the responses of the first and second diaphragms, these diaphragms may have similar shapes and may be rigidly connected about their peripheries. To maintain substantially equal shapes upon deflection, one or both diaphragms may include a reinforcing member. 
     According to another aspect of the invention, an implantable microphone is provided that generates a first and second outputs for subsequent summation. The microphone includes a first diaphragm operative to receive pressure variations in overlying media and generate a corresponding first output signal. This first output signal may include an acceleration component and an acoustic component. A cancellation surface is operative to generate a second output signal indicative of acceleration. A summation device combines the first and second output signals to generate a combined output signal that is operative to actuate an actuator of a hearing instrument. 
     The summation device may remove at least a portion of the second output from the first output such that an acceleration component of the combined output signal is less than the acceleration component of the first output signal. This may entail subtracting the second signal form the first signal. 
     In one arrangement, the first and second outputs are electrical outputs and the summation device is an electric summation device. This arrangement may further include a first filter for filtering the first output and/or a second filter for filtering the second output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a fully implantable hearing instrument as implanted in a wearer&#39;s skull. 
         FIG. 2  illustrates a perspective view one embodiment of an implantable housing. 
         FIG. 3  illustrates the implantable housing of  FIG. 2  as implanted relative to patient tissue. 
         FIG. 4  illustrates a signal flow block diagram of one embodiment of an implantable hearing instrument. 
         FIG. 5  illustrates a mathematical model of the present invention. 
         FIG. 6  illustrates a signal flow block diagram of another embodiment of an implantable hearing instrument. 
         FIG. 7  illustrates a cross-sectional view of one embodiment of the present invention. 
         FIGS. 8A and 8B  illustrate a cross-sectional view of a second embodiment of the present invention. 
         FIG. 9  illustrates a performance plot of the embodiment of  FIGS. 8A and 8B . 
         FIG. 10  illustrates a cross-sectional view of a third embodiment of the present invention. 
         FIG. 11  illustrates a cross-sectional view of a fourth embodiment of the present invention. 
         FIG. 12  illustrates a signal flow block diagram of a further embodiment of an implantable hearing instrument. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the accompanying drawings, which at least assist in illustrating the various pertinent features of the present invention. In this regard, the following description of a hearing instrument is presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain the best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. 
     Hearing Instrument System: 
       FIGS. 1–3  illustrate one application of the present invention. As illustrated, the application comprises a fully implantable hearing instrument system. As will be appreciated, certain aspects of the present invention may be employed in conjunction with semi-implantable hearing instruments as well as fully implantable hearing instruments, and therefore the illustrated application is for purposes of illustration and not limitation. 
     In the illustrated system, an implanted biocompatible housing  100  (i.e., implant housing) is located subcutaneously on a patient&#39;s skull. The implant housing  100  includes a receiver  118  (e.g., comprising a coil element), an energy storage device (not shown), a microphone including a microphone diaphragm  10 , and a signal processor (not shown) including a speech signal-processing (SSP) unit (i.e., in addition to processing circuitry and/or a microprocessor). Various additional processing logic and/or circuitry components may also be included in the implant housing  100  as a matter of design choice. The signal processor is electrically interconnected via wire  106  to an electromechanical transducer  108 . Alternatively, however, the transducer  108  may be any type of transducer operative to stimulate a component of the auditory system such as the tympanic membrane, ossicies  120  or cochlea. 
     The transducer  108  is supportably connected to a positioning system  110 , which in turn, is connected to a bone anchor  116  mounted within the patient&#39;s mastoid process (e.g., via a hole drilled through the skull). The transducer  108  includes a connection apparatus  112  for connecting the transducer  108  to the ossicles  120  of the patient. In a connected state, the connection apparatus  112  provides a communication path for acoustic stimulation of the ossicles  120 , e.g., transmission of axial vibrations to the incus  122 . 
     As shown in  FIGS. 2 and 3 , an optional compliant base member  132  is utilized to reduce non-ambient vibrations that may be transmitted from an implant wearer&#39;s skull (i.e., skull-borne vibrations) and/or tissue to the implant housing  100  and, hence, the microphone diaphragm  10 . The compliant base member  132  is designed to receive the implant housing  100  within a cup-shaped recess  140  and hold the implant housing  100  such that the microphone diaphragm  10  is positioned to receive ambient acoustic signals through overlying tissue. Further, the compliant base member  32  includes a channel  48  through the periphery of the recess  40  that allows wire  106  to be routed from the implant housing  100  to the transducer free of obstruction. In  FIG. 3  the compliant base member  132  and implant housing  100  are shown as they would appear in use in relation to a patient&#39;s skull  144 , and overlying tissue/skin  142  (e.g., shown in cut-away relation). 
     During normal operation, acoustic signals are received subcutaneously at the microphone diaphragm  10 . Upon receipt of the acoustic signals, the implanted signal processor processes the signals (e.g., using the SSP unit) to provide a processed audio drive signal via wire  106  to the transducer  108 . As will be appreciated, the SSP unit may utilize digital processing to provide frequency shaping, amplification, compression, and other signal conditioning, including conditioning based on patient-specific fitting parameters. The audio drive signals cause the transducer  108  to transmit vibrations (e.g., axial) at acoustic frequencies to the connection apparatus  112  to effect the desired sound sensation via mechanical stimulation of the incus  122  of the patient. 
     An external charger (not shown) may be utilized to transcutaneously re-charge the energy storage device within the implant housing  100 . Such an external charger may include a power source and a transmitter that is operative to transcutaneously transmit, for example, RF signals to the implanted receiver  118 . In this regard, the implanted receiver  118  may also include, for example, rectifying circuitry to convert a received signal into an electrical signal for use in charging the energy storage device. The external transmitter and implanted receiver  118  may each comprise coils for inductive coupling of signals there between. In addition to being inductively coupled with the inductive coil  118  for charging purposes, such an external charger may also provide program instructions to the processor(s) of the implantable hearing instrument. 
     The block diagram  FIG. 4 , illustrates how pressures resulting from ambient acoustic sounds and tissue-borne acceleration are combined at an implanted microphone diaphragm  10  of an implantable microphone assembly  8 . As shown, the implanted microphone diaphragm  10  is exposed to pressure in overlying tissue  142  that is generated externally to the patient, represented by ambient sound source  40 . This ambient signal (e.g., sound) from the sound source  40  passes through and is filtered by the tissue  142  overlying the microphone diaphragm  10 . The deflection of the microphone diaphragm  10  by the pressure associated with the ambient sound results in a desired signal component, or, microphone sound response  42 . This microphone sound response  42  is also mixed with the pressure generated by acceleration in the overlying tissue  142  caused by one or more acceleration sources  50 . The pressure from the acceleration source  50  is likewise filtered by the tissue  142  overlying the microphone diaphragm  10  and results in an undesired signal component or microphone vibration response  52  (Hmv). 
     The net effect is that the signals  42 ,  52  are summed by the normal action of the microphone diaphragm  10 . That is, pressure associated with each of the ambient signal and acceleration signal, which arrive at the microphone diaphragm  10  through the overlying tissue  142 , deflect the diaphragm  10  and thereby generate a microphone output signal  54 . The microphone output signal  54  is a combination of the pressure associated with the two received signals  42 ,  52 . 
     The microphone output signal  54  is processed by the processor  104  of the implantable hearing instrument. Such processing may include, without limitation, functions such as band pass filtering, equalization and compression. Once the hearing instrument processing is performed on the microphone output signal  54 , the output  56  of the processor  104  drives the transducer  108 , which may include, for example, a middle ear transducer or cochlear implant electrode array. 
     The acceleration source  50  may comprise any source of tissue-borne vibrations and may include biological sources and mechanical sources. Such biological sources may include, without limitation, chewing and speaking. One example of a mechanical source includes feedback signals from the transducer  108 , which in the normal course of its operation may vibrate surrounding tissue. Such vibration may subsequently be conducted to the location of the microphone diaphragm  10 . Such a feedback path  58  is illustrated with a dotted line in the block diagram of  FIG. 4 . If the feedback is of sufficient strength and phase, feedback oscillation may occur (see e.g. Nyquist). If of sufficient power, such acceleration signals will present themselves as impairments to the performance of the hearing instrument. Further, if they are of sufficient power, such acceleration signals may saturate the microphone. 
     As noted above, tissue-borne vibration and ambient sound signals each induce pressure fluctuations within the tissue  142  overlying the microphone diaphragm  10 . As further noted, the microphone diaphragm  10  detects the combination of these pressure fluctuations as a single varying pressure. In order to detect desired signal components (e.g., the microphone sound response  42 ) with sufficient sensitivity, the implanted microphone needs to compensate for undesired signal components (e.g., the microphone vibration response  52 ). Stated otherwise, the microphone assembly  8  needs to separate ambient acoustic signals from tissue-borne vibration-induced signals. In order to separate these signals, one element of the microphone assembly  8  is designed to be preferentially sensitive to vibration and preferentially insensitive to acoustic stimulation. 
     The present invention utilizes a reference or ‘cancellation surface’ that is primarily sensitive to tissue-borne vibration (i.e., acceleration) while being substantially insensitive to ambient acoustic signals. In this regard, output from the cancellation surface may be removed from a combined output  54  from a diaphragm  10  that is sensitive to pressure variations associated with both ambient acoustic signals and acceleration signals. Accordingly, by monitoring the differences in, for example motion, force, distance, velocity, volume or other properties of the diaphragm  10  and the cancellation surface, variation in the diaphragm caused by ambient acoustic signals may be effectively extracted from a combined output signal  54  of the microphone diaphragm  10 . Accordingly, this difference (e.g., the ambient acoustic signal) may be output to the implanted signal processor  104  for additional processing and/or output to the transducer  108  for use in stimulating a component of a patient&#39;s auditory system. 
       FIG. 5  shows a schematic/mathematical depiction of the basic operating principle of a microphone assembly  8  that utilizes a cancellation surface  16 . As shown, the microphone assembly  8  can be modeled as a spring mass system where the diaphragm  10  and a mass of overlying tissue is a first mass M 1  having a first spring constant k 1 . The diaphragm  10  may be positioned immediately adjacent and facing to the skin of the patient such that a combined force F 1  including ambient acoustic signals and acceleration acts upon M 1 . The cancellation surface  16  is disposed within the microphone assembly  8  such that it is substantially isolated from ambient acoustic signals (e.g., within an implant housing). In this regard, the cancellation surface  16  is represented by M 2 , has a second spring constant k 2 , and is acted upon by F 2 , which is the force due to acceleration. 
     As will be appreciated, the response of the two systems M 1  and M 2  is governed by simple harmonics. It can be shown mathematically that when the microphone assembly  8  measures a frequency significantly higher that the resonant frequencies of the systems M 1 , k 1  and M 2 , k 2 , the difference Δ between the systems (e.g., velocity in one embodiment) may be determined and is independent of spring rates and masses of the systems. Accordingly, the difference Δ between the cancellation surface  16  and the diaphragm  10  will represent the pressure applied to the diaphragm  10  by ambient acoustic signals. That is, the difference Δ represents the ambient acoustic signal applied to the diaphragm  12  free of pressure variations caused by acceleration. In this regard, by monitoring the difference Δ of the systems M 1  and M 2  the acoustic signal may be determined substantially free of acceleration. Accordingly, the difference Δ may be supplied to the processor  104  and/or transducer  108  for use in hearing augmentation. 
       FIG. 6  illustrates a block diagram of one embodiment of a microphone assembly  8  that utilizes a cancellation surface. In this arrangement the cancellation surface, which is capable of responding to acceleration due to vibration while being substantially isolated from ambient sound signals, is represented as an accelerometer  60 . As shown, the accelerometer  60  generates an accelerometer output signal  64  in response to the acceleration source  50 , which is also represented by Hav (accelerometer vibration response) on  FIG. 6 . The accelerometer output signal  64  is substantially free of effects of the ambient acoustic signals from the sound source  40 . Similar to the situation discussed in  FIG. 4 , the output signal  54  of the microphone diaphragm  10  is a combination of the pressures associated with the sound source  40  and acceleration source  50 . The microphone output signal  54  may optionally be filtered by a microphone post-filter  66  (e.g., transfer function/rational polynomial Gm) operative to adjust, one or more characteristics of the microphone output signal  54  (e.g., gain, phase, etc.). Likewise, the accelerometer output signal  64  may optionally be filtered by an accelerometer post-filter (Ga) to adjust one or more characteristics of the accelerometer output signal  64 . Such post-filtering will be more fully discussed herein. 
     The output signals  54 ,  64  are combined in a summer  70 . The post filter and summer embodiments may comprise mechanical, pneumatic, electrical analog, digital or software, or combinations thereof. Generally, the values of the post-filter coefficients/weights are selected so that to a substantial degree in the frequency range of interest (i.e., an acoustic hearing range) the post filter outputs  72 ,  74  have substantially equal magnitude and/or phase where:
 
Hmv Gm=−Hav Ga  (Eq 1)
 
     The sign change in this equation may be provided in the output/response of the accelerometer  60 , the microphone diaphragm  10 , either of the filters  66 ,  68 , or, the summer  70  may have the polarity inverted on one input. In this regard, the summer  70  performs as a subtractor. If the equation holds to a substantial degree in the frequency range of interest, then the response of the microphone assembly  8 , consisting of microphone sound response Hms(s)  42 , microphone vibration response Hmv(s)  52 , accelerometer vibration response Hav(s)  64 , microphone post-filter output  72 , accelerometer post-filter output  74  and summer output  76  is essentially just the response of the microphone diaphragm  10  to the sound source  40 . That is, the response of the microphone assembly  8  is the response of the microphone diaphragm  10  to the sound source  40  alone. There is a large degree of flexibility of how a microphone assembly may be constructed, based on various design choices presented by Equation 1. Some of the possible design choices are presented herein. 
     In one case, the acceleration response of the microphone diaphragm  10  and accelerometer  60  are made substantially identical, while the post filters  66 ,  68  are a substantially equal value of −G (including the possibility of G=k, a constant, and further including the case k=1) for both post filters  66 ,  68 . That is:
 
Hav=Hmv, Gm=Ga=−G  (Eq2)
 
(Where, equality is assumed to mean substantial equality over the frequency range of interest.) This includes the case where the acceleration responses of the microphone diaphragm  10  and accelerometer  60  are mechanically selected to be substantially equal, but have opposite sign due to the mechanical construction, and where the output signals  54 ,  64  are mechanically summed, with essentially no filtering. Where there is no filtering, the post-filters  66 ,  68  may be eliminated.
 
     While making the acceleration response of the microphone diaphragm  10  and accelerometer  70  equal is relatively easy for frequencies substantially above the resonant frequency of the microphone diaphragm  10 , achieving a resonant frequency low enough to cover the frequency range of interest while maintaining sufficient desired signal strength can be problematic. The frequency range of interest usually includes the band of 1 kHz and higher, whereas the resonant frequency of the microphone diaphragm is generally limited to higher frequencies due to the limited volume of the microphone assembly (i.e., for generating an acoustic output in response to received via overlying tissue) and the mass of the overlying tissue, coupled with the necessity to maintain sufficient acoustic sensitivity. 
     In another case, the acceleration response  52  (Hmv) of the microphone diaphragm is corrected by the post-filter  66  to be an essentially flat gain k. That is:
 
Hmv Gm=k, Hav Ga=−k  (Eq 3)
 
This has the advantage of flattening the sound response of the microphone diaphragm  10  as well, since the ratio of the sound pressure response to acceleration response is approximately constant. This is normally advantageous to any subsequent processing by the processor  104 , as it optimizes the dynamic range of an analog to digital converter (not shown), which converts the generally analog output signals to digital values for processing, and optimizes the numeric processes involved in equalization and any intervening processing. A similarly flattened signal is generated by the accelerometer branch of the microphone assembly  8 , so that output of the accelerometer post-filter  68  is corrected to the same essentially flat gain k, but have opposite sign. Making the outputs  72 ,  74  of each of the microphone diaphragm branch and accelerometer branch essentially flat with frequency also has the advantage of reducing the sensitivity of the cancellation process to variations in any of the subcomponents. This approach also has the advantage of being simple to adjust, in that the response of each of the microphone diaphragm and accelerometer branches can be adjusted to have a flat response independently.
 
     In another case, no post filtering is provided to the microphone diaphragm branch other than a gain k with an allpass filter. That is:
 
Hmv Gm=k, Hav Ga=−k  (Eq 4)
 
This has the advantage of not altering the sound response of the microphone assembly  8 . It has the further advantage of concentrating the filter coefficients into a single branch. This may be important, depending on the implementation, in minimizing, say, the processing power required, and may be a preferred embodiment for systems in which the post-filters  66 ,  68  and second summer  70  are implemented digitally or in software. The allpass function is chosen to correct for any phase shifts due to the filtering process, as often it is not possible to exactly invert the filter Hav and remain causal. An example of such an allpass filter may be a time delay, with a time delay selected to correct for any time delays introduced into the accelerometer branch.
 
     It should be noted that in most implementations, the responses  42 ,  52  (Hmv and Hav) of the microphone diaphragm  10  in the frequency range of interest will change with changing thickness of tissue over the microphone diaphragm  10 . Both the sound and acceleration responses of the microphone diaphragm  10  can change in a complex manner, with the strength and position of resonances changing. In general, the frequency of the resonances will decrease with increasing skin thickness, approximately inversely proportional to the square root of the skin thickness. The amplitude of the response will increase approximately proportional to the thickness of the skin. Generally, the response of a simple microphone diaphragm, operated as a membrane or plate, has multiple, complicated, resonances when incorporated into an implanted microphone assembly. Such resonances, unless matched in the accelerometer response, will result in a reduction of the cancellation of the microphone accelerometer response. 
     The resonances of the microphone diaphragm  10  can be moved to a frequency range above the frequency range of interest (i.e., an acoustic frequency range) by increasing the tension and/or stiffness of the microphone diaphragm  10  substantially, resulting in a flatter and easer-to-match response, but this is found to reduce the acoustic sensitivity of the microphone assembly  8 . By using mechanical filtering, the response of the microphone diaphragm is considerably simplified, making matching the response with the accelerometer response much easier. Such filtering can be achieved mechanically by, for instance, changing the distribution of stiffness and/or the damping of the diaphragms used. 
     The goal of mechanical filtering in general is to simplify the task of matching the microphone acceleration response with the accelerometer response, or in subsequently matching the output  72  of the microphone post-filter  66  to the output  74  of the accelerometer post-filter  68 . With a microphone acceleration response achieved, the accelerometer  70  is designed to have a similar response. Then, either or both of the responses can be further post filtered by e.g. pneumatic, mechanical, electrical or digital means so that the filtered responses result in cancellation of the microphone acceleration response to a desired degree. In any case, it is desirable that the response of the microphone diaphragm to acceleration and the response of the accelerometer to acceleration be closely matched such that post-filtering may be facilitated or eliminated. 
       FIGS. 7–11  disclose various embodiments of vibration compensating microphone assemblies. As shown in  FIGS. 7 ,  8 A,  8 B,  10  and  11 , the microphone assemblies are constructed in a manner that substantially matches the magnitude and phase of the cancellation surface/accelerometer response to the acceleration response of the microphone diaphragm. 
       FIG. 7  shows a first embodiment of a vibration compensating microphone assembly  8 . In this embodiment, the microphone assembly  8  measures the difference in motion between the center of a microphone diaphragm  10  and a cancellation mass  16  (i.e., an accelerometer). The cancellation mass is also know variously as a proof mass or a seismic mass. As shown, the microphone assembly  8  includes a housing  20  that defines an internal chamber  22 . The chamber  22  includes an aperture across which the microphone diaphragm  10  is sealably disposed. A compressible foam electret material  25  is interconnected to the center of the microphone diaphragm  10 . Interconnected to an opposing surface of the electret material  25  is the cancellation mass  16  that forms the cancellation surface for the depicted microphone assembly  8 . 
     In this embodiment, the resonant frequency of the microphone diaphragm  10 , which is mass loaded due to the surrounding media (i.e., tissue), is selected to be significantly below the lowest frequency of interest. Likewise, the resonant frequency of the cancellation mass  16  is also chosen to be significantly below the lowest frequency of interest. Furthermore, it will be noted that the size of the cancellation mass  16  may be selected for resonant frequency purposes. Ideally, the resonant frequency of the microphone diaphragm  10  and the cancellation mass  16  are substantially equal. However, it will be appreciated that the resonant frequency of the microphone diaphragm  10  and cancellation mass  16  need not be the same. 
     The difference in motion between the microphone diaphragm  10  and the cancellation mass  16  is measured using the electret material  25 . In this regard, the electret material  25  acts as a sensor that measures the relative movement between the connected portions of the cancellation mass  16  and diaphragm  10 . If the mass of the cancellation mass  16  is substantially equal to the mass (i.e., tissue) overlying the diaphragm  10 , the responses of these elements  10 ,  16  to acceleration from an acceleration source may be substantially equal. In this regard, the resulting force on the electret material  25  may correspond primarily to ambient acoustic signals acting on the diaphragm  10 . The output of the electret likewise may be indicative of the response of the microphone diaphragm  10  to ambient acoustic signals free of acceleration. 
     As shown, the electret material  25  acts as a piezo-active material. Accordingly, by measuring the changes in voltage between the membrane  12  and the cancellation surface  16 , differences Δ in motion between the diaphragm  10  and the cancellation surface  16  may be directly monitored thereby generating an output signal that may be utilized for hearing augmentation purposes. As will be appreciated, in order to measure a voltage across the foam electret material  25 , the cancellation surface  16  may form a first electrode and the diaphragm  10  may form a second electrode. 
       FIGS. 8A and 8B  illustrate another embodiment of a vibration compensating microphone assembly  8 . As shown, the microphone assembly  8  forms what may be termed a trapped volume acceleration microphone assembly. In this regard, the microphone assembly  8  utilizes two elements moving in parallel. One of the elements is responsive to acoustic signals and acceleration/vibration while the other element is only responsive to acceleration/vibration. Specifically, the acoustic and acceleration sensitive element is the microphone diaphragm  10 , which, as shown, also serves as a hermetic seal for a microphone housing  20  isolating an internal chamber of the microphone assembly  8  from the user&#39;s body. The vibration sensitive element is a mass loaded second diaphragm or cancellation diaphragm  18  (i.e., accelerometer) disposed inside and parallel to the microphone diaphragm  10 . The mass loading allows the cancellation diaphragm  18  to move in response to acceleration forces applied to the housing  20 . 
     Generally, the movement of the microphone diaphragm  10  and the cancellation diaphragm  18  in response to acceleration will be substantially equal, if the mass loading on these elements  10 ,  18  is substantially equal. Accordingly, displacement of the diaphragm  10  due to acoustic signals will result in relative movement between the diaphragm  10  and the cancellation diaphragm  18 . This relative movement is indicative of the acoustic signal. Accordingly, by measuring the relative movement (e.g., displacement of diaphragm  10 ) the acoustic signal may be isolated. 
     The microphone diaphragm  10  and the cancellation diaphragm  18  together form an enclosure  30  having a finite volume. The microphone assembly  8  may monitor changes in the physical configuration of the enclosure  30  to extract an acoustic signal from the acoustic and acceleration signals received by the microphone diaphragm  10  and provide the extracted acoustic to an implanted hearing system for use in hearing augmentation. That is, rather than measuring acceleration directly and subtracting the acceleration from a combined acoustic and acceleration signal, changes in a physical configuration of the enclosure  30  may be detected and utilized as an acoustic output signal. 
     One method of measuring changes in the physical configuration includes monitoring changes in the volume of the enclosure  30 . Generally, any fluid trapped between the microphone diaphragm  10  and the cancellation diaphragm  18  will tend to be forced inward or outward in response to the relative movement of these diaphragms  10 ,  18 . The change in the volume of the enclosure  30  may be measured in any appropriate manner. Examples include measuring the pressure in the trapped volume using a conventional microphone element or by the detection of the displacement by measuring the changes in electrical capacitance between the two diaphragms. 
     As shown in  FIGS. 8A and 8B , a microphone element  32  is acoustically coupled to the enclosure  30  via an aperture  34  through the bottom surface of the cancellation diaphragm  18 . The size and dimension of the aperture  34  may be selected to, in conjunction with the microphone element  32 , provide a concentrated mass near the center of the cancellation diaphragm  18 . Furthermore, if additional mass is desired or required for mass loading the cancellation diaphragm  18  (e.g., for resonant frequency purposes), a distributed mass may also be utilized. This may be supplied by a layer of silicone gel, or other appropriate materials that may be applied to the bottom surface of the cancellation diaphragm  18 . It should be will be noted that the microphone element  32  could be acoustically interconnected to the enclosure  30  through an edge of the enclosure (i.e., at a common perimeter of the diaphragms  10 ,  18 ). This may provide benefits in matching the resonant frequencies and/or providing for more equal movement of the diaphragms  10 ,  18 . 
     The microphone diaphragm  10  and cancellation diaphragm  18  are clamped to a microphone housing  20  utilizing an outer clamp ring  24  and an inner-clamp ring  26 . To maintain a space between the external diaphragm  12  and the cancellation diaphragm  18 , a spacer washer may be utilized (not shown). Such a spacer washer may also electrically isolate the diaphragms  12 ,  18  and/or may be placed near the peripheries of the diaphragms  12 ,  18 . Further, such a spacer washer may be a compliant member that provides damping and can provide a resonant frequency boost wherever needed in the acoustic spectrum. For example, an elastomeric spacer may allow acoustic resonance to be placed as 2–4 kHz, thereby increasing gain in that segment of the speech band where most speech information is contained. 
     As will be appreciated, the peripheries of the diaphragms  10 ,  18  will typically accelerate equally and will therefore not contribute to a net change in volume due to acceleration. At the same time, motion of the perimeter of the surface of the microphone diaphragm  10  does not contribute to the acoustic sensitivity of the microphone assembly  8 . At frequencies significantly above the resonant frequency of both diaphragms  10 ,  18 , the center of the diaphragms  10 ,  18  will stay largely motionless due to the inertia of the mass loading of each diaphragm  10 ,  18 . Hence, they will not contribute to a net change in volume due to acceleration. Further, if the structure of each diaphragm  10 ,  18  is selected such that their deformities under acceleration are similar, then they will not contribute to a net change in volume due to acceleration as both the perimeter and center of the diaphragms  10 ,  18  must move equally as discussed above. 
     It has been found that making the microphone diaphragm  10  much stiffer except at the periphery removes complicating resonances to frequencies that are above the frequency range of interest while minimally impacting the acoustic sensitivity of the microphone assembly  8 . As shown, this is achieved by attaching a reinforcing plate or reinforcing disc  35  to the surface of the microphone diaphragm  10 . Generally, the reinforcing disc  35  will have a stiffness that is greater than the stiffness of the diaphragm  10 . Such attachment may be by permanent coupling or may utilize a viscous coupling (such as a thin layer of silicone or grease), which results in sufficient shear dissipation to dampen high resonances. It will be further appreciated that the use of the disc  35  also allows the microphone diaphragm  10  to maintain a shape that remains substantially the same as the cancellation diaphragm  18 . That is, the displacement of the microphone diaphragm  10  (see  FIG. 8B ) due to ambient acoustic signals will not significantly deform the shape of the microphone diaphragm  10  in relation to the cancellation diaphragm  18 . Further, it has been determined that the correlation between the response of the microphone diaphragm  10  to acceleration and the response of the cancellation diaphragm  18  to acceleration is highest when the shapes of these members  10 ,  18  are substantially the same. 
     In order to achieve the greatest degree of reduction of the acceleration signal, it may be desirable that the net displacement of each diaphragm  10 ,  18  be similar. This can be achieved by a variety of methods, one of which is to ensure that the diaphragms  10 ,  18  have similar shapes. In this regard, every point on the microphone diaphragm  10  may have a corresponding point on the cancellation diaphragm  18  that moves an equal amount in relation to acceleration. As a result, there will be very little net change in the enclosure  30  due to acceleration. Further, if the microphone diaphragm  10  is formed as a thin plate having restoring forces due to bending and stretching, the cancellation diaphragm  18  desirably should be made in a similar manner such that both diaphragms  10 ,  18  exhibit similar response to applied forces. As a further example, if the microphone diaphragm  12  behaves as a membrane having restoring forces due to tension alone, then the cancellation diaphragm  18  should also behave as a membrane. The areas of stiffness and mass loading should desirably be the same as well. In this regard, it may be preferable that the entire bottom surface of the cancellation diaphragm  18  be covered to replicate the mass loading of the diaphragm  10 . 
     Mass loading the entire bottom surface of the cancellation diaphragm  18  may be achieved using an elastomeric material such as a silicone gel. Alternatively, the mass loading may comprise a dense metal backing. For instance, a tungsten backing or tungsten diaphragm  18  may allow for more easily replicating the mass loading on the microphone diaphragm  10 . Tungsten has a density that is approximately 8 times greater than water, which is the major component of tissue. Accordingly, a 1 mm tungsten backing provides a mass loading to the cancellation diaphragm  18  that is roughly equivalent of the mass loading of the microphone diaphragm  10  by 8 mm of tissue. Further, it will be noted that the cancellation diaphragm  18  may be made of the same material as microphone diaphragm  10  (e.g. titanium) or be made of other materials such as metal foils, plastic films, elastomers, and closed cell foams so long as the cancellation diaphragm is designed to have properties that approximate displacement of the mass loaded (i.e., tissue loaded) microphone diaphragm  10 . 
     When a pressure originating from an external acoustic sound is presented to the microphone diaphragm  10 , the position of the microphone diaphragm  10  will change. That is, the microphone diaphragm  10  will be displaced. The cancellation diaphragm  18  may deform as well, but to a lesser degree. In any case, the displacement of the diaphragm  10  will result in pressurization of the enclosure  30 . As shown in  FIG. 8B , this pressurization is measured by the microphone element  32  directly. However, it will be appreciated that distance, force, or a combination of distance and force may be utilized to monitor the change in volume of the enclosure  30 . In any case, the changes in enclosure volume represent the acoustic signals received by the microphone diaphragm  10 . The microphone element  32  may then generate an output signal that may be processed by the signal processor  104  and utilized by a transducer to stimulate a component of the auditory system. See for example  FIG. 1 . 
     Advantageously, in a pressure type microphone assembly  10  such as shown in  FIGS. 8A and 8B , the finite volume defined by the enclosure  30  between the diaphragms  10 ,  18  may be filled with an acoustic media, which acts substantially like a fluid. This acoustic media may be, for example, gas, liquid, an elastomer, or a gel. To advantage, combinations of acoustic media may be used in the enclosure  30 . For instance, a moderate viscosity incompressible acoustic fluid may be used to fill most of the enclosure  30 , while a gas bubble is used to provide the very low viscosity coupling to the microphone element  32  (or another sensor detecting change in physical configuration). For example, when using a mixture of media, a majority of the enclosure may be filled with an incompressible gel leaving a small gas-filled pocket or bubble at the input port of a pressure sensor. As will be appreciated, use of such a bubble may protect the sensor from becoming filled with gel as well as provide for enhanced pressure sensitivity. 
       FIG. 9  shows the results of acoustic and vibration testing of the microphone assembly  8  of  FIGS. 8A and 8B  in comparison with a simple microphone (i.e., that does not account for acceleration) when these microphones are disposed beneath a tissue-like material having a thickness of about 8 mm. As shown, the vibration sensitivity of the microphone assembly  8  of  FIGS. 8A and 8B  is superior to that of the simple microphone at all frequencies over the entire frequency range associated with human hearing. 
     Though discussed herein above in relation to utilizing a standard microphone element  32  in determining pressure differences between the microphone diaphragm  10  and the cancellation diaphragm  18 , it will be appreciated that numerous other methodologies may be implemented in determining relative movement between a diaphragm and a cancellation surface, including electrically monitoring such changes. For instance, as shown in  FIG. 10  first and second electrodes  90 ,  92  are disposed on opposing sides of an electrically active material  94  (e.g., a foam electret or piezo-electric member) that is disposed between an external diaphragm  10  and a cancellation mass  16 , respectively. To advantage, these electrodes  90 ,  92  may be incorporated into the diaphragm  10  and/or cancellation mass  16 . As shown, the first electrode  90  is formed by electrically interconnecting the diaphragm  10  to an amplifier and the second electrode  92  is formed from a conductive plate disposed between the electrically active material  94  and the cancellation mass  16 . 
     As shown, the space between the diaphragm  10  and the electrically active material  94  is filled with an elastomeric material  96 , which also interconnects the cancellation mass to the housing  20 . However, it will be appreciated that there may be an enclosed space between these elements, or, that the electrically active material  94  may extend entirely between the diaphragm  10  and the second conductor  92 . Further, it will be noted that a second diaphragm may also be utilized instead of a cancellation mass  16 . 
     In this embodiment, deflections of the electrically active material  94  caused by relative movement between the external diaphragm and the cancellation mass  16  will result in a measurable electric output. That is, changes in voltage between the electrodes  90 ,  92  may be representative of movement of the external diaphragm  10  and absent from the cancellation mass  16 . As will be appreciated, in this embodiment the diaphragm  10  and the cancellation mass  16  form what may be considered electrical plates of a capacitor. In this regard, changes in the physical configuration between the two elements  10 ,  16  are governed by the equation:
 
 Q=C×V   (Eq 5)
         where Q is the electrical charge on the electrodes  90 ,  92 ;   where C is the capacitance; and   where V is the voltage on the electrodes  90 ,  92 .       

     As will be appreciated, changes in voltage is then given by:
 
 DV=dQ/C−Q×dC/C   2 
 
     Thus, the changes in charge dQ with a capacitance dC will result in a change in voltage that may be measured and/or utilized as an input signal for a microphone and/or transducer. 
     In another embodiment, an inductive system (not shown) may be utilized for monitoring the physical configuration between the two diaphragms (i.e., in a closed space embodiment). For instance, changes due to inductance from the motion of a permeable core relative to a coil may be utilized to monitor changes within the enclosure. For instance, a flat “pancake” coil may be disposed on one of the diaphragms. Accordingly, movement of the conductive microphone diaphragm will cause a current within the pancake coil that may be measured and/or utilized as an input to a transducer. 
     In other embodiments, force may be utilized to measure the changes in the physical configuration between two diaphragms. As will be appreciated displacement cannot occur without force, nor force without displacement. In some cases, however, the force may be considerably larger than the displacement and therefore measuring force rather than displacement may be more desirable. Furthermore, it will be appreciated that a device utilized as a displacement sensor (e.g., pressure sensor) can be used as a force sensor with the addition of a suitable restoring force such as a spring or gas pressure. In this regard, force may be measured by measuring: displaced electrical charge using piezo-electric elements; displaced electrical charge using piezo active elements, such as foam electrets; voltage changes through strain gauges; optical variations using the photo-optic properties of fiber optics, plastics, or moiré detection. 
     Furthermore, it will be appreciated that the measurements of the physical configuration between two diaphragms may be combined. Such combinations include, but are not limited to, measuring the physical configuration at a single point (e.g., the center of the diaphragms), at several points, distributed over a large area (e.g., utilizing large electrodes and/or disk or ring-shaped electrodes). Furthermore, these values may be combined/processed by a weighted average or using other mathematical functions. As explained above, such processing may be performed by using post filters  66 ,  68 . See  FIG. 6 . 
       FIG. 11  illustrates another embodiment of a vibration compensating microphone assembly that is operative to provide a cancellation response indicative of acceleration that may subsequently be removed from a combined response from a microphone diaphragm, which includes an acceleration response and an ambient sound response. However, as opposed to mechanically/pneumatically combining the cancellation response with the combined response, the responses may be combined electrically. 
     As shown, the microphone assembly  8  utilizes a first diaphragm  10  that is positioned to be responsive to acoustic signals and acceleration/vibration received through overlying tissue and generate a first output indicative of the acoustic and acceleration signals. More specifically, the microphone diaphragm  10  deflects relative to a first enclosed space  30 A. This deflection results in a pressure fluctuation that is monitored by a first microphone element  32 A. Accordingly, the microphone element  32 A generates a first electrical output corresponding to the movement of the microphone diaphragm  10 . As shown, the microphone diaphragm  10  also serves as a hermetic seal for a microphone housing  20 . 
     The microphone assembly also includes a cancellation diaphragm  18  that is mass loaded with a cancellation mass  16  (e.g., proof mass). The cancellation diaphragm is a vibration sensitive element that is disposed inside of the microphone housing  20  such that it is substantially isolated from ambient acoustic signals. The mass loading allows the cancellation diaphragm  18  to deflect in response to acceleration forces applied to the housing  20 . Specifically, the cancellation diaphragm deflects relative to a second enclosed space  30 B in response to acceleration. This deflection results in a pressure fluctuation in the second enclosed space  30 B that is monitored by a second microphone element  32 B. Accordingly, the second microphone element  32 B generates a second electrical output corresponding to the movement of the cancellation diaphragm  18 . 
     In the embodiment of  FIG. 11 , the resonant frequency of the microphone diaphragm  10 , which is mass loaded due to the surrounding media (i.e., tissue), is again selected to be significantly below the lowest frequency of interest. Likewise, the resonant frequency of the cancellation diaphragm  18  is also chosen to be significantly below the lowest frequency of interest. In this regard, one or more mechanical properties of the cancellation diaphragm  18  and/or the size of the cancellation mass  16  may be selected to achieve a desired resonant frequency. Ideally, the resonant frequency of the microphone diaphragm  10  and the cancellation diaphragm  18  are substantially equal. However, it will be appreciated that the resonant frequency of the microphone diaphragm  10  and cancellation diaphragm  18  need not be the same. In this latter regard, it will be noted that electrically combining the first and second outputs allows individually processing/filtering one or more characteristics (e.g., gain) of each signal. Such individual processing/filtering reduces the need to mechanically match the acceleration response of the cancellation diaphragm  18  with the acceleration response of the microphone diaphragm  10 . In this regard, the size of the microphone diaphragm  10  and cancellation diaphragm  18  need not be the same. Such individual processing/filtering may be performed by using post filters  66 ,  68  as briefly discussed above in relation to the acceleration and microphone branches of  FIG. 6 . 
     The post-filters  66 ,  68  discussed in relation to  FIG. 6  can be implemented in a number of technologies. As mentioned above, these can be mechanical, pneumatic, electrical analog, digital or software implementations. These implementations are well known to those skilled in the art. A particular useful subset of these filters  66 ,  68  include, but are not limited to: mechanical filters using masses, spring constants, and dampers; acoustic filters using tubing of appropriate lengths and diameters; electrical filters designed as, especially, second order physical system analogs, such as biquad or Sallen-Key implementations; digital or software implementations using lattice filters, finite impulse response, infinite impulse response, Fourier transforms, or polyphase subband processors. The digital and software implementations may be implemented in ASICs, FPGAs, DSPs, general-purpose processors, or random logic. 
     It will be appreciated that there may be some advantage into splitting the post-filters  66 ,  68  into several stages, and implementing, for instance, part of the filter electrically while a second part is implemented in, for example, software. For instance, implementing some of the filtering electrically can reduce the dynamic range requirements of the software implementation by reducing differences between the peaks of the response functions and the valleys. Stated otherwise, this allows ‘pre-whitening’ the transfer function. This can be desirably carried over to pre-whitening the actual input signal spectrum, so that the output of the first stage of a multiple stage post-filter has reduced differences between the peaks of the output spectrum and the valleys. 
     Optimum settings for the filter parameters may be determined by a number of approaches. These include, but are not limited to, minimization of the following error norms:
         L-k-norm, where k is 1 through infinity. This subsumes the absolute value of the error (L-1-norm), the RMS value (L-2-norm), and the maximum value (L-infinity-norm).   Weighted averages of any of the above error norms, where the weighting is performed over frequency.   A metric of the degree of oscillation of the system, for instance, House Ear Institutes “PCR” metric, which measures the ratio of power concentrated in the strongest 5 spectral lines vs. the total power.       

     The number of free parameters of the post-filters  66 ,  68  determines largely how complete the cancellation can be. For example, in the case relating to equation 4 above, in which Gm=k*allpass, Ga=−k Hmv/Hav, the value of Hmv/Hav may be approximated by using a single fixed number, α. This single fixed value may be chosen to be the optimum gain that minimizes one of the error norms above. By way of further example, where the error norm selected is the RMS error, it will be appreciated that unless Hmv and Hav are identical except for magnitude, a single gain adjustment cannot cause complete cancellation. Therefore, it may be preferable to choose to approximate the value of Hmv/Hav in the usual way, that is, using the ratio of two polynomials (also called a rational polynomial). This may be done in either the s- or the z-domain. When this is done, the numerator polynomial is chosen to have a degree of n while the denominator is chosen have a degree of m. Having n=2 and m=2 gives a good approximation to the response of a well-damped microphone and accelerometer. Such a rational polynomial has 6 free parameters. Other values (degrees) of n and m may be used. When the error is compared with the number of degrees, the majority of the error can be removed using just the n=2, m=2 rational polynomical, but typically more and more error can be removed with higher and higher values of n or m. 
     It will be appreciated that reducing the degree of the rational polynomial reduces the complexity of the post-filter, and either the number of components required or the amount of time and memory space needed. Thus, obtaining a good fit with a minimum degree polynomial is important to minimize size and/or power requirements. 
     When operating the microphone diaphragm  10  and accelerometer  60  above their resonant frequency, they are relatively insensitive to changes in such variables as skin thickness and density. As mentioned elsewhere in this disclosure, operating in this regime can reduce acoustic sensitivity, and it may simply be difficult to achieve the needed mechanical resonances at a low enough value to cover the frequency range of interest. 
     To achieve better cancellation between the microphone and accelerometer acceleration responses, the coefficients for the post filters  66 ,  68  can be adjusted for best cancellation. This is more easily accommodated in an (at least partial) electrical or software implementation than in a purely mechanical or pneumatic approach. 
     In order to accommodate changes to the microphone acceleration response due to changes in, for instance, skin thickness and density, due to posture, diurnal variations, etc., it is useful to have an adaptive implementation, such as that illustrated in block diagram  FIG. 12 . In this manner, the output  76  of the second summer  70  is compared with the output  64  of the accelerometer  60  using an adaptive algorithm  78 . The filter parameters of one or both post filters  66 ,  68  may be adjusted by the adaptive algorithm  78  to eliminate any accelerometer component from the output  76 . Those skilled in the art will appreciate that there are many variations that may be incorporated with the use of an adaptive filter(s), see, for instance “Introduction to Adaptive Filters,” by Simon Haykin 2 nd  edition, 1992, Macmillan Press. One embodiment is that of an adaptive algorithm using any one of a number of variations on the LMS (least mean squares) algorithm, since in practice this is found to be relatively robust and requires relatively modest resources. 
     Those skilled in the art will appreciate variations of the above-described embodiments that fall within the scope of the invention. For instance, it may be advantageous to fill the space between opposing diaphragms with a high density flexible material (i.e., a lossy energy dissipating material) to improve sensitivity and/or reduce the resonant frequency of the diaphragm(s). In one embodiment, a titanium powder may be added to a silicone elastomer or gel to prove a material that may be formed into a material having an extremely low resonant frequency. As a result, the invention is not limited to the specific examples and illustrations discussed above, but only by the following claims and their equivalents.