Patent Publication Number: US-2023156386-A1

Title: Micro-electro-mechanical systems (mems) microphone assembly

Description:
TECHNICAL FIELD 
     Aspects as disclosed herein generally relate to a microphone such as a micro-electro-mechanical systems (MEMS) microphone for a microphone assembly that can be adapted to form a microphone with different directivity patterns (e.g., uni-directional vs. omni-directional) and/or frequency response shapes. The disclosed MEMS microphone assembly may be used for any number of applications including, but not limited to, active noise cancellation (ANC) techniques and voice pickup in hands-free phone applications. These aspects and others will be discussed in more detail herein. 
     BACKGROUND 
     U.S. Pat. No. 10,154,330 to Baumhauer et al. provides a micro-electro-mechanical systems (MEMS) microphone assembly. 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. 
     U.S. Pat. No. 9,955,246 to Reese et al. provides a micro-electro-mechanical systems (MEMS) microphone assembly. The assembly includes an enclosure, a single micro-electro-mechanical systems (MEMS) transducer, a substrate layer, and an application housing. The single MEMS transducer is positioned within the enclosure. The substrate layer supports the single MEMS transducer. The application housing supports the substrate layer and defining at least a portion of a first transmission mechanism to enable a first side of the single MEMS transducer to receive an audio input signal and at least a portion of a second transmission mechanism to enable a second side of the single MEMS transducer to receive the audio input signal. 
     SUMMARY 
     In at least one embodiment, a microphone assembly including an enclosure, a first printed circuit board (PCB) and a microphone assembly is provided. The microphone assembly includes a sub-casing, a micro-electro-mechanical systems (MEMS) transducer, and a second PCB. The MEMS transducer positioned in the sub-casing and the second PCB supports the MEMS transducer. The first PCB defines a first acoustic path positioned below the second PCB and the MEMS transducer. The second PCB defines a first audio port positioned directly below the MEMS transducer. The enclosure defines a first acoustic opening that is positioned directly below the first acoustic path to enable an audio input signal to pass through the first audio port and to an underside of the MEMS transducer. The enclosure defines a second acoustic opening that is positioned at a distance of between 3 to 30 mm from the first acoustic opening. 
     In at least another embodiment, a microphone assembly including an enclosure, a first printed circuit board (PCB), a post, and a microphone sub-assembly is provided. The microphone sub-assembly includes a sub-casing, a micro-electro-mechanical systems (MEMS) transducer, at least one port hole. The micro-electro-mechanical systems (MEMS) transducer is positioned in the sub-casing to receive an audio input signal. The at least one port is positioned on a topside of the sub-casing. The post is positioned over the at least one port hole to seal the sub-casing. 
     In at least another embodiment, a microphone assembly including an enclosure, a first printed circuit board (PCB) and a microphone sub-assembly is provided. The microphone sub-assembly includes a sub-casing, a micro-electro-mechanical systems (MEMS) transducer, at least one port hole, and a cover. The micro-electro-mechanical systems (MEMS) transducer is positioned in the sub-casing to receive an audio input signal. The at least one port is positioned on a topside of the sub-casing. a cover positioned over the at least one port hole and being moveable about the first port hole to provide a plurality of frequency responses based on a location of the cover relative to the first port hole. 
    
    
     
       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 accompany drawings in which: 
         FIG.  1    illustrates one example of an amplitude response curve for a microphone; 
         FIG.  2    illustrates an example of different cut-off frequencies in various automotive audio applications; 
         FIG.  3    illustrates one example of a MEMS microphone for achieving a single cut off frequency; 
         FIG.  4    illustrates one example of a microphone assembly; 
         FIG.  5    illustrates a microphone assembly in accordance to one embodiment; 
         FIG.  6    illustrates a detailed implementation of a microphone sub-assembly in accordance to one embodiment; 
         FIG.  7    illustrates one example of frequency response curves in accordance to one embodiment; 
         FIG.  8    illustrates one example of various frequency responses in accordance to one embodiment; 
         FIG.  9    illustrates one example of various directivities in accordance to one embodiment; 
         FIG.  10    illustrates another detailed implementation of the microphone assembly in accordance to one embodiment; and 
         FIGS.  11 A- 11 B  illustrate respective top views of a sub-casing of the microphone sub-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. 
     It is recognized that directional terms that may be noted herein (e.g., “upper”, “lower”, “inner”, “outer”, “top”, “bottom”, etc.) simply refer to the orientation of various components of a transducer assembly in connection with the microphone assembly as illustrated in the accompanying figures. Such terms are provided for context and understanding of the embodiments disclosed herein. 
     A microphone assembly may include, but not limited to, a microphone sub-assembly including a micro-electro-mechanical systems (MEMS) based microphone element (e.g., transducer) or an electret condenser microphone (ECM), a printed circuit board (PCB) assembly on which the microphone element is mounted, and a housing (or casing) that encloses the foregoing features. Thus, the microphone assembly is understood to be larger in size than the MEMS microphone element package (or the ECM). 
     Customer applications may require microphone elements to provide different frequency response shapes and/or directivities. This means that different microphone element models may be needed in overall microphone assemblies for various specific applications. Embodiments as set forth herein attempt to extend acoustic design parameters from the microphone element level (e.g., MEMS mic package design) (or sub-assembly level) to the assembly level. That is, by modifying structural and geometric parameters of the assembly housing/casing, a single common microphone element package may be used in multiple microphone subassembly designs. For example, microphone assembly designs in a vehicle may currently require different and dedicated MEMS microphone elements to satisfy hands-free communication or active noise cancellation (ANC) applications. Embodiments as set forth herein, may provide a single common MEMS microphone element model that can be used for both applications, in which only some design features on the module housing level need to be modified. 
     For automotive applications, the overall microphone assembly may include a protective housing and is installed into the car body. For other applications, microphone elements and the PCB may be mounted directly on a housing of the product (e.g., cellphone). In this case, the final product housing (e.g., the cellphone) may enclose (or store) the microphone assembly to the protective housing of the microphone assembly. 
       FIG.  1    illustrates one example of an amplitude response curve for a microphone as a function of frequency. Starting from the lowest frequency defined on the plot, the curve typically presents a rising response shape characterized by a cutoff frequency. The cutoff frequency for the microphone may be generally defined as a frequency point at which a sensitivity is 3 dB (or other suitable value) below its normal sensitivity measured at 1 kHz.  FIG.  2    illustrates one example of cutoff frequencies for various applications (e.g., a microphone used in connection with ANC or a microphone used in connection with a hands-free communication implementations). These microphone-based applications may be applicable to vehicles. As shown, the cutoff frequency for microphones used in ANC is less than 20 Hz and the cutoff frequency for microphones used for hands-free applications is greater than 200 Hz. 
       FIG.  3    illustrates one example of a MEMS microphone transducer  100  for achieving a single cut off frequency. The transducer  100  includes a diaphragm  102 , a backplate  104 , and a baffle  106 . The diaphragm  102  generally moves under acoustic pressure excitation. The backplate  104  is provided with multiple holes to thereby allow acoustic waves to pass therethrough which is considered acoustically transparent. The diaphragm  102  and the backplate  104  form a capacitor that transfers the mechanical motion of the diaphragm  102  to electrical (e.g., voltage) output of the transducer  100 . The diaphragm  102  defines a path  108  that creates the cutoff frequency of an audio output signal. When assembled in a MEMS microphone package (or microphone assembly), the baffle  106  and a package casing (not shown in  FIG.  3   ) ensures that the external sound waves may only reach from one side of the diaphragm  102  to the other through the path  108 . It may be easier for low frequency sound waves with longer wavelengths to pass through the path  108  to reach both sides of the diaphragm  102 . This may result in less to no net pressure differences between the two sides of the diaphragm  102  (e.g., no net motion) at low frequencies than at high frequencies. This may effectively form a first-order low pass filter with a cut off frequency proportional to the size of the path  108 . Therefore, to change the cutoff frequency of the MEMS microphone transducer  100 , the size of the path  108  needs to be adjusted. As such, existing implementations may require the use of different MEMS elements for each individual application with a specific cut-off frequency target or a specific frequency response. 
       FIG.  4    illustrates one example of a microphone assembly  200  (or assembly  200 ). The microphone assembly  200  includes a protective housing or enclosure  202 , a microphone sub-assembly  204 , electronic circuitry  206 , and a printed circuit board (PCB)  208 . The electronic circuitry  206  may be mounted on the PCB  208  together with the microphone sub-assembly  204  and perform any number of audio processing applications such as, but not limited to, ANC, hands free operation, etc. The electronic circuitry  206  may interface with the microphone sub-assembly  204  to perform operations related to the foregoing audio applications. 
     The microphone sub-assembly  204  includes a sub-casing  220 , a MEMS transducer (or microphone)  222 , an application-specific integrated circuit (ASIC)  224 , and a PCB base  226 . The MEMS transducer  222  and the ASIC  224  are positioned on the PCB base  226 . The sub-casing  220  encloses the MEMS transducer  222 , the ASIC  224 , and seals to (typically by soldering) the PCB base  226  along its perimeter. It is recognized that the MEMS transducer  222  may also be implemented as an ECM. The PCB base  226  defines a first audio port  228 . The PCB  208  defines a first acoustic path  230  and the base part of the housing  202  defines a first acoustic opening  237 . The first acoustic path  230  and the first acoustic opening  237  axially align vertically with the first audio port  228 . While not shown, the MEMS transducer  222  includes a diaphragm that oscillates or is excited in response to an audio pressure that impinges on the diaphragm. An underside of the diaphragm is exposed to the environment to enable the audio signal to enter into the first audio port  228  provided by the PCB base  226 , the first acoustic path  230  provided by the PCB  208 , and the first acoustic opening  237  provided by the housing  202 . The ASIC  224  provides an electrical output indicative of the sound captured by the MEMS transducer  222 . 
       FIG.  5    illustrates a microphone assembly  300  in accordance to one embodiment. The assembly  300  includes the protective housing  202 , the microphone sub-assembly  204 , the electronic circuitry  206 , and the PCB  208 . Similarly, as noted above in connection with  FIG.  4   , the electronic circuitry  206  may be mounted on the PCB  208  together with the microphone sub-assembly  204  and perform any number of audio processing applications such as, but not limited to, ANC, hands free operation, etc. The electronic circuit  206  may interface with the microphone sub-assembly  204  to perform operations related to the foregoing audio applications. 
     Similarly, as noted in connection with  FIG.  4   , the microphone assembly  204  includes the sub-casing  220 , the microphone transducer  222  (MEMS or ECM), the ASIC  224 , and the PCB base  226 . The MEMS transducer  222  and the ASIC  224  are positioned on the PCB base  226 . The PCB base  226  defines a first audio port  228   a  that is positioned directly below the microphone transducer  222 . A first acoustic path  230  is defined by the PCB  208 . A base  227  of the enclosure  202  defines a first acoustic opening  237   a  and a second acoustic opening  237   b . When assembled, the first audio port  228   a  provided by the PCB base  226 , the first acoustic path  230  provided by the PCB  208 , and the first acoustic opening  237   a  provided by the enclosure  202  are axially aligned in the vertical direction. The second acoustic opening  237   b  opens directly to a cavity or volume  302  defined by the enclosure  202 . The first acoustic opening  237   a  and the second acoustic opening  237   b  may be positioned on the same surface of the enclosure  202  (e.g., the base  227 ) and separated by a distance, d. The distance d may be preferably within the range of 3 to 30 mm. When the microphone assembly  300  is configured as a uni-directional microphone, the microphone output sensitivity is proportional to the value of d. If d is too small, this may lead to a very low microphone sensitivity that may not be optimal. When the microphone assembly  300  is configured as an omni-directional microphone with various cutoff frequencies, the assembly  300  relies on the sound pressure amplitudes presented at the first and second acoustic openings  237   a ,  237   b  being similar to one another. If d is too large, this may lead to differences in sound pressure amplitudes presented at the first and second acoustic openings  237   a ,  237   b . It is recognized that the second acoustic opening  237   b  may also be positioned along any of one of vertically extending side walls of the enclosure  202  while maintaining an accumulated distanced in the preferred range. 
     The first acoustic opening  237   a , the first acoustic path  230  and the first audio port  228   a  enable the underside of the microphone transducer  222  to be vented to a sound field external to the assembly  300 . The sub-casing  220  defines at least one second audio port  228   b  (hereafter port hole  228   b ) positioned on a top side thereof. Similarly, the second acoustic opening  237   b  may enable a sound field external to the assembly  300  to enter into the cavity or volume  302  defined by the enclosure  202 . This may subsequently enable the topside of the microphone transducer  222  to be vented to the external sound field through port hole  228   b . When the microphone assembly  300  is configured as an omnidirectional microphone with various cutoff frequencies, the size of the opening area of the port hole  228   b  determines the cutoff frequency. If the port hole  228   b  is of a circular shape, a preferred diameter range is between 0.01 mm and 1 mm. This results in a cut-off frequency that may be suitable for hands-free microphone applications. If the port hole  228   b  is of other geometric shapes and/or in a of plurality forms, the effective total opening area is preferred to be in the range equivalent to that provided by a circular port hole with a diameter from 0.01 mm to 1 mm. 
     A post  304  may be positioned on the top side of the sub-casing  220  and mounted directly on top of the port hole  228   b . The post  304  may be integrated with the enclosure  202 . The post  304  extends from a top side of the sub-casing  220  to an underside of a top portion  309  of the enclosure  202 . When provided, the post  304  may serve as a sealing mechanism and seal the port hole  228   b . A first acoustic resistance element  310   a  (e.g., cloth, sintered material, foam, micro-machined or laser drilled hole arrays, etc.) may be positioned below the base  227  of the enclosure  202 . The first acoustic resistance element  310   a  may be placed directly underneath or above the first acoustic opening  237   a . A second acoustic resistance element  310   b  (e.g., cloth, sintered material, foam, micro-machined or laser drilled hole arrays, etc.) may also be positioned below the base  227  of the enclosure  202 . The second acoustic resistance element  310   b  may be placed directly underneath or above the second acoustic opening  237   b . The addition of the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  also act as an ingress protection mechanism to prevent foreign particles and moisture in the external environment from entering into the interior of the microphone assembly  300 . 
     It is recognized that any one or more of the post  304 , the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  may or may not be utilized on the assembly  300 . The utilization of the post  304 , the first acoustic resistance element  310   a , and the second acoustic resistance element  310   b  may provide differing frequency responses or directivity patterns for the assembly  300 . For example, in the event the post  304 , the first acoustic resistance element  310   a , and the second acoustic resistance element  310   b  are not implemented on the assembly  300 , the port hole  228   b  is unsealed and both sides of the microphone diaphragm are exposed to the external sound field. In this case, the port hole  228   b  effectively acts as the path  108  explained in connection with  FIG.  3   . Thus, the microphone subassembly  204  allows a rising frequency response with a cutoff frequency determined by the size of the opening area of the port hole  228   b . The microphone assembly  300  functions as an omni-directional microphone with a rising frequency response as illustrated in the waveform  402  of the plot depicted in  FIG.  7   . The waveform  402  illustrates a cutoff frequency higher than 200 Hz that may be preferable for applications such as hands-free communications. 
     In the event the post  304  is utilized to seal the port hole  228   b  in the assembly  300  and the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  are absent, only the underside of the microphone diaphragm is exposed to the external sound field. In such a case, the microphone sub-assembly  204  and thus the microphone assembly  300  perform as an omni-directional microphone with a flat frequency response as illustrated in the waveform  404  of the plot depicted in  FIG.  7   . The waveform  404  illustrates a cutoff frequency that is lower than 20 Hz that may be preferable for applications such as ANC. 
     In the event the microphone assembly  300  is arranged as an omni-directional microphone with various cutoff frequencies by either utilizing the post  304  or not utilizing the post  304 , it may not be necessary to implement the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b . However, in practice, it may be preferable to include the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  with small resistance values to serve as an ingress protection mechanism to prevent foreign particles and moisture intrusion. 
     Without the post  304 , but with the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b , the microphone assembly  300  may also be configured to provide a uni-directional (cardioid) response characteristic. In the case where the audio source or talker is located to the left of the assembly  300 , it may be desirable to point a pickup sensitivity beam (polar pattern) toward the left side (θ=0′), but discriminate against the pickup of unwanted sound from other directions (e.g., from the right or behind the microphone assembly  300 ). Without the post  304 , the microphone assembly  300  allows the external sound or audio signal to enter the first acoustic opening  237   a  thereby reaching the underside of the transducer  222  (thus the diaphragm). Similarly, the external sound or audio signal is transmitted through the second acoustic opening  237   b  thereby reaching the upper side of the transducer  222  (thus the diaphragm). The output of the microphone sub-assembly  204  may be a function of the subtraction or “acoustical gradient” between the two acoustic pressures impinging on the two sides of the transducer  222  (or the diaphragm). Due to the differences in the transmission paths, there will be a relative phase delay corresponding to a time difference for a sound source to reach the two sides of the transducer  222 . Such a phase delay enables the microphone assembly  300  to achieve desirable performance, like certain polar patterns. 
     To achieve the desired cardioid directivity shape, a certain amount of the acoustic resistance level, R sb , of the second acoustic resistance element  310   b  may be needed to satisfy a certain mathematical relationship determined by the delay distance, d, and an acoustic compliance, C v . In general, R sb  should be proportional to the quotient of d/C v , where the value of C v  is determined by the combined air volumes of the second acoustic opening  237   b , the cavity  302 , the port hole  228   b  and a volume  231  enclosed by the sub-casing  220  of the microphone element  204 . It should be noted that the first acoustic resistance element  310   a  may be omitted or included. The acoustic resistance value for the first acoustic resistance element  310   a  may be smaller than that of the second acoustic resistance element  310   b  and may be used to prevent foreign particles and moisture intrusion. 
       FIG.  8    depicts an example of simulated frequency response shapes of the microphone assembly  300  when configured as a uni-directional microphone as set forth in  FIG.  5    in accordance to one embodiment. In particular,  FIG.  8    plots the frequency response ratios in dB of the electrical outputs of the microphone assembly  300  to the acoustical inputs transmitted from various directions indicated by 1 to the first acoustic opening  237   a  versus the frequency. 
       FIG.  9    is a simulated plot that illustrates one example of a cardioid polar directivity or spatial filtering attributed to the microphone assembly  300  as described above in connection with  FIG.  5   . 
       FIG.  6    illustrates a detailed view of the microphone sub-assembly  204  in accordance to one embodiment. The transducer  222  includes a diaphragm  102  that is moveable and a backplate  104  that is immovable but acoustically transparent owning to the plurality number of through holes distributed in the backplate  104 . The transducer  222  and the ASIC  224  are positioned on the PCB base  226  and enclosed in the cavity  231  formed by the sub-casing  220 . As noted above, the PCB base  226  defines the first audio port  228   a , and the sub-casing  220  defines the port hole  228   b . The external audio signal can reach the underside of the diaphragm  102  through the first audio port  228   a  and the cavity  231  that is sealed between the transducer  222  and the PCB base  226 . Similarly, the external audio signal can reach the topside of the diaphragm  102  through the port hole  228   b  and the cavity  231  that is sealed between the transducer  222 , the sub-casing  220  and the PCB base  226 . In addition, the audio signal can also reach from one side of the diaphragm  102  to the other side through the path  108  defined by the diaphragm  102 . The diaphragm  102  is excited by the net acoustic pressure (i.e., the pressure difference between the two sides) that is applied. As noted previously, for the external audio signal, both the path  108  and the audio port  228   b  function as leak paths with a cut-off frequency value that is based on their relative size. This aspect may result in a rising frequency response shape of the microphone assembly  300 . Since it is generally more difficult to alter the size of the path  108  in addition to the overall size of the microphone sub-assembly  204 , the microphone assembly  300  discloses an approach to adjust the cutoff frequency of the rising response by adjusting the sealing or unsealing status (i.e., the opening area) of the port hole  228   b . For example, the path  108  as defined by the transducer  222  of the microphone sub-assembly  204  may be omitted or ignored if kept small so that, with the port hole  228   b  being sealed, the cutoff frequency of the rising response of the microphone sub-assembly  204  may be lower than 20 Hz. 
     In light of the foregoing, the assembly  300  provides a plurality of frequency response shapes or directivities while utilizing a single microphone sub-assembly  204  in which the overall size of the path  108  as defined by the diaphragm  102  is fixed. Based on the utilization status of the post  304 , the first acoustic resistance element  310   a , the second acoustic resistance element  310   b  and on the values for the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b , the microphone assembly  300  may behave as an omni-directional microphone with a flat frequency response (i.e., with a cutoff frequency lower than 20 Hz), an omni-directional microphone with a rising frequency response, or a uni-directional microphone with a cardioid polar directivity. 
       FIG.  10    illustrates another detailed view of the sub-casing  220  of the microphone sub-assembly  204  in accordance to one embodiment. A moveable cover  400  is placed over the port hole  228   b . In one example, the cover  400  may correspond to adhesive tape or other suitable material. Similar to the functionality of the post  304  in  FIG.  5   , the cover  400  may serve as a sealing mechanism and seal the port hole  228   b . In another example, the cover  400  may slide relative to the port hole  228   b . A first guide rail and a second guide rail may be axially spaced apart from one another and positioned on the top-side of the sub-casing  220 . The cover  400  may slide along the first guide rail and the second guide rail to provide varying levels of coverage of the port hole  228   b  ranging from 0% to 100%. The port hole  228   b  may be fully open when the cover  400  is moved away from the port hole  228   b . Conversely, the port hole  228   b  may be completely closed or covered by the cover  400 . The cover  400  may also be positioned at any number of positions relative to the port hole  228   b  such that the port hole  228   b  is uncovered, partially covered, or completely covered. 
     As noted above, the external audio signal reaches both the underside of the diaphragm  102  (e.g., via the first audio port  228   a  and the first acoustic path  230 ) and the top side of the diaphragm  102  (e.g., via the second acoustic opening  237   b  and the port hole  228   b ) through the second audio port  228   b . It is easier for low frequency sound waves with longer wavelengths to pass through a small opening. Thus, depending on the size of the port hole  228   b , positioning the cover  400  at different positions related to the port hole  228   b  may provide an omni-directional microphone with varying frequency responses without significantly altering the overall design of the microphone sub-assembly  204  and the microphone assembly  300 . For example, in the event the cover  400  is not implemented or is positioned to leave the second audio port  228   b  substantially open on the assembly  300 , the assembly  300  performs as an omni-directional microphone with a rising frequency response as illustrated in the waveform  402  of the plot depicted in  FIG.  7   . In the event the cover  400  fully covers the port hole  228   b , the microphone assembly  300  behaves as an omni-directional microphone with a flat frequency response as illustrated in the waveform  404  of the plot depicted in  FIG.  7   . In the event the cover  400  partially covers the second audio port  228   b , the microphone assembly  300  performs as an omni-directional microphone with a rising frequency response as illustrated in the waveform  403  of the plot depicted in  FIG.  7    whose cutoff frequency is between the waveforms  402  and  404 . 
     It is recognized that the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  may also be implemented on the microphone assembly  300  regardless of whether the cover  400  is provided. The first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  may be provided simply to prevent foreign particles and moisture intrusion. In the event the cover  400  is not utilized while the implementation of the first acoustic resistance element  310   a  and the second acoustic resistance element  310   b  is provided, as long as the acoustic resistance value of the second acoustic resistance element  310   b  satisfies the mathematical relationship described previously, the microphone assembly  300  behaves as a uni-directional microphone with a cardioid directivity pattern. 
       FIG.  11 A  depicts a top view of the sub-casing  220  and the port hole  228   b  of the microphone sub-assembly  204 .  FIG.  11 B  depicts another top view of the sub-casing  220  and the port hole  228   b . In this case, the port hole  228   b  may be formed as a plurality of holes  420  that take on a desired pattern. One or more of the plurality of holes  420  may be sealed (or covered) to provide a progressively adjustable cut-off frequency. For example, this aspect may enable the assembly  300  to provide an omni-directional microphone with varying frequency responses without significantly altering the overall design of the microphone sub-assembly  204 . It is recognized that the number of holes  420  may vary based on the desired criteria of a particular implementation. 
     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.