Patent Publication Number: US-9420367-B2

Title: Passive acoustical filters with filled expansion chamber

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation of patent application Ser. No. 14/601,926, entitled PASSIVE ACOUSTICAL FILTERS INCORPORATING INSERTS THAT REDUCE THE SPEED OF SOUND, filed Jan. 21, 2015, which claims priority from Provisional Patent Application No. 61/929,788, entitled PASSIVE AUDIO EAR FILTERS, filed Jan. 21, 2014, both incorporated herein by reference. 
     This patent is related to patent application Ser. No. 14/601,901, entitled PASSIVE AUDIO EAR FILTERS WITH MULTIPLE FILTER ELEMENTS, filed Jan. 21, 2015, which also claims priority from Provisional Patent Application No. 61/929,788. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     1. Field 
     This disclosure relates generally to passive audio ear filters that can attenuate certain sound frequencies and allow other frequencies to pass through unchanged. 
     2. Description of the Related Art 
     Exposure to sound at certain sound pressure levels and sound frequencies can, over time, cause hearing loss. 
     Humans&#39; perception to sound varies with both frequency and sound pressure level (SPL). For example, humans do not perceive low and high frequency sounds as well as they perceive midrange frequencies sounds (e.g., 500 Hz to 6,000 Hz). Further, human hearing is more responsive to sound at high frequencies compared to low frequencies.  FIG. 1  illustrates equal loudness contours defined in ISO (International Standards Organization) Standard 226(2003). The X axis represents sound frequency measured in Hertz (Hz) and the Y axis represents sound pressure level measured in decibels (dB) relative to a pressure level of 2×10 −5  Pascal. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours.  FIG. 1  shows equal loudness contours for loudness levels of 20, 40, 60, 80, and 100 phon. Each equal-loudness contour defines the sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones.  FIG. 1  also shows the hearing threshold level. 
     Hearing protection that attenuates sound equally at all frequencies, or otherwise without regard to the variation of hearing sensitivity with frequency, may attenuate potentially damaging sounds at the expense of pleasurable sounds. For example, an ear filter providing uniform attenuation of 20 dB would reduce loudness by about 20 phon at 1 kHz and 40 phon at 20 Hz. Thus the relative loudness of low frequency sounds would be substantially reduced relative to the loudness of higher frequency sounds. However, with attention to the hearing response curves, it is possible to design ear filters that attenuate damaging sound levels and maintain, or even enhance, desired sounds. 
     There are many situations where people desire protection from sounds at certain frequencies, while allowing sounds at other frequencies to reach their ears. For example, at a concert, concert goers might want to enjoy the music, but also be protected from the mid-range and high levels of sound frequencies that cause damage to a person&#39;s hearing. On an airplane, passengers might wish to block out the roar of the engine, but not conversation. At a sports event, fans might desire to hear the action of the game, but receive protection from the roar of the crowd. While sleeping, people might want protection from all auditory disturbances. These are just a few common examples where people wish to hear some, but not all, of the sound frequencies in their environment. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph of equal-loudness contours. 
         FIG. 2  is a schematic cross-sectional view of an exemplary low-pass filter element. 
         FIG. 3  is a schematic cross-sectional view of an exemplary high-pass filter element. 
         FIG. 4  is a schematic cross-sectional view of an exemplary band-reject filter element. 
         FIG. 5  is a schematic diagram of a passive acoustical filter including a series of low-pass filter elements in parallel with a high-pass filter element. 
         FIG. 6  is a graph of attenuation curves relating to the passive acoustical filter of  FIG. 5 . 
         FIG. 7  is a schematic diagram of an exemplary high-pass filter element. 
         FIG. 8  is a graph of attenuation curves for two low-pass filter elements such as the low-pass filter element shown in  FIG. 7 . 
         FIG. 9  is a schematic diagram of a passive acoustical filter including a low-pass filter element in parallel with a high-pass filter element. 
         FIG. 10  is a graph of attenuation curves relating to the passive acoustical filter of  FIG. 9 . 
         FIG. 11  is a schematic diagram of a passive acoustical filter including two low-pass filter elements in series. 
         FIG. 12  is a graph of attenuation curves relating to the passive acoustical filter of  FIG. 11 . 
         FIG. 13A  is a graphical image of components of a representative music acoustical filter. 
         FIG. 13B  is a schematic cross-sectional view of a music acoustical filter assembled form the components shown in  FIG. 13A . 
         FIG. 14  is a schematic diagram of the music acoustical filter assembled form the components shown in  FIG. 13A . 
         FIG. 15  is a graphical image of the passive acoustical filter assembled from the components shown in  FIG. 13A . 
         FIG. 16  is an exploded graphical image of another music acoustical filter. 
         FIG. 17  is a graphical image of the music acoustical filter assembled from the components shown in  FIG. 16 . 
         FIG. 18  is a graphical image of an exemplary music acoustical filter within an earbud. 
     
    
    
     DETAILED DESCRIPTION 
     Passive acoustical filters incorporating multiple filter elements can protect the ear from damaging sound frequencies while allowing less damaging sound frequencies to reach the ear. In this document, the term “filter element” refers to an acoustic filter that provides a single filter function such as a low-pass, high-pass, band-pass, or band-reject filter function. The term “passive acoustical filter” refers to a filter device that includes one or more filter element coupled between an ambient and a listener&#39;s ear. Two or more filter elements are considered to be “coupled between the ambient and the listener&#39;s ear” if ambient sound must pass through at least one of the filter elements before reaching the listener&#39;s ear. Two or more filter elements are considered to be “in series” if ambient sound must pass consecutively through all of the two or more filter elements before reaching the listener&#39;s ear. Two or more filter elements are considered to be “in parallel” if the filter elements provide alternate paths for ambient sound to reach the listener&#39;s ear. Unless otherwise stated, ambient sound may be divided approximately equally between parallel filter elements. The term “earbud” means an apparatus configured to fit, at least partially, within and be supported by a user&#39;s ear. Typically, a portion of an earbud fits against or within the user&#39;s outer ear canal. Other portions of an earbud may fit within the concha or pinna of the user&#39;s ear. 
     Such passive acoustical filters can be designed compactly in order to fit within an earbud, headphone, or other apparatus that can be placed into or outside an ear. Further, such passive acoustical filters can be designed to attenuate certain damaging and/or disturbing sound frequencies associated with specific environments and/or activities. For example, a passive acoustical filter may include a series of low-pass filter elements in parallel with a high-pass filter element and can attenuate damaging mid-range frequencies associated with attending a concert. For further example, a passive acoustical filter may provide a low-pass filter element and a high-pass filter element in parallel, and can attenuate mid-range frequencies associated with attending a sports event or participating in motor sport activities. Yet another exemplary passive acoustical filter may include a series of low-pass filter elements, and can attenuate mid and high-range frequencies associated with sounds disturbing to sleep. Other passive acoustical filters are possible that provide different types of filter elements alone, in parallel, in series, and/or in parallel/series combinations to attenuate unwanted frequencies associated with specific activities and/or environments, while allowing desired frequencies to pass through. 
       FIG. 2  is a schematic cross-sectional view of an exemplary low-pass filter element. The low-pass filter element  200  includes a main branch  210  and a single expansion chamber  220 . The expansion chamber  220  may be filled with an insert  225 . In operation, ambient sound can enter an opening  230  of the low-pass filter  200 , and certain sound frequencies can be filtered as the ambient sound passes through the main branch  210  and expansion chamber  220 . The resulting filtered sound can exit the opening  240  at the opposite end of the low-pass filter element  200 . 
     The following equation can be used to calculate the dimensions needed for a low-pass filter element to achieve a desired cutoff frequency (i.e., the frequency at which the filter starts to have effect): 
     
       
         
           
             
               f 
               c 
             
             = 
             
               ( 
               
                 cS 
                 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     ⁡ 
                     
                       ( 
                       
                         
                           S 
                           1 
                         
                         - 
                         S 
                       
                       ) 
                     
                   
                 
               
               ) 
             
           
         
       
         
         
           
             Where: 
             fc=the cutoff frequency of the low-pass filter element  200 ; 
             c=speed of sound within the expansion chamber  220 ; 
             S=diameter of the main branch  210 ; 
             S 1 =diameter of the expansion chamber  220 ; and 
             L=length of the expansion chamber  220 . 
           
         
       
    
     The cutoff frequency of the low pass filter element  200  is dictated by the shape and size of the filter element, as well as the medium within expansion chamber of the filter element. The conduction of sound waves through a medium is dependent upon the ratio of the bulk modulus of the medium to the density of the medium, and is governed by the Newton-Laplace equation: 
     
       
         
           
             c 
             = 
             
               
                 K 
                 ρ 
               
             
           
         
       
         
         
           
             Where: 
             c is the speed of sound; 
             K is the bulk modulus of the medium; and 
             ρ is the density of the medium. 
           
         
       
    
     The speed of sound in air is 343 meters per second (at 20° C., 1 atm). In some applications, the size of a filter element having a desired cutoff frequency can be reduced by filling the expansion chamber with an insert made of a material having a ratio of bulk modulus to density (K/ρ) less than that of air. Filing the expansion chamber with such a material will slow down the speed of sound. For example, in the case of a low-pass filter element, filling the expansion chamber with a material that lowers the speed of sound by a factor of two can reduce the length of the expansion chamber equally by a factor of two. A reticulated material that has a K/ρ ratio lower than air may be placed inside the acoustical filters to reduce the speed of sound, while still allowing passage of sound through the filter. In this context, “reticulated” means forming or formed like a network or a web. Suitable reticulated materials may include open-cell or closed-cell foams made of polyurethane, polyester, polystyrene, or other plastic. Other suitable reticulated materials include organic fibers like cotton, bamboo, and yarn. 
       FIG. 3  is a schematic cross-sectional view of an exemplary high-pass filter element. The high-pass filter element  300  includes a main branch  310  and a side branch  320 . The side branch may optionally be filled with an insert  325 . In operation, ambient sound can enter an opening  330  of the high-pass filter element  300 , and certain sound frequencies can be filtered as the ambient sound passes through the main branch  310  and side branch  320 . The resulting filtered sound can exit the opening  340  at the opposite end of the high-pass filter element  300 . 
     The following equation can be used to calculate the dimensions needed for a high-pass filter element to achieve a desired cutoff frequency: 
     
       
         
           
             
               f 
               c 
             
             = 
             
               ( 
               
                 
                   c 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     a 
                     2 
                   
                 
                 
                   2 
                   ⁢ 
                   SL 
                 
               
               ) 
             
           
         
       
         
         
           
             Where: 
             fc=the cutoff frequency of the high pass filter element  300 ; 
             c=speed of sound in the insert  325 ; 
             S=diameter of the main branch  310 ; 
             L=the length of the side branch  320 ; and 
             a=radius of the side branch  320 . 
           
         
       
    
       FIG. 4  is a schematic cross-sectional view of an exemplary band-reject filter element. The band-reject filter element  400  includes a main branch  410 , a volume neck  420 , and an expansion chamber  450 . The expansion chamber and the volume next may be filled with an insert  455 . In operation, ambient sound can enter an opening  430  of the band-reject filter element  400 , and certain sound frequencies can be filtered as the ambient sound passes through the main branch  410 , volume neck  420  and expansion chamber  450 . The resulting filtered sound can exit the opening  440  at the opposite end of the band-reject filter element  400 . 
     The following equation can be used to calculate the dimensions needed for a band-reject filter element to achieve a desired cutoff frequency: 
     
       
         
           
             
               f 
               c 
             
             = 
             
               
                 ( 
                 
                   c 
                   
                     2 
                     ⁢ 
                     π 
                   
                 
                 ) 
               
               ⁢ 
               
                 
                   ( 
                   
                     
                       S 
                       b 
                     
                     LV 
                   
                   ) 
                 
               
             
           
         
       
         
         
           
             Where: 
             fc=the cutoff frequency of the band-reject filter element  400 ; 
             c=speed of sound in the insert  455 ; 
             L=the length of the volume neck  420 ; 
             S b =area of the volume neck  420 ; and 
             V=volume of the expansion chamber  450 . 
           
         
       
    
     As will be described below, a combination of acoustical filter elements can be used to achieve a target response curve for a particular environment and/or activities that protects the ear from certain damaging or disturbing sound frequencies while allowing other sound frequencies to reach the ear. 
       FIG. 5  is a schematic diagram of a passive acoustical filter  500  that can be used within an earbud, headphone or other apparatus to protect a human&#39;s auditory system from hearing damage and/or audio disturbance associated with attending musical events. The passive acoustical filter  500  includes a double-stage low-pass filter element  510  arranged in parallel with a single stage high-pass filter element  520 . The configuration of a double-stage low-pass filter element in parallel with a single stage high-pass filter element will be referred to herein as a “music acoustical filter”. The double-stage low-pass filter element  510  consists of two low-pass filter elements in series. The low-pass filter elements include respective expansion chambers  514  connected by a main branch  512 . Either or both of the expansion chambers may be filled with respective inserts  516 . The high-pass filter element  520  includes a main branch  522  and a side branch  524  which may be filled with an insert  526   526 . In operation, ambient sound can enter the passive acoustical filter  500  at one opening  530  and certain sound frequencies can be filtered as the ambient sound passes through the high-pass filter element  520  and the dual-stage low-pass filter element  510 . The resulting filtered sound can exit the passive acoustic filter  500  at the opposite opening  540 . The filtered sound exiting at  540  may be approximately the sum of the sound passing through the double-stage low-pass filter element  510  and the sound passing through the high-pass filter element  520 . 
     Exemplary dimensions for the double stage low-pass filter element  510 , are set forth in Table 1 below: 
                     TABLE 1               Exemplary dimensions for the double-       stage low-pass filter element 505                                                Main branch 512 diameter   0.5 mm           Expansion chamber 514 diameter   2.6 mm           Expansion chamber 514 length    14 mm                        
These dimensions can be derived from the equation for the low-pass filter element set forth above. Further, these dimensions can be useful for creating a compact passive acoustic filter that can be inserted into ear buds, headphones or other apparatuses that fit into or outside an ear and for achieving an exemplary cutoff frequency for each low-pass filter of approximately 300 Hz. Unless otherwise stated, the term “approximately” means plus-or-minus 20%. In this example, the resonant frequencies of the two low-pass filter elements are the same. However, in other music filter designs, the resonant frequencies and dimensions of the two low-pass filter elements may differ.
 
     Further, inserts  516  made from a reticulated material, for example a foam material having a density of 0.5 g/cm 3  and bulk modulus 27 kPA, can be used the expansion chambers  514  of the low-pass filter element  510  to reduce the speed of sound (e.g., to 200 m/s). The foam material or other reticulated material can optionally be inserted in both expansion chambers  514  of the double stage low-pass filter  510  to reduce the speed of sound and to help keep the dimensions of the music acoustic filter compact. For the double-stage low-pass filter  510  having the dimensions disclosed in Table 1, filling the expansion chambers  512  with foam inserts  516  of these parameters would lower the cutoff frequency from approximately 300 Hz to approximately 200 Hz. To achieve a low-pass filter element having a cutoff frequency of 200 Hz or less without the use of a material to reduce the speed of sound in the expansion chambers, would require the length of diameter of the expansion chamber by 50% from the dimensions disclosed in Table 1. Such a double-stage low pass filter may be too large to include within an earbud. 
     Passive acoustical filter  500  also includes a high-pass filter element  520  having a main branch  522 , and single side branch  524 . Exemplary dimensions for the high-pass filter element  520 , are set forth in Table 2 below: 
                     TABLE 2               Exemplary Dimensions for the high-pass filter element 520                                                Side branch 524 diameter    0.5 mm           Side branch 524 length   3.0           Main branch 522 diameter   0.55 mm                        
These dimensions can be derived from the equation for the high-pass filter element set forth above. Further, these dimensions can be useful for creating a compact passive acoustic filter that can be inserted into ear buds, headphones or other apparatuses that fit into or outside an ear and for achieving an exemplary cutoff frequency of approximately 18,200 Hz.
 
     The dimensions described above for the passive acoustical filter  500  are non-limiting examples, and other dimensions that can achieve similar hearing protection may also be used. Additionally, the dimensions described above can be tailored to attenuate frequencies associated with specific genres of music. Further, the frequency response of a music acoustical filter can be selected to suit the venue of a musical event. For example, the cut-off frequencies of the low-pass filter elements may be 50 to 3000 Hz, and the cutoff frequency of the high pass filter element may be 1,000 to 25,000 Hz. 
       FIG. 6  is a graph of transmission curves relating to a music acoustic filter having the general configuration of the passive acoustic filter  500  as shown in  FIG. 5 . The transmission curves are plotted along an X axis that represents sound frequency measured in hertz (Hz) and a Y axis that represents transmission (inverse of attenuation) of a passive acoustic filter or filter element measured in decibels (dB). The curve  610  represents the transmission of a loss-less double-stage low-pass filter element having the dimensions given in Table 1, including the incorporation of inserts  516  made from a reticulated material in the expansion chambers  514  of the double-stage low-pass filter elements. The curve  620  represents the transmission of a loss-less high-pass filter element having the dimensions given in Table 2. The curve  630  represents the combined transmission of the double stage low-pass filter in parallel with the single stage high-pass filter, including frequency independent attenuation that occurs in the small diameter main branches of the filter elements. The curve  640  is the 80 Phon loudness level from  FIG. 1 , normalized to 0 dB at 20 Hz. The curve  640  represents the sound pressure level required to produce apparently equal loudness as a function of frequency. As illustrated in  FIG. 6 , the performance of the passive acoustic filter (as summarized by the transmission curve  630 ) compliments the 80 Phon equal loudness curve  640 , in that the filter provides highest attenuation (lowest transmission) at frequencies where the human ear is most sensitive, and lowest attenuation (highest transmission) for low and high frequencies where the ear is less sensitive. 
       FIG. 7  is a schematic diagram of a passive acoustical filter  700  that can be used within an ear bud, headphone or other apparatus to protect a human&#39;s auditory system from hearing damage and/or audio disturbance associated with air and train travel. The passive acoustical filter  700  includes a single stage high-pass filter element having a main branch  710  and a side branch  720 . The configuration of a single high-pass filter element will be referred to herein as a “travel acoustic filter”. In operation, ambient sound can enter the passive acoustical filter  700  at one opening  730 , and certain sound frequencies can be filtered as the ambient sound passes through the main branch  710  and side branch  720 . The resulting filtered sound can exit the passive acoustical filter  700  at the opposite opening  740 . 
     Exemplary dimensions for the passive acoustical filter  700 , in accordance with an embodiment of the disclosed subject matter, are set forth in Table 3 below: 
                     TABLE 3               Exemplary dimensions for the passive acoustical filter 700                                                Side branch 720 diameter   2 mm           Side branch 710 length   20 mm            Main branch 710 diameter   2 mm                        
The above disclosed dimensions can be derived from the equation for the high-pass filter element set forth above. Further, these dimensions can be useful for creating a compact filter that can be inserted into earbuds, headphones or other apparatuses that fit into or outside an ear, and for achieving an exemplary cutoff frequency of approximately 1850 Hz.
 
     An insert  725 , made from a reticulated material, having dimensions approximate to the side branch  720  may optionally be placed into the side branch  720  to slow down the speed of sound and reduce the cutoff frequency to approximately 925 Hz. 
     The dimensions described above for the travel acoustical filter are non-limiting examples, and other dimensions that can achieve similar hearing protection may also be used. 
       FIG. 8  is a graph of transmission curves relating to a travel acoustic filter having the general configuration of the passive acoustic filter  700  as shown in  FIG. 7 . The transmission curves are plotted along an X axis that represents sound frequency measured in hertz (Hz) and a Y axis that represents transmission (inverse of attenuation) of a passive acoustic filter or filter element measured in decibels (dB). Specifically,  FIG. 8 , includes a curve  810  that represents the transmission of a loss-less high-pass filter (e.g., the high-pass filter shown in  FIG. 7  and defined in Table 3) as a function of frequency.  FIG. 8  also includes a curve  820  that represents the transmission of a realistic travel acoustic filter including attenuation that occurs in the narrow diameter main branch of the filter A travel acoustical filter can be designed to attenuate low and mid-range frequencies, while allowing high-range frequencies to pass through the filter unchanged or attenuated by a predetermined amount. 
       FIG. 9  is a schematic diagram of a passive acoustical filter  900  that can be used to protect from hearing damage and/or audio disturbance associated with attending a sports event or participating in motor sport activities (e.g., jet skiing, wave running, motorcycling). The passive acoustical filter  900  includes a single stage low-pass filter element  910  and a single stage high-pass filter element  920  in parallel. The configuration of a single stage low-pass filter element and a single stage high-pass filter element in parallel will be referred to herein as a “sports filter”. The low-pass filter element  910  includes a main branch  912  and an expansion chamber  914 . The expansion chamber may optionally be filled with a medium  916 . The high-pass filter element includes a main branch  922  and a side branch  924 . In operation, ambient sound can enter the passive acoustical filter  900  at one opening  930 , and certain sound frequencies can be filtered as the ambient sound passes through the low-pass filter element  910  and the high-pass filter element  920 . The filtered sound can exit the passive acoustical filter  900  at the opposite opening  940 . 
     Exemplary dimensions for the low-pass filter element  910  are set forth in Table 4 below: 
                     TABLE 4               Exemplary Dimensions for the low-pass filter element 910                                                    Main branch 912 diameter   0.8 mm               Expansion chamber 914 diameter   5 mm           Expansion chamber 914 length   12 mm                        
The disclosed dimensions can be derived from the equation for the low-pass filter set forth above. Further, these dimensions can be useful for creating a compact sports filter that can be inserted into ear buds, headphones or other apparatuses that fit into or outside an ear, and for achieving an exemplary cutoff frequency for the low-pass filter element of approximately 240 Hz.
 
     Further, a insert  916  of reticulated material having dimensions approximate to the expansion chamber  914  can optionally be placed in the expansion chamber of the low-pass filter  910  to reduce the speed of sound and thus lower the cut-off frequency of the low-pass filter element  910  to approximately 160 Hz. 
     Exemplary dimensions for the high-pass filter element  920  are set forth in Table 5 below: 
                     TABLE 5               Exemplary dimensions for the high-pass filter element 920                                                Side branch 924 diameter   0.8 mm           Side branch 924 length    12 mm           Main branch 922 diameter   0.8 mm                        
The disclosed dimensions can be derived from the equation for the high-pass filter set forth above. Further, these dimensions can be useful for creating a compact sports filter that can be inserted into earbuds, headphones or other apparatuses that fit into or outside an ear, and for achieving an exemplary cutoff frequency for the high-pass filter element of approximately 4.5 kHz.
 
     The dimensions described above for the low-pass filter element  910  and the high-pass filter element  920  of the passive acoustical filter  900  are non-limiting examples, and other dimensions that can achieve similar hearing protection for sports events and motor activities may also be used. Additionally, the dimensions described above can be tailored for indoor versus outdoor sports stadium use. Because indoor stadiums generate greater midrange sound frequencies than outdoor stadiums, an indoor sports acoustical filter may be designed to attenuate more midrange frequencies (e.g., the midrange sound frequencies generated by peoples&#39; voices) relative to the bass and treble sounds (e.g., from feet stomping, sound systems and fireworks). For example, the cut-off frequency of the low-pass filter element may be 50 to 3000 Hz, and the cutoff frequency of the high pass filter element may be 1,000 to 20,000 Hz. 
       FIG. 10  is a graph of transmission curves relating to a sports acoustic filter having the general configuration of the passive acoustic filter  900  as shown in  FIG. 9 . Like  FIG. 6  and  FIG. 8 , the transmission curves are plotted along an X axis that represents sound frequency measured in hertz (Hz) and a Y axis that represents transmission (inverse of attenuation) of a passive acoustic filter or filter element measured in decibels (dB). The curve  1010  represents the transmission of a loss-less low-pass filter element having the dimensions given in Table 4 and the expansion chamber filled with a medium to lower the cut-off frequency to approximately 160 Hz. The curve  1020  represents the transmission of a loss-less high-pass filter element having the dimensions given in Table 5. The curve  1030  represents the combined transmission of the double stage low-pass filter in parallel with the single stage high-pass filter, including attenuation that occurs in the narrow diameter main branches of the low-pass and high-pass filter elements. The curve  1040  is the 80 Phon loudness level from  FIG. 1 , normalized to 0 dB at 20 Hz. The curve  1040  represents the sound pressure level required to produce apparently equal loudness as a function of frequency. As illustrated in  FIG. 10 , the performance of the sports acoustic filter (as summarized by the transmission curve  1030 ) compliments the 80 Phon equal loudness curve  1040 , in that the filter provides highest attenuation (lowest transmission) at frequencies where the human ear is most sensitive, and lowest attenuation (highest transmission) for low and high frequencies where the ear is less sensitive. 
       FIG. 11  is a schematic diagram of a passive acoustical filter  1100  that can be used within earbuds, headphones or other apparatuses to protect a human&#39;s auditory system from auditory disturbance during sleep. Acoustical filter  1100  includes a double stage low-pass filter  1110  including two low-pass filter elements in series. A filter configuration consisting of two low-pass filter elements in series will be referred to herein as a “sleep filter”. The low pass filter elements have respective expansion chambers  1114  connected by a main branch  1112 . Either or both of the expansion chambers  1114  may be filled inserts  1116  made from a reticulated material. In operation, ambient sound can enter the passive acoustical filter  1100  at one opening  1130 , and certain sound frequencies can be filtered as the ambient sound passes through the main branch  11112  and two expansion chambers  1114 . The filtered sound can exit the passive acoustical filter  1100  at the opposite opening  1140 . 
     Exemplary dimensions for the sleep filter, in accordance with an embodiment of the disclosed subject matter, are set forth in the table below: 
                     TABLE 6               Exemplary Dimensions for the passive acoustical filter 1100                                                    Main branch 1112 diameter   0.5 mm               Expansion chamber 1114 diameter   4 mm           Expansion chamber 1114 length   13 mm                        
These dimensions can be derived from the equation for the low-pass filter set forth above. Further, these dimensions can be useful for creating a compact filter that can be inserted into earbuds, headphones or other apparatuses that fit into or outside an ear, and for achieving an exemplary cutoff frequency for the low-pass filter of approximately 130 Hz.
 
     Further, inserts  1116  made from a reticulated material (e.g., foam) having dimensions approximate to the expansion chambers  1114  can optionally be inserted into the expansion chambers  1114  of the double stage low-pass filter  1110  to reduce the speed of sound and lower the cutoff frequency to 90 Hz. 
     The dimensions described above for the double stage low-pass filter of the sleep filter are non-limiting examples, and other dimensions that can achieve similar hearing protection from sleep disturbances may also be used. For example, the cut-off frequency of each low-pass filter element may be 50 to 3000 Hz. 
       FIG. 12  is a graph of transmission curves relating to a sleep acoustic filter having the general configuration of the passive acoustic filter  1100  as shown in  FIG. 11 . The transmission curves are plotted along an X axis that represents sound frequency measured in hertz (Hz) and a Y axis that represents transmission (inverse of attenuation) of a passive acoustic filter or filter element measured in decibels (dB). Specifically,  FIG. 12  includes a curve  1210  that represents the transmission of an exemplary loss-less double-stage low-pass filter (e.g., the double stage low-pass filter shown in  FIG. 11  and defined in Table 6) as a function of frequency.  FIG. 12  also includes a curve  1220  that represents the transmission of the exemplary double-stage low-pass filter including attenuation that occurs in the narrow main branch of the filter and the use of the reticulated inserts to fill the expansion chambers. A sleep acoustical filter can be designed to attenuate mid- and high-range frequencies, while allowing low-range frequencies to pass through the filter unchanged or with a predetermined attenuation. 
     Other passive acoustical filters may include a band-reject filter element to attenuate frequencies at a specific frequency range. The dimensions of the band-reject filter element can be tailored to filter out sound at a targeted frequency range on the sound frequency spectrum. For example, a band-reject filter element, similar to the example shown in  FIG. 4 , can be used to filter our sound frequencies associated with a consistent whine or hum (e.g., power generation plant), while allowing other frequencies to pass through. 
     Other passive acoustical filters may include a band-pass filter element can be used to allow only a specific range of sound frequencies to pass through unchanged, while attenuating sounds at all other frequencies. A band-pass filter element may be realized, for example, by placing a low-pass filter element and a high-pass filter element in series, with the cutoff frequency of the high-pass filter element set lower than the cutoff frequency of the low-pass filter element. 
     Passive acoustical filters, such as those described above, may be disposed within a housing that can be substantially contained within the ear (e.g., in the ear canal, concha, and/or pinna. Passive acoustical filters, such as those described above, may be disposed outside the ear in a headphone apparatus or any other apparatus that allows ambient sound to travel through an acoustical filter. 
       FIG. 13A  is a graphical image of components of an exemplary musical acoustical filter  1300 . The musical acoustic filter  1300  includes housing  1310  made of a plastic or metal material, a center divider  1320 , two inserts  1330  made of foam or other reticulated material, and first and second end caps  1340 ,  1350 . In this example, the housing  1310  is a cylindrical tube. The housing  1310  may have a cross-sectional shape other than cylindrical, such as square or hexagonal. The reticulated inserts  1330  may include an adhesive backing for attaching to the center divider  1320 . The end caps  1340 ,  1350  incorporate apertures for the input and output of the music acoustical filter  1300  and channels that connect the other elements of the music acoustical filter  1300 . In this context, the term “channels” means any passage through which acoustic waves may pass. Channels may commonly be cylindrical passages or tubes. As shown in the cross-sectional schematic view of  FIG. 13B , the center divider  1320  together with the walls of the outer tube  1310  form two expansion chambers  1360  that are substantially filled by the reticulated inserts  1330 . Additionally, first and second longitudinal channels  1322 ,  1324  run through the length the center divider  1320 . 
       FIG. 14  is a schematic diagram of the music acoustical filter  1300 . The central portion of the device (comprising the outer tube  1310 , center divider  1320 , and reticulated inserts  1330 , as shown in  FIG. 13A ) includes two expansions chambers  1360  that are substantially filled by reticulated inserts  1330 . The two expansion chambers are connected in series by the first longitudinal channel  1322  (running the length of the center divider  1320 ) and channels  1412  (formed in the end caps  1340 ,  1350 ) to form a two-stage low pass filter. The second longitudinal channel  1324  (running the length of the center divider  1320 ) forms the main branch of a high-pass filter element, which is connected in parallel to the two-stage low-pass filter element by additional channels  1412  in the end caps  1340 ,  1350 . One of the end caps (end cap  1340  as shown in  FIG. 14 ) includes a side branch  1424  of the high-pass filter element. Ambient sound can enter the music acoustical filter  1300  through an aperture  1430  in end cap  1340  and certain sound frequencies can be filtered as the ambient sound passes through the high-pass filter element and the dual-stage low-pass filter element. The resulting filtered sound can exit the music acoustical filter  1300  through an aperture  1440  in end cap  1350 . The filtered sound exiting at  1440  may be approximately the sum of the sound passing through the double-stage low-pass filter element and the sound passing through the high-pass filter element. The dimensions of the music acoustical filter  1300  may be selected to provide the desired filter function, as previously discussed with respect to the music acoustical filter  500  of  FIG. 5 .  FIG. 15  is a graphical image of the exemplary music acoustical filter  1300  assembled from the components shown in  FIG. 13A . The outer tube  1310  and end caps  1340 ,  1350  are visible. 
       FIG. 16  shows an exploded view of another exemplary music acoustical filter  1600 . The exploded music acoustical filter  1600  includes a double stage low-pass filter formed by a first body piece  1610  and a second body piece  1620 . The first and second body pieces  1610 ,  1620  may formed from a plastic material such as polycarbonate. Each of the first body piece  1610  and the second body piece  1620  includes a symmetric half of a double-stage low-pass filter element  1630  in the shape of an “S.” First body piece  1610  includes half-cylindrical recesses forming two expansion chambers  1632  and connecting channels  1634 ,  1636 ,  1638 . Second body piece  1620  includes mirror images recesses (not visible). When assembled, the first body piece  1610  and the second body piece  1620  collectively form two cylindrical expansion chambers connected in series by cylindrical main channels. 
     The second body piece  1620  also includes a cavity  1622  for placement of an insert  1640  which may be formed from polycarbonate or another plastic. When the insert  1640  is placed into the cavity  1622 , an interstitial space between the insert  1640  and the second body piece  1620  forms a main branch of a high-pass filter element. A side branch of the high-pass filter element is formed by the two holes  1642 . Two holes are used in this example to provide sufficient cross-sectional area for the side branch. A passage  1624  in the body piece  1620  may couple the high-pass filter element to the channel  1638 . A groove  1644  in the insert  1640  provides an outlet to the ambient for the side branch of the high-pass filter element. 
     An end cap  1650  may fit over the end portions of first and second body pieces  1610  and  1620  to hold the pieces in alignment. The end cap may optionally include a scrim cloth designed to protect the filter from ear surface contaminants. 
       FIG. 17  shows the exemplary music filter  1600  assembled from the components shown in  FIG. 16 .  FIG. 17  shows the first body piece  1610 , the second body piece  1620 , the inset  1640 , and the end cap  1650 . 
       FIG. 18  shows a transparent view of an exemplary earbud  1800  containing a passive acoustical filter  1810 , which may be, for example, the music acoustic filter  1600  (as shown in  FIG. 17 ) or the music acoustical filter  1300  (as shown in  FIG. 15 ). The music acoustical filter  1810  is shown inserted into the neck of the exemplary earbud  1800 . An outer removable cap  1815  may be positioned on top of the filter and can be inserted into the ear. The exemplary earbud  1800  may be, for example, the earbud described in Design patent application Ser. No. 29/485,359. 
     Passive acoustical filters as described above can be designed to be interchangeable, so they can each fit into the same sized earbud, headphone or other apparatus. Designing various passive acoustic filters for interchangeability can allow for the different types of acoustic filters to be swapped in and out of the earbud, headphone or other apparatus, in order to match the type of auditory protection desired for different environments and/or activities. 
     Closing Comments 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of elements, it should be understood that those elements may be combined in other ways to accomplish the same objectives. Elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.