Patent Publication Number: US-10771875-B2

Title: Gradient micro-electro-mechanical systems (MEMS) microphone

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/147,194 filed Jan. 3, 2014, now U.S. Pat. No. 10,154,330, issued Dec. 11, 2018, which, in turn, claims the benefit of U.S. provisional application Ser. No. 61/842,858 filed on Jul. 3, 2013, the disclosures of which are hereby incorporated in their entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     Aspects as disclosed herein generally relate to a microphone such as a gradient based micro-electro-mechanical systems (MEMS) microphone for forming a directional and noise canceling microphone. 
     BACKGROUND 
     A dual cell MEMS assembly is set forth in U.S. Publication No. 2012/0250897 (the &#39;897 publication”) to Michel et al. The &#39;897 publication discloses, among other things, a transducer assembly that utilizes at least two MEMS transducers. The transducer assembly defines either an omnidirectional or directional microphone. In addition to at least first and second MEMS transducers, the assembly includes a signal processing circuit electrically connected to the MEMS transducers, a plurality of terminal pads electrically connected to the signal processing circuit, and a transducer enclosure housing the first and second MEMS transducers. The MEMS transducers may be electrically connected to the signal processing circuit using either wire bonds or a flip-chip design. The signal processing circuit may be comprised of either a discrete circuit or an integrated circuit. The first and second MEMS transducers may be electrically connected in series or in parallel to the signal processing circuit. The first and second MEMS transducers may be acoustically coupled in series or in parallel. 
     SUMMARY 
     In at least one embodiment, a micro-electro-mechanical systems (MEMS) microphone assembly is provided. The assembly includes an enclosure, a MEMS transducer, and a plurality of substrate layers. The single MEMS transducer is positioned within the enclosure. The plurality of substrate layers support the single MEMS transducer. The plurality of substrate layers define a first transmission mechanism to enable a first side of the single MEMS transducer to receive an audio input signal and a second transmission mechanism to enable a second side of the single MEMS transducer to receive the audio input signal. 
     In at least another embodiment, a MEMS microphone assembly is provided. The assembly includes an enclosure, a MEMS transducer, and a plurality of substrate layers. The single MEMS transducer is positioned within the enclosure. The plurality of substrate layers include a first substrate layer to support the single MEMS transducer. The first substrate layer is configured to electrically couple the single MEMS transducer to an end user circuit board. The plurality of substrate layers define at least one transmission mechanism that is acoustically coupled to the single MEMS transducer to enable an audio input to pass to the single MEMS transducer. 
     In at least another embodiment, a MEMS microphone assembly is provided. The assembly includes a first enclosure, a single first (MEMS) transducer, a second enclosure a single second MEMS transducer, and a plurality of substrate layers. The single first MEMS transducer is positioned within the first enclosure. The single second MEMS transducer is positioned within the second enclosure. The plurality of substrate layers including a first substrate layer and a second substrate layer support the single first MEMS transducer and the single second MEMS transducer. The plurality of substrate layers define a first transmission mechanism to enable the single first MEMS transducer to receive an audio input signal and a second transmission mechanism to enable the second first MEMS transducer to receive the audio input signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present disclosure are pointed out with particularity in the appended claims. However, other features of the various embodiments will become more apparent and will be best understood by referring to the following detailed description in conjunction with the ac-company drawings in which: 
         FIG. 1  depicts a cross sectional view of a gradient MEMS microphone assembly in accordance to one embodiment; 
         FIG. 2  depicts a microphone of  FIG. 1  in accordance to one embodiment; 
         FIGS. 3A-3B  depict the microphone assembly as coupled to an end-user assembly in accordance to various embodiments; 
         FIG. 4  depicts an exploded view of the microphone assembly and a portion of the end-user assembly in accordance to one embodiment; 
         FIG. 5  depicts one example of spatial filtering attributed to the microphone assembly of  FIG. 1 ; 
         FIG. 6  depicts one example of frequency response of the microphone assembly as set forth in  FIG. 1  in accordance to one embodiment; 
         FIG. 7  depicts another cross-sectional view of a gradient MEMS microphone assembly as coupled to another end-user assembly in accordance to one embodiment; 
         FIG. 8  depicts another cross-sectional view of a gradient MEMS microphone assembly in accordance to one embodiment; 
         FIG. 9  depicts another cross-sectional view of a gradient MEMS microphone assembly in accordance to one embodiment 
         FIG. 10  depicts another cross-sectional view of a gradient MEMS microphone assembly in accordance to one embodiment; 
         FIG. 11  depicts another cross-sectional view of another gradient MEMS microphone assembly in accordance to one embodiment; 
         FIG. 12  depicts another cross-sectional view of an electrical-gradient MEMS based micro-phone assembly in accordance to one embodiment; and 
         FIG. 13  depicts another cross-sectional view of an electrical-gradient MEMS based micro-phone assembly in accordance to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     The performance of MEMS type condenser microphones has improved rapidly and such microphones are gaining a larger market share from established electrets condenser microphones (ECM). One area in which MEMS microphone technology lags behind ECM is in the formation of gradient microphone structures. Such structures including ECM have, since the 1960&#39;s been used to form, far-field directional and near-field noise-canceling (or close-talking) microphone structures. A directional microphone allows spatial filtering to improve the signal-to-random incident ambient noise ratio, while noise-canceling microphones take advantage of a speaker&#39;s (or talker&#39;s) near-field directionality in addition to the fact that the gradient microphone is more sensitive to near-field speech than to far-field noise. The acoustical-gradient type of ECM as set forth herein uses a single microphone with two sound ports leading to opposite sides of its movable diaphragm. Thus, the sound signals from two distinct spatial points in the sound field are subtracted acoustically across a diaphragm of a single MEMS microphone. In contrast, an electrical-gradient based microphone system includes a two single port ECM that is used to receive sound at the two distinct spatial points, respectively. Once sound (e.g., an audio input signal) is received at the two distinct spatial points, then their outputs are subtracted electronically outside of the microphone elements themselves. 
     Unfortunately, a gradient type or based MEMS microphone (including directional and noise-canceling versions) have been limited to electrical-gradient technology. The embodiments disclosed herein provide for, but not limited to, an acoustical-gradient type MEMS microphone implementation. Further, the disclosure provided herein generally illustrates the manner in which an acoustical-gradient type MEMS microphone implementation can be achieved by, but not limited to, (i) providing a thin mechano-acoustical structure (e.g., outside of the single two port MEMS microphone) that is compatible with surface-mount manufacture technology and a thin form factor for small space constraint in consumer products (e.g., cell phone, laptops, etc.) and (ii) providing advantageous acoustical performance as will be illustrated herein. 
       FIG. 1  depicts a cross sectional view of a gradient MEMS microphone assembly (“assembly”)  100  in accordance to one embodiment. The assembly  100  includes a single MEMS microphone (“microphone”)  101  including a single micro-machined MEMS die transducer (“transducer”)  102  with a single moving diaphragm (“diaphragm”)  103 . It is recognized that a single transducer  102  may be provided with a multiple number of diaphragms  103 . A microphone enclosure (“enclosure”)  112  is positioned over the transducer  102  and optionally includes a base  113 . 
     The base  113 , when provided, defines a first acoustic port  111  and a second acoustic port  115 . The first acoustic port  111  is positioned below the diaphragm  103 . A first acoustic cavity  104  is formed between the base  113  and one side of the diaphragm  103 . A second acoustic cavity  105  is formed at an opposite side of the diaphragm  103 . The second acoustic port  115  abuts the second acoustic cavity  105 . The diaphragm  103  is excited in response to an audio signal pressure gradient that is generated between the first and the second acoustic cavities  104 ,  105 . 
     A plurality of substrate layers  116  supports the microphone  101 . The plurality of substrate layers  116  include a first substrate layer  121  and a second substrate layer  122 . In one example, the first substrate layer  121  may be a polymer such as PCABS or other similar material. The second structure layer  122  may be a printed circuit board (PCB) and directly abuts the enclosure  112  and/or the base  113 . The second substrate layer  122  may also be a polyimide or other suitable material. The plurality of substrate layers  116  mechanically and electrically support the microphone  101  and enable the assembly  100  to form a standalone component for attachment to an end user assembly (not shown). The plurality of substrate layers  116  form or define a first transmission mechanism (generally shown at “ 108 ”) and a second transmission mechanism (generally shown at “ 109 ”). The first transmission mechanism  108  generally includes a first sound aperture  106 , a first acoustic tube  110 , and a first acoustic hole  117 . The second transmission mechanism  109  generally includes a second sound aperture  107 , a second acoustic tube  114 , and a second acoustic hole  118 . An audio input signal (or sound) is generally received at the first sound aperture  106  and at the second sound aperture  107  and subsequently passed to the microphone  101 . This will be discussed in more detail below. 
     The base  113  defines a first acoustic port  111  and a second acoustic port  115 . As noted above, the base  113  may be optionally included in the microphone  101 . If the base  113  is not included in the microphone  101 , the first acoustic hole  117  may directly provide sound into the first acoustic cavity  104 . In addition, the second acoustic hole  118  may directly provide sound into the second acoustic cavity  105 . 
     The second substrate layer  122  is substantially planar to support the microphone  101 . The first and the second acoustic tubes  110  and  114  extend longitudinally over the first substrate layer  121 . The first sound aperture  106  is separated from the second sound aperture  107  at a delay distance d. The first and the second sound apertures  106  and  107 , respectively, are generally perpendicular to the first and the second acoustic tubes  110  and  114 , respectively. The first and the second acoustic holes  117 ,  118  are generally aligned with the first and the second acoustic ports  111  and  115 , respectively. 
     A first acoustic resistance element  119  (e.g., cloth, sintered material, foam, micro-machined or laser drilled hole arrays, etc.) is placed on the first substrate layer  121  and about (e.g., across or within) the first sound aperture  106 . A second acoustic resistance element  120  (e.g., cloth, sintered material, foam, micro-machined or laser drilled hole arrays, etc.) is placed on the first substrate layer  121  about (e.g., across or within) the second sound aperture  107 . It is recognized that the first and/or second acoustic resistance elements  119  and  120  may be formed directly within the transducer  102  while the transducer  102  undergoes its micromachining process. Alternatively, the first and/or the second acoustic resistance elements  119  and  120  may be placed anywhere within the first and the second transmission mechanisms  108  and  109 , respectively. 
     In general, at least one of the first and the second acoustic resistance elements  119 ,  120  are arranged to cause a time delay with the sound (or ambient sound) that is transmitted to the first sound aperture  106  and/or the second sound aperture  107  and to cause directivity (e.g., spatial filtering) of the assembly  100 . In one example, the second acoustic resistance element  120  includes a resistance that is greater than three times the resistance of the first acoustic resistance element  119 . In addition, the second acoustic cavity  105  may be three times larger than the first acoustic cavity  104 . 
     In general, the first and the second acoustic resistance elements  119 ,  120  are formed based on the size restrictions of the acoustical features such as apertures, holes, or tube cross-sections of the first and the second transmission mechanisms  108  and  109 . The first transmission mechanism  108  enables sound to enter into the microphone  101  (e.g., into the first acoustic cavity  104  on one side of the diaphragm  103 ). The second transmission mechanism  109  and the second acoustic port  115  (if the base  113  is provided) enable the sound to enter into the microphone  101  (e.g., into the second acoustic cavity  105  on one side of the diaphragm  103 ). In general, the microphone  101  (e. g., acoustic gradient microphone) receives the sound from a sound source and such a sound is routed to opposing sides of the moveable diaphragm  103  with a delay in time with respect to when the sound is received. The diaphragm  103  is excited by the signal pressure gradient between the first acoustic cavity  104  and the second acoustic cavity  105 . 
     The delay is generally formed by a combination of two physical aspects. First, for example, the acoustic sound (or wave) takes longer to reach one entry point (e.g., the second acoustic aperture  107 ) into the microphone  101  than another entry point (e.g., the second acoustic aperture  106 ) since the audio wave travels at a speed of sound in the first transmission mechanism  108  and the second transmission mechanism  109 . This effect is governed by the spacing or the delay distance, d between the first sound aperture  106  and the second sound aperture  107  and an angle of the sound source, θ. In one example, the delay distance d may be 12.0 mm. Second, the acoustic delay created internally by a combination of resistances (e.g., resistance values of the first and the second acoustic resistance elements  119  and  120 ) and acoustic compliance (volumes) creates the desired phase difference across the diaphragm. 
     If the sound source is positioned to the right of the assembly  100 , any sound generated therefrom will first reach the first sound aperture  106 , and after some delay, the sound will enter into the second sound aperture  107  with an attendant relative phase delay in the sound thereof. Such a phase delay assists in enabling the microphone  101  to achieve desirable performance. As noted above, the first and the second sound apertures  106  and  107  are spaced at the delay distance “d”. Thus, the first acoustic tube  110  and the second acoustic tube  114  are used to transmit the incoming sound to the first acoustic hole  117  and the second acoustic hole  118 , respectively, and then on to the first acoustic port  111  and the second acoustic port  115 , respectively. 
     In general, the sound or audio signal that enters from the second sound aperture  107  and subsequently into the second acoustic cavity  105  induces pressure on a back side of the diaphragm  103 . Likewise, the audio signal that enters from the first sound aperture  106  and subsequently into the first acoustic cavity  104  induces pressure on a front side of the diaphragm  103 . Thus, the net force and deflection of the diaphragm  103  is a function of the subtraction or “acoustical gradient” between the two pressures applied on the diaphragm  103 . The transducer  102  is operably coupled to an ASIC  140  via wire bonds  142  or other suitable mechanism to provide an output indicative of the sound captured by the microphone  101 . An electrical connection  144  (see  FIGS. 3A-3B ) is provided on the second substrate layer  122  to provide an electrical output from the microphone  101  via a connector  147  (see  FIGS. 3A-3B ) to an end user assembly  200  (see  FIGS. 3A-3B ). This aspect will be discussed in more detail in connection with  FIGS. 3A-3B . The plurality of substrate layers include a shared electrical connection  151  which enable the first substrate layer  121  and the second substrate layer  122  to electrically communicate with one another and to electrically communicate with the end user assembly  200 . 
     In general, the assembly  100  may be a stand-alone component that is surface mountable on an end-user assembly. Alternatively, a first coupling layer  130  and a second coupling layer  132  (e.g., each a gasket and/or adhesive layer) may be used to couple the assembly  100  to the end user assembly  200 . The second substrate layer  122  extends outwardly to enable other electrical or MEMS components to be provided thereon. It is recognized that the base  113  may be eliminated and that the ASIC  140  and transducer  102  (e.g., their respective die(s)) may be bonded directly to the second substrate layer  122 . In this case, the first acoustic port  111  and the second acoustic port  115  no longer exist. Of course, other arrangements are feasible, such as the first sound aperture  106  being led directly to the first acoustic cavity  104  and the second sound aperture  107  being led directly into the second acoustic cavity  105 . Additionally, the transducer  102  may be inverted and bump bonded directly to the base  113  or to the second substrate layer  122 . 
     It may be desirable to form a “far field” directional type microphone where the audio source or talker is, for example, farther than 0.25 meters from the first sound aperture  106 . In this case, it may be desirable to point a pickup sensitivity beam (polar pattern) toward the talker&#39;s general direction, but discriminate against the pickup of noise and room reverberation coming from other directions (e.g., from the left or behind the microphone). The second acoustic resistance element  120  (e.g., the larger resistance value) is placed into the plurality of substrate layers  116 , and forms, for example, a cardioid polar directionality (see  FIG. 5 ) instead of a bi-directional polar directivity, otherwise. 
     The appropriate level of acoustic resistance (e.g., Rs), used for the second acoustic resistance  120 , depends on the desired polar shape, the delay distance d, and on the combined air volumes (acoustic compliance, Ca) of the second acoustic tube  114 , the second acoustic hole  118 , the second acoustic port  115  and the second acoustic cavity  105 . The second acoustic tube  114  adds a significant air volume that augments the volume of the second acoustic cavity  105 . Thus, for a given acoustic resistance value and the delay distance d, such a condition decreases the need to configure the second acoustic cavity  105  and hence the microphone  101  to be larger. Of course, the second acoustic tube  114  enables in achieving the large delay distance “d” as needed above. It should be noted that the first acoustic resistance element  119  may be omitted or included. The acoustic resistance for the first acoustic resistance element  119  may be smaller than that of the second acoustic resistance element  120  and may be used to prevent debris and moisture intrusion or mitigate wind disturbances. The resistance value of Rs for the second acoustic resistance element  120  is generally proportional to d/Ca. In general, the acoustical compliance is a volume or cavity of air that forms a gas spring with equivalent stiffness, and whereas its acoustical compliance is the inverse of its acoustical stiffness. 
     It should be noted that electroacoustic sensitivity is proportional to the delay distance d and hence a larger d means higher acoustical signal-to-noise ratio (SNR), which is a strong factor to the directional microphone due to the distant talker or speaker. Thus, in the assembly  100 , the enhancement of SNR is enabled due to the first and second acoustic tubes  110  and  114  which allow for a large “d”, while achieving the originally desired polar directionality that is needed in customer applications. 
     The assembly  100  may support near field (&lt;0.25 meters) capability with a smaller delay distance “d” and still achieve high levels of acoustic noise canceling. While the gradient noise-canceling acoustic sensitivity of the microphone  101  and hence acoustical signal-to-noise ratio (SNR) will decrease, this is generally not a concern as the speaker is close. 
     The assembly  100  as set forth herein not only provides high levels of directionality or noise canceling, but a high SNR when needed. Further, the assembly  100  yields a relatively flat and wide-bandwidth frequency response which is quite surprising given the long length of the first and second acoustic tube  110  and  114 . The assembly  100  may be either SMT bonded within, or SMT bonded or connected to an end-used board or housing which may be external to the assembly  100 . 
     In general, it should be noted that “air volumes” or “acoustic cavities” are positioned proximate to the diaphragm  103  to allow motion thereof. These acoustic cavities can take varied shapes and be formed within (i) portions of the second acoustic cavity  105  in the enclosure  112 , (ii) the first acoustic cavity  104  in the transducer  102 , or (iii) the first and the second transmission mechanisms  108  and  109  when the second substrate layer  122  is formed. 
     It is recognized that the first and the second transmission mechanism  108  or  109  and the first and second acoustic tubes  110  or  114  may also utilize a multiplicity of acoustically parallel tubes or holes or ports with the same origin and terminal points, for example, a bifurcated tube. Moreover, such a parallel transmission implementation of tubes could have a single origin, but multiple terminal points. For example, a single “first tube” leading from the microphone  101  to the first sound aperture  106  could be replaced by parallel tubes leading from the same origin point at the microphone  101  to a multiplicity of separated first sound apertures  106 . 
     It is also recognized that to further enhance the effective delay distance, d between the first and the second sound apertures  106 ,  107  when the assembly  100  is mated to the ported end-user housing, physical baffles (not shown) may be placed on an exterior of the end user housing between the two ports so as to increase the traveling wave distance between the two ports. 
     It also recognized that while the assembly  100  provides two acoustical transmission lines leading to two substantially separated sound apertures thus forming a first-order gradient microphone system, similar structures may be used to form higher-order gradient microphone system with a greater number of transmission lines and sound apertures. 
       FIG. 2  depicts the microphone  101  of  FIG. 1  in accordance to one embodiment. In general, the microphone  101  is a base element MEMS microphone that includes a microphone die with at least two ports (e.g., first and second acoustic ports  111  and  115 ) to allow sound to impinge on a front (or top) and a back (or bottom) of the diaphragm  103 . 
       FIGS. 3 a -3 b    depict the microphone assembly  100  as coupled to an end user assembly  200 . The end user assembly  200  includes an end user housing  202  and an end user circuit board  204 . In one example the end user assembly  200  may be a cellular phone, speaker phone or other suitable device that requires a microphone for receiving audio data. The end user housing  202  may be a portion of a handset or housing of the speaker phone, etc. The end user housing  202  defines a first user port  206  and a second user port  207  that is aligned with the first sound aperture  106  and the second sound aperture  107 , respectively. The sound initially passes through the first user port  206  and the second user port  207  and into the first transmission mechanism  108  and the second transmission mechanism  109 , respectively, and subsequently into the microphone  101  as described above. 
     As shown, the microphone assembly  100  may be a standalone product that is coupled to the end user assembly  200 . The first coupling layer  130  and the second coupling layer  132  couple the microphone assembly  100  to the end user assembly  200 . In addition, the first coupling layer  130  and the second coupling layer  132  are configured to acoustically seal the interface between the microphone assembly  100  and the end user assembly  200 . The second substrate layer  122  includes a flexible board portion  146 . The flexible board portion  146  is configured to flex in any particular orientation to provide the electrical connection  144  (e.g., wires) and a connector  147  to the end user circuit board  204 . It is recognized that the electrical connection  144  need not include wires for electrically coupling the microphone  101  to the end user circuit board  204 . For example, the electrical connection  144  may be an electrical contact that is connected directly with the connector  147 . The connector  147  is then mated directly to the end user circuit board  204 . This aspect is depicted in  FIG. 3B . It is also recognized that any microphone assembly as described herein may or may not include the flexible board portion  146  for providing an electrical interface to the end user circuit board  204 . This condition applies to any embodiment as provided herein. 
       FIG. 4  depicts an exploded view of the microphone assembly  100  in addition to the end user housing  202  of the end user assembly  200  in accordance to one embodiment. A first acoustic seal  152  (not shown in  FIGS. 1 and 3 ) is positioned over the first substrate layer  121  to prevent the sound from leaking from the first acoustic tube  110  and the second acoustic tube  114 . The end user housing  202  is provided to be coupled with the microphone assembly  100 . 
       FIG. 5  is a plot  170  that illustrates one example of polar directivity or spatial filtering attributed to the microphone  101  (or assembly  100 ) as noted above in connection with  FIG. 1 .  FIG. 5  generally represents a free field 1 meter microphone measurement polar directivity response. 
       FIG. 6  depicts an example of a simulated frequency response shape of the microphone assembly  100  as set forth in  FIG. 1  in accordance to one embodiment. In particular, the  FIG. 6  is a plot of the ration in dB of the electrical output from the ASIC  140  to the acoustical input to the first sound aperture  106  versus the frequency. 
       FIG. 7  depicts another cross-sectional view of a gradient MEMS microphone assembly  300  as coupled to another end user assembly  400 . In general, the microphone assembly  300  may be implemented as a surface mountable standalone package that is reflow soldered on the end user circuit board  204 . The microphone assembly  300  includes a first extended substrate  302  and a second extended substrate  304  that acoustically couples the microphone  101  to the end user housing  202  for receiving sound from a speaker (or talker). For example, the first extended substrate  302  defines a first extended channel  306  for receiving sound from the first user port  206 . The sound is then passed into the first transmission mechanism  108  and subsequently into the first acoustic cavity  104  of the microphone  101 . The second extended substrate  304  defines a second extended channel  308  for receiving sound from the second user port  207 . The sound is then passed into the second transmission mechanism  109  and subsequently into the second acoustic cavity  105  of the microphone  101 . 
     It is recognized that the first acoustic resistance element  119  may be placed at any location about the first transmission mechanisms  108 . The second acoustic resistance element  120  may optionally be placed anywhere along the second transmission mechanism  109 . Additionally, the first and the second acoustic resistance elements  119 ,  120  may optionally be placed anywhere along the first and the second user ports  206  and  207 . This condition applies to any embodiment as provided herein. The first coupling layer  130  may be placed at the interface of the second substrate layer  122  and the first extended substrate  302  and at the interface of the first extended substrate  302  and the end user housing  202 . The second coupling layer  132  may be placed at the interface of the second substrate layer  122  and the second extended substrate  304  and at the interface of the second extended substrate  304  and the end user housing  202 . As shown, the flexible board portion  146  is provided at two locations to form an electrical connection  310  with the end user circuit board  204 . The electrical connection  310  may comprise a surface mount technology (SMT) electrical connection. 
       FIG. 8  depicts another view of a gradient MEMS microphone assembly  500  as coupled to another end user assembly  600 . The microphone assembly  500  may also be implemented as a surface mountable standalone package that is reflow soldered on the end user circuit board  204 . The microphone assembly  500  includes a plurality of electrical legs  502  that protrude therefrom for being reflowed soldered to contacts  504  on the end user circuit board  204 . In general, the microphone assembly  500  may include any number of the features as disclosed herein. It is also recognized that the microphone assembly  500  may include the first and the second resistance elements  119  and  120 . Additionally, the first and the second coupling layers  130 ,  132  may be provided at the interface between the first and the second sound apertures  106 ,  107  and the first and the second user ports  206 ,  207 . 
       FIG. 9  depicts another cross-sectional view of a gradient MEMS microphone assembly  550  as coupled to another end user assembly  650 . In general, the assembly  550  (e.g., the first substrate layer  121 ) may be electrically coupled to the end user circuit board  204  via surface mount contacts  552  and  554  (e.g., the assembly  550  is surface mounted to the end user circuit board  204 ). The end user circuit board  204  defines a first board channel  556  and a second board channel  557 . The first board channel  556  and the second board channel  557  of the end user circuit board  204  are aligned with the first sound aperture  106  and the second sound aperture  107  in addition to the first user port  206  and the second user port  207  such that each of the assembly  550 , the end user circuit board  204  and the end user housing  202  enable acoustic communication therebetween. First and second coupling layers  580  and  582  are provided to mechanically couple the end user circuit board  204  to the end user housing  202 . Further, the first and the second coupling layers  580  and  582  acoustically seal the interface between the end user circuit board  204  and the end user housing  202 . 
       FIG. 10  depicts a cross-sectional view of another gradient MEMS microphone assembly  700  in accordance to one embodiment. As shown, the first sound aperture  106  is directly coupled to the first acoustic port  111 . In this case, the first transmission mechanism  108  includes the first sound aperture  106  and the first acoustic port  111 , while the second transmission mechanism  109  includes the second sound aperture  107 , the second acoustic tube  114 , and the second acoustic hole  118 . This differs from the microphone assemblies noted above as the first acoustic tube  110  and the first acoustic hole  117  is not provided in the first transmission mechanism  108  of the assembly  700 . It is recognized that the first transmission mechanism  108  and the second transmission mechanism  109  is still separated by a delay distance, d. The delay distance however as illustrated in connection with the assembly  700  may not be as large as the delay distance, d used in connection with the other embodiments as disclosed herein. This condition may create a small amount of degradation of the high frequency response for the assembly  700 . 
       FIG. 11  depicts a cross-sectional view of another gradient MEMS microphone assembly  800  in accordance to one embodiment. As shown, the enclosure  112  is directly attached to the second substrate structure layer  122  (i.e., the base  113  is removed (see  FIG. 1  for comparison)). Additionally, the first acoustic port  111  and the second acoustic port  115  are removed (see  FIG. 1  for comparison). Accordingly, a sound wave that enters into the first sound aperture  106  will travel into the first acoustic tube  110  and into the first acoustic hole  117 . The sound wave also enters directly into the first acoustic cavity  104  which induces pressure on the front side of the diaphragm  103 . Likewise, the sound wave will travel the delay distance, d and enter into the second sound aperture  107  and further travel into the second acoustic tube  114 . The sound wave will enter into the second acoustic hole  118  and subsequently into the second acoustic cavity  105  which induces pressure on the rear side of the diaphragm  103 . As noted above, the net force and deflection of the diaphragm  103  is a function of the subtraction or “acoustical gradient” between the two pressures applied on the diaphragm  103 . The microphone  101  produces an electrical output that is indicative of the sound wave. 
       FIG. 12  depicts a cross-sectional view of an electrical-gradient MEMS microphone assembly  850  in accordance to one embodiment. The assembly includes the microphone  101  and a microphone  101 ′. The microphone  101 ′ includes a transducer  102 ′, a diaphragm  103 ′, a first acoustic cavity  104 ′, a first acoustic port  111 ′, an enclosure  112 ′, and a base  113 ′. As shown, the sound wave that enters into the second sound aperture  107  travels through the second acoustic tube  114  and through the second acoustic hole  118 . From there, the sound wave travels through the first acoustic port  111 ′ and into the first acoustic cavity  104 ′ toward the front of the diaphragm  103 ′. In general, each diaphragm  103  and  103 ′ experiences pressure from the incoming sound wave thereby enabling each microphone  101  and  101 ′ to generate an electrical output indicative of the incoming sound wave. The electrical outputs are subtracted from each other outside in another integrated circuit that is positioned outside of the assembly  850 . Alternatively, one of the microphones  101  or  101 ′ may provide an electrical output that is conveyed to (via circuit traces within the second substrate layer  122 ) to the other microphone  101  or  101 ′ for the subtraction operation as noted above to be executed. As shown, the assembly  850  in response to receiving sound at the two distinct spatial points, electronically subtracts the outputs from microphone elements  101  and  101 ′. This differs from the assemblies  100 ,  700  and  800  as such assembles require a pressure differential of the sound wave to be present across the diaphragm  103 . 
       FIG. 13  depicts a cross-sectional view of an electrical gradient MEMS microphone  870  in accordance to another embodiment. The microphone assembly  870  is generally similar to the microphone assembly  850 . However, the enclosures  112  and  112 ′ are coupled together via a dividing wall  852 . The dividing wall  852  may be solid or include apertures (or be mechanically compliant) to enable acoustical transmission between the microphones  101  and  101 ′ at certain frequencies. Such acoustical transmission can be used to provide advantageous combined microphone performance in sensitivity, polar directivity, signal-to-noise ratio (SNR), and/or frequency response and bandwidth. This implementation may provide cost savings in comparison to the assembly  850  of  FIG. 11 . For example, a single housing may be formed and include the enclosure  112  and  112 ′. It is recognized that while multiple ASICs  140  and  140 ′ are illustrated, a single ASIC may be provided for both microphones  101  and  101 ′. Each of the foregoing aspects may reduce cost associated with assembling the assembly  850 . 
     It is recognized that while two acoustical transmission mechanisms  108  and  109  are provided which lead to two substantially separated sound apertures thus forming a first-order gradient microphone system, similar structures employing the concepts disclosed herein may be employed to form higher-order gradient microphone systems with a greater number of transmission mechanisms  108  and  109  and sound apertures  106  and  107 . 
     It is further recognized that the first and the second transmission mechanisms  108  or  109  and the first and second acoustic tubes  110  and  114  may utilize a multiplicity of acoustically parallel apertures or tubes or holes or ports with the same origin and terminal points, for example a bifurcated tube. Moreover, such parallel transmission mechanisms, aperture, tubes, or hole may have a single origin but multiple terminal points. For example, a single “first tube” leading from the microphone  101  to a “first sound aperture” could be replaced by parallel tubes leading from the same origin point at the microphone  101  to a multiplicity of separated “first sound apertures.” 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.