Patent Publication Number: US-8995701-B2

Title: Microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electroacoustic transducer having an air chamber in back of a diaphragm, the electroacoustic transducer actively canceling a sound pressure generated in the air chamber such that the air chamber functions as a large-volume air chamber even if the air chamber has a small volume, and thus enhancing bass response. 
     2. Related Background Art 
     Electroacoustic transducers, for example, unidirectional dynamic microphones, omnidirectional dynamic microphones, headphones, and speakers may each have an air chamber to prevent sound waves from entering from the outside. Such an electroacoustic transducer has a diaphragm that vibrates in response to sound waves or generates sound waves as being driven by audio signals. The air chamber is provided in back of the diaphragm. The air chamber functions as an acoustic capacitance. Specifically, a large air chamber functions as a lowly resilient spring, while a small air chamber as a highly resilient spring. Thus, in the case where an acoustic capacitance having a small stiffness is required, namely, the diaphragm can be moved easily, a large-volume air chamber is needed. 
     The air chamber is explained in more detail in the case of an omnidirectional or unidirectional dynamic microphone as an example herein. In the omnidirectional or unidirectional dynamic microphone, an acoustic resistance and an air chamber should be provided in a rear portion or in back of a diaphragm in order to obtain omnidirectional components. The stiffness of the air chamber is dominant in a low frequency range. If the air chamber has a small volume, the stiffness is high and directional frequency response is low. Thus, the volume of the air chamber must be increased to reduce the stiffness. 
     In a hand-held wireless microphone, a transmitter circuit and a power battery should be housed in a grip, and thus a large air chamber cannot be provided like a wired microphone. Accordingly, the air chamber in the rear portion of the diaphragm is limited in volume and omnidirectional components should be obtained in a small air chamber. This results in poor directional frequency response and sound quality in bass sound. Specifically, if a small air chamber responds to bass sound to vibrate a diaphragm, a large back pressure is applied to the diaphragm. The diaphragm is then difficult to vibrate, thus increasing the lowest responding frequency level and reducing the bass output level. 
     The inventor of the present invention invented and filed a patent application of a dynamic microphone reducing an acoustic impedance in a back air chamber in an equivalent manner to allow pickup of bass sound even in a small-volume back air chamber (refer to Japanese Unexamined Patent Application Publication No. 2009-232176). In the invention disclosed in Japanese Unexamined Patent Application Publication No. 2009-232176, the back air chamber is provided in back of a diaphragm of a main microphone unit and a sub-microphone unit is disposed in front of the main microphone unit in a casing that supports the main microphone unit. Audio signals (voltage signals) output from the sub-microphone unit drive a membrane composed of a piezoelectric element in the back air chamber, thus reducing the acoustic impedance in the back air chamber in an equivalent manner. 
     According to the invention disclosed in Japanese Unexamined Patent Application Publication No. 2009-232176, sound waves from a sound source directed to the sub-microphone unit disposed in front of the main microphone unit are converted into audio signals in the sub-microphone unit. The audio signals drive the membrane composed of the piezoelectric element in the back air chamber. The output signals from the sub-microphone unit disposed in front of the diaphragm of the main microphone unit feedforward-controls the membrane composed of the piezoelectric element. In response to the sound waves from the sound source reaching the sub-microphone unit, a pressure change in the back air chamber is estimated and the membrane is driven based on the estimation. Thus, the membrane cannot be driven properly in accordance with the pressure change in the back air chamber. A further improvement is required for acoustic impedance control in the back air chamber at high accuracy. 
     SUMMARY OF THE INVENTION 
     In view of the circumstances above, an object of the prevent invention is to provide an electroacoustic transducer changing a volume of an air chamber properly in accordance with a change in the sound pressure of the air chamber that changes in response to vibration of a diaphragm in an electroacoustic transducer unit, thereby accurately controlling the acoustic impedance of the air chamber. 
     A main feature of the present invention provides an electroacoustic transducer having a diaphragm, an electroacoustic transducer unit including the diaphragm; and an air chamber accommodating the diaphragm of the electroacoustic transducer unit and having a variable volume in response to vibration of the diaphragm. The air chamber includes a sound pressure detector detecting a sound pressure in the air chamber; and a volume adjuster driven by output signals from the sound pressure detector, changing the volume of the air chamber in response to the output signals, and controlling an acoustic impedance of the air chamber. A control system from the sound pressure detector to the volume adjuster configures a feedback control system increasing the volume of the air chamber with an increase in the sound pressure in the air chamber. 
     As the diaphragm in the electroacoustic transducer unit vibrates, the volume of the air chamber changes and the sound pressure in the air chamber changes. The feedback control is then performed in which the sound pressure detector detects the sound pressure change and the detection signals drive the volume adjuster to eliminate the sound pressure change. This control reduces the acoustic impedance of the air chamber in an equivalent manner, thus enhancing directional frequency response particularly in bass sound. Even if an air chamber or an enclosure has a small volume in a speaker or headphones as the electroacoustic transducer, sound can be played at a sufficient sound pressure level to bass sound. Even if an air chamber is limited in size due to a power battery loaded in a microphone casing of a microphone, such as a wireless microphone, as the electroacoustic transducer, audio signals can be converted at a predetermined signal level to bass sound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an electroacoustic transducer according to an embodiment of the present invention; 
         FIG. 2  is an acoustic equivalent circuit diagram of the embodiment; 
         FIG. 3  is a schematic cross-sectional view illustrating an example property tester of an electroacoustic transducer; 
         FIG. 4  is a graph illustrating observed results with the tester; 
         FIG. 5  is a graph illustrating other observed results with the tester; 
         FIG. 6  is a graph illustrating still other results with the tester; 
         FIG. 7  is a schematic cross-sectional view of an electroacoustic transducer according to another embodiment of the present invention; and 
         FIG. 8  is a schematic cross-sectional view of an electroacoustic transducer according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of an electroacoustic transducer according to the present invention are explained below with reference to the attached drawings. 
     First Embodiment 
     An embodiment shown in  FIG. 1  is explained, in which the technological concept of the present invention is applied to headphones. In  FIG. 1 , each headphone  10  has a cup-shaped housing  12 ; a baffle plate  13  fixed to the internal periphery of the housing  12  proximate to an open end; a speaker unit  11  serving as a driver unit attached to the baffle plate  13  and surrounded by the housing  12 ; and an ear pad  14  mounted on the open end of the housing  12 . The ear pad  14  of the headphone  10  covers an ear  21  of a user and is pressed against the side of the user&#39;s head, as commonly known.  FIG. 1  illustrates a state where the headphone  10  is worn on one of the ears of the user. Headphones generally include right and left headphones to be worn on right and left ears. The right and left headphones are connected by a headband or a neckband.  FIG. 1  illustrates an example ear-covering type headphone having the ear pad  14  covering an ear  21 . An ear-mounted type may be employed, in which the ear pad  14  is mounted on the ear  21 . 
     As shown in  FIG. 1 , the headphone  10  is worn on the side of the user&#39;s head. Then, an air chamber  15  is surrounded by the baffle plate  13 , a diaphragm (not shown in the drawing) included in the speaker unit  11 , a portion of the housing  12 , the ear pad  14 , and the side of the user&#39;s head. Sound is output from the speaker unit  11  toward the air chamber  15  and sound waves reach the eardrum of the user&#39;s inner ear. The pressure in the air chamber  15 , namely the sound pressure, changes according to the sound waves. The air chamber  15  is provided with a sound pressure detector  16  that detects the sound pressure. An omnidirectional microphone is suitable, but a unidirectional microphone may be used as the sound pressure detector  16 . 
     The speaker unit  11  generates sound driven by sound signals input from a sound source, such as a CD player or an MP3 player. The speaker unit  11  is also driven by detection signals from the sound pressure detector  16 . In the example shown in  FIG. 1 , the detection signals from the sound pressure detector  16  are input to an adder  18  through a circuit block  17 , such as an amplifier; are added to sound signals  20  in the adder  18 ; and are input to the speaker unit  11  through an amplifier  19 . The amplifier  19 , which serves as a drive circuit of the speaker unit  11 , drives the speaker unit  11  using the is detection signals from the sound pressure detector  16  added in the adder  18  and the sound signals  20 . Throughout the specification, the sound signals refer to generally-called audio signals, which are electrically converted music, voice, and sound of nature. 
     In the embodiment configured as shown in  FIG. 1 , the speaker unit  11  is driven by the sound signals  20  and then outputs sound according to the sound signals  20 . The sound pressure in the air chamber  15  changes according to the output sound. The sound pressure detector  16  detects a change in the sound pressure and outputs detection signals associated with the sound pressure. The detection signals are input to the speaker unit  11  through the circuit block  17 , the adder  18 , and the amplifier  19 . The speaker unit  11  is then driven by the detection signals, and thereby the sound pressure in the air chamber  15  is maintained at a constant level. 
     Specifically, the speaker unit  11  and the air chamber  15  are provided in the embodiment shown in  FIG. 1 , the speaker unit  11  being an electroacoustic transducer unit provided with a diaphragm which is vibrated by audio signals and generate sound, the air chamber  15  being provided with the diaphragm of the speaker unit  11  and having a variable volume in response to vibration of the diaphragm. The air chamber  15  includes the sound pressure detector  16  that detects the sound pressure in the air chamber  15  and a volume adjuster that is driven by output signals from the sound pressure detector  16  so as to change the volume of the air chamber  15  according to the output signals and thus control the acoustic impedance of the air chamber  15 . In the embodiment, the speaker unit  11  also serves as the volume adjuster. The sound pressure detector  16  detects the sound pressure in the air chamber  15  and feeds back the detection signals to the speaker unit  11 , which is then controlled such that the sound pressure in the air chamber  15  does not change. 
     Specifically, in the case where the sound pressure detector  16  detects an increase in the sound pressure in the air chamber  15 , the diaphragm of the speaker unit  11  is controlled so as to retract from the air chamber  15 , and thereby the acoustic impedance in the air chamber  15  is reduced in an equivalent manner. Thus, even if the air chamber  15  has a small volume, the diaphragm of the speaker unit  11  can be vibrated in response to bass audio signals without resistance, thus improving the directional frequency response properties in bass sound. 
       FIG. 2  illustrates an acoustic equivalent circuit included in the electroacoustic transducer of the embodiment explained above with reference to  FIG. 1 . In  FIG. 2 , reference symbol P 1  represents a sound pressure of a front sound source, namely the air chamber  15  in the front; reference symbol P 2  represents a sound pressure of a back sound source, namely an air chamber in back of the diaphragm of the speaker unit  11 ; reference symbol m 0  represents the mass of the diaphragm; reference symbol s 0  represents the stiffness of the diaphragm; reference symbol m 1  represents the mass in the back air chamber; reference symbol r 1  represents an acoustic resistance in the back air chamber; reference symbol s 1  represents the stiffness in the back air chamber; and reference symbol Ps 1  represents a sound pressure generated by the stiffness s 1 . In the case where the back air chamber is small and the stiffness s 1  is high, the stiffness s 1  is dominant in bass sound and the sound pressure Ps 1  is high. The diaphragm thus moves barely. This leads to poor directional frequency response in bass sound. It is desirable to minimize the stiffness s 1  in the back air chamber to minimize the sound pressure Ps 1  so that only the acoustic resistance r 1  functions effectively. To this end, it is preferred that the volume in the back air chamber be increased as much as possible. As explained above, however, many factors limit the volume in the back air chamber. 
     According to the embodiment of the present invention shown in  FIG. 1 , the sound pressure in the air chamber  15  changes due to vibration of the diaphragm of the speaker unit  11 , which is an electroacoustic transducer unit; the microphone unit  16 , which is a sound pressure detector, then detects the change in the sound pressure; and the detection signals drive the speaker unit  11  to reduce the acoustic impedance of the air chamber  15  in an equivalent manner. Thereby, the stiffness s 1  is reduced in an equivalent manner and the sound pressure Ps 1  is reduced, thus enhancing the directional frequency response in bass sound. In addition, the sound pressure detection signals from detection of the sound pressure in the air chamber  15  are fed back to the speaker unit  11 , which also functions as a volume adjuster, so as to prevent a fluctuation in the sound pressure in the air chamber  15 . Thus, the equivalent acoustic impedance of the air chamber  15  can be controlled at high accuracy. 
     In order to demonstrate the advantageous effects of the technological concept of the present invention incorporated into the electroacoustic transducer, frequency property tests were conducted. A tester was compliant with the EIAJ RC-8160 standard.  FIG. 3  illustrates a schematic view of the tester. A unidirectional dynamic microphone  29  was used as a device under test. Test sound waves were output to the dynamic microphone  29  from a speaker unit  25  placed 50 cm forward the microphone  29 . The sound waves were received and electroacoustically transduced by the microphone  29 . The transduced signals were then recorded. 
     A device to form a space corresponding to the air chamber explained above was provided in back of the microphone  29 . The space forming device had a housing  24  and the dynamic speaker unit  25  included in the housing  24 . The speaker unit  25  had a diaphragm  26 . The housing  24  was partitioned by the diaphragm  26  into a front air chamber  27  and a back air chamber. A microphone unit  28  as a sound pressure detector was disposed in the air chamber  27 . Detection signals from the microphone unit  28  were fed back to the speaker unit  25  as a volume adjuster through a circuit block  30 , including an amplifier. The detection signals thus drove the speaker unit  25 . This feedback control system was turned on or off as desired. The volume of the air chamber  27  in a natural state where the feedback control system was turned off and the speaker unit  25  was not driven was adjusted as desired by moving the mounting position of the dynamic microphone  29 , for example. 
     The tests were performed using the tester under the following three conditions: 
     (1) Assuming a regular dynamic microphone, the volume of the air chamber  27  was set at 30 cc. The feedback control system was turned off. The acoustic impedance in the air chamber was not controlled. 
     (2) The volume of the air chamber  27  was set at 2 cc. The feedback control system was turned off. The acoustic impedance in the air chamber was not controlled. 
     (3) The volume of the air chamber  27  was set at 2 cc. The feedback control system was turned on. The acoustic impedance in the air chamber was controlled. These conditions satisfy the technological concept of the present invention. 
       FIG. 4  illustrates the observed results of condition (1);  FIG. 5  illustrates the observed results of condition (2); and  FIG. 6  illustrates the observed results of condition (3). In the observed results illustrated in each drawing, a thick line represents a case where the speaker outputting test sound waves was placed at an angle of 0 degrees to the center axis, namely in the front; a medium thickness line represents a case where the speaker was placed at an angle of 90 degrees to the center axis; and a thin line represents a case where the speaker was placed at an angle of 180 degrees to the center axis. 
     As demonstrated in comparison of  FIG. 5  and  FIG. 6 , turning the feedback control system on improves the directional frequency response in bass sound. Thus, the responding frequency level is expanded to a bass range and the bass output level is increased. It is demonstrated in comparison of  FIG. 4  and  FIG. 6  that turning the feedback control system on improves the frequency response in bass sound more than the case of an increase in the volume of the air chamber  27 . These results evidentially show that the electroacoustic transducer satisfying the technological concept of the present invention can exhibit the projected advantageous effects. 
     Second Embodiment 
     An embodiment shown in  FIG. 7  is explained, in which the technological concept of the present invention is applied to a speaker system. An electroacoustic transducer in the embodiment shown in  FIG. 7  is an example speaker system in which a speaker unit  41  is built into an enclosure  40 . The inside of the enclosure  40  is partitioned by a partitioning plate  46 . The front plate of the enclosure  40  is a baffle plate  44 . An air chamber  43  is defined between the baffle plate  44  and the partitioning plate  46 . The speaker unit  41  as an electroacoustic transducer unit is attached to the baffle plate  44  and is located inside the air chamber  43 . The air chamber  43  is located in back of a diaphragm  42  of the speaker unit  41 . A volume adjuster  47  is attached to the partitioning plate  46 . Similar to the speaker unit  41 , the volume adjuster  47  has a structure similar to that of a dynamic speaker unit. The volume adjuster  47  has a diaphragm  48 , whose front surface faces the air chamber  43 . A microphone unit  45  as a sound pressure detector is disposed in the air chamber  43 . 
     Detection signals from the microphone unit  45  are input to the volume adjuster  47  through an amplifier  49 . The detection signals are configured to drive the volume adjuster  47 . The speaker unit  41  is driven by audio signals from an audio signal source (not shown in the drawing) to vibrate the diaphragm  42  and generate sound. The vibration of the diaphragm  42  changes the volume of the air chamber  43  as well as the sound pressure in the air chamber  43 . The microphone unit  45  detects the change in the sound pressure. The sound pressure detection signals output from the microphone unit  45  drive the volume adjuster  47  through the amplifier  49 , vibrate the diaphragm  48  of the volume adjuster  47 , and change the volume of the air chamber  43 . 
     A feedback control system is thereby formed in which the signals of the sound pressure change in the air chamber  43  detected in the microphone unit  45  as the sound pressure detector are input to the volume adjuster  47  and the sound pressure change in the air chamber  43  is cancelled. Thus, the acoustic impedance in the air chamber  43  can be reduced in an equivalent manner. Even if the air chamber  43  has a small volume, the diaphragm  42  of the speaker unit  41  can be vibrated in response to bass audio signals without resistance, thus improving directional frequency response properties in bass sound. The microphone unit  45  as the sound pressure detector is disposed in the air chamber  43  to directly detect the sound pressure in the air chamber  43  for feedback control with the detection signals, thus allowing acoustic impedance control in the air chamber  43  at high accuracy. 
     In  FIG. 7 , the speaker unit  41  for sound playback is smaller than the volume adjuster  47  for acoustic impedance control of the air chamber  43  composed of the speaker unit. The sizes of these components may be determined as desired. These components may have the same size. Alternatively, the volume adjuster  47  may be smaller. To allow easy movement of the diaphragm  48  of the volume adjuster  47 , a hole open to the air may be provided to the space in which the volume adjuster  47  is disposed. 
     Third Embodiment 
       FIG. 8  illustrates an embodiment of the technological concept of the present invention that is applied to a dynamic microphone. In  FIG. 8 , the dynamic microphone  50  has a microphone casing  51  that also serves as a grip. A dynamic microphone unit  52 , which is an electroacoustic transducer unit, is appropriately mounted in the front end of the microphone casing  51 . The microphone unit  52  has a unit casing  54 . A diaphragm  53  that vibrates in response to sound waves is disposed inside the internal periphery in the front end of the unit casing  54 . The diaphragm  53  has a voice coil, which is disposed in a magnetic gap formed by magnetic circuit members, such as a permanent magnet and a yoke. The diaphragm  53  vibrates in response to sound waves along with the voice coil, which then outputs audio signals corresponding to the sound waves due to electromagnetic conversion. 
     The unit casing  54  is provided with an air chamber  56  in back of the diaphragm  53  and the magnetic circuit members. The back surface of the diaphragm  53  is connected to the air chamber  56  through an appropriate hole. A sound pressure detector  55  composed of an omnidirectional microphone unit, for example, is disposed in the air chamber  56 . Furthermore, a volume adjuster  57  is disposed in the air chamber  56 , the volume adjuster  57  being driven by the output signals from the sound pressure detector  55  and changing the volume of the air chamber  56  in response to the output signals to control the acoustic impedance of the air chamber  56 . Similar to the volume adjuster in the previous embodiment, the volume adjuster  57  may employ a structure similar to a dynamic speaker. The output signals from the sound pressure detector  55  are amplified in an amplifier  58  and the amplified signals drive the volume adjuster  57 . 
     A connector  59  is provided in the rear end of the microphone casing  51  to connect a cable connector. In the microphone casing  51 , a power battery compartment  60  is provided between the microphone unit  52  and the connector  59 . The dynamic microphone  50 , which is provided with the power battery compartment  60 , is a microphone that requires a power source, similar to a wireless microphone. The volume of the air chamber  56  is thus limited by the power battery compartment  60 . Accordingly, the stiffness of the air chamber  56  is high and the diaphragm  53  of the microphone unit  52  moves barely in bass sound, thus leading to poor directional frequency response in bass sound. In the case of a pin-type wireless microphone in particular, the entire size is small, in which a power battery should to be installed, thus further reducing the volume of the air chamber and further lowering the directional frequency response in bass sound. In the embodiment shown in  FIG. 8 , the volume adjuster  57  is provided in the air chamber  56  and is driven by the output signals from the sound pressure detector  55  through the amplifier  58 . A control system from the sound pressure detector  55  to the volume adjuster  57  through the amplifier  58  configures a feedback control system in which the volume adjuster  57  increases the volume of the air chamber  56  as the sound pressure in the air chamber  56  increases so as to reduce the acoustic impedance of the air chamber  56  in an equivalent manner. 
     According to the embodiment of the microphone shown in  FIG. 8 , the diaphragm  53  of the dynamic microphone unit  52  vibrates in response to received sound waves, and then the volume of the air chamber  56  fluctuates and the sound pressure of the air chamber  56  fluctuates. The fluctuation in the sound pressure is fed back to the volume adjuster  57 , which is then driven to control the sound pressure in the air chamber  56  at a constant level. Accordingly, even if the air chamber  56  has a small volume, the acoustic impedance of the air chamber  56  is reduced in an equivalent manner. Thus, the diaphragm  53  can vibrate properly according to the sound pressure and provide a microphone excellent in directional frequency response. 
     It is mainly bass sound in which the directional frequency response is lowered due to the small volume of the air chamber. It is thus preferred in each embodiment explained above that the detection signals from the sound pressure detector be input to the volume adjuster through a low-pass filter so as to reduce the acoustic impedance in bass sound in the air chamber in an equivalent manner. 
     Applying the technological concept of the present invention to a speaker sufficiently increases the volume or pressure of bass sound, even if an enclosure attached to the speaker has a small volume, thus providing a compact and high-performance speaker system. 
     Furthermore, applying the technological concept of the present invention to a microphone provides a high-performance microphone capable of electroacoustically transducing sound at a high level even in bass sound, even if an air chamber is extremely small in back of a diaphragm of a microphone unit, such as in a pin-type wireless microphone, as an electroacoustic transducer unit. 
     The volume adjuster of the present invention is not limited by a drive type. In addition to the configuration similar to the dynamic speaker employed in each of the embodiments, a member similar to an electromagnetic actuator may be used to drive a diaphragm facing an air chamber in order to control the volume of the air chamber. Alternatively, a piezoelectric element, such as a piezoelectric bimorph, may be used to control the volume of the air chamber.