Patent Publication Number: US-8995693-B2

Title: Noise mitigating microphone system and method

Description:
PRIORITY 
     This patent application is a continuation of utility patent application Ser. No. 12/546,073, entitled, “Noise Mitigating Microphone System and Method,” filed on Aug. 24, 2009, and naming Kieran P. Harney, Jason Weigold, and Gary Elko as inventors, the disclosure of which is incorporated herein, in its entirety, by reference, which is a continuation in part of utility patent application Ser. No. 11/492,314 entitled “Noise Mitigating Microphone System and Method” filed Jul. 25, 2006, and naming Kieran P. Harney, Jason Weigold, and Gary Elko as inventors, the disclosure of which is incorporated herein, in its entirety, by reference. 
     Utility patent application Ser. No. 11/492,314, in turn, claims priority from provisional U.S. patent application No. 60/710,515, filed Aug. 23, 2005, entitled, “MICROPHONE SYSTEM,” and naming Kieran Harney as the inventor, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to microphones and, more particularly, the invention relates to improving the performance of microphones. 
     BACKGROUND OF THE INVENTION 
     Condenser microphones typically have a diaphragm that forms a capacitor with an underlying backplate. Receipt of an audible signal causes the diaphragm to vibrate to form a variable capacitance signal representing the audible signal. It is this variable capacitance signal that can be amplified, recorded, or otherwise transmitted to another electronic device. 
     Problems arise, however, when the microphone is subjected to a mechanical shock. Specifically, mechanical shocks can cause the diaphragm to vibrate in a manner that degrades the microphone output signal. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, a microphone system has a base coupled with first and second microphone apparatuses. The first microphone apparatus is capable of producing a first output signal having a noise component, while the second microphone apparatus is capable of producing a second output signal. The first microphone apparatus may have a first back-side cavity and the second microphone may have a second back-side cavity. The first and second back-side cavities may be fluidly unconnected. The system also has combining logic operatively coupled with the first microphone apparatus and the second microphone apparatus. The combining logic uses the second output signal to remove at least a portion of the noise component from the first output signal. 
     The second output signal may have, among other things, data relating to the mechanical response of the first microphone apparatus. Moreover, the second microphone apparatus may have a diaphragm and a cap acoustically sealing the diaphragm. Alternatively, the diaphragm may be exposed to a space to which another diaphragm in the system is exposed. In some embodiments, the second microphone apparatus has a microphone and a low pass filter. 
     Various embodiments of the first microphone apparatus have a first microphone, while the second microphone apparatus has a second microphone. The second microphone may be configured to have a low frequency cut-off that is greater than the low frequency cut-off of the first microphone. In addition, the first microphone may have a first diaphragm and a first circumferential gap defined at least in part by the first diaphragm, while the second microphone may have a second diaphragm and a second circumferential gap defined at least in part by the second diaphragm. The second circumferential gap illustratively is greater than the first circumferential gap. This second gap effectively mitigates low frequency audio components while the filter, if used, substantially removes or mitigates remaining audio component. 
     To remove at least a portion of the noise components produced by mechanical shock, the second microphone apparatus may have a microphone and a signal transformation module (e.g., an adaptive filter). 
     In accordance with another embodiment of the invention, a microphone system has a base coupled with first and second microphone apparatuses. The first microphone apparatus is capable of producing a first output signal and has a first microphone with a first mechanical response. In a similar manner, the second microphone apparatus is capable of producing a second output signal and has a second microphone with a second mechanical response. The first microphone apparatus may have a first back-side cavity and the second microphone may have a s back-side cavity. The first and second back-side cavities may be fluidly unconnected. The system also has combining logic operatively coupled with the first and second microphone apparatuses. The combining logic combines the first and second output signals to produce an output audio signal. The first and second mechanical responses illustratively are effectively the same. 
     Among other things, the combining logic may include a subtractor that subtracts the second output signal from the first output signal. In other embodiments, the combining logic may have an adder. 
     In accordance with another embodiment of the invention, a method of producing an output audio signal from a microphone system provides a base having a first microphone for generating a first microphone output signal (having an audio component and a mechanical component) in response to an input audio signal and a mechanical signal. The base also has a second microphone for generating a second microphone output signal in response to the mechanical signal. The first and second microphones may have back-side cavities that are fluidly unconnected to one another. The method uses information from the second microphone output signal to remove at least a portion of the mechanical component from the first microphone output signal. 
     The second microphone output signal may have a second audio component. In that case, the method may remove at least a portion of the second audio component from the second microphone output signal. In addition, the method may adaptively filter the second microphone output signal. Among other ways, the method may remove at least a portion of the second audio component from the second microphone output signal before adaptively filtering. 
     Illustrative embodiments of the invention are implemented as a computer program product having a computer usable medium with computer readable program code thereon. The computer readable code may be read and utilized by a computer system in accordance with conventional processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein: 
         FIG. 1  schematically shows a base having a microphone system configured in accordance with illustrative embodiments of the invention. 
         FIG. 2  schematically shows a cross-sectional view of a MEMS microphone that may be used with illustrative embodiments of the invention. 
         FIG. 3A  schematically shows a plan view of the microphone system in accordance with a first embodiment of the invention. 
         FIG. 3B  schematically shows a plan view of the microphone system in accordance with a second embodiment of the invention. 
         FIG. 3C  schematically shows a cross-sectional view of the microphone system shown in  3 A, in accordance with illustrative embodiments of the present invention. 
         FIG. 3D  schematically shows a cross-sectional view of an alternative microphone system with a lid, in accordance with embodiments of the present invention. 
         FIG. 3E  schematically shows a cross-sectional view of an alternative microphone system made on a single die, in accordance with further embodiments of the present invention. 
         FIG. 3F  schematically shows a cross-sectional view of an additional alternative microphone system in accordance with further embodiments of the present invention. 
         FIG. 4A  schematically shows the frequency response for the primary microphone in the microphone system of illustrative embodiments of the invention. 
         FIG. 4B  schematically shows the frequency response for the correction microphone in the microphone system of illustrative embodiments of the invention. 
         FIG. 5  schematically shows additional details of illustrative embodiments of the microphone system, including filters and combination logic. 
         FIG. 6  shows a process used by the microphone system of  FIG. 1  to produce an audible signal in accordance with illustrative embodiments of the invention. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In illustrative embodiments, a microphone system has a primary microphone and a correction microphone coupled to the same base to both receive the same noise signals (e.g., mechanical shock signals) and react in a corresponding manner. To improve the quality of the output audio signal it produces, the microphone system uses noise signals detected by the correction microphone to remove significant amounts of noise from the signal produced by the primary microphone. As a result, the output audio signal should have less noise than if not processed and noise is present. Details of illustrative embodiments are discussed below. 
       FIG. 1  schematically shows a mobile telephone acting as a base  10  for supporting a microphone system  12  configured in accordance with illustrative embodiments of the invention. To that end, the mobile telephone (also identified by reference number  10 ) has a plastic body  14  containing the microphone system  12  for producing an output audio signal, an earpiece  16 , and various other components, such as a keypad, transponder logic and other logic elements (not shown). As discussed in greater detail below, the microphone system  12  has a primary microphone  18 A and a correction microphone  18 B that are both fixedly secured in very close proximity to each other, and fixedly secured to the telephone body  14 . More generally, both microphones  18 A and  18 B illustratively are mechanically coupled to each other (e.g., via the base  10  or a direct connection) to ensure that they receive substantially the same mechanical signals. For example, if the telephone  10  is dropped to the ground, both microphones  18 A and  18 B should receive substantially identical mechanical/inertial signals representing the movement and subsequent shock(s) (e.g., if the telephone  10  bounces several times after striking the ground) of the telephone  10 . 
     In alternative embodiments, the microphone system  12  is not fixedly secured to the telephone body  14 —it may be movably secured to the telephone body  14 . Since they are mechanically coupled, both microphones  18 A and  18 B nevertheless still should receive substantially the same mechanical signals as discussed above. For example, the two microphones  18 A and  18 B may be formed on a single die that is movably connected to the telephone body  14 . Alternatively, the microphones  18 A and  18 B may be formed by separate dies packaged together or separately. 
     The base  10  may be any structure that can be adapted to use a microphone. Those skilled in the art thus should understand that other structures may be used as a base  10 , and that the mobile telephone  10  is discussed for illustrative purposes only. For example, among other things, the base  10  may be a movable or relatively small device, such as the dashboard of an automobile, a computer monitor, a video recorder, a camcorder, or a tape recorder. The base  10  also may be a surface, such as the substrate of a single chip or die, or the die attach pad of a package. Conversely, the base  10  also may be a large or relatively unmovable structure, such as a building (e.g., next to the doorbell of a house). 
       FIG. 2  schematically shows a cross-sectional view of a MEMS microphone (identified by reference number  18 ) generally representing the structure of one embodiment of the primary and correction microphones  18 A and  18 B. Among other things, the microphone  18  includes a static backplate  22  that supports and forms a capacitor with a flexible diaphragm  24 . In illustrative embodiments, the backplate  22  is formed from single crystal silicon, while the diaphragm  24  is formed from deposited polysilicon. A plurality of springs  26  (not shown well in  FIG. 2 , but more explicitly shown in  FIGS. 3A and 3B ) movably connect the diaphragm  24  to the backplate  22  by means of various other layers, such as an oxide layer  28 . To facilitate operation, the backplate  22  has a plurality of throughholes  30  that lead to a back-side cavity  32 . Depending on the embodiment and its function, the microphone  18  may have a cap  34 . 
     Audio signals cause the diaphragm  24  to vibrate, thus producing a changing capacitance. On-chip or off-chip circuitry (not shown) converts this changing capacitance into electrical signals that can be further processed. It should be noted that discussion of the microphone of  FIG. 2  is for illustrative purposes only. Other MEMS or non-MEMS microphones thus may be used with illustrative embodiments of the invention. 
     One function of the primary microphone  18 A is to produce a primary signal having an audio component and a (zero or non-zero) noise component. This noise component can include, among other things, 1) a mechanical portion and 2) audio responses to the mechanical portion of the noise component. For example, the mechanical portion of the noise component could be the response of the microphone when it is dropped to the ground (i.e., its diaphragm  24  moves as an inertial response). As another example, the audio response to the mechanical portion of the noise signal may be the initial sound and resultant of echoes generated when the microphone/base  10  strikes the ground. Alternatively, the mechanical portion of the noise component could be the response of the microphone to wind (e.g., wind entering the mouthpiece of the telephone). The primary microphone  18 A also may be packaged or capped, as shown, (e.g., a post-processing cap or in-situ cap) with a through-hole to permit ingress of audio signals. 
     One function of the correction microphone  18 B is to generate a correction signal that can be used to substantially mitigate much of the noise component of the primary signal. This mitigation may remove a significant portion, or a relatively small portion, of the noise component of the primary signal. Various embodiments, however, preferably remove substantially all of the discussed noise components. Removing the noise component should enhance the quality (e.g., the signal to noise ratio) of the ultimate output signal. 
     The overall amount and type of mitigation may depend on the application. For example, some embodiments remove the mechanical portion of a noise component only. Other embodiments remove both the mechanical portion and its audio response. Yet other embodiments may remove the audio response portion of the noise signal only. 
     The correction microphone  18 B may be considered to act as an effective accelerometer within the microphone system  12 . Accordingly, in this context, the term “microphone” may be used generally to include other devices, such as inertial sensors. Regardless its exact name, the correction microphone  18 B assists in mitigating inertial based noise (i.e., signals causing undesired diaphragm displacement and related noise). In some embodiments, rather than using the discussed correction microphone  18 B, the microphone system  12  thus has an accelerometer, such as one or more one, two, or three axis IMEMS accelerometers produced and distributed by Analog Devices, Inc. of Norwood, Mass. 
     The primary microphone  18 A and correction microphone  18 B preferably are formed to have substantially identical responses to audio and noise signals discussed herein. To that end, illustrative embodiments produce two microphones  18 A and  18 B using substantially identical fabrication processes and materials (e.g., silicon-on-insulator technology, or conventional non-SOI surface micromachining processes that deposit layers on a silicon wafer substrate). Accordingly, to the extent they can as consistent with various discussed embodiments, the microphones  18 A and  18 B should have substantially identical diaphragm masses, backplates, hole sizes, material, etc. . . . Alternative embodiments, however, may use different microphones  18 A and  18 B that are calibrated to perform the functions discussed herein. 
     As discussed in greater detail below with regard to  FIG. 6 , illustrative embodiments combine the correction signal with the primary signal to remove the noise component from the primary signal. Among other ways, illustrative embodiments may subtract the correction signal from the primary signal. Accordingly, to avoid subtracting the intended audio signal from the primary signal, illustrative embodiments of the correction signal substantially do not include the noted audio component (e.g., it may include a significantly mitigated version of the audio component). If the correction signal substantially has the audio component, it would undesirably cancel or otherwise substantially mitigate the audio component from the primary signal, thus substantially undercutting one advantage of various embodiments of the system. 
     Various embodiments therefore physically shield the correction microphone  18 B from the input audio signal. In so doing, the correction microphone diaphragm  24  receives mechanical (or related) signals, but does not receive the audio signal. To physically shield the diaphragm  24 , the correction microphone  18 B may 1) have a cap  34  that provides an acoustic seal (i.e., shielding the correction microphone diaphragm  24 ) to the diaphragm  24 , 2) be contained within a sealed package, or 3) have some other physical means for preventing the input audio signal from contacting its diaphragm  24 . 
     Other embodiments, however, logically shield the diaphragm  24  of the correction microphone  18 B from the input audio signal. If that diaphragm  24  is logically shielded, then the diaphragms  24  of both of the correction microphone  18 B and the primary microphone  18 A may be exposed to a common space (e.g., the space through which the desired audio signal traverses). In other words, both diaphragms  24  may receive essentially the same audio input signal.  FIGS. 3A and 3B  schematically show two embodiments that provide this functionality. 
       FIG. 3A  schematically shows a plan view of the microphone system  12  in accordance with a first embodiment that logically shields the correction microphone diaphragm  24 . Specifically, the microphone system  12  includes the primary and correction microphones  18 A and  18 B fixedly secured to an underlying printed circuit board  36 , and logic  38  (see  FIG. 5 ) for improving the quality of audio signals received by the primary microphone  18 A. Because it is a plan view,  FIG. 3A  shows the respective diaphragms  24  of the microphones  18  and  18 B and their springs  26 . This configuration of having a diaphragm  24  supported by discrete springs  26  produces a gap between the outer parameter of the diaphragm  24  and the inner parameter of the structure to which each spring  26  connects. This gap is identified in  FIG. 3A  as “gap  1 ” for the primary microphone  18 A, and “gap  2 ” for the correction microphone  18 B. 
     As mentioned above, and as shown in  FIG. 3C , in some embodiments, the primary (e.g.,  18 A) and correction microphones (e.g.,  18 B) may be formed on or secured to a single substrate or base, for example, the circuit board  36 . To that end, the circuit board  36  may be used to segment or isolate the back-side cavities  32 A and  32 B of the primary microphone  18 A and the correction microphone  18 B from one another. In other words, in some embodiments, the back-side cavities  32 A/ 32 B may be fluidly disconnected from one another (e.g., they do not share a common volume; the back-side cavities are not fluidly connected). In this manner, the primary microphone  18 A and the correction microphone  18 B may by exposed to the same audio signal without interference from one another (e.g., the motion of the diaphragms  24 , changes in pressure within the back-side cavities  32 A/ 32 B, etc.). 
     In addition to the segmented back-side cavities  32 A/ 32 B mentioned above, one or both of the microphones  18 A/ 18 B may also have caps (e.g., caps  34 A and  34 B, respectively). For example, the primary microphone  18 A may have a cap  34 A with an opening to allow the audio signal to reach the diaphragm  24 , and the correction microphone  18 B may have a closed cap  34 B (e.g., with no opening) to physically shield the microphone  18 B and/or diaphragm  24  from the audio signal. Alternatively, both the primary microphone  18 A and the correction microphone  18 B may have caps  34 A with openings to allow the audio signal to reach the diaphragms  24 . In such embodiments, the correction microphone  18 B may be logically shielded as described above. 
     As shown in  FIG. 3D , some embodiments of the present invention may also include a lid  320  that spans and covers the microphones  18 A and  18 B, with segmented back volumes, to seal the microphones  18 A/ 18 B as a single package. The lid  320  may have one or more openings  322  that allow the audio signal to reach the primary microphone  18 A. In such embodiments, the microphones  18 A and  18 B may or may not also have the caps  34 A/ 34 B described above (e.g., the primary microphone may have no cap or an open cap  34 A and the correction microphone may have either an open cap  34 A, a closed cap  34 B, or no cap). 
     In some embodiments (e.g., those having no cap or an open cap on the correction microphone  18 B), the system may have a divider  340  located between the primary microphone  18 A and the correction microphone  18 B. This divider  340  physically shields the correction microphone  18 B from the audio signal entering through the opening  322  in the lid  320 . Alternatively, if the correction microphone  18 B is not physically shielded (e.g., there is no divider  340  and/or the correction microphone  18 B has no cap or a cap with an opening), the correction microphone  18 B may be logically shielded as discussed above. 
     Although the microphones are described above as being separate dies, the microphones may also be formed on the same die and maintain segmented volumes, as shown in  FIG. 3E . In such embodiments, the microphones  330 A/ 330 B may be formed on or otherwise secured to a package base  310 , such as an FR-4 base. The package base  310  may then, in turn, be secured to the telephone or the circuit board  36  discussed above. Like the embodiments described above, embodiments formed on the same die may also include caps  34 A/ 34 B. For example, the primary microphone  18 A may have an open cap  34 A and the secondary microphone may have a closed cap  34 B. 
       FIG. 3F  shows a cross-sectional view of an additional embodiment of the present invention, in which the audio signal enters the primary microphone  18 A through the bottom of the microphone  18 A. To that end, the substrate  36  may have an opening  360  (a “bottom port”) to the back-side cavity  32 A, which effectively becomes a front volume. The area behind the diaphragm now serves the function of a back volume. The opening  360  thus allows the audio signal to enter what formerly acted as a back volume, the back-side cavity  32 A, and interact with the diaphragm  24  (e.g., cause the diaphragm to vibrate), as described above. 
     Additionally, the substrate  36  may also have a similar opening (not shown) for the correction microphone  18 B. If the substrate  36  does not have an opening for the correction microphone  18 B, the correction microphone will be physically shielded from the audio signal (e.g., if the lid  350 B described below does not have an opening). If the substrate  36  does have an opening for the correction microphone  18 B, the correction microphone may be logically shielded, as discussed above. 
     The primary and correction microphones  18 A/ 18 B of this embodiment may also have lids  350 A and  350 B. In a manner similar to the caps  34 A/ 34 B and lid  320  discussed above, the lids  350 A/ 350 B may or may not have openings to allow the audio signal to reach the microphones  18 A/ 18 B from the top. For example, the primary microphone lid  350 A may have an opening and the correction microphone lid  350 B may not. Alternatively, the correction microphone lid  350 B may have an opening and the primary microphone lid  350 A may not, or both may have or not have an opening. 
     As mentioned in greater detail below and as shown in  FIGS. 4A and 4B , some embodiments of the present invention can utilize microphones having different low frequency cut-offs. It is important to note that segmented and/or fluidly unconnected back-side cavities aid in creating the different low frequency cut-offs shown in  FIGS. 4A and 4B . In particular, the segmented and/or fluidly unconnected back-side cavities may allow each of the microphones  18 A/ 18 B to have separate and independent air leakage rates past the diaphragms  24 , as described in greater detail below. 
     As known by those skilled in the art, it is generally desirable to minimize the size of that gap (e.g., gap  1 ) to ensure that the microphone can respond to low-frequency audio signals. In other words, if the gap is too large, the microphone may not be capable of detecting audio signals having relatively low frequencies. Specifically, with respect to the frequency response of a microphone, the location of its low frequency cut-off (e.g., the 3 dB point) is a function of this gap.  FIG. 4A  schematically shows an illustrative frequency response curve of the primary microphone  18 A when configured in accordance with illustrative embodiments of the invention. As shown, the low frequency cut-off is F 1 , which preferably is a relatively low frequency (e.g., 50-100 Hz, produced by an appropriately sized gap, such as a gap of about 1 micron). 
     It should be noted that this gap is anticipated to have no greater than a negligible impact on the inertial response of the microphone to a mechanical signal. Accordingly, although the microphone substantially does not detect audio signals having frequencies below the low frequency cut-off, it still can detect low-frequency inertial signals. For example, a microphone having a gap sized to produce a low frequency cut-off of approximately 350 Hz still should detect a mechanical signal having a frequency of 150 Hz. 
     In accordance with one embodiment of the invention, gap  2  (of the correction microphone  18 B) is larger than gap  1  (of the primary microphone  18 A). Accordingly, as shown in  FIG. 4B  (showing the frequency response of the correction microphone  18 B), the low frequency cut-off F 2  (e.g., 2-2.5 KHz, produced by an appropriately sized gap, such as about 5-10 microns) of the correction microphone  18 B is much higher than the low frequency cut-off F 1  of the primary microphone  18 A. As a result, the correction microphone  18 B does not adequately detect a wider range of low-frequency audio signals. In other words, increasing the size of gap  2  effectively acts as an audio high pass filter for the correction microphone  18 B. As discussed in greater detail below, illustrative embodiments use this effective high pass filter in combination with a subsequent low pass filter  46  to significantly mitigate the response of the correction microphone  18 B to an input audio signal. Accordingly, the correction microphone  18 B does not require some means to shield it from an input audio signal (e.g., a cap  34 ). 
     There are various ways to make gap  2  larger than gap  1  while still ensuring that both microphones  18 A and  18 B have substantially identical responses to noise signals. Among other ways, the diaphragms  24  may be formed to have substantially identical masses. To that end, the diaphragm  24  of the correction microphone  18 B may be thicker than the diaphragm  24  of the primary microphone  18 A, while the diameter of the diaphragm  24  of the correction microphone  18 B is smaller than the diameter of the diaphragm  24  of the primary microphone  18 A. 
     In other embodiments, the diaphragm masses may be different. In that case, internal or external logic may be used to compensate for the mass differences. For example, if the mass of the correction microphone diaphragm  24  is half that of the primary microphone diaphragm  24 , then logic may multiply the signal from the correction microphone  18 B by a scalar value (e.g., a scalar of two). Logic therefore causes the effective vibration output of the two microphones to be effectively the same. Stated another way, the mechanical responses of the two microphones may be considered to be effectively the same if 1) they do, in fact, have the same diaphragm masses, or 2) if logic compensates for diaphragm mass differences to effectively cause them to appear the same (e.g., applying a scalar). In yet other embodiments, the two microphones may be entirely different and thus, other logic is required to ensure accurate results consistent with those discussed herein. 
       FIG. 3B  schematically shows another embodiment in which the gaps discussed above are substantially identical. Despite having identical gaps, the correction microphone  18 B still is configured to have a frequency response as shown in  FIG. 4B  (i.e., having a higher low frequency cut-off). To that end, the diaphragm  24  of the correction microphone  18 B has one or more perforations or through-holes that effectively increase the low frequency cut-off. Specifically, the low frequency cut-off is determined by the amount of area defined by the gap and the hole(s) through the diaphragm  24 . This area thus is selected to provide the desired cutoff frequency. 
     In general terms, the embodiments shown in  FIGS. 3A and 3B  are two of a wide variety of means for controlling the air leakage past the respective diaphragms  24 . In other words, those embodiments control the rate at which air flows past the diaphragm, thus controlling the respective low frequency cut-off points. Those skilled in the art therefore can use other techniques for adjusting the desired low frequency cut-off of either microphone  18 A and  18 B. 
     As noted above, illustrative embodiments combine the correction signal with the primary signal to remove the noise component from the primary signal. To that end,  FIG. 5  schematically shows various elements of the microphone system  12  for accomplishing those ends. In general, the microphone system  12  has a primary microphone apparatus  40  having the primary microphone  18 A, and a correction microphone apparatus  42  having the correction microphone  18 B and two subsequent processing stages  46  and  48  (i.e., logic  38 ). Summation logic  44  (also referred to as “combining logic  44 ”) combines the outputs from the two microphone apparatuses to generate an output audio signal that preferably has a relatively low noise component. 
     As noted above, the correction microphone apparatus  42  generates a noise component for mitigating the noise component of the primary signal. To that end, the correction microphone apparatus  42  has 1) a low pass filter  46  for substantially mitigating audio components in the correction signal received from the correction microphone  18 B, and 2) a signal transformation module  48  for normalizing the audio response to the mechanical portion of the noise component. 
     More specifically, although they illustratively are very similar, the two microphones  18 A and  18 B still may have some differences. For example, due to the tolerances and limits of their fabrication process, the microphones  18 A and  18 B may have some minor differences, such as the diaphragm thickness. In fact, as noted herein, some embodiments use different types of devices to serve the function of one or both of the microphones  18 A and  18 B (e.g., the correction microphone  18 B may be a conventional accelerometer). As another example, the microphones  18 A and  18 B are spaced from each other. The correction microphone  18 B therefore may receive a slightly time delayed version of an audio and/or noise signal. 
     Unless normalized, these differences can cause the noise components of the two microphones  18 A and  18 B to vary. If they vary too much, the output signal may be corrupted or have a less desirable signal to noise ratio. Illustrative embodiments thus compensate for the impact of these and other differences between the two microphones  18 A and  18 B to ensure that the two microphones  18 A and  18 B have substantially identical noise components. As noted above, this process may be referred to herein as a “normalization” process. 
     To that end, the signal transformation module  48  compensates for differences between the primary microphone  18 A and the correction microphone  18 B. In illustrative embodiments, the signal transformation module  48  is a conventional adaptive filter. In alternative embodiments, the signal transformation module  48  is a fixed filter. Other devices may be used to achieve the noted results. The respective filters may be any conventionally known filter used for the noted purposes. For example, if used, the adaptive filter may be a least mean squared adaptive filter, also referred to in the art as an “LMS” filter. 
     Accordingly, in illustrative embodiments, the correction microphone apparatus  42  generates a signal having no greater than a negligible amount of the audio signal, thus substantially comprising a noise component. It is this noise component that is used to remove the corresponding noise component generated by the primary microphone apparatus  40 . 
     The microphone system  12  therefore has combining logic  44  to combine the two signals. Among other things, as noted above, the combining logic  44  may include conventional subtraction logic that subtracts the signal generated by the correction microphone apparatus  42  from the signal generated by the primary microphone apparatus  40 . In alternative embodiments, the combining logic  44  may include an adder. For example, in such embodiments, the microphones  18 A and  18 B may be positioned within the base  10  to generate signals that are  180  degrees out of phase. More specifically, it is contemplated that one microphone could be oriented so that the top surface of its diaphragm  24  faces upwardly, while the other microphone could be oriented so that the top surface of its diaphragm  24  faces downwardly. Of course, those skilled in the art should understand that other combining logic  44  may be used to facilitate system implementation. 
     It should be noted that the signal generated by the correction microphone apparatus  42  may be considered to be the above noted correction signal (i.e., as processed by the filters). In a similar manner, the signal generated by the primary microphone apparatus  40  also may be considered to be the above noted primary signal (i.e., as processed by any intervening logic elements, not shown). Accordingly, for simplicity, the output signals of the primary microphone apparatus  40  and the correction microphone apparatus  42  respectively are referred to as the primary signal and correction signal. 
       FIG. 6  shows a process of generating an output audio signal in accordance with illustrative embodiments of the invention. The process begins at step  600  by substantially mitigating the audio component from the signal generated by the correction microphone  18 B. To that end, the correction output signal is filtered by the low pass filter  46 . As noted above, when using the embodiments of  FIGS. 3A ,  3 B, or other related embodiment, the correction microphone  18 B naturally filters signals having frequencies that are less than the low frequency cut-off of the low pass filter  46 . For example, if the frequency response of the correction microphone  18 B has a low frequency cut-off of about 200 Hz, then the low pass filter  46  should similarly have a high frequency cut-off of about 190-200 Hz or greater. 
     Accordingly, after executing step  600 , the correction output signal should have substantially no non-negligible audio component corresponding to an input audio signal. This step may be skipped, however, for those embodiments that shield the diaphragm  24  from the input audio signal. 
     Before, contemporaneously with, or after executing step  600 , the process normalizes the audio response to the mechanical portion of the noise component (step  602 ). Embodiments that do not remove this audio response may skip this step. In the general case, however, the signal transformation module  48  may be retained in the system as an all pass filter that is selectively activated. Alternatively, the signal transformation module  48  may be eliminated. 
     At this point in the process, the correction microphone apparatus  42  should have generated a correction signal having a noise component that is substantially identical to the noise component in the primary signal. Both signals thus are forwarded to the summation logic  44  to remove/mitigate the noise component from the primary signal (step  604 ), thus ending the process. In other words, step  604  removes both the mechanical portion of the noise signal, as well as its associated audio response. 
     As noted above, depending upon the orientation of the microphones  18 A and  18 B, the summation logic  44  may subtract or add the two signals. Of course, other logic may be used in place of, or in addition to, the discussed subtraction and addition logic. Accordingly, discussion of specific subtraction or addition logic is illustrative only and not intended to limit all embodiments of the invention. 
     Illustrative embodiments therefore should significantly improve signal to noise ratios over conventional single microphone systems known to the inventors. It should be reiterated that various of the components shown in the drawings are illustrative and not intended to limit the scope of all embodiments. For example, additional components may be used to optimize operation. As another example, the microphone system  12  may have more than two microphones  18 A and  18 B or microphone apparatuses. Instead, among other things, the microphone system  12  may have three or more microphones, or three or more microphone apparatuses. Moreover, one or more of the microphone apparatuses may include just microphones (e.g., the correction microphone  18 B may have just a cap  34  with no audio input port). 
     Various embodiments of the invention may at least have portions implemented at least in part in any conventional computer programming language. For example, some embodiments may be implemented in a procedural programming language (e.g., “C”), or in an object oriented programming language (e.g., “C++”). Other embodiments of the invention may be implemented as preprogrammed hardware elements (e.g., application specific integrated circuits, FPGAs, and digital signal processors), or other related components. 
     In an alternative embodiment, some portions of the disclosed apparatus and methods (e.g., see the flow chart described above) may be implemented as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented with wireless techniques (e.g., WIFI, microwave, infrared or other transmission techniques). The series of computer instructions can embody all or part of the functionality previously described herein with respect to the system. 
     Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies. 
     Among other ways, such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software. 
     Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.