Patent Publication Number: US-11665470-B2

Title: Audio speaker with back volume containing adsorptive material

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 16/883,802, filed May 26, 2020, which is a continuation of U.S. patent application Ser. No. 16/409,682, filed on May 10, 2019 (now issued as U.S. Pat. No. 10,694,284), which is a continuation of U.S. patent application Ser. No. 14/715,407, filed on May 18, 2015 (now issued as U.S. Pat. No. 10,349,167), and these applications are specifically incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Embodiments related to an audio speaker having a speaker housing surrounding a back volume that is divided into several cavities by a permeable partition, are disclosed. More particularly, an embodiment related to a multi-cavity back volume within a speaker housing, the multi-cavity back volume having a cavity defined between the speaker housing and a permeable partition, and that may be filled with an adsorptive material to adsorb gas during sound generation, is disclosed. 
     BACKGROUND INFORMATION 
     A portable consumer electronics device, such as a mobile phone, a tablet computer, or a portable media device, typically includes a system enclosure surrounding internal system components, such as audio speakers. Such devices may have small form factors with limited internal space, and thus, the integrated audio speakers may be micro speakers, also known as microdrivers, that are miniaturized implementations of loudspeakers having a broad frequency range. Due to their small size, micro speakers tend to have limited space available for a back volume. Furthermore, given that acoustic performance in the low frequency audio range usually correlates directly with the back volume size, micro speakers tend to have limited performance in the bass range. However, the low frequency acoustic performance of portable consumer electronics devices having micro speakers may be increased by increasing the back volume size as much as possible within the internal space available in the system enclosure. 
     SUMMARY 
     Portable consumer electronics devices, such as mobile phones, have continued to become more and more compact. As the form factor of such devices shrinks, system enclosures become smaller and the space available for speaker integration is reduced. More particularly, the space available for a speaker back volume decreases, and along with it, low frequency acoustic performance diminishes. However, as described below, the effective back volume of a portable consumer electronics device may be increased without increasing the actual physical size of the back volume. More particularly, an adsorbent material may be incorporated within the back volume to lower the frequency of the natural resonance peak and thereby make bass sounds louder. The adsorbent material may reduce the spring rate of the speaker by adsorbing and desorbing air molecules as pressure fluctuates within the back volume during sound generation. Such adsorption/desorption may increase system efficiency at lower frequencies to produce more audio power. Thus, the audio speaker may produce better sound in the same form factor, or produce equivalent sound in a smaller form factor. However, directly incorporating an adsorbent material within the back volume to improve acoustic performance may cause side effects. In particular, incorporating loose adsorbent particles directly within the back volume may create a system that is physically unbalanced and susceptible to damage as the particles shift, e.g., due to the mobile device being carried or moved by a user. Furthermore, attempting to mitigate these effects by packaging the adsorbent particles in a secondary enclosure, e.g., a mesh bag, that is then located in the back volume may cost precious enclosure space, as the secondary enclosure walls occupy vertical clearance in the back volume. Thus, for adsorbent materials to be used in a speaker back volume to enhance acoustic performance within the smallest possible form factor, an audio speaker having a speaker enclosure that physically isolates an adsorbent material from sensitive driver components without adding additional system thickness may be needed. 
     In an embodiment, an audio speaker includes a housing defining a back volume behind a speaker driver. The back volume may be divided into several cavities by a permeable partition that is coupled with the housing along a perimeter of the partition. More particularly, the permeable partition may divide the back volume into a rear cavity and an adsorption cavity. The rear cavity may be defined between, or enclosed by, the speaker driver, the housing, and a first side of the permeable partition. Similarly, the adsorption cavity may be defined between the housing and a second side of the permeable partition. An adsorptive filler that is to adsorb a gas may be directly loaded into the adsorption cavity. That is, the adsorptive filler directly contacts the inside of a wall of the housing and the adsorption cavity side of the permeable partition. Furthermore, the permeable partition may include one or more holes between the first side and the second side (extending through the partition from the adsorption cavity side to the other side) to place the rear cavity in fluid (gaseous) communication with the adsorption cavity, while at the same time preventing the adsorptive filler from passing from the adsorption cavity into the rear cavity. For example, each of the holes in the permeable partition may have dimensions smaller than a size of the adsorptive filler such that the adsorptive filler is retained within the adsorption cavity by the permeable partition. In an embodiment, retention of the adsorptive filler within the adsorption cavity may be further enhanced by providing a seal, e.g., a hermetic seal, between the permeable partition and the housing. 
     The rear cavity and the adsorption cavity of the back volume may be adjacent to each other, but may have a relative orientation that differs in various embodiments. For example, the speaker driver may include a diaphragm that moves along a central axis, and the rear cavity may be directly behind the diaphragm along the central axis. However, the adsorption cavity may be laterally offset from the rear cavity away from the central axis. That is, a normal vector emerging from the first side of the permeable partition and pointing into the rear cavity is oriented in a direction that is orthogonal to the central axis, e.g., the rear cavity and the adsorption cavity may each be flat and thin and positioned side-by-side. Alternatively, the adsorption cavity may be directly behind the rear cavity along the central axis such that the normal vector from the first side of the permeable partition points into the rear cavity in a direction parallel to the central axis. 
     In an embodiment, the adsorptive filler may partially, substantially, or completely fill the adsorption cavity. For example, the adsorptive filler may cover a portion of the housing within the adsorption cavity, e.g., may coat an inner surface of the housing. The adsorptive filler may include unbound particles, such as a granular composition of one or more of a zeolite material and/or an activated carbon material. The granular materials may be inserted into the adsorption cavity through a fill port extending through the housing wall between the adsorption cavity and a surrounding environment outside of the housing, and a plug may be located in the fill port to seal the adsorption cavity and prevent the adsorptive filler from passing through the fill port into the surrounding environment (and the plug may also perform as a hermetic seal to prevent gaseous flow out through the fill port). 
     A method of forming an audio speaker with a multi-cavity back volume includes overmolding the housing around the permeable partition. More particularly, the method may include molding the housing around the perimeter of the permeable partition such that the back volume is created with the rear cavity and the adsorption cavity placed in fluid communication through the holes in the permeable partition. The speaker driver may be mounted in a driver port, which may be integrally formed with the housing, such that the rear cavity is defined between the speaker driver, the housing, and the first side of the permeable partition. Furthermore, a fill port may be integrally formed with the housing to allow the adsorptive filler to be poured, injected, sprayed, or otherwise inserted into the adsorption cavity through the fill port. After filling the adsorption cavity with the adsorptive filler, the fill port may be sealed by a plug such that the adsorption cavity is defined between the plug, the housing, and the second side of the permeable partition. Accordingly, the audio speaker may be built with a directly filled adsorption cavity separated from a rear cavity by an acoustically transparent barrier that allows sound to drive the adsorption and desorption of air molecules within an adsorptive filler and thereby increase the overall output power of the audio speaker, especially in the low frequency audio range. 
     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 sectional view of an audio speaker of an electronic device in accordance with an embodiment. 
         FIGS.  3 A- 3 B  are cross-sectional views, taken about line A-A of  FIG.  2   , of a permeable partition of an audio speaker in accordance with an embodiment. 
         FIG.  4    is a cross-sectional view, taken about line B-B of  FIG.  3 A , of a sealed portion of an audio speaker in accordance with an embodiment. 
         FIGS.  5 A- 5 B  are cross-sectional views, taken about line C-C of  FIG.  2   , of an adsorption cavity of an audio speaker in accordance with an embodiment. 
         FIGS.  6 A- 6 B  are cross-sectional views, taken about line D-D of  FIG.  2   , of a plugged portion of an audio speaker in accordance with an embodiment. 
         FIG.  7    is a sectional view of an audio speaker of an electronic device in accordance with an embodiment. 
         FIG.  8    is a flowchart of a method of forming an audio speaker having a multi-cavity back volume in accordance with an embodiment. 
         FIG.  9    is a schematic view of an electronic device having a micro speaker in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe an audio speaker having a speaker housing surrounding a back volume that is divided into several cavities (one of which contains an adsorbent material) by a permeable partition. However, while some embodiments are described with specific regard to integration within mobile electronics devices, such as handheld devices, the embodiments are not so limited and certain embodiments may also be applicable to other uses. For example, an audio speaker as described below may be incorporated into other devices and apparatuses, including desktop computers, laptop computers, or motor vehicles, to name only a few possible applications. 
     In various embodiments, description is made with reference to the figures. However, certain embodiments 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 may denote a relative position or direction. For example, “forward” may indicate a first direction away from a reference point. Similarly, “behind” or “rear” may indicate a location in a second direction from the reference point opposite from the first direction. However, such terms are not intended to limit the use of an audio speaker to a specific configuration described in the various embodiments below. For example, a micro speaker may be oriented to radiate sound in any direction with respect to an external environment, including upward toward the sky and downward toward the ground. 
     In an aspect, an audio speaker may include a speaker housing that incorporates a multi-cavity back volume in a space efficient manner. The housing may at least partially define each cavity in the back volume, and the cavities may be in fluid communication with each other, i.e., air and sound may flow between the cavities. For example, a permeable partition may extend directly across the back volume such that the cavities are defined by the housing and the partition wall. The permeable partition may be supported by the housing, and in an embodiment, the permeable partition is sealed to the housing by a hermetic seal. At least one of the cavities, i.e., an “adsorption cavity,” may contain an adsorbent material, such as a volume of unbound adsorbent particles, which adsorb and desorb air as pressure fluctuates in the adsorption cavity due to sound traveling between the cavities during sound generation. Thus, a multi-cavity back volume may be formed in a space efficient manner since the housing may form part of the enclosure that constrains the adsorption material, thereby eliminating a need for a secondary enclosure, e.g., a mesh bag, to constrain the adsorptive particles within the adsorption cavity. 
     In an aspect, an audio speaker may have a permeable partition separating an adsorption cavity, in which adsorptive filler is located, from a rear cavity, in which audio speaker components are located. The permeable partition may include holes that are large enough to permit air to flow in and out of the adsorption cavity. Thus, the permeable partition may be acoustically transparent. However, the holes may be small enough to prevent an adsorptive material from sifting outward into the rear cavity. Accordingly, the permeable partition separating the cavities may allow the adsorptive material to adsorb and desorb air molecules in response to pressure variations to lower the natural resonance peak of the audio speaker while reducing the likelihood that the adsorptive material will intrude into and damage sensitive speaker components, such as a voicecoil or a diaphragm housed in the rear cavity. 
     Referring to  FIG.  1   , a pictorial view of an electronic device is shown in accordance with an embodiment. 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 or a tablet computer. 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, and photos. Electronic device  100  may also include hardware to facilitate such capabilities. For example, an integrated microphone  102  may pick up the voice of a user during a call, and an audio speaker  106 , e.g., a micro speaker, may deliver a far-end voice to the near-end user during the call. Audio speaker  106  may also emit sounds associated with music files played by a music player application running on electronic device  100 . A display  104  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 sectional view of an audio speaker of an electronic device is shown in accordance with an embodiment. In an embodiment, an audio speaker  106  includes an enclosure, such as a speaker housing  204 , which supports a speaker driver  202 . Speaker driver  202  may be a loudspeaker used to convert an electrical audio signal into a sound. For example, speaker driver  202  may be a micro speaker having a diaphragm  206  supported relative to housing  204  by a speaker surround  208 . Speaker surround  208  may flex to permit axial motion of diaphragm  206  along a central axis  210 . For example, speaker driver  202  may have a motor assembly attached to diaphragm  206  to move diaphragm  206  axially with pistonic motion, i.e., forward and backward, along central axis  210 . The motor assembly may include a voicecoil  212  that moves relative to a magnetic assembly  214 . In an embodiment, magnetic assembly  214  includes a magnet, such as a permanent magnet, attached to a top plate at a front face and to a yoke at a back face. The top plate and yoke may be formed from magnetic materials to create a magnetic circuit having a magnetic gap within which voicecoil  212  oscillates forward and backward. Thus, when the electrical audio signal is input to voicecoil  212 , a mechanical force may be generated that moves diaphragm  206  to radiate sound forward along central axis  210  into a surrounding environment outside of housing  204 . 
     Movement of diaphragm  206  to radiate sound forward toward the surrounding environment may cause sound to be pushed in a rearward direction. For example, sound may propagate through a gas filling a space enclosed by housing  204 . More particularly, sound may travel through air in a back volume  216  behind diaphragm  206 . Back volume  216  may influence acoustic performance. In particular, the size of back volume  216  may influence the natural resonance peak of audio speaker  106 . For example, increasing the size of back volume  216  may result in the generation of louder bass sounds. 
     In an embodiment, back volume  216  within housing  204  may be separated into several cavities. For example, back volume  216  may be separated into a rear cavity  218  and an adsorption cavity  220  by a permeable partition  222 . Rear cavity  218  may be located directly behind speaker driver  202 . That is, speaker driver  202  may be suspended or supported within rear cavity  218  such that sound radiating backward from diaphragm  206  propagates directly into rear cavity  218 . Accordingly, at least a portion of rear cavity  218  may be defined by a rear surface of diaphragm  206 , and similarly, by a rear surface of speaker surround  208 . Furthermore, given that permeable partition  222  may extend across a cross-sectional area of back volume  216  between several walls of housing  204 , rear cavity  218  may be further defined by an internal surface of housing  204  and a first side  224  of permeable partition  222 . 
     Back volume  216  may include adsorption cavity  220  separated from rear cavity  218  by permeable partition  222 . That is, adsorption cavity  220  may be adjacent to rear cavity  218  on an opposite side of permeable partition  222 . In an embodiment, adsorption cavity  220  is defined by an internal surface of housing  204  that surrounds back volume  216 , and may also be defined by a second side  226  of permeable partition  222 . Thus, rear cavity  218  and adsorption cavity  220  may be immediately adjacent to one another across permeable partition  222 . 
     In an embodiment, adsorption cavity  220  may be placed in fluid communication with the surrounding environment through a fill port  228 . For example, fill port  228  may be a hole through a wall of housing  204  that places adsorption cavity  220  in fluid communication with the surrounding environment. The port may be formed during molding of housing  204 , or through a secondary operation, as described further below. To isolate adsorption cavity  220  from the surrounding environment, a plug  230  may be located in fill port  228 , e.g., after filling adsorption cavity  220  with an adsorptive filler  232 , to prevent leakage of the adsorptive filler  232  into the surrounding environment. Thus, adsorption cavity  220  may be partially defined by a surface of plug  230 . 
     Audio speaker  106  may have a form factor with any number of shapes and sizes. For example, audio speaker  106 , and thus housing  204 , may have an external contour that appears to be a combination of hexahedrons, cylinders, etc. One such external contour could be a thin box, for example. Furthermore, housing  204  may be thin-walled, and thus, a cross-sectional area of a plane passing across housing  204  at any point may have a geometry corresponding to the external contour, including rectangular, circular, and triangular, etc. Accordingly, permeable partition  222  extending across back volume  216  within housing  204  may also have a variety of profile shapes. For example, in the case where audio speaker  106  is a hexahedron, e.g., a low-profile box having a rectangular profile extruded in a direction orthogonal to central axis  210 , permeable partition  222  may have a rectangular profile. 
     Adsorptive filler  232  may be packaged in adsorption cavity  220  by directly filling, e.g., packing, adsorption cavity  220  with a loose adsorptive material and/or by coating inner surfaces of housing  204  with an adsorptive material. Directly filling adsorption cavity  220  may be distinguished from indirectly filling adsorption cavity  220  in that the loose adsorptive material may be poured, injected, or other transferred into adsorption cavity  220  in a loose and unconstrained manner such that the adsorptive material may move freely within adsorption cavity  220 . That is, the adsorptive material may be constrained only by the walls that define adsorption cavity  220 , e.g., an inner surface of housing  204 , and not by a separate constraint, e.g., a bag, pouch, box, etc. that is filled with adsorptive material prior to or after inserting the separate constraint into adsorption cavity  220 . Potential processes for filling adsorption cavity  220  with adsorptive material are described further below, but in an embodiment, at least a portion of the space of adsorption cavity  220  is filled with adsorptive filler  232 , and at least a portion of an inner surface of housing  204  within adsorption cavity  220  is covered by adsorptive filler  232 . The adsorptive filler  232  may be any appropriate adsorptive material that is capable of adsorbing a gas located in back volume  216 . For example, adsorptive filler  232  may include an adsorptive material such as a zeolite, activated carbon, silica, alumina, etc., which are configured to adsorb air molecules. The adsorptive material may be in a loose granular form. More particularly, the adsorptive filler  232  may include unbound particles that are able to move freely within adsorption cavity  220 , e.g., the particles may shake around during device use. Thus, permeable partition  222  may act as a barrier to prevent adsorptive filler  232  from shaking out of adsorption cavity  220  into rear cavity  218  behind speaker driver  202 . 
     Referring to  FIG.  3 A , a cross-sectional view, taken about line A-A of  FIG.  2   , of a permeable partition of an audio speaker is shown in accordance with an embodiment. In an embodiment, permeable partition  222  may include a film or a sheet extending across back volume  216  between opposing walls of an inner surface  302  of housing  204 . For example, permeable partition  222  may be a thin plastic or metallic plate. A thickness of permeable partition  222  may be on the order of 0.1 to 5 millimeter. A perimeter  304  of permeable partition  222  may be, for example, an outer edge of first side  224  or second side  226  of permeable partition  222 . Alternatively, perimeter  304  may be an outer wall extending between the sides of permeable partition  222 , i.e., may be an outer rim of permeable partition  222 . In any case, perimeter  304  may be coincident with inner surface  302  such that permeable partition  222  is sealed against housing  204  to prevent a gas, e.g., air, and/or adsorptive filler  232 , from leaking around permeable partition  222  between adsorption cavity  220  and rear cavity  218 . Inner surface  302  may provide an inner dimension of a housing wall of housing  204 . More particularly, housing  204  may include a housing wall with a thickness extending from inner surface  302  adjacent to perimeter  304  of permeable partition  222  and an outer surface that provides an outer dimension of the housing wall. Thus, housing  204  may have a housing wall that forms a case or enclosure around permeable partition  222 . 
     In an embodiment, permeable partition  222  is to allow a gas, e.g., air, to flow freely between rear cavity  218  and adsorption cavity  220  of back volume  216  through partition thickness. Permeable partition  222  may include one or more holes  306  that are larger than the constituent molecules of air, and thus allows air to flow from first side  224  to second side  226  of permeable partition  222 . Thus, permeable partition  222  may be acoustically transparent, since air molecules may freely transmit pressure changes caused by sound generation. Several holes  306  of the same or different cross-section dimensions may be formed through permeable partition  222 . For example, permeable partition  222  may include a first set of several circular holes  306  with diameters on the order of 5 nanometer to 100 micrometer, and a second set of holes or slots, e.g., transverse slits  308 , having widths on the order of 5 nanometer to 100 micrometer and lengths on the order of 5 nanometer to 500 micrometer. These hole shapes and dimensions are provided by way of example only, and may be varied within the scope of this description to provide passages through which air may pass from rear cavity  218  to adsorption cavity  220  to create an acoustically transparent permeable partition  222 . 
     Referring to  FIG.  3 B , a cross-sectional view, taken about line A-A of  FIG.  2   , of a permeable partition of an audio speaker is shown in accordance with an embodiment. In an embodiment, permeable partition  222  includes a mesh structure extending across back volume  216  from perimeter  304  at inner surface  302  of housing  204 . The mesh structure may include several strands  310  of flexible material, e.g., plastic or metal strands  310 , which are woven, knitted, etc., to form a web-like structure with holes  306  between strands  310 . As an example, permeable partition  222  may be a polyester mesh. Other processes, such as expanding plastic sheeting to produce a porous sheet, may also be used to form permeable partition  222  with acoustically transparent passages. The pitch and spacing between strands  310  may be selected and formed to create holes  306  having dimensions that allow air to flow across permeable partition  222  between rear cavity  218  and adsorption cavity  220 . The dimension of strands  310  may be selected accordingly. For example, mesh strands  310  may have a diameter on the order of 0.1 to 5 mm. 
     In an embodiment, the holes  306  in permeable partition  222  are sized to prevent adsorptive filler  232  from escaping adsorption cavity  220  through the partition. For example, holes  306  and/or slits  308  of permeable partition  222  may include dimensions that are smaller than a maximum dimension of adsorptive filler  232  particles. In an embodiment, adsorptive filler  232  includes particles having outer dimensions on the order of 10 nanometer to 500 micrometer. Thus, depending on the size range of adsorptive filler  232  particles that are used to fill adsorption cavity  220 , permeable partition  222  may include holes  306  on the order of 9 nanometer to 499 nanometer. In an example, if the outer dimension of adsorptive filler  232  particles in adsorption cavity  220  range between 50 to 150 micrometer, then holes  306  and/or slits  308  may have a maximum dimension of less than 50 micrometer, e.g., 40 micrometer, to retain even the smallest particles in adsorption cavity  220 . 
     Referring to  FIG.  4   , a cross-sectional view, taken about line B-B of  FIG.  3 A , of a sealed portion of an audio speaker is shown in accordance with an embodiment. Permeable partition  222  may be sealed along perimeter  304  to prevent escape of adsorptive filler  232  around an outer edge of permeable partition  222  into rear cavity  218 . More particularly, partition wall  402  may engage a slot formed in an inner surface  302  of housing  204  such that the surface of the slot overlaps a portion of first side  224  or second side  226 . The overlap of permeable partition  222  and housing  204  may be sufficient to prevent adsorptive filler  232  from escaping adsorption cavity  220 . For example, the slot in housing  204  and overlap between permeable partition  222  and housing  204  may be formed by overmolding housing  204  around permeable partition  222 . In such case, permeable partition  222  may be joined to housing  204  along the overmolded surfaces such that a seal is formed to resist movement of adsorptive filler  232  around permeable partition  222 . The joint may have gaps that range in dimension from zero to the smallest particle dimension of adsorptive filler  232 . When the gap is essentially zero, the joint may form a hermetic seal. 
     Optionally, the barrier between adsorption cavity  220  and rear cavity  218 , and more particularly the joint between perimeter  304  and housing  204 , may be enhanced by forming a seal  404  at the interface between housing  204  and permeable partition  222 . For example, a bead of adhesive or epoxy, e.g., silicone, may be applied at the interface to form a hermetic seal that prevents the migration of both adsorptive filler  232  and air around the outer edge of permeable partition  222 . Thus, seal  404  may be applied on one or both of first side  224  or second side  226  of permeable partition  222  to form an airtight seal between permeable partition  222  and housing  204 . 
     A hermetic seal may be formed with or without a separate seal  404  at the interface between housing  204  and permeable partition  222 . That is, when the gap between housing  204  and permeable partition  222  is essentially zero, the joint may form a hermetic seal that ensures that sound airflow (due to the speaker driver in operation) does not pass around perimeter  304  of partition wall  402 , and is instead confined to the predetermined holes  306  in partition wall  402 . However, the addition of separate seal  404 , e.g., a bead of sealant, may assist in forming the hermetic seal, and furthermore, may provide a mechanical bond between housing  204  and partition wall  402  to secure the components relative to each other. 
     Referring to  FIG.  5 A , a cross-sectional view, taken about line C-C of  FIG.  2   , of an adsorption cavity of an audio speaker is shown in accordance with an embodiment. As described above, adsorption cavity  220  may include adsorptive filler  232 , which may include any combination of adsorptive materials, including activated carbon and zeolite materials. Adsorptive filler  232  may be in a particulate form, e.g., granular powder, and thus may move freely within adsorption cavity  220  and directly contact surfaces of permeable partition  222  and housing  204 . As shown in  FIG.  5 A , adsorptive filler  232  may partially fill adsorption cavity  220 , i.e., the total volume of adsorptive filler  232  may be less than the volume of adsorption cavity  220 . This may be the case, for example, when a layer of adsorptive filler  232  is coated over inner surface  302  using known coating techniques. 
     In an embodiment, adsorptive filler  232  may cover at least a portion of inner surface  302 . For example, gravity may cause adsorptive filler  232  particles to settle to a bottom side of adsorption cavity  220  and the particles may cover the bottom inner surface  302  of housing  204  within adsorption cavity  220 . In an alternative embodiment, at least a portion of the adsorptive filler  232  particles may adhere to inner surface  302  through native surface adhesion, or through the addition of an adhesive therebetween. For example, adsorptive filler  232  particles may be coated over an adhesive layer on inner surface  302  such that the particles adhere around a periphery of adsorption cavity  220 , as shown in  FIG.  5 A . That is, particles may at least partially cover several internal walls of adsorption cavity  220 . Thus, adsorptive filler  232  may cover inner surface  302  around adsorption cavity  220 . 
     Referring to  FIG.  5 B , a cross-sectional view, taken about line C-C of  FIG.  2   , of an adsorption cavity of an audio speaker is shown in accordance with an embodiment. In an embodiment, adsorptive filler  232  substantially or completely fills adsorption cavity  220 . For example, adsorptive filler  232  may be packed into adsorption cavity  220  such that at least 50% of the volume of adsorption cavity  220  is filled by the packed particulate volume. This packing volume may include the spaces between the particles. For example, a volume of the particulate matter and a volume of the interstitial spaces between particles may be at least 75% of the volume of adsorption cavity  220 , and in some cases at least 90% of the volume of adsorption cavity  220 . In an embodiment, none of the volume of adsorption cavity  220  is occupied by secondary containers, such as mesh bag walls or films, surrounding adsorptive filler  232 . 
     Referring to  FIG.  6 A , a cross-sectional view, taken about line D-D of  FIG.  2   , of a plugged portion of an audio speaker is shown in accordance with an embodiment. In an embodiment, housing  204  includes plug  230  disposed within fill port  228  to prevent adsorptive filler  232  from leaking out of adsorption cavity  220 . For example, plug  230  may be shaped to engage fill port  228  such that a surface of plug  230  forms a press fit against a surface of fill port  228  to create a stopper that seals adsorption cavity  220 . Plug  230  may also be welded, e.g., chemically or thermally, to housing  204  within fill port  228  to form a permanently sealed plug. 
     Referring to  FIG.  6 B , a cross-sectional view, taken about line D-D of  FIG.  2   , of a plugged portion of an audio speaker is shown in accordance with an embodiment. In an embodiment, plug  230  may have a grommet shape with a central body and upper and lower collars that seal against an inner surface  302  and an outer surface of housing  204 . The central body may seal against housing  204  within fill port  228 . The grommet plug may be formed prior to insertion within fill port  228 . Alternatively, plug  230  may be formed from an adhesive, e.g., a temperature or light-cured adhesive, an epoxy, or a molten plastic or metal that is flowed into fill port  228  and thereafter cured or cooled such that plug  230  hardens in place to create a permanently sealed plug  230  to seal adsorption cavity  220 . 
     Referring to  FIG.  7   , a sectional view of an audio speaker of an electronic device is shown in accordance with an embodiment. Rear cavity  218  and adsorption cavity  220  may have different relative orientations in various embodiments. For example, as shown in  FIG.  2   , in an embodiment, adsorption cavity  220  is located lateral to rear cavity  218 , i.e., is laterally offset from rear cavity  218  away from central axis  210 . That is, permeable partition  222  may cross back volume  216  along a plane such that a normal vector  250  emerging from first side  224  and pointing into a rear cavity  218  is oriented in a direction orthogonal to central axis  210 . For example, rear cavity  218  and adsorption cavity  220  may each be flat and thin and positioned side-by-side. As a result, sound emitted rearward from diaphragm  206  may propagate directly toward a rear wall of rear cavity  218 , rather than be radiated directly toward permeable partition  222 . 
     In an embodiment, audio speaker  106  includes axially arranged back volume  216  cavities. For example, adsorption cavity  220  may be located directly behind rear cavity  218 . That is, central axis  210  may intersect rear cavity  218  behind diaphragm  206  and adsorption cavity  220  on an opposite side of permeable partition  222 . Accordingly, permeable partition  222  may cross back volume  216  along a plane such that normal vector  250  emerging from first side  224  and pointing into rear cavity  218  is oriented in a direction that is parallel to central axis  210 . For example, rear cavity  218  and adsorption cavity  220  may each be flat and thin and positioned forward-and-behind along central axis. Thus, sound emitted rearward by diaphragm  206  may propagate along central axis  210  directly through rear cavity  218  and permeable partition  222  into adsorption cavity  220 . 
     Permeable partition  222  may be oriented at any angle relative to central axis  210 . That is, although first face may face a direction orthogonal to, or parallel to, central axis  210 , in an embodiment, permeable partition  222  is oriented at an oblique angle relative to central axis  210 . Thus, adsorption cavity  220  may be some combination of lateral to, or directly behind, adsorption cavity  220  within the scope of this description. In any case, rear cavity  218  and adsorption cavity  220  may be adjacent to one another such that opposite sides of permeable partition  222  define a portion of each cavity. 
     Referring to  FIG.  8   , a flowchart of a method of forming an audio speaker having a multi-cavity back volume is shown in accordance with an embodiment. At operation  802 , housing  204  may be overmolded around permeable partition  222 . For example, permeable partition  222  may be a thin-walled sheet, plate, or mesh having the structure described above, and may be loaded into an injection mold before molding housing  204  around perimeter  304  of permeable partition  222 . The overmolded enclosure may include housing  204  with a multi-cavity back volume  216  defined on either side of permeable partition  222 . In particular, permeable partition  222  may separate rear cavity  218  on first side  224  from adsorption cavity  220  on second side  226 , and a seal, e.g., a hermetic seal, may be formed around perimeter  304  due to an overlap between housing  204  and partition wall  402 . Thus, housing  204  may include rear cavity  218  separate from, and in fluid communication with, adsorption cavity  220  through holes  306  of permeable partition  222 . Furthermore, the cavities of back volume  216  may be formed directly between the surfaces of housing  204  and permeable partition  222 . 
     At operation  804 , speaker driver  202  and/or other audio components may be mounted in audio speaker  106 . For example, diaphragm  206  and/or speaker surround  208  may be mounted within a speaker port formed in housing  204 . The speaker port may be a circular port that speaker surround  208  is sealed against to support diaphragm  206 , for example. The speaker port may be formed during the molding process, e.g., the injection mold may include geometry that defines speaker port. Alternatively, the speaker port may be formed in a secondary operation, e.g., by drilling or milling a port of the desired size and geometry through a wall of housing  204  that encloses rear cavity  218 . Thus, speaker driver  202  may partially define rear cavity  218  of back volume  216 . For example, with speaker surround  208  mounted around the speaker port, a rear surface of diaphragm  206  may define a portion of the enclosure around rear cavity  218 . 
     At operation  806 , adsorptive material may be loaded into adsorption cavity  220 . In an embodiment, fill port  228  is formed through housing  204 . For example, fill port  228  may be formed during the molding process, e.g., the injection mold may include geometry that defines fill port  228 . Alternatively, fill port  228  may be formed in a secondary operation, e.g., by drilling or milling a port of the desired size and geometry through a wall of housing  204  that encloses adsorption cavity  220 . After forming fill port  228 , adsorptive filler  232  may be inserted through fill port  228  to at least partially fill adsorption cavity  220 . For example, adsorptive filler  232  in the form of a powder or other granular material may be poured into adsorption cavity  220  through fill port  228 . As described above, adsorptive filler  232  may partially, substantially, or completely fill adsorption cavity  220 . For example, the adsorptive particulate (including interstitial spaces between particles) may have a volume that is at least 95% of the volume of adsorption cavity  220 , and in some cases more than 98% of the volume of adsorption cavity  220 . 
     In an embodiment, inserting adsorptive filler  232  may involve injecting a combination of adsorptive material and adhesive material such that the adsorptive filler  232  adheres to inner surface  302  of housing  204  around adsorption cavity  220 . The combination of adhesive and granules may be injected, for example, through a pressurized delivery nozzle that is inserted through fill port  228  and then directed toward inner surface  302  to spray the adsorptive composition against the cavity wall. Thus, inserting adsorptive filler  232  into adsorption cavity  220  may include coating a portion of inner surface  302  of housing  204  around adsorption cavity  220  with adsorptive filler  232 . 
     At operation  808 , fill port  228  may be sealed to enclose adsorption cavity  220 . For example, plug  230  may be press fit into fill port  228 , or alternatively, plug  230  may be flowed and cured within fill port  228  to seal fill port  228  against the escape of air and/or adsorptive filler  232  from adsorption cavity  220 . As a result, plug  230  may partially define adsorption cavity  220 , along with inner surface  302  of housing  204  and second side  226  of permeable partition  222 . Accordingly, adsorption cavity  220  may be fully enclosed to prevent the escape of adsorptive filler  232 . 
     Referring to  FIG.  9   , a schematic view of an electronic device having a micro speaker 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. Thus, the diagrammed circuitry is provided by way of example and not limitation. Electronic device  100  may include one or more processors  902  that execute instructions to carry out the different functions and capabilities described above. Instructions executed by the one or more processors  902  of electronic device  100  may be retrieved from local memory  904 , 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, to perform the different functions introduced above, e.g., phone or telephony and/or music play back. For example, processor  902  may directly or indirectly implement control loops and provide drive signals to voicecoil  212  of audio speaker  106  to drive diaphragm  206  motion and generate sound. 
     Audio speaker  106  having the structure described above may include back volume  216  separated by an acoustically transparent barrier, e.g., permeable partition  222 , into two cavities: rear cavity  218  directly behind speaker driver  202  and adsorption cavity  220  adjacent to rear cavity  218  across permeable partition  222 . Furthermore, adsorption cavity  220  may be directly filled with an adsorptive material such that back volume  216  includes an adsorptive volume defined directly between a system housing  204  and the acoustically transparent barrier. The adsorptive volume may reduce the overall spring rate of back volume  216  and lower the natural resonance peak of audio speaker  106 . That is, adsorptive filler  232  may adsorb and desorb randomly traveling air molecules as pressure fluctuates within back volume  216  in response to a propagating sound. As a result, audio speaker  106  may have a higher efficiency at lower frequencies, as compared to a speaker having a back volume  216  without adsorptive material. Thus, the overall output power of audio speaker  106  may be improved. More particularly, audio speaker output, e.g., during telephony or music play back, may be louder, especially within the low frequency audio range. Accordingly, audio speaker  106  having the structure described above may produce louder, richer sound within the bass range using the same form factor as a speaker back volume without multiple cavities, or may produce equivalent sound within the bass range within a smaller form factor. Furthermore, since adsorption cavity  220  is defined directly between housing  204  and permeable partition  222 , which are sealed together, the form factor of audio speaker  106  may be smaller than, e.g., a speaker back volume that holds a secondary container, e.g., a mesh bag, filled with an adsorbent material. 
     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.