TECHNICAL FIELD 
     This description relates to increasing the mechanical or acoustic impedance of a headphone cushion to reduce the audibility of outside sounds without substantially increasing the axial stiffness of the cushion. 
     BACKGROUND 
     For background, reference is made to commonly owned U.S. Pat. Nos. 4,922,452 and 6,597,792, the entire contents of which are hereby incorporated by reference. 
     SUMMARY 
     In a first aspect, a headset including an earcup having a front opening adapted to be adjacent to the ear of the user, a baffle disposed within the earcup to define front and rear cavities, a cushion extending around the periphery of the front opening of the earcup and constructed and arranged to accommodate the ear of the user, the cushion having a first density, an inner radial portion, and an outer radial portion opposite the inner radial portion, a cushion cover substantially surrounding the cushion to form a headphone cushion assembly, and a high impedance component having a second density and being disposed proximate the outer radial portion to increase the transmission loss of the cushion along a radial direction. 
     In various embodiments, the headset can include a transducer inside the earcup. The second density can be substantially higher than the first density. In some embodiments the high impedance component is interposed between the outer radial portion of the cushion and the cushion cover. In others embodiments, the high impedance component is interposed between the inner radial portion of the cushion and the cushion cover. In some embodiments, the high impedance component is disposed adjacent the cushion cover. In some embodiments, the high impedance component includes a substantially rigid ring. In still further embodiments, the high impedance component includes a colloidal ring, such as, for example, a gel layer. In some embodiments, the high impedance component includes polyurethane foam. In some embodiments, the cushion cover includes a plurality of openings extending along the inner radial portion of the cushion to acoustically add the volume of the cushion to the volume of the earcup and enhance passive attenuation of the headset. In some embodiments, the cushion cover includes an acoustically transparent mesh along the inner radial portion of the cushion to acoustically add the volume of the cushion to the volume of the earcup and enhance passive attenuation of the headset. In some specific embodiments, the outer radial portion of the cushion has an average area density greater than about 0.03 g/cm 2  and the headphone cushion assembly has an axial stiffness per contact area less than about 8 gf/mm/cm 2 . In some embodiments, the headphone cushion assembly has an axial stiffness per contact area less than about 4 gf/mm/cm 2 . 
     The headphone cushion assembly may be a substantially toroidal shape, such as for example, circumaural or is supra-aural. In some embodiments, the headset further includes a microphone inside the earcup adjacent to a driver; and active noise reducing circuitry intercoupling the microphone and the driver constructed and arranged to provide active noise cancellation. In some embodiments, the inner radial portion of the cushion cover is constructed and arranged to furnish additional damping to help smooth an audio response at an ear of a user and control stability when the headset is not being worn on a head of the user. In some embodiments, the cushion cover includes a plurality of openings such that the volume of the cushion is acoustically added to the volume of the earcup. In some specific embodiments, the cushion adheres to the cushion cover with a peel strength greater than about 0.1 gf/mm, and in other embodiments, the foam adheres to the cushion cover with a peel strength greater than about 0.4 gf/mm. In some embodiments, the cushion includes open cell foam and has a bulk density between about 2 pcf and about 6 pcf, and can have an elastic modulus between about 1 kPa and about 10 kPa, or between about 2 kPa and about 5 kPa. In some embodiments, the high impedance component includes a silicone material. 
     In a second aspect, an apparatus for blocking sound includes an earcup having a front opening adapted to be adjacent the ear of a user; and a headphone cushion assembly extending around the periphery of the front opening of the earcup, the cushion assembly having an inner radial portion, and an outer radial portion opposite the inner radial portion and the ratio of radial stiffness to axial stiffness per contact area of the headphone cushion assembly is greater than about 10 cm 2 . In some embodiments, a stiffening component is attached to the outer radial portion of the headphone cushion assembly. In still other embodiments, a stiffening component is attached to the outer radial portion of the headphone cushion assembly. In various embodiments, the stiffening component includes a substantially rigid support ring and/or a gel layer. In some embodiments, the headphone cushion assembly may be a substantially toroidal shape. 
     In another aspect, a headphone cushion assembly includes a cushion comprising an open cell foam and adapted to be adjacent the ear of the user; an inner cushion cover substantially covering the inner portion of the cushion proximate the ear of the user; the inner cushion cover comprising a plurality of openings, and an outer cushion cover substantially covering the outer part of the cushion distal to the ear of the user, the outer cushion cover comprising a first layer having an average area density less than about 0.03 g/cm 2  and a second layer attached to the first layer, the second layer having an average area density greater than about 0.045 g/cm 2 . 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of a headphone assembly on a head. 
         FIG. 2A  is a perspective drawing of one embodiment of a headphone cushion including a stiffening component and  FIG. 2B  is plan view of one embodiment of a headphone cushion. 
         FIG. 3  is a sectional view of a headphone cushion including a stiffening ring. 
         FIG. 4  is a sectional view of a headphone cushion including a high density layer. 
         FIG. 5  is a drawing of an outer cover including a high density layer. 
         FIG. 6  is a sectional view of an earcup assembly. 
         FIG. 7  is a graph of sound attenuation through a headphone assembly including a stiffening ring as measured on a test fixture. 
         FIG. 8  is a graph of sound attenuation through a headphone assembly including a stiffening ring as measured on a head. 
         FIG. 9  is a graph of sound attenuation through a headphone assembly including a high density layer as measured on a test fixture. 
         FIG. 10  is a graph of sound attenuation through a headphone assembly including a high density layer as measured on a head. 
         FIG. 11  is a sectional view of a test method for measuring axial stiffness. 
         FIG. 12  is a sectional view of a test method for measuring radial stiffness. 
         FIG. 13  is a sectional view of a test method for measuring peel strength. 
         FIG. 14  is a sectional view of an earcup assembly including active noise reducing circuitry. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a diagrammatic view one embodiment of a headphone assembly  100  worn by a user on a human head  102  having ears  104 . The headphone assembly  100  includes suspension assembly  106 , transducer assembly  108 , stiffening component  110 , headphone cushion  112 , audio opening  114 , and cover  116 . Headphone assembly  100  is shown covering and substantially surrounding ears  104  and accordingly, is referred to as circumaural headphones. Alternatively, headphone assembly  100  may be an on-the-ear (supra-aural) set of headphones. Stiffening component  110  serves to increase the impedance of the outer cover of the cushion thus reducing the sound transmission through headphone assembly  110 , thereby improving the isolation from outside noise for the headphones listener. In some embodiments, the stiffening component does not appreciable change the axial stiffness of the cushion so as not to impact the comfort of the headphone assembly to the user. An earcup assembly is formed by the combination of transducer assembly  108 , headphone cushion  112 , and cover  116 . Optionally, stiffening component  110  may be included in the earcup assembly. The earcup assembly may have a substantially toroidal shape to fit over or on the ear  104 . 
     The stiffening component  110  may be shaped in the form of a support ring that encircles the headphone cushion  112 . Cover  116  may extend over the exterior portion of headphone cushion  112 . Cover  116  may extend over the interior portion of headphone cushion  112 . Interior cavity  118  is formed by transducer assembly  108 , headphone cushion  112 , and head  102 . Headphone cushion  112  may be constructed of open cell foam. If headphone cushion  112  is constructed of open cell foam, audio openings  114  allow the volume of the headphone cushion  112  to combine with interior volume  118 . This combined volume is useful for tuning the audio characteristics of headphone assembly  100 . Audio openings  114  are constructed and arranged to furnish additional damping to help smooth the audio response of headphone assembly  100  and control stability when headphone assembly  100  is not being worn. For a description of tuning using audio openings and combined volume, reference is made to U.S. Pat. Nos. 4,922,542 and 6,597,792. 
     The bulk density of foam is defined as the density of the foam in its expanded state. In some implementations, headphone cushion  112  may have a bulk density of about 2 to about 6 pounds-mass per cubic foot (pcf). In one implementation, the headphone cushion  112  includes a foam having a bulk density of about 5 pcf. In some implementations, the headphone cushion  112  includes a foam having an elastic modulus between 1 and 10 kiloPascals (kPa). In one implementation, the headphone cushion  112  includes a foam having an elastic modulus between about 2 and about 5 kPa. High stiffness foam is useful to reduce sound transmission through headphone cushion  112 . However, foam that is too stiff may reduce the comfort of the headphones. 
     Referring to  FIGS. 2A and 2B , in one embodiment of a headphone cushion assembly  200  includes gasket  202 , inside cover  204 , outside cover  206 , stiffening ring  208 , and front surface  210 . The headphone cushion assembly for only one ear is depicted but it is understood by persons of ordinary skill in the art that headphone cushion assemblies for two ears are included in a set of headphones. Front surface  210  fits against the head of the listener while the headphone is in use. Gasket  202  fits between the headphone cushion assembly  200  and transducer assembly  108  to affect a seal at the interface. Inside cover  204  and outside cover  206  may be one continuous piece of material in some embodiments. Inside cover  204  and outside cover  206  may be made of plastic, leather, leatherette, or leather-like plastic (also known as pleather) material. In  FIG. 2A , stiffening ring  208  is attached to the outside of outside cover  206 . Alternatively, stiffening ring  208  may be attached to the inside of outside cover  206 . Headphone cushion assembly  200  may have a substantially toroidal shape to fit over or on the shape of the human ear. In some embodiments, the headphone cushion assembly  200  further includes a plurality of openings  212  ( FIG. 2B ) disposed along the inside cover  204  to expose the underlying foam and thereby increase the effective volume of the earcup by the volume of the underlying foam. In these embodiments, passive attenuation is enhanced and additional damping is provided to help smooth the audio response and control stability of the feedback loop of the active noise reduction system, as more fully explained in commonly owned U.S. Pat. No. 6,597,792. 
     Referring to  FIG. 3 , there is shown a section drawing of another embodiment of a headphone cushion assembly. In  FIG. 3 , Headphone cushion assembly  300  includes opening  302 , gasket  304 , outside cover  306 , inside cover  308 , stiffening ring  310 , headphone cushion  312 , and front surface  314 . In this embodiment, stiffening ring  310  is attached to the inside of outside cover  306 . 
     The radial stiffness of headphone cushion assembly  300  is measured by compressing one side of headphone cushion assembly  300  in a direction along the radius of it&#39;s toroidal shape and measuring the force necessary to compress headphone cushion assembly  300  a known distance. Stiffness is calculated by dividing the force by the distance compressed. Likewise, the axial stiffness is calculated in a direction along the axis of the toroidal shape. The radial directions are perpendicular to the axial direction. To achieve high attenuation simultaneously with good comfort, the ratio of radial stiffness to axial stiffness per contact area should be greater than 10 cm 2 . 
     Referring to  FIG. 4 , there is shown a section drawing of another embodiment of a headphone cushion assembly. To increase the mechanical impedance of the outer cushion cover, a high density layer  400  is attached to the inside of outside cover  306 . Outside cover  306  forms a first layer. High density layer  400  forms a second layer. In one embodiment, outside cover  306  has an average area density of less than 0.03 g/cm 2  and high density layer  400  has an average area density greater than 0.045 g/cm 2 . The high density layer may be a highly compliant, massive, and dissipative material. The high density layer may be silicone gel. The high density layer may optionally be applied to only the outside of outside cover  306  or to both the inside and outside of outside cover  306 . 
     Referring to  FIG. 5 , there is shown a headphone cushion cover before it is spread around a headphone cushion. In this state, the headphone cushion cover is a flat piece of cloth or similar material shown as cover  500 . High density layer  400  is shown attached to cover  500 . The average area density is defined as the mass per unit area averaged over the area shown in  FIG. 5 . For example, the average area density of cover  500  is the total mass of cover  500  divided by the area of cover  500  as shown in  FIG. 5 . The average area density of high density layer  400  is the total mass of high density later  400  divided by the area of layer  400  as shown in  FIG. 5 . 
     Referring to  FIG. 6 , there is shown a section drawing of a headphone cushion assembly pressed between top plate  630  and bottom plate  640 . Bottom plate  640  is immovable as shown by hash marks  650 . Cover  600  covers cushion  670 . Outside portion  680  of cover  600  is outside of the headphone cushion assembly and extends from the contact point between cover  600  and top plate  630  to the contact point between cover  600  and bottom plate  640 . Inside portion  690  of cushion  600  is inside of the headphone cushion assembly and extends from the contact point between cover  600  and top plate  630  to the contact point between cover  600  and bottom plate  640 . Audio openings  660  are also shown in cover  600 . 
     In one embodiment, the headphone assembly has audio openings in the portion of the cover that extends over the interior surface of the headphone cushion. The audio openings function to acoustically add the volume of the headphone cushion  112  to the interior volume  118  which enhances passive attenuation. The audio openings are approximately 30% of the total surface area of the interior surface of the cover. The approximate volume of the interior cavity is 100 cc, the half-mass of the headphone assembly is 95 g, and the stiffness of the headphone cushion is 100 g-force/mm. The approximate volume of the open-cell foam in the headphone cushion is 40 cc, so the combined volume of the interior cavity and headphone cushion is 140 cc. 
     At frequencies above the resonance of the axial bouncing mode of the headphone, a second mode of radial, through-cushion transmission may exist—especially in low-impedance cushions with audio openings. Increased radial stiffness through the addition of a stiffening ring, or increased mass and damping through the application of a silicone gel can improve the cushion&#39;s attenuation of outside noise. Increased cushion cover stiffness, mass, and damping generally correlate with higher attenuation. The axial stiffness affects the comfort of the headphones. Low axial stiffness is desired to improve comfort. For a headphone cushion assembly without a stiffening ring, the axial stiffness is approximately 80 gf/mm. For the same headphone cushion with a stiffening ring, the axial stiffness is approximately 100 gf/mm. The stiffening ring increases the radial stiffness much more than the axial stiffness. This difference in stiffness creates headphones that have both excellent comfort and high attenuation of outside noise. 
     Referring to  FIG. 7 , there is shown a graph of measured sound attenuation (in dB) vs frequency (in Hertz) through one embodiment of a headphone assembly while the headphone assembly is mounted on a test fixture. As opposed to the human head, the test fixture is flat so that it does not have leaks between the headphone cushion and the test fixture. Also, the fixture is rigid compared with the much more compliant surface (the skin) of a human test subject. The shapes of the curves in  FIG. 7  depend on the physical dimensions and material properties of the headphone assembly under test. Curve  700  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, but no interior cover. Curve  702  shows the sound attenuation through a headphone assembly that has both an exterior cover and an interior cover over the headphone cushion. Curve  704  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, holes in the interior cover (or no interior cover), and a stiffening ring attached to the outside of the exterior cover. Curve  704  shows the benefit of high attenuation from the stiffening ring above approximately 500 Hz. The attenuation of the headphones with the stiffening ring and holes in the interior cover is approximately equal to the attenuation from the headphone assembly with both inside and outside covers. The advantage of using holes in the interior cover and the stiffening ring rather than interior and exterior covers is that the volume of the headphone cushion can be used to help tune the audio characteristics of the headphones. Since the volume encapsulated by the cushion may be utilized, the headphone assembly may be made smaller and still achieve performance similar to a larger set of headphones that has no holes in the interior cover. 
     Referring to  FIG. 8 , there is shown a graph of measured sound attenuation (in dB) vs frequency (in Hertz) through one embodiment of a headphone assembly while the headphone assembly is mounted on human heads. The curves in  FIG. 8  represent data averaged from many individual heads. The set of headphones does not perfectly fit on each head, so leaks occur between the set of headphones and the heads. The shapes of the curves in  FIG. 8  depend on the physical dimensions of the heads, and the physical dimensions and material properties of the set of headphones under test. Curve  800  shows the sound attenuation through a set of headphones that has an exterior cover over the headphone cushion, but no interior cover. Curve  802  shows the sound attenuation through a set of headphones that has both an exterior cover and an interior cover over the headphone cushion. Curve  804  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, holes in the interior cover (or no interior cover), and a stiffening ring attached to the outside of the exterior cover. Curve  804  shows the benefit of high attenuation from the stiffening ring above approximately 500 Hz. 
     Referring to  FIG. 9 , there is shown a graph of measured sound attenuation (in dB) vs frequency (in Hertz) through one embodiment of a headphone assembly while the headphone assembly is mounted on a test fixture. The shapes of the curves in  FIG. 9  depend on the physical dimensions and material properties of the headphone assembly under test. Curve  900  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, but no interior cover. Curve  902  shows the sound attenuation through a headphone assembly that has both an exterior cover and an interior cover over the headphone cushion. Curve  904  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, holes in the interior cover (or no interior cover), and a high density layer attached to the inside of the exterior cover. Curve  904  shows the benefit of high attenuation from the high density layer above approximately 500 Hz. The attenuation of the headphones with the high density layer and holes in the interior cover is approximately equal to the attenuation from the headphone assembly with both inside and outside covers. 
     Referring to  FIG. 10 , there is shown a graph of measured sound attenuation (in dB) vs frequency (in Hertz) through one embodiment of a headphone assembly while the headphone assembly is mounted on human heads. The curves in  FIG. 10  represent data averaged from many individual heads. The shapes of the curves in  FIG. 10  depend on the physical dimensions of the heads, and the physical dimensions and material properties of the set of headphones under test. Curve  1000  shows the sound attenuation through a set of headphones that has an exterior cover over the headphone cushion, but no interior cover. Curve  1002  shows the sound attenuation through a set of headphones that has both an exterior cover and an interior cover over the headphone cushion. Curve  1004  shows the sound attenuation through a headphone assembly that has an exterior cover over the headphone cushion, holes in the interior cover (or no interior cover), and a high density layer attached to the inside of the exterior cover. Curve  1004  shows the benefit of high attenuation from the high density layer above approximately 500 Hz. 
     Referring to  FIG. 11 , there is shown a sectional view of a test method for axial stiffness. Force  1100  is applied to moveable plate  1110  which pushes on top plate  1120 . Bottom plate  1130  is held immovable as shown by hash marks  1140 . Headphone cushion assembly  1180  includes cushion  1150 , cover  1160 , and attachment plate  1170 . Headphone cushion assembly  1180  is pressed between top plate  1120  and bottom plate  1130  during the axial stiffness test. Distance  1195  is the distance between top plate  1120  and bottom plate  1130 . Audio openings  1190  are also shown in cover  1160 . The steps of the axial stiffness test procedure are as follows. Determine the nominal clamp force of a headset (adjusted for medium size) as the force applied by the ear cushions to parallel plates with outer surfaces spaced 138 mm apart. Place headphone cushion assembly  1180  between top plate  1120  and bottom plate  1130 . Apply a series of known forces  1100  to top plate  1120  in the direction perpendicular to top plate  1120 . The range of forces  1100  should include the nominal clamp force of the corresponding headset. Record the resulting distances  1195  and forces  1100 . Calculate the axial stiffness of the headphone cushion assembly as the slope of the forces  1100  as a function of distances  1195  in gf/mm at the nominal clamp force of the corresponding headset. Determine the contact area of the headphone cushion assembly as the total area of cover  1160  which is in contact with bottom plate  1130  when the nominal clamp force of the corresponding headset is applied as force  1100 . Calculate the axial stiffness per contact area as the axial stiffness divided by the contact area of the cushion in gf/mm/cm 2 . Forces  1100  should be applied at less than or equal to 100 gf/min. Alternatively, forces  1100  may be applied rapidly if two minutes settling time is allowed before measurement of the forces  1100  and distances  1195 . 
     Referring to  FIG. 12 , there is shown a sectional view of a test method for radial stiffness. Top plate  1220  and bottom plate  1230  are held immovable as shown by hash marks  1240 . Headphone cushion assembly  1280  includes cushion  1250 , cover  1260 , and attachment plate  1270 . Top plate  1220  and bottom plate  1230  have adhesive surfaces to hold headphone cushion assembly  1280  in place between top plate  1220  and bottom plate  1230 . Distance  1295  is the distance between top plate  1220  and bottom plate  1230 . Indenter  1297  pushes on the headphone cushion assembly in a radial direction. Indenter  1297  is a rigid cylinder with a diameter of 3 mm. Resultant force  1200  pushes back on indenter  1297 . Audio openings  1290  are also shown in cover  1260 . Before the radial test procedure is performed, distance  1295  must be determined. Using the test setup in  FIG. 11 , set force  1100  to 150 gf and measure resultant distance  1195 . Set distance  1295  in  FIG. 12  equal to resultant distance  1195  from the test setup in  FIG. 11  with force  1100  equal to 150 gf. The steps of the radial stiffness test procedure are as follows. Clamp headphone cushion assembly  1280  between top plate  1220  and bottom plate  1230 . Position the axis of indenter  1297  in the central plane of cushion  1250 , and along a direction perpendicular to the curvature of the cover  1260 &#39;s outer surface when viewed along a direction perpendicular to plates  1220  and  1230 . Push indenter  1297  3.8 mm (from the position of initial contact) into headphone cushion assembly  1280 . After 2 minutes settling time, record the resultant force  1200  on indenter  1297 . Calculate the radial stiffness of the headphone cushion assembly as the resultant force  1200  divided by the 3.8 mm indenting distance in gf/mm. 
     Referring to  FIG. 13 , there is shown a sectional view of a test method for peel strength. Force  1300  is applied to pull up cover sample  1310  from foam sample  1320 . Foam sample  1320  is mounted to plate  1330  which is held immovable as shown by hash marks  1340 . Cover sample  1310  is a rectangular piece of outer cover material from the headphone cushion assembly with a width greater than 100 mm and a length greater than 150 mm. Foam sample  1320  is a rectangular piece of foam from the headphone cushion assembly which has a width and length larger than cover sample  1310 . Cover sample  1310  is placed over foam sample  1320  such that the inner surface of cover  1310  contacts foam sample  1320 . 10 kPa of force is then applied evenly to cover sample  1310  on foam sample  1320  for 2 minutes to allow cover sample  1310  to adhere to foam sample  1320 . The steps of the peel strength test procedure are as follows. Using a load cell with a resolution of at least 0.01 N to measure force  1300 , peel cover sample  1310  from foam sample  1320  at a rate of 60 mm/min in the direction perpendicular to foam sample  1320 . According to one test protocol, cover sample  1310  can be peeled so that the angle between cover sample  1310  and foam sample  1320  remains within 10° of perpendicular. Record average force  1300  as the average force measured over a peel distance of 100 mm. The peel direction should be perpendicular to the direction of gravity. Calculate the peel strength as average force  1300  divided by the width of the cover sample  1310  in gf/mm. 
     Referring to  FIG. 14 , there is shown a sectional view of an earcup assembly with noise reducing circuitry. Reference is made to U.S. Pat. No. 6,597,792, the entire contents of which are hereby incorporated by reference. Driver  1400  is seated in earcup  1410  with driver plate  1420  extending rearward from a lip  1430  of earcup  1410  to a ridge  1440  with microphone  1450  closely adjacent to driver  1400  and covered by a wire mesh resistive cover  1460 . Cushion  1470  covers the front opening of earcup  1410  and includes foam  1480 . 
     Other implementations are also within the scope of the following claims.
A headset including an earcup having a front opening adapted to be adjacent to the ear of the user, a baffle disposed within the earcup to define front and rear cavities, a cushion extending around the periphery of the front opening of the earcup and constructed and arranged to accommodate the ear of the user, the cushion having a first density, an inner radial portion, and an outer radial portion opposite the inner radial portion, a cushion cover substantially surrounding the cushion to form a headphone cushion assembly, and a high impedance component having a second density and located near the outer radial portion to increase the transmission loss of the cushion along a radial direction.