Patent Publication Number: US-11664006-B2

Title: Sound output device

Description:
Cross-Reference To Related Applications 
     This application claims the benefit under 35 U.S.C. § 371 as a U.S. National Stage Entry of International Application No. PCT/JP2019/029288, filed in the Japanese Patent Office as a Receiving Office on Jul. 25, 2019, which claims priority to Japanese Patent Application Number JP2018-147179, filed in the Japanese Patent Office on Aug. 3, 2018, each of which is hereby incorporated by reference in its entirety. 
     FIELD 
     The present invention relates to a sound output device. 
     BACKGROUND 
     When earphones or headphones are worn, a need exists for reduction of sound (extraneous noise) reaching a pinna from the outside of the earphones or the headphones. Thus, a noise cancelling system that removes noise by signal processing based on an audio signal output from a microphone provided in a housing of an earphone or a headphone has been known. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2016-086281 
     Patent Literature 2: Japanese Patent Application Laid-open No. 2017-120447 
     Patent Literature 3: National Publication of International Patent Application No. 2017-509284 
     SUMMARY 
     Technical Problem 
     The above noise cancelling system has room for improvement regarding system stability and noise attenuation. 
     The present disclosure proposes a sound output device capable of further reducing extraneous noise. 
     Solution to Problem 
     For solving the problem described above, a sound output device according to one aspect of the present disclosure has an acoustic path connecting a first space on a front surface of a driver unit and an outside of a housing including the driver unit separately from a second space on a back surface of the driver unit, and a microphone disposed in the vicinity of an opening where the acoustic path is connected to the outside of the housing. 
     Advantageous Effects of Invention 
     The present disclosure can further reduce extraneous noise. Note that the present disclosure does not necessarily have to be limited to the effect described above and may provide any effect described in the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a view illustrating a configuration example of a noise cancelling system using a feedback technique. 
         FIG.  1 B  is a view illustrating the configuration example of the noise cancelling system using the feedback technique. 
         FIG.  1 C  is a view illustrating the configuration example of the noise cancelling system using the feedback technique. 
         FIG.  2    is a view illustrating a bode plot. 
         FIG.  3 A  is a view illustrating a configuration example of a noise cancelling system using an FF technique. 
         FIG.  3 B  is a view illustrating the configuration example of the noise cancelling system using the FF technique. 
         FIG.  3 C  is a view illustrating the configuration example of the noise cancelling system using the FF technique. 
         FIG.  4 A  is a view illustrating a configuration of an earphone example according to an existing technique. 
         FIG.  4 B  is a view illustrating the configuration of the earphone example according to the existing technique. 
         FIG.  4 C  is a view illustrating the configuration of the earphone example according to the existing technique. 
         FIG.  5 A  is a view illustrating a configuration of an earphone example according to a first embodiment. 
         FIG.  5 B  is a view illustrating the configuration of the earphone example according to the first embodiment. 
         FIG.  5 C  is a view illustrating the configuration of the earphone example according to the first embodiment. 
         FIG.  5 D  is a view illustrating a configuration of another earphone example according to the first embodiment. 
         FIG.  5 E  is a view illustrating the configuration of the earphone example according to the first embodiment. 
         FIG.  6    is a view for explaining an effect according to the first embodiment. 
         FIG.  7 A  is a view illustrating a configuration of an earphone example according to a first modification of the first embodiment. 
         FIG.  7 B  is a view schematically illustrating a structure of a driver unit example. 
         FIG.  8    is a view illustrating a configuration of an earphone example according to a second modification of the first embodiment. 
         FIG.  9    is a view illustrating a configuration of an earphone example according to a third modification of the first embodiment. 
         FIG.  10    is a view illustrating a configuration of a headphone example according to a second embodiment. 
         FIG.  11    is a view illustrating a configuration of a headphone example according to a first modification of the second embodiment. 
         FIG.  12    is a view illustrating a configuration of a headphone example according to a second modification of the second embodiment. 
         FIG.  13    is a view illustrating a configuration of a headphone example according to a third modification of the second embodiment. 
         FIG.  14    is a view illustrating a configuration of a headphone example according to a fourth modification of the second embodiment. 
         FIG.  15 A  is a view for explaining a position where a microphone is disposed. 
         FIG.  15 B  is a view for explaining another position where the microphone is disposed. 
         FIG.  15 C  is a view for explaining another position where the microphone is disposed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail based on the drawings. Note that the same components are denoted by the same reference numerals to omit the overlapping description in the following respective embodiments. 
     [Outline of the Present Disclosure] 
     Examples of a sound output device according to the present disclosure include an over-ear (or on-ear) type headphone (hereinafter headphone) that delivers, to a pinna from the vicinity thereof, sound generated with a diaphragm vibrating according to an audio signal in a driver unit, and a classic (or in-ear) type earphone (hereinafter earphone) that directly delivers the sound to the pinna. The sound output device is also provided with a microphone capable of collecting sound (extraneous noise) reaching from the outside of a housing including the driver unit. The sound output device corresponds to a noise cancelling system capable of reducing noise included in the sound delivered to the pinna by using an audio signal based on the noise collected by the microphone. 
     Before describing the present disclosure, a basic configuration of the noise cancelling system applied to the headphone and the earphone will be described in order to facilitate understanding. 
     (Feedback Noise Cancelling System) 
     First, a noise cancelling system using an existing feedback (hereinafter FB) technique will be described.  FIGS.  1 A,  1 B, and  1 C  are views illustrating a configuration example of the feedback noise cancelling system. 
       FIG.  1 A  is a block diagram illustrating a configuration of an electrical circuit example of the FB noise cancelling system. In this example, an over-head type headphone  10   FB  used by being worn on a head  30  of a listener is used as the sound output device. The headphone  10   FB  includes a microphone  100   a  and a driver unit  106 . The driver unit  106  includes, for example, a diaphragm, and generates air vibration based on an audio signal supplied thereto with the diaphragm vibrating according to the audio signal, thereby outputting sound. 
     In the headphone  10   FB , a space on the pinna side of the driver unit  106  and a space facing this space via the driver unit  106  are typically separated by a partition wall or the like. Note that a surface on the pinna side of the driver unit  106  is hereinafter referred to as a front surface and a surface facing the front surface as a back surface. 
     The microphone  100   a  is disposed in the front-surface space of the driver unit  106  on the inside of a housing (housing portion) of the headphone  10   FB  so as to collect sound within the space. In other words, the microphone  100   a  directly collects the sound within the space, i.e., sound to be guided to the pinna of the listener. An audio signal based on the sound collected by the microphone  100   a  is supplied to a filter  102   a  corresponding to the FB technique, which will be described in detail later, through a microphone amplifier  101 . The audio signal filtered by the filter  102   a  is supplied to an adder  104 . 
     Meanwhile, an input signal according to an audio signal as a sound source is supplied to the adder  104  through an equalizer  103  having a characteristic described in detail later. The adder  104  supplies an audio signal obtained by adding the output of the filter  102   a  and the output of the equalizer  103  to a power amplifier  105 . The power amplifier  105  power-amplifies the supplied audio signal and supplies this signal to the driver unit  106 . The driver unit  106  is driven according to the audio signal supplied from the power amplifier  105 , outputting sound. The microphone  100   a  collects the sound output by the driver unit  106  and sound (extraneous noise) reaching from the outside of the headphone  10   FB . 
       FIG.  1 B  is a view for explaining each sound related to the headphone  10   FB . In  FIG.  1 B , a noise  22  is the extraneous noise from a noise source outside the headphone  10   FB . Additionally, a noise  23  is the noise  22  entering the inside of the headphone  10   FB . In the headphone  10   FB , the noise  23  and a sound pressure  21  generated based on the audio signal in the driver unit  106  reach the pinna on the head  30  on which the headphone  10   FB  is worn. 
     A control point  20  indicates a position to reduce the noise  23  in the noise cancelling system including the headphone  10   FB . In the case of the FB technique, the control point  20  is located at the microphone  100   a  as illustrated in  FIG.  1 B . Thus, typically, the microphone  100   a  is placed at a position close to the pinna, e.g., on the front surface of the diaphragm of the driver unit  106 . 
       FIG.  1 C  is a view defining a transfer function for each portion of the configuration illustrated in  FIG.  1 A . Note that the driver unit  106  is illustrated as “driver  106 ” in  FIG.  1 C . As shown in parentheses attached to the name of each block, “M” represents the transfer function of a microphone/microphone amplifier  101   a ′ combining the microphone  100   a  and the microphone amplifier  101 , “−β” represents the transfer function of the filter  102   a , “A” represents the transfer function of the power amplifier, “D” represents the transfer function of the driver  106 , and “E” represents the transfer function of the equalizer  103 . Additionally, “H” represents a spatial transfer function  120  that is a transfer function from the driver  106  to the microphone  100   a . Note that each transfer function is represented by a complex number. 
     Moreover, “N” represents the noise  23  that is the external noise  22  illustrated in  FIG.  1 B  entering the inside of the headphone  10   FB . A reason why the noise  22  is transmitted to the inside of the headphone  10   FB  is considered that the noise leaks as a sound pressure from, for example, a gap in an earpad portion of the headphone  10   FB  (an earpiece portion in the case of in-ear type) disposed in contact with skin. The reason may be also that the noise is transmitted to the inside of the housing of the headphone  10   FB  as a result of vibration of the housing upon receiving a sound pressure from a hole formed in communication with the outside from the front surface of the headphone  10   FB . 
     An adder  121  indicates that the output of the driver unit  106  and the noise  23  are collected by the microphone  100   a , and corresponds to the control point  20 . That is, the spatial transfer function “H” is equivalent to a transfer function from the driver unit  106  to the control point  20 . Additionally, sound obtained by adding the output of a driver unit  106   b  and the noise  23  reaches the pinna as a sound pressure. The sound pressure is represented by “P”. Additionally, the input signal is represented by “S”. 
     A relation among the respective blocks in  FIG.  1 C  can be expressed by the following equation (1) using the transfer functions. 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       
                         1 
                         
                           1 
                           + 
                           
                             A 
                             ⁢ 
                             D 
                             ⁢ 
                             H 
                             ⁢ 
                             M 
                             ⁢ 
                             β 
                           
                         
                       
                       ⁢ 
                       N 
                     
                     + 
                     
                       
                         
                           A 
                           ⁢ 
                           H 
                           ⁢ 
                           D 
                         
                         
                           1 
                           + 
                           
                             A 
                             ⁢ 
                             D 
                             ⁢ 
                             H 
                             ⁢ 
                             M 
                             ⁢ 
                             β 
                           
                         
                       
                       ⁢ 
                       E 
                       ⁢ 
                       S 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Focusing on “N” representing the noise  23  in the equation (1), it is understood that the noise  23  is attenuated to “1/(1+ADHMβ)”. For the system of the equation (1) to operate stably without oscillation, a condition expressed by the following equation (2) needs to be satisfied. 
     
       
         
           
             
               
                 
                   
                      
                     
                       1 
                       
                         1 
                         + 
                         
                           A 
                           ⁢ 
                           D 
                           ⁢ 
                           H 
                           ⁢ 
                           M 
                           ⁢ 
                           β 
                         
                       
                     
                      
                   
                   &lt; 
                   1 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In combination with 1&lt;&lt;|ADMHβ| in general, the equation (2) can be interpreted as follows. 
     “−ADMHβ” obtained by disconnecting one point in a loop portion related to “N” representing the noise  23  in  FIG.  1 C  is referred to as an open loop, which has a characteristic as indicated by, for example, a bode plot of  FIG.  2   . When the open loop is targeted, the condition according to the above equation (2) needs to satisfy the following two conditions (1) and (2). 
     (1) The gain should be lower than 0 [dB] when the phase passes a point of 0 [deg]. 
     (2) The phase should not include a point of 0 [deg] when the gain is 0 [dB] or higher. 
     When the above conditions (1) and (2) are not satisfied, positive feedback is effected in the loop to cause oscillation (howling). In  FIG.  2   , margins Pa and Pb represent phase margins, and margins Ga and Gb represent gain margins. When the margins Pa and Pb and the margins Ga and Gb are small, the risk of oscillation is increased depending on, for example, individual differences in face shape or variations in wearing state of the headphone  10   FB . 
     Next, reproduction of sound according to the input signal from the headphone  10   FB  will be described in addition to the above-described function of reducing the noise reaching from the outside. The input signal “S” in  FIG.  1 C  is an audio signal based on original sound to be reproduced by the driver unit  106  of the headphone  10   FB , and includes an audio signal such as a music signal, sound of a microphone outside the housing (a use example as a hearing aid function), and a speech signal through communication (a use example as a headset). 
     Focusing on the input signal “S” in the above equation (1), the sound pressure “P” is expressed by the following equation (4) by setting the transfer function “E” of the equalizer  103  as in the following equation (3).
 
 E =(1+ADHMβ)  (3)
 
     
       
         
           
             
               
                 
                   
                     P 
                     = 
                     
                       
                         
                           1 
                           
                             1 
                             + 
                             
                               A 
                               ⁢ 
                               D 
                               ⁢ 
                               H 
                               ⁢ 
                               M 
                               ⁢ 
                               β 
                             
                           
                         
                         ⁢ 
                         N 
                       
                       + 
                     
                   
                   ⁢ 
                   AHDS 
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     When the microphone  100   a  is placed very close to the pinna, the transfer function “H” can be considered as a transfer function from the driver unit  106  to the microphone  100   a  (pinna). Here, the transfer functions “A” and “D” are the transfer functions of the power amplifier  105  and the driver unit  106 , respectively. Thus, it is understood that a characteristic similar to that of a headphone with no noise reducing function is obtained. Note that the equalizer  103  at this time has a substantially inverse characteristic from the open loop characteristic as viewed on a frequency axis. 
     (Feed-Forward Noise Cancelling System) 
     Next, a noise cancelling system using an existing feed-forward (hereinafter FF) technique will be described.  FIGS.  3 A,  3 B, and  3 C  are views illustrating a configuration example of the FF noise cancelling system. 
       FIG.  3 A  is a block diagram illustrating a configuration of an electrical circuit example of the FF noise cancelling system. In the configuration illustrated in  FIG.  3 A , the equalizer  103  is omitted and a filter  102   b  having a characteristic corresponding to the FF technique is provided instead of the filter  102   a  as compared with the above configuration illustrated in  FIG.  1 A . The input signal is directly input into the adder  104 . Additionally, in a headphone  10   FF , a microphone  100   b  for collecting extraneous noise is placed on a surface of the housing of the headphone  10   FF . An omni-directional microphone is used as the microphone  100   b.    
       FIG.  3 B  is a view for explaining each sound related to the headphone  10   FF . In  FIG.  3 B , the microphone  100   b  collects the noise  22  from the noise source outside the headphone  10   FF . Moreover, in the example of  FIG.  3 B , a control point  20 ′ is placed at a position close to the pinna on the front surface of the driver unit  106  similarly to the headphone  10   FB  illustrated in  FIG.  1 B . In the FF technique, the control point  20 ′ can be set at any pinna position of the listener. 
       FIG.  3 C  is a view defining a transfer function for each portion of the configuration illustrated in  FIG.  3 A . Note that the driver unit  106  is illustrated as “driver  106 ” in  FIG.  3 C . In this example, “M” represents the transfer function of a microphone/microphone amplifier  101   b ′ combining the microphone  100   b  and the microphone amplifier  101 . Additionally, “−α” represents the transfer function of the filter  102   b , and “H” represents the spatial transfer function  120  from the driver unit  106  to an adder  132  corresponding to the control point  20 . Moreover, “F” represents a spatial transfer function  130  of the noise  22  as the extraneous noise reaching the control point  20  (the adder  132 ) through the housing of the headphone  10   FF , and “F′” represents a spatial transfer function  131  of the noise  22  reaching the microphone  100   b.    
     A relation among the respective blocks in  FIG.  3 C  can be expressed by the following equation (5) using the transfer functions.
 
 P=−F ′ADHMα N+FN +ADHS  (5)
 
     Here, the spatial transfer function “F” (the spatial transfer function  130 ) is expressed as in the following equation (6) in consideration of an ideal state. In this case, the above equation (5) can be expressed as in the following equation (7).
 
 F=F ′ADHMα  (6)
 
 P =ADHS  (7)
 
     According to the equation (7), the input signal “S” is left in the sound pressure “P”, which does not include the noise “N”. Thus, it is understood that the noise is cancelled, and sound equivalent to that in a normal headphone operation (i.e., an operation in a state in which the external noise  22  is not present) can be listened to. 
     Unfortunately, it is practically difficult to configure the perfect filter  102   b  having the transfer function “−α” that perfectly satisfies the equation (6). Especially in a mid-to-high frequency range, the characteristic changes due to large individual differences in wearing state and ear shape among listeners, and depending on the position of the source of the noise  22 , and the position of the microphone  100   b . Thus, in the mid-to-high frequency range, the active noise reducing process according to  FIG.  3 C  is not normally performed, but passive sound isolation is often performed by, for example, increasing sealing performance against external noise in the housing of the headphone  10   FF . 
     Note that the equation (6) means that the spatial transfer function “F′” (the spatial transfer function  131 ) from the noise source of the noise  22  to the pinna position is imitated in the electrical circuit including the transfer function “−α” of the filter  102   b.    
     As described above, in the FF technique, the control point  20 ′ can be set at any pinna position of the listener. Meanwhile, the transfer function “−α” of the filter  102   b  is typically fixed, and it is necessary to design the filter  102   b  in a limited manner aiming at some target characteristic in design stage. In this case, there is a possibility that a sufficient noise cancelling effect cannot be obtained due to the pinna shape of each listener being different from that expected at the time of design, or that a noise component is added in non-reverse phase, resulting in a phenomenon such as occurrence of unusual sound. 
     Based on the above description, while the FF technique typically achieves a low risk of oscillation and high stability, it is difficult to achieve sufficient noise attenuation. Meanwhile, the FB technique, which is expected to achieve high attenuation, is inferior to the FF technique regarding the stability of the system. 
     A noise cancelling system using a method of adaptive signal processing has also been proposed. The noise cancelling system using the method of adaptive signal processing is typically provided with a microphone on, for example, both of the inside and the outside of the headphone housing. The microphone provided on the inside of the headphone is used in analyzing an error signal intended for cancellation with a filtered component, and generating a new adaptive filter by updating its coefficients. Basically, noise outside the headphone housing is digitally filtered and the obtained sound is reproduced in the driver unit. Thus, it can be roughly said that the noise cancelling system using the method of adaptive signal processing uses the FF technique. Unfortunately, the noise cancelling system using the method of adaptive signal processing has a problem of system stability and a cost-effectiveness problem due to a large processing scale. 
     Therefore, the present disclosure intends to improve the characteristics by the noise cancellation using the FF technique. 
     First Embodiment 
     Next, a first embodiment will be described. In the first embodiment, the sound output device according to the present disclosure will be described as an in-ear type earphone (hereinafter referred to as earphone). First, a configuration of an earphone performing the noise cancellation using the FF technique, according to an existing technique, will be described in contrast to the earphone according to the present disclosure.  FIGS.  4 A,  4 B, and  4 C  are views illustrating a configuration of an earphone example according to the existing technique. 
     In  FIG.  4 A , an earphone  60   a  according to the existing technique includes a sound output port  56  that guides sound output from the driver unit  106  to the pinna, and a cylindrical portion  59  to which a wire for supplying an audio signal to the driver unit  106  is connected. For example, an opening of the sound output port  56  has a smaller area than the front surface of the driver unit  106 . The driver unit  106  is a dynamic-type driver unit including a voice coil, a magnet, and the diaphragm and outputting sound with the diaphragm vibrating according to the audio signal input into the voice coil. 
     A partition wall  53   a  for separating the front surface and the back surface of the driver unit  106  is disposed within a housing  50   a  of the earphone  60   a . The inside of the housing  50   a  of the earphone  60   a  is divided into a space  54   a  (first space) on the front surface side of the driver unit  106  and a space  55   a  (second space) on the back surface side thereof by the driver unit  106  and the partition wall  53   a.    
     Here, the front surface of the driver unit  106  is a surface of the driver unit  106  on a side spatially directly connected to the sound output port  56 . The back surface of the driver unit  106  is a surface of the driver unit  106  on an opposite side to the front surface. 
     As illustrated in  FIG.  4 A , a vent hole  57   a  connecting the front-surface space  54   a  and the outside, and a vent hole  57   b  connecting the back-surface space  55   a  and the outside are disposed at predetermined positions of the housing  50   a . The vent hole  57   a  is provided for lessening a pressure load on an eardrum, reducing individual differences in output sound, or the like when the earphone  60   a  is worn on the pinna of the listener to output sound. In the example of  FIG.  4 A , the vent hole  57   a  is disposed in a wall of the housing  50   a  constituting the front-surface space  54   a . Additionally, the vent hole  57   b  is provided for lessening a load on the diaphragm of the driver unit  106  in, for example, outputting sound. 
     Actually, a ventilation resistance body  56   a  made of, for example, compressed urethane or non-woven fabric is provided within the sound output port  56 . Moreover, an earpiece  58  made of urethane or silicone rubber is typically attached to the sound output port  56  to adjust a size for the pinna and improve adhesion to the pinna. 
     The microphone  100   b  for sound collection using the FF technique is also disposed on, for example, the surface of the housing  50   a  of the earphone  60   a.    
       FIG.  4 B  is a view illustrating an action example of the noise  22  for the earphone  60   a  having the configuration in  FIG.  4 A . The noise  22  is collected by the microphone  100   b  as indicated by a path A. The noise  22  is also input into the front-surface space  54   a  from the vent hole  57   a  and guided to the pinna through the sound output port  56  from the front-surface space  54   a  as indicated by a path B. 
       FIG.  4 C  illustrates an example of an acoustic equivalent circuit of a sound isolation path for performing sound isolation of the noise  22  based on the structure in  FIG.  4 B . In  FIG.  4 C , a capacitor C e  is an ear canal volume of the pinna where the earphone  60   a  is worn, and a sound pressure supplied to the capacitor C e  is an inner-ear sound pressure. The noise  22  from the noise source is supplied to the capacitor C e  through acoustic resistance R 1  by the vent hole  57   a  and acoustic resistance R 2  by the ventilation resistance body  56   a.    
       FIGS.  5 A,  5 B, and  5 C  are views illustrating a configuration of an earphone example according to the first embodiment. In an earphone  60   b  according to the first embodiment illustrated in  FIG.  5 A , a partition wall  53   b  separates the front surface and the back surface of the driver unit  106  to form a front-surface space  54   b  and a back-surface space  55   b.    
     Here, in the earphone  60   b  according to the first embodiment, the front-surface space  54   b  and the outside of a housing  50   b  are connected by an acoustic path  70  that is separated from the back-surface space  55   b . The noise  22  is collected by the microphone  100   b  as indicated by the path A. The noise  22  is also input from a connection portion of the acoustic path  70  on the surface of the housing  50   b  of the earphone  60   b  as indicated by a path C. The connection portion is an opening formed in the surface of the housing  50   b . The noise  22  is input into the front-surface space  54   a  through the acoustic path  70  and guided to the pinna through the sound output port  56  from the front-surface space  54   a . For example, an opening of the sound output port  56  has a smaller area than the front surface of the driver unit  106 . 
     For example, a cylinder that is opened at an end connected to the partition wall  53   b  and an end connected to the outside of the housing  50   b  can be used as the acoustic path  70 . Additionally, in the first embodiment, the acoustic path  70  is disposed at a position not in contact with the driver unit  106 . A ventilation resistance body  52  made of, for example, urethane foam or non-woven fabric is preferably provided within the acoustic path  70  or around the connection portion (opening). The connection portion (opening) may be also covered with a lid made of metal or synthetic resin where a plurality of holes are formed. 
     Note that the acoustic path  70  may have a shape other than the cylindrical shape, such as a shape whose cross section has an oval, rectangular, triangular, or pentagonal or more polygonal shape. Additionally, the acoustic path  70  is not limited to the shape directly connecting the partition wall  53   b  and a connection position with the outside of the housing  50   b  and may have any shape that is topologically equivalent. 
       FIG.  5 C  illustrates an example of an acoustic equivalent circuit of a sound isolation path for performing sound isolation of the noise  22 , according to the first embodiment based on the structure in  FIG.  5 B . In  FIG.  5 C , the noise  22  from the noise source is supplied to the capacitor C e  through inductance L by the acoustic path  70  and the acoustic resistance R 2  by the ventilation resistance body  56   a.    
     When  FIG.  5 C  and  FIG.  4 C  described above are compared, the inductance L by the acoustic path  70  is connected in the equivalent circuit in  FIG.  5 C  instead of the acoustic resistance R 1  by the vent hole  57   a  in the equivalent circuit in  FIG.  4 C . Meanwhile, the acoustic resistance R 2  by the ventilation resistance body  56   a  is considered to be common in  FIG.  4 C  and  FIG.  5 C . In the equivalent circuit in  FIG.  5 C , a mid-to-high-frequency component is attenuated by the inductance L. Thus, a high passive attenuation effect can be expected. 
     In the earphone  60   b  according to the first embodiment, the microphone  100   b  for noise collection using the FF technique is further disposed in the vicinity of the connection portion (opening) where the acoustic path  70  is connected to the outside of the housing  50   b  of the earphone  60   b  on the surface of the housing  50   b . The external noise  22  collected by the microphone  100   b  can be thereby collected in a state close to the noise  22  reaching the pinna through the acoustic path  70 . Consequently, the noise cancelling effect according to the FF technique can be further improved. 
     In this case, examples of the vicinity include a state in which an end of a sound collection surface of the microphone  100   b  and an end of the connection portion (opening) of the acoustic path  70  on the surface of the housing  50   b  of the earphone  60   b  are in contact with each other. In addition to this state, the vicinity can include a state in which the end of the sound collection surface of the microphone  100   b  and the end of the connection portion (opening) are distant from each other by about several millimeters. For example, it is assumed that the sound collection surface of the microphone  100   b  has a diameter of 4 mm, and the surface of the housing  50   b  of the earphone  60   b  where the microphone  100   b  and the connection portion (opening) of the acoustic path  70  are provided has a diameter of 10 mm. In this case, when the microphone  100   b  and the connection portion (opening) of the acoustic path  70  are placed on this surface, the microphone  100   b  can be considered to be in the vicinity of the connection portion (opening) of the acoustic path  70 . 
     The microphone  100   b  may be also located in the acoustic path  70  as illustrated in  FIG.  5 D . In this case, the microphone  100   b  that is placed at a position distant from the connection portion (opening) of the acoustic path  70  by about several millimeters can be considered to be in the vicinity of the connection portion (opening) of the acoustic path  70 . 
     When the microphone  100   b  is located in the acoustic path  70 , the microphone  100   b  that is located on the inside of the connection portion (opening) of the acoustic path  70  and closer to the connection portion (opening) than the ventilation resistance body  52  can be considered to be in the vicinity of the connection portion (opening) of the acoustic path  70 . 
     Moreover, when the microphone  100   b  is located in the acoustic path  70 , the microphone  100   b  that satisfies a condition as described below can be also considered to be in the vicinity of the connection portion (opening) of the acoustic path  70 . 
     That is, referring to  FIG.  5 E , “Dx” represents the transfer function of sound output from the driver unit  106 , reaching a portion  73  connected to the acoustic path  70  through the front-surface space  54   b  from the driver unit  106  as indicated by a path R. Additionally, “Dy” represents the transfer function of the sound reaching the microphone  100   b  through the front-surface space  54   b  and the acoustic path  70  from the driver unit  106  as indicated by a path S. In this case, when the microphone  100   b  is placed at a position where |Dx|/|Dy| that is a ratio of absolute values of Dx and Dy is higher than about 10 [dB], the microphone  100   b  can be considered to be in the vicinity of the connection portion (opening) of the acoustic path  70 . 
     Here, when the microphone  100   b  is mounted at a predetermined position with respect to the connection portion (opening) of the acoustic path  70  on the surface of the housing  50   b  of the earphone  60   b , the microphone  100   b  needs to be located at a position not causing howling in the earphone  60   b . Such a position can be obtained by, for example, experiments. 
     The vicinity may also include a position of the microphone  100   b  where a difference between a characteristic of sound collected by the microphone  100   b  and a characteristic of sound at the connection portion (opening) of the acoustic path  70  on the surface of the housing  50   b  is equal to or less than a predetermined value. In this case, a measurable value in the transfer function, such as a frequency characteristic, can be used as the characteristic. 
     Note that a direction of the connection portion (opening) of the acoustic path  70  and a direction perpendicular to the sound collection surface of the microphone  100   b  are preferably substantially equal to each other. 
       FIG.  6    is a view for explaining the effect according to the first embodiment. In  FIG.  6   , the horizontal axis represents a frequency [Hz] displayed on a logarithmic scale. The vertical axis represents an active noise reduction amount [dB]. The active noise reduction amount is a noise reduction amount obtained when the noise cancelling system in  FIGS.  3 A to  3 C  is operated based on noise reduction amounts in the earphones  60   a  and  60   b  obtained in passive sound isolation, i.e., when the noise cancelling system is not operated, as a reference value (Ref). 
     In  FIG.  6   , a characteristic line  90  shows a characteristic of the earphone  60   a  according to the existing technique, described using  FIGS.  4 A to  4 C . Additionally, a characteristic line  91  shows a characteristic of the earphone  60   b  according to the first embodiment, described using  FIGS.  5 A to  5 C . When the characteristic lines  90  and  91  in  FIG.  6    are compared, it is understood that the characteristic line  91  has a larger active noise reduction amount than the characteristic line  90 . Especially in a frequency band  80  from approximately 2 [kHz] to approximately 4 [kHz], a reduction effect of 10 [dB] or more can be observed in the active noise reduction amount indicated by the characteristic line  91  with respect to the active noise reduction amount indicated by the characteristic line  90 . 
     As described above, disposing the microphone  100   b  in the vicinity of the connection portion (opening) of the acoustic path  70  on the surface of the housing  50   b  allows the noise reaching the pinna from the outside to be further reduced in the FF noise cancelling system. 
     First Modification of the First Embodiment 
     Next, a first modification of the first embodiment will be described. An earphone according to the first modification of the first embodiment will be described using  FIGS.  7 A and  7 B .  FIG.  7 A  is a view illustrating a configuration of an example of an earphone  60   c  according to the first modification of the first embodiment. 
     As illustrated in  FIG.  7 A , the earphone  60   c  according to the first modification of the first embodiment is provided with a vent hole  71  in, for example, the center of the driver unit  106  so as to penetrate the front surface and the back surface of the driver unit  106 . The acoustic path  70  is connected to the vent hole  71  or configured including the vent hole  71  to connect the front-surface space  54   a  and the outside of a housing  50   c  of the earphone  60   c  separately from a back-surface space  55   c  that is separated from the front-surface space  54   a  by the partition wall  53   a.    
       FIG.  7 B  is a view schematically illustrating a structure of an example of the driver unit  106 . In the example of  FIG.  7 B , the driver unit  106  includes a frame  1061 , a diaphragm  1062 , and a ventilation resistance body  1063 . The frame  1061  includes, for example, a magnet and a voice coil connected to the diaphragm  1062 . The diaphragm  1062  vibrates according to the audio signal input into the voice coil to output sound. Here, a doughnut-shaped magnet having a hollow center is used as the magnet so as to form a hole in the center of the diaphragm  1062 . The vent hole  71  can be thereby formed penetrating the front surface and the back surface of the driver unit  106 . 
     The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  70  is connected to the surface of the housing  50   c  of the earphone  60   c  in a similar manner to the above first embodiment. Configuring the earphone  60   c  as described above also allows the noise reaching the pinna from the outside to be further reduced in the FF noise cancelling system in a similar manner to the above first embodiment. 
     Second Modification of the First Embodiment 
     Next, a second modification of the first embodiment will be described.  FIG.  8    is a view illustrating a configuration of an earphone example according to the second modification of the first embodiment. An earphone  60   d  according to the second modification of the first embodiment illustrated in  FIG.  8    is provided by adding the microphone  100   a  for the FB noise cancelling system to the front-surface space  54   b  in, for example, the earphone  60   b  according to the first embodiment described using  FIG.  5 A . 
     In this configuration, the electrical circuit of the noise cancelling system includes the microphone amplifier, the filter  102   a , and the equalizer  103  in  FIG.  1 A , and the microphone amplifier  101  and the filter  102   b  in  FIG.  3 A . 
     The second modification of the first embodiment enables improvement in stability while reducing the gain and decreasing the noise attenuation in the signal processing circuit using the FB technique, and further enables noise removal using the FF technique. As a result, the noise attenuation in the entire system can be increased, and the system can be stably operated. 
     While it has been described that the microphone  100   a  for the FB noise cancelling system is added to the earphone  60   b  according to the first embodiment, the configuration is not limited to this example. For example, the microphone  100   a  may be also added to the front-surface space  54   a  (see  FIG.  7 A ) of the earphone  60   c  according to the first modification of the first embodiment. The same applies to a configuration in  FIG.  9    described below. 
     Third Modification of the First Embodiment 
     Next, a third modification of the first embodiment will be described.  FIG.  9    is a view illustrating a configuration of an earphone example according to the third modification of the first embodiment. Note that  FIG.  9    shows an example in which the configuration according to the third modification of the first embodiment is applied to the configuration of the earphone  60   c  according to the first modification of the first embodiment described using  FIG.  7 A . 
     While it has been described that the acoustic path  70  has a cylindrical shape in the first embodiment and the first and second modifications of the first embodiment described above, the shape is not limited to this example. An earphone  60   e  according to the third modification of the first embodiment illustrated in  FIG.  9    includes an acoustic path  70 ′ that connects the front-surface space  54   a  of the driver unit  106  and the surface of a housing  50   e  of the earphone  60   e . The acoustic path  70 ′ is shaped such that the opening at the connection portion where the acoustic path  70 ′ is connected to the surface of the housing  50   e  has a larger area than an opening at a connection portion where the acoustic path  70 ′ is connected to the front-surface space  54   a.    
     To be more specific, the acoustic path  70 ′ has a so-called trumpet shape in which its diameter is increased nonlinearly from the driver unit  106  toward the surface of the housing  50   e . In other words, a longitudinal cross section of the acoustic path  70 ′ according to the third modification of the first embodiment is curved symmetrically to the longitudinal center. The acoustic path  70 ′ is not limited to this shape, and the longitudinal cross section thereof may be also curved asymmetrically to the longitudinal center. 
     The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  70 ′ is connected to the surface of the housing  50   e  of the earphone  60   e  in a similar manner to the above first embodiment. Configuring the earphone  60   e  as described above also allows the noise reaching the pinna from the outside to be further reduced in the FF noise cancelling system in a similar manner to the above first embodiment. 
     Additionally, in the third modification of the first embodiment, the acoustic path  70 ′ is shaped such that the opening in the surface of the housing  50   e  has a larger area than the opening connected to the front-surface space  54   a  as described above. This makes directivity of the acoustic path  70 ′ against the noise  22  input thereinto close to that of the omni-directional microphone  100   b . Thus, improvement in the noise reducing effect according to the FF technique can be expected. 
     Note that the acoustic path  70 ′ according to the third modification of the first embodiment can be similarly applied to the earphone  60   b  according to the first embodiment and the earphone  60   d  according to the third modification of the first embodiment described above. 
     Second Embodiment 
     Next, a second embodiment will be described. The second embodiment is an example in which the present disclosure is applied to an over-ear (or on-ear) type headphone.  FIG.  10    is a view illustrating a configuration of a headphone example according to the second embodiment. In a headphone  10   a  according to the second embodiment illustrated in  FIG.  10   , a housing  1000  is divided into the front surface and the back surface of the driver unit  106  by a partition wall  1002 , and the front surface side of the driver unit  106  has an open structure. On the front surface side, an end of the housing  1000  covers the pinna on the head  30  of the listener via an earpad  1001  made of urethane or the like. The front surface of the driver unit  106 , a portion of the housing  1000 , the earpad  1001 , and the head  30  of the listener form the front-surface space (first space) of the driver unit  106 . 
     Additionally, in the example of  FIG.  10   , a first back-surface space  1010  (second space) is formed by the partition wall  1002  on the back surface side of the driver unit  106  in the housing  1000  of the headphone  10   a . Moreover, in the example of  FIG.  10   , a partition wall  1003  is disposed in the first back-surface space  1010  so as to form a second back-surface space  1011  (third space) including a back-surface portion of the driver unit  106 . 
     In the headphone  10   a  according to the second embodiment, the front-surface space of the driver unit  106  and the outside of the housing  1000  are connected by an acoustic path  72  that is separated from the first back-surface space  1010  through the first back-surface space  1010 . The connection portion (opening) may be covered with a lid made of metal or synthetic resin where a plurality of holes are formed. For example, a cylinder that is opened at an end connected to the partition wall  1002  and an end connected to the outside of the housing  1000  can be used as the acoustic path  72  similarly to the acoustic path  70  in the above first embodiment. Additionally, in the second embodiment, the acoustic path  72  is disposed at a position not in contact with the driver unit  106 . A ventilation resistance body made of, for example, urethane foam or non-woven fabric is preferably provided within the acoustic path  72 . 
     The microphone  100   b  for noise collection using the FF technique is disposed in the vicinity of the connection portion (opening) where the acoustic path  72  is connected to the housing  1000  of the headphone  10   a  on the surface of the housing  1000  of the headphone  10   a . The external noise  22  collected by the microphone  100   b  can be thereby collected in a state close to the noise  22  reaching the pinna through the acoustic path  72  (see a path F in  FIG.  10   ). Consequently, the noise cancelling effect according to the FF technique can be further improved. 
     Note that the definition of the vicinity described in the first embodiment can be applied to the vicinity in this case. Here, in the headphone  10   a , the area of the surface of the housing  1000  where the connection portion of the acoustic path  72  and the microphone  100   b  are provided can be made larger than that of the above earphone  60   b  or the like. Thus, a larger distance margin of, for example, several tens millimeters can be provided between the end of the sound collection surface of the microphone  100   b  and the end of the opening of the acoustic path  72  in the surface of the housing  1000  as compared with that in the example of the above earphone  60   b.    
     Note that a direction of the connection portion (opening) of the acoustic path  72  and a direction perpendicular to the sound collection surface of the microphone  100   b  are preferably substantially equal to each other in this case as well. 
     First Modification of the Second Embodiment 
     Next, a first modification of the second embodiment will be described.  FIG.  11    is a view illustrating a configuration of a headphone example according to the first modification of the second embodiment. In a headphone  10   b  illustrated in  FIG.  11   , the housing  1000  is divided into the front surface and the back surface of the driver unit  106  by the partition wall  1002 , and the second back-surface space  1011  is formed by the partition wall  1003  within the first back-surface space  1010  formed by the housing  1000  and the partition wall  1002  on the back surface of the driver unit  106  in a similar manner to the headphone  10   a  described using  FIG.  10   . 
     In the headphone  10   b  according to the first modification of the second embodiment, the front-surface space of the driver unit  106  and the outside of the housing  1000  are connected by the acoustic path  72  that is separated from the second back-surface space  1011  and the first back-surface space  1010 . 
     The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  72  is connected to the housing  1000  of the headphone  10   b  on the surface of the housing  1000  of the headphone  10   b  in a similar manner to the above second embodiment. The external noise  22  collected by the microphone  100   b  can be thereby collected in a state close to the noise  22  reaching the pinna through the acoustic path  72  (see a path G in  FIG.  11   ). Consequently, the noise cancelling effect according to the FF technique can be further improved. 
     Second Modification of the Second Embodiment 
     Next, a second modification of the second embodiment will be described.  FIG.  12    is a view illustrating a configuration of a headphone example according to the second modification of the second embodiment. A headphone  10   c  illustrated in  FIG.  12    corresponds to the earphone  60   c  (see  FIG.  7 A ) according to the above first modification of the first embodiment, and is provided with the vent hole  71  in, for example, the center of the driver unit  106  so as to penetrate the front surface and the back surface of the driver unit  106 . The acoustic path  72  is connected to the vent hole  71  or configured including the vent hole  71  to connect the front-surface space of the driver unit  106  and the outside of the housing  1000  of the headphone  10   c  through the second back-surface space  1011  and the first back-surface space  1010 . 
     Since the driver unit  106  has the same structure as that described using  FIG.  7 B , the detailed description thereof is omitted here. 
     The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  72  is connected to the housing  1000  of the headphone  10   b  on the surface of the housing  1000  of the headphone  10   b  in a similar manner to the above second embodiment. The external noise  22  collected by the microphone  100   b  can be thereby collected in a state close to the noise  22  reaching the pinna through the acoustic path  72  (see a path H in  FIG.  12   ). Consequently, the noise cancelling effect according to the FF technique can be further improved. 
     Third Modification of the Second Embodiment 
     Next, a third modification of the second embodiment will be described.  FIG.  13    is a view illustrating a configuration of a headphone example according to the third modification of the second embodiment. A headphone  10   d  according to the third modification of the second embodiment illustrated in  FIG.  13    is provided by adding the microphone  100   a  for the FB noise cancelling system to the front-surface space of the driver unit  106  in, for example, the headphone  10   a  according to the second embodiment described using  FIG.  10   . 
     In this example, the electrical circuit of the noise cancelling system includes the microphone amplifier, the filter  102   a , and the equalizer  103  in  FIG.  1 A , and the microphone amplifier  101  and the filter  102   b  in  FIG.  3 A  in a similar manner to the above second modification of the first embodiment. 
     The third modification of the second embodiment enables improvement in stability while reducing the gain and decreasing the noise attenuation in the signal processing circuit using the FB technique, and further enables noise removal using the FF technique. As a result, the noise attenuation in the entire system can be increased, and the system can be stably operated. 
     While it has been described that the microphone  100   a  for the FB noise cancelling system is added to the headphone  10   a  according to the second embodiment, the configuration is not limited to this example. For example, the microphone  100   a  may be also added to the front-surface space of the driver unit  106  in the headphone  10   b  according to the first modification of the second embodiment and the headphone  10   c  according to the second modification of the second embodiment. The same applies to a configuration in  FIG.  14    described below. 
     Fourth Modification of the Second Embodiment 
     Next, a fourth modification of the second embodiment will be described.  FIG.  14    is a view illustrating a configuration of a headphone example according to the fourth modification of the second embodiment. Note that  FIG.  14    shows an example in which the configuration according to the fourth modification of the second embodiment is applied to the configuration of the headphone  10   c  according to the second modification of the second embodiment described using  FIG.  12   . 
     A headphone  10   e  illustrated in  FIG.  14    corresponds to the earphone  60   e  (see  FIG.  9   ) according to the above third modification of the first embodiment. An acoustic path  72 ′ that connects the front-surface space of the driver unit  106  and the surface of  1000  of the headphone  10   d  is shaped such that the opening at the connection portion where the acoustic path  72 ′ is connected to the surface of the housing  1000  has a larger area than an opening at a connection portion where the acoustic path  72 ′ is connected to the front-surface space of the driver unit  106 . 
     To be more specific, the acoustic path  72 ′ has a so-called trumpet shape in which its diameter is increased nonlinearly from the driver unit  106  toward the surface of the housing  1000  similarly to the acoustic path  70 ′ in  FIG.  9   . In other words, a longitudinal cross section of the acoustic path  72 ′ according to the fourth modification of the second embodiment is curved symmetrically to the longitudinal center. The acoustic path  72 ′ is not limited to this shape, and the longitudinal cross section thereof may be also curved asymmetrically to the longitudinal center. 
     The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  72 ′ is connected to the surface of the housing  1000  of the headphone  10   e  in a similar manner to the above first embodiment. Configuring the headphone  10   e  as described above also allows the noise reaching the pinna from the outside to be further reduced in the FF noise cancelling system in a similar manner to the above second embodiment. 
     Additionally, in the fourth modification of the second embodiment, the acoustic path  72 ′ is shaped such that the opening in the surface of the housing  1000  has a larger area than the opening connected to the front-surface space of the driver unit  106  as described above. This makes directivity of the acoustic path  72 ′ against the noise  22  input thereinto close to that of the omni-directional microphone  100   b . Thus, improvement in the noise reducing effect according to the FF technique can be expected. 
     Note that the acoustic path  72 ′ according to the fourth modification of the third embodiment can be similarly applied to the headphone  10   a  according to the second embodiment, the headphone  10   b  according to the first modification of the second embodiment, and the headphone  10   d  according to the third modification of the second embodiment described above. 
     Fifth Modification of the Second Embodiment 
     Next, a fifth modification of the second embodiment will be described. In the fifth modification of the second embodiment, a position where the microphone  100   b  is disposed will be described using  FIGS.  15 A to  15 C . Here, the headphone  10   c  according to the second modification of the second embodiment described using  FIG.  12    will be described as an example. 
       FIG.  15 A  shows an example in which the microphone  100   b  for noise collection using the FF technique is disposed on an inner surface of the acoustic path  72 , more specifically, on an inner wall of the acoustic path  72 . In this case, the microphone  100   b  is preferably placed such that the sound collection surface is located in the vicinity of the connection position of the acoustic path  72  with the housing  1000 . Additionally, when the microphone  100   b  is disposed on the inner wall of the acoustic path  72 , for example, the sound collection surface of the microphone  100   b  is preferably disposed parallel to the inner wall of the acoustic path  72 . 
       FIG.  15 B  shows an example in which the microphone  100   b  is arranged flush with the surface of the connection portion (opening) where the acoustic path  72  is connected to the housing  1000  in the housing  1000  of the headphone  10   c . In other words, the sound collection surface of the microphone  100   b  is placed toward the outside of the housing  1000  in the example of  FIG.  15 B . The microphone  100   b  is disposed in the vicinity of the connection portion (opening) where the acoustic path  72  is connected to the housing  1000  in the example of  FIG.  15 B  as well. Additionally, the flush surface is, for example, a surface without an edge of a predetermined angle or more with respect to the surface of the connection portion (opening). 
       FIG.  15 C  shows an example in which the microphone  100   b  is placed in the opening at the connection portion where the acoustic path  72  is connected to the housing  1000 . In this case, the diameter of the opening is increased according to need such that the microphone  100   b  does not close the acoustic path  72 . The arrangement in  FIG.  15 C  is considered to be more advantageous than the arrangement examples in  FIGS.  15 A and  15 B  in a sense that the microphone  100   b  is placed in the vicinity of the opening at the connection portion where the acoustic path  72  is connected to the housing  1000 . 
     While the headphone  10   c  has been described as an example, the respective positions of the microphone  100   b  described using  FIGS.  15 A to  15 C  can be also applied to the headphones  10   a ,  10   b ,  10   d , and  10   e  illustrated in  FIGS.  10 ,  11 ,  13 , and  14   , respectively. 
     Moreover, the respective positions of the microphone  100   b  described using  FIGS.  15 A to  15 C  can be similarly applied to the earphones  60   b ,  60   c ,  60   d , and  60   e  illustrated in  FIGS.  5 A,  7 A,  8 , and  9   , respectively, in the first embodiment and its respective modifications. 
     The present disclosure can be also configured as follows. 
     (1) A sound output device comprising: 
     an acoustic path connecting a first space on a front surface of a driver unit and an outside of a housing including the driver unit separately from a second space on a back surface of the driver unit; and 
     a microphone disposed in the vicinity of an opening where the acoustic path is connected to the outside of the housing. 
     (2) The sound output device according to the above (1), wherein 
     the acoustic path connects the first space and the outside separately from the second space while penetrating the driver unit and a portion of the second space. 
     (3) The sound output device according to the above (1), wherein 
     the acoustic path connects the first space and the outside separately from the second space without contacting the driver unit. 
     (4) The sound output device according to any one of the above (1) to (3), wherein 
     the second space includes a third space connected to the back surface of the driver unit, and 
     the acoustic path connects the first space and the outside separately from the third space and the second space. 
     (5) The sound output device according to any one of the above (1) to (4), wherein 
     in the acoustic path, an area of an end connected to the outside and an area of an end connected to the first space are substantially equal to each other. 
     (6) The sound output device according to any one of the above (1) to (4), wherein 
     in the acoustic path, an area of a first end connected to the outside is larger than an area of a second end connected to the first space. 
     (7) In the sound output device according to the above (6), the acoustic path has a sectional area that increases nonlinearly from the second end toward the first end. 
     (8) The sound output device according to any one of the above (1) to (7), wherein 
     the microphone is disposed in the vicinity of the opening on a surface of the housing. 
     (9) The sound output device according to any one of the above (1) to (7), wherein 
     the microphone is disposed on an inner surface of the acoustic path. 
     (10) The sound output device according to any one of the above (1) to (7), wherein 
     the microphone is disposed in the opening of the acoustic path. 
     (11) The sound output device according to any one of the above (1) to (10), further comprising 
     a microphone disposed at a position enabling direct collection of sound in the first space. 
     (12) The sound output device according to any one of the above (1) to (11), wherein 
     the housing is shaped such that the first space is opened in a direction of the front surface of the driver unit. 
     (13) The sound output device according to any one of the above (1) to (11), wherein 
     the housing is shaped such that an opening having a smaller area than an area of the front surface of the driver unit is disposed in a direction of the front surface of the driver unit in the first space. 
     (14) In the sound output device according to any one of the above (1) to (13), the microphone is placed at a position where a difference between a characteristic of sound at the opening and a characteristic of sound collected by the microphone is equal to or less than a predetermined value. 
     REFERENCE SIGNS LIST 
     
         
           10   a ,  10   b ,  10   c ,  10   d ,  10   e ,  10   FB ,  10   FF  HEADPHONE 
           20 ,  20 ′ CONTROL POINT 
           21  SOUND PRESSURE 
           22 ,  23  NOISE 
           50   a ,  50   b ,  50   c ,  50   e ,  1000  HOUSING 
           53   a ,  53   b ,  1002 ,  1003  PARTITION WALL 
           60   a ,  60   b ,  60   c ,  60   d ,  60   e  EARPHONE 
           70 ,  70 ′,  72 ,  72 ′ ACOUSTIC PATH 
           101   a ′,  101   b ′ MICROPHONE/MICROPHONE AMPLIFIER 
           100   a ,  100   b  MICROPHONE 
           101  MICROPHONE AMPLIFIER 
           102   a ,  102   b  FILTER 
           103  EQUALIZER 
           105  POWER AMPLIFIER 
           106  DRIVER UNIT 
           120 ,  130 ,  131  SPATIAL TRANSFER FUNCTION