Patent Publication Number: US-11659341-B2

Title: Systems and methods for suppressing sound leakage

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/170,874, filed on Feb. 8, 2021, which is a continuation-in-part application of U.S. patent application Ser. No. 17/074,762 (now U.S. Pat. No. 11,197,106), filed on Oct. 20, 2020, which is a continuation-in-part of U.S. patent application Ser. No. 16/813,915 (now U.S. Pat. No. 10,848,878), filed on Mar. 10, 2020, which is a continuation of U.S. patent application Ser. No. 16/419,049 (issued as U.S. Pat. No. 10,616,696), filed on May 22, 2019, which is a continuation of U.S. patent application Ser. No. 16/180,020 (issued as U.S. Pat. No. 10,334,372), filed on Nov. 5, 2018, which is a continuation of U.S. patent application Ser. No. 15/650,909 (issued as U.S. Pat. No. 10,149,071), filed on Jul. 16, 2017, which is a continuation of U.S. patent application Ser. No. 15/109,831 (issued as U.S. Pat. No. 9,729,978), filed on Jul. 6, 2016, which is a U.S. National Stage entry under 35 U.S.C. § 371 of International Application No. PCT/CN2014/094065, filed on Dec. 17, 2014, designating the United States of America, which claims priority to Chinese Patent Application No. 201410005804.0, filed on Jan. 6, 2014; U.S. patent application Ser. No. 17/170,874 is also a continuation-in-part application of U.S. patent application Ser. No. 16/833,839, filed on Mar. 30, 2020, which is a continuation of U.S. application Ser. No. 15/752,452 (issued as U.S. Pat. No. 10,609,496), filed on Feb. 13, 2018, which is a national stage entry under 35 U.S.C. § 371 of International Application No. PCT/CN2015/086907, filed on Aug. 13, 2015, the entire contents of each of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This application relates to a bone conduction device, and more specifically, relates to methods and systems for reducing sound leakage by a bone conduction device. 
     BACKGROUND 
     A bone conduction speaker, which may be also called a vibration speaker, may push human tissues and bones to stimulate the auditory nerve in cochlea and enable people to hear sound. The bone conduction speaker is also called a bone conduction headphone. 
     An exemplary structure of a bone conduction speaker based on the principle of the bone conduction speaker is shown in  FIGS.  1 A and  1 B . The bone conduction speaker may include an open housing  110 , a panel  121 , a transducer  122 , and a linking component  123 . The transducer  122  may transduce electrical signals to mechanical vibrations. The panel  121  may be connected to the transducer  122  and vibrate synchronically with the transducer  122 . The panel  121  may stretch out from the opening of the housing  110  and contact with human skin to pass vibrations to auditory nerves through human tissues and bones, which in turn enables people to hear sound. The linking component  123  may reside between the transducer  122  and the housing  110 , configured to fix the vibrating transducer  122  inside the housing  110 . To minimize its effect on the vibrations generated by the transducer  122 , the linking component  123  may be made of an elastic material. 
     However, the mechanical vibrations generated by the transducer  122  may not only cause the panel  121  to vibrate, but may also cause the housing  110  to vibrate through the linking component  123 . Accordingly, the mechanical vibrations generated by the bone conduction speaker may push human tissues through the bone board  121 , and at the same time a portion of the vibrating board  121  and the housing  110  that are not in contact with human issues may nevertheless push air. Air sound may thus be generated by the air pushed by the portion of the vibrating board  121  and the housing  110 . The air sound may be called “sound leakage.” In some cases, sound leakage is harmless. However, sound leakage should be avoided as much as possible if people intend to protect privacy when using the bone conduction speaker or try not to disturb others when listening to music. 
     Attempting to solve the problem of sound leakage, Korean patent KR10-2009-0082999 discloses a bone conduction speaker of a dual magnetic structure and double-frame. As shown in  FIG.  2   , the speaker disclosed in the patent includes: a first frame  210  with an open upper portion and a second frame  220  that surrounds the outside of the first frame  210 . The second frame  220  is separately placed from the outside of the first frame  210 . The first frame  210  includes a movable coil  230  with electric signals, an inner magnetic component  240 , an outer magnetic component  250 , a magnet field formed between the inner magnetic component  240 , and the outer magnetic component  250 . The inner magnetic component  240  and the out magnetic component  250  may vibrate by the attraction and repulsion force of the coil  230  placed in the magnet field. A vibration board  260  connected to the moving coil  230  may receive the vibration of the moving coil  230 . A vibration unit  270  connected to the vibration board  260  may pass the vibration to a user by contacting with the skin. As described in the patent, the second frame  220  surrounds the first frame  210 , in order to use the second frame  220  to prevent the vibration of the first frame  210  from dissipating the vibration to outsides, and thus may reduce sound leakage to some extent. 
     However, in this design, since the second frame  220  is fixed to the first frame  210 , vibrations of the second frame  220  are inevitable. As a result, sealing by the second frame  220  is unsatisfactory. Furthermore, the second frame  220  increases the whole volume and weight of the speaker, which in turn increases the cost, complicates the assembly process, and reduces the speaker&#39;s reliability and consistency. 
     SUMMARY 
     The embodiments of the present application disclose methods and system of reducing sound leakage of a bone conduction speaker. 
     In one aspect, the embodiments of the present application disclose a method of reducing sound leakage of a bone conduction speaker, including: 
     providing a bone conduction speaker including a panel fitting human skin and passing vibrations, a transducer, and a housing, wherein at least one sound guiding hole is located in at least one portion of the housing; 
     the transducer drives the panel to vibrate; 
     the housing vibrates, along with the vibrations of the transducer, and pushes air, forming a leaked sound wave transmitted in the air; 
     the air inside the housing is pushed out of the housing through the at least one sound guiding hole, interferes with the leaked sound wave, and reduces an amplitude of the leaked sound wave. 
     In some embodiments, one or more sound guiding holes may locate in an upper portion, a central portion, and/or a lower portion of a sidewall and/or the bottom of the housing. 
     In some embodiments, a damping layer may be applied in the at least one sound guiding hole in order to adjust the phase and amplitude of the guided sound wave through the at least one sound guiding hole. 
     In some embodiments, sound guiding holes may be configured to generate guided sound waves having a same phase that reduce the leaked sound wave having a same wavelength; sound guiding holes may be configured to generate guided sound waves having different phases that reduce the leaked sound waves having different wavelengths. 
     In some embodiments, different portions of a same sound guiding hole may be configured to generate guided sound waves having a same phase that reduce the leaked sound wave having same wavelength. In some embodiments, different portions of a same sound guiding hole may be configured to generate guided sound waves having different phases that reduce leaked sound waves having different wavelengths. 
     In another aspect, the embodiments of the present application disclose a bone conduction speaker, including a housing, a panel and a transducer, wherein: 
     the transducer is configured to generate vibrations and is located inside the housing; 
     the panel is configured to be in contact with skin and pass vibrations; 
     At least one sound guiding hole may locate in at least one portion on the housing, and preferably, the at least one sound guiding hole may be configured to guide a sound wave inside the housing, resulted from vibrations of the air inside the housing, to the outside of the housing, the guided sound wave interfering with the leaked sound wave and reducing the amplitude thereof. 
     In some embodiments, the at least one sound guiding hole may locate in the sidewall and/or bottom of the housing. 
     In some embodiments, preferably, the at least one sound guiding sound hole may locate in the upper portion and/or lower portion of the sidewall of the housing. 
     In some embodiments, preferably, the sidewall of the housing is cylindrical and there are at least two sound guiding holes located in the sidewall of the housing, which are arranged evenly or unevenly in one or more circles. Alternatively, the housing may have a different shape. 
     In some embodiments, preferably, the sound guiding holes have different heights along the axial direction of the cylindrical sidewall. 
     In some embodiments, preferably, there are at least two sound guiding holes located in the bottom of the housing. In some embodiments, the sound guiding holes are distributed evenly or unevenly in one or more circles around the center of the bottom. Alternatively or additionally, one sound guiding hole is located at the center of the bottom of the housing. 
     In some embodiments, preferably, the sound guiding hole is a perforative hole. In some embodiments, there may be a damping layer at the opening of the sound guiding hole. 
     In some embodiments, preferably, the guided sound waves through different sound guiding holes and/or different portions of a same sound guiding hole have different phases or a same phase. 
     In some embodiments, preferably, the damping layer is a tuning paper, a tuning cotton, a nonwoven fabric, a silk, a cotton, a sponge, or a rubber. 
     In some embodiments, preferably, the shape of a sound guiding hole is circle, ellipse, quadrangle, rectangle, or linear. In some embodiments, the sound guiding holes may have a same shape or different shapes. 
     In some embodiments, preferably, the transducer includes a magnetic component and a voice coil. Alternatively, the transducer includes piezoelectric ceramic. 
     The design disclosed in this application utilizes the principles of sound interference, by placing sound guiding holes in the housing, to guide sound wave(s) inside the housing to the outside of the housing, the guided sound wave(s) interfering with the leaked sound wave, which is formed when the housing&#39;s vibrations push the air outside the housing. The guided sound wave(s) reduces the amplitude of the leaked sound wave and thus reduces the sound leakage. The design not only reduces sound leakage, but is also easy to implement, doesn&#39;t increase the volume or weight of the bone conduction speaker, and barely increase the cost of the product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are schematic structures illustrating a bone conduction speaker of prior art; 
         FIG.  2    is a schematic structure illustrating another bone conduction speaker of prior art; 
         FIG.  3    illustrates the principle of sound interference according to some embodiments of the present disclosure; 
         FIGS.  4 A and  4 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  4 C  is a schematic structure of the bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  4 D  is a diagram illustrating reduced sound leakage of the bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  4 E  is a schematic diagram illustrating exemplary two-point sound sources according to some embodiments of the present disclosure; 
         FIG.  5    is a diagram illustrating the equal-loudness contour curves according to some embodiments of the present disclosure; 
         FIG.  6    is a flow chart of an exemplary method of reducing sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIGS.  7 A and  7 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  7 C  is a diagram illustrating reduced sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIGS.  8 A and  8 B  are schematic structure of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  8 C  is a diagram illustrating reduced sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIGS.  9 A and  9 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  9 C  is a diagram illustrating reduced sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIGS.  10 A and  10 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  10 C  is a diagram illustrating reduced sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  10 D  is a schematic diagram illustrating an acoustic route according to some embodiments of the present disclosure; 
         FIG.  10 E  is a schematic diagram illustrating another acoustic route according to some embodiments of the present disclosure; 
         FIG.  10 F  is a schematic diagram illustrating a further acoustic route according to some embodiments of the present disclosure; 
         FIGS.  11 A and  11 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  11 C  is a diagram illustrating reduced sound leakage of a bone conduction speaker according to some embodiments of the present disclosure; and 
         FIGS.  12 A and  12 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIGS.  13 A and  13 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  14    illustrates an equivalent model of a vibration generation and transferring system of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  15 A  illustrates a structure of a contact surface of a vibration unit of a bone conduction speaker according to some embodiments of the present disclosure; 
         FIG.  15 B  illustrates a vibration response curve of a bone conduction speaker according to some embodiments of the present disclosure; and 
         FIG.  16    illustrates a structure of a contact surface of a vibration unit of a bone conduction speaker according to some embodiments of the present disclosure. 
     
    
    
     The meanings of the mark numbers in the figures are as followed: 
       110 , open housing;  121 , panel;  122 , transducer;  123 , linking component;  210 , first frame;  220 , second frame;  230 , moving coil;  240 , inner magnetic component;  250 , outer magnetic component;  260 ; panel;  270 , vibration unit;  10 , housing;  11 , sidewall;  12 , bottom;  21 , panel;  22 , transducer;  23 , linking component;  24 , elastic component;  30 , sound guiding hole. 
     DETAILED DESCRIPTION 
     Followings are some further detailed illustrations about this disclosure. The following examples are for illustrative purposes only and should not be interpreted as limitations of the claimed invention. There are a variety of alternative techniques and procedures available to those of ordinary skill in the art, which would similarly permit one to successfully perform the intended invention. In addition, the figures just show the structures relative to this disclosure, not the whole structure. 
     To explain the scheme of the embodiments of this disclosure, the design principles of this disclosure will be introduced here.  FIG.  3    illustrates the principles of sound interference according to some embodiments of the present disclosure. Two or more sound waves may interfere in the space based on, for example, the frequency and/or amplitude of the waves. Specifically, the amplitudes of the sound waves with the same frequency may be overlaid to generate a strengthened wave or a weakened wave. As shown in  FIG.  3   , sound source  1  and sound source  2  have the same frequency and locate in different locations in the space. The sound waves generated from these two sound sources may encounter in an arbitrary point A. If the phases of the sound wave  1  and sound wave  2  are the same at point A, the amplitudes of the two sound waves may be added, generating a strengthened sound wave signal at point A; on the other hand, if the phases of the two sound waves are opposite at point A, their amplitudes may be offset, generating a weakened sound wave signal at point A. 
     This disclosure applies above-noted the principles of sound wave interference to a bone conduction speaker and disclose a bone conduction speaker that can reduce sound leakage. 
     Embodiment One 
       FIGS.  4 A and  4 B  are schematic structures of an exemplary bone conduction speaker. The bone conduction speaker may include a housing  10 , a panel  21 , and a transducer  22 . The transducer  22  may be inside the housing  10  and configured to generate vibrations. The housing  10  may have one or more sound guiding holes  30 . The sound guiding hole(s)  30  may be configured to guide sound waves inside the housing  10  to the outside of the housing  10 . In some embodiments, the guided sound waves may form interference with leaked sound waves generated by the vibrations of the housing  10 , so as to reducing the amplitude of the leaked sound. The transducer  22  may be configured to convert an electrical signal to mechanical vibrations. For example, an audio electrical signal may be transmitted into a voice coil that is placed in a magnet, and the electromagnetic interaction may cause the voice coil to vibrate based on the audio electrical signal. As another example, the transducer  22  may include piezoelectric ceramics, shape changes of which may cause vibrations in accordance with electrical signals received. 
     Furthermore, the panel  21  may be connected to the transducer  22  and configured to vibrate along with the transducer  22 . The panel  21  may stretch out from the opening of the housing  10 , and touch the skin of the user and pass vibrations to auditory nerves through human tissues and bones, which in turn enables the user to hear sound. In some embodiments, the panel  21  may be in contact with human skin directly, or through a vibration transfer layer made of specific materials (e.g., low-density materials). The linking component  23  may reside between the transducer  22  and the housing  10 , configured to fix the vibrating transducer  122  inside the housing. The linking component  23  may include one or more separate components, or may be integrated with the transducer  22  or the housing  10 . In some embodiments, the linking component  23  is made of an elastic material. 
     The transducer  22  may drive the panel  21  to vibrate. The transducer  22 , which resides inside the housing  10 , may vibrate. The vibrations of the transducer  22  may drives the air inside the housing  10  to vibrate, producing a sound wave inside the housing  10 , which can be referred to as “sound wave inside the housing.” Since the panel  21  and the transducer  22  are fixed to the housing  10  via the linking component  23 , the vibrations may pass to the housing  10 , causing the housing  10  to vibrate synchronously. The vibrations of the housing  10  may generate a leaked sound wave, which spreads outwards as sound leakage. 
     The sound wave inside the housing and the leaked sound wave are like the two sound sources in  FIG.  3   . In some embodiments, the sidewall  11  of the housing  10  may have one or more sound guiding holes  30  configured to guide the sound wave inside the housing  10  to the outside. The guided sound wave through the sound guiding hole(s)  30  may interfere with the leaked sound wave generated by the vibrations of the housing  10 , and the amplitude of the leaked sound wave may be reduced due to the interference, which may result in a reduced sound leakage. Therefore, the design of this embodiment can solve the sound leakage problem to some extent by making an improvement of setting a sound guiding hole on the housing, and not increasing the volume and weight of the bone conduction speaker. 
     In some embodiments, one sound guiding hole  30  is set on the upper portion of the sidewall  11 . As used herein, the upper portion of the sidewall  11  refers to the portion of the sidewall  11  starting from the top of the sidewall (contacting with the panel  21 ) to about the ⅓ height of the sidewall. 
       FIG.  4 C  is a schematic structure of the bone conduction speaker illustrated in  FIGS.  4 A- 4 B . The structure of the bone conduction speaker is further illustrated with mechanics elements illustrated in  FIG.  4 C . As shown in  FIG.  4 C , the linking component  23  between the sidewall  11  of the housing  10  and the panel  21  may be represented by an elastic element  23  and a damping element in the parallel connection. The linking relationship between the panel  21  and the transducer  22  may be represented by an elastic element  24 . 
     Outside the housing  10 , the sound leakage reduction is proportional to
 
(∫∫ S     hole     Pds−∫∫   S     housing     P   d   ds ),  (1)
 
wherein S hole  is the area of the opening of the sound guiding hole  30 , S housing  is the area of the housing  10  (e.g., the sidewall  11  and the bottom  12 ) that is not in contact with human face.
 
     The pressure inside the housing may be expressed as P=P a +P b +P c +P e , (2) wherein P a , P b , P c  and P e  are the sound pressures of an arbitrary point inside the housing  10  generated by side a, side b, side c and side e (as illustrated in  FIG.  4 C ), respectively. As used herein, side a refers to the upper surface of the transducer  22  that is close to the panel  21 , side b refers to the lower surface of the panel  21  that is close to the transducer  22 , side c refers to the inner upper surface of the bottom  12  that is close to the transducer  22 , and side e refers to the lower surface of the transducer  22  that is close to the bottom  12 . 
     The center of the side b, O point, is set as the origin of the space coordinates, and the side b can be set as the z=0 plane, so P a , P b , P c  and P e  may be expressed as follows: 
                         P   a     ⁡     (     x   ,   y   ,   z     )       =         -   j     ⁢   ω   ⁢     ρ   0     ⁢     ∫       ∫     S   a       ⁢           W   a     ⁡     (       x   a   ′     ,     y   a   ′       )       ·       e     j   ⁢     kR   ⁡     (       x   a   ′     ,     y   a   ′       )             4   ⁢   π   ⁢           ⁢     R   ⁡     (       x   a   ′     ,     y   a   ′       )             ⁢     dx   a   ′     ⁢     dy   a   ′             -     P     a   ⁢   R           ,           (   3   )                     P   b     ⁡     (     x   ,   y   ,   z     )       =         -   j     ⁢   ω   ⁢     ρ   0     ⁢     ∫       ∫     S   b       ⁢           W   b     ⁡     (       x   ′     ,     y   ′       )       ·       e     jkR   ⁡     (       x   ′     ,     y   ′       )           4   ⁢   π   ⁢           ⁢     R   ⁡     (       x   ′     ,     y   ′       )             ⁢     dx   ′     ⁢     dy   ′             -     P     b   ⁢   R           ,           (   4   )                     P   c     ⁡     (     x   ,   y   ,   z     )       =         -   j     ⁢   ω   ⁢     ρ   0     ⁢     ∫       ∫     S   c       ⁢           W   c     ⁡     (       x   c   ′     ,     y   c   ′       )       ·       e     j   ⁢     kR   ⁡     (       x   c   ′     ,     y   c   ′       )             4   ⁢   π   ⁢           ⁢     R   ⁡     (       x   c   ′     ,     y   c   ′       )             ⁢     dx   c   ′     ⁢     dy   c   ′             -     P   cR         ,           (   5   )                     P   e     ⁡     (     x   ,   y   ,   z     )       =         -   j     ⁢   ω   ⁢     ρ   0     ⁢     ∫       ∫     S   e       ⁢           W   e     ⁡     (       x   e   ′     ,     y   e   ′       )       ·       e     j   ⁢     kR   ⁡     (       x   e   ′     ,     y   e   ′       )             4   ⁢   π   ⁢           ⁢     R   ⁡     (       x   e   ′     ,     y   e   ′       )             ⁢     dx   e   ′     ⁢     dy   e   ′             -     P   eR         ,           (   6   )               
wherein R(x′, y′)=√{square root over ((x−x′) 2 +(y−y′) 2 +z 2 )} is the distance between an observation point (x, y, z) and a point on side b (x′, y′, 0); S a , S b , S c  and S e  are the areas of side a, side b, side c and side e, respectively;
 
R(x′ a , y′ a )=√{square root over ((x−x a ′) 2 +(y−y a ′) 2 +(z−z a ) 2 )} is the distance between the observation point (x, y, z) and a point on side a (x′ a , y′ α , z a );
 
R(x′ c , y′ c )=√{square root over ((x−x c ′) 2 +(y−y c ′) 2 +(z−z c ) 2 )} is the distance between the observation point (x, y, z) and a point on side c (x′ c , y′ c , z c );
 
     R(x′ e , y′ e )=√{square root over ((x−x e ′) 2 +(y−y e ′) 2 +(z−z e ) 2 )} is the distance between the observation point (x, y, z) and a point on side e (x′ e , y′ e , z e ); 
     k=ω/u (u is the velocity of sound) is wave number, ρ 0  is an air density, co is an angular frequency of vibration; 
     P aR , P bR , P cR  and P eR  are acoustic resistances of air, which respectively are: 
                       P     a   ⁢   R       =       A   ·           z   a     ·   r     +     j   ⁢           ⁢     ω   ·     z   a     ·     r   ′           φ       +   δ       ,           (   7   )                   P   bR     =       A   ·           z   b     ·   r     +     j   ⁢           ⁢     ω   ·     z   b     ·     r   ′           φ       +   δ       ,           (   8   )                   P   cR     =       A   ·           z   c     ·   r     +     j   ⁢           ⁢     ω   ·     z   c     ·     r   ′           φ       +   δ       ,           (   9   )                   P   eR     =       A   ·           z   e     ·   r     +     j   ⁢           ⁢     ω   ·     z   e     ·     r   ′           φ       +   δ       ,           (   10   )               
wherein r is the acoustic resistance per unit length, r′ is the sound quality per unit length, z a  is the distance between the observation point and side a, z b  is the distance between the observation point and side b, z c  is the distance between the observation point and side c, z e  is the distance between the observation point and side e.
 
     W a  (x, y), W b  (x, y), W c  (x, y), W e  (x, y) and W d  (x, y) are the sound source power per unit area of side a, side b, side c, side e and side d, respectively, which can be derived from following formulas (11):
 
 F   e   =F   a   =F−k   1  cos ω t−∫∫   S     a     W   a ( x,y ) dxdy−∫∫   S     e     W   e ( x,y ) dxdy−f  
 
 F   b   =−F+k   1  cos ω t+∫∫   S     b     W   b ( x,y ) dxdy−∫∫   S     e     W   e ( x,y ) dxdy−L  
 
 F   c   =F   d   =F   b   −k   2  cos ω t−∫∫   S     c     W   c ( x,y ) dxdy−f−γ   (11)
 
 F   d   =F   b   −k   2  cos ω t−∫∫   S     d     W   d ( x,y ) dxdy  
 
wherein F is the driving force generated by the transducer  22 , F a , F b , F c , F d , and F e  are the driving forces of side a, side b, side c, side d and side e, respectively. As used herein, side d is the outside surface of the bottom  12 . S d  is the region of side d, f is the viscous resistance formed in the small gap of the sidewalls, and f= Δs(dv/dy).
 
     L is the equivalent load on human face when the panel acts on the human face, γ is the energy dissipated on elastic element  24 , k 1  and k 2  are the elastic coefficients of elastic element  23  and elastic element  24  respectively,   is the fluid viscosity coefficient, dv/dy is the velocity gradient of fluid, Δs is the cross-section area of a subject (board), A is the amplitude, φ is the region of the sound field, and δ is a high order minimum (which is generated by the incompletely symmetrical shape of the housing); 
     The sound pressure of an arbitrary point outside the housing, generated by the vibration of the housing  10  is expressed as: 
                       P   d     =       -   j     ⁢           ⁢     ωρ   0     ⁢     ∫     ∫           W   d     ⁡     (       x   d   ′     ,     y   d   ′       )       ·       e     j   ⁢     kR   ⁡     (       x   d   ′     ,     y   d   ′       )             4   ⁢   π   ⁢           ⁢     R   ⁡     (       x   d   ′     ,     y   d   ′       )             ⁢     dx   d   ′     ⁢     dy   d   ′               ,           (   12   )               
wherein R(x′ d , y′ d )=√{square root over ((x−x d ′) 2 +(y−y d ′) 2 +(z−z d ) 2 )} is the distance between the observation point (x, y, z) and a point on side d (x′ d , y′ d , z d ).
 
     P a , P b , P c  and P e  are functions of the position, when we set a hole on an arbitrary position in the housing, if the area of the hole is S hole , the sound pressure of the hole is ∫∫ S     hole   Pds. 
     In the meanwhile, because the panel  21  fits human tissues tightly, the power it gives out is absorbed all by human tissues, so the only side that can push air outside the housing to vibrate is side d, thus forming sound leakage. As described elsewhere, the sound leakage is resulted from the vibrations of the housing  10 . For illustrative purposes, the sound pressure generated by the housing  10  may be expressed as ∫∫ S     housing   P d ds. 
     The leaked sound wave and the guided sound wave interference may result in a weakened sound wave, i.e., to make ∫∫ S     hole   Pds and ∫∫ S     housing   P d ds have the same value but opposite directions, and the sound leakage may be reduced. In some embodiments, ∫∫ S     hole   Pds may be adjusted to reduce the sound leakage. Since ∫∫ S     hole   Pds corresponds to information of phases and amplitudes of one or more holes, which further relates to dimensions of the housing of the bone conduction speaker, the vibration frequency of the transducer, the position, shape, quantity and/or size of the sound guiding holes and whether there is damping inside the holes. Thus, the position, shape, and quantity of sound guiding holes, and/or damping materials may be adjusted to reduce sound leakage. 
     Additionally, because of the basic structure and function differences of a bone conduction speaker and a traditional air conduction speaker, the formulas above are only suitable for bone conduction speakers. Whereas in traditional air conduction speakers, the air in the air housing can be treated as a whole, which is not sensitive to positions, and this is different intrinsically with a bone conduction speaker, therefore the above formulas are not suitable to an air conduction speaker. 
     According to the formulas above, a person having ordinary skill in the art would understand that the effectiveness of reducing sound leakage is related to the dimensions of the housing of the bone conduction speaker, the vibration frequency of the transducer, the position, shape, quantity and size of the sound guiding hole(s) and whether there is damping inside the sound guiding hole(s). Accordingly, various configurations, depending on specific needs, may be obtained by choosing specific position where the sound guiding hole(s) is located, the shape and/or quantity of the sound guiding hole(s) as well as the damping material. 
       FIG.  5    is a diagram illustrating the equal-loudness contour curves according to some embodiments of the present disclose. The horizontal coordinate is frequency, while the vertical coordinate is sound pressure level (SPL). As used herein, the SPL refers to the change of atmospheric pressure after being disturbed, i.e., a surplus pressure of the atmospheric pressure, which is equivalent to an atmospheric pressure added to a pressure change caused by the disturbance. As a result, the sound pressure may reflect the amplitude of a sound wave. In  FIG.  5   , on each curve, sound pressure levels corresponding to different frequencies are different, while the loudness levels felt by human ears are the same. For example, each curve is labeled with a number representing the loudness level of said curve. According to the loudness level curves, when volume (sound pressure amplitude) is lower, human ears are not sensitive to sounds of high or low frequencies; when volume is higher, human ears are more sensitive to sounds of high or low frequencies. Bone conduction speakers may generate sound relating to different frequency ranges, such as 1000 Hz-4000 Hz, or 1000 Hz-4000 Hz, or 1000 Hz-3500 Hz, or 1000 Hz-3000 Hz, or 1500 Hz-3000 Hz. The sound leakage within the above-mentioned frequency ranges may be the sound leakage aimed to be reduced with a priority. 
       FIG.  4 D  is a diagram illustrating the effect of reduced sound leakage according to some embodiments of the present disclosure, wherein the test results and calculation results are close in the above range. The bone conduction speaker being tested includes a cylindrical housing, which includes a sidewall and a bottom, as described in  FIGS.  4 A and  4 B . The cylindrical housing is in a cylinder shape having a radius of 22 mm, the sidewall height of 14 mm, and a plurality of sound guiding holes being set on the upper portion of the sidewall of the housing. The openings of the sound guiding holes are rectangle. The sound guiding holes are arranged evenly on the sidewall. The target region where the sound leakage is to be reduced is 50 cm away from the outside of the bottom of the housing. The distance of the leaked sound wave spreading to the target region and the distance of the sound wave spreading from the surface of the transducer  20  through the sound guiding holes  30  to the target region have a difference of about 180 degrees in phase. As shown, the leaked sound wave is reduced in the target region dramatically or even be eliminated. 
     According to the embodiments in this disclosure, the effectiveness of reducing sound leakage after setting sound guiding holes is very obvious. As shown in  FIG.  4 D , the bone conduction speaker having sound guiding holes greatly reduce the sound leakage compared to the bone conduction speaker without sound guiding holes. 
     In the tested frequency range, after setting sound guiding holes, the sound leakage is reduced by about 10 dB on average. Specifically, in the frequency range of 1500 Hz-3000 Hz, the sound leakage is reduced by over 10 dB. In the frequency range of 2000 Hz-2500 Hz, the sound leakage is reduced by over 20 dB compared to the scheme without sound guiding holes. 
     A person having ordinary skill in the art can understand from the above-mentioned formulas that when the dimensions of the bone conduction speaker, target regions to reduce sound leakage and frequencies of sound waves differ, the position, shape and quantity of sound guiding holes also need to adjust accordingly. 
     For example, in a cylinder housing, according to different needs, a plurality of sound guiding holes may be on the sidewall and/or the bottom of the housing. Preferably, the sound guiding hole may be set on the upper portion and/or lower portion of the sidewall of the housing. The quantity of the sound guiding holes set on the sidewall of the housing is no less than two. Preferably, the sound guiding holes may be arranged evenly or unevenly in one or more circles with respect to the center of the bottom. In some embodiments, the sound guiding holes may be arranged in at least one circle. In some embodiments, one sound guiding hole may be set on the bottom of the housing. In some embodiments, the sound guiding hole may be set at the center of the bottom of the housing. 
     The quantity of the sound guiding holes can be one or more. Preferably, multiple sound guiding holes may be set symmetrically on the housing. In some embodiments, there are 6-8 circularly arranged sound guiding holes. 
     The openings (and cross sections) of sound guiding holes may be circle, ellipse, rectangle, or slit. Slit generally means slit along with straight lines, curve lines, or arc lines. Different sound guiding holes in one bone conduction speaker may have same or different shapes. 
     A person having ordinary skill in the art can understand that, the sidewall of the housing may not be cylindrical, the sound guiding holes can be arranged asymmetrically as needed. Various configurations may be obtained by setting different combinations of the shape, quantity, and position of the sound guiding. Some other embodiments along with the figures are described as follows. 
     In some embodiments, the leaked sound wave may be generated by a portion of the housing  10 . The portion of the housing may be the sidewall  11  of the housing  10  and/or the bottom  12  of the housing  10 . Merely by way of example, the leaked sound wave may be generated by the bottom  12  of the housing  10 . The guided sound wave output through the sound guiding hole(s)  30  may interfere with the leaked sound wave generated by the portion of the housing  10 . The interference may enhance or reduce a sound pressure level of the guided sound wave and/or leaked sound wave in the target region. 
     In some embodiments, the portion of the housing  10  that generates the leaked sound wave may be regarded as a first sound source (e.g., the sound source  1  illustrated in  FIG.  3   ), and the sound guiding hole(s)  30  or a part thereof may be regarded as a second sound source (e.g., the sound source  2  illustrated in  FIG.  3   ). Merely for illustration purposes, if the size of the sound guiding hole on the housing  10  is small, the sound guiding hole may be approximately regarded as a point sound source. In some embodiments, any number or count of sound guiding holes provided on the housing  10  for outputting sound may be approximated as a single point sound source. Similarly, for simplicity, the portion of the housing  10  that generates the leaked sound wave may also be approximately regarded as a point sound source. In some embodiments, both the first sound source and the second sound source may approximately be regarded as point sound sources (also referred to as two-point sound sources). 
       FIG.  4 E  is a schematic diagram illustrating exemplary two-point sound sources according to some embodiments of the present disclosure. The sound field pressure p generated by a single point sound source may satisfy Equation (13): 
                     p   =         j   ⁢   ω   ⁢     ρ   0         4   ⁢   π   ⁢           ⁢   r       ⁢     Q   0     ⁢   exp   ⁢           ⁢     j   ⁡     (       ω   ⁢           ⁢   t     -   kr     )           ,           (   13   )               
where ω denotes an angular frequency, ρ 0  denotes an air density, r denotes a distance between a target point and the sound source, Q 0  denotes a volume velocity of the sound source, and k denotes a wave number. It may be concluded that the magnitude of the sound field pressure of the sound field of the point sound source is inversely proportional to the distance to the point sound source.
 
     It should be noted that, the sound guiding hole(s) for outputting sound as a point sound source may only serve as an explanation of the principle and effect of the present disclosure, and the shape and/or size of the sound guiding hole(s) may not be limited in practical applications. In some embodiments, if the area of the sound guiding hole is large, the sound guiding hole may also be equivalent to a planar sound source. Similarly, if an area of the portion of the housing  10  that generates the leaked sound wave is large (e.g., the portion of the housing  10  is a vibration surface or a sound radiation surface), the portion of the housing  10  may also be equivalent to a planar sound source. For those skilled in the art, without creative activities, it may be known that sounds generated by structures such as sound guiding holes, vibration surfaces, and sound radiation surfaces may be equivalent to point sound sources at the spatial scale discussed in the present disclosure, and may have consistent sound propagation characteristics and the same mathematical description method. Further, for those skilled in the art, without creative activities, it may be known that the acoustic effect achieved by the two-point sound sources may also be implemented by alternative acoustic structures. According to actual situations, the alternative acoustic structures may be modified and/or combined discretionarily, and the same acoustic output effect may be achieved. 
     The two-point sound sources may be formed such that the guided sound wave output from the sound guiding hole(s) may interfere with the leaked sound wave generated by the portion of the housing  10 . The interference may reduce a sound pressure level of the leaked sound wave in the surrounding environment (e.g., the target region). For convenience, the sound waves output from an acoustic output device (e.g., the bone conduction speaker) to the surrounding environment may be referred to as far-field leakage since it may be heard by others in the environment. The sound waves output from the acoustic output device to the ears of the user may also be referred to as near-field sound since a distance between the bone conduction speaker and the user may be relatively short. In some embodiments, the sound waves output from the two-point sound sources may have a same frequency or frequency range (e.g., 800 Hz, 1000 Hz, 1500 Hz, 3000 Hz, etc.). In some embodiments, the sound waves output from the two-point sound sources may have a certain phase difference. In some embodiments, the sound guiding hole includes a damping layer. The damping layer may be, for example, a tuning paper, a tuning cotton, a nonwoven fabric, a silk, a cotton, a sponge, or a rubber. The damping layer may be configured to adjust the phase of the guided sound wave in the target region. The acoustic output device described herein may include a bone conduction speaker or an air conduction speaker. For example, a portion of the housing (e.g., the bottom of the housing) of the bone conduction speaker may be treated as one of the two-point sound sources, and at least one sound guiding holes of the bone conduction speaker may be treated as the other one of the two-point sound sources. As another example, one sound guiding hole of an air conduction speaker may be treated as one of the two-point sound sources, and another sound guiding hole of the air conduction speaker may be treated as the other one of the two-point sound sources. It should be noted that, although the construction of two-point sound sources may be different in bone conduction speaker and air conduction speaker, the principles of the interference between the various constructed two-point sound sources are the same. Thus, the equivalence of the two-point sound sources in a bone conduction speaker disclosed elsewhere in the present disclosure is also applicable for an air conduction speaker. 
     In some embodiments, when the position and phase difference of the two-point sound sources meet certain conditions, the acoustic output device may output different sound effects in the near field (for example, the position of the user&#39;s ear) and the far field. For example, if the phases of the point sound sources corresponding to the portion of the housing  10  and the sound guiding hole(s) are opposite, that is, an absolute value of the phase difference between the two-point sound sources is 180 degrees, the far-field leakage may be reduced according to the principle of reversed phase cancellation. 
     In some embodiments, the interference between the guided sound wave and the leaked sound wave at a specific frequency may relate to a distance between the sound guiding hole(s) and the portion of the housing  10 . For example, if the sound guiding hole(s) are set at the upper portion of the sidewall of the housing  10  (as illustrated in  FIG.  4 A ), the distance between the sound guiding hole(s) and the portion of the housing  10  may be large. Correspondingly, the frequencies of sound waves generated by such two-point sound sources may be in a mid-low frequency range (e.g., 1500-2000 Hz, 1500-2500 Hz, etc.). Referring to  FIG.  4 D , the interference may reduce the sound pressure level of the leaked sound wave in the mid-low frequency range (i.e., the sound leakage is low). 
     Merely by way of example, the low frequency range may refer to frequencies in a range below a first frequency threshold. The high frequency range may refer to frequencies in a range exceed a second frequency threshold. The first frequency threshold may be lower than the second frequency threshold. The mid-low frequency range may refer to frequencies in a range between the first frequency threshold and the second frequency threshold. For example, the first frequency threshold may be 1000 Hz, and the second frequency threshold may be 3000 Hz. The low frequency range may refer to frequencies in a range below 1000 Hz, the high frequency range may refer to frequencies in a range above 3000 Hz, and the mid-low frequency range may refer to frequencies in a range of 1000-2000 Hz, 1500-2500 Hz, etc. In some embodiments, a middle frequency range, a mid-high frequency range may also be determined between the first frequency threshold and the second frequency threshold. In some embodiments, the mid-low frequency range and the low frequency range may partially overlap. The mid-high frequency range and the high frequency range may partially overlap. For example, the mid-high frequency range may refer to frequencies in a range above 3000 Hz, and the mid-low frequency range may refer to frequencies in a range of 2800-3500 Hz. It should be noted that the low frequency range, the mid-low frequency range, the middle frequency range, the mid-high frequency range, and/or the high frequency range may be set flexibly according to different situations, and are not limited herein. 
     In some embodiments, the frequencies of the guided sound wave and the leaked sound wave may be set in a low frequency range (e.g., below 800 Hz, below 1200 Hz, etc.). In some embodiments, the amplitudes of the sound waves generated by the two-point sound sources may be set to be different in the low frequency range. For example, the amplitude of the guided sound wave may be smaller than the amplitude of the leaked sound wave. In this case, the interference may not reduce sound pressure of the near-field sound in the low-frequency range. The sound pressure of the near-field sound may be improved in the low-frequency range. The volume of the sound heard by the user may be improved. 
     In some embodiments, the amplitude of the guided sound wave may be adjusted by setting an acoustic resistance structure in the sound guiding hole(s)  30 . The material of the acoustic resistance structure disposed in the sound guiding hole  30  may include, but not limited to, plastics (e.g., high-molecular polyethylene, blown nylon, engineering plastics, etc.), cotton, nylon, fiber (e.g., glass fiber, carbon fiber, boron fiber, graphite fiber, graphene fiber, silicon carbide fiber, or aramid fiber), other single or composite materials, other organic and/or inorganic materials, etc. The thickness of the acoustic resistance structure may be 0.005 mm, 0.01 mm, 0.02 mm, 0.5 mm, 1 mm, 2 mm, etc. The structure of the acoustic resistance structure may be in a shape adapted to the shape of the sound guiding hole. For example, the acoustic resistance structure may have a shape of a cylinder, a sphere, a cubic, etc. In some embodiments, the materials, thickness, and structures of the acoustic resistance structure may be modified and/or combined to obtain a desirable acoustic resistance structure. In some embodiments, the acoustic resistance structure may be implemented by the damping layer. 
     In some embodiments, the amplitude of the guided sound wave output from the sound guiding hole may be relatively low (e.g., zero or almost zero). The difference between the guided sound wave and the leaked sound wave may be maximized, thus achieving a relatively large sound pressure in the near field. In this case, the sound leakage of the acoustic output device having sound guiding holes may be almost the same as the sound leakage of the acoustic output device without sound guiding holes in the low frequency range (e.g., as shown in  FIG.  4 D ). 
     Embodiment Two 
       FIG.  6    is a flowchart of an exemplary method of reducing sound leakage of a bone conduction speaker according to some embodiments of the present disclosure. At  601 , a bone conduction speaker including a panel  21  touching human skin and passing vibrations, a transducer  22 , and a housing  10  is provided. At least one sound guiding hole  30  is arranged on the housing  10 . At  602 , the panel  21  is driven by the transducer  22 , causing the vibration  21  to vibrate. At  603 , a leaked sound wave due to the vibrations of the housing is formed, wherein the leaked sound wave transmits in the air. At  604 , a guided sound wave passing through the at least one sound guiding hole  30  from the inside to the outside of the housing  10 . The guided sound wave interferes with the leaked sound wave, reducing the sound leakage of the bone conduction speaker. 
     The sound guiding holes  30  are preferably set at different positions of the housing  10 . 
     The effectiveness of reducing sound leakage may be determined by the formulas and method as described above, based on which the positions of sound guiding holes may be determined. 
     A damping layer is preferably set in a sound guiding hole  30  to adjust the phase and amplitude of the sound wave transmitted through the sound guiding hole  30 . 
     In some embodiments, different sound guiding holes may generate different sound waves having a same phase to reduce the leaked sound wave having the same wavelength. In some embodiments, different sound guiding holes may generate different sound waves having different phases to reduce the leaked sound waves having different wavelengths. 
     In some embodiments, different portions of a sound guiding hole  30  may be configured to generate sound waves having a same phase to reduce the leaked sound waves with the same wavelength. In some embodiments, different portions of a sound guiding hole  30  may be configured to generate sound waves having different phases to reduce the leaked sound waves with different wavelengths. 
     Additionally, the sound wave inside the housing may be processed to basically have the same value but opposite phases with the leaked sound wave, so that the sound leakage may be further reduced. 
     Embodiment Three 
       FIGS.  7 A and  7 B  are schematic structures illustrating an exemplary bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21 , and a transducer  22 . The housing  10  may cylindrical and have a sidewall and a bottom. A plurality of sound guiding holes  30  may be arranged on the lower portion of the sidewall (i.e., from about the ⅔ height of the sidewall to the bottom). The quantity of the sound guiding holes  30  may be 8, the openings of the sound guiding holes  30  may be rectangle. The sound guiding holes  30  may be arranged evenly or evenly in one or more circles on the sidewall of the housing  10 . 
     In the embodiment, the transducer  22  is preferably implemented based on the principle of electromagnetic transduction. The transducer  22  may include components such as magnetizer, voice coil, and etc., and the components may be located inside the housing and may generate synchronous vibrations with a same frequency. 
       FIG.  7 C  is a diagram illustrating reduced sound leakage according to some embodiments of the present disclosure. In the frequency range of 1400 Hz-4000 Hz, the sound leakage is reduced by more than 5 dB, and in the frequency range of 2250 Hz-2500 Hz, the sound leakage is reduced by more than 20 dB. 
     In some embodiments, the sound guiding hole(s) at the lower portion of the sidewall of the housing  10  may also be approximately regarded as a point sound source. In some embodiments, the sound guiding hole(s) at the lower portion of the sidewall of the housing  10  and the portion of the housing  10  that generates the leaked sound wave may constitute two-point sound sources. The two-point sound sources may be formed such that the guided sound wave output from the sound guiding hole(s) at the lower portion of the sidewall of the housing  10  may interfere with the leaked sound wave generated by the portion of the housing  10 . The interference may reduce a sound pressure level of the leaked sound wave in the surrounding environment (e.g., the target region) at a specific frequency or frequency range. 
     In some embodiments, the sound waves output from the two-point sound sources may have a same frequency or frequency range (e.g., 1000 Hz, 2500 Hz, 3000 Hz, etc.). In some embodiments, the sound waves output from the first two-point sound sources may have a certain phase difference. In this case, the interference between the sound waves generated by the first two-point sound sources may reduce a sound pressure level of the leaked sound wave in the target region. When the position and phase difference of the first two-point sound sources meet certain conditions, the acoustic output device may output different sound effects in the near field (for example, the position of the user&#39;s ear) and the far field. For example, if the phases of the first two-point sound sources are opposite, that is, an absolute value of the phase difference between the first two-point sound sources is 180 degrees, the far-field leakage may be reduced. 
     In some embodiments, the interference between the guided sound wave and the leaked sound wave may relate to frequencies of the guided sound wave and the leaked sound wave and/or a distance between the sound guiding hole(s) and the portion of the housing  10 . For example, if the sound guiding hole(s) are set at the lower portion of the sidewall of the housing  10  (as illustrated in  FIG.  7 A ), the distance between the sound guiding hole(s) and the portion of the housing  10  may be small. Correspondingly, the frequencies of sound waves generated by such two-point sound sources may be in a high frequency range (e.g., above 3000 Hz, above 3500 Hz, etc.). Referring to  FIG.  7 C , the interference may reduce the sound pressure level of the leaked sound wave in the high frequency range. 
     Embodiment Four 
       FIGS.  8 A and  8 B  are schematic structures illustrating an exemplary bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21 , and a transducer  22 . The housing  10  is cylindrical and have a sidewall and a bottom. The sound guiding holes  30  may be arranged on the central portion of the sidewall of the housing (i.e., from about the ⅓ height of the sidewall to the ⅔ height of the sidewall). The quantity of the sound guiding holes  30  may be 8, and the openings (and cross sections) of the sound guiding hole  30  may be rectangle. The sound guiding holes  30  may be arranged evenly or unevenly in one or more circles on the sidewall of the housing  10 . 
     In the embodiment, the transducer  21  may be implemented preferably based on the principle of electromagnetic transduction. The transducer  21  may include components such as magnetizer, voice coil, etc., which may be placed inside the housing and may generate synchronous vibrations with the same frequency. 
       FIG.  8 C  is a diagram illustrating reduced sound leakage. In the frequency range of 1000 Hz-4000 Hz, the effectiveness of reducing sound leakage is great. For example, in the frequency range of 1400 Hz-2900 Hz, the sound leakage is reduced by more than 10 dB; in the frequency range of 2200 Hz-2500 Hz, the sound leakage is reduced by more than 20 dB. 
     It&#39;s illustrated that the effectiveness of reduced sound leakage can be adjusted by changing the positions of the sound guiding holes, while keeping other parameters relating to the sound guiding holes unchanged. 
     Embodiment Five 
       FIGS.  9 A and  9 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21  and a transducer  22 . The housing  10  is cylindrical, with a sidewall and a bottom. One or more perforative sound guiding holes  30  may be along the circumference of the bottom. In some embodiments, there may be 8 sound guiding holes  30  arranged evenly of unevenly in one or more circles on the bottom of the housing  10 . In some embodiments, the shape of one or more of the sound guiding holes  30  may be rectangle. 
     In the embodiment, the transducer  21  may be implemented preferably based on the principle of electromagnetic transduction. The transducer  21  may include components such as magnetizer, voice coil, etc., which may be placed inside the housing and may generate synchronous vibration with the same frequency. 
       FIG.  9 C  is a diagram illustrating the effect of reduced sound leakage. In the frequency range of 1000 Hz-3000 Hz, the effectiveness of reducing sound leakage is outstanding. For example, in the frequency range of 1700 Hz-2700 Hz, the sound leakage is reduced by more than 10 dB; in the frequency range of 2200 Hz-2400 Hz, the sound leakage is reduced by more than 20 dB. 
     Embodiment Six 
       FIGS.  10 A and  10 B  are schematic structures of an exemplary bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21  and a transducer  22 . One or more perforative sound guiding holes  30  may be arranged on both upper and lower portions of the sidewall of the housing  10 . The sound guiding holes  30  may be arranged evenly or unevenly in one or more circles on the upper and lower portions of the sidewall of the housing  10 . In some embodiments, the quantity of sound guiding holes  30  in every circle may be 8, and the upper portion sound guiding holes and the lower portion sound guiding holes may be symmetrical about the central cross section of the housing  10 . In some embodiments, the shape of the sound guiding hole  30  may be circle. 
     The shape of the sound guiding holes on the upper portion and the shape of the sound guiding holes on the lower portion may be different; One or more damping layers may be arranged in the sound guiding holes to reduce leaked sound waves of the same wave length (or frequency), or to reduce leaked sound waves of different wave lengths. 
       FIG.  10 C  is a diagram illustrating the effect of reducing sound leakage according to some embodiments of the present disclosure. In the frequency range of 1000 Hz-4000 Hz, the effectiveness of reducing sound leakage is outstanding. For example, in the frequency range of 1600 Hz-2700 Hz, the sound leakage is reduced by more than 15 dB; in the frequency range of 2000 Hz-2500 Hz, where the effectiveness of reducing sound leakage is most outstanding, the sound leakage is reduced by more than 20 dB. Compared to embodiment three, this scheme has a relatively balanced effect of reduced sound leakage on various frequency range, and this effect is better than the effect of schemes where the height of the holes are fixed, such as schemes of embodiment three, embodiment four, embodiment five, and so on. 
     In some embodiments, the sound guiding hole(s) at the upper portion of the sidewall of the housing  10  (also referred to as first hole(s)) may be approximately regarded as a point sound source. In some embodiments, the first hole(s) and the portion of the housing  10  that generates the leaked sound wave may constitute two-point sound sources (also referred to as first two-point sound sources). As for the first two-point sound sources, the guided sound wave generated by the first hole(s) (also referred to as first guided sound wave) may interfere with the leaked sound wave or a portion thereof generated by the portion of the housing  10  in a first region. In some embodiments, the sound waves output from the first two-point sound sources may have a same frequency (e.g., a first frequency). In some embodiments, the sound waves output from the first two-point sound sources may have a certain phase difference. In this case, the interference between the sound waves generated by the first two-point sound sources may reduce a sound pressure level of the leaked sound wave in the target region. When the position and phase difference of the first two-point sound sources meet certain conditions, the acoustic output device may output different sound effects in the near field (for example, the position of the user&#39;s ear) and the far field. For example, if the phases of the first two-point sound sources are opposite, that is, an absolute value of the phase difference between the first two-point sound sources is 180 degrees, the far-field leakage may be reduced according to the principle of reversed phase cancellation. 
     In some embodiments, the sound guiding hole(s) at the lower portion of the sidewall of the housing  10  (also referred to as second hole(s)) may also be approximately regarded as another point sound source. Similarly, the second hole(s) and the portion of the housing  10  that generates the leaked sound wave may also constitute two-point sound sources (also referred to as second two-point sound sources). As for the second two-point sound sources, the guided sound wave generated by the second hole(s) (also referred to as second guided sound wave) may interfere with the leaked sound wave or a portion thereof generated by the portion of the housing  10  in a second region. The second region may be the same as or different from the first region. In some embodiments, the sound waves output from the second two-point sound sources may have a same frequency (e.g., a second frequency). 
     In some embodiments, the first frequency and the second frequency may be in certain frequency ranges. In some embodiments, the frequency of the guided sound wave output from the sound guiding hole(s) may be adjustable. In some embodiments, the frequency of the first guided sound wave and/or the second guided sound wave may be adjusted by one or more acoustic routes. The acoustic routes may be coupled to the first hole(s) and/or the second hole(s). The first guided sound wave and/or the second guided sound wave may be propagated along the acoustic route having a specific frequency selection characteristic. That is, the first guided sound wave and the second guided sound wave may be transmitted to their corresponding sound guiding holes via different acoustic routes. For example, the first guided sound wave and/or the second guided sound wave may be propagated along an acoustic route with a low-pass characteristic to a corresponding sound guiding hole to output guided sound wave of a low frequency. In this process, the high frequency component of the sound wave may be absorbed or attenuated by the acoustic route with the low-pass characteristic. Similarly, the first guided sound wave and/or the second guided sound wave may be propagated along an acoustic route with a high-pass characteristic to the corresponding sound guiding hole to output guided sound wave of a high frequency. In this process, the low frequency component of the sound wave may be absorbed or attenuated by the acoustic route with the high-pass characteristic. 
       FIG.  10 D  is a schematic diagram illustrating an acoustic route according to some embodiments of the present disclosure.  FIG.  10 E  is a schematic diagram illustrating another acoustic route according to some embodiments of the present disclosure.  FIG.  10 F  is a schematic diagram illustrating a further acoustic route according to some embodiments of the present disclosure. In some embodiments, structures such as a sound tube, a sound cavity, a sound resistance, etc., may be set in the acoustic route for adjusting frequencies for the sound waves (e.g., by filtering certain frequencies). It should be noted that  FIGS.  10 D- 10 F  may be provided as examples of the acoustic routes, and not intended be limiting. 
     As shown in  FIG.  10 D , the acoustic route may include one or more lumen structures. The one or more lumen structures may be connected in series. An acoustic resistance material may be provided in each of at least one of the one or more lumen structures to adjust acoustic impedance of the entire structure to achieve a desirable sound filtering effect. For example, the acoustic impedance may be in a range of 5 MKS Rayleigh to 500 MKS Rayleigh. In some embodiments, a high-pass sound filtering, a low-pass sound filtering, and/or a band-pass filtering effect of the acoustic route may be achieved by adjusting a size of each of at least one of the one or more lumen structures and/or a type of acoustic resistance material in each of at least one of the one or more lumen structures. The acoustic resistance materials may include, but not limited to, plastic, textile, metal, permeable material, woven material, screen material or mesh material, porous material, particulate material, polymer material, or the like, or any combination thereof. By setting the acoustic routes of different acoustic impedances, the acoustic output from the sound guiding holes may be acoustically filtered. In this case, the guided sound waves may have different frequency components. 
     As shown in  FIG.  10 E , the acoustic route may include one or more resonance cavities. The one or more resonance cavities may be, for example, Helmholtz cavity. In some embodiments, a high-pass sound filtering, a low-pass sound filtering, and/or a band-pass filtering effect of the acoustic route may be achieved by adjusting a size of each of at least one of the one or more resonance cavities and/or a type of acoustic resistance material in each of at least one of the one or more resonance cavities. 
     As shown in  FIG.  10 F , the acoustic route may include a combination of one or more lumen structures and one or more resonance cavities. In some embodiments, a high-pass sound filtering, a low-pass sound filtering, and/or a band-pass filtering effect of the acoustic route may be achieved by adjusting a size of each of at least one of the one or more lumen structures and one or more resonance cavities and/or a type of acoustic resistance material in each of at least one of the one or more lumen structures and one or more resonance cavities. It should be noted that the structures exemplified above may be for illustration purposes, various acoustic structures may also be provided, such as a tuning net, tuning cotton, etc. 
     In some embodiments, the interference between the leaked sound wave and the guided sound wave may relate to frequencies of the guided sound wave and the leaked sound wave and/or a distance between the sound guiding hole(s) and the portion of the housing  10 . In some embodiments, the portion of the housing that generates the leaked sound wave may be the bottom of the housing  10 . The first hole(s) may have a larger distance to the portion of the housing  10  than the second hole(s). In some embodiments, the frequency of the first guided sound wave output from the first hole(s) (e.g., the first frequency) and the frequency of second guided sound wave output from second hole(s) (e.g., the second frequency) may be different. 
     In some embodiments, the first frequency and second frequency may associate with the distance between the at least one sound guiding hole and the portion of the housing  10  that generates the leaked sound wave. In some embodiments, the first frequency may be set in a low frequency range. The second frequency may be set in a high frequency range. The low frequency range and the high frequency range may or may not overlap. 
     In some embodiments, the frequency of the leaked sound wave generated by the portion of the housing  10  may be in a wide frequency range. The wide frequency range may include, for example, the low frequency range and the high frequency range or a portion of the low frequency range and the high frequency range. For example, the leaked sound wave may include a first frequency in the low frequency range and a second frequency in the high frequency range. In some embodiments, the leaked sound wave of the first frequency and the leaked sound wave of the second frequency may be generated by different portions of the housing  10 . For example, the leaked sound wave of the first frequency may be generated by the sidewall of the housing  10 , the leaked sound wave of the second frequency may be generated by the bottom of the housing  10 . As another example, the leaked sound wave of the first frequency may be generated by the bottom of the housing  10 , the leaked sound wave of the second frequency may be generated by the sidewall of the housing  10 . In some embodiments, the frequency of the leaked sound wave generated by the portion of the housing  10  may relate to parameters including the mass, the damping, the stiffness, etc., of the different portion of the housing  10 , the frequency of the transducer  22 , etc. 
     In some embodiments, the characteristics (amplitude, frequency, and phase) of the first two-point sound sources and the second two-point sound sources may be adjusted via various parameters of the acoustic output device (e.g., electrical parameters of the transducer  22 , the mass, stiffness, size, structure, material, etc., of the portion of the housing  10 , the position, shape, structure, and/or number (or count) of the sound guiding hole(s) so as to form a sound field with a particular spatial distribution. In some embodiments, a frequency of the first guided sound wave is smaller than a frequency of the second guided sound wave. 
     A combination of the first two-point sound sources and the second two-point sound sources may improve sound effects both in the near field and the far field. 
     Referring to  FIGS.  4 D,  7 C, and  10 C , by designing different two-point sound sources with different distances, the sound leakage in both the low frequency range and the high frequency range may be properly suppressed. In some embodiments, the closer distance between the second two-point sound sources may be more suitable for suppressing the sound leakage in the far field, and the relative longer distance between the first two-point sound sources may be more suitable for reducing the sound leakage in the near field. In some embodiments, the amplitudes of the sound waves generated by the first two-point sound sources may be set to be different in the low frequency range. For example, the amplitude of the guided sound wave may be smaller than the amplitude of the leaked sound wave. In this case, the sound pressure level of the near-field sound may be improved. The volume of the sound heard by the user may be increased. 
     Embodiment Seven 
       FIGS.  11 A and  11 B  are schematic structures illustrating a bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21  and a transducer  22 . One or more perforative sound guiding holes  30  may be set on upper and lower portions of the sidewall of the housing  10  and on the bottom of the housing  10 . The sound guiding holes  30  on the sidewall are arranged evenly or unevenly in one or more circles on the upper and lower portions of the sidewall of the housing  10 . In some embodiments, the quantity of sound guiding holes  30  in every circle may be 8, and the upper portion sound guiding holes and the lower portion sound guiding holes may be symmetrical about the central cross section of the housing  10 . In some embodiments, the shape of the sound guiding hole  30  may be rectangular. There may be four sound guiding holds 30 on the bottom of the housing  10 . The four sound guiding holes  30  may be linear-shaped along arcs, and may be arranged evenly or unevenly in one or more circles with respect to the center of the bottom. Furthermore, the sound guiding holes  30  may include a circular perforative hole on the center of the bottom. 
       FIG.  11 C  is a diagram illustrating the effect of reducing sound leakage of the embodiment. In the frequency range of 1000 Hz-4000 Hz, the effectiveness of reducing sound leakage is outstanding. For example, in the frequency range of 1300 Hz-3000 Hz, the sound leakage is reduced by more than 10 dB; in the frequency range of 2000 Hz-2700 Hz, the sound leakage is reduced by more than 20 dB. Compared to embodiment three, this scheme has a relatively balanced effect of reduced sound leakage within various frequency range, and this effect is better than the effect of schemes where the height of the holes are fixed, such as schemes of embodiment three, embodiment four, embodiment five, and etc. Compared to embodiment six, in the frequency range of 1000 Hz-1700 Hz and 2500 Hz-4000 Hz, this scheme has a better effect of reduced sound leakage than embodiment six. 
     Embodiment Eight 
       FIGS.  12 A and  12 B  are schematic structures illustrating a bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21  and a transducer  22 . A perforative sound guiding hole  30  may be set on the upper portion of the sidewall of the housing  10 . One or more sound guiding holes may be arranged evenly or unevenly in one or more circles on the upper portion of the sidewall of the housing  10 . There may be 8 sound guiding holes  30 , and the shape of the sound guiding holes  30  may be circle. 
     After comparison of calculation results and test results, the effectiveness of this embodiment is basically the same with that of embodiment one, and this embodiment can effectively reduce sound leakage. 
     Embodiment Nine 
       FIGS.  13 A and  13 B  are schematic structures illustrating a bone conduction speaker according to some embodiments of the present disclosure. The bone conduction speaker may include an open housing  10 , a panel  21  and a transducer  22 . 
     The difference between this embodiment and the above-described embodiment three is that to reduce sound leakage to greater extent, the sound guiding holes  30  may be arranged on the upper, central and lower portions of the sidewall  11 . The sound guiding holes  30  are arranged evenly or unevenly in one or more circles. Different circles are formed by the sound guiding holes  30 , one of which is set along the circumference of the bottom  12  of the housing  10 . The size of the sound guiding holes  30  are the same. 
     The effect of this scheme may cause a relatively balanced effect of reducing sound leakage in various frequency ranges compared to the schemes where the position of the holes are fixed. The effect of this design on reducing sound leakage is relatively better than that of other designs where the heights of the holes are fixed, such as embodiment three, embodiment four, embodiment five, etc. 
     Embodiment Ten 
     The sound guiding holes  30  in the above embodiments may be perforative holes without shields. 
     In order to adjust the effect of the sound waves guided from the sound guiding holes, a damping layer (not shown in the figures) may locate at the opening of a sound guiding hole  30  to adjust the phase and/or the amplitude of the sound wave. 
     There are multiple variations of materials and positions of the damping layer. For example, the damping layer may be made of materials which can damp sound waves, such as tuning paper, tuning cotton, nonwoven fabric, silk, cotton, sponge or rubber. The damping layer may be attached on the inner wall of the sound guiding hole  30 , or may shield the sound guiding hole  30  from outside. 
     More preferably, the damping layers corresponding to different sound guiding holes  30  may be arranged to adjust the sound waves from different sound guiding holes to generate a same phase. The adjusted sound waves may be used to reduce leaked sound wave having the same wavelength. Alternatively, different sound guiding holes  30  may be arranged to generate different phases to reduce leaked sound wave having different wavelengths (i.e., leaked sound waves with specific wavelengths). 
     In some embodiments, different portions of a same sound guiding hole can be configured to generate a same phase to reduce leaked sound waves on the same wavelength (e.g., using a pre-set damping layer with the shape of stairs or steps). In some embodiments, different portions of a same sound guiding hole can be configured to generate different phases to reduce leaked sound waves on different wavelengths. 
     The above-described embodiments are preferable embodiments with various configurations of the sound guiding hole(s) on the housing of a bone conduction speaker, but a person having ordinary skills in the art can understand that the embodiments don&#39;t limit the configurations of the sound guiding hole(s) to those described in this application. 
     In the past bone conduction speakers, the housing of the bone conduction speakers is closed, so the sound source inside the housing is sealed inside the housing. In the embodiments of the present disclosure, there can be holes in proper positions of the housing, making the sound waves inside the housing and the leaked sound waves having substantially same amplitude and substantially opposite phases in the space, so that the sound waves can interfere with each other and the sound leakage of the bone conduction speaker is reduced. Meanwhile, the volume and weight of the speaker do not increase, the reliability of the product is not comprised, and the cost is barely increased. The designs disclosed herein are easy to implement, reliable, and effective in reducing sound leakage. 
     In general, a sound quality of a bone conduction speaker may be affected by various factors, such as, a physical property of components of the bone conduction speaker, a vibration transfer relationship between the components, a vibration transfer relationship between the bone conduction speaker and external environment, a vibration transfer efficiency of the vibration transfer system, or the like. The components of the bone conduction speaker may include a vibration generation element (such as the transducer  22 ), a component for fixing the speaker (such as headset bracket/headset lanyard), a vibration transfer component (such as the panel  21  and a vibration transfer layer covering an outer side of the panel  21 ). The vibration transfer relationships between the components and between the bone conduction speaker and external environment may be determined by the manner that the bone conduction speaker is in contact with a user (such as clamping force, contacting area, contacting shape).  FIG.  14    is an equivalent diagram illustrating the vibration generation and vibration transfer system of the bone conduction speaker. The equivalent system of a bone conduction speaker may include a fixed end  1401 , a sensor terminal  1402 , a vibration unit  1403 , and a transducer  1404 . The fixed end  1401  may be connected to the vibration unit  1403  through a transfer relationship K1 (i.e., k 4  in  FIG.  14   ); the sensor terminal  1402  may be connected to the vibration unit  1403  through the transfer relationship K2 (i.e., R 3  and k 3  in  FIG.  14   ); the vibration unit  1403  may be connected to the transducer  1404  through the transfer relationship K3 (R 4 , k 5  in  FIG.  14   ). 
     The vibration unit  1403  may include a panel (e.g., the panel  21 ) and a transducer (e.g., the transducer  22 ). The transfer relationships K1, K2 and K3 may be used to describe the relationships between the corresponding components in the equivalent system of the bone conduction speaker (described in detail below). Vibration equations of the equivalent system may be expressed as:
 
 m   3   x   3   ″+R   3   x   3   ′−R   4   x   4 ′+( k   3   +k   4 ) x   3   +k   5 ( x   3   −x   4 )= f   3 ,  (14),
 
 m   4   x   4   ″+R   4   x   4   ″−k   5 ( x   3   −x   4 )= f   4 ,  (15),
 
where, m 3  is an equivalent mass of the vibration unit  1403 ; m 4  is an equivalent mass of the transducer  1404 ; x 3  is an equivalent displacement of the vibration unit  1403 ; x 4  is an equivalent displacement of the transducer  1404 ; k 3  is an equivalent elastic coefficient formed between the sensor terminal  1402  and the vibration unit  1403 ; k 4  is an equivalent elastic coefficient formed between the fixed ends  1401  and the vibration unit  1403 ; k 5  is an equivalent elastic coefficient formed between the transducer  1404  and the vibration unit  1403 ; R 3  is an equivalent damping formed between the sensor terminal  1402  and the vibration unit  1403 ; R 4  is an equivalent damping formed between the transducer  1404  and the vibration unit  1403 ; f 3  and f 4  are interaction forces between the vibration unit  1403  and the transducer  1404 . The equivalent amplitude of the vibration unit A 3  is:
 
                       A   3     =       -         m   4     ⁢     ω   2               (         m   3     ⁢     ω   2       +     j   ⁢   ω   ⁢     R   3       -     (       k   3     +     k   4     +     k   5       )       )                 (         m   4     ⁢     ω   2       +     j   ⁢   ω   ⁢     R   4       -     k   5       )     -       k   5     ⁡     (       k   5     -     j   ⁢   ω   ⁢     R   4         )                   ·     f   0         ,             (   16   )     ,               
where f 0  is a unit driving force, and co is a vibration frequency. The factors affecting the frequency response of the bone conduction speaker may include the vibration generation (including but not limited to, the vibration unit, the transducer, the housing, and the connection means between each other, such as m 3 , m 4 , k 5 , R 4  in equation (16)), and the vibration transfer (including but not limited to, the way being in contact with skin, the property of headset bracket/headset lanyard, such as k 3 , k 4 , R 3  in equation (16)). The frequency response and the sound quality of the bone conduction speaker may also be affected by changes of the structure of each component and the parameter of the connection between each component of the bone conduction speaker; for example, changing the size of the clamping force may be equivalent to changing k 4 , changing the bond with glue may be equivalent to changing R 4  and k 5 , and changing hardness, elasticity, damping of relevant materials may be equivalent to changing k 3  and R 3 .
 
     In an embodiment, the location of the fixed end  1401  may refer to a point or an area relatively fixed at a location in the vibration process, and the point or area may be deemed as the fixed end. The fixed end may be consisted of certain components, or may also be determined by the structure of the bone conduction speaker. For example, the bone conduction speaker may be suspended, adhered, or absorbed around a user&#39;s ear, or may attach to a man&#39;s skin through special design for the structure or the appearance of the bone conduction speaker. 
     The sensor terminal  1402  may be an auditory system of a person for receiving a sound signal. The vibration unit  1403  may be used to protect, support, and connect the transducer. The vibration unit  1403  may include a vibration transfer layer for transmitting vibrations to a user, a panel being in contact with a user directly or indirectly, and a housing for protecting and supporting other vibration generation components. The transducer  1404  may generate sound vibrations. 
     The transfer relationship K1 may connect the fixed end  1401  and the vibration unit  1403 , which refers to the vibration transfer relationship between the fixed end and the vibration generation portion. K1 may be determined based on the shape and the structure of the bone conduction speaker. For example, the bone conduction speaker may be fixed on a user&#39;s head by a U-shaped headset bracket/the headset lanyard. The bone conduction speaker may also be set on a helmet, a fire mask or a specific mask, a glass, or the like. Different structures and shapes of the bone conduction speaker may affect the transfer relationship K1. Further, the structure of the bone conduction speaker may include the material, mass, etc., of different parts of the bone conduction speaker. The transfer relationship K2 may connect the sensor terminal  1402  and the vibration unit  1403 . 
     K2 may depend on the component of the transfer system. The transfer may include but not limited to transferring sound through a user&#39;s tissue to the user&#39;s auditory system. For example, when the sound is transferred to the auditory system through the skin, subcutaneous tissue, bones, etc., the physical properties of various parts and mutual connection relationships between the various parts may have impacts on K2. Further, the vibration unit  1403  may be in contact with tissue. In various embodiments, the contact surface may be the vibration transfer layer or the side surface of the panel. The shape and the size of the contact surface, and the force between the vibration unit  1403  and tissue may influence the transfer coefficient K2. 
     The transfer coefficient K3 between the vibration unit  1403  and the transducer  1404  may be dependent on the connection property inside the vibration generation unit of the bone conduction speaker. The transducer and the vibration unit may be connected rigidly or flexibly, or changing the relative position of the connector between the vibration unit, and the transducer may affect the transducer for transferring vibrations to the vibration unit, especially the transfer efficiency of the panel, thereby affecting the transfer relationship K3. 
     When the bone conduction speaker is used, the sound generation and transferring process may affect the sound quality that a user feels. For example, the fixed end, the sense terminal, the vibration unit, the transducer and transfer relationship K1, K2 and K3, etc., mentioned above, may have impacts on the sound quality. It should be noted that K1, K2, and K3 are merely descriptions for the connection manners involved in different parts of the apparatus or the system may include but not limited to physical connection manner, force conduction manner, sound transfer efficiency, etc. 
     The descriptions of the equivalent system of bone conduction speaker are merely a specific embodiment, and it should not be considered as the only feasible embodiment. Apparently, those skilled in the art, after understanding the basic principles of bone conduction speaker, may make various modifications and changes on the type and detail of the vibrations of the bone conduction speaker, but these changes and modifications are still in the scope described above. For example, K1, K2, and K3 described above may refer to a simple vibration or mechanical transfer mode, or they may also include a complex non-linear transfer system. The transfer relationship may be formed by a direct connection between each portion or may be transferred via a non-contact manner. 
     The transfer relationship K2 between the sensor terminal  1402  and the vibration unit  1403  may also affect the frequency response of the bone conduction system. The volume of a sound heard by a user&#39;s ear depends on the energy received by a user&#39;s cochlea. The energy may be affected by various parameters during its transmission, which may be expressed by the following equation:
 
 P=∫∫   S   α·f ·(α, R )· L·ds,   (17),
 
where P is linear to the energy received by the cochlea, S is the area of a contact surface between the bone conduction speaker and a user&#39;s face, α is a coefficient for dimension change, f(α, R) denotes an effect of an acceleration a of a point on the contact surface and tightness R of contact between contact surface and a user&#39;s skin on energy transmission, L refers to the damping of any contacting points on the transmission of mechanical wave, i.e., a transmission impedance of a unit area.
 
     In terms of (17), the transmission impedance L may have an impact on the sound transmission, and the vibration transmission efficiency of the bone conduction system may relate to the transmission impedance L. The frequency response curve of the bone conduction system may be a superposition of frequency response curves of multiple points on the contact surface. Factors that change the impedance may include the size of the energy transmission area, the shape of the energy transmission area, the roughness of the energy transmission area, the force on the energy transmission area, or a distribution of the force on the energy transmission area, etc. For example, the transmission effect of sound may change when changing the structure and shape of the vibration unit  1403 , thus changing the sound quality of the bone conduction speaker. Merely by way of example, the transmission effect of sound may be changed by changing the corresponding physical characteristic of the contact surface of the vibration unit  1403 . 
     A well-designed contact surface may have a gradient structure, and the gradient structure may refer to an area with various heights on the contact surface. The gradient structure may be a convex/concave portion or a sidestep that exists on an outer side (towards a user) or inner side (backward a user) of the contact surface. An embodiment of a vibration unit of the bone conduction speaker may be illustrated in  FIG.  15 A . A convex/concave portion (not shown in  FIG.  15 A ) may exist on a contact surface  1501  (an outer side of the contact surface). During the operation of the bone conduction speaker, the convex/concave portion may be in contact with a user&#39;s face, changing the forces between different positions on the contact surface  1501  and a user&#39;s face. A convex portion may be in contact with a user&#39;s face in a tighter manner; thus the force on the skin and tissue of a user that contact with the convex portion may be larger, and the force on the skin and tissue that contact with a concave portion may be smaller accordingly. For example, three points A, B, and C on the contact surface  1501  in  FIG.  15 A  may be located on a non-convex portion, an edge of a convex portion, and a convex portion, respectively. When being in contact with a user&#39;s skin, clapping forces F A , F B , and F C  on the three points may be F C &gt;F A &gt;F B . In some embodiments, a clamping force on the point B may be 0; i.e., the point B may not be in contact with the skin of a user. The skin and tissue of a user&#39;s face may have different impedances and responses under different forces. The part of a user&#39;s face under a larger force may correspond to a smaller impedance rate and have a high-pass filtering characteristic for an acoustic wave. The part under a smaller force may correspond to a larger impedance rate, and have a low-pass filtering characteristic for an acoustic wave. Different parts of the contact surface  1501  may correspond to different impedance characteristics L. Different parts may correspond to different frequency responses for sound transmission. The transmission effect of the sound via the entire contact surface may be equivalent to a sum of transmission effect of the sound via each part of the contact surface. A smooth curve may be formed when the sound transmits into a user&#39;s brain, which may avoid exorbitant harmonic peak under a low frequency or a high frequency, thus obtaining an ideal frequency response across the whole bandwidth. Similarly, the material and thickness of the contact surface  1501  may have an effect on the transmission effect of the sound, thus affecting the sound quality. For example, when the contact surface is soft, the transmission effect of the sound in the low frequency range may be better than that in the high frequency range, and when the contact surface is hard, the transmission effect of the sound in the high frequency range may be better than that in the low frequency range. 
       FIG.  15 B  shows response curves of the bone conduction speaker with different contact areas. The dotted line corresponds to the frequency response of the bone conduction speaker having a convex portion on the contact surface. The solid line corresponds to the frequency response of the bone conduction speaker having a non-convex portion of the contact surface. In a low-intermediate frequency range, the vibration of the non-convex portion may be weakened relative to that of the convex portion, which may form one “pit” on the frequency response curve, indicating that the frequency response is not ideal and may influence the sound quality. 
     The above descriptions of the  FIG.  15 B  are merely the explanation for a specific embodiment, and those skilled in the art, after understanding the basic principles of bone conduction speaker, may make various modifications and changes on the structure and the components to achieve different frequency response effects. 
     It should be noted that for those skilled in the art, the shape and the structure of the contact surface may not be limited to the descriptions above. In some embodiments, the convex portion or the concave portion may be located at an edge of the contact surface or may be located at the center of the contact surface. The contact surface may include one or more convex portions or concave portions. The convex portion and/or concave portion may be located on the contact surface. The material of the convex portion or the concave portion may be different from the material of the contact surface, such as flexible material, rigid material, or a material easy to produce a specific force gradient. The material may be memory material or non-memory material; the material may be a single material or composite material. The structure pattern of the convex portion or concave portion of the contact surface may include but not limited to axial symmetrical pattern, central symmetrical pattern, symmetrical rotational pattern, asymmetrical pattern, etc. The structure pattern of the convex portion or the concave portion on the contact surface may include one pattern, two patterns, or a combination of two or patterns. The contact surface may include but not limited to a certain degree of smoothness, roughness, waviness, or the like. The distribution of the convex portions or the concave portions on the contact surface may include but not limited to axial symmetry, the center of symmetry, rotational symmetry, asymmetry, etc. The convex portion or the concave portion may be set at an edge of the contact surface or may be distributed inside the contact surface. 
     It should be noted that, the gradient structure on the contact surface in a bone conduction speaker disclosed in the present disclosure is also applicable for an air conduction speaker. For example, the air conduction speaker may include a gradient structure that exists on an outer side (towards a user) or inner side (backward a user) of a contact surface between the air conduction speaker and the user&#39;s face. In some embodiments, the gradient structure on the outer side of the contact surface may match the shape of the user&#39;s auricle (e.g., the shape of fossa triangularis, the shape of anthelix, etc.) such that the user such can wear the air conduction speaker more comfortably. Optionally or additionally, the air conduction speaker or the bone conduction speaker may include one or more sound guiding holes. The one or more sound guiding holes may be configured to guide sound waves inside a housing of the air conduction speaker or the bone conduction speaker through the one or more sound guiding holes to an outside of the housing. The one or more sound guiding holes may be located on a same wall or different walls of the housing. Merely by way of example, the one or more sound guiding holes may include two sound guiding holes. One sound guiding hole may be located on the contact surface of the air conduction speaker. The other sound guiding hole may be located on a wall (e.g., a sidewall) of the housing different from the contact surface. 
       1604 - 1611  in  FIG.  16    are embodiments of the structure of the contact surface. 
       1604  in  FIG.  16    shows multiple convex portions with similar shapes and structures on the contact surface. The convex portions may be made of a same material or similar materials as other parts of the panel, or different materials. In particular, the convex portions may be made of a memory material and the material of the vibration transfer layer, wherein the proportion of the memory material may be not less than 10%. Preferably, the proportion may be not less than 50%. The area of a single convex portion may be 1%-80% of the total area, preferably 5%-70%, and more preferably 8%-40%. The sum of the area of the convex portions may be 5%-80% of the total area, preferably 10%-60%. There may be at least one convex portion, preferably one convex portion, more preferably two convex portions, and further preferably at least five convex portions. The shapes of the convex portions may be circular, oval, triangular, rectangular, trapezoidal, irregular polygons or other similar patterns, wherein the structures of the convex portions may be symmetrical, or asymmetrical, the distribution of the convex portions may be symmetrically distributed or asymmetrically distributed, the number of the convex portions may be one or more, the heights of the convex portions may be the same or different, and the height distribution of the convex portions may form a certain gradient. 
       1605  in  FIG.  16    shows an embodiment of convex portions on the contact surface with two or more structure patterns. There may be one or more convex portions of different patterns. Shapes of the two or more convex portions may be circular, oval, triangular, rectangular, trapezoidal, irregular polygons, other shapes, or a combination of any two or more shapes. The material, quantity, size, symmetry of the convex portions may be similar to that as illustrated in  1604 . 
       1606  in  FIG.  16    shows an embodiment that the convex portions may be distributed at edges of the contact surface or in the contact surface. The number of the convex portions located at edges of the contact surface may be 1% to 80% of the total number of the convex portions, preferably 5%-70%, more preferably 10%-50%, and more preferably 30%-40%. The material, quantity, size, shape, or symmetry of the convex portions may be similar to  1604 . 
       1607  in  FIG.  16    shows a structure pattern of concave portions on the contact surface. The structures of the concave portions may be symmetrical or asymmetrical, the distribution of the concave portions may be symmetrical or asymmetrical, the number of the concave portions may be one or more than one, the shapes of the concave portions may be same or different, and the concave portions may be hollow. The area of a single concave portion may be not less than 1%-80% of the total area of the contact surface, preferably 5%-70%, and more preferably 8%-40%. The sum of the area of all concave portions may be 5%-80% of the total area, preferably 10%-60%. There may be at least one concave, preferably one, more preferably two, and more preferably at least five. The shapes of the concave portions may be circular, oval, triangular, rectangular, trapezoidal, irregular polygons or other similar patterns. 
       1608  in  FIG.  16    shows a contact surface including convex portions and concave portions. There may be one or more convex portions and one or more concave portions. The ratio of the number of the concave portions to the convex portions may be 0.1%-100%, preferably 1%-80%, more preferably 5%-60%, further preferably 10%-20%. The material, quantity, size, shape, or symmetry of each convex portion or each concave portion may be similar to  1604 . 
       1609  in  FIG.  16    shows an embodiment of the contact surface having a certain waviness. The waviness may be formed by two or more convex/concave portions. Preferably, the distances between adjacent convex/concave portions may be equal. More preferably, the distances between convex/concave portions may be presented in an arithmetic progression. 
       1610  in  FIG.  16    shows an embodiment of a convex portion having a large area on the contact surface. The area of the convex portion may be 30%-80% of the total area of the contact surface. Preferably, a part of an edge of the convex portion may substantially contact with a part of an edge of the contact surface. 
       1611  in  FIG.  16    shows a first convex portion having a large area on the contact surface, and a second convex portion on the first convex portion may have a smaller area. The area of the convex portion having a larger area may be 30%-80% of the total area, and the area of the convex portion having a smaller area may be 1%-30% of the total area, preferably 5%-20%. The area of the smaller area may be 5%-80% that of the larger area, preferably 10%-30%. 
     The above descriptions of the contact surface structure of the bone conduction speaker are merely a specific embodiment, and it may not be considered the only feasible implementation. Apparently, those skilled in the art, after understanding the basic principles of bone conduction speaker, may make various modifications and changes in the type and detail of the contact surface of the bone conduction speaker, but these changes and modifications are still within the scope described above. For example, the count of the convex portions and the concave portions may not be limited to that of the  FIG.  16   , and modifications made on the convex portions, the concave portions, or the patterns of the contact surface may remain in the descriptions above. Moreover, the contact surface of at least one vibration unit of the bone conduction speaker may have the same or different shapes and materials. The effect of vibrations transferred via different contact surfaces may have differences due to the properties of the contact surfaces, which may result in different sound effects. 
     It&#39;s noticeable that above statements are preferable embodiments and technical principles thereof. A person having ordinary skill in the art is easy to understand that this disclosure is not limited to the specific embodiments stated, and a person having ordinary skill in the art can make various obvious variations, adjustments, and substitutes within the protected scope of this disclosure. Therefore, although above embodiments state this disclosure in detail, this disclosure is not limited to the embodiments, and there can be many other equivalent embodiments within the scope of the present disclosure, and the protected scope of this disclosure is determined by following claims.