Abstract:
The invention relates to an arrangement for improving leak tolerance in an earpiece. The invention can be applied preferably in teleterminals, particularly in mobile stations. One idea of the invention is that an acoustic return path ( 314, 313, 311, 312, 320 ) is directed from the back part of the earpiece capsule ( 301 ) to a volume ( 307 ) between the earpiece ( 300 ) and the user&#39;s ear. By means of the solution according to the invention an optimum, controlled load is achieved particularly for low frequencies, such that a change in the volume ( 307 ) between the earpiece and the ear only has a minor effect on the volume and quality of the sensed sound. By means of the solution a good leak tolerance is achieved, even though the volume to be arranged behind the earpiece capsule is small.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and an arrangement for improving leak tolerance in an earpiece. The invention can be applied preferably in teleterminals, particularly in mobile stations. 
     2. Description of the Related Art 
     Teleterminals conventionally contain a receiver part which has an earpiece for reproducing the received acoustic signal. The earpiece has been conventionally designed in such a way that it forms the maximum sound volume and the best quality of sound when the earpiece is sealed against the user&#39;s ear. If there is a gap, i.e. a leak between the earpiece and the user&#39;s ear, this usually causes a significant weakening of the sensed sound pressure. Additionally, the frequency distribution of the sensed sound does not then correspond to the original acoustic signal but low frequencies are attenuated to a greater extent than high frequencies. The ability of an earpiece to maintain its acoustic properties when the gap between the earpiece and the ear changes, is called its leak tolerance. 
     The problem described above is extremely serious particularly in mobile stations, because the mobile station is rarely completely sealed against the user&#39;s ear. On the other hand, standards relating to mobile stations are primarily based on measurements where the gap between the mobile station and the artificial ear has been arranged so that there is a tight seal. In order to ensure that the volume and frequency distribution of the reproduced sound are according to the specifications also in real operating conditions, extremely good leak tolerance is required from the earpiece arrangement. 
     To improve leak tolerance the following ways are prior known. The leak tolerance can be improved by arranging a loose coupling to the membrane which produces the sound waves in the earpiece capsule and by loading it by a relatively large volume situated behind the earpiece capsule. Most preferably the volume behind the earpiece capsule has been arranged to be open, in which case the aforementioned volume becomes as large as possible. Another way to improve the leak tolerance is to lower the acoustic output impedance of the arrangement by using an acoustic return path. 
     FIG. 1 shows an earpiece  100  according to prior art. It comprises an earpiece capsule  101  which converts an electric signal into an acoustic sound. The earpiece capsule  101  is connected to the housing  103  of the earpiece by its edges  102 . The sound wave formed by the earpiece capsule is generated in the volume  104  between the earpiece  101 , the edges  102  and the housing  103 , from which it is transferred to the external volume of the housing through holes  105 . Between the housing  103  and the ear  106  there remains a volume  107  which thus in an optimal situation is closed. If the earpiece is a part of a mobile station, the housing  103  is preferably the cover of the mobile station. 
     In the solution shown in FIG. 1, leak tolerance has been improved by arranging an acoustic return path from the back part of the earpiece capsule to the front part of it. The acoustic return path is formed of the holes  114  in the back part of the earpiece capsule, the volume  113  behind the earpiece capsule, the volume  112  on the sides of the earpiece capsule and the holes  115  in the front edge of the earpiece capsule. The volume arranged for said acoustic return path has been closed by a special casing  110  in the solution of FIG. 1, but the volume can also be formed by a normal casing of a device, such as a mobile station, and the components inside it. In the solution shown in FIG. 1 an acoustic volume  113  has been arranged for the acoustic path and it can contain material attenuating high frequency components. The ability to improve leak tolerance in the solution shown in FIG. 1 is based on the fact that the return path arranged especially for low frequencies operates as an acoustic load for the earpiece capsule at low frequencies, in which case the changes in external load have a smaller relative effect on the acoustic total load of the earpiece capsule. 
     A disadvantage of the above described solution according to prior art is that the load caused by the acoustic return path is difficult to optimize. The leak tolerance to be achieved depends essentially on the size of the volume arranged behind the earpiece capsule. In small-sized devices, such as mobile stations, it is often impossible to provide a sufficiently large volume to achieve the optimum acoustic load. 
     FIG. 2 shows an acoustic equivalent circuit of an earpiece according to FIG.  1 . In it, the earpiece capsule forms a pressure wave by operating as an acoustic source  201  and comprising an internal impedance  204 . The pressure wave propagates to the outside of the earpiece arrangement through holes in the casing, such that the holes form an impedance  205  and the external volume forms a load impedance  206 . The interface between the earpiece and the external volume has been marked by  207  in FIG.  1 . The acoustic return path operates as a feedback impedance  215 . The load impedance  206  consists mainly of the load caused by the ear and the load resulting from the leak between the earpiece and the ear. It can be noted from the equivalent circuit that the changes in the load impedance  206  have a major impact on the acoustic power which is transferred to the load, and by means of the feedback impedance, the effect of load variation can be reduced to only a minor extent. 
     OBJECT OF THE INVENTION 
     The aim of the present invention is to devise an earpiece solution which achieves good leak tolerance in a small-sized device, such as a mobile station. 
     BRIEF SUMMARY OF THE INVENTION 
     One idea of the invention is that the acoustic return path is directed from the back part of the earpiece capsule to the volume between the earpiece and the user&#39;s ear. By means of the solution according to the invention, an optimum, controlled load especially for low frequencies is achieved, in which case a change in the volume between the earpiece and the ear only has a minor effect on the volume and the frequency distribution of the sensed sound. By means of the solution a good leak tolerance is achieved, though the volume to be arranged behind the earpiece capsule is small. 
     A method according to the invention for improving leak tolerance in an earpiece such that the sound formed by an earpiece capsule is directed through the first acoustic path to the first volume which is confined by the user&#39;s ear and the housing part between the earpiece capsule and the ear, is characterized in that from the back part of the earpiece capsule a sound formed by the earpiece capsule is directed to said first volume through the second acoustic path. 
     An arrangement according to the invention for improving leak tolerance of an earpiece, which comprises 
     an earpiece capsule, 
     a housing part situated between the earpiece capsule and the user&#39;s ear for confining the first acoustic volume between said housing part and user&#39;s ear and 
     the first acoustic path arranged between the front part of the earpiece capsule and the first acoustic volume for directing sound from the front part of the earpiece capsule to the first acoustic volume, 
     is characterized in that the arrangement comprises additionally 
     the second acoustic path arranged from the back part of the earpiece capsule to said first volume. 
     A mobile station according to the invention, whose earpiece comprises 
     an earpiece capsule, 
     a housing part situated between the earpiece capsule and the user&#39;s ear for confining the first acoustic volume between said housing part and user&#39;s ear and 
     the first acoustic path arranged between the front part of the earpiece capsule and the first acoustic volume for directing sound from the front part of the earpiece capsule to the first acoustic volume, 
     is characterized in that the earpiece comprises additionally 
     the second acoustic path arranged from the back part of the earpiece capsule to said first volume for improving leak tolerance. 
     Preferable embodiments of the invention have been presented in dependent claims. 
     By the front and back parts of the earpiece capsule one means herein the front and back parts of a membrane which forms sound waves and is situated in the earpiece capsule, and sound waves generated in these front and back parts are in opposite phases from each other. 
     By the earpiece one means herein the earpiece capsule and acoustic and mechanical structures connected to it. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the following the invention is described in more detail by means of the attached drawings in which 
     FIG. 1 shows an earpiece arrangement according to prior art, 
     FIG. 2 shows an acoustic equivalent circuit of the earpiece arrangement according to FIG. 1, 
     FIG. 3 shows an arrangement according to the invention for improving leak tolerance of an earpiece, 
     FIG. 4 shows an acoustic equivalent circuit of the earpiece arrangement according to FIG. 3, 
     FIG. 5 shows a block diagram of a prior known mobile station to which the present invention can preferably be applied and 
     FIG. 6 shows a mobile station according to the invention from the front and the side. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 were already described above in the context of the description of prior art. 
     FIG. 3 shows an arrangement according to the invention for improving leak tolerance of an earpiece. It comprises an earpiece capsule  301  which converts an electric signal into an acoustic sound. The earpiece capsule  301  is connected to the housing part  303  of the earpiece by its edges  302 . The sound wave formed by the earpiece capsule is generated in the volume  304  between the earpiece capsule  301 , the edges  302  and the housing part  303 , from which it is transferred to the external volume of the housing through holes  305  which form the first acoustic path. Between the housing part  303  and the ear  306  the first volume  307  is confined. If the earpiece is a part of a mobile station, the housing part is preferably the cover of the mobile station. 
     In the solution shown in FIG. 3 the leak tolerance has been improved according to the invention by arranging the second acoustic path from the back part of the earpiece capsule  301  to the first volume  307 . The second acoustic path is formed of the holes  314  in the back part of the earpiece capsule, the volume  313  behind the earpiece capsule, the volume  312  on the side of the earpiece capsule and holes  320  arranged in the housing part. 
     The volume arranged on the sides of the earpiece capsule has been closed by a special casing  310  in the solution of FIG.  3 . Additionally, in the solution shown in FIG. 3, behind the earpiece capsule an optimum acoustic volume  313  has been arranged which can contain material for attenuating high frequency components. Volumes  313  and  312  need, however, not necessarily be volumes separated from each other but they can form one combined volume. 
     The ability to improve leak tolerance in the solution shown in FIG. 3 is based, according to the invention, on the fact that the acoustic return path arranged particularly for low frequencies operates as an acoustic load for the earpiece capsule at low frequencies, in which case the changes in external load have a smaller relative effect on the acoustic total load of the earpiece capsule. When the second acoustic path has been, according to the invention, directed to the first volume between the ear and the housing part, it is possible to use the small volume behind the earpiece capsule for arranging the second acoustic path and still gain the optimum load at low frequencies. Thus, for example, applied to a mobile station, the arrangement according to the invention does not cause any significant enlargement in the size of the mobile station or have any effect on its shape. 
     The holes  320  of the housing part  303  which have been arranged for forming the second acoustic path are preferably in the same size range as the holes  305  arranged for the first acoustic path. These additional holes  320  of the housing part are the only detail which is visible to the user resulting from the arrangement according to the invention. Thus the arrangement according to the invention has no significant impact on the appearance of the device. 
     FIG. 4 shows a simplified acoustic equivalent circuit of an earpiece according to FIG.  3 . In it, the earpiece capsule forms a pressure wave and functions thus as an acoustic source  401  to which an internal impedance  404  is further connected. The pressure wave propagates to the outside of the earpiece arrangement through the holes in the casing, in which case the holes form an acoustic impedance  405  and the external volume a load impedance  406 . The interface between the earpiece and the external volume has been marked by  407  in FIG.  4 . The acoustic return path operates herein as a feedback impedance  420 . From the equivalent circuit one can notice now that the effects of the changes in the load impedance  406  on the acoustic power which is transferred to the load can be minimized by means of a feedback impedance  420 , because the feedback impedance compensates for the effect of the holes  305  in the casing, that is the effect of the acoustic impedance  405 . If the situation according to the invention is compared with the solution according to prior art shown in FIG. 2, one can further notice that to achieve a certain pressure level (corresponding to the voltage of an electric circuit) to the listener&#39;s ear  406  at a certain impedance value seen from the acoustic source, less acoustic volume velocity (corresponding to current in an electrical circuit) is needed in the solution according to the invention. This means that in order to achieve a given level of audibility, a smaller movement of the membrane in the earpiece capsule is needed. 
     Next, the application of the present invention to a mobile station is studied. At first, by means of FIG. 5, the operation of a conventional mobile station is described and thereafter, the mechanical structure of a mobile station according to the invention is described by means of FIG.  6 . 
     FIG. 5 shows a block diagram of a mobile station according to an embodiment as an example of the invention. The mobile station comprises the parts which are typical of the device, such as a microphone  531 , a keyboard  537 , a display  536 , an earpiece  501 , a transmission/reception coupling  538 , an antenna  539  and a control unit  535 . Additionally, the figure shows transmission and reception blocks  534 ,  541  which are typical of the mobile station. 
     The transmission block  534  comprises operations needed for speech coding, channel coding, ciphering and modulation, and RF operations. The reception block  541  comprises corresponding RF operations and operations needed for demodulation, deciphering, channel decoding and speech decoding. A signal which comes from the microphone  531 , which has been amplified at an amplification stage  532  and converted into a digital form in an A/D converter is transferred to the transmission block  534 , typically to a speech coding element included in the transmission block. The transmission signal which has been shaped, modulated and amplified by the transmission block is directed via the transmission/reception coupling  538  to the antenna  539 . The signal to be received is brought from the antenna via the transmission/reception coupling  538  to the reception block  541  which demodulates the received signal and performs the deciphering and the channel decoding. The speech signal received as a final result is transferred via a D/A converter  542  to an amplifier  543  and further to an earpiece  501 . The control unit  535  controls the operation of the mobile station, reads control commands given by the user from the keyboard  537  and delivers messages to the user via the display  536 . 
     When an earpiece arrangement according to the invention is used, the frequency response of the earpiece may differ from the frequency response of the arrangement according to prior art. The frequency response can be compensated analogically by using a filter which is included in the amplifier  543 . Another alternative is to perform the compensation in the context of digital signal processing in the digital signal processor (DSP) of block  541 . When the frequency response is corrected on the digital signal processor, component changes are not necessarily needed, but the correction can be performed by making the necessary additions to the program which controls the digital signal processor. 
     FIG. 6 shows a mechanical structure of a mobile station  600  according to the invention viewed from the front and the side. The side view has been enlarged by  2 : 1  compared to the front view and it shows a partial cross section A—A at the position of an earpiece according to the invention. The front view shows a microphone  631 , a keyboard  637  and a display  636  which are included in a conventional mobile station. On the top part of the mobile station one can see holes  605  which form the first acoustic path leading from the front part of the earpiece capsule to the outside of the device and holes  620  which are a part of the second acoustic path. In the cross-sectional view one can see additionally volumes  612  and  613  which are a part of the second acoustic path. The volumes  612  and  613  need not necessarily be separated but they can also form one combined volume. The housing of the earpiece between the earpiece and the user&#39;s ear is in the mobile station preferably the cover  603  of the device to which other mechanical parts of the earpiece are connected. 
     Above, an embodiment of the solution according to the invention has been described. The principle according to the invention can naturally be modified within the frame of the scope defined by the claims, for example, by modification of the details of the implementation and ranges of use.