Patent Publication Number: US-11647324-B2

Title: Audio speaker having a rigid adsorptive insert

Description:
RELATED APPLICATIONS 
     The present application is a continuation application of co-pending U.S. patent application Ser. No. 16/268,267 filed Feb. 5, 2019, which is a continuation application of U.S. patent application Ser. No. 15/198,852, filed Jun. 30, 2016, which claims the benefit of U.S. Provisional Patent Application No. 62/210,766, filed Aug. 27, 2015, and those applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments related to an audio speaker having an adsorptive insert in a speaker back volume, are disclosed. More particularly, an embodiment includes an adsorptive insert having a rigid open-pore body formed by bonded adsorptive particles. The rigid open-pore body includes interconnected macropores that transport air from the speaker back volume to adsorptive micropores in the bonded adsorptive particles during sound generation. 
     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. The low frequency acoustic performance of portable consumer electronics devices having micro speakers may be increased, however, 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. The effective back volume of a portable consumer electronics device may, however, 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 can 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. 
     Directly incorporating an adsorbent material within the back volume to improve acoustic performance may, however, cause negative 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 such as a mesh bag 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 an adsorptive insert that is physically stable and efficiently utilizes the available back volume may be needed. 
     In an embodiment, an audio speaker includes a physically stable adsorptive insert that is located in, and occupies a substantial portion of, a speaker back volume. The audio speaker incudes a speaker housing having a speaker port and an inner surface. A loudspeaker may be mounted in the speaker port to define the back volume between the loudspeaker and the inner surface. The adsorptive insert that is located in the back volume includes adsorptive particles bound together to form a rigid open-pore body having an outer surface surrounding a spatial volume. The spatial volume occupied by the monolithic open-pore body may be a same order of magnitude as the back volume, e.g., the spatial volume may occupy a majority of the back volume. In an embodiment, the rigid open-pore body includes macropores along the outer surface and between the bonded adsorptive particles, and the macropores are interconnected to transport air from the back volume to micropores within the bonded adsorptive particles. The rigid open-pore body may have a lower porosity than loosely packed, i.e., not bonded, adsorptive particles. For example, the interconnected macropores may occupy less than 60% of the spatial volume of the open-pore body. In an embodiment, the bonded adsorptive particles occupy a majority of the spatial volume, e.g., more than 75% of the spatial volume. 
     All of the outer surface of the open-pore body may be spaced apart from the inner surface of the speaker housing. For example, spacers may be located between the inner surface and the outer surface. In an embodiment, the spacers include an open-cell spacer that allows air to move freely from the back volume to the open-pore body through channels within the open-cell spacer. To that end, the open-cell spacer may be an open-cell foam material that includes a first porous surface disposed against the outer surface and a second porous surface exposed to air in the back volume between the inner surface and outer surface. The first porous surface may be placed in fluid communication with the second porous surface through the interconnected channels to transport air from the back volume to the macropores along the outer surface. 
     In an embodiment, substantially all of (and not necessarily all of) the outer surface of the open-pore body may be spaced apart from the inner surface of the speaker housing. For example, the adsorptive insert may include one or more protrusions extending from a surrounding portion of the outer surface, and the protrusions may be spacers. That is, the protrusions may have respective apices disposed against the inner surface to stabilize the open-pore body within the back volume and maintain a spaced apart relationship between the open-pore body and the speaker housing. As such, the apices may represent a portion of the outer surface that is in contact with, and not spaced apart from, the inner surface. The apices may, however, have a combined surface area that is substantially less than the total outer surface area. For example, the combined surface area of the apices may be less than 10% of the total surface area of the outer surface to ensure that at least 90% of the outer surface is spaced apart from the inner surface and placed in fluid communication with the back volume. 
     In an embodiment, a portion of the outer surface of the open-pore body conforms to an opposing portion of the inner surface of the speaker housing. For example, part of the outer surface that is spaced apart from the inner surface may include an outer contour opposing an inner contour of the inner surface, and the contours may have matching shapes. The outer contour and inner contour may both include curvatures or corners that are negative shapes of each other. Thus, the open-pore body may conform to the speaker housing to efficiently utilize the back volume. 
     In an embodiment, an audio speaker includes an adsorptive insert with a hierarchical open-pore body. For example, the open-pore body, which may be formed from bonded adsorptive particles, may include a core region and a shell region surrounding the core region. The shell region can include the outer surface surrounding the spatial volume of the hierarchical open-pore body. Furthermore, macropores may be interconnected throughout the open-pore body, within both the core region and the shell region. The macropores in the shell region, however, may be larger on average than the macropores in the core region. For example, interconnected macropores in the shell region may occupy less than 60% of the shell volume, while interconnected macropores in the core region may occupy less than 30% of the shell volume. Thus, the hierarchical macroscopic network may funnel air from the back volume through smaller and smaller macropores to micropores in the bonded adsorptive particles of the core region. 
     In an embodiment, a method of fabricating an audio speaker includes assembling a loudspeaker, a speaker housing, and an adsorptive insert. The method may include forming, e.g., through plastic or metal molding processes, the speaker housing having a speaker port and an inner surface defining a rear cavity. The method may also include forming a rigid open-pore body, by bonding adsorptive particles together. Various bonding techniques may be used to bond the adsorptive particles, including techniques that employ one or more of heat or pressure, e.g., sintering techniques. As a result of the bonding techniques, the rigid open-pore body may be a monolithic structure having an outer surface surrounding a spatial volume. Furthermore, as a result of the bonding process, a network of interconnected macropores may be located along the outer surface and between the bonded adsorptive particles. Optionally, the rigid open-pore body may be shaped by removing bonded adsorptive particles from the outer surface to create an outer contour that has a shape matching and conforming to a same shape of an inner contour of the inner surface of the speaker housing. The adsorptive insert having the rigid open-pore body may be inserted into the rear cavity. In an embodiment, the rigid open-pore body is spaced apart from the speaker housing by positioning a spacer, e.g., an open-cell spacer, between the rigid open-pore body and the speaker housing. Furthermore, the loudspeaker may be located in the speaker port to define a back volume between the loudspeaker and the inner surface. The back volume may be a same order of magnitude as the spatial volume occupied by the open-pore body. Thus, during sound generation by the loudspeaker, air may be transported from the back volume, through the open-cell spacer, and into the interconnected macropores of the open-pore body to be adsorbed and/or desorbed by micropores in the bonded adsorptive particles. 
     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 having an adsorptive insert within a speaker housing in accordance with an embodiment. 
         FIG.  3    is a cross-sectional view, taken about line A-A of  FIG.  2   , of an open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  4    is a cross-sectional view, taken about line C-C of  FIG.  3   , of bonded adsorptive particles of an open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  5    is a side view of an adsorptive particle in accordance with an embodiment. 
         FIG.  6    is a cross-sectional view, taken about line B-B of  FIG.  2   , of an open-cell spacer between a speaker housing and an open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  7    is a cross-sectional view, taken about line A-A of  FIG.  2   , of a hierarchical open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  8    is a cross-sectional view, taken about line D-D of  FIG.  7   , of a core of a hierarchical open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  9    is a cross-sectional view, taken about line E-E of  FIG.  7   , of a middle shell of a hierarchical open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  10    is a cross-sectional view, taken about line F-F of  FIG.  7   , of an outer shell of a hierarchical open-pore body of an adsorptive insert in accordance with an embodiment. 
         FIG.  11    is a sectional view of an audio speaker having an adsorptive insert and speaker housing with conforming curved contours in accordance with an embodiment. 
         FIG.  12    is a sectional view of an audio speaker having an adsorptive insert and speaker housing with conforming angular contours in accordance with an embodiment. 
         FIG.  13    is a sectional view of an audio speaker having an adsorptive insert with protrusions to space apart an open-pore body from a speaker housing in accordance with an embodiment. 
         FIG.  14    is a flowchart of a method of forming an audio speaker having an adsorptive insert within a speaker housing in accordance with an embodiment. 
         FIG.  15    is a schematic view of an electronic device having an audio speaker in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe an audio speaker having a speaker housing surrounding a back volume and a rigid adsorptive insert in the back volume. 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, “front” may indicate a first direction away from a reference point. Similarly, “lateral” may indicate a location in a second direction orthogonal to the first direction. However, such terms are provided to establish relative frames of reference, and are not intended to limit the use or orientation of an audio speaker (or components of the audio speaker) to a specific configuration described in the various embodiments below. 
     In an aspect, an audio speaker includes an adsorptive insert in a speaker back volume. The adsorptive insert includes adsorptive particles, e.g., zeolite or activated carbon particles, that are bound together to form a rigid open-pore body with a network of interconnected passages, or macropores, between the bonded adsorptive particles. Furthermore, the adsorptive particles may each include micropores that are sized to adsorb air, e.g., oxygen, nitrogen, or other constituent molecules of air. Thus, the rigid open-pore body provides a transportation network for air to be moved, e.g., by pressure waves during sound generation, from the back volume, through the macropores, and into (or out of) the micropores. The rigid open-pore body may be a hierarchical open-pore body having a network of air passages that include macropores that reduce in size from an outer surface of the rigid open-pore body toward a core at the center of the rigid open-pore body. Such a hierarchical open-pore body may allow air to migrate more easily to the center of the rigid open-pore body, allowing free movement of air in an open-pore body occupying a spatial volume that has a same order of magnitude as the speaker back volume. Accordingly, the adsorptive insert having a rigid open-pore body allows for the adsorption and desorption of air molecules in response to pressure variations, which can lower the natural resonance peak of the audio speaker. 
     In an aspect, a rigid open-pore body of an adsorptive insert in a speaker back volume is spaced apart from an inner surface of a speaker housing that defines the speaker back volume. For example, an outer surface of the rigid open-pore body may be entirely spaced apart from the inner surface. Full separation may be achieved by placing one or more spacers between the outer surface of the rigid open-pore body and the inner surface of the speaker housing. Alternatively, the outer surface of the rigid open-pore body may be substantially separated from the inner surface, i.e., the outer surface and the inner surface may contact each other minimally, as in the case where one or more protrusions extend from the rigid open-pore body to contact the inner surface at apices that have contact surface areas that are one or more orders of magnitude smaller than a total surface area of the outer surface. Thus, the rigid open-pore body may be maximally exposed to air, and the adsorptive insert may also be stabilized and/or cushioned within the speaker housing to reduce the likelihood of damage to sensitive speaker components, such as a voicecoil or a diaphragm of a loudspeaker mounted in the speaker housing. 
     In an aspect, a method of manufacturing an audio speaker having an adsorptive insert within a speaker housing includes operations for bonding adsorptive particles together to form a rigid open-pore body that includes a network of macropores to transport air from a speaker back volume to micropores of the bonded adsorptive particles. The operations for bonding adsorptive particle may include processing techniques to form a hierarchical open-pore body having a network of air pathways that includes macropores that reduce in size from an outer surface of the rigid open-pore body toward a core at a center of the rigid open-pore body. Furthermore, the operations may include removing portions of the bonded adsorptive particle to shape the rigid open-pore body such that an outer contour of the adsorptive insert conforms to an inner contour of the speaker housing, e.g., the components may include matching corner or curvature geometries. Thus, an adsorptive insert may be formed that efficiently utilizes the available back volume by conforming to the internal shape of the speaker housing. 
     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 having an adsorptive insert within a speaker housing is shown in accordance with an embodiment. Audio speaker  106  includes an enclosure, which may be a speaker housing  202  that supports a loudspeaker  204 . More particularly, speaker housing  202  may include a speaker port  205 , e.g., a hole formed in a wall of speaker housing  202 , and loudspeaker  204  may be mounted on speaker housing  202  in speaker port  205 . Loudspeaker  204  may be any of a variety of electroacoustic transducers, such as micro speakers, that include a speaker driver to convert an electrical audio signal into a sound. For example, loudspeaker  204  may be a micro speaker having a diaphragm  206  supported relative to speaker housing  202  and/or a speaker frame  210 . Diaphragm  206  may be connected to speaker housing  202  by a surround  208 . Surround  208  flexes to permit axial motion of diaphragm  206  along a central axis to produce sound. For example, loudspeaker  204  may have a motor assembly attached to diaphragm  206  to move diaphragm  206  axially with pistonic motion, i.e., forward and backward, along the central axis. 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 the central axis into a surrounding environment outside of speaker housing  202 . Similarly, oscillation of diaphragm  206  radiates sound rearward into a back volume  216  between loudspeaker  204  and speaker housing  202 . 
     Back volume  216  may be a spatial volume defined between loudspeaker  204  and an inner surface  218  of speaker housing  202 . For example, when loudspeaker  204  is mounted in speaker port  205 , back volume  216  may include the volume of air behind diaphragm  206  and within a rear cavity defined by the inner surface  218  of speaker housing  202 , including the volume of the rear cavity that is not occupied by loudspeaker  204  components, e.g., voicecoil  212 , frame  210 , and magnetic assembly  214 . Sound generated by the movement of diaphragm  206  propagates through back volume  216 , and thus, the size of back volume  216  may influence acoustic performance. Generally speaking, increasing the size of back volume  216 , i.e., increasing the spatial volume occupied by air in back volume  216 , may result in the generation of louder bass sounds by audio speaker  106 . 
     Acoustic performance of audio speaker  106  may also be influenced by an adsorptive insert  220  located within back volume  216 . Adsorptive insert  220  may include adsorptive materials capable of adsorbing constituent molecules of a gas, e.g., air, located in back volume  216 . For example, adsorptive insert  220  may include zeolite, activated carbon, silica, alumina, etc., having a porous structure that accommodates, i.e., adsorbs/desorbs, air molecules. Adsorption (and desorption) of air molecules by the adsorptive material in adsorptive insert  220  can influence pressure changes within back volume  216  and hence increase the effective back volume  216 . That is, the adsorption/desorption can cause audio speaker  106  to operate as though it includes a larger back volume  216  than it actually has. 
     In an embodiment, adsorptive insert  220  includes an open-pore body  222  formed from the adsorptive materials. For example, the adsorptive materials may be bonded together to form a monolithic open-pore structure. The adsorptive materials may include beads, powders, etc. in a raw form. The raw adsorptive particle may then be processed as described below to fix the relative position of the adsorptive material constituents into a single agglomerated mass, e.g., a brick. The agglomerated body includes an outer surface  224  surrounding a spatial volume. Here, the term “agglomerated” is not used to merely describe the aggregation or agglomeration of several particles into a small grain structure, but rather, in an embodiment, the spatial volume occupied by open-pore body  222  is on a same order of magnitude, i.e., at least 10% of, the spatial volume occupied by back volume  216 . Thus, open-pore body  222  of adsorptive insert  220  may be a monolithic mass composed of adsorptive materials that do not shift relative to each other during use. Such a structure may be contrasted with a bag of loosely packed adsorptive grains in which each grain is formed from aggregated adsorptive powders. 
     In addition to being a monolithic structure, open-pore body  222  may be rigid. In an embodiment, adsorptive materials bonded along outer surface  224  may be adjoined with one another such that outer surface  224  does not deform under external pressures, e.g., when knocked against frame  210  or magnetic assembly  214  if audio speaker  106  is dropped to the ground. More specifically, in an embodiment, only an outer shell region of open-pore body  222  is rigid. For example, adsorptive material making up an outer thickness, e.g., 2-5 mm, of open-pore body  222  may resist deformation while adsorptive material inward from the outer shell region, i.e., a core region, may be composed of loosely packed or weakly bonded adsorptive material that may not resist deformation and may shift relative to each other during an impact. In another embodiment, adsorptive materials throughout open-pore body  222 , e.g., in the outer shell region and the core region, may be bonded such that the entire body is rigid and resistant to deformation during an impact. Thus, at least an outer surface  224  of open-pore body  222  may be considered to be solid in the sense that a portion of open-pore body  222  may be hard, compact, and not loosely packed. The term “solid,” however, is not intended to exclude the porous structures described below. 
     The solid portions of open-pore body  222  may be shaped to conform to inner surface  218  of speaker housing  202 . For example, speaker housing  202  may have corners (as in the case of a polyhedral inner surface  218  shape) or curvatures (as in the case of a curved inner surface  218  shape), and outer surface  224  of open-pore body  222  may include corresponding portions that are similar or identical in shape to the corners or curvatures of inner surface  218 . Furthermore, in addition to being shaped to conform to inner surface  218  of speaker housing  202 , open-pore body  222  may be shaped to conform to other components of audio speaker  106 . For example, open-pore body  222  may include a loudspeaker receptacle  226 , which may be a recess in a portion of outer surface  224  facing loudspeaker  204 . Loudspeaker receptacle  226  may be sized to receive a portion of loudspeaker  204 , e.g., a lower portion of magnetic assembly  214 . Thus, the outer shape of open-pore body  222  may be modified to efficiently and/or maximally utilize the available space of back volume  216 . 
     Open-pore body  222  may be spaced apart from inner surface  218  of speaker housing  202  to maximally expose outer surface  224  to air within back volume  216 . For example, the entirety of outer surface  224  may be separated from inner surface  218  by a gap that may be consistent, or may vary, along outer surface  224 . In the case of a varying gap distance, a portion of outer surface  224  on a top surface of open-pore body  222  may be farther from a top wall  230  of speaker housing  202  adjacent to speaker port  205 , than a portion of outer surface  224  on a bottom surface of open-pore body  222  is from a bottom wall  232  of speaker housing  202 . By contrast, the distance between all side portions of outer surface  224  may be equidistant from opposing side wall  234  portions of inner surface  218 . In an embodiment, portions of outer surface  224  and inner surface  218  that are in an opposing and spaced apart relationship are separated by a distance at least equal to the mean free path of air molecules at standard atmospheric pressure, and may be at least 500 micron. 
     Portions of outer surface  224  and inner surface  218  that are in a spaced apart relationship may nonetheless be connected through an intermediate spacer. More particularly, one or more spacers, such as an open-cell spacer  228 , may be used to separate open-pore body  222  from inner surface  218  and/or loudspeaker  204 . Open-cell spacer  228  is one embodiment of a spacer, but it is not intended to be limiting. For example, dabs of adhesive may be located between open-pore body  222  and speaker housing  202  at discrete locations to attach outer surface  224  to inner surface  218 . The adhesive spacers may maintain the spaced apart relationship over a distance equal to an adhesive film thickness. Alternatively, structures such as felt or foam spacers may be used to separate open-pore body  222  from speaker housing  202 . 
     In an embodiment, the spacers are located between open-pore body  222  and inner surface  218  on one or more surfaces of open-pore body  222 . For example, at least two spacers may be placed on different side portions of outer surface  224  such that the spacers resist motion in opposite directions, e.g., a spacer on a left side portion of open-pore body  222  may be squeezed when open-pore body  222  accelerates to the right and a spacer on a right side portion of open-pore body  222  may be squeezed when open-pore body  222  accelerates to the left. Similarly, opposing spacers may be located on top and bottom portions of outer surface  224 . 
     In an embodiment, the spacers are permeable by air and allow air to move freely through them from back volume  216  to outer surface  224 . Thus, portions of outer surface  224  that are in contact with spacer surfaces receive air from back volume  216  through the spacer for adsorption/desorption within the bonded adsorptive particles. As such, a spacer may cover a substantial portion of outer surface  224 , e.g., may completely encompass open-pore body  222 , without restricting the transfer of air molecules between back volume  216  and open-pore body  222 . An open-cell spacer  228  is an embodiment of a spacer that facilitates air transfer between back volume  216  and open-pore body  222 , and is described in more detail below. 
     Referring to  FIG.  3   , a cross-sectional view, taken about line A-A of  FIG.  2   , of an open-pore body of an adsorptive insert is shown in accordance with an embodiment. Open-pore body  222  may be a monolithically formed rigid mass, as described above. Furthermore, outer surface  224  may be shaped such that the cross-sectional profile of open-pore body  222  is rectangular, to fit within a corresponding rear cavity portion, e.g., a rectangular cuboid cavity, a pyramidal cavity, etc., of speaker housing  202 . In an embodiment, the spatial volume occupied by open-pore body  222  may be on a same order of magnitude as a spatial volume occupied by back volume  216 . For example, open-pore body  222  may be sized to fill at least 10% of back volume  216 . In an embodiment, open-pore body  222  may be sized to fill a majority of back volume  216 . Accordingly, the spatial volume occupied by open-pore body  222  (a spatial volume surrounded by outer surface  224  and not accounting for a porosity or density of open-pore body  222  within the spatial volume envelope), may substantially fill the spatial volume occupied by back volume  216  (the spatial volume between the inner surface  218  of speaker housing  202  and loudspeaker  204 ). The ratio of the spatial volume of open-pore body  222  to the spatial volume of back volume  216  may be greater than 0.5 (50% fill), such as more than 0.75 (75% fill) or more than 0.90 (90% fill). In an embodiment, the ratio is less than 1.0 (100% fill) because outer surface  224  and inner surface  218  are spaced apart from each other. 
     The constituent adsorptive material of open-pore body  222  may be adsorptive particles, and more particularly, thousands to millions of adsorptive particles bound together to form a rigid, monolithic structure. Because the adsorptive particles may be bound together using one or more of the processing techniques described below, the density of open-pore body  222  may be greater than the density of the constituent adsorptive particles if they were loosely packed together. For example, whereas if open-pore body  222  were formed from loosely packed adsorptive particles that were not bonded together, the density of open-pore body  222  would be expected to be less than 40%, it is contemplated that open-pore body  222  formed from bonded adsorptive particles may include a rigid structure in which the bonded adsorptive particles occupy at least 40% of the spatial volume surrounded by outer surface  224 . More particularly, open-pore body  222  may include a porous structure having macropores  302  along outer surface  224  and between the bonded adsorptive particles, but the macropores  302  may occupy less than 60% of the spatial volume, such that the bonded adsorptive particles occupy more than 40%, and optionally a majority, of the spatial volume surrounded by outer surface  224 . 
     Open-pore body  222  may be considered “open-pored” because the macropores  302  between bonded adsorptive particles are interconnected throughout the rigid body. That is, the macropores  302 , which are represented as circular holes in the cross-sectional view of  FIG.  3   , but which in fact may be voids of any shape, may be interconnected in a three-dimensional network of passages that allow for air to flow from one macropore  302  to another. As such, macropores  302  may instead be conceptualized as interstitial spaces, or interstices having varying geometries, that separate one adsorptive particle from one or more other adjacent adsorptive particles in the rigidly-bound structure of open-pore body  222 . Accordingly, at a macroscopic level, a cross-section of open-pore body  222  having a uniform porosity may include bonded adsorptive particles occupying at least 50% of the cross-sectional area and macropores  302  occupying no more than 50% of the cross-sectional area. 
     Referring to  FIG.  4   , a cross-sectional view, taken about line C-C of  FIG.  3   , of bonded adsorptive particles of an open-pore body of an adsorptive insert is shown in accordance with an embodiment. The average diameter or dimension across a macropore  302  between adjacent adsorptive particles  402  may be at least equal to the mean free path of air molecules at standard atmospheric pressure. For example, the average dimension may be at least 75 nm. Open-pore body  222  may include macropores  302  having an average pore dimension, however, on the order of tens of microns, e.g., 10 microns, up to on the order of hundreds of microns, e.g., 500 microns. In an embodiment, the pore dimension is uniform within a tolerance of an order of magnitude throughout the cross-section of open-pore body  222 . For example, the pore dimensions may be in a range of 10 to 50 microns throughout the cross-section. Accordingly, macropores  302  distributed along outer surface  224  and throughout open-pore body  222  provide a network of passages through which air may be transported by pressure waves during sound generation. More particularly, air may be transported from back volume  216  surrounding outer surface  224  into open-pore body  222  through macropores  302  along outer surface  224 . After entering open-pore body  222 , the air may further circulate or travel through the interconnected macropores  302  to the surfaces of bonded adsorptive particles  402 , where the air molecules may then be adsorbed/desorbed by adsorptive particles  402 . 
     Referring to  FIG.  5   , a side view of an adsorptive particle is shown in accordance with an embodiment. Adsorption/desorption of air molecules by bonded adsorptive particles  402  occurs based on micropores  502  within adsorptive particle  402 . Similar to the macroscopic structure of open-pore body  222 , which includes outer surface  224  surrounding a spatial volume and macropores  302  along outer surface  224  and within the spatial volume, each adsorptive particle  402  may include a particle surface  504  surrounding a particle spatial volume and micropores  502  along the particle surface  504  and within the particle spatial volume. Adsorptive particle surface  504  may be spherical (as shown) or may have any other surface morphology. Accordingly, adsorptive particle  402  includes a porous structure with micropores  502  suited to adsorb and desorb the constituent molecules of air, e.g., nitrogen, oxygen, carbon dioxide, etc. As discussed above, numerous adsorptive materials are known for this purpose, including zeolite, activated carbon, and other molecular sieve materials. Adsorptive particles  402  formed from such materials are contemplated to be within the scope of this disclosure. For example, in an embodiment, bonded adsorptive particles  402  include zeolites having micropores  502  with pore dimensions, i.e., average pore diameters, in a range of 2-10 angstroms. Accordingly, micropores  502  may adsorb/desorb constituent molecules of air transported to particle surfaces  504  during sound generation to alter the frequency of the natural resonance peak of audio speaker  106 . 
     Referring to  FIG.  6   , a cross-sectional view, taken about line B-B of  FIG.  2   , of an open-cell spacer between a speaker housing and an open-pore body of an adsorptive insert is shown in accordance with an embodiment. One or more open-cell spacer  228  may separate outer surface  224  of open-pore body  222  from inner surface  218  of speaker housing  202 . Open-cell spacer  228  may have a thickness between inner surface  218  and outer surface  224  that is at least twice an average diameter of the pores within open-cell spacer  228 . For example, open-cell spacer  228  may be formed from an open-cell foam, e.g., a reticulated foam of polyurethane, ceramic, or metal, with several interconnected pores forming channels  602  that provide an air path  604  from back volume  216  to outer surface  224 . In an embodiment, the interconnected pores may have an average diameter of 100 micron, and accordingly, the thickness of open-cell spacer  228  and the distance between outer surface  224  and inner surface  218  may be at least 200 microns, such as 500 microns or more. 
     The interconnected pores of open-cell spacer  228  may form channels  602  to create routes of air ingress and egress along every side of open-cell spacer  228 . More particularly, open-cell spacer  228  may include channels  602  that interconnect at least one side exposed to back volume  216  to another side opposing macropores  302  along outer surface  224 . In an embodiment, open-cell spacer  228  is a rectangular cuboid block of open-cell foam having a surface pressed against speaker housing  202  in a rearward direction, a support surface  606  opposing and pressed against outer surface  224  in a frontward direction, and four lateral surfaces  608  exposed to air within back volume  216  between speaker housing  202  and open-pore body  222 . Support surface  606  and lateral surfaces  608  may be porous, in that each surface may include a terminal end of at least one of several pores or channels  602  that are interconnected across open-cell spacer  228  to create air path  604  from lateral surface  608  to support surface  606 . More particularly, lateral surfaces  608  may not act as barriers to air flow in a lateral direction between speaker housing  202  and outer surface  224 , but may instead be air permeable, allowing air to flow laterally from one lateral surface  608  to another lateral surface  608 . Accordingly, the porous surfaces of lateral surface  608  and support surface  606  may be in fluid communication through channels  602  to transport air from back volume  216  to macropores  302  along outer surface  224 , and into the macroscopic network of passages in open-pore body  222 . 
     Referring to  FIG.  7   , a cross-sectional view, taken about line A-A of  FIG.  2   , of a hierarchical open-pore body of an adsorptive insert is shown in accordance with an embodiment. In an embodiment, adsorptive particles  402  of open-pore body  222  are bonded to form a rigid, tiered structure. For example, rather than having a substantially uniform porosity throughout open-pore body  222 , adsorptive insert  220  may include open-pore body  222  having macropores  302  that vary in size between a central core  702  region and one or more shell regions surrounding the core  702  region. For example, open-pore body  222  may have a three-level hierarchical structure with core  702  region surrounded by a middle shell  704 , and an outer shell  706  surrounding middle shell  704 . The porosity of open-pore body  222  may vary between one or more of core  702 , middle shell  704 , and outer shell  706 . For example, macropores  302  along outer surface  224  on outer shell  706  may be larger, on average, than macropores  302  at a center of open-pore body  222  in core  702 . Similarly, macropores  302  in middle shell  704  may be smaller, on average, than macropores  302  in outer shell  706 , and larger, on average, than macropores  302  in core  702 . As in the embodiments of open-pore body  222  described above, an open-pore body  222  having a hierarchical structure may allow for air to be transported from back volume  216  through the interconnected macropores  302  of open-pore body  222  from outer surface  224  into micropores  502  in the bonded adsorptive particles  402  of the outer shell  706  region, middle shell  704  region, and core  702  region. 
     Referring to  FIG.  8   , a cross-sectional view, taken about line D-D of  FIG.  7   , of a core of a hierarchical open-pore body of an adsorptive insert is shown in accordance with an embodiment. Core  702  region of open-pore body  222  may include adsorptive particles  402  bonded together and separated by intervening macropores  302 , as described above. In an embodiment, the porosity of core  702  region and the average dimension of macropores  302  may be similar to the values described with respect to  FIG.  4   . For example, the average pore dimension of macropores  302  may be in a range of 10 to 50 microns throughout the cross-section of core  702 , and macropores  302  may occupy less than 60% of a spatial volume occupied by core  702  region, i.e., a core volume. In an embodiment, bonded adsorptive particles  402  may be at least twice as dense as would be the case if the adsorptive particles  402  were loosely packed. Thus, macropores  302  may occupy less than 30% of the core volume, e.g., macropores  302  may occupy between 10-20% of the core volume. 
     Referring to  FIG.  9   , a cross-sectional view, taken about line E-E of  FIG.  7   , of a middle shell of a hierarchical open-pore body of an adsorptive insert is shown in accordance with an embodiment. Middle shell  704  region of open-pore body  222  may include several adsorptive particles  402  bonded together and separated by intervening macropores  302  with average separation distances higher than those of core  702  region. In an embodiment, the average dimension of macropores  302  may be at least twice the corresponding values of core  702  region. For example, when core  702  region includes an average macropore  302  dimension in a range of 10 to 50 microns, middle shell  704  region may include an average macropore  302  dimension in a range of 20 to 100 microns. Similarly, a porosity of middle shell  704  region may be greater than a porosity of core  702  region. For example, when macropores  302  of core  702  region occupy less than 30% of the core volume, macropores  302  of middle shell  704  may occupy more than 30%, e.g., 30-50%, of a spatial volume occupied by middle shell  704 , i.e., a middle shell volume. 
     Referring to  FIG.  10   , a cross-sectional view, taken about line F-F of  FIG.  7   , of an outer shell of a hierarchical open-pore body of an adsorptive insert is shown in accordance with an embodiment. Outer shell  706  region of open-pore body  222  may include several adsorptive particles  402  bonded together and separated by intervening macropores  302  with average separation distances higher than those of middle shell  704  region. In an embodiment, the average dimension of macropores  302  may be larger than the corresponding values of middle shell  704  region. Furthermore, the average dimension of macropores  302  in outer shell  706  region may be at least an order of magnitude larger than macropores  302  in the core  702  region. For example, when core  702  region includes an average macropore  302  dimension in a range of 10 to 50 microns and middle shell  704  region includes an average macropore  302  dimension in a range of 20 to 100 microns, outer shell  706  region may include an average macropore  302  dimension in a range 100 to 500 microns. Similarly, a porosity of outer shell  706  region may be greater than a porosity of both core  702  region and middle shell  704  region, but less than a porosity associated with loosely packed adsorptive particles  402 , e.g., less than 60%. For example, when macropores  302  of core  702  region occupy less than 30% of the core volume, and macropores  302  of middle shell  704  occupy more than 30%, e.g., 30-50%, of the middle shell volume, macropores  302  of outer shell  706  may occupy between 50-60% of the outer shell volume. 
     The value of porosity and average pore dimension of various shells are provided above by way of example, but those values and the shell configuration may vary. For example, in an embodiment, open-pore body  222  may include only two regions, e.g., core  702  and outer shell  706  regions, or may include more than three regions, e.g., may have more than two shell regions. Accordingly, the configuration of open-pore body  222  may be altered within the scope of the description to provide a porous structure having pores that decrease in size (on average) from outer surface  224  toward a center such that air transported from back volume  216  into open-pore body  222  is funneled into smaller and smaller passages within the network of interconnected macropores  302 . 
     Referring to  FIG.  11   , a sectional view of an audio speaker having an adsorptive insert and speaker housing with conforming curved contours is shown in accordance with an embodiment. As described above, all of outer surface  224  of open-pore body  222  may be spaced apart from inner surface  218  of speaker housing  202 . For example, open-cell spacers  228  may be located between opposing portions of inner surface  218  and outer surface  224  to maintain speaker housing  202  and open-pore body  222  in a spaced apart relationship. In addition to being spaced apart from each other, all or part of outer surface  224  may conform to opposing portions of inner surface  218 . For example, outer surface  224  may include one or more outer contour  1102  opposing corresponding inner contour(s)  1104  of inner surface  218 . Outer contour  1102  and inner contour  1104  may be conforming. That is, a shape of outer surface  224  along outer contour  1102  may essentially be a negative of a shape of inner surface  218  along inner contour  1104 . Accordingly, outer surface  224  may include a curvature  1106  along outer contour  1102  having a radius of curvature that is similar or identical to a radius of curvature of a corresponding curvature  1106  along inner contour  1104  of inner surface  218 . Thus, outer contour  1102  may conform with inner contour  1104 , and the curved surfaces may be spaced apart from each other by an intervening spacer. 
     Referring to  FIG.  12   , a sectional view of an audio speaker having an adsorptive insert and speaker housing with conforming angular contours is shown in accordance with an embodiment. Open-pore body  222  may be entirely surrounded by open-cell spacer  228 , and thus, the entirety of outer surface  224  may be in a spaced apart relationship with inner surface  218 . In addition to separating open-pore body  222  from inner surface  218 , open-cell spacer  228  may permit ingress/egress of air from back volume  216 . Additionally, the spacer may provide cushioning or shock absorption between open-pore body  222  and other structures. For example, open-cell spacer  228  may extend over loudspeaker receptacle  226  that conforms with loudspeaker (not shown) and can absorb mechanical impacts from loudspeaker  204  in the event that audio speaker  106  is dropped or otherwise jolted. Thus, open-pore body  222  may be shaped to closely conform with inner surface  218 , as well as other surfaces of audio speaker components, while being cushioned from impacts therewith by open-cell spacers  228 . 
     Still referring to  FIG.  12   , the conforming contours need not be curves (as in the case of curvature  1106  shown in  FIG.  11    and loudspeaker receptacle  226  shown in  FIG.  12   ), but may be angular. For example, outer surface  224  may include a corner  1202  along outer contour  1102  having one or more angles (as in the case of a pyramidal corner) and the corner may be similar or identical to an angular configuration of a corresponding corner  1202  along inner contour  1104  of inner surface  218 . Accordingly, outer contour  1102  may conform to inner contour  1104 , and the contours may be curved, angular, or otherwise-shaped surfaces that are spaced apart from each other by an intervening spacer. 
     Referring to  FIG.  13   , a sectional view of an audio speaker having an adsorptive insert with protrusions to space apart an open-pore body from a speaker housing is shown in accordance with an embodiment. In an embodiment, substantially all (but not the entirety) of outer surface  224  of open-pore body  222  is spaced apart from inner surface  218  of speaker housing  202 . More particularly, adsorptive insert  220  may include one or more protrusions  1302  extending from a surrounding portion of outer surface  224  to contact inner surface  218  and thereby maintain the surrounding portion of outer surface  224  in a spaced apart relationship with inner surface  218 . Protrusions  1302  may be formed by removing material of open-pore body  222  around protrusions  1302  to a level of the surrounding portion of outer surface  224 . Alternatively, protrusions  1302  may be separately formed porous structures that are adhered or otherwise attached to outer surface  224 . In any case, protrusions  1302  may include respective apices that are disposed against inner surface  218 . For example, protrusion  1302  may include a conical structure extending from a base at a surrounding portion of outer surface  224  and terminating in an apex  1304 , e.g., a pointed or flattened region of protrusion  1302 . Apex  1304  may include a surface area that is substantially less than the entire surface area of outer surface  224 . In an embodiment, a surface area of each apex  1304  may be less than 1% of the entire surface area of outer surface  224  to ensure that no more than 10% of outer surface  224  (including apex  1304  surface area) is pressed against inner surface  218 . As such, at least 90% of outer surface  224  of open-pore body  222  may be exposed to back volume  216  to allow air to migrate through macropores  302  along outer surface  224  into the macroscopic network of open-pore body  222 . As described above, open-pore body  222  having protrusions  1302  that act as spacers between outer surface  224  and inner surface  218  may also occupy a majority of back volume  216 , and include contours (such as corners  1202 ) that conform to opposing contours of speaker housing  202  or loudspeaker  204 . 
     Referring to  FIG.  14   , a flowchart of a method of forming an audio speaker having an adsorptive insert within a speaker housing is shown in accordance with an embodiment. At operation  1402 , speaker housing  202  may be formed, e.g., in a plastic or metal molding process, to include speaker port  205  and inner surface  218 . A rear cavity may be defined within inner surface  218  when speaker housing  202  is in an assembled condition. For example, speaker housing  202  may include multiple components, e.g., two halves, which are joined together along corresponding edges using adhesives, welding, or other processes to form speaker housing  202  and enclose the rear cavity. 
     At operation  1404 , open-pore body  222  may be formed from adsorptive particles  402 . Adsorptive particles  402  may be bound together into a rigid monolithic structure. As described below, adsorptive particles  402  may be bonded using several processing techniques. For example, compaction, sintering, spark plasma sintering, extrusion, and scaffolding techniques may be used to transform loose adsorbent particles, e.g., in powder form, into open-pore body  222 . Several of the described processing techniques include methods of applying heat or pressure to the adsorptive particles  402  to bond the particles into a monolith having interconnected macropores  302  that occupy less than 60% of a spatial volume occupied by open-pore body  222 . Furthermore, the bonded adsorptive particles  402  may occupy a majority of the spatial volume. Accordingly, a rigid structure may be formed from adsorptive particles  402  and have a macroscopic porosity that is less than a porosity, on a per unit volume basis, of the adsorptive particles  402  if they were loosely packed. 
     At operation  1406 , the monolithically formed open-pore body  222  is, optionally, shaped with secondary processing techniques. For example, adsorptive particles  402  along outer surface  224  may be removed using known machining techniques, e.g., mechanical milling, laser cutting, or electrical discharge machining, to shape a portion of outer surface  224  into outer contour  1102  that has a same shape, or conforms with, an inner contour  1104  of a portion of inner surface  218 . Shaping of the monolithically formed open-pore body  222  may be achieved in other manners, including stamping, grinding, etc. Thus, open-pore body  222  formed by binding adsorptive particles  402  together may be subsequently shaped to achieve a predetermined shape, which optionally conforms to a shape of the rear cavity of speaker housing  202 . 
     At operation  1408 , the open-pore body  222  having the desired shape is inserted into the rear cavity of speaker housing  202 . Insertion may be through speaker port  205 . Alternatively, speaker housing  202  may have multiple components, e.g., halves, which are assembled around adsorptive insert  220 . For example, bottom wall  232  of speaker housing  202  opposite from speaker port  205  may be a cap such that open-pore body  222  may be inserted upward into rear cavity, and the bottom wall  232  cap may be glued or otherwise fastened to the mating side walls  234  of speaker housing  202  to seal open-pore body  222  within the rear cavity. 
     At operation  1410 , one or more open-cell spacer  228  may be located between outer surface  224  of open-pore body  222  and inner surface  218  of speaker housing  202 . Several spacers may be located along inner surface  218 , e.g., by bonding a back surface opposite from support surface  606  to the inner surface  218  of speaker housing  202 , prior to inserting open-pore body  222  into the rear cavity. Alternatively, a single open-cell spacer  228  may surround a portion of open-pore body  222 , e.g., as in the case of a sleeve placed around all sides of open-pore body  222 , or a pouch placed around the entirety of open-pore body  222 , prior to inserting open-pore body  222  into the rear cavity. Accordingly, the assembled audio speaker may include porous surfaces of open-cell spacer  228  that are in fluid communication through interconnected pores or channels  602  to transport air in the rear cavity to macropores  302  along outer surface  224  of open-pore body  222 . Furthermore, open-cell spacer(s)  228  may cushion and fasten open-pore body  222  in a spaced apart relationship with speaker housing  202 . 
     At operation  1412 , loudspeaker  204  is mounted in speaker port  205  to define back volume  216  between loudspeaker  204  and inner surface  218  of speaker housing  202 . Thus, by fully enclosing the rear cavity, back volume  216  may be defined between loudspeaker  204  and inner surface  218 , may encompass air and open-pore body  222 . Accordingly, air within the defined back volume  216  may be exchanged with open-pore body  222  for adsorption/desorption by micropores  502  within the bonded adsorptive particles  402  during sound generation by loudspeaker  204 . 
     Several processing techniques for bonding loose adsorptive particles  402  together into a rigid monolith, as used in operation  1404 , are now described. In an embodiment, adsorptive particles  402  may be bound together to form a rigid open-pore body  222  using a compaction method. In the compaction method, the adsorptive particles  402  may be loaded into a die having a desired shape, e.g., a cubical shape having a rectangular cross-sectional profile slightly smaller than a rectangular cross-sectional profile of speaker housing  202 . Inward pressure may then be applied to the adsorptive particles  402  through compression from the die to cause the particles to fuse together. Optionally, a chemical binder, e.g., a polymer, may be dispersed between the adsorptive particles  402  such that the pressure activates the binder to cause fusion of the adsorptive particles  402 . Accordingly, the pressure-fused adsorptive particles  402  may form a monolithic rigid structure having a macroscopic porosity lower than the loose adsorptive particles  402 . 
     In an embodiment, adsorptive particles  402  may be bound together to form a rigid open-pore body  222  using a sintering method that employs heat and pressure. For example, the adsorptive particles  402  may be compacted in a die of the desired shape to create a “green” material, which may be subsequently heated below the liquefaction point. As heat and inward pressure are applied over time, necks may form between the particles, causing the particles to become bonded and merged into a rigid structure. The sintering process may reduce the porosity, and increase the strength and rigidity, of the green material. Accordingly, a monolithically formed rigid open-pore body  222  having a macroscopic porosity less than the porosity of loosely packed adsorptive particles  402  may be formed. 
     A sintering process may also be used to form a hierarchical open-pore body  222 . For example, a first die may be used to form core  702  region of open-pore body  222  having a first porosity, which depends on the heat and inward pressure applied. Subsequently, the rigid core  702  region may be loaded into a second die and additional adsorptive particles  402  may be loaded around core  702  region. The additional adsorptive particles  402  may have the same or different size, shape, or micropore porosity as the raw adsorptive particles  402  used to form core  702  region. A different heat and inward pressure may be used to sinter the second layer of material around the core  702  region to form a rigid middle shell  704  region. For example, lower pressures may be applied during the firing process to create a more porous middle shell  704  region. Subsequently, the rigid core  702  and middle shell  704  regions may be loaded into a third die and additional adsorptive particles  402  may be loaded around the middle shell  704 . The additional adsorptive particles  402  may have the same or different size, shape, or micropore porosity as the raw adsorptive particles  402  used to form the core  702  and middle shell  704  regions. A different heat and inward pressure may be used to sinter the third layer of material around the core  702  and middle shell  704  regions to form a rigid outer shell  706 . For example, lower temperatures may be applied during a shorter firing process to create a more porous outer shell  706  region. As described above, the differences in raw material sizes and porosity, as well as the differences in sintering process parameters, may result in a tiered structure that is monolithic in the sense that it can be stably handled as a single structure, but which may include a hierarchical macroscopic network to funnel air from larger diameter macropores  302  in the outer shell  706  region to smaller diameter macropores  302  in the core  702  region. 
     Other sintering techniques may be used to form one or more layers of a rigid open-pore body  222 . For example, a die may be loaded with adsorptive particles  402  and compacted to form a green material. Spark plasma sintering may then be used to selectively apply electric charge to different regions of the green material to form different porous structures. For example, a first electric current may be applied only to a core  702  region of the monolith during formation, and then a second electric current may be applied only to a shell region around the core  702  region. These regionally applied currents may create different degrees of porosity throughout a monolithically formed structure, e.g., a less porous core  702  region surrounded by a more porous shell region. 
     In an embodiment, extrusion techniques may be used to form a rigid open-pore body  222 . Adsorptive particles  402  in powder form may be mixed with a chemical binder and then extruded through a die to form, e.g., a monolithic open-pore body  222  having a cylindrical shape. The open-pore body  222  may then be shaped using machining techniques to remove material and shape the open-pore body  222  into the desired final structure that conforms with speaker housing  202 . 
     In an embodiment, scaffolding techniques may be used to form a rigid open-pore body  222 . A scaffold having a macroscopic structure may be formed from a polymer. For example, a polymer may be shaped into sponge-like structure having interconnected pores or passages. Adsorptive particles  402 , e.g., adsorptive powders, may then be sprayed onto the sponge-like scaffold to surround  208  the polymer scaffold and partially fill the scaffold pores. In an embodiment, the sprayed adsorptive material may include interconnected macropores  302 . The macroscopic porosity of the sprayed structure may vary depending on the porosity of the initial polymer scaffold. Thus, a rigid open-pore body  222  may be formed from the coated scaffold. 
     The above processing techniques are provided by way of example and not limitation. For example, other processes, such as mixing adsorptive particles  402  with a chemical binder and then applying a catalyst to cause solidification of the binder and bonding of the adsorptive particles  402  may be used. Thus, a person of ordinary skill in the art will appreciate that numerous processing techniques may be used to bond adsorptive particles  402  to form a rigid open-pore body  222  having interconnected macropores, which may be used as a component of adsorptive insert  220 . 
     Referring to  FIG.  15   , a schematic view of an electronic device having an audio 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  1502  that execute instructions to carry out the different functions and capabilities described above. Instructions executed by the one or more processors  1502  of electronic device  100  may be retrieved from local memory  1504 , 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  1502  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 may adsorb and desorb randomly traveling air molecules as pressure fluctuates due to the generated sound. As a result, audio speaker  106  may have a higher efficiency at lower frequencies, as compared to a speaker without an adsorptive insert. Thus, audio speaker  106  may produce loud, rich sound, comparable to that of a much larger speaker, but within the form factor of a micro speaker. 
     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.