Abstract:
A headphone includes a housing defining an enclosed volume, an electro-acoustic transducer dividing the enclosed volume into a front volume and a rear volume, a first port in the housing coupling the front volume to an ear canal of a user, a second port in the housing coupling the front volume to space outside the ear, a third port in the housing coupling the rear volume to space outside the ear, and an ear tip configured to surround the first port and seal the ear canal from space outside the ear. The second port has a diameter and a length that provide an acoustic mass with an acoustic impedance with a high reactive component and a low resistive component.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/268,210, filed on May 2, 2014, which is a continuation of U.S. application Ser. No. 14/085,029, filed on Nov. 20, 2013, now U.S. Pat. No. 8,755,550, which is a continuation of application Ser. No. 12/857,462, filed on Aug. 16, 2010, now U.S. Pat. No. 8,594,351, which is a continuation-in-part of U.S. application Ser. No. 11/428,057, filed on Jun. 30, 2006, now U.S. Pat. No. 7,916,888, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This description relates generally to earphones and more specifically to earphone including port structures to equalize the frequency response. 
     As shown in  FIG. 1 , a human ear  10  includes an ear canal  12  which leads to the sensory organs (not shown). The pinna  11 , the part of the ear outside the head, includes the concha  14 , the hollow next to the ear canal  12 , defined in part by the tragus  16  and anti-tragus  18 . An earphone is generally designed to be worn over the pinna, in the concha, or in the ear canal. 
     SUMMARY 
     In general, in one aspect an earphone includes a first acoustic chamber including a reactive element and a resistive element in parallel, a second acoustic chamber separated from the first acoustic chamber by an acoustic transducer, and a housing to support the apparatus from the concha of a wearer&#39;s ear and to extend the second acoustic chamber into the ear canal of the wearer&#39;s ear. 
     Implementations may include one or more of the following features. 
     An acoustic damper is in the second acoustic chamber. The acoustic damper covers an opening in the second acoustic chamber. A portion of the acoustic damper defines a hole. A wall of the second acoustic chamber defines a hole that couples the second acoustic chamber to free space. 
     A cushion surrounds a portion of the housing to couple the housing to the concha and ear canal of the users ear. The cushion includes an outer region formed of a first material having a first hardness, and an inner region formed of a second material having a second hardness. The first material has a hardness of around 3 shore A to 12 shore A. The first material has a hardness of around 8 shore A. The second material has a hardness of around 30 shore A to 90 shore A. The second material has a hardness of around 40 shore A. A first region of the cushion is shaped to couple the second acoustic chamber to the ear canal, and a second region of the cushion is shaped to retain the apparatus to the ear, the second region not extending into the ear canal. The cushion is removable. A set of cushions of different sizes is included. 
     The reactive element and the resistive element cause the first acoustic chamber to have a resonance of between around 30 Hz and around 100 Hz. The resistive element includes a resistive port. The reactive element includes a reactive port. The reactive port includes a tube coupling the first acoustic chamber to free space. The reactive port has a diameter of between around 1.0 to around 1.5 mm and a length of between around 10 to around 20 mm. The reactive port has a diameter of around 1.2 mm. The reactive port and the resistive port couple to the first acoustic chamber at about radially opposite positions. The reactive port and the resistive port are positioned to reduce pressure variation on a face of the transducer exposed to the first acoustic chamber. A plurality of reactive or resistive ports are about evenly radially distributed around a center of the acoustic transducer. A plurality of resistive ports are about evenly radially distributed around a center of the acoustic transducer, and the reactive port couples to the first acoustic chamber at about the center of the acoustic transducer. A plurality of reactive ports are about evenly radially distributed around a center of the acoustic transducer, and the resistive port couples to the first acoustic chamber at about the center of the acoustic transducer. 
     The first acoustic chamber is defined by a wall conforming to a basket of the acoustic transducer. The first acoustic chamber has a volume less than about 0.4 cm 3 , including volume occupied by the transducer. The first acoustic chamber has a volume less than about 0.2 cm 3 , excluding volume occupied by the transducer. The second acoustic chamber is defined by the transducer and the housing, the housing defines a first and a second hole, the first hole being at an extremity of the wall extending into the wearer&#39;s ear canal, and the second hole being positioned to couple the acoustic chamber to free space when the apparatus is positioned in the wearer&#39;s ear; and an acoustic damper is positioned across the first hole and defines a third hole having a smaller diameter than the first hole. 
     A circuit is included to adjust a characteristic of signals provided to the acoustic transducer. A set of earphones includes a pair of earphones. 
     In general, in one aspect, a cushion includes a first material and a second material and is formed into a first region and a second region. The first region defines an exterior surface shaped to fit the concha of a human ear. The second region defines an exterior surface shaped to fit the ear canal of a human ear. The first and second regions together define an interior surface shaped to accommodate an earphone. The first material occupies a volume adjacent to the interior surface. The second material occupies a volume between the first material and the first and second outer surfaces. The first and second materials are of different hardnesses. 
     Implementations may include one or more of the following features. The first material has a hardness in the range of about 3 shore A to about 12 shore A. The first material has a hardness of about 8 shore A. The second material has a hardness in the range of about 30 shore A to about 90 shore A. The first material has a hardness of about 40 shore A. 
     In general, in another aspect, an earphone includes a first acoustic chamber having a first reactive port and a first resistive port in a parallel configuration to couple the first chamber with outside atmosphere, a second acoustic chamber separated from the first acoustic chamber by an acoustic transducer. The second acoustic chamber includes a second acoustic chamber port to provide both pressure equalization of the second chamber and equalization of the earphone to a predetermined frequency response. The earphone also includes a housing to support the earphone from the concha of a wearer&#39;s ear and to extend the second acoustic chamber into the ear canal of the wearer&#39;s ear, the housing and the transducer define the second acoustic chamber. The second acoustic chamber port can include a plurality of ports. The earphone can include a cushion as described above. 
     In general, in another aspect, an earphone includes a first acoustic chamber having a first reactive port and a first resistive port in arranged in a parallel configuration to couple the first chamber with outside atmosphere, a second acoustic chamber separated from the first acoustic chamber by an acoustic transducer. The second acoustic chamber includes a second reactive port and a second resistive port to provide both pressure equalization of the second chamber and equalization of the earphone to a predetermined frequency response, and a housing to support the apparatus from the concha of a wearer&#39;s ear and to extend the second acoustic chamber into the ear canal of the wearer&#39;s ear. The second reactive and second resistive ports can be arranged in a parallel configuration in some embodiments and arranged in a series configuration in other embodiments. The earphone can include a cushion as described above. 
     Other features and advantages will be apparent from the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a human ear. 
         FIG. 2A  is a perspective view of an earphone located in the ear. 
         FIG. 2B  is an isometric view of an earphone. 
         FIG. 3A  is a schematic cross section of an earphone. 
         FIG. 3B  is an exploded isometric view of an earphone. 
         FIG. 3C-3G  are schematic cross sections of multiple embodiments of an earphone. 
         FIGS. 4A-4C and 6  are graphs of earphone frequency response. 
         FIG. 5  is a circuit diagram for a passive electrical equalization circuit of an earphone. 
         FIGS. 7A-7D  are isometric views of portions of an earphone. 
         FIGS. 8A and 8B  are side views of a cushion. 
         FIG. 8C  is a top view of a cushion. 
         FIG. 8D  is an isometric view of a cushion. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 2A and 2B , an earphone  100  has a first region  102  designed to be located in the concha  14  of the wearer&#39;s ear  10 , and a second region  104  to be located in the ear canal  12 . ( FIGS. 2A and 2B  show a wearer&#39;s left ear and corresponding earphone  100 . A complementary earphone may fit the right ear, not shown. In some examples, only one earphone is provided. In some examples, a left earphone and a right earphone may be provided together as a pair.) A cushion  106  couples the acoustic components of the earphone to the physical structure of a wearer&#39;s ear. A plug  202  connects the earphone to a source of audio signals, such as a CD player, cell phone, MP3 player, or PDA (not shown), or may have multiple plugs (not shown) allowing connection to more than one type of device at a time. A circuit housing  204  may include circuitry for modifying the audio signal, for example, by controlling its volume or providing equalization. The housing  204  may also include switching circuitry, either manual or automatic, for connecting the signals output by one or another of the above mentioned sources to the earphone. A cord  206  conveys audio signals from the source to the earphones. In some examples, the signals may be communicated wirelessly, for example, using the Bluetooth protocol, and the cord  206  would not be included. Alternatively or additionally, a wireless link may connect the circuitry with one or more of the sources. 
     As shown in  FIGS. 3A and 3B , the first region  102  of the earphone includes a rear chamber  112  and a front chamber  114  defined by shells  113  and  115 , respectively, on either side of a driver  116 . In some examples, a 16 mm diameter driver is used. Other sizes and types of acoustic transducers could be used depending, for example, on the desired frequency response of the earphone. The front chamber  114  extends ( 126 ) to the entrance to the ear canal  12 , and in some embodiments into the ear canal  12 , through the cushion  106  and ends at acoustic resistance element  118 . In some examples, the resistance element  118  is located within the extended portion  126  of the front chamber  114 , rather than at the end, as illustrated. An acoustic resistance element dissipates a proportion of acoustic energy that impinges on or passes through it. In some examples, the front chamber  114  includes a pressure equalization (PEQ) hole  120 . The PEQ hole  120  serves to relieve air pressure that could be built up within the ear canal  12  and front chamber  114  when the earphone  100  is inserted into the ear  10 . The rear chamber  112  is sealed around the back side of the driver  116  by the shell  113 . In some examples, the rear chamber  112  includes a reactive element, such as a port (also referred to as a mass port)  122 , and a resistive element, which may also be formed as a port  124 . U.S. Pat. No. 6,831,984 describes the use of parallel reactive and resistive ports in a headphone device, and is incorporated here by reference. Although we refer to ports as reactive or resistive, in practice any port may have both reactive and resistive effects. The term used to describe a given port indicates which effect is dominant. In the example of  FIG. 3B , the reactive port is defined by spaces in an inner spacer  117 , the shell  113 , and an outer cover  111 . A reactive port like the port  122  is, for example, a tube-shaped opening in what may otherwise be a sealed acoustic chamber, in this case rear chamber  112 . A resistive port like the port  124  is, for example, a small opening in the wall of an acoustic chamber covered by a material providing an acoustical resistance, for example, a wire or fabric screen that allows some air and acoustic energy to pass through the wall of the chamber. 
     Each of the cushion  106 , cavities  112  and  114 , driver  116 , damper  118 , hole  120 , and ports  122  and  124  have acoustic properties that may affect the performance of the earphone  100 . These properties may be adjusted to achieve a desired frequency response for the earphone  100 . 
     Further embodiments of an earphone are shown in  FIGS. 3C-3G . As shown in  FIG. 3C , an earphone  200  includes a resistive port  205  to replace the pressure equalization hole  120  of earphone  100  in  FIG. 3A . The remaining elements of earphone  200  substantially correspond to those of earphone  100  in  FIG. 3A , and are denoted by the same referenced numbers. The resistive port  205  extends from the front chamber  114  to the outside atmosphere. The resistive port  205  may be a single port or multiple ports and includes a material disposed within the port opening to provide acoustic resistance, such as a wire cloth, for example, 70×088 Dutch twill wire cloth, available from Cleveland Wire of Cleveland, Ohio. The resistive port  205  may be appropriately sized and the resistive element within the port  205  appropriately configured to equalize a desired frequency response for the earphone  200  and also provide the pressure equalization function of provided by the PEQ  120  in earphone  100 . The resistive port  205  may be a single, circular opening with a diameter of between 3 and 6 mm. In one specific embodiment, the resistive port  205  is made up of two identical ports with a combined effective area equivalent to a circle having a diameter of about 5 mm. 
     As shown in  FIG. 3D , an earphone  225  includes a port  230  extending from the front chamber  114  to the outside atmosphere to replace the pressure equalization hole  120  of earphone  100  in  FIG. 3A . The remaining elements of earphone  225  substantially correspond to those of earphone  100  in  FIG. 3A  as described above, and are denoted by the same referenced numbers. The port  230  includes both resistive and reactive elements in a series configuration. The port  230  may be appropriately sized and the resistive element configured to equalize a desired frequency response for the earphone  200  and also provide the pressure equalization function provided by the PEQ  120  in earphone  100 . In one embodiment, the resistive-reactive port  230  is predominantly resistive such that the reactance of the port  230  does not begin to affect the total port impedance until the frequencies are greater than about 1 kHz. 
     As shown in  FIG. 3E , an earphone  250  includes a reactive port  255  and resistive port  260  in a parallel configuration, which together, replace the pressure equalization hole  120  of earphone  100  in  FIG. 3A . The remaining elements of earphone  250  correspond to earphone  100  in  FIG. 3A  as described above, and are denoted by the same referenced numbers. The ports  255 ,  260  extend from the front chamber  114  to the outside atmosphere. The ports  255 ,  260  may be appropriately sized and the resistive element of resistive port  260  configured to equalize a desired frequency response for the earphone  250  and also provide the pressure equalization function of the PEQ  120  of earphone  100 . 
     As shown in  FIG. 3F , an earphone  275  includes a resistive port  280  to replace the pressure equalization hole  120  of earphone  100  in  FIG. 3A , and a reactive port  285  in a parallel configuration. The remaining elements of earphone  275  correspond to earphone  100  in  FIG. 3A  as describe above, and are denoted by the same referenced numbers. The resistive port  280  extends from the front chamber  114  to the outside atmosphere and is located in the first region  102  of the earphone  275 . The reactive port  285  is located in the extended portion  126  of the chamber  114 . The reactive port  285  also extends through and is formed by an opening in the lower portion  110  of the cushion  106 . The opening in the lower portion  110  of the cushion  106  substantially aligns with the opening in the extended portion  126  when the cushion  106  is attached to extended portion  126 . Either the extended portion  126  of the front chamber  114  or the cushion  106  can include features to orient the relative rotational position of the front portion  126  and cushion  106  to align the front portion and cushion portions forming the reactive port  285 . The ports  280 ,  285  may be appropriately sized and the resistive element of resistive port  280  configured to equalize a desired frequency response for the earphone  275  and also provide the pressure equalization function of the PEQ  120  of earphone  100 . 
     As shown in  FIG. 3G , an earphone  300  includes a reactive port  305  to replace the pressure equalization hole  120  of earphone  100  in  FIG. 3A , and a resistive port  310 . The remaining elements of earphone  300  correspond to earphone  100  in  FIG. 3A , and are denoted by the same referenced numbers. The reactive and resistive port positions for earphone  300  are reversed as compared with the reactive and resistive port positions of earphone  275  ( FIG. 3F ). The reactive port  305  and the resistive port  310  extend from the front chamber  114  to the outside atmosphere and are arranged in a parallel configuration. The reactive port  305  is located in the first region  102  of the earphone  300 . The resistive port  310  is located in the extended portion  126  of the front chamber  114 . The resistive port  310  also extends through and is formed by an opening in the lower portion  110  of the cushion  106 . The opening in the lower portion  110  of the cushion  106  substantially aligns with the opening in the extended portion  126  when the cushion  106  is attached to extended portion  126 . Either the extended portion  126 , or the cushion  106  can include features to orient the relative rotational position of the extended portion  126  and cushion  106  to align the nozzle and cushion portions of the resistive port  310 . The ports  305 ,  310  may be appropriately sized and the resistive element of resistive port  310  configured to equalize a desired frequency response for the earphone  300  and also provide the pressure equalization function of the PEQ  120  of earphone  100 . 
     Additional elements, such as active or passive equalization circuitry, may also be used to adjust the frequency response. 
     The effects of the cavities  112  and  114  and the ports  122  and  124  of earphone  100  are shown by graph  400  in  FIG. 4A . The frequency response of a traditional earbud headphone (that is, one that does not extend into the ear canal and does not provide a seal to the ear canal) is shown as curve  404  in  FIG. 4A . Traditional ear bud designs have less low frequency response than may be desired, as shown by section  404   a , which shows decreased response below around 200 Hz. To increase low frequency response and sensitivity, a structure  126 , sometimes referred to as a nozzle, may extend the front chamber  114  into the ear canal, facilitating the formation of a seal between the cushion  106  and the ear canal. Sealing the front chamber  114  to the ear canal decreases the low frequency cutoff, as does enclosing the rear of transducer  116  with rear chamber  112  including the ports  122  and  124 . Together with a lower portion  110  of the cushion, the lower portion  126  (or nozzle) of the front chamber  114  provides better seal to the ear canal than earphones that merely rest in the concha, as well as a more consistent coupling to the user&#39;s ears, which reduces variation in response among users. The tapered shape and pliability of the cushion allow it to form a seal in ears of a variety of shapes and sizes. The nozzle and cushion design is described in more detail below. 
     In some examples, the rear chamber  112  has a volume of 0.28 cm 3 , which includes the volume of the driver  116 . Excluding the driver, the rear chamber  112  has a volume of 0.08 cm 3 . An even smaller rear chamber may be formed by simply sealing the rear surface of the driver  116  (e.g., sealing the basket of a typical driver, see the cover  702  in  FIG. 7A ). Other earbud designs often have rear cavities of at least 0.7 cm 3 , including 0.2 cm 3  for the driver. 
     The reactive port  122  resonates with the back chamber volume. In some examples, it has a diameter in the range of about 1.0-1.5 mm and a length in the range of about 10-20 mm long. In some embodiments, the reactive port is tuned to resonate with the cavity volume around the low frequency cutoff of the earphone. In some embodiments, this is in the low frequency range between 30 Hz and 100 Hz. In some examples, the reactive port  122  and the resistive port  124  provide acoustical reactance and acoustical resistance in parallel, meaning that they each independently couple the rear chamber  112  to free space. In contrast, reactance and resistance can be provided in series in a single pathway, for example, by placing a resistive element such as a wire mesh screen inside the tube of a reactive port. In some examples, a parallel resistive port is made from a 70×088 Dutch twill wire cloth, for example, that available from Cleveland Wire of Cleveland, Ohio, and has a diameter of about 3 mm. Parallel reactive and resistive elements, embodied as a parallel reactive port and resistive port, provides increased low frequency response compared to an embodiment using a series reactive and resistive elements. The parallel resistance does not substantially attenuate the low frequency output while the series resistance does. The frequency response of an earphone having a combination of a small back chamber with parallel reactive and resistive ports and a front chamber with a nozzle is shown by curve  416  in  FIG. 4A . Using a small rear cavity with parallel ports allows the earphone to have improved low frequency output and a desired balance between low frequency and high frequency output. Various design options for the ports are discussed below. 
     High frequency resonances in the front chamber structure, for example, those represented by peaks  416   a , can be damped by placing an acoustical resistance (sometimes referred to as a damper or acoustical damper), element  118  in  FIGS. 3A and 3B , in series with the output of the nozzle  126 , as shown in  FIG. 3A . In some examples, a stainless steel wire mesh screen of 70×800 Dutch twill wire cloth is used. In some examples, a small hole  128  is formed in the center of the screen  118 . In some examples, the screen  118  is about 4 mm in diameter, and the hole is about 1 mm. Other sizes may be appropriate for other nozzle geometries or other desired frequency responses. The hole  128  in the center of the screen  118  slightly lowers the acoustical resistance of the screen  118 , but does not block low frequency volume velocity significantly, as can be seen in region  422   a  of curve  422 . The curve  416  is repeated from  FIG. 4A , showing the effects of an undamped nozzle and small back chamber with reactive and resistive ports in parallel. Curve  422  has substantially more low frequency output than curve  418   a , which shows the effects of a damper  118  without a hole. A screen with a hole in it provides damping of the higher frequency resonances (compare peaks  422   b  to peaks  416   a ), though not as much as a screen without a hole (compare peaks  422   b  to peaks  418   b ), but substantially increases low frequency output, nearly returning it to the level found without the damper. 
     The PEQ hole  120  of earphone  100  is located so that it will not be blocked when in use. For example, the PEQ hole  120  is not located in the cushion  106  that is in direct contact with the ear, but away from the ear in the front chamber  114 . The primary purpose of the hole is to avoid an over-pressure condition when the earphone  100  is inserted into the user&#39;s ear  10 . Additionally, the hole can used to provide a fixed amount of leakage that acts in parallel with other leakage that may be present. This helps to standardize response across individuals. In some examples, the PEQ hole  120  has a diameter of about 0.50 mm. Other sizes may be used, depending on such factors as the volume of the front chamber  114  and the desired frequency response of the earphones. The frequency response effect of the known leakage through the PEQ hole  120  is shown by a graph  424  in  FIG. 4C . Curve  422  is repeated from  FIG. 4B , showing the response with the other elements (small rear chamber with parallel reactive and resistive ports, front chamber with nozzle, and screen damper with small hole in center across nozzle opening) but without the PEQ hole  120 , while curve  428  shows the response with the PEQ hole providing a known amount of leakage. Adding the PEQ hole makes a trade off between some loss in low frequency output and more repeatable overall performance. 
     Some or all of the elements described above can be used in combination to achieve a particular frequency response (non-electronically). In some examples, additional frequency response shaping may be used to further tune sound reproduction of the earphones. One way to accomplish this is passive electrical equalization using circuitry like that shown in  FIG. 5 . For example, if a resonance remained at 1.55 KHz after tuning the acoustic components of the earphones, a passive equalization circuit  500  including resistors  502  and  504  and capacitors  506  and  508  connected as indicated may be used. In circuit  500 , the output resistance  510  represents the nominal 32 ohm electrical impedance of standard earphones, and the input voltage source  512  represents the audio signal input to the headphones, for example, from a CD player. Graph  514  in  FIG. 6  shows the electrical frequency response curve  516  that results from circuit  500 , indicating a dip  516   a  in response at 1.55 KHz corresponding to a Q factor of 0.75, with an 8 db decrease in output voltage at the dip frequency compared to the response at low frequencies. The actual values of the resistors and capacitors, and the resulting curve, will depend on the specific equalization needs based on the details of the acoustic components of the earphone. Such circuitry can be housed in-line with the earphones, for example, inside the circuit housing  204  ( FIG. 2A ). 
     Options for the design of the ports  122  and  124  are shown in  FIGS. 7A-7D . As shown in  FIG. 7A , a reactive port  122   a  extends out from the back cover  702  of the rear chamber  112 . A resistive port  124   a  is located on the opposite side of the cover  702 . Such a reactive port could be bent or curved to provide a more compact package, as shown by a curved port  122   b  formed in the inner spacer  117  in  FIG. 7B . In some examples, as shown in  FIGS. 3B, 7C, and 7D , the full tube of the port is formed by the assembly of the inner spacer  117  with the outer shell  113 , which also may form the outer wall of the rear chamber  112 . In the example of  FIGS. 7C and 7D , an opening  704  in the inner spacer  117  is the beginning of the port  122 . The port curves around the circumference of the earphone to exit at an opening  706  in the outer shell  113 . A portion of the shell  113  is cut away in  FIG. 7D  so that the beginning opening  704  can be seen.  FIG. 7C  also shows an opening  708  for the resistive port  124 . In some examples, arranging ports symmetrically around the rear chamber  112  as shown in  FIG. 7A  has advantages, for example, it helps to balance pressure differences across the rear chamber  112  (which would appear across the back of the diaphragm of the driver  116 ,  FIG. 7B ) that could otherwise occur. Pressure gradients across the driver diaphragm could induce rocking modes. Some examples may use more than one reactive port or resistive port, or both types of ports, evenly radially distributed around the rear chamber  112 . A single resistive port (or single reactive port) could be centrally located, with several reactive (or resistive) ports evenly distributed around it. 
     The cushion  106  is designed to comfortably couple the acoustic elements of the earphone to the physical structure of the wearer&#39;s ear. As shown in  FIGS. 8A-8D , the cushion  106  has an upper portion  802  shaped to make contact with the tragus  16  and anti-tragus  18  of the ear (see  FIGS. 1 and 2A ), and a lower portion  110  shaped to enter the ear canal  12 , as mentioned above. In some examples, the lower portion  110  is shaped to fit within but not apply significant pressure on the flesh of the ear canal  12 . The lower portion  110  is not relied upon to provide retention of the earphone in the ear, which allows it to seal to the ear canal with minimal pressure. A void  806  in the upper portion  802  receives the acoustic elements of the earphone (not shown), with the nozzle  126  ( FIG. 3 ) extending into a void  808  in the lower portion  110 . In some examples, the cushion  106  is removable from the earphone  100 , and cushions of varying external size may be provided to accommodate wearers with different-sized ears. 
     In some examples, the cushion  106  is formed of materials having different hardnesses, as indicated by regions  810  and  812 . The outer region  810  is formed of a soft material, for example, one having a durometer of 8 shore A, which provides good comfort because of its softness. Typical durometer ranges for this section are from 3 shore A to 12 shore A. The inner region  812  is formed from a harder material, for example, one having a durometer of 40 shore A. This section provides the stiffness needed to hold the cushion in place. Typical durometer ranges for this section are from 30 shore A to 90 shore A. In some examples, the inner section  812  includes an O-ring type retaining collar  809  to retain the cushion on the acoustic components. The stiffer inner portion  812  may also extend into the outer section to increase the stiffness of that section. In some examples, variable hardness could be arranged in a single material. 
     In some examples, both regions of the cushion are formed from silicone. Silicone can be fabricated in both soft and more rigid durometers in a single part. In a double-shot fabrication process, the two sections are created together with a strong bond between them. Silicone has the advantage of maintaining its properties over a wide temperature range, and is known for being successfully used in applications where it remains in contact with human skin. Silicone can also be fabricated in different colors, for example, for identification of different sized cushions, or to allow customization. In some examples, other materials may be used, such as thermoplastic elastomeric (TPE). TPE is similar to silicone, and may be less expensive, but is less resistant to heat. A combination of materials may be used, with a soft silicone or TPE outer section  812  and a hard inner section  810  made from a material such as ABS, polycarbonate, or nylon. In some examples, the entire cushion may be fabricated from silicone or TPE having a single hardness, representing a compromise between the softness desired for the outer section  812  and the hardness needed for the inner section  810 . 
     Other embodiments are within the scope of the following claims.