Patent Publication Number: US-9888307-B2

Title: Microphone assembly having an acoustic leak path

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 62/263,460, filed Dec. 4, 2015, and this application hereby incorporates herein by reference that provisional patent application in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments related to electronic devices having water resistant acoustic ports and a microphone assembly are disclosed. More particularly, embodiments related to electronic devices having a volume of air trapped between a water resistant membrane and a microphone of a microphone assembly are disclosed. 
     Background Information 
     An electronic device, such as a computer and/or mobile device, may be exposed to water, e.g., rain or water in a swimming pool. Water resistant acoustic ports, i.e., acoustic ports covered by water resistant membranes, are used to protect electronic components within such electronic devices from water ingress. In some cases, such membranes may not allow air exchange between an environment surrounding the electronic device and an enclosed volume within the electronic device. Sometimes, such membranes can exchange air, but at a rate that does not prevent air from being trapped within the enclosed volume, as in the case of rapid pressure changes, e.g., in an ascending elevator. More particularly, when a microphone is located in the enclosed volume behind the water resistant membrane, a volume of air may be trapped between the membrane and the microphone (including the interior volume of the microphone component). The trapped air may be vented to the enclosed volume within the electronic device to avoid negatively affecting an acoustic response of the membrane, e.g., to avoid distorting the natural deflection of the membrane when a sound is received through the water resistant acoustic port. 
     SUMMARY 
     Although venting air behind a water resistant membrane may avoid negative effects on the acoustic response of the membrane, it can cause other problems. More particularly, a vent used to equalize pressure between a trapped volume of air and an enclosed volume in an electronic device may also provide a path for sound to propagate from the enclosed volume into the trapped volume. Thus, sounds generated within the enclosed volume, e.g., audio generated by a speaker or electrical noise generated by capacitors, may propagate into the trapped volume and distort pick up by a microphone in the trapped volume. 
     In an embodiment, an electronic device includes a microphone assembly having a trapped volume of air between a water resistant membrane and a microphone. The microphone assembly also includes a barometric equalization element having a channel to vent air from the trapped volume to an encased space within the electronic device. The channel also defines an acoustic leak path that attenuates sound generated within the encased space to prevent the sound from entering the trapped volume of air and distorting the microphone pick up. 
     In an embodiment, an electronic device includes a casing having a casing wall separating a surrounding environment from an encased space, and the casing wall includes an acoustic port. A water resistant membrane may cover the acoustic port. In an embodiment, the water resistant membrane is impermeable to both water and air. A microphone may be located in the encased space behind the membrane. Thus, a trapped volume of air may be disposed between the membrane and an enclosure wall of the microphone. More particularly, the enclosure wall may be disposed between the trapped volume and the encased space. A barometric equalization element may be incorporated in the microphone assembly to equalize pressure within the trapped volume and the encased space. More particularly, the barometric equalization element may include a channel having an entrance port in fluid communication with the trapped volume and an exit port in fluid communication with the encased space. In an embodiment, the channel defines an acoustic leak path between the trapped volume and the encased space which leaks air between the volumes and attenuates predetermined wavelengths of sound generated within the encased space, e.g., acts essentially as a low-pass acoustic filter. 
     The barometric equalization element of the electronic device may include a channel having a geometry to attenuate predetermined sound wavelengths, e.g., wavelengths above a desired threshold. For example, the channel may include a length along the acoustic leak path between the entrance port and the exit port, and the channel length may be at least 20 times greater, e.g., at least 100 times greater, than a width of the exit port. The acoustic leak path through the channel may also include a nonlinear path, e.g., a tortuous or circuitous path, to allow the long channel to fit compactly within the electronic device. Accordingly, the channel may include several linear channel segments connected by one or more channel bends arranged along a tortuous path. Alternatively, or additionally, the channel may include one or more curvilinear channel segments arranged along a circuitous path. In an embodiment, the width of the exit port is less than a width of the channel at an intermediate point between the entrance port and the exit port. 
     The barometric equalization element may be fabricated and structured in numerous manners to provide a channel as described above. For example, the barometric equalization element may be disposed between the membrane and the microphone of the microphone assembly. In an embodiment, the membrane is mounted on the barometric equalization element such that an acoustic passage of the barometric equalization element is aligned with the acoustic port in the casing wall along an axis. The barometric equalization element may have an essentially planar profile, such that the channel extends along a plane oriented transverse to the axis between the trapped volume and the encased space. In an embodiment, the essentially planar barometric equalization element includes a first plate having a groove extending along the acoustic leak path and a second plate mounted on the first plate and covering the groove such that the channel is defined between the second plate and a groove surface of the groove. In another embodiment, the essentially planar barometric equalization element includes an open-cell foam layer having a plurality of interconnected pores within a material matrix such that the channel is defined through the interconnected pores. The barometric equalization element may be constructed from preformed and readily available materials. For example, the barometric equalization element may include a plurality of tubes, e.g., hypotube components that are used in medical needle manufacturing. The tubes may be interconnected by one or more chambers such that the channel extends along the acoustic leak path through respective inner lumens of the tubes and respective cavities of the chambers. 
     In an embodiment, an electronic device includes several barometric equalization elements arranged to attenuate sound having wavelengths above and below the predetermined threshold. More particularly, the trapped volume may be disposed between the membrane and the enclosure wall of the microphone, and a diaphragm of the microphone may be located in the trapped volume to divide the trapped volume into a front compartment and a rear compartment. That is, the diaphragm may include a front face facing the front compartment of the trapped volume and a rear face facing the rear compartment of the trapped volume. A first barometric equalization element may include a first channel having a first entrance port in fluid communication with the front compartment, and a second barometric equalization element may include a second channel having a second entrance port in fluid communication with the rear compartment. Furthermore, the first channel may define a first acoustic leak path having a nonlinear path, e.g., a tortuous path, between the front compartment and the encased space, and the second channel may define a second acoustic leak path between the rear compartment and the encased space. Thus, in addition to attenuating higher frequencies, low frequency sound entering the trapped volume through the barometric equalization elements may arrive at the front face and the rear face at the same time to cause a noise cancellation effect. Accordingly, distortion of the microphone pick up may be mitigated for both higher and lower frequencies. In addition to mitigating distortion across a wide range of frequencies, preventing leakage of speaker signal into a microphone can realize other benefits. For example, in the case of a telephone call, having a speaker signal leak into a microphone may either degrade the quality of the phone call or prevent the ability to transmit on the microphone and receive on the speaker at the same time. Thus, the embodiments described below may mitigate call degradation and/or improve simultaneous transmit/receive of the microphone and speaker. 
     The barometric equalization elements may be fabricated and structured in numerous manners to provide channels as described above. For example, the first channel may include a first exit port spaced apart from the first entrance port by a first channel length, and the second channel may include a second exit port spaced apart from the second entrance port by a second channel length equal to the first channel length. Furthermore, the exit ports may be located near each other to receive sound from the encased space at essentially the same location. For example, the first entrance port may be spaced apart from the second entrance port by a first distance, and the first exit port may be spaced apart from the second exit port by a second distance that is less than the first distance. More particularly, the first exit port may be adjacent to the second exit port. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial view of an electronic device in accordance with an embodiment. 
         FIG. 2  is a schematic view of an electronic device in accordance with an embodiment. 
         FIG. 3  is a pictorial view of an electronic device in accordance with an embodiment. 
         FIG. 4  is a sectional view of a microphone assembly of an electronic device in accordance with an embodiment. 
         FIG. 5  is an exploded view of a barometric equalization element in accordance with an embodiment. 
         FIG. 6  is a sectional view, taken about line A-A of  FIG. 5 , of a first plate of a barometric equalization element in accordance with an embodiment. 
         FIGS. 7A-7B  are detail views, taken from Detail A of  FIG. 6 , of a channel of a barometric equalization element in accordance with an embodiment. 
         FIG. 8  is a sectional view of a barometric equalization element in accordance with an embodiment. 
         FIG. 9  is a detail view of an exit port of a barometric equalization element in accordance with an embodiment. 
         FIG. 10  is a perspective view of a barometric equalization element in accordance with an embodiment. 
         FIG. 11  is a sectional view, taken about line B-B of  FIG. 10 , of a barometric equalization element in accordance with an embodiment. 
         FIG. 12  is a pictorial sectional view of a microphone assembly of an electronic device in accordance with an embodiment. 
         FIG. 13  is a sectional view of a microphone assembly of an electronic device in accordance with an embodiment. 
         FIG. 14  is a pictorial view of a barometric equalization element in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe electronic devices and/or electroacoustic transducer components having barometric equalization elements that vent air and attenuate sound having wavelengths in a predetermined range. Some embodiments are described with specific regard to integration within mobile devices such as mobile phones. The embodiments are not so limited, however, and certain embodiments may also be applicable to other uses. For example, a barometric equalization element may be incorporated into other devices and apparatuses, including desktop computers, laptop computers, tablet computers, wearable computers, wristwatch devices, or motor vehicles, to name only a few possible applications. 
     In various embodiments, description is made with reference to the figures. Certain embodiments, however, may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions, and processes, in order to provide a thorough understanding of the embodiments. In other instances, well-known processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the description. Reference throughout this specification to “one embodiment,” “an embodiment,” or the like, means that a particular feature, structure, configuration, or characteristic described is included in at least one embodiment. Thus, the appearance of the phrase “one embodiment,” “an embodiment,” or the like, in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The use of relative terms throughout the description, such as “in front of” and “behind” may denote a relative position or direction. For example, an acoustic membrane may be described as being “in front of” a barometric equalization element and “behind” an acoustic port in a casing, opposite from a surrounding environment. Nonetheless, such terms are not intended to limit the use of an acoustic membrane to a specific configuration described in the various embodiments below. For example, an acoustic membrane may be located on the same side of the acoustic port as the surrounding environment. 
     In an aspect, an electronic device includes a microphone assembly having a barometric equalization element that vents a trapped volume of air from between a water resistant membrane and a microphone to an encased space outside of the microphone assembly. In addition to venting the air to equalize pressure within the device, the barometric equalization element also includes a narrow and long channel that acts as an acoustic low-pass filter. Thus, air can equalize without admitting a predetermined range of audible frequencies to the microphone pick up. In an embodiment, the channel includes an acoustic leak path, e.g., a nonlinear leak path, such as a tortuous path like a maze, between the trapped volume of air and the encased space. The acoustic leak path may be integrally formed in one or more of the microphone assembly components, e.g., in a layer of the barometric equalization element and/or in a printed circuit board (PCB) used to mount the microphone. 
     Referring to  FIG. 1 , a pictorial view of an electronic device is shown in accordance with an embodiment. An electronic device  100  may be a smartphone device. Alternatively, it could be any other portable or stationary device or apparatus, such as a laptop computer, a tablet computer, a wearable computer, a wristwatch device, etc. Electronic device  100  may include various capabilities to allow the user to access features involving, for example, calls, voicemail, music, e-mail, internet browsing, scheduling, or photos. Electronic device  100  may also include hardware to facilitate such capabilities. For example, a casing  102  may contain a microphone  104  to pick up the voice of a user during a call, and an audio speaker  106 , e.g., a micro speaker, to deliver a far-end voice to the near-end user during the call. Speaker  106  may also emit sounds associated with music files played by a music player application running on electronic device  100 . A display  108  may present the user with a graphical user interface to allow the user to interact with electronic device  100  and/or applications running on electronic device  100 . Other conventional features are not shown but may of course be included in electronic device  100 . 
     Referring to  FIG. 2 , a schematic view of an electronic device is shown in accordance with an embodiment. As described above, electronic device  100  may be one of several types of portable or stationary devices or apparatuses with circuitry suited to specific functionality. Accordingly, the diagrammed circuitry is provided by way of example and not limitation. Electronic device  100  may include one or more processors  202  to execute instructions to carry out the different functions and capabilities described above. Instructions executed by processor(s)  202  of electronic device  100  may be retrieved from a local memory  204 , and may be in the form of an operating system program having device drivers, as well as one or more application programs that run on top of the operating system. The instructions may cause electronic device  100  to perform the different functions introduced above, e.g., phone and/or music play back functions. To perform such functions, processor(s)  202  may directly or indirectly implement control loops and receive input signals from and/or provide output signals to other electronic components, such as microphone  104  or speaker  106 . 
     Referring to  FIG. 3 , a pictorial view of an electronic device is shown in accordance with an embodiment. Casing  102  may provide a shell within which the various internal components of electronic device  100  are located. More particularly, casing  102  may include a casing wall  302  having a thickness between an outer surface and an inner surface. Thus, the inner surface of casing wall  302  may surround an encased space  304  to receive the various components of electronic device  100 . Furthermore, the outer surface of casing wall  302  may face a surrounding environment  306  and separate surrounding environment  306  from encased space  304 . 
     Electronic device  100  may include one or more ports through casing wall  302  to place the encased space  304  in acoustic communication with surrounding environment  306 . For example, speaker  106  may be mounted on the inner surface of casing wall  302  to generate and emit sound outward into surrounding environment  306 . Thus, a speaker port  308  may be located in and through casing wall  302  to provide an acoustic pathway between speaker  106  and surrounding environment  306 . Similarly, a vent port  310  may be located in and through casing wall  302  to provide a barometric equalization path between encased space  304  and surrounding environment  306 . One or more of speaker port  308  and vent port  310  may be covered by a barrier to prevent the ingress of particles and/or water into encased space  304  from surrounding environment  306 . For example, electronic device  100  may include a vent cover  312  covering vent port  310 , and vent cover  312  may include a mesh or other barrier material to repel incoming particles and/or water. 
     In addition to having ports for communicating air and/or sound outward from encased space  304  into surrounding environment  306 , electronic device  100  may include an acoustic port  314  in and through casing wall  302  to receive sounds from surrounding environment  306 . Acoustic port  314  may have a cross-sectional area large enough to admit particulate and/or water from surrounding environment  306 , and thus, a water resistant membrane may be used to waterproof electronic device  100 . More particularly, a membrane  316  having high water resistance may be used to cover acoustic port  314 . Membrane  316  may exhibit high water resistance as a result of having a porosity that resists water ingress. More particularly, membrane  316  may include pores having cross-sectional dimensions that are smaller than a dimension of water molecules. In an embodiment, membrane  316  is also impermeable to air. For example, leakage through membrane  316  may only result from molecular diffusivity. Accordingly, any pores within membrane  316  may have a mean cross-sectional dimension less than the mean free path of air at ambient pressure, e.g., less than 50 nm. Thus, air impermeable channels may inhibit the passage of air across acoustic membrane  316 , and reduce the likelihood of gas and water exchange between surrounding environment  306  and encased space  304 . 
     Membrane  316  may be acoustically transparent in addition to being air impermeable. More particularly, membrane  316  may be a reactive membrane that deflects when sound from surrounding environment  306  impinges upon it, and transmits the sound to a space behind membrane  316 . In an embodiment, the transmitted sound is directed toward microphone  104  mounted within encased space  304 . Thus, microphone  104  and membrane  316  may act in combination to provide a microphone assembly to pick up sound for electronic device  100 . That is, the microphone assembly may be mounted on casing  102  behind acoustic port  314  to pick up sound from surrounding environment  306 . 
     Microphone  104  may include an enclosure wall  318  to mount on casing  102  and to separate encased space  304  from a trapped volume  320  of air. More particularly, trapped volume  320  may be a sub-volume of encased space  304  that is disposed between membrane  316  and the enclosure wall  318 . Accordingly, enclosure wall  318  may be used as a reference structure situated between trapped volume  320  and encased space  304 . Enclosure wall  318  may not merely be referential, however. That is, enclosure wall  318  may be a rigid barrier and as membrane  316  reacts to incoming sound waves, air within trapped volume  320  may compress to generate a pressure difference between trapped volume  320  and encased space  304  on the other side of enclosure wall  318 . Accordingly, trapped volume  320  may be vented to encased space  304  to prevent a pressure buildup within trapped volume  320  from affecting an acoustic response of membrane  316 . 
     Equalization of pressure within trapped volume  320  and encased space  304  may be achieved using a barometric equalization element  326 . As described below, barometric equalization element  326  may be incorporated in the microphone assembly and may include a passage for leaking air between trapped volume  320  and encased space  304  to equalize the pressures therein. The passage may also provide a route for sound to enter trapped volume  320  from encased space  304 . 
     Electronic device  100  may house noisy components. As described above, speaker  106  may generate sound in encased space  304  and the sound may propagate through barometric equalization element  326  into trapped volume  320  where it could be picked up by microphone  104 . Similarly, electronic device  100  may include one or more electronic components  322  mounted on a printed circuit board (PCB)  324  as is known in the art. For example, electronic component  322  may be a capacitor that generates electronic noise, and the noise may propagate through barometric equalization element  326  to trapped volume  320 . Thus, as described further below, the passage of barometric equalization element  326  may be designed to provide an acoustic leak path that vents air and also to resist passage of certain wavelengths of sound, i.e., to attenuate frequencies that could negatively affect the microphone pick up. The acoustic leak path may include one or more curves, bends, undulations, etc., between trapped volume  320  and encased space  304 . Thus, the acoustic leak path may be an acoustic leak path  328  having a nonlinear path. Furthermore, barometric equalization element  326  may include a passage that is long and narrow, since the nonlinear shape of acoustic leak path  328  can allow a long path to be fit within a compact area. 
     Referring to  FIG. 4 , a sectional view of a microphone assembly of an electronic device  100  is shown in accordance with an embodiment. The microphone assembly may include microphone  104  mounted on barometric equalization element  326 , and barometric equalization element  326  may be mounted on membrane  316 . Thus, barometric equalization element  326  may be disposed between membrane  316  and microphone  104 . An attachment between these components may be direct or indirect. For example, a rear surface of membrane  316  may interface directly with a front surface of barometric equalization element  326 . Alternatively, one or more additional components may be located between these components. For example, microphone  104  may be a MEMS microphone, and thus, enclosure wall  318  of microphone  104  may be mounted on, and electrically connected with, a transducer PCB  402 . Transducer PCB  402  may receive electrical audio signals from microphone  104 , e.g., when a diaphragm  404  of microphone  104  is moved by pressure variations within trapped volume  320 . Accordingly, transducer PCB  402  may be located between barometric equalization element  326  and microphone  104 . 
     In an embodiment, the microphone assembly may be mounted on the inner surface of casing wall  302  and located to receive sound through acoustic port  314  to actuate diaphragm  404  of microphone  104 . More particularly, barometric equalization element  326  may include one or more acoustic passage  406  axially aligned with acoustic port  314  such that sound propagating through acoustic port  314  will cause deflection of membrane  316  over acoustic passage  406 . Barometric equalization element  326  may include several acoustic passages  406  located between a peripheral region behind casing wall  302  and one or more support ribs  408  behind acoustic port  314 . Support ribs  408  may provide a stiffening effect on membrane  316  to limit the deflection of membrane  316 , e.g., caused by sound and/or impinging water. This support can prevent damage to membrane  316  caused by deflection beyond a failure point. 
     Barometric equalization element  326  may include a channel  410  to place trapped volume  320  in fluid communication with encased space  304 . More particularly, channel  410  may include an entrance port  412  in fluid communication with trapped volume  320 , and an exit port  414  at an opposite end of channel  410  in fluid communication with encased space  304 . As used here, the terms “entrance” and “exit” are not intended to be limiting since it is possible for air to pass through channel  410  both from trapped volume  320  to encased space  304 , and from encased space  304  to trapped volume  320 . More particularly, channel  410  is not necessarily directional. 
     As described further below, barometric equalization element  326  may include an essentially planar shape, e.g., a thin plate-like shape. In an embodiment, acoustic passage  406  of barometric equalization element  326  is axially aligned with acoustic port  314  (e.g., along an axis running in a left-to-right axial direction in  FIG. 4 ) and extends through a thickness of barometric equalization element  326 . Thus, a planar surface of barometric equalization element  326 , e.g., a front or rear surface of the element, may be parallel to diaphragm  404 . Furthermore, channel  410  may extend along a plane oriented transverse to the axis that acoustic port  314  and acoustic passage  406  are aligned on. More particularly, barometric equalization element  326  may include a passage wall ( FIG. 10 ) extending in a thickness direction between a front and rear surface of barometric equalization element  326 . The passage wall may surround acoustic passage  406 , and face support ribs  408 . Likewise, barometric equalization element  326  may include an outer wall laterally outward from the passage wall and surrounding the body of barometric equalization element  326 . Entrance port  412  may be located on the passage wall and exit port  414  may be located on the outer wall, and thus, channel  410  may extend from entrance port  412  to exit port  414  in a transverse direction. Channel  410 , which may be long and narrow, may include a variety of geometries described below. 
     Referring to  FIG. 5 , an exploded view of a barometric equalization element is shown in accordance with an embodiment. Barometric equalization element  326  may include a first plate  502  having an upper planar surface separated from a bottom planar surface by a thickness. Acoustic passages  406  may be formed in the thickness direction through a central region of first plate  502  and separated from each other by support ribs  408 . Thus, channel  410  may extend along the upper surface from entrance port  412  at acoustic passage  406  to exit port  414  along an outer wall of first plate  502 . Accordingly, channel  410  may include a channel length, i.e., the length of acoustic leak path  328  through channel  410  between entrance port  412  and exit port  414 . 
     In an embodiment, channel  410  is formed by a combination of first plate  502  and a second plate  504 . For example, a groove  506  may be formed in the upper surface of first plate  502 , and second plate  504  may be mounted on first plate  502  to cover groove  506 . Accordingly, channel  410  may be defined between a surface of groove  506  recessed below the upper surface of first plate  502  and a lower surface of second plate  504 . For example, the lower surface of second plate  504  may be flat and extend over groove  506  to form a cross-sectional area of channel  410 . Alternatively, a corresponding groove  506  may be formed in the lower surface of second plate  504  to mate with groove  506  in first plate  502  and form a cross-sectional area of channel  410 . By way of example, when both grooves  506  are semi-circular the combination of grooves  506  forms channel  410  having a circular cross-sectional area. 
     An essentially planar barometric equalization element  326  may be formed in numerous ways. In an embodiment, first plate  502  and second plate  504  are fabricated from a sheet of material, e.g., stainless steel sheet metal may be cut into the desired plate shape. Subsequently, one or both of first plate  502  and second plate  504  may be masked to prepare the plate blank for an etching process to form groove  506  on an upper or lower surface. Alternatively, groove  506  may be formed in one or both of the plates using known micromachining processes, e.g., using an electrical discharge machining process. Acoustic passage  406  may be formed though the plates using known machining processes, such as laser cutting, or via chemical etching processes. Acoustic passage  406  may have an identical profile in first plate  502  and second plate  504  ( FIG. 5 ) or one plate may include acoustic passages  406  that are framed by support ribs  408  and another plate may include an acoustic passage  406  without support ribs  408 , e.g., a circular hole. After fabricating grooves  506  within the plates, the corresponding sides of the plates, i.e., the upper surface of first plate  502  and/or the lower surface of second plate  504  may be tinned and pressed together to join the plates. That is, first plate  502  having groove  506  may be attached to second plate  504  and the combined surfaces of the plates may form channel  410 . 
     Still referring to  FIG. 5 , acoustic leak path  328  may include a tortuous path  508  between entrance port  412  and exit port  414 . A tortuous path  508 , i.e., a path with several bends, curves, switchbacks, etc., allows for a maximum channel length to be achieved within a given area. As a straightforward example, when channel  410  extends along a straight path from entrance port  412  at acoustic passage  406  to exit port  414  at an outer wall of first plate  502 , the channel length would be approximately half of a planar length of first plate  502 . When channel  410  extends along tortuous path  508 , however, the channel length can be many times longer than the planar length of first plate  502 . 
     In an embodiment, channel  410  includes several linear channel segments  510  connected by one or more channel bends  512 . More particularly, linear channel segment  510  may extend along a straight segment of tortuous path  508 , and channel bends  512  may extend along an angular or curved segment of tortuous path  508  that connects two straight segments. Accordingly, tortuous path  508  may be a serpentine path (in the case of channel bends  512  extending along radial curves), a zig-zag path (in the case of channel bends  512  extending along angular bends), or another undulating or meandering path having several reversals of direction between entrance port  412  and exit port  414 . Thus, tortuous path  508  is longer than a straight line between the ports. 
     Referring to  FIG. 6 , a sectional view, taken about line A-A of  FIG. 5 , of a first plate of a barometric equalization element is shown in accordance with an embodiment. The channel length at which entrance port  412  is spaced apart from exit port  414  along acoustic leak path  328 , may have a predetermined relationship relative to a cross-sectional dimension of channel  410 . For example, the channel length may be substantially greater than a dimension measured between opposing sides of a groove surface  602  of groove  506  formed in first plate  502 . In an embodiment, the channel length is at least 20 times greater than a width  604  across channel  410  between opposing groove surfaces  602 . The channel length may be at least 100 times greater than width  604 , e.g., at least 1000 times greater than width  604 . In addition to having a relationship between the channel length and a cross-sectional dimension of channel  410 , the cross-sectional dimension may be constrained to be less than a predetermined dimension. For example, width  604  may be less than 50 micron, e.g., less than 40 micron. By way of example, rectangular channel  410  shown in  FIG. 6  may have width  604  and/or a height of 35 microns. The channel length between entrance port  412  and exit port  414  along tortuous path  508 , however, may be 1000 times width  604 , i.e., 35 millimeters in this example. 
     As described below, a cross-section of channel  410  may be uniform or non-uniform. For example, channel  410  may have a same width  604  at entrance port  412 , exit port  414 , and every point between the ports. Alternatively, channel  410  may have a different width  604  at entrance port  412 , exit port  414 , and/or one or more intermediate points between the ports. In an embodiment, the relationship between the channel length and the channel  410  cross-sectional dimension may be specific to exit port  414 . For example, the channel length may be at least 20 times, e.g., 100 to upwards of 1000 times, greater than width  604  located at exit port  414 . 
     Referring to  FIG. 7A , a detail view, taken from Detail A of  FIG. 6 , of a channel of a barometric equalization element is shown in accordance with an embodiment. The cross-sectional area of channel  410  may vary. For example, rather than being composed of flat surfaces, groove  506  formed in first plate  502  may be curved, e.g., a cross-section of groove surface  602  may include a semi-circle  702 , such that a cross-sectional dimension across channel  410  is a diameter, i.e., twice a radius  704  of the semi-circle  702 . Thus, channel  410  may have a semi-circular or an elliptical cross-section and the channel length may be at least 20 times greater than the diameter. 
     Referring to  FIG. 7B , a detail view, taken from Detail A of  FIG. 6 , of a channel of a barometric equalization element is shown in accordance with another embodiment. In an embodiment, groove  506  may include a v-groove  706 , such that a cross-sectional dimension across channel  410  is a height  708 . Thus, the channel length may be at least 20 times greater than the height  708 . 
     Referring to  FIG. 8 , a sectional view of a barometric equalization element is shown in accordance with an embodiment. In an embodiment, channel  410  defines acoustic leak path  328  that includes a circuitous path  802 . A circuitous path  802  may be a path that extends in a curved manner along a winding course without reversing direction. Thus, channel  410  may include a curvilinear channel segment  804  extending from entrance port  412  at acoustic passage  406  and winding outward around acoustic passage  406  to exit port  414  at the outer wall of barometric equalization element  326 . Channel  410  may include a single curvilinear channel segment  804  arranged in a spiral fashion between the ports, or several curvilinear channel segments  804  may be interconnected, e.g., by bends or straight segments as the channel  410  extends along the circuitous path  802 . 
     As described above, a cross-sectional dimension of channel  410  may vary along acoustic leak path  328 . For example, exit port  414  may include a diameter  806  that differs from a channel width  808  at a location of channel  410  between entrance port  412  and exit port  414 . In an embodiment, variations in cross-sectional dimensions of channel  410  may occur gradually, e.g., channel  410  may taper smoothly from the location having channel width  808  to exit port  414  having diameter  806 . 
     Still referring to  FIG. 8 , variations in the cross-sectional dimension of channel  410  may also be used to create one or more cavities  820  separated by one or more restrictions  822 . Cavities  820  may be regions along channel  410  having a first, larger dimension, and restrictions  822  may be regions along channel  410  having a second, smaller dimension. In the case of channel  410  forming a circular lumen through barometric equalization element  326 , cavities  820  may have larger diameters than restrictions  822 . Furthermore, diameters may vary from cavity to cavity, and from restriction to restriction. Accordingly, channel  410  may include a continuous, smooth wall between entrance port  412  and exit port  414  that transitions from larger to smaller dimensions to define cavities  820  and restrictions  822  with various spatial volumes. Channel  410  having a varying diameter may be fabricated using known shaping processes, e.g., chemical etching. Accordingly, cavities  820  may be placed along channel  410  to tune channel  410  in a predetermined manner to create a desired frequency response. For example, cavities  820  may act as springs to lower the cut-off frequency of the low-pass filter. 
     Referring to  FIG. 9 , a detail view of an exit port of a barometric equalization element is shown in accordance with an embodiment. Changes in cross-sectional dimensions of channel  410  at different locations may occur abruptly. In an embodiment, a width of exit port  414  is less than channel width  808  of channel  410  at an intermediate point between entrance port  412  and exit port  414 . For example, channel  410  may have channel width  808  at a location adjacent to exit port  414 , and channel  410  may be shaped to have an abrupt restriction in cross-sectional area to reduce channel width  808  to diameter  806  at exit port  414 . Thus, a portion of channel  410 , i.e., the portion having a larger channel width  808 , may include a cross-sectional dimension that is greater than 1/20 of the channel length and another portion of channel  410 , i.e., exit port  414 , may include a cross-sectional dimension that is less than 1/20 of the channel length. 
     Referring to  FIG. 10 , a perspective view of a barometric equalization element is shown in accordance with an embodiment. Barometric equalization element  326  may include an essentially planar shape having one or more open-cell foam layer  1002 . Barometric equalization element  326  may include acoustic passage  406  located in a central region to be aligned with acoustic port  314 . Acoustic passage  406  is shown without support ribs  408 , however, support ribs  408  may be incorporated in barometric equalization element  326  as described above. 
     In an embodiment, open-cell foam layer  1002  may be laminated with a top layer  1004  and a bottom layer  1006  that sandwich open-cell foam layer  1002  to form a plate-like structure. As such, top layer  1004  may include a flat upper surface that may be mounted on membrane  316 , and bottom layer  1006  may include a flat bottom surface that may be mounted on microphone  104 . A material of top layer  1004  and bottom layer  1006  may be selected to facilitate such mounting and/or attachment between the microphone assembly components. Top layer  1004  and bottom layer  1006  may also be formed from a material that is impermeable to air. Air within acoustic passage  406  may therefore be vented through open-cell foam layer  1002  between top layer  1004  and bottom layer  1006 . More particularly, air may be vented from a passage wall  1008  of open-cell foam layer  1002  facing acoustic passage  406  to an outer foam wall  1010  laterally outward from passage wall  1008  (and surrounding open-cell foam layer  1002 ) without passing through top layer  1004  or bottom layer  1006 . Thus, air may vent from trapped volume  320  through channel  410  formed in open-cell foam layer  1002  to encased space  304 . 
     Referring to  FIG. 11 , a sectional view, taken about line B-B of  FIG. 10 , of a barometric equalization element is shown in accordance with an embodiment. Channel  410  may be defined within interconnected pores  1102  of open-cell foam layer  1002 . More particularly, open-cell foam layer  1002  may include an open-cell foam that has a matrix  1104  of foam material surrounding or encapsulating several interconnected pores  1102 . Such structure is known in the art for open-cell foams. A porosity of matrix  1104  may be varied during manufacturing. For example, an amount of compression of the foam material may be controlled to fabricate open-cell foam layer  1002  having interconnected pores  1102  of a predetermined average diameter. The average diameter of interconnected pores  1102  may be controlled to be less than a predetermined threshold, e.g., less than 40 micron. Similarly, interconnected pores  1102  may provide a continuous channel  410  of air through open-cell foam layer  1002  and a length of the continuous channel  410  may have a predetermined ratio to the average diameter of interconnected pores  1102 . For example, the length of the continuous channel  410  may be at least 20 times greater than the average diameter of interconnected pores  1102 . 
     Referring to  FIG. 12 , a pictorial sectional view of a microphone assembly of an electronic device is shown in accordance with an embodiment. A microphone assembly of electronic device  100  may include a first barometric equalization element  1202 , which can have a structure similar to barometric equalization element  326  discussed above. Thus, first barometric equalization element  1202  may vent air from trapped volume  320  and may attenuate audio frequencies above a predetermined threshold that could otherwise enter trapped volume  320  from encased space  304  and distort microphone pick up. In an embodiment, a second barometric equalization element  1204  is incorporated in the microphone assembly to provide a noise cancellation effect that prevents audio frequencies below the predetermined threshold from distorting microphone pick up when they enter trapped volume  320  from encased space  304 . 
     The microphone assembly that reduces an impact of audio frequencies above and below the predetermined threshold on microphone pick up may include several components that are similar to those described above. For example, electronic device  100  may include a casing wall  302  having acoustic port  314 , and the microphone assembly may include microphone  104  and membrane  316 . The air impermeable membrane  316  may be mounted on casing wall  302  to cover and waterproof acoustic port  314 . Furthermore, as discussed above, microphone  104  may include diaphragm  404  in trapped volume  320  that moves according to pressure variations to pick up sound. Thus, diaphragm  404  may divide trapped volume  320  into several compartments. That is, a portion of trapped volume  320  in front of diaphragm  404  and between membrane  316  and diaphragm  404  may be referred to as a front compartment  1206 , and a portion of trapped volume  320  behind diaphragm  404  and between diaphragm  404  and enclosure wall  318  may be referred to as a rear compartment  1208 . 
     In an embodiment, first barometric equalization element  1202  and second barometric equalization element  1204  are in fluid communication with respective compartments of trapped volume  320 . For example, first barometric equalization element  1202  may include a first channel having a first entrance port  1210  in fluid communication with front compartment  1206 . Similarly, second barometric equalization elements  1204  may include a second channel having a second entrance port  1212  in fluid communication with rear compartment  1208 . As described above, a first channel may extend along a first acoustic leak path  1214 , e.g., a first nonlinear acoustic leak path, between front compartment  1206  and encased space  304 , and the second channel may extend along a second acoustic leak path  1216 , e.g., a second nonlinear acoustic leak path, between the rear compartment  1208  and encased space  304 . At least one of first acoustic leak path  1214  or second acoustic leak path  1216  may also include tortuous path  508  and/or circuitous path  802 , as described above, between a respective compartment of trapped volume  320  and encased space  304 . Thus, one or more of first barometric equalization element  1202  or second barometric equalization element  1204  may vent air and attenuate audio frequencies above a predetermined threshold. 
     First barometric equalization element  1202  and second barometric equalization element  1204  may each provide pathways for low-frequency sound to propagate from encased space  304  into trapped volume  320 . In an embodiment, to reduce the likelihood that such sound propagation may negatively impact microphone pick up, an acoustic resistance of first barometric equalization element  1202  and second barometric equalization element  1204  may be controlled such that sound waves of the same frequency that enter the first channel and the second channel at the same time also exit and impinge upon diaphragm  404  at the same time, albeit on opposite sides of diaphragm  404 . 
     Acoustic resistance of the barometric equalization elements  326  may be matched in a number of ways. In an embodiment, the first channel includes a first exit port  1218  spaced apart from first entrance port  1210  by a first channel length, and the second channel includes a second exit port  1220  spaced apart from the second entrance port  1212  by a second channel length. The first channel length and the second channel length may be equal to provide an equivalent path length for sound entering each channel to reach diaphragm  404 . Similarly, a cross-sectional dimension of the first and second channels may be equal at corresponding locations along the acoustic leak paths  1214 ,  1216 . Also, the nonlinear acoustic leak paths of the first and second channels may be identical and/or similar, e.g., mirror images of each other, to make the respective acoustic resistances of the barometric equalization elements  326  the same. 
     The noise cancellation effect may also be facilitated by locating first exit port  1218  adjacent to second exit port  1220  such that essentially the same sound waves from encased space  304  enter both barometric equalization elements at the same location and/or time. For example, a distance between first exit port  1218  and second exit port  1220  may be less than a predetermined threshold. In an embodiment, the threshold is a distance that provides better than ¼ wavelength spacing below 15 kHz. Thus, the first exit port  1218  may be separated from second exit port  1220  by a distance less than 6 mm, e.g., less than 1 mm. 
     In an embodiment, the exit ports  1218 ,  1220  of the barometric equalization elements  1202 ,  1204  may be nearer to each other than the entrance ports  1210 ,  1212  of the elements. Therefore, in addition to receiving sounds from encased space  304  at approximately the same location, the barometric equalization elements may emit the sounds into the respective compartments at different locations that direct the sounds toward diaphragm  404 . Accordingly, the first entrance port  1210  may be spaced apart from the second entrance port  1212  by a first distance  1222 , and the first exit port  1218  may be spaced apart from the second exit port  1220  by a smaller second distance  1224 . That is, first distance  1222  may be greater than second distance  1224 . 
     Referring to  FIG. 13 , a sectional view of a microphone assembly of an electronic device is shown in accordance with an embodiment. The microphone assembly, having first barometric equalization element  1202  and second barometric equalization element  1204  to provide a noise cancellation effect, may also include diaphragm  404  separating front compartment  1206  from rear compartment  1208 . Diaphragm  404  may extend laterally between internal sides of enclosure wall  318  and be suspended within trapped volume  320  by a surround element. Furthermore, diaphragm  404  may have a thickness, and thus, a front face  1302  of diaphragm  404  may face front compartment  1206 , and a rear face  1304  of diaphragm  404  may face rear compartment  1208 . First barometric equalization element  1202  and second barometric equalization element  1204  may incorporate any of the elements features described above. For example, first barometric equalization element  1202  may be located behind membrane  316  and may include support ribs  408  to alter an effective stiffness of membrane  316  and limit membrane movement. Second barometric equalization element  1204 , however, may be mounted on enclosure wall  318  behind diaphragm  404 , and thus, may not include support ribs  408  or acoustic passage  406 . Each barometric equalization element may nonetheless include a respective channel  410 , i.e., the first channel and the second channel. As shown, the exit ports of the channels  410  may be separated by a same distance as the entrance ports of the channels  410 . Nonetheless, a length of the channels  410  may be the same to provide an equivalent sound path length for noise cancellation. 
     Several embodiments of barometric equalization element  326  structures are described above ( FIG. 5  and  FIG. 10 ). One skilled in the art and equipped with this description would be able to derive other embodiments within the scope of the invention. For example, channel  410  may be incorporated in other components of the microphone assembly to vent air between trapped volume  320  and encased space  304 . By way of example, groove  506  may be formed in a front surface of transducer PCB  402  shown in  FIG. 4 , and transducer PCB  402  may be mounted directly on membrane  316  to allow for the elimination of a separate barometric equalization element plate-like component. More particularly, air may be vented through channel  410  formed between a rear surface of membrane  316  and a groove surface  602  formed in transducer PCB  402 . The channel  410  may also attenuate sound as described below. 
     Referring to  FIG. 14 , a pictorial view of a barometric equalization element is shown in accordance with an embodiment. Barometric equalization element  326  may be fabricated using readily available and low cost components. For example, barometric equalization element  326  may include several preformed tubes  1402 , e.g., stainless steel hypotubes that are mass-produced for the medical industry, interconnected by one or more chambers  1404 . Each tube  1402  may include an inner lumen  1406  and, since tube  1402  may be straight, inner lumen  1406  may provide a linear channel segment  510  of channel  410 . Each chamber  1404  may include a cavity  1408  and, since cavity  1408  may provide an air path between non-coaxial inner lumens  1406 , cavity  1408  may provide a channel bend  512  of channel  410 . Thus, a series of parallel tubes  1402  may be interconnected by several chambers  1404  to provide a tortuous air path from entrance port  412  to exit port  414  that includes narrow segments (inner lumens  1406 ) and wide segments (cavities  1408 ). More particularly, channel  410  may extend along acoustic leak path  328  through inner lumens  1406  of tubes  1402  and cavities  1408  of chambers  1404 . 
     In an embodiment, barometric equalization element  326  having tubes  1402  and chambers  1404  may be formed using an injection molding process. For example, preformed tubes  1402  may be loaded into an injection mold as mold inserts, and aligned with chamber  1404  inserts. A body  1410 , e.g., a polymer block, may be injection molded around the inserts to fix the components relative to each other and to hermetically seal the joints at which the tubes  1402  and chambers  1404  meet. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.