Patent Publication Number: US-2023164486-A1

Title: Noise-cancelling headphone

Description:
TECHNICAL FIELD 
     The present invention relates to a noise-cancelling headphone. 
     BACKGROUND ART 
     A noise-cancelling headphone collects so-called noise with a microphone and mutes (cancels) the noise by using a cancelling sound corresponding to a cancelling signal in antiphase to the collected noise. 
     One of the types of noise cancellation is a feed-forward type (hereinafter referred to as “FF type”). An FF type noise-cancelling headphone collects noise outside (around) the noise-cancelling headphone and generates a cancelling signal. 
     Herein, a cancelling signal generating circuit and the like in the FF type noise-cancelling headphone are designed on the assumption that an ear pad of the noise-cancelling headphone and a head are brought into close contact with each other without any gap when the noise-cancelling headphone is worn on the head of a user. However, a shape of a user&#39;s head varies from person to person. For this reason, a gap is generated between the ear pad and the head for some users. The FF type noise-cancelling headphone do not assume that noise comes into the user&#39;s ear through the gap. Therefore, the FF type noise-cancelling headphone does not enable the noise through the gap to be cancelled. 
     A measure to solve such a problem of the FF type noise-cancelling headphone includes a hybrid type noise-cancelling headphone (For example, see PTL 1). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP2012-023637 A 
     The hybrid type noise-cancelling headphone collects noise in a space (front air chamber) between an ear pad and a head and generates a cancelling signal. That is, the hybrid type noise-cancelling headphone outputs a cancelling sound corresponding to a cancelling signal generated by collecting external noise and outputs a cancelling sound corresponding to the cancelling signal generated by collecting the noise in the front air chamber. Thus, the hybrid type noise-cancelling headphone achieves a higher cancelling effect than the FF type noise-cancelling headphone. 
     However, the hybrid type noise-cancelling headphone requires, in addition to the circuit for generating the cancelling signal corresponding to the external noise, a circuit (error correcting noise cancelling circuit) for generating the cancelling signal corresponding to the noise in the front air chamber. Further, the hybrid type noise-cancelling headphone requires an adder that adds the cancelling signal corresponding to the external noise and the cancelling signal corresponding to the noise in the front air chamber. Therefore, the hybrid type noise-cancelling headphone has a more complex circuit configuration and is also more expensive. 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to achieve a high cancelling effect with a simple configuration. 
     Solution to Problem 
     A noise-cancelling headphone according to the present invention includes: a headphone unit configured to output a sound wave corresponding to an audio signal; a baffle plate to which the headphone unit is attached; an ear pad attached to the baffle plate; a housing attached to the baffle plate; a first microphone configured to collect external noise outside the housing, a first buffer amplifier unit configured to perform impedance conversion to a signal from the first microphone and output the impedance-converted signal; a second microphone configured to collect internal noise inside a front air chamber formed by the headphone unit, the baffle plate, the ear pad, and a head of a user when the noise-cancelling headphone is worn on the user&#39;s head; a second buffer amplifier unit configured to perform impedance conversion to a signal from the second microphone and output the impedance-converted signal; and a noise-cancelling signal generation circuit configured to generate a noise-cancelling signal, based on a combined signal by combining the signal from the first buffer amplifier unit with the signal from the second buffer amplifier unit. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     The present invention achieves a high cancelling effect with a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of an embodiment of a noise-cancelling headphone according to the present invention. 
         FIG.  2    is a schematic diagram illustrating a state in which the noise-cancelling headphone in  FIG.  1    is worn on a head of a user. 
         FIG.  3    is a schematic diagram illustrating a configuration of a noise-cancelling circuit included in the noise-cancelling headphone in  FIG.  1   . 
         FIG.  4    is a graph illustrating a cancelling effect of the noise-cancelling headphone in  FIG.  1   . 
         FIG.  5    is a graph illustrating a cancelling effect of a conventional feedforward type noise-cancelling headphone. 
         FIG.  6    is a schematic diagram illustrating a state of cancellation in the conventional feedforward type noise-cancelling headphone in an ideal using state. 
         FIG.  7    is a schematic diagram illustrating a state of cancellation in the conventional feedforward type noise-cancelling headphone in an actual using state. 
         FIG.  8    is a schematic diagram illustrating a state of cancellation in the noise-cancelling headphone according to the present invention. 
         FIG.  9    is a schematic diagram illustrating a configuration of a noise-cancelling circuit included in a conventional hybrid type noise-cancelling headphone. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a noise-cancelling headphone according to the present invention are described below with reference to the attached drawings. 
       FIG.  1    is a perspective view of an embodiment of the noise-cancelling headphone according to the present invention. 
     A noise-cancelling headphone  1  is worn on a head of a user of the noise-cancelling headphone  1  and outputs a sound wave corresponding to an audio signal from a sound source (not illustrated) such as a portable music player toward an ear of the user. 
     The “user” is a user of the noise-cancelling headphone  1 . 
     The noise-cancelling headphone  1  includes a left sound emitting unit  10 , a right sound emitting unit  20 , and a connecting member  30 . The left sound emitting unit  10  and the right sound emitting unit  20  constitute a pair of sound emitting units. The pair of sound emitting units is worn around the ear of the user on a temporal region HD of the user and outputs the sound wave corresponding to the audio signal from the sound source. In a state in which the noise-cancelling headphone  1  is worn on the head of the user (hereinafter referred to as “worn state”), the noise-cancelling headphone  1  forms a front air chamber SF (see  FIG.  2   ) described later between the noise-cancelling headphone  1  and the temporal region HD. 
     The left sound emitting unit  10  is worn around a left ear LE (see  FIG.  2   ) on the temporal region HD of the user and outputs the sound wave corresponding to the audio signal from the sound source. The left sound emitting unit  10  includes a housing  11 , an ear pad  12 , a baffle plate  13 , and a headphone unit  14  (see  FIG.  2   ). 
     The housing  11  is attached to the baffle plate  13  and accommodates the headphone unit  14  and the like. The housing  11  has a cup shape. The housing  11  is made of, for example, a synthetic resin such as acrylonitrile-butadiene-styrene (ABS) resin. The housing  11  includes a sound collecting hole  11   h . The sound collecting hole  11   h  allows communication between the outside and the inside (rear air chamber SR described later (see  FIG.  2   )) of the housing  11 . The sound collecting hole  11   h  is a first sound collecting hole in the present invention. 
     The ear pad  12  is attached to the baffle plate  13  and functions as a cushion between the baffle plate  13  and the temporal region HD. The ear pad  12  has an elliptical ring shape. The ear pad  12  abuts on the temporal region HD of the user. An elastic material such as urethane foam, which is easily deformable, is used for the ear pad  12 . 
     The baffle plate  13  holds the headphone unit  14 . The baffle plate  13  separates the front air chamber SF and the rear air chamber SR. The baffle plate  13  includes a sound collecting hole  13   h  (see  FIG.  2   ). The sound collecting hole  13   h  allows communication between the front air chamber SF and the rear air chamber SR. The sound collecting hole  13   h  is a second sound collecting hole in the present invention. 
     The right sound emitting unit  20  is worn around a right ear on the temporal region HD of the user and outputs the sound wave corresponding to the audio signal from the sound source. The right sound emitting unit  20  includes a housing  21 , an ear pad  22 , and a baffle plate  23 . The housing  21 , the ear pad  22 , and the baffle plate  23  have the same functions and configurations as the housing  11 , the ear pad  12 , and the baffle plate  13 , respectively. Thus, a specific description of the configuration and the like of the right sound emitting unit  20  is omitted. 
     The connecting member  30  connects the left sound emitting unit  10  and the right sound emitting unit  20  and supports the left sound emitting unit  10  and the right sound emitting unit  20 . The connecting member  30  fixes the noise-cancelling headphone  1  to the head of the user. In the worn state of the noise-cancelling headphone  1 , the connecting member  30  applies side pressure to the right and left sound emitting units  10  and  20  toward the temporal region HD side of the user and fixes the right and left sound emitting units  10  and  20  to the temporal region HD of the user. 
       FIG.  2    is a schematic diagram illustrating a state (worn state) in which the noise-cancelling headphone  1  is worn on the head (temporal region) of the user. 
     For convenience of description, the figure schematically illustrates the temporal region HD and the left ear LE. The figure illustrates that the front air chamber SF is formed between the noise-cancelling headphone  1  and the temporal region HD. The figure also illustrates that the front air chamber SF and the rear air chamber SR are separated by the baffle plate  13 . 
     The “front air chamber SF” is an acoustic space (a space formed by the headphone unit  14 , the baffle plate  13 , the ear pad  12 , and the temporal region HD) surrounded by the head (temporal region HD) of the user and the noise-cancelling headphone  1  in the worn state of the noise-cancelling headphone  1 . 
     The “rear air chamber SR” is an acoustic space (a space formed by the housing  11 , the baffle plate  13 , and the headphone unit  14 ) surrounded by the housing  11 , the baffle plate  13 , and the headphone unit  14 . 
     Further,  FIG.  2    illustrates that a part of the ear pad  12  does not abut on the temporal region HD. That is, the figure indicates that a gap exists between the part of the ear pad  12  and the temporal region HD. The front air chamber SF communicates with the outside of the noise-cancelling headphone  1  through the gap. 
     Herein, in general, since shapes of human temporal region HD vary from person to person, the gap is likely to occur between the temporal region HD and the ear pad  12 . When the gap occurs between the ear pad  12  and the temporal region HD, sound insulation of the noise-cancelling headphone  1  deteriorates and noise (hereinafter referred to as “external noise”) outside the noise-cancelling headphone  1  enters the front air chamber SF (acoustic space) through the gap. 
     As illustrated in the figure, the noise-cancelling headphone  1  includes the headphone unit  14 , a substrate  15 , a first microphone  16 , and a second microphone  17  inside the housing  11  (rear air chamber SR). 
     The headphone unit  14  converts the audio signal from the sound source into the sound wave corresponding to the audio signal and outputs the sound wave. The headphone unit  14  is attached to the baffle plate  13 . 
     The substrate  15  mounts a noise-cancelling circuit (hereinafter referred to as “NC circuit”). The NC circuit is described in detail later. 
     The first microphone  16  collects external noise outside the housing  11  and generates a noise signal corresponding to the external noise. The first microphone  16  is, for example, a condenser microphone. 
     The “external noise” is sound that reaches the inside (rear air chamber SR) of the housing  11  and the front air chamber SF from a sound source different from the sound source such as the portable music player, for example, the outside of the noise-cancelling headphone  1 . That is, the “external noise” is so-called noise. 
     The first microphone  16  is attached to the housing  11 . The first microphone  16  is disposed at a position away from the headphone unit  14  and near the sound collecting hole  11   h  in the rear air chamber SR. A sound collecting portion of the first microphone  16  is exposed to the outside of the housing  11  through the sound collecting hole  11   h . The first microphone  16  collects the external noise through the sound collecting hole  11   h . The first microphone  16  is connected to the NC circuit mounted on the substrate  15  via a signal line L 1 . 
     Note that the configuration in which the sound collecting portion of the first microphone is exposed to the outside is not limited to the configuration to be exposed through the sound collecting hole (sound collecting hole  11   h ). That is, the first microphone may be disposed inside the sound collecting hole in such a way that the sound collecting portion (sound collecting surface) is continuous to an outer surface of the housing, for example. 
     The second microphone  17  collects noise (hereinafter referred to as “internal noise”) inside the front air chamber SF at a position close to the ear of the user and generates a noise signal corresponding to the internal noise. The second microphone  17  is, for example, a condenser microphone. 
     The “internal noise” is external noise that enters the front air chamber SF through the gap between the ear pad  12  and the temporal region HD or through the ear pad  12 . 
     The second microphone  17  is attached to the baffle plate  13 . The second microphone  17  is disposed at a position not to be overlapped with the ear pad  12  and the headphone unit  14  in the rear air chamber SR. A sound collecting portion of the second microphone  17  is exposed to the front air chamber SF through the sound collecting hole  13   h.  The second microphone  17  collects the internal noise through the sound collecting hole  13   h.  The second microphone  17  is connected to the NC circuit mounted on the substrate  15  via a signal line L 2 . 
     Note that the configuration in which the sound collecting portion of the second microphone is exposed to the front air chamber is not limited to the configuration to be exposed through the sound collecting hole (sound collecting hole  13   h ). That is, the second microphone may be disposed inside the sound collecting hole in such a way that the sound collecting portion (sound collecting surface) is continuous to one surface of the baffle plate on the front air chamber side, for example. 
     Further, the second microphone may be disposed inside the front air chamber as long as the internal noise can be collected. 
       FIG.  3    is a schematic diagram illustrating a configuration of the NC circuit included in the noise-cancelling headphone  1 . 
     The NC circuit includes a first buffer amplifier unit  151 , a second buffer amplifier unit  152 , a microphone signal amplifying unit  153 , a noise-cancelling signal generating circuit  154 , a noise-cancelling signal amplifying unit  155 , a musical sound input terminal  156 , and a musical sound signal amplifying unit  157 . 
     The first buffer amplifier unit  151  performs impedance conversion to the noise signal from the first microphone  16 . The first buffer amplifier unit  151  outputs the impedance-converted noise signal to the microphone signal amplifying unit  153 . 
     The second buffer amplifier unit  152  performs impedance conversion to the noise signal from the second microphone  17 . The second buffer amplifier unit  152  outputs the impedance-converted noise signal to the microphone signal amplifying unit  153 . 
     The microphone signal amplifying unit  153  amplifies a signal (hereinafter referred to as “combined signal”) including the output signal (noise signal corresponding to the external noise) from the first buffer amplifier unit  151  and the output signal (noise signal corresponding to the internal noise) from the second buffer amplifier unit  152 . The microphone signal amplifying unit  153  outputs the amplified combined signal to the noise-cancelling signal generating circuit (hereinafter referred to as “NC signal generating circuit”)  154 . 
     The NC signal generating circuit  154  generates a noise-cancelling signal, based on the output signal (combined signal amplified by the microphone signal amplifying unit  153 ) from the microphone signal amplifying unit  153 . That is, the NC signal generating circuit  154  generates the noise-cancelling signal including a noise-cancelling signal based on the external noise collected by the first microphone  16  (noise-cancelling signal based on the signal from the first buffer amplifier unit  151 ) and a noise-cancelling signal based on the internal noise collected by the second microphone  17  (noise-cancelling signal based on the signal from the second buffer amplifier unit  152 ). In other words, the noise-cancelling signal generated by the NC signal generating circuit  154  includes the noise-cancelling signal in antiphase to the external noise and the noise-cancelling signal in antiphase to the internal noise. The NC signal generating circuit  154  outputs the noise-cancelling signal to the noise-cancelling signal amplifying unit  155  (hereinafter referred to as “NC signal amplifying unit”). 
     Herein, the noise-cancelling signal in antiphase to the external noise is a signal to cancel the external noise. The noise-cancelling signal in antiphase to the internal noise is a signal to cancel the internal noise. 
     Further, the noise-cancelling signal in antiphase to the external noise is a first noise-cancelling signal in the present invention. The noise-cancelling signal in antiphase to the internal noise is a second noise-cancelling signal in the present invention. 
     The NC signal amplifying unit  155  amplifies the noise-cancelling signal (the first noise-cancelling signal, the second noise-cancelling signal) from the NC signal generating circuit  154 . An output unit (not illustrated) of the NC signal amplifying unit  155  is connected to one input unit (not illustrated) of the headphone unit  14 . 
     The audio signal (musical sound) from the sound source such as a portable music player is input to the musical sound input terminal  156 . The musical sound input terminal  156  outputs the audio signal to the musical sound signal amplifying unit  157 . 
     The musical sound signal amplifying unit  157  amplifies the audio signal from the musical sound input terminal  156 . An output unit (not illustrated) of the musical sound signal amplifying unit  157  is connected to the other input unit (not illustrated) of the headphone unit  14 . 
     The headphone unit  14  converts the audio signal from the sound source into the sound wave (sound wave corresponding to the audio signal) and outputs the sound wave as described above, and also converts the first noise-cancelling signal into a sound wave and converts the second noise-cancelling signal into a sound wave, and outputs the sound waves. That is, the headphone unit  14  outputs the sound wave corresponding to the musical sound signal and the sound waves corresponding to the noise-cancelling signals amplified by the NC signal amplifying unit  155 . 
     Note that the second buffer amplifier unit may be an impedance conversion unit included in the second microphone. That is, for example, the second microphone includes a microphone unit that converts the internal noise into the noise signal and the impedance conversion unit that performs impedance conversion to the noise signal and outputs the impedance-converted noise signal. The impedance conversion unit of the second microphone functions as the second buffer amplifier unit. According to this configuration, the number of components included in the NC circuit is reduced. 
       FIG.  4    is a graph illustrating a cancelling effect of the noise-cancelling headphone  1 . 
     The figure indicates a frequency characteristic (thin lines) when a noise-cancelling function of the noise-cancelling headphone  1  is off and the frequency characteristic (thick lines) when the same function is on. In the figure, the solid lines indicate the frequency characteristic of the left sound emitting unit  10  and the dashed lines indicate the frequency characteristic of the right sound emitting unit  20 . 
     As illustrated in the figure, the noise-cancelling headphone  1  exhibits a cancelling effect of attenuating a gain by about 8 dB to 30 dB, particularly in a low frequency band (around 30 Hz to 500 Hz). 
       FIG.  5    is a graph illustrating a cancelling effect of the conventional FF type noise-cancelling headphone. 
     The figure indicates the frequency characteristic (thin lines) when the noise-cancelling function of the conventional FF type noise-cancelling headphone is off and the frequency characteristic (thick lines) when the same function is on. In the figure, the solid lines indicate the frequency characteristic of the left sound emitting unit and the dashed lines indicate the frequency characteristic of the right sound emitting unit. 
     As illustrated in the figure, the conventional FF type noise-cancelling headphone exhibits the cancelling effect of attenuating a gain by about 6 dB to 24 dB in a low frequency band (around 80 Hz to 400 Hz). However, compared with the noise-cancelling headphone  1 , the cancelling effect by the conventional FF type noise-cancelling headphone appears in a narrow frequency band, and the attenuation amount of the gain is small. In other words, the noise-cancelling headphone  1  exhibits the large cancelling effect in a wide range of frequency bands, as compared with the conventional FF type noise-cancelling headphone. 
     Herein, the principle in which the noise-cancelling headphone according to the present invention exhibits a higher canceling effect than the conventional FF type noise-cancelling headphone is described. 
       FIG.  6    is a schematic diagram illustrating a state of cancellation in the conventional FF type noise-cancelling headphone in an ideal using state. 
       FIG.  7    is a schematic diagram illustrating a state of cancellation in the conventional FF type noise-cancelling headphone in an actual using state. 
     The noise-cancelling headphone is worn on the head of the user, thereby causing a so-called passive effect in which noise in a high frequency band (high frequency component) is muted (cancelled) due to shielding by the ear pad. That is, in the ideal using state (hereinafter referred to as “ideal state”), when the noise-cancelling headphone is worn on the head of the user, the high frequency component is muted (cancelled) by the ear pad, and the high frequency component does not reach the front air chamber. In contrast, the passive effect is not sufficiently obtained for noise in a low frequency band (low frequency component) of the external noise, and thus such noise reaches the front air chamber in a state where the sound volume is reduced. That is, a headphone without noise-cancelling function has little passive effect. The noise-cancelling signal generating circuit (NC signal generating circuit) included in the conventional FF type noise-cancelling headphone functions to cancel the low frequency component of the external noise that reaches the front air chamber due to the little passive effect. The conventional noise-cancelling signal generating circuit performs, with respect to the collected external noise, signal processing in consideration of the volume reduction amount due to the passive effect in the ideal state, generates a noise signal, and generates a noise-cancelling signal in antiphase to the generated noise signal. As illustrated in  FIG.  6   , external noise in a low frequency band (N 1 ) is cancelled by a generated noise-cancelling signal (C 1 ). As a result, the external noise is muted (cancelled) to a predetermined muting level (L 1 ). 
     However, the conventional noise-cancelling signal generating circuit (NC signal generating circuit) in the ideal state is designed assuming the passive effect in the ideal state in which the head (temporal region) of the user and the ear pad is in close contact without any gap (the state in which the front air chamber is sealed). The conventional noise-cancelling signal generating circuit in the ideal state generates a cancelling signal, based on a reference characteristic with this assumption as a standard. That is, when the passive effect as assumed is not obtained due to the gap between the head (temporal region) of the user and the ear pad, the conventional noise-cancelling signal generating circuit generates the cancelling signal (C 1 ), based on the external noise (N 1 ) when the passive effect is obtained according to the reference characteristic. As a result, as illustrated in  FIG.  7   , a part (L 2 ) of the external noise in the low frequency band remains in the front air chamber without being cancelled by the generated cancelling signal (C 1 ). That is, the part of the external noise is not cancelled, and the user cannot obtain a sufficient effect of the noise-cancelling headphone. Note that, in  FIG.  7   , N 2  indicates the external noise that actually reaches the front air chamber. 
       FIG.  8    is a schematic diagram illustrating a state of cancellation in the noise-cancelling headphone according to the present invention. 
     In the noise-cancelling headphone according to the present invention, the second microphone collects, as the internal noise, the part (L 2 ) of the external noise in the low frequency band that remains without being cancelled in the conventional FF type noise-cancelling headphone in the actual using state illustrated in  FIG.  7   , and an antiphase noise-cancelling signal (C 2 ) including the collected internal noise is generated. As a result, the noise that is not completely cancelled and remains, as the internal noise, in the conventional FF type noise-cancelling headphone in the actual using state is muted to the predetermined muting level (L 1 ). 
     Thus, the noise-cancelling headphone according to the present invention generates the cancelling signal corresponding to the internal noise collected by the second microphone in addition to the external noise collected by the first microphone. As a result, the noise-cancelling headphone according to the present invention exhibits a higher cancelling effect than the conventional FF type noise-cancelling headphone. 
       FIG.  9    is a schematic diagram illustrating a configuration of an NC circuit included in the conventional hybrid type noise-cancelling headphone. 
     A front air chamber SFA and a rear air chamber SRA illustrated in the figure are spaces corresponding to the front air chamber SF and the rear air chamber SR illustrated in  FIG.  3   , respectively. 
     The NC circuit included in the conventional hybrid type noise-cancelling headphone (hereinafter referred to as “conventional NC circuit”) includes a microphone signal amplifying unit  153 A, an NC signal generating circuit  154 A, an NC signal amplifying unit  155 A, a musical sound input terminal  156 A, a musical sound signal amplifying unit  157 A, an error correcting NC circuit  18 A, and an adder  19 A. That is, the conventional NC circuit includes the error correcting NC circuit  18 A and the adder  19 A as compared with the NC circuit included in the noise-cancelling headphone  1  illustrated in  FIG.  3   . 
     The musical sound input terminal  156 A, the musical sound signal amplifying unit  157 A, a first microphone  16 A, and a second microphone  17 A illustrated in the figure have the same functions and configurations as the musical sound input terminal  156 , the musical sound signal amplifying unit  157 , the first microphone  16 , and the second microphone  17  illustrated in  FIG.  3   , respectively. Therefore, a specific description of each of the musical sound input terminal  156 A, the musical sound signal amplifying unit  157 A, the first microphone  16 A, and the second microphone  17 A are omitted. 
     The microphone signal amplifying unit  153 A amplifies a noise signal from the first microphone  16 A. 
     The NC signal generating circuit  154 A generates a noise-cancelling signal, based on an output signal from the microphone signal amplifying unit  153 A. The NC signal generating circuit  154 A generates a noise-cancelling signal in antiphase to the external noise collected by the first microphone  16 A. The NC signal generating circuit  154 A outputs the noise-cancelling signal to the NC signal amplifying unit  155 A via the adder  19 A. The adder  19 A and the NC signal amplifying unit  155 A are described later. 
     In particular, the error correcting NC circuit  18 A generates an error correcting signal for cancelling noise (hereinafter referred to as “error noise”) that has not been completely muted by the cancelling signal corresponding to the noise signal from the first microphone  16 A. The second microphone  17 A collects the error noise, generates a noise signal corresponding to the error signal, and outputs the noise signal to the error correcting NC circuit  18 A. The error correction NC circuit  18 A outputs the error correcting signal to the adder  19 A. 
     The adder  19 A adds the noise-cancelling signal from the NC signal generating circuit  154 A and the error correcting signal from the error correcting NC circuit  18 A. The adder  19 A outputs the added signal to the NC signal amplifying unit  155 A. 
     The NC signal amplifying unit  155 A amplifies the added signal (noise-cancelling signal and error correcting signal) generated by adding in the adder  19 A. An output unit (not illustrated) of the NC signal amplifying unit  155 A is connected to one input unit (not illustrated) of a headphone unit  14 A. 
     As illustrated in  FIG.  9   , the NC circuit included in the conventional hybrid type noise-cancelling headphone includes the second microphone  17 A, the error correcting NC circuit  18 A, and the adder  19 A, in addition to the NC circuit included in the conventional FF type noise-cancelling headphone. 
     In contrast, the NC circuit (see  FIG.  3   ) included in the noise-cancelling headphone  1  includes the second microphone  17  and the second buffer amplifier unit  152 , in addition to the NC circuit included in the conventional FF type noise-cancelling headphone. That is, the second microphone  17  and the second buffer amplifier unit  152  function as a filter to mute the noise through the gap between the head (temporal region) of the user and the ear pad. Further, the second microphone  17  and the second buffer amplifier unit  152  automatically correct the noise-cancelling signal with respect to noise unique to a user caused by an individual variation of the gap between the head (temporal region) of the user and the ear pad. 
     Note that the second buffer amplifier unit may be the impedance conversion unit included in the second microphone as described above. In this case, the NC circuit included in the noise-cancelling headphone according to the present invention is configured by adding only the second microphone to the NC circuit included in the conventional FF type noise-cancelling headphone. That is, the noise-cancelling headphone according to the present invention reduces the signal processing according to the noise signal and achieves the cancelling effect equivalent to the conventional hybrid type with a simple NC circuit. 
     According to the embodiments described above, the NC circuit included in the noise-cancelling headphone according to the present invention does not require an expensive error correcting NC circuit nor an adder with a complex configuration unlike the NC circuit included in the conventional hybrid type noise-cancelling headphone. That is, the NC circuit noise-cancelling headphone according to the present invention has a simple configuration as compared with the NC circuit included in the conventional hybrid type noise-cancelling headphone. Meanwhile, the noise-cancelling headphone according to the present invention achieves a higher cancelling effect than the conventional FF type noise-cancelling headphone. As described above, the noise-cancelling headphone according to the present invention achieves a high cancelling effect with a simple configuration. 
     REFERENCE SIGNS LIST 
     
         
           1  NOISE-CANCELLING HEADPHONE 
           10  LEFT SOUND EMITTING UNIT 
           11  HOUSING 
           11   h  SOUND COLLECTING HOLE (FIRST SOUND COLLECTING HOLE) 
           12  EAR PAD 
           13  BAFFLE PLATE 
           13   h  SOUND COLLECTING HOLE (SECOND SOUND COLLECTING HOLE) 
           14  HEADPHONE UNIT 
           15  SUBSTRATE 
           151  FIRST BUFFER AMPLIFIER UNIT 
           152  SECOND BUFFER AMPLIFIER UNIT 
           153  MICROPHONE SIGNAL AMPLIFYING UNIT 
           154  NOISE-CANCELLING SIGNAL GENERATING CIRCUIT (NC SIGNAL GENERATING CIRCUIT) 
           155  NOISE-CANCELLING SIGNAL AMPLIFYING UNIT (NC SIGNAL AMPLIFYING UNIT) 
           156  MUSICAL SOUND INPUT TERMINAL 
           157  MUSICAL SOUND SIGNAL AMPLIFYING UNIT 
           16  FIRST MICROPHONE 
           17  SECOND MICROPHONE 
           20  RIGHT SOUND EMITTING UNIT 
           21  HOUSING 
           22  EAR PAD 
           23  BAFFLE PLATE 
           30  CONNECTING MEMBER 
         SF FRONT AIR CHAMBER 
         SR REAR AIR CHAMBER