Abstract:
An open-air type earphone having a duct that communicates between an inside and an outside of the earphone and applies an inductance component to an acoustic signal generated by an electroacoustic transducer. The earphone includes the electroacoustic transducer to convert an electric signal into an acoustic signal, a housing to accommodate the electroacoustic transducer, and a variable duct unit that inwardly extends from the housing to communicate between the earphone and the surrounding atmosphere, and to adjust an inductance component for the acoustic signal generated by the electroacoustic transducer. Since a length or sectional area of the duct can be varied at an end of the housing, a frequency characteristics, particularly, a loss bass characteristic of the earphone, can be easily adjusted according to a user&#39;s taste, a genre of music, and the like.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2005-0133157, filed on Dec. 29, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present general inventive concept relates to an earphone, and more particularly, to an open-air type earphone having a duct that communicates between an inner portion of the earphone and an outer portion of the earphone and applies an inductance component to an acoustic signal generated by an electroacoustic transducer. 
     2. Description of the Related Art 
     Earphones are tiny speakers that fit into a user&#39;s ears and have an electroacoustic transducer that converts an electric signal into an acoustic signal. 
     Earphones can be classified as a closed-air type earphone and an open-air type earphone according to the shape of a housing in which an electroacoustic transducer is contained. Closed-air type earphones are configured such that a housing is hermetically closed from the surrounding atmosphere, and open-air type earphones are configured such that small back holes are formed along an edge of a rear portion of a housing to communicate between the inside and the outside of the housing. 
     In closed-air type earphones, since the sound pressure in the ear changes according to how tight the earphone fits into the ear, the sound quality can also vary. However, in the open-air type earphones, since the inside and the outside of a housing communicate with each other, the sound pressure inside the ear can be maintained constant over a wide range of frequencies from a low frequency to a high frequency. Additionally, acoustic resistance materials, e.g., urethane foams, may be embedded in back holes formed in the housing of the open-air type earphones to reduce external noise. 
     Resonance in the open-air type earphone occurs at a frequency between a middle frequency and a high frequency of an acoustic signal according to the size of the back holes. This resonance results in a sound pressure peak between the middle frequency and the high frequency, thereby degrading frequency characteristics of the open-air type earphones. In an effort to address these problems, U.S. Pat. No. 4,742,887 describes an open-air type earphone having a duct. 
       FIG. 1  is a cross-sectional view illustrating a conventional open-air type earphone. 
     Referring to  FIG. 1 , the conventional open-air type earphone includes an electroacoustic transducer  102  including a permanent magnet, a voice coil, and a diaphragm, and a housing  104  accommodating the electroacoustic transducer  102 . Back holes  106  are formed in the back of the housing  104  and are covered by acoustic resistance materials such as non-woven fabrics. A duct  108  extends from a side of the housing  104 . 
     In the conventional open-air type earphone having the back holes  106 , since the frequency response decreases at frequencies below the resonant frequency of the vibration system consisting of the voice coil and the diaphragm, the resonant frequency of the electroacoustic transducer  102  should be as small as possible in order to improve the low frequency characteristic. 
     The resonant frequency of the electroacoustic transducer  102  may be decreased by increasing the compliance or the equivalent mass of the electroacoustic transducer  102 . Here, the compliance is a measure of the flexibility of a moving body. For example, a high compliance speaker is very soft at a cone support portion. 
     In particular, in order to increase the compliance of the electroacoustic transducer  102 , it is necessary to either (1) select a material of high compliance for the diaphragm or (2) decrease the thickness of the diaphragm. However, there are limits regarding the compliance of the material that can be used for the diaphragm and the extent to which the thickness of the diaphragm can be reduced. Further, by increasing the equivalent mass of the electroacoustic transducer  102 , the sensitivity and acoustic characteristic of the earphone in the high frequency range is deteriorated. 
     In the conventional open-air type earphone of  FIG. 1 , the compliance and the equivalent mass of the electroacoustic transducer  102  are improved by extending a portion of the housing  104  to form the duct  108 . Since the duct  108  adds an equivalent mass to the vibration system, the resonant frequency of the vibration system is reduced by the amount corresponding to the added equivalent mass. That is, this reduction of the resonant frequency of the vibration system is achieved irrespective of the compliance and the equivalent mass of the vibration system. Accordingly, the low frequency characteristic of the conventional open-air type earphone can be improved due to the duct  108 . 
     The low frequency characteristic of the earphone is basically determined by the equivalent mass of the duct  108  and the resonant frequency of the vibration system, but also is determined by how tight the earphone fits in the ear. That is, the low frequency characteristic is changed according to the leakage of sound when an acoustic signal generated by the earphone is transmitted to the ear. The low frequency component of the acoustic signal is reduced when there is a great deal of sound leakage. 
     Additionally, since the hearing sensitivity of different users varies based on ear structure, the low frequency characteristic of the earphone is also affected by the ear structure as well as the equivalent mass of the duct  108  and the resonant frequency of the vibration system. 
     Users may also want to adjust the low frequency characteristic of the earphone according to the music genre. Here, the low frequency ranges from 20 to 200 Hz, and can be divided into deep bass ranging from 20 to 40 Hz, middle bass ranging from 40 to 400 Hz, and upper bass ranging from 100 to 200 Hz. For example, deep bass is particularly important when listening to classical music, whereas upper bass is particularly important when listening to hip-hop or dance music. 
     Accordingly, the low frequency characteristic should be adjusted according to a user&#39;s physical feature (i.e., the ear structure), taste, and music genre. 
     SUMMARY OF THE INVENTION 
     The present general inventive concept provides an open-air type earphone having a low frequency characteristic which can be adjusted according to a user&#39;s physical feature, taste, and a genre of music. 
     Additional aspects of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other aspects of the present general inventive concept are achieved by providing an earphone including an electroacoustic transducer to convert an electric signal into an acoustic signal, a housing to accommodate the electroacoustic transducer therein, and a variable duct unit that extends inwardly from the housing to communicate between the transducer and a surrounding atmosphere, and to adjust an inductance component for the acoustic signal generated by the electroacoustic transducer. 
     The variable duct unit may include an extended portion extending from a side of the housing, and a duct mounted in the extended portion and sliding in a longitudinal direction of the housing. 
     The variable duct unit may include an extended portion extending from a side of the housing, a plurality of sub ducts mounted in the extended portion, and an opening unit to open and close one or more of the plurality of sub ducts. 
     The foregoing and/or other aspects of the present general inventive concept are achieved by providing an earphone, including a rounded housing having a transducer disposed therein, an extended portion extending away from a side of the housing, and a duct disposed in the extended portion and having at least one of an adjustable cross sectional area and an adjustable length. 
     The foregoing and/or other aspects of the present general inventive concept are achieved by providing an earphone, including a circular housing having a first side with a speaker unit and a second side having back holes extending therethrough, an elongated portion extending from a rounded side of the housing, and a movable duct disposed in the elongated portion and which is movable between at least first and second positions with respect to the housing such that a frequency characteristic is adjustable by moving the duct. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a cross-sectional view illustrating a conventional open-air type earphone; 
         FIG. 2  is a perspective view illustrating an open-air type earphone; 
         FIG. 3  is a circuit diagram illustrating an acoustic analytic model of the open-air type earphone of  FIG. 2 ; 
         FIG. 4  is a graph illustrating response characteristics when the open-air type earphone of  FIG. 2  includes a foam cover versus when the open-air type earphone does not include the foam cover; 
         FIG. 5  is a graph illustrating response characteristics when the open-air type earphone of  FIG. 2  fits in the ear tightly versus when the open-air type earphone fits into the ear loosely; 
         FIG. 6  is a perspective view illustrating an earphone according to an embodiment of the present general inventive concept having a distance between a duct and a housing that is adjustable; 
         FIG. 7  is a plan view illustrating the earphone of  FIG. 6 ; 
         FIG. 8  illustrates a Helmholtz resonator model, an acoustic model, and an analogous circuit of an open-air type earphone; 
         FIG. 9  is a graph illustrating frequency characteristics corresponding to the states in which the distance between the duct and the housing of the earphone is adjusted as illustrated in  FIG. 6 ; 
         FIG. 10  is a perspective view illustrating an earphone according to another embodiment of the present general inventive concept; 
         FIG. 11  is a plan view illustrating the earphone of  FIG. 10  having sub ducts that are selectable using a moving slit; and 
         FIG. 12  is a plan view illustrating an earphone according to yet another embodiment of the present general inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
       FIG. 2  is a perspective view illustrating an open-air type earphone. 
     Referring to  FIG. 2 , the open-air type earphone includes a housing  202  having back holes  204  with a predetermined length L Back     —     Hole  and a gross sectional area ΣS Back     —     Hole  formed therein, and a duct  206  having a predetermined sectional area S duct  and a predetermined length L duct  contained therein. 
       FIG. 3  is a circuit diagram illustrating an acoustic analytic model of the open-air type earphone of  FIG. 2 . 
     Referring to  FIG. 3 , subscript “a” represents an acoustic parameter, “Ca_box” represents a capacitance of the housing  202 , “Ma_duct” represents an inductance of the duct  206 , “Ra_loss” represents a sum of resistances of the housing  202 , the duct  206 , and other serial components, “R_Hole” represents air-flow resistance of materials, for example, nonwoven fabrics, covering the back holes  204 , and “Ma_Hole” represents an inductance of the back holes  204 . 
     These variables are calculated as follows.
 
 Ca _box= V   box   /ρ·c   2  
 
 Ma _duct=ρ· c   2   /S   duct  
 
R_Hole; value obtained by measurement
 
 Ma _Hole=ρ· L   Back     —     Hole   /ΣS   Back     —     Hole  
 
where “V box ” represents a volume of the housing  202 , “ρ” represents an air density, “c” represents a sound velocity in air (345 m/s), “Ravc” represents a resistance of a voice coil, “Ras” represents a suspension resistance, “Cas” represents a suspension compliance, and “Mad” represents a mass of a diaphragm.
 
     These variables can be obtained by Thiele &amp; Small Parameter as follows.
 
 i=Eg/Revc ; current in voice coil
 
 F=BL·I=Eg·BL/Revc·Sd ; force generated by coil
 
 Pag=F/Sd=Eg·BL/Revc·Sd ; pressure generated by diaphragm
 
                 R   avc     =       1     R   evc       ·       (     BL   Sd     )     2         ;         resistance of voice coil   Mad=Mmd/Sd   2 ; mass of diaphragm   Mas (ω)= Mad+Mar (ω); diaphragm mass plus radiation mass
 
 Mas   ˜   =Mad+Mar   ˜ ; approximate value of  Mas (ω)
 
 Cas=Cms·Sd   2 ; suspension compliance
 
               Ras   =       Rms     Sd   2       ≅           Mas   ⁡     (   0   )       /   Cas       Qms         ;         suspension resistance 
     As mentioned above, the Ra_loss is the sum of the resistances of the housing  202 , the duct  206 , and other serial components, and is given by: 
             Ra_loss   =       Q   loss         w   box     ·   Ca_box             
where “Q loss ” represents a total box loss of the housing  202 , and ranges from 3 to 7 according to the damping degree of the housing  202 , and “ω Box ” represents a resonant frequency 2*π* of the duct  206 .
 
       FIG. 4  is a graph illustrating response characteristics when the open-air type earphone of  FIG. 2  includes a foam cover versus when the open-air type earphone of  FIG. 2  does not include the foam cover.  FIG. 5  is a graph illustrating frequency response characteristics when the open-air type earphone of  FIG. 2  fits in the ear tightly versus when the open-air type earphone of  FIG. 2  fits loosely in the ear. The foam cover may be an earphone cover made of sponge used to increase tightness between the open-air type earphone of  FIG. 2  and the ear. The graph of  FIG. 4  illustrates the frequency response characteristics of the open-air type earphone of  FIG. 2  measured using a head and torso system. 
     A curve  402  indicated by a thick solid line in  FIG. 4  illustrates the frequency response characteristic when the open-air type earphone of  FIG. 2  does not include the foam cover, and a curve  404  indicated by a thin dashed line in  FIG. 4  illustrates the frequency response characteristic when the open-air type earphone of  FIG. 2  includes the foam cover. 
     A curve  502  indicated by a thick solid line in  FIG. 5  illustrates the frequency response characteristic when the open-air type earphone of  FIG. 2  fits in the ear loosely. A curve  504  indicated by a thin dashed line in  FIG. 5  illustrates the frequency response characteristic when the open-air type earphone of  FIG. 2  fits in the ear tightly. 
     Referring to  FIGS. 4 and 5 , the low frequency characteristic of the open-air type earphone varies substantially with the presence of the foam cover and how tightly the earphone fits in the ear, as compared with other frequency characteristics. 
     In other words, the frequency response characteristic is changed according to the state of the earphone and a condition in which the earphone is used. Accordingly, a user should adjust the low frequency characteristic according to the state of the earphone, a condition in which the earphone is used, and the genre of music being reproduced. 
     Referring back to  FIG. 2 , the open-air type earphone according to embodiments of the present general inventive concept enables a user to adjust a low frequency characteristic according to the state of the earphone, a condition in which the earphone is used, a user taste or preference, or music being listened to by varying the length L duct  and the sectional area S duct  of the duct  206  installed in the housing  202 . 
       FIG. 6  is a perspective view illustrating an earphone according to an embodiment of the present general inventive concept.  FIG. 6  illustrates cases in which a distance between a housing  602  and a duct  606  is adjusted. Referring to  FIG. 6 , an extended portion  604  extends from a side of the housing  602  in a longitudinal direction. The extended portion  604  contains the duct  606 . The duct  606  can be moved inside the extended portion  604  in the longitudinal direction. The duct  606  has a predetermined length and has a first hole formed toward the housing  602  and a second hole formed perpendicular to the longitudinal direction. An inside and outside of the housing  602  communicate with each other through the first and second holes. 
     Fixing grooves  606   a  are formed at constant intervals on an outer surface of the duct  606 . Fixing protrusions  604   a  are formed on an inner surface of the extended portion  604  to correspond to and engage the fixing grooves  606   a  of the duct  606 . The duct  606  can be fixed by the fixing grooves  606   a  and the fixing protrusions  604   a.    
     The duct  606  has a projection  606   b  which has the second hole. The projection  606   b  projects from a surface of the extended portion  604  through an opening of the extended portion  604  such that a user can easily move the duct  606  by hand. A lower side of the duct  606  is closed and thus the duct  606  communicates with the surrounding atmosphere through the second hole. 
     Referring to  FIG. 6 , the duct  606  can be adjusted to three positions. A distance between the duct  606  and the housing  602  is changed according to the positions of the duct  606 . For example, upper, middle, and lower perspective views of  FIG. 6  illustrate cases in which the distance between the protrusion  606   b  of the duct  606  and a portion of the housing  602  where the housing  602  meets the extended portion  604  is adjusted to 12 mm, 8 mm, and 4 mm, respectively. The distances are measured from a free end of the protrusion  606   b  via the inside of the duct  606  to the portion of the housing  602  where the housing  602  meets the extended portion  604 . 
       FIG. 7  is a plan view illustrating the earphone of  FIG. 6 . Left, middle, and right plan views of  FIG. 7  correspond to the upper, middle, and lower perspective views of  FIG. 6 , respectively. 
     Referring to  FIGS. 6 and 7 , a frequency characteristic of the earphone is changed by adjusting the distance between the duct  606  and the housing  602 . 
       FIG. 8  illustrates a Helmholtz resonator model, an acoustic model, and an analogous circuit of the earphone of  FIG. 6 . The open-air type earphone can be modelled as a Helmholtz resonator  802  (left) as illustrated in  FIG. 8 . 
     The Helmholtz resonator  802  of  FIG. 8  includes a box  802   a  having a volume V, and a duct  802   b  having a length L and a sectional area S, the duct  802   b  being connected to the box  802   a . The box  802   a  of the Helmholtz resonator  802  corresponds to the housing  602  of the open-air type earphone, and the duct  802   b  corresponds to the duct  606  of the open-air type earphone. 
     The Helmholtz resonator  802  may be represented as an acoustic model (middle) and an acoustic analogous circuit (right) having an acoustic impedance Z (that is, a resistance R, an inductance M, and a capacitance C). Referring to  FIG. 8 , “P” represents sound pressure input to the Helmholtz resonator  802 , and “U” represents volume velocity in the Helmholtz resonator  802 . 
             Z   =       P   U     =     R   +     j   ⁢           ⁢     ω   ·   M       +     1     jω   ·   C                     where               M   =       ρ   ·     L   ′       S       ,     C   =     V     ρ   ·     c   2           ,         
and L′ is an effective length and is increased by an effect of air radiation and mass loading.
 
 L′=L+ 0.85 ·d ; with flange at inlet of duct
 
 L′=L+ 0.725 ·d ; without flange at inlet of duct,
 
where “d” represents a diameter of the duct  802   b.  
 
     That is, when the sectional area S of the duct  802   b  increases or the length L of the duct  802   b  decreases, the inductance M of the Helmholtz resonator  802  decreases, and vice versa. That is, the frequency characteristic of the open-air type earphone can be adjusted by adjusting the sectional area S and the length L of the duct  802   b.    
       FIG. 9  is a graph illustrating the frequency characteristics when the distance between the duct  606  and the housing  602  is adjusted as illustrated in  FIG. 6 . In particular,  FIG. 9  illustrates the frequency response characteristics when the earphone is mounted in an infinite baffle. 
     Referring to  FIG. 9 , curves  902 ,  904 , and  906  correspond to the upper, middle, and lower perspective views of  FIG. 6 , respectively, which illustrate the states in which the distance between the duct  606  and the housing  602  are adjusted to 12 mm, 8 mm, and 4 mm. The distance may be measured between a proximal end of the duct  606  and a portion of the housing  602  where the housing  602  meets the extended portion  604 . The curve  906  is suitable for hip-hop, dance music, or the like, which uses strong bits, and the curve  902  is suitable for big classic, Rock, Jazz, or the like, which requires deep bass rather than strong bass. 
     Referring to  FIG. 9 , the frequency characteristic, particularly, the low frequency characteristic of the earphone is significantly changed by adjusting the distance between the duct  606  and the housing  602 . 
       FIG. 10  is a perspective view illustrating an earphone according to another embodiment of the present general inventive concept. Referring to  FIG. 10 , the earphone includes three fixed sub ducts  102   a ,  102   b , and  102   c  having different lengths, and holes of the sub ducts  102   a ,  102   b , and  102   c  are opened and closed using a moving slit  104   a.    
       FIG. 11  is a plan view illustrating the earphone of  FIG. 10  when one of the sub ducts  102   a ,  102   b , and  102   c  is selected using the moving slit  104   a . The moving slit  104   a  is formed on a rotating grip  104 , and one of the sub ducts  102   a ,  102   b , and  102   c  can be selected by rotating the rotating grip  104 . As can be seen from  FIG. 11 , the moving slit  104   a  can be positioned to correspond to the sub duct  102   a  to adjust deep bass frequency characteristics, the sub duct  102   b  to adjust middle bass frequency characteristics, and the sub duct  102   c  to adjust upper bass frequency characteristics. Therefore, the deep bass, middle bass and upper bass frequency characteristics can be emphasized by the positions of the moving slit  104   a.    
       FIG. 12  is a plan view illustrating an earphone according to yet another embodiment of the present general inventive concept. Referring to  FIG. 12 , the earphone includes three sub ducts  122   a ,  122   b , and  122   c  having the same length, and a rotating cover  124  having a slit  124   a  that opens and closes the sub ducts  122   a ,  122   b , and  122   c . A combination of the sub ducts  122   a ,  122   b , and  122   c  can be selected by rotating the rotating cover  124 . That is, a number of the sub ducts  122   a ,  122   b , and  122   c  can be opened/closed by rotating the rotating cover  124 . Accordingly, air can be moved between a housing and the number of sub ducts  122   a ,  122   b , and  122   c , thereby adjusting the frequency characteristics of the earphone. As can be seen from  FIG. 12 , the rotating cover  124  can be moved to position the slit  124   a  to correspond to one sub duct to adjust deep bass frequency characteristics, two sub ducts to adjust middle bass frequency characteristics, and three sub ducts to adjust upper bass frequency characteristics. 
     As described above, since a duct extends from a side of the housing and a length and sectional area of the duct can be varied, a frequency characteristic, particularly, a low frequency characteristic, of an open-air type earphone of embodiments of the present general inventive concept can be easily adjusted according to a user&#39;s taste, a genre of music, a presence of the foam cover, or a distance between the earphone and an ear of a user. 
     Since an acoustic inductance can be changed using mechanical elements, a frequency characteristic of an open-air type earphone of embodiments of the present general inventive concept can be adjusted simply and efficiently. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.