Patent Publication Number: US-8116472-B2

Title: Noise control device

Description:
TECHNICAL FIELD 
     The present invention relates to a noise control device, and particularly to a noise control device for reducing noises respectively arriving in a plurality of spaces which are acoustically independent from each other. 
     BACKGROUND ART 
     In recent years, a so-called noise-canceling headphone has entered the market in response to the growing needs of improvement in comfortability in an environment where there is too much noise, typically an aircraft cabin or the like. The noise-canceling headphone is a headphone apparatus using an active noise control technique in which a control sound in antiphase to a noise is actively outputted, whereby the noise is reduced (e.g., Patent Document 1). 
     Hereinafter, a conventional noise-canceling headphone will be described with reference to  FIG. 20 .  FIG. 20  shows a configuration of the conventional noise-canceling headphone. Here,  FIG. 20  shows a view seen from above a head of a user  90 . In  FIG. 20 , the user  90  faces upward. 
     As shown in  FIG. 20 , the noise-canceling headphone comprises a headband  91 , a left ear case  92   a , a right ear case  92   b , a left ear speaker  93   a , a right ear speaker  93   b , a left ear microphone  94   a , a right ear microphone  94   b , a left ear control section  95   a  and a right ear control section  95   b . The left ear case  92   a  is placed near a left ear of the user  90 . The right ear case  92   b  is placed near a right ear of the user  90 . The left ear case  92   a  and the right ear case  92   b  are connected by the headband  91 . The left ear speaker  93   a  is provided within the left ear case  92   a . The right ear speaker  93   b  is provided within the right ear case  92   b . The left ear microphone  94   a  is provided within the left ear case  92   a . The right ear microphone  94   b  is provided within the right ear case  92   b.    
     Here, the left ear case  92   a  and the right ear case  92   b  have spaces formed therein, respectively. These spaces are acoustically independent from each other. Here, being acoustically independent means that an acoustic state is such that a gain of an electroacoustic transfer function between the spaces is sufficiently small. 
     The left ear microphone  94   a  detects a noise arriving in the left ear case  92   a . The left ear microphone  94   a  outputs, as a detection signal e L  to the left ear control section  95   a , a noise signal based on the detected noise. The left ear control section  95   a  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  such that the level is lowered. The left ear control section  95   a  outputs the generated control signal to the left ear speaker  93   a . Similarly, the right ear microphone  94   b  detects a noise arriving in the right ear case  92   b . The right ear microphone  94   b  outputs, as a detection signal e R  to the right ear control section  95   b , a noise signal based on the detected noise. The right ear control section  95   b  generates, based on the detection signal e R , a control signal for controlling a level of the detection signal e R  such that the level is lowered. The right ear control section  95   b  outputs the generated control signal to the right ear speaker  93   b.    
     Next, configurations of the left ear control section  95   a  and the right ear control section  95   b  as well as processes performed by the left ear control section  95   a  and the right ear control section  95   b  will be described in detail with reference to  FIG. 21 .  FIG. 21  shows, by blocks of signal processing, the configuration of the noise-canceling headphone of  FIG. 20 . It is assumed for  FIG. 21  that components, which are denoted by the same reference numerals as those used for components in  FIG. 20 , have the same functions as those of the components in  FIG. 20 , and descriptions thereof will be omitted. 
     A block  921   a  in the left ear case  92   a  indicates an electroacoustic transfer function H L  from an input of the left ear speaker  93   a  to an output of the left ear microphone  94   a . A block  921   b  within the right ear case  92   b  indicates an electroacoustic transfer function H R  from an input of the right ear speaker  93   b  to an output of the right ear microphone  94   b . An adder  922   a  adds an output signal of the block  921   a  to a noise signal N L  indicating the noise arriving in the left ear case  92   a . A signal outputted from the adder  922   a  is the aforementioned detection signal e L . An adder  922   b  adds an output signal of the block  921   b  to a noise signal N R  indicating the noise arriving in the right ear case  92   b . A signal outputted from the adder  922   b  is the aforementioned detection signal e R . 
     First, a process performed for the left ear of the user  90  will be described. The left ear control section  95   a  comprises a feedback control filter  951   a  and a phase inverter  952   a . For the feedback control filter  951   a , a filter coefficient indicating a transfer function C L  is set. The detection signal e L  outputted from the adder  922   a  is inputted to the feedback control filter  951   a . The phase inverter  952   a  inverts a phase of an output signal of the feedback control filter  951   a . An output signal from the phase inverter  952   a  is inputted to the block  921   a . Here, a transfer function from the noise signal N L  to the detection signal e L  is represented by an equation (1). 
     
       
         
           
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     Here, the transfer function C L  of the feedback control filter  951   a  is set, as shown in an equation (2), so as to have an inverse characteristic to that of the electroacoustic transfer function H L  at the left ear. Note that, a indicates a filter gain of a fixed frequency. 
     
       
         
           
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     When the noise arrives in the left ear case  92   a , the left ear microphone  94   a  outputs, as is clear from the equation (1), N L /(1+C L ×H L ) as the detection signal e L . The detection signal e L  is inputted to the feedback control filter  951   a . At this point, the control signal generated at the feedback control filter  951   a  is C L ×N L /(1+C L +H L ). Since the transfer function C L  is set as shown in the equation (2), the control signal is N L /(H L ×(1+1/α)). The control signal is inputted to the block  921   a  after a phase of the control signal is inverted at the phase inverter  952   a . Accordingly, a cancellation sound, which is −H L ×N L /(H L ×(1+1/α))=−N L /(1+1/α), is radiated from the left ear speaker  93   a  to the vicinity of the left ear. As a result, the greater the filter gain α, the nearer to −N L  the cancellation sound becomes, whereby the noise arriving near the left ear is canceled. 
     Next, a process performed for the right ear of the user  90  will be described. The right ear control section  95   b  comprises a feedback control filter  951   b  and a phase inverter  952   b . For the feedback control filter  951   b , a filter coefficient indicating a transfer function C R  is set. The detection signal e R  outputted from the adder  922   b  is inputted to the feedback control filter  951   b . The phase inverter  952   b  inverts a phase of an output signal of the feedback control filter  951   b . An output signal from the phase inverter  952   b  is inputted to the block  921   b . Note that, the process performed for the right ear is different from the above-described process performed for the left ear only in that the transfer function C R  of the right ear control section  95   b  has an inverse characteristic to that of the electroacoustic transfer function H R  at the right ear. Other than this, the process performed for the right ear is the same as that of the process performed for the left, and therefore a description thereof will be omitted. 
     There is a known conventional technique in which the noise reduction function illustrated in  FIG. 21  and an audio signal outputting function are combined.  FIG. 22  shows a configuration in which the noise reduction function and the audio signal outputting function are combined. It is assumed for  FIG. 22  that components, which are denoted by the same reference numerals as those used for components in  FIG. 20 , have the same functions as those of the components in  FIG. 20 , and descriptions thereof will be omitted. 
     A configuration shown in  FIG. 22  is a result of adding, to the configuration shown in  FIG. 20 , an audio signal output section  97 , a left ear audio signal canceling section  98   a , a right ear audio signal canceling section  98   b , subtractors  99   a  and  99   b , and adders  100   a  and  100   b . The audio signal output section  97  outputs audio signals such as music. As shown in  FIG. 22 , the audio signal output section  97  outputs an audio signal A L  to the left ear and an audio signal A R  to the right ear. The left ear audio signal canceling section  98   a  generates, based on a filter coefficient indicating a transfer function simulating the electroacoustic transfer function H L , a cancellation signal for canceling the audio signal A L . The subtractor  99   a  subtracts, from the detection signal e L , the cancellation signal for canceling the audio signal A L . An output signal from the subtractor  99   a  is inputted to the left ear control section  95   a . A control signal outputted from the left ear control section  95   a  is added to the audio signal A L  by the adder  100   a . An output signal from the adder  100   a  is inputted to the left ear speaker  93   a . The left ear speaker  93   a  outputs a sound based on the control signal and the audio signal A L . 
     Here, the detection signal e L  from the left ear microphone  94   a  contains the audio signal A L . However, the subtractor  99   a  subtracts, from the detection signal e L , the cancellation signal for canceling the audio signal A L . As a result, the audio signal A L  is not inputted to the left ear control section  95   a , and the same process as that described in  FIG. 21  is performed at the left ear control section  95   a.    
     The right ear audio signal canceling section  98   b  generates, based on a filter coefficient indicating a transfer function simulating the electroacoustic transfer function H R , a cancellation signal for canceling the audio signal A R . The subtractor  99   b  subtracts, from the detection signal e R , the cancellation signal for canceling the audio signal A R . An output signal from the subtractor  99   b  is inputted to the right ear control section  95   b . A control signal outputted from the right ear control section  95   b  is added to the audio signal A R  by the adder  100   b . An output signal from the adder  100   b  is inputted to the right ear speaker  93   b . The right ear speaker  93   b  outputs a sound based on the control signal and the audio signal A R . Other than the above, the process for the right ear is the same as the above-described process for the left ear, and therefore a description thereof will be omitted. As described above, the configuration shown in  FIG. 22  allows noise reduction and stereo audio signal reproduction to be performed concurrently. 
     Usually, in a radio frequency band, a phase lag occurs in each of the electroacoustic transfer functions H L  and H R . For this reason, there is a problem that even if, e.g., the transfer function C L  is set to have an inverse characteristic to that of the electroacoustic transfer function H L , the transfer function C L  does not have the inverse characteristic to that of the electroacoustic transfer function H L  in the radio frequency band, whereby noise reduction effect deteriorates. For this problem, there is a conventionally suggested configuration as shown in  FIG. 23  for widening a frequency band in which a noise reduction effect is obtained.  FIG. 23  shows a configuration of a noise-canceling headphone capable of widening a frequency band in which a noise reduction effect is obtained. The configuration shown in  FIG. 23  is a result of adding, to the configuration shown in  FIG. 20 , a left ear high frequency control section  101   a , a right ear high frequency control section  101   b  and adders  102   a  and  102   b.    
     As shown in  FIG. 23 , the left ear control section  95   a  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  such that the level is lowered, the control signal having a frequency which is no higher than a predetermined frequency. In other words, the left ear control section  95   a  generates a cancellation signal for canceling a noise arriving in the left ear case  92   a , the noise having the frequency which is no higher than the predetermined frequency. Here, the predetermined frequency is lower than a frequency at which a phase lag of the electroacoustic transfer function H L  occurs. The left ear control section  95   a  outputs the generated control signal to the adder  102   a . The left ear high frequency control section  101   a  generates, based on the detection signal e L , a control signal for controlling the level of the detection signal e L  such that the level is lowered, the control signal having a frequency which is higher than the predetermined frequency. In other words, the left ear high frequency control section  101   a  generates a cancellation signal for canceling a noise arriving in the left ear case  92   a , the noise having the frequency which is higher than the predetermined frequency. The left ear high frequency control section  101   a  outputs the generated control signal to the adder  102   a . The adder  102   a  adds the control signal generated at the left ear control section  95   a  to the control signal generated at the left ear high frequency control section  101   a . A signal resulting from the addition at the adder  102   a  is inputted to the left ear speaker  93   a . The left ear speaker  93   a  outputs sounds based on the control signals generated at the left ear control section  95   a  and the left ear high frequency control section  101   a . As a result, the sounds, which are based on the control signals, and the noises are canceled by each other near the left ear. 
     On the other hand, the right ear control section  95   b  generates, based on the detection signal e R , a control signal for controlling a level of the detection signal e R  such that the level is lowered, the control signal having a frequency which is no higher than a predetermined frequency. In other words, the right ear control section  95   b  generates a cancellation signal for canceling a noise arriving in the right ear case  92   b , the noise having the frequency which is no higher than the predetermined frequency. Here, the predetermined frequency is lower than a frequency at which a phase lag of the electroacoustic transfer function H R  occurs. The right ear control section  95   b  outputs the generated control signal to the adder  102   b . The right ear high frequency control section  101   b  generates, based on the detection signal e R , a control signal for controlling the level of the detection signal e R  such that the level is lowered, the control signal having a frequency which is higher than the predetermined frequency. In other words, the right ear high frequency control section  101   b  generates a cancellation signal for canceling a noise arriving in the right ear case  92   b , the noise having a frequency which is higher than the predetermined frequency. The right ear high frequency control section  101   b  outputs the generated control signal to the adder  102   b . The adder  102   b  adds the control signal generated at the right ear control section  95   b  to the control signal generated at the right ear high frequency control section  101   b . A signal resulting from the addition at the adder  102   b  is inputted to the right ear speaker  93   b . The right ear speaker  93   b  outputs sounds based on the control signals generated at the right ear control section  95   b  and the right ear high frequency control section  101   b . As a result, the sounds, which are based on the control signals, and the noises are canceled by each other near the right ear. 
     As described above, separately for a high frequency band higher than the predetermined frequency in which a phase lag of the electroacoustic transfer function occurs, controls are performed using the left ear high frequency control section  101   a  and the right ear high frequency control section  101   b  for each of which a filter coefficient is set based on the electroacoustic transfer function whose phase is lagged. This allows a frequency band, in which the noise reduction effect is obtained, to be widened.
     [Patent Document 1] (PCT) International Publication WO94/17512   

     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     As described above, in a headphone apparatus or the like, a space formed within the left ear case  92   a  and a space formed within the right ear case  92   b  are acoustically independent from each other. For this reason, it is usual in the conventional manner that control is separately performed for each of the right ear and the left ear. Therefore, in the above-described conventional noise-canceling headphone, the control for the left ear is performed by the left ear control section  95   a  and the control for the right ear is performed by the right ear control section  95   b.    
     Described here is a case where processing at the left ear control section  95   a  and processing at the right ear control section  95   b  are performed by two arithmetic processing circuits (not shown). These arithmetic processing circuits are CPUs, for example. When the processing is performed by two arithmetic processing circuits, there is a problem of increasing costs due to the necessity to provide the two arithmetic processing circuits. 
     In order to reduce the costs, it is conceivable to perform the processing at the left ear control section  95   a  and right ear control section  95   b  by a single arithmetic processing circuit. In this case, however, the amount of arithmetic processing to be performed increases as compared to the case where two arithmetic processing circuits are provided. For this reason, input/output delays at the left ear control section  95   a  and the right ear control section  95   b  increase. This consequently causes a problem that the above-described noise reduction effect to be obtained is extremely reduced. 
     Therefore, an object of the present invention is to provide a noise control device, which is capable of sufficiently producing the noise reduction effect without increasing an input/output delay at a control section even in the case where the processing is performed by a single arithmetic processing circuit. 
     Solution to the Problems 
     A first aspect of the present invention is a noise control device for reducing noises respectively arriving in a plurality of spaces which are acoustically independent from each other. The noise control device comprises: a plurality of sound output means, which are respectively provided in the plurality of spaces so as to respectively correspond to the plurality of spaces, each for outputting a sound to a corresponding space; first noise detection means, which is provided in at least one of the plurality of spaces, for detecting a noise arriving in the at least one of the plurality of spaces; and first signal generation means which is a single means for generating, based on the noise detected by one of the first noise detection means, a cancellation signal for canceling the noise, and outputting the generated cancellation signal to each of the plurality of sound output means. 
     In a second aspect of the present invention based on the above first aspect, the first signal generation means generates the cancellation signal such that a level of the cancellation signal increases in accordance with a decrease in a frequency of the cancellation signal. 
     In a third aspect of the present invention based on the above first aspect, the noise control device further comprises: second noise detection means, provided in a space which is not one of the plurality of spaces and in which a noise source generating the noise is present, for detecting the noise arriving from the noise source; and second signal generation means for generating, based on the noise detected by the second noise detection means, a cancellation signal for canceling the noise, and outputting the generated cancellation signal to each of the plurality of sound output means. 
     In a fourth aspect of the present invention based on the above first aspect, a plurality of the first noise detection means are provided in the plurality of spaces, respectively. The noise control device further comprises third signal generation means, which are provided respectively corresponding to the plurality of the first noise detection means, each for generating, based on the noise detected by a corresponding one of the first noise detection means, the cancellation signal having a higher frequency than a predetermined frequency, and outputting the generated cancellation signal to one of the sound output means which is provided in a same space as that of the corresponding one of the first noise detection means. The first signal generation means generates, based on the noise detected by one of the plurality of first noise detection means, the cancellation signal having a frequency no higher than the predetermined frequency, and outputs the generated cancellation signal to each of the plurality of sound output means. 
     In a fifth aspect of the present invention based on the above fourth aspect, the predetermined frequency is lower than a frequency at which a phase lag occurs in an electroacoustic transfer function from an input of each sound output means to an output of a corresponding one of the first noise detection means which is provided in a same space as that of said each sound output means. 
     In a sixth aspect of the present invention based on the above first aspect, a plurality of the first noise detection means are provided in the plurality of spaces, respectively. The noise control device further comprises switching means for switching, among outputs of the plurality of the first noise detection means, an output of first noise detection means to which an input of the first signal generation means is to be connected. In accordance with an operation by a user, the switching means switches the output of the first noise detection means to which the input of the first signal generation means is to be connected, to an output of first noise detection means which is most closely provided to the noise source generating the noise. 
     In a seventh aspect of the present invention based on the above first aspect, a plurality of the first noise detection means are provided in the plurality of spaces, respectively. The noise control device further comprises: switching means for switching, among outputs of the plurality of the first noise detection means, an output of first noise detection means to which an input of the first signal generation means is to be connected; and level detection means for detecting a level of the noise detected by each of the plurality of the first noise detection means. The switching means switches the output of the first noise detection means to which the input of the first signal generation means is to be connected, to an output of first noise detection means for which a highest level has been detected by the level detection means. 
     In an eighth aspect of the present invention based on the above first aspect, a plurality of the first noise detection means are provided in the plurality of spaces, respectively. The noise control device further comprises: switching means for switching, among outputs of the plurality of the first noise detection means, an output of first noise detection means to which an input of the first signal generation means is to be connected; and calculation means for calculating a cross-correlation function for noises respectively detected by the plurality of the first noise detection means. The switching means switches the output of the first noise detection means, based on the cross-correlation function calculated by the calculation means. 
     In a ninth aspect of the present invention based on the above first aspect, the noise control device further comprises: audio signal output means for outputting an audio signal to each of the plurality of sound output means; fourth signal generation means for generating a cancellation signal for canceling the audio signal outputted from the audio signal output means; and an adder for adding a signal, which is based on a sound detected by one of the first noise detection means, to the cancellation signal generated by the fourth signal generation means, and outputting the added signal to the first signal generation means. The signal based on the sound detected by one of the first noise detection means contains a signal which is based on the noise arriving in a space in which said one of the first noise detection means is provided, and contains the audio signal outputted from the audio signal output means via the sound output means provided in a same space as that of said one of the first noise detection means. 
     A tenth aspect of the present invention is an integrated circuit for reducing noises respectively arriving in a plurality of spaces which are acoustically independent from each other. The integrated circuit comprises: an input terminal to which an output from one of noise detection means is inputted, which noise detection means is provided in at least one of the plurality of spaces and detects a noise arriving in the at least one of the plurality of spaces in which the noise detection means is provided; signal generation means which is a single means for generating, based on the output from said one of the noise detection means which is inputted to the input terminal, a cancellation signal for canceling the noise detected by said one of the noise detection means; and an output terminal for outputting the cancellation signal, which is generated by the signal generation means, to each of sound output means which are respectively provided in the plurality of spaces so as to respectively correspond to the plurality of spaces and each of which outputs a sound to a corresponding space. 
     An eleventh aspect of the present invention is a headphone apparatus for reducing noises respectively arriving in two spaces which are acoustically independent from each other and which are respectively formed near left and right ears of a user. The headphone apparatus comprises: left ear sound output means, which is provided at a space formed near the left ear, for outputting a sound in the space; right ear sound output means, which is provided at a space formed near the right ear, for outputting a sound in the space; noise detection means, which is provided in at least one of the two spaces, for detecting a noise arriving in the at least one of the two spaces; and signal generation means which is a single means for generating, based on the noise detected by one of the noise detection means, a cancellation signal for canceling the noise, and outputting the generated cancellation signal to the left ear sound output means and to the right ear sound output means. 
     Effect of the Invention 
     According to the above first aspect, a noise reduction control is performed for the plurality of spaces which are acoustically independent from each other, by using a common cancellation signal generated by the single first signal generation means. In other words, according to this aspect, the single first signal generation means is used in common for the plurality of acoustically independent spaces. Here, the noises respectively arriving in the plurality of acoustically independent spaces are highly correlated to each other in a low frequency band. For this reason, when the single first signal generation means is used in common for the plurality of acoustically independent spaces, the noises respectively arriving in the plurality of acoustically independent spaces can be sufficiently reduced. As a result, according to this aspect, the number of first signal generation means each of which performs a large amount of processing can be reduced to 1, while sufficiently producing the noise reduction effect. Consequently, according to this aspect, a noise control device, which is capable of preventing an increase in an input/output delay at the first signal generation means even in the case where the processing at the first signal generation means is performed by a single arithmetic processing circuit, can be provided. 
     According to the above second aspect, an increase in the noise, which the user may feel due to the cancellation sound having a low correlation in other frequency bands than the low frequency band, can be avoided without newly providing a control circuit. 
     According to the above third aspect, the noise reduction effect can be further enhanced. 
     According to the above fourth aspect, since the first and second signal generation means respectively generate cancellation signals having different frequency bands from each other, processing loads on the first and second signal generation means can be reduced. 
     According to the above fifth aspect, an optimal control can be performed in accordance with the phase lag of the electroacoustic transfer function. This allows a frequency band, in which the noise reduction effect is obtained, to be further widened. 
     According to the above sixth to eighth aspects, an optimal noise reduction effect can be produced in accordance with an arrival direction of the noise. 
     According to the above ninth aspect, noise reduction and audio signal reproduction can be performed concurrently without affecting the audio signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary calculation result of a coherence function. 
         FIG. 2  shows a configuration of a noise control device according to a first embodiment. 
         FIG. 3  shows, by blocks of signal processing, an exemplary configuration of the noise control device shown in  FIG. 2 . 
         FIG. 4A  shows a noise reduction effect near a left ear. 
         FIG. 4B  shows a noise reduction effect near a right ear. 
         FIG. 5  shows another exemplary configuration of a control section  15  shown in  FIG. 3 . 
         FIG. 6  shows another exemplary configuration of the control section  15  shown in  FIG. 3 . 
         FIG. 7  shows a configuration in which the noise control device shown in  FIG. 2  further comprises an external microphone  14   c , a feedforward control section  16  and an adder  17 . 
         FIG. 8  shows a configuration in which a noise reduction function and an audio signal outputting function are combined. 
         FIG. 9  shows a configuration of a noise control device according to a second embodiment. 
         FIG. 10  shows a configuration of a control section  15   a.    
         FIG. 11  shows a configuration which is a result of further adding, to the configuration of the noise control device shown in  FIG. 9 , an echo canceling section  26  and a subtractor  27 . 
         FIG. 12  shows a configuration of a noise control device according to a third embodiment. 
         FIG. 13A  shows a state where there is a noise source at a left ear side of a user  10 . 
         FIG. 13B  shows a waveform, along a temporal axis, of a noise which is detected by a left ear microphone  14   a  in an environment illustrated in  FIG. 13A . 
         FIG. 13C  shows a waveform, along a temporal axis, of a noise which is detected by a right ear microphone  14   b  in the environment illustrated in  FIG. 13A . 
         FIG. 14A  shows a frequency characteristic of a detection signal e R  of the right ear microphone  14   b  in the case where a control is performed using a detection signal e L  of the left ear microphone  14   a.    
         FIG. 14B  shows a frequency characteristic of the detection signal e L  of the left ear microphone  14   a  in the case where a control is performed using the detection signal e R  of the right ear microphone  14   b.    
         FIG. 15  shows a configuration which is a result of newly adding a microphone determination section  31  and a switching control section  32  to the configuration shown in  FIG. 12 . 
         FIG. 16  shows a result of analyzing, when control is performed, frequencies of the detection signal e L  of the left ear microphone  14   a  and the detection signal e R  of the right ear microphone  14   b , and a result of analyzing, when the control is not performed, the frequencies of the detection signal e L  of the left ear microphone  14   a  and the detection signal e R  of the right ear microphone  14   b.    
         FIG. 17  shows a configuration which is a result of newly having, in the configurations shown in  FIGS. 12 and 15 , an echo canceling section  26  described in the second embodiment. 
         FIG. 18  shows a configuration of a first use form in which the noise control device according to the first embodiment is used. 
         FIG. 19  shows a configuration of a second use form which is further developed from the noise control device according to the second embodiment. 
         FIG. 20  shows a configuration of a conventional noise-canceling headphone. 
         FIG. 21  shows, by blocks of signal processing, the configuration of the noise-canceling headphone of  FIG. 20 . 
         FIG. 22  shows a configuration in which a noise reduction function and an audio signal outputting function are combined. 
         FIG. 23  shows a configuration of a noise-canceling headphone capable of widening a frequency band in which a noise reduction effect can be maintained. 
     
    
    
     
       
         
           
               
             
               
                   
               
               
                 DESCRIPTION OF THE REFERENCE CHARACTERS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                  11 
                 headband 
               
               
                   
                  12a 
                 left ear case 
               
               
                   
                  12b 
                 right ear case 
               
               
                   
                  13a 
                 left ear speaker 
               
               
                   
                  13b 
                 right ear speaker 
               
               
                   
                  14a 
                 left ear microphone 
               
               
                   
                  14b 
                 right ear microphone 
               
               
                   
                  14c 
                 external microphone 
               
               
                   
                  15, 15a, 15b, 15c 
                 control section 
               
               
                   
                 151 
                 feedback control filter 
               
               
                   
                 152 
                 phase inverter 
               
               
                   
                 153 
                 echo canceling filter 
               
               
                   
                 154, 20, 27, 34 
                 subtractor 
               
               
                   
                 155 
                 filtered X filter 
               
               
                   
                 156 
                 coefficient update section 
               
               
                   
                 157 
                 adaptive filter 
               
               
                   
                 158, 159 
                 low-pass filter 
               
               
                   
                  16 
                 feedforward control section 
               
               
                   
                  17, 21a, 21b 
                 adder 
               
               
                   
                  18 
                 audio signal output section 
               
               
                   
                  19 
                 audio signal canceling section 
               
               
                   
                  25a 
                 left ear high frequency control section 
               
               
                   
                  25b 
                 right ear high frequency control section 
               
               
                   
                  26 
                 echo canceling section 
               
               
                   
                  30, 33 
                 switching section 
               
               
                   
                  31 
                 microphone determination section 
               
               
                   
                  32 
                 switching control section 
               
               
                   
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to describing noise control devices according to the embodiments of the present invention, a concept of the present invention will be described. In a headphone apparatus or the like, spaces which are acoustically independent from each other are formed near right and left ears of a user, respectively. For these spaces, a correlation between a noise arriving in the space formed near the left ear and a noise arriving in the space formed near the right ear is obtained using a coherence function. 
     The coherence function indicates a degree of correlation between the two noises. To be specific, when it is assumed that: the coherence function is γ 2  (f); a power spectrum of a noise signal N L  based on the noise near the left ear is S LL  (f); a power spectrum of a noise signal N R  based on the noise near the right ear is S RR  (f); and a cross spectrum of the noise signals N L  and N R  is S LR  (f), the coherence function γ 2  (f) can be represented by an equation (3). Here, f is a frequency. 
     
       
         
           
             
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       γ 
                       2 
                     
                     ⁡ 
                     
                       ( 
                       f 
                       ) 
                     
                   
                   = 
                   
                     
                       
                          
                         
                           
                             S 
                             LR 
                           
                           ⁡ 
                           
                             ( 
                             f 
                             ) 
                           
                         
                          
                       
                       2 
                     
                     
                       
                         
                           S 
                           LL 
                         
                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                       ⁢ 
                       
                         
                           S 
                           RR 
                         
                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     When the coherence function was calculated based on the equation (3), a result as shown in  FIG. 1  was obtained.  FIG. 1  shows an exemplary calculation result of the coherence function. The result in  FIG. 1  shows that a value of the coherence function increases in accordance with a decrease in a frequency of the noises. Here, the greater the value of the coherence function, the higher is the correlation between the two noises. Thus, it is understood from the result shown in  FIG. 1  that the correlation between the noise near the left ear and the noise near the right ear increases in accordance with a decrease in the frequency of the noises. Note that, the result in  FIG. 1  shows that the correlation is extremely high, particularly in a low frequency band no higher than 100 Hz. 
     As described above, it has been discovered with respect to the acoustically independent spaces respectively formed near the left and right ears of the user that the correlation between the noise near the left ear and the noise near the right ear increases in accordance with a decrease in the frequency of the noises. This discovery means that when a cancellation signal for canceling a noise arriving in one of the spaces is used for the other space, a noise in a low frequency band can be canceled from a noise arriving in the other space. In other words, this discovery means that when a cancellation signal for canceling a noise arriving in one of the spaces is used for the other space, a noise arriving in the other space is sufficiently reduced. 
     Accordingly, in the present invention, for the acoustically independent spaces respectively formed near the left and right ears of the user, a cancellation signal for canceling a noise arriving in one of the spaces is used for the other space. That is, in the present invention, a control section for generating the cancellation signal is used in common for the two acoustically independent spaces. This allows the present invention to reduce, while producing a sufficient noise reduction effect, the number of control sections each of which performs a great amount of arithmetic processing. Consequently, the present invention can provide a noise control device capable of preventing an increase in an input/output delay at a control section even in the case where the processing at the control section is performed by a single arithmetic processing circuit. 
     First Embodiment 
     Hereinafter, a noise control device according to a first embodiment of the present invention will be described with reference to the drawings. First, a configuration of the noise control device according to the present embodiment will be described with reference to  FIG. 2 .  FIG. 2  shows the configuration of the noise control device according to the first embodiment. Note that,  FIG. 2  shows a configuration in the case where the noise control device according to the present embodiment is applied in a headphone apparatus.  FIG. 2  and later-described  FIGS. 3 ,  7  and  8  each is a diagram which shows a view seen from above a head of a user  10  and in which the user  10  faces upward. 
     As shown in  FIG. 2 , the noise control device comprises a headband  11 , a left ear case  12   a , a right ear case  12   b , a left ear speaker  13   a , a right ear speaker  13   b , a left ear microphone  14   a  and a control section  15 . The left ear case  12   a  is placed near a left ear of the user  10 , and the left ear case  12   a  has a space formed therein. The right ear case  12   b  is placed near a right ear of the user  10 , and the right ear case  12   b  has a space formed therein. The left ear case  12   a  and the right ear case  12   b  are connected by the headband  11 . The left ear speaker  13   a  is provided within the left ear case  12   a . The right ear speaker  13   b  is provided within the right ear case  12   b . The left ear speaker  13   a  has a same characteristic as that of the right ear speaker  13   b . The left ear microphone  14   a  is provided within the left ear case  12   a.    
     The spaces respectively formed within the left ear case  12   a  and the right ear case  12   b  are acoustically independent from each other. As described above, being acoustically independent means that an acoustic state is such that a gain of an electroacoustic transfer function between the spaces is sufficiently small. In other words, the acoustic state is such that when a sound radiated from a speaker provided in one of the spaces has arrived in the other space, a level of the sound having arrived in the other space is sufficiently small. For example, the acoustically independent spaces are, in the headphone apparatus in  FIG. 2 , a space formed near one ear and a space formed near the other ear. As another example, the acoustically independent spaces are spaces respectively formed in adjacent rooms separated by a wall or the like. 
     Next, operations of the noise control device according to the present embodiment will be described. The left ear microphone  14   a  detects a noise arriving in the left ear case  12   a . The left ear microphone  14   a  outputs, as a detection signal e L  to the control section  15 , a noise signal based on the detected noise. The control section  15  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  such that the level is lowered. The control section  15  outputs the generated control signal to the left ear speaker  13   a  and to the right ear speaker  13   b . Thus, in the noise control device according to the present embodiment, the single control section  15  is used in common for the two acoustically independent spaces. 
     Near the left ear, a sound based on the control signal generated by the control section  15  is outputted from the left ear speaker  13   a . As a result, the sound based on the control signal and the noise are canceled by each other near the left ear. Thus, the control signal is a cancellation signal for canceling the noise. 
     In the case where the sound based on the control signal and the noise are not entirely canceled near the left ear, a control error is detected by the left ear microphone  14   a , which control error is a residual component occurring as a result of synthesizing the sound based on the control signal and the noise. The left ear microphone  14   a  outputs, as the detection signal e L  to the control section  15 , an error signal based on the control error. Thus, near the left ear, the left ear microphone  14   a , the control section  15  and the left ear speaker  13   a  form a feedback loop. The feedback loop causes the noise control device to operate such that the control error attenuates. 
     Near the right ear, a sound is outputted from the right ear speaker  13   b , the sound being the same as the sound which is based on the control signal and which is outputted near the left ear. As shown in  FIG. 1 , a noise arriving in the right ear case  12   b  is highly correlated, in the low frequency band, to a noise arriving in the left ear case  12   a . For this reason, near the right ear, the noise in the low frequency band which has a high correlation is canceled by the sound which is based on the control signal and which is outputted near the left ear. Thus, the control section  15  generates the cancellation signal to be used in common in the vicinity of each of the right and left ears. The control section  15  corresponds to first signal generation means of the present invention. 
     Further, the noise control device according to the present embodiment comprises: a microphone amplifier for amplifying the detection signal e L  detected by the left ear microphone  14   a : and a speaker amplifier for amplifying the control signal of the control section  15  so as to drive the left ear speaker  13   a  and the right ear speaker  13   b . However, these components are omitted in  FIG. 2 . 
     Next, a configuration and processing of the control section  15  will be described in detail with reference to  FIG. 3 .  FIG. 3  shows, by blocks of signal processing, an exemplary configuration of the noise control device shown in  FIG. 2 . It is assumed for  FIG. 3  that components, which are denoted by the same reference numerals as those used for components in  FIG. 2 , have the same functions as those of the components in  FIG. 2 , and descriptions thereof will be omitted. 
     A block  121   a  in the left ear case  12   a  indicates an electroacoustic transfer function H L  from an input of the left ear speaker  13   a  to an output of the left ear microphone  14   a . A block  121   b  in the right ear case  12   b  indicates an electroacoustic transfer function H R  from an input of the right ear speaker  13   b  to an output of a right ear microphone  14   b . An adder  122   a  adds an output signal of the block  121   a  to the noise signal N L  indicating the noise arriving in the left ear case  12   a . A signal outputted from the adder  122   a  is the aforementioned detection signal e L . 
     The control section  15  comprises a feedback control filter  151  and a phase inverter  152 . For the feedback control filter  151 , a filter coefficient indicating a transfer function C L  is set. The detection signal e L  outputted from the adder  122   a  is inputted to the feedback control filter  151 . The phase inverter  152  inverts a phase of an output signal of the feedback control filter  151 . An output signal from the phase inverter  152  is inputted to the block  121   a  and to the block  121   b . Here, a transfer function from the noise signal N L  to the detection signal e L  is represented by an equation (4). 
     
       
         
           
             
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       e 
                       L 
                     
                     
                       N 
                       L 
                     
                   
                   = 
                   
                     1 
                     
                       1 
                       + 
                       
                         
                           C 
                           L 
                         
                         ⁢ 
                         
                           H 
                           L 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Note that, the transfer function C L  of the feedback control filter  151  is, as shown in an equation (5), set so as to have an inverse characteristic to that of the electroacoustic transfer function H L  at the left ear. Here, α indicates a filter gain of a fixed frequency. 
     
       
         
           
             
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     C 
                     L 
                   
                   = 
                   
                     α 
                     
                       H 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Here, as is clear from the equation (1), when a noise arrives in the left ear case  12   a , the left ear microphone  14   a  outputs N L /(1+C L ×H L ) as the detection signal e L . The detection signal e L  is inputted to the feedback control filter  151 . At this point, the control signal generated at the feedback control filter  151  is C L ×N L /(1+C L ×H L ). Since the transfer function C L  is set as shown in the equation (5), the control signal is N L (H L ×(1+1/α)). The control signal is inputted to the block  121   a  after a phase of the control signal is inverted by the phase inverter  152 . Accordingly, a cancellation sound radiated from the left ear speaker  13   a  to the vicinity of the left ear is −H L ×N L /(H L ×(1+1/α))=−N L /(1+1/α). As a result, the greater the filter gain α, the nearer to −N L  the cancellation sound becomes, whereby the noise arriving near the left ear is canceled. 
     On the other hand, a cancellation sound radiated from the right ear speaker  13   b  to the vicinity of the right ear is −H R ×N L /(H L ×(1+1/α)). Here, the left ear speaker  13   a  and the right ear speaker  13   b  have a same characteristic. That is, a relationship H L ≈H R  is realized. Also, as shown in  FIG. 1 , a relational equation N L ≈N R  is realized for the noise in the low frequency band. Further, when it is assumed that the filter gain α is large and a relational equation 1/α≈0 is realized, an equation (6) is realized for the noise in the low frequency band. As a result, the noise in the low frequency band is canceled near the right ear. 
     
       
         
           
             
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       N 
                       R 
                     
                     + 
                     
                       ( 
                       
                         
                           H 
                           R 
                         
                         ⁢ 
                         
                           
                             - 
                             
                               N 
                               L 
                             
                           
                           
                             
                               H 
                               L 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   1 
                                   / 
                                   α 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                   ≅ 
                   
                     
                       N 
                       R 
                     
                     - 
                     
                       N 
                       L 
                     
                   
                   ≅ 
                   0 
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     As described above, the noise control device according to the present embodiment performs a control so as to reduce the noises for the two acoustically independent spaces, by using the common control signal generated by the single control section  15 . In other words, the noise control device according to the present embodiment uses the control section  15  for common use between the two acoustically independent spaces. Here, the noises respectively arriving in the two acoustically independent spaces are highly correlated to each other in the low frequency band as shown in  FIG. 1 . For this reason, the noise arriving in the left ear case  12   a  can be canceled for all the frequency bands, and the noise arriving in the right ear case  12   b  can be canceled for the low frequency band. In other words, even if the control section  15  is used in common for the two acoustically independent spaces, the noises respectively arriving in the two acoustically independent spaces can be reduced sufficiently. Thus, the noise control device according to the present embodiment can reduce, while sufficiently producing the noise reduction effect, the number of control sections  15  each of which performs a large amount of arithmetic processing. Consequently, according to the present embodiment, the noise control device, which is capable of preventing an input/output delay at the control section  15  from increasing even in the case where the processing at the control section  15  is performed by a single arithmetic processing circuit, can be provided. 
     Further, the noise control device according to the present embodiment performs a control for the two acoustically independent spaces. Therefore, in the noise control device according to the present embodiment, there is no necessity to take into account a leak of the cancellation sound (crosstalk) from the right ear speaker  13   b  to the left ear microphone  14   a . Accordingly, the noise control device according to the present embodiment provides an advantage that there is no necessity to provide a circuit for controlling the leak of the cancellation sound. 
     In the processing at the control section  15  illustrated in  FIG. 3 , the sound outputted near the right ear is the same as the sound which is based on the control signal and which is outputted near the left ear. Thus, a cancellation sound, which has a low correlation and which is in a frequency band different from the low frequency band, is outputted near the right ear. Here, in the case where the cancellation sound, which is in a frequency band in which the correlation thereof is low, is outputted near the right ear, there may be a case where the cancellation sound does not have a same amplitude as and is not in antiphase to the noise arriving in the right ear case  12   b , since the cancellation sound has a high frequency. In the case where the cancellation sound does not have a same amplitude as and is not antiphase to the noise arriving in the right ear case  12   b , the cancellation sound is, in the frequency band thereof, superimposed on the noise, and the noise is increased accordingly. In other words, the user  10  feels that the noise has increased in this frequency band. In this case, it is preferred to cause the control section  15  to generate a control signal whose characteristic corresponds to a frequency characteristic of the coherence function shown in  FIG. 1 . Since, in this case, a frequency characteristic of the cancellation sound corresponds to the frequency characteristic of the coherence function, the increase in the noise which the user  10  feels can be avoided without newly providing a control circuit. 
     Note that, the characteristic of the control signal, which corresponds to the frequency characteristic of the coherence function, is such that a level of the control signal increases in accordance with a decrease in a frequency of the control signal. This characteristic may, e.g., simulate the frequency characteristic of the coherence function, or may be such that in the case where a predetermined frequency is set as a reference frequency, the level of the control signal is at a fixed value when the frequency of the control signal is no higher than the reference frequency, and the level of the control signal decreases from the fixed value in accordance with an increase in the frequency of the control signal from the reference frequency. 
       FIGS. 4A  and B show noise reduction effects in the case where the control section  15  generates a control signal having the characteristic corresponding to the frequency characteristic of the coherence function. In  FIGS. 4A  and B, a reference frequency is set to 150 Hz, and the control signal used herein has a characteristic such that the level of the control signal is at a fixed value when the frequency thereof is no higher than 150 Hz, and the level of the control signal decreases from the fixed value in accordance with an increase in the frequency thereof from 150 Hz.  FIG. 4A  shows a noise reduction effect near the left ear.  FIG. 4B  shows a noise reduction effect near the right ear. As shown in  FIG. 4A , near the left ear, a level of the noise in a low frequency band no higher than 150 Hz is sufficiently reduced when a control is performed, as compared to when the control is not performed. Also as shown in  FIG. 4B , near the right ear, a level of the noise in the frequency band no higher than 150 Hz is reduced when the control is performed as compared to when the control is not performed. Although the amount of the reduced level near the right ear is smaller than that near the left ear, it is clearly understood that the sufficient noise reduction effect, which is no smaller than 10 db, is obtained. 
     Still further, the configuration of the above-described control section  15  is not limited to the configuration shown in  FIG. 3 . The control section  15  may further comprise, as shown in  FIG. 5 , an echo cancellation filter  153  and a subtractor  154 .  FIG. 5  shows another exemplary configuration of the control section  15  shown in  FIG. 3 . The echo cancellation filter  153  is a filter for canceling echo which contributes to howling. For the echo cancellation filter  153 , a filter coefficient indicating a transfer function E L  is set. The subtractor  154  subtracts an output signal of the echo cancellation filter  153  from the detection signal e L  outputted from the adder  122   a . An output signal from the subtractor  154  is inputted to the feedback control filter  151 . An output signal from the phase inverter  152  is inputted to the echo cancellation filter  153  and the blocks  121   a  and  121   b . Here, a transfer function from the noise signal N L  to the detection signal e L  is represented by an equation (7). 
     
       
         
           
             
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       e 
                       L 
                     
                     
                       N 
                       L 
                     
                   
                   = 
                   
                     
                       1 
                       - 
                       
                         
                           C 
                           L 
                         
                         ⁢ 
                         
                           E 
                           L 
                         
                       
                     
                     
                       1 
                       + 
                       
                         
                           C 
                           L 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               H 
                               L 
                             
                             - 
                             
                               E 
                               L 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Here, the transfer function E L  of the echo cancellation filter  153  is set so as to simulate the electroacoustic transfer function H L  at the left ear. In this case, the denominator of the equation (7) is 1, and the control section  15  always operates stably. Further, the transfer function C L  of the feedback control filter  151  is set, as shown in the equation (5), so as to have an inverse characteristic to that of the electroacoustic transfer function H L  at the left ear. In this case, the right-hand side of the equation (7) is 0, and the noise near the left ear is canceled. Thus, when the control section  15  has the configuration shown in  FIG. 5 , the feedback loop is stabilized. Consequently, an occurrence of an unusual sound due to oscillation, such as howling, can be suppressed. 
     Still further, the above-described control section  15  may have a structure shown in  FIG. 6 .  FIG. 6  shows another exemplary configuration of the control section  15  shown in  FIG. 3 . In  FIG. 6 , the control section  15  comprises a filtered X filter  155 , a coefficient update section  156 , an adaptive filter  157  and the phase inverter  152 . The filtered X filter  155  is a filter for which a filter coefficient simulating the electroacoustic transfer function H L  is set. The coefficient update section  156  sequentially calculates a filter coefficient based on the LMS algorithm, thereby updating a filter coefficient to be set for the adaptive filter  157 . The adaptive filter  157  is a filter for which the set filter coefficient can be sequentially updated. It is assumed here that each component of the control section  15  shown in  FIG. 6  is structured by a digital circuit. In the case where each component of the control section  15  is structured by a digital circuit, the control section  15  comprises, although not shown in  FIG. 6 , an analogue/digital converter, a digital/analogue converter, an anti-aliasing filter and the like. 
     The coefficient update section  156  sequentially calculates, based on an update equation shown as an equation (8), the filter coefficient such that a level of the detection signal e L  outputted from the adder  122   a  is lowered.
 
[equation 8]
 
 w ( k+ 1)= w ( k )+2 μe   L ( k )×( k )  (8)
 
Here, w(k) is a filter coefficient vector at a sampling time k; μ is an adaptive step size; e L (k) is the detection signal at the sampling time k; and x(k) is an input vector at the sampling time k. Also, x(k) is a result of converting an output signal of the filtered X filter  155  into a vector from a sampling time k−m+1 to the sampling time k (m is the number of filter taps of the adaptive filter  157 ). The filter coefficient calculated by the coefficient update section  156  is set as a filter coefficient for the adaptive filter  157 . The coefficient update section  156  terminates the calculation at a point when the detection signal e L  has become small and converged. By using the filter coefficient which is set for the adaptive filter  157  at this termination point, the noises near both the right and left ears can be reduced, similarly to the processing illustrated in  FIG. 3 . Note that, the echo cancellation filter  153  and the subtractor  154  shown in  FIG. 5  may be further added to the configuration shown in  FIG. 6 .
 
     Although, in the noise control device shown in  FIG. 2 , the left ear microphone  14   a  for detecting a noise is provided within the left ear case  12   a , the present invention is not limited thereto. Such a microphone for detecting a noise may be provided not within the left ear case  12   a  but within the right ear case  12   b . In this case, the filter coefficient for the feedback control filter  151 , which is a component of the control section  15  shown in  FIG. 3 , is set so as to have an inverse characteristic to that of the electroacoustic transfer function H R  at the right ear. 
     Further, the noise control device shown in  FIG. 2  is applied in a headphone apparatus. However, the present invention is not limited thereto. The noise control device according to the present embodiment may be applied in any device as long as there is a necessity in said any device to reduce noises arriving in acoustically independent spaces. 
     Still further, in the noise control device shown in  FIG. 2 , the two spaces within the left ear case  12   a  and the right ear case  12   b  are assumed to be the acoustically independent spaces. However, the number of spaces is not limited to 2. There may be three or more acoustically independent spaces. In such a case, the spaces are each provided with a speaker; at least one of the spaces is provided with a microphone; and only one control section  15  is provided. The control section  15  generates a control signal for canceling a noise detected by the microphone, and outputs a common control signal to the speaker provided in each space. 
     Still further, in the noise control device shown in  FIG. 2 , the noise canceling control is performed only by the feedback control using the detection signal e L  of the left ear microphone  14   a  provided within the left ear case  12   a . However, the noise control device shown in  FIG. 2  may further comprise, as shown in  FIG. 7 , an external microphone  14   c , a feedforward control section  16  and an adder  17 .  FIG. 7  shows a configuration in which the noise control device shown in  FIG. 2  further comprises the external microphone  14   c , the feedforward control section  16  and the adder  17 . 
     The external microphone  14   c  is provided outside the left ear case  12   a . An external space of the left ear case  12   a  is not acoustically independent but has a noise source. The external microphone  14   c  detects a noise which is present outside the left ear case  12   a . In other words, the external microphone  14   c  detects a noise arriving from the noise source. The external microphone  14   c  outputs an external noise signal, which is based on the detected external noise, as an external detection signal e o  to the feedforward control section  16 . Based on a filter coefficient indicating a transfer function G which has been set, the feedforward control section  16  generates, as a control signal, a cancellation signal for canceling the external detection signal e o . Thus, the feedforward control section  16  generates the cancellation signal for canceling the external noise. The feedforward control section  16  corresponds to second signal generation means of the present invention. 
     The transfer function G of the feedforward control section  16  may be set such that an equation (9) is satisfied when an electroacoustic transfer function from a position of the external microphone  14   c  to a position of the left ear microphone  14   a  is H. Note that, H L  in the equation (9) is an electroacoustic transfer function from an input of the left ear speaker  13   a  to an output of the left ear microphone  14   a.  
 
[equation 9]
 
 H+H   L   G= 0  (9)
 
     As is clear from the equation (9), the transfer function G of the feedforward control section  16  is set such that G=−H/H L . By having this configuration, a noise reduction effect by feedforward control is further obtained in addition to the noise reduction effect by the feedback control. Consequently, a further enhanced noise reduction effect is obtained. 
     Although the noise control device shown in  FIG. 2  has a configuration which has only a noise reduction function, the control device may have a configuration in which the noise reduction function and an audio signal outputting function are combined.  FIG. 8  shows the configuration in which the noise reduction function and the audio signal outputting function are combined. It is assumed for  FIG. 8  that components, which are denoted by the same reference numerals as those used for components in  FIG. 2 , have the same functions as those of the components in  FIG. 2 , and descriptions thereof will be omitted. 
     The configuration shown in  FIG. 8  is a result of adding, to the configuration shown in  FIG. 2 , an audio signal output section  18 , an audio signal canceling section  19 , a subtractor  20  and adders  21   a  and  21   b . The audio signal output section  18  outputs stereo audio signals such as music. As shown in  FIG. 8 , the audio signal output section  18  outputs an audio signal A L  to the left ear and an audio signal A R  to the right ear. The audio signal canceling section  19  generates, based on a filter coefficient indicating a transfer function simulating the electroacoustic transfer function H L , a cancellation signal for canceling the audio signal A L . Thus, the audio signal canceling section  19  generates the cancellation signal for canceling the audio signal A L . The audio signal canceling section  19  corresponds to fourth signal generation means of the present invention. The subtractor  20  subtracts, from the detection signal e L , the cancellation signal for canceling the audio signal A L . An output signal of the subtractor  20  is inputted to the control section  15 . A control signal outputted from the control section  15  is added by the adder  21   a  to the audio signal A L . An output signal from the adder  21   a  is inputted to the left ear speaker  13   a . The left ear speaker  13   a  outputs a sound based on the control signal and the audio signal A L . Similarly, the control signal outputted from the control section  15  is added by the adder  21   b  to the audio signal A R . An output signal from the adder  21   b  is inputted to the right ear speaker  13   b . The right ear speaker  13   b  outputs a sound based on the control signal and the audio signal A R . 
     Here, the detection signal e L  from the left ear microphone  14   a  contains the audio signal A L . However, the subtractor  20  subtracts, from the detection signal e L , the cancellation signal for canceling the audio signal A L . Consequently, the audio signal A L  is not inputted to the control section  15 , and the same processing as that illustrated in  FIG. 3  is performed by the control section  15 . 
     As described above, according to the configuration shown in  FIG. 8 , noise reduction and stereo audio signal reproduction can be performed concurrently. Further, according to the configuration shown in  FIG. 8 , noises respectively arriving near both the ears can be reduced without affecting audio signals. Note that, the audio signal output section  18  may output not only stereo audio signals but also monaural signals to both the ears. Further, the audio signal output section  18  may downmix multichannel audio signals, e.g., DVD contents, and output resultant signals to both the ears. 
     Second Embodiment 
     Hereinafter, a noise control device according to a second embodiment of the present invention will be described with reference to the drawings. Usually, in a high frequency band, a phase lag of each of the aforementioned electroacoustic transfer functions H L  and H R  occurs. Accordingly, there is a case where even if the transfer function C L  of the control section  15  described in the first embodiment is set so as to have the inverse characteristic to that of the electroacoustic transfer function H L , the transfer function C L  does not have the inverse characteristic in a high frequency band, whereby the noise reduction effect decreases. For this reason, in the present invention, for a frequency band which is higher than a predetermined frequency and in which a phase lag of the electroacoustic transfer function occurs, a control is separately performed by using a high frequency control section for which a filter coefficient based on the electroacoustic transfer function having the phase lag is set. 
     Hereinafter, the noise control device according to the second embodiment will be described with reference to  FIG. 9 .  FIG. 9  shows a configuration of the noise control device according to the second embodiment. It is assumed for  FIG. 9  that components, which are denoted by the same reference numerals as those used for components of the noise control device according to the first embodiment shown in  FIG. 2 , have the same functions as those of the components of the noise control device shown in  FIG. 2 , and detailed descriptions thereof will be omitted. Note that, FIG.  9  and later-described  FIG. 11  each are a diagram which shows a view seen from above a head of the user  10  and in which the user  10  faces upward. 
     As shown in  FIG. 9 , the noise control device comprises the headband  11 , the left ear case  12   a , the right ear case  12   b , the left ear speaker  13   a , the right ear speaker  13   b , the left ear microphone  14   a , the right ear microphone  14   b , a control section  15   a , the adders  21   a  and  21   b , a left ear high frequency control section  25   a , and a right ear high frequency control section  25   b . The configuration shown in  FIG. 9  is different from the first embodiment shown in  FIG. 2  in that the configuration shown in  FIG. 9  newly comprises the right ear microphone  14   b , the adders  21   a  and  21   b , the left ear high frequency control section  25   a , and the right ear high frequency control section  25   b , and also, the control section  15  according to the first embodiment shown in  FIG. 2  is replaced with the control section  15   a . In this configuration, the right ear microphone  14   b  is provided within the right ear case  12   b , and detects a noise arriving in a space formed near the right ear of the user  10 . 
     Next, operations of the noise control device according to the present embodiment will be described. The left ear microphone  14   a  detects a noise arriving in the left ear case  12   a . The left ear microphone  14   a  outputs a noise signal, which is based on the detected noise, as the detection signal e L  to the control section  15   a  and to the left ear high frequency control section  25   a . The control section  15   a  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  such that the level is lowered, the control signal having a frequency no higher than a predetermined frequency. In other words, the control section  15   a  generates a cancellation signal for canceling a noise arriving in the left ear case  12   a , the noise having the frequency no higher than the predetermined frequency. Here, the predetermined frequency is lower than a frequency at which a phase lag of the electroacoustic transfer function H L  occurs. The control section  15   a  outputs the generated control signal to the adders  21   a  and  21   b . The left ear high frequency control section  25   a  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  such that the level is lowered, the control signal having a higher frequency than the predetermined frequency. In other words, the left ear high frequency control section  25   a  generates a cancellation signal for canceling a noise arriving in the left ear case  12   a , the noise having the higher frequency than the predetermined frequency. The left ear high frequency control section  25   a  outputs the generated control signal to the adder  21   a . The adder  21   a  adds the control signal generated by the control section  15   a  to the control signal generated by the left ear high frequency control section  25   a . A signal resulting from the addition at the adder  21   a  is inputted to the left ear speaker  13   a . The left ear speaker  13   a  outputs sounds based on the control signals generated by the control section  15   a  and the left ear high frequency control section  25   a . As a result, the sounds, which are based on the control signals, and the noises are canceled by each other near the left ear. 
     In the case where the sounds, which are based on the control signals, and the noises are not entirely canceled near the left ear, a control error is detected by the left ear microphone  14   a , which control error is a residual component occurring as a result of synthesizing the sounds, which are based on the control signals, and the noises. The left ear microphone  14   a  outputs an error signal, which is based on the control error, as the detection signal e L  to the control section  15   a  and to the left ear high frequency control section  25   a . Thus, the left ear microphone  14   a , the control section  15   a , the adder  21   a  and the left ear speaker  13   a  form a feedback loop near the left ear. Further, another feedback loop is formed near the left ear by the left ear microphone  14   a , the left ear high frequency control section  25   a , the adder  21   a  and the left ear speaker  13   a . These two feedback loops cause the noise control device to operate in such a manner that the control error near the left ear further attenuates as compared to the first embodiment. 
     Near the right ear, the right ear microphone  14   b  detects a noise arriving in the right ear case  12   b . The right ear microphone  14   b  outputs a noise signal, which is based on the detected noise, as the detection signal e R  to the right ear high frequency control section  25   b . The right ear high frequency control section  25   b  generates, based on the detection signal e R , a control signal for controlling a level of the detection signal e R  such that the level is lowered, the control signal having a higher frequency than a predetermined frequency. In other words, the right ear high frequency control section  25   b  generates a cancellation signal for canceling a noise arriving in the right ear case  12   b , the noise having the higher frequency than the predetermined frequency. The right ear high frequency control section  25   b  outputs the generated control signal to the adder  21   b . The adder  21   b  adds the control signal generated by the control section  15   a  to the control signal generated by the right ear high frequency control section  25   b . A signal resulting from the addition at the adder  21   b  is inputted to the right ear speaker  13   b . The right ear speaker  13   b  outputs sounds based on the control signals generated by the control section  15   a  and the right ear high frequency control section  25   b . Here, as shown in  FIG. 1 , the noise arriving in the right ear case  12   b  is highly correlated, in the low frequency band, to the noise arriving in the left ear case  12   a . Accordingly, near the right ear: a noise in the low frequency band, which has a high correlation, is canceled by the sound based on the control signal generated by the control section  15   a ; and the sound, which is based on the control signal generated by the right ear high frequency control section  25   b , and a noise, which is in a frequency band of the control signal, are canceled by each other. Thus, the control section  15   a  generates a cancellation signal for common use between the vicinities of the left and right ears. The control section  15   a  corresponds to the first signal generation means of the present invention. Also, the left ear high frequency control section  25   a  and the right ear high frequency control section  25   b  each generate a cancellation signal for canceling a noise in a high frequency band, and each correspond to third signal generation means of the present invention. Here, there is only one control section  15   a  for the spaces formed for the left and right ears. Further, the left ear high frequency control section  25   a  and the right ear high frequency control section  25   b  are provided respectively corresponding to the two spaces formed for the left and right ears. 
     In the case where the sounds, which are based on the control signals, and the noises are not entirely canceled near the right ear, a control error is detected by the right ear microphone  14   b , which control error is a residual component occurring as a result of synthesizing the sounds, which are based on the control signals, and the noises. The right ear microphone  14   b  outputs an error signal, which is based on the control error, as the detection signal e R  to the right ear high frequency control section  25   b . Thus, a feedback loop is formed near the right ear by the right ear microphone  14   b , the right ear high frequency control section  25   b , the adder  21   b  and the right ear speaker  13   b . This feedback loop causes the noise control device to operate in such a manner that the control error near the right ear attenuates. 
     Next, the configuration of the control section  15   a  will be described with reference to  FIG. 10 .  FIG. 10  shows the configuration of the control section  15   a . Here,  FIG. 10  shows, by way of example, the configuration in which the control section  15   a  is realized using an adaptive filter. The configuration of the control section  15   a  shown in  FIG. 10  is a result of adding, to the configuration of the control section  15  shown in  FIG. 6 , low-pass filters  158  and  159 . The low-pass filter  158  attenuates, from an output signal of the filtered X filter  155 , a high frequency component higher than a predetermined frequency. The low-pass filter  159  attenuates, from an output signal of the left ear microphone  14   a , a high frequency component higher than the predetermined frequency. For this reason, in the coefficient update section  156 , a filter coefficient is rarely updated for the high frequency component higher than the predetermined frequency. This allows the filter coefficient calculated by the coefficient update section  156  to converge to such a filter coefficient that a gain is obtained only in a low frequency band no higher than the predetermined frequency. The filter coefficient calculated by the coefficient update section  156  is set as a filter coefficient for the adaptive filter  157 . Accordingly, the control signal generated at the control section  15   a  is a signal which is generated based on a filter coefficient having an inverse characteristic to that of the electroacoustic transfer function H R  and which has the frequency no higher than the predetermined frequency. 
     The left ear high frequency control section  25   a  and the right ear high frequency control section  25   b  are realized by replacing, in the configuration of the control section  15   a  shown in  FIG. 10 , the low-pass filters  158  and  159  with high-pass filters. The high-pass filters each attenuate a low-frequency component of an inputted signal, which is no higher than a predetermined frequency. For this reason, in the coefficient update section  156 , a filter coefficient is rarely updated for the low-frequency component no higher than the predetermined frequency. Also, in the coefficient update section  156 , a filter coefficient, which has an inverse characteristic to that of an electroacoustic transfer function having a phase lag in the high frequency band higher than the predetermined frequency, is updated. This allows the filter coefficient calculated by the coefficient update section  156  to converge to the filter coefficient which has the inverse characteristic to that of the electroacoustic transfer function having a phase lag and which allows a gain to be obtained only in the high frequency band higher than the predetermined frequency. The filter coefficient calculated by the coefficient update section  156  is set as a filter coefficient for the adaptive filter  157 . Accordingly, the control signal generated at the left ear high frequency control section  25   a  is generated based on the filter coefficient having the inverse characteristic to that of the electroacoustic transfer function H L  having a phase lag, and has the higher frequency than the predetermined frequency. Also, the control signal generated at the right ear high frequency control section  25   b  is generated based on the filter coefficient having the inverse characteristic to that of the electroacoustic transfer function H R  having a phase lag, and has the higher frequency than the predetermined frequency. 
     As described above, separately for the high frequency band higher than the predetermined frequency in which the phase of the electroacoustic transfer function is lagged, the noise control device according to the present embodiment performs a control using the left ear high frequency control section  25   a  and the right ear high frequency control section  25   b  for each of which the filter coefficient based on the electroacoustic transfer function having a phase lag is set. In other words, the control section  15   a , and the left and right ear high frequency control sections  25   a  and  25   b , divide the frequency band, and the control signal is generated for each divided frequency band. This enables an optimal control to be performed in accordance with the phase lag of the electroacoustic transfer function. This consequently allows a frequency band, in which the noise reduction effect is obtained, to be further widened as compared to the first embodiment. Moreover, according to the noise control device of the present embodiment, the control section  15   a  is only required to generate the control signal whose frequency is no higher than the predetermined frequency. This reduces a processing load of the control section  15   a  as compared to a processing load of the control section  15  according to the first embodiment. 
     Note that, the configuration of the noise control device shown in  FIG. 9  may additionally have an echo canceling section  26  and a subtractor  27  as shown in  FIG. 11 .  FIG. 11  shows a configuration which is a result of adding, to the configuration of the noise control device shown in  FIG. 9 , the echo canceling section  26  and the subtractor  27 . The echo canceling section  26  cancels echo which contributes to howling, and has the same function as that of the echo cancellation filter  153  shown in  FIG. 5 . For the echo canceling section  26 , a filter coefficient indicating the transfer function E L  is set. The transfer function E L  is set so as to simulate the electroacoustic transfer function H L  at the left ear. The echo canceling section  26  processes, based on the filter coefficient indicating the transfer function E L , an output signal from the adder  21   a , and outputs the processed signal to the subtractor  27 . The subtractor  27  subtracts, from the detection signal e L , outputted from the left ear microphone  14   a , the output signal of the echo canceling section  26 . By additionally having the echo canceling section  26  as described above, processing can be stabilized for the feedback loop including the control section  15   a  and the feedback loop including the left ear high frequency control section  25   a . Consequently, an occurrence of an unusual sound due to oscillation, such as howling, can be suppressed. 
     Third Embodiment 
     Hereinafter, the noise control device according to a third embodiment of the present invention will be described with reference to the drawings. The noise control device according to the present embodiment is, as compared to the above second embodiment, further capable of producing an optimal noise reduction effect in accordance with an arrival direction of noise. 
     A configuration of the noise control device according to the third embodiment will be described with reference to  FIG. 12 .  FIG. 12  shows a configuration of the noise control device according to the third embodiment. As shown in  FIG. 12 , the noise control device comprises the headband  11 , the left ear case  12   a , the right ear case  12   b , the left ear speaker  13   a , the right ear speaker  13   b , the left ear microphone  14   a , the right ear microphone  14   b , the control section  15   a , the adders  21   a  and  21   b , the left ear high frequency control section  25   a , the right ear high frequency control section  25   b , and a switching section  30 . The configuration shown in  FIG. 12  is different from the second embodiment shown in  FIG. 9  in that the configuration shown in  FIG. 12  newly comprises the switching section  30 . It is assumed for  FIG. 12  that components, which are denoted by the same reference numerals as those used for components shown in  FIG. 9 , have the same functions as those of the components shown in  FIG. 9 , and descriptions thereof will be omitted. Note that,  FIG. 12  and later-described  FIGS. 13A ,  15  and  17  each are a diagram which shows a view seen from above a head of the user  10  and in which the user  10  faces upward. Hereinafter, a description will be given with a focus on the aforementioned difference. 
     The switching section  30  switches, between an output of the left ear microphone  14   a  and an output of the right ear microphone  14   b , an output of a microphone to be connected to an input of the control section  15   a . The switching section  30  is provided with terminals a to c. The input of the control section  15   a  is connected to the terminal c. The output of the left ear microphone  14   a  is connected to the terminal a. The output of the right ear microphone  14   b  is connected to the terminal b. The switching section  30  switches a connection state by connecting the terminals a and c, or by connecting the terminals b and c. Which connection state is to be used is determined based on an operation by the user  10 .  FIG. 12  shows the connection state of the switching section  30  in which the terminals a and c are connected. 
     Next, a relationship between the connection state of the switching section  30  and a noise reduction operation will be described with reference to FIGS.  12  and  13 A-C. It is assumed in the following description that there is an environment where a noise source is present at the left ear side of the user  10  as shown in  FIG. 12 .  FIGS. 13A-C  are diagrams for describing the relationship between the connection state of the switching section  30  and the noise reduction operation.  FIG. 13A  shows a state where there is a noise source at the left ear side of the user  10 .  FIG. 13B  shows a waveform, along a temporal axis, of a noise detected by the left ear microphone  14   a  in the environment illustrated in  FIG. 13A .  FIG. 13C  shows a waveform, along a temporal axis, of a noise detected by the right ear microphone  14   b  in the environment illustrated in  FIG. 13A . 
     In the environment where the noise source is present at the left ear side of the user  10 , a noise generated from the noise source is transmitted from the left side to the right side of the user  10 . Generally speaking, a distance between the left and right ears of the user  10  is 15 cm. Accordingly, when it is assumed that a sound velocity is 340 m/sec, there is a time lag of approximately 0.4 ms between a timing at which a noise is detected by the left ear microphone  14   a  and a timing at which the noise is detected by the right ear microphone  14   b . In other words, as shown in  FIGS. 13B and 13C , the timing of detection at the right ear microphone  14   b  is delayed, by approximately 0.4 ms, from the timing of detection at the left ear microphone  14   a.    
     When the connection state of the switching section  30  is such that the terminals a and c are connected as shown in  FIG. 12 , the control section  15   a  generates a control signal by using the detection signal e L  of the left ear microphone  14   a . Here, it is ideal that the right ear speaker  13   b  radiates, at a same timing as that when a noise arrives near the left ear, a sound based on the control signal generated by using the detection signal e L  of the left ear microphone  14   a . Accordingly, the noise to be controlled arrives near the right ear when 0.4 ms have passed after the timing of radiation, from the right ear speaker  13   b , of the sound based on the control signal. 
     On the other hand, when the connection state of the switching section  30  is such that the terminals b and c are connected, the control section  15   a  generates a control signal by using the detection signal e R  of the right ear microphone  14   b . Here, it is ideal that the right ear speaker  13   b  radiates, at a same timing as that when the noise arrives near the right ear, a sound based on the control signal generated by using the detection signal e R  of the right ear microphone  14   b . In other words, the timing at which the noise arrives near the right ear is the same as the timing at which the right ear speaker  13   b  radiates, near the right ear, the sound based on the control signal. 
     In reality, however, there is a delay time from when the microphone detects a noise to when the speaker outputs the sound based on the control signal, due to a processing delay such as a processing delay at the control section  15   a  or a group delay of an electroacoustic transfer function. 
     Accordingly, in the case where the connection state of the switching section  30  is such that the terminals a and c are connected as shown in  FIG. 12 , if the delay time due to the aforementioned processing delay is approximately 0.4 ms, the delay time due to the processing delay is compensated for by the delay time occurring in the case of the connection state shown in  FIG. 12 . To be specific, in reality, in the case of the connection state shown in  FIG. 12 , the timing at which the right ear speaker  13   b  radiates the sound based on the control signal is the same as the timing at which the noise arrives near the right ear. 
     Note that, near the left ear, the delay time caused by the aforementioned processing delay is not compensated for. In other words, in the case of the connection state shown in  FIG. 12 , near the left ear, the timing at which the left ear speaker  13   a  radiates the sound based on the control signal is delayed, by the above processing delay (0.4 ms), from the timing at which the noise arrives near the left ear. Accordingly, a level of noise reduction is lower near the left ear than near the right ear. 
     On the other hand, when the connection state of the switching section  30  is such that the terminals b and c are connected, the timing at which the right ear speaker  13   b  radiates, near the right ear, the sound based on the control signal is delayed, by the above processing delay (0.4 ms), from the timing at which the noise arrives near the right ear. 
     Note that, near the left ear, the timing at which the left ear speaker  13   a  radiates the sound based on the control signal is delayed from the timing at which the noise arrives near the left ear, by the sum (0.8 ms) of the above processing delay (0.4 ms) and the delay time (0.4 ms) for the noise to arrive near the right ear from the left ear. In other words, a level of noise reduction is lower near the left ear than near the right ear. 
     Provided below is a comparison, between the case where the connection state of the switching section  30  is such that the terminals a and c are connected and the case where the connection state of the switching section  30  is such that the terminals b and c are connected, about the delay time between the timing at which the speaker radiates the sound based on the control signal and the timing at which the noise arrives. As described above, in the case where the connection state of the switching section  30  is such that the terminals a and c are connected, the delay time near the right ear is 0, and the delay time near the left ear is the aforementioned processing delay (0.4 ms). On the other hand, as described above, in the case where the connection state of the switching section  30  is such that the terminals b and c are connected, the delay time near the right ear is the aforementioned processing delay (0.4 ms), and the delay time near the left ear is the sum (0.8 ms) of the above processing delay (0.4 ms) and the delay time (0.4 ms) for the noise to arrive near the right ear from the left ear. Accordingly, the level of noise reduction is higher in the case where the connection state of the switching section  30  is such that the terminals a and c are connected, i.e., in the case of performing a control by using the left ear microphone  14   a  which is a nearest microphone to the noise source. 
       FIG. 14A  shows a frequency characteristic of the detection signal e R  of the right ear microphone  14   b  in the case where a control is performed using the detection signal e L  of the left ear microphone  14   a , in the environment where the noise source is present at the left ear side of the user  10 .  FIG. 14B  shows a frequency characteristic of the detection signal e L  of the left ear microphone  14   a  in the case where a control is performed using the detection signal e R  of the right ear microphone  14   b , in the environment where the noise source is present at the left ear side of the user  10 . It is understood from these diagrams that in the case where the detection signal shown in  FIG. 14A  is used when the control is performed, a frequency band, in which a sound pressure level decreases as compared to when the control is not performed, is wider, and an amount, by which the sound pressure level decreases as compared to when the control is not performed, is greater. In other words, the detection signal shown in  FIG. 14A  is superior with respect to the width of the frequency band in which the noise is reduced and to the amount of the noise reduction. 
     When it is assumed that there is an environment where there is a noise source at the right ear side of the user  10 , the switching section  30  may switch, in accordance with an operation by the user  10 , the output of the microphone to be connected to the input of the control section  15   a , to the output of the right ear microphone  14   b  which is the nearest microphone to the noise source. Further, even if the noise control device has three or more microphones, the switching section  30  may switch, in accordance with an operation by the user  10 , the output of the microphone to be connected to the input of the control section  15   a , to the output of a nearest microphone to the noise source. 
     As described above, in the noise control device according to the present embodiment, the switching section  30  may switch, in accordance with an operation by the user  10 , the output of the microphone to be connected to the input of the control section  15   a , to the output of the nearest microphone to the noise source. This produces an optimal noise reduction effect in accordance with an arrival direction of the noise. 
     In the above description, the switching section  30  switches the connection in accordance with an operation by the user  10 . However, in the case where the user  10  is unable to specify a position of the noise source, a microphone determination section  31  and a switching control section  32  may be newly added.  FIG. 15  shows a configuration which is a result of newly adding the microphone determination section  31  and the switching control section  32  to the configuration shown in  FIG. 12 . 
     In  FIG. 15 , the microphone determination section  31  refers to the detection signal e L  of the left ear microphone  14   a  and the detection signal e R  of the right ear microphone  14   b , thereby determining whether the nearest microphone to the noise source is the left ear microphone  14   a  or the right ear microphone  14   b . Hereinafter, a manner of the determination performed by the microphone determination section  31  will be described. It is assumed here that an initial state of the noise control device shown in  FIG. 15  is such that the terminals a and c or the terminals b and c are connected in the switching section  30 . The microphone determination section  31  analyzes a frequency of the detection signal e L  of the left ear microphone  14   a  and a frequency of the detection signal e R  of the right ear microphone  14   b . The microphone determination section  31  compares, at a frequency f in a frequency band for which the control section  15   a  performs a control, a sound pressure level of the detection signal e L  of the left ear microphone  14   a  and a sound pressure level of the detection signal e R  of the right ear microphone  14   b.    
     Here, as described above, regardless of whether the terminals a and c are connected or the terminals b and c are connected in the switching section  30 , the level of noise reduction is lower for an ear which is nearer to the noise source than the other ear. In other words, regardless of whether the terminals a and c are connected or the terminals b and c are connected in the switching section  30 , the sound pressure level of the detection signal of the nearer microphone to the noise source is higher than the sound pressure level of the detection signal of the other microphone. Therefore, the microphone determination section  31  determines that a microphone whose sound pressure level is higher is the nearest microphone to the noise source. 
       FIG. 16  shows: a result of analyzing, when the control is not performed, the frequencies of the detection signal e L  of the left ear microphone  14   a  and the detection signal e R  of the right ear microphone  14   b ; a result of analyzing, when the control is performed, the frequency of the detection signal e L  of the left ear microphone  14   a ; and a result of analyzing, when the control is performed, the frequency of the detection signal e R  of the right ear microphone  14   b . In an example shown in  FIG. 16 , when the control is not performed, the sound pressure level of the detection signal e L  of the left ear microphone  14   a  is the same as the sound pressure level of the detection signal e R  of the right ear microphone  14   b . On the other hand, when the control is performed, the sound pressure level of the detection signal e L  of the left ear microphone  14   a  is higher than that of the detection signal e R  of the right ear microphone  14   b . Therefore, in the example shown in  FIG. 16 , the microphone determination section  31  determines that the left ear microphone  14   a  is the nearest microphone to the noise source. 
     Based on a determination result provided by the microphone determination section  31 , the switching control section  32  controls the switching section  30  such that the output of the microphone to be connected to the input of the control section  15   a  is switched to the output of the nearest microphone to the noise source. 
     As described above, by having the configuration shown in  FIG. 15 , the output of the microphone to be connected to the input of the control section  15   a  can be automatically switched to the output of the nearest microphone to the noise source, even if the user  10  is unable to specify the position of the noise source. 
     Note that, in the configuration shown in  FIG. 15 , the switching operation by the microphone determination section  31  and the switching control section  32  may be performed only when the noise control device performs an initial operation, or may be performed regularly. 
     Further, in the configuration shown in  FIG. 15 , the microphone determination section  31  compares the sound pressure levels of the detection signals of the left ear microphone  14   a  and the right ear microphone  14   b . However, the present invention is not limited thereto. The microphone determination section  31  may perform a determination by using a cross-correlation function related to the detection signals. In such a case, the microphone determination section  31  first calculates the cross-correlation function for the detection signals of the left ear microphone  14   a  and the right ear microphone  14   b . The microphone determination section  31  uses the cross-correlation function, thereby calculating a time lag between the detection signals, based on a characteristic of the cross-correlation function in which a maximum value of the time lag between the detection signals is taken. The microphone determination section  31  evaluates a noise arrival direction from the calculated time lag, and determines the nearest microphone to the noise source. Still further, the microphone determination section  31  may determine the nearest microphone to the noise source, based on, e.g., seat position information in a vehicle such as an aircraft. The seat position information may indicate, e.g., a right or left side seat, or aisle or window seat. In the case of, e.g., a window seat, a noise source exists at a window side, and therefore the microphone determination section  31  determines that the nearest microphone to the window is the nearest microphone to the noise source. 
     Although the configurations shown in  FIGS. 12 and 15  each comprise the left ear high frequency control section  25   a  and the right ear high frequency control section  25   b , these components may be omitted therefrom. 
     Still further, the configurations shown in  FIGS. 12 and 15  may each newly comprise, as shown in  FIG. 17 , the echo canceling section  26  described in the second embodiment.  FIG. 17  shows a configuration which is a result of newly having, to the configurations shown in  FIGS. 12 and 15 , the echo canceling section  26  described in the second embodiment. In this case, as shown in  FIG. 17 , the configuration shown in  FIG. 12  newly comprises the echo canceling section  26 , a switching section  33  and a subtractor  34 . The switching section  33  switches a connection of the echo canceling section  26  such that the echo canceling section  26  is connected to an output of the adder  21   a  or an output of the adder  21   b . The switching section  33  is provided with terminals a to c. An input of the echo canceling section  26  is connected to the terminal c. The output of the adder  21   a  is connected to the terminal a. The output of the adder  21   b  is connected to the terminal b. The switching section  33  switches a connection state thereof by either connecting the terminals a and c or connecting the terminals b and c. Note that, the switching section  33  switches the connection state thereof in conjunction with the switching section  30 . To be specific, when the connection state of the switching section  30  is such that the terminals a and c are connected, the connection state of the switching section  33  is also such that the terminals a and c are connected. Further, when the connection state of the switching section  30  is such that the terminals b and c are connected, the connection state of the switching section  33  are also such that the terminals b and c are connected. The subtractor  34  subtracts an output signal of the echo canceling section  26  from an output signal of the switching section  30 . 
     Fourth Embodiment 
     Hereinafter, a noise control device according to a fourth embodiment of the present invention will be described with reference to the drawings. Described in the present embodiment are other forms of noise control devices which are further developed using the noise control devices according to the above first to third embodiments. 
     A first use form will be described with reference to  FIG. 18 .  FIG. 18  shows a configuration of the first use form in which the noise control device according to the first embodiment is used. The configuration shown in  FIG. 18  is a result of adding a control section  15   b  to the configuration shown in  FIG. 2 . It is assumed for  FIG. 18  that components, which are denoted by the same reference numerals as those used for components of the noise control device of the first embodiment in  FIG. 2 , have the same functions as those of the components of the noise control device in  FIG. 2 , and descriptions thereof will be omitted.  FIG. 18  shows a view seen from above a head of the user  10 . In  FIG. 18 , the user  10  faces upward. 
     The control section  15   b  has the same configuration as that of the control section  15  described with reference to  FIG. 3 , except that a filter coefficient, which has an inverse characteristic to that of the electroacoustic transfer function H R  of the right ear speaker  13   b , is set for a feedback control filter of the control section  15   b . The control section  15   b  generates, based on the detection signal e L , a control signal for controlling a level of the detection signal e L  detected by the left ear microphone  14   a  such that the level is lowered. The control signal generated by the control section  15   b  is outputted to the right ear speaker  13   b.    
     According to the configuration shown in  FIG. 18 , even if there is a significant characteristic difference between the left ear speaker  13   a  and the right ear speaker  13   b , noise can be reduced for both the left and right ears. Further, since only one microphone is used for detecting a noise, there is an advantage of reducing a cost for a microphone, as compared to the above-described conventional art. 
     Next, a second use form will be described with reference to  FIG. 19 .  FIG. 19  shows a configuration of the second use form which is further developed from the noise control device according to the second embodiment. The configuration shown in  FIG. 19  is a result of adding a control section  15   c  to the configuration shown in  FIG. 9 . It is assumed for  FIG. 19  that components, which are denoted by the same reference numerals as those used for components of the noise control device according to the second embodiment in  FIG. 9 , have the same functions as those of the components of the noise control device in  FIG. 9 , and descriptions thereof will be omitted.  FIG. 19  shows a view seen from above a head of the user  10 . In  FIG. 19 , the user  10  faces upward. 
     The control section  15   c  has the same configuration as that of the control section  15   a  described with reference to  FIG. 10 , except for the filtered X filter  155  for which a filter coefficient simulating the electroacoustic transfer function H R  is set. The control section  15   c  generates, based on the detection signal e L , a control signal for controlling the level of the detection signal e L  detected by the left ear microphone  14   a  such that the level is lowered. The control signal generated by the control section  15   c  is outputted to the adder  21   b . The control section  15   a  generates, based on the detection signal e R , a control signal for controlling the level of the detection signal e R  detected by the right ear microphone  14   b  such that the level is lowered. The control signal generated by the control section  15   a  is outputted to the adder  21   a . The adder  21   a  adds the control signal generated by the control section  15   a  to a control signal generated by the left ear high frequency control section  25   a , and outputs the added signal to the left ear speaker  13   a . The adder  21   b  adds the control signal generated by the control section  15   c  to a control signal generated by the right ear high frequency control section  25   b , and outputs the added signal to the right ear speaker  13   b.    
     In the above configuration, the left ear high frequency control section  25   a , for example, is designed by taking the electroacoustic transfer function H L  into account. For this reason, when a characteristic of the left ear microphone  14   a  deteriorates due to aged deterioration or the like, the control signal generated by the left ear high frequency control section  25   a  is not always capable of canceling a noise. As a result, the feedback loop formed by the left ear microphone  14   a , the left ear high frequency control section  25   a , the adder  21   a  and the left ear speaker  13   a  does not operate as designed, and this results in a failure to reduce a noise in a high frequency band near the left ear. Similarly, the control section  15   c  is designed by taking into account the electroacoustic transfer function H R  which has the same value as that of the electroacoustic transfer function H L . For this reason, when a characteristic of the left ear microphone  14   a  deteriorates due to aged deterioration or the like, the control signal generated by the control section  15   c  is not always capable of canceling a noise, and this results in a failure to reduce a noise in a low frequency band near the right ear. 
     However, if the right ear microphone  14   b  does not deteriorate in characteristic, and operates properly, the control section  15   a  and the high frequency control section  25   b  each output a control signal which is capable of canceling a noise. As a result, a noise in a low frequency band arriving near the left ear and a noise in a high frequency band arriving near the right ear can be reduced. As described above, in the configuration shown in  FIG. 19 , the microphone in the feedback loop including the control section  15   a  is used as the right ear microphone  14   b , and the microphone in the feedback loop including the control section  15   c  is used as the left ear microphone  14   a . As a result, even when one of the microphones deteriorates in characteristic, a risk of entirely losing the noise reduction effect can be avoided. 
     Next, a third use form will be described. A configuration of the third use form is a result of modifying the configuration of the second embodiment shown in  FIG. 9  such that the frequency band of the control signal generated by each of the left ear high frequency control section  25   a  and the right ear high frequency control section  25   b  is the same as that of the control section  15   a . In this configuration, although a frequency band in which the noise is reduced is the frequency band of the control signal generated by the control section  15   a , the level of noise reduction is further increased. 
     In each of the noise control devices according to the above-described first to fourth embodiments, components, other than the headband  11 , the left ear case  12   a , the right ear case  12   b , the left ear speaker  13   a , the right ear speaker  13   b , the left ear microphone  14   a , the right ear microphone  14   b  and the external microphone  14   c , may be realized as a single chip by using, e.g., an integrated circuit such as LSI or a dedicated signal processing circuit. Also, each of the noise control devices according to the above first to fourth embodiments may be realized by using chips respectively corresponding to the functions of the above-described components. For example, in the configuration shown in  FIG. 2 , the control section  15  is realized by an integrated circuit. Here, the integrated circuit comprises an input terminal, to which an output from the left ear microphone  14   a  is inputted, and an output terminal for outputting the control signal generated by the control section  15  to the left ear speaker  13   a  and to the right ear speaker  13   b . Although LSI is mentioned above, the integrated circuit may be referred to as an IC, a system LSI, a super LSI or an ultra LSI, depending on an integration density thereof. The integrated circuit technology is not necessarily limited to LSI. The integrated circuit may be realized as a dedicated circuit or a general-purpose processor. Further, an FPGA (Field Programmable Gate Array), which can be programmed after LSI production, or a reconfigurable processor, which enables connections or settings of circuit cells in an LSI to be reconfigured, may be used. Still further, if a new circuit integration technology to be replaced with the LSI technology is developed as a result of an advance in the semiconductor technology, or is developed based on a technology derived from the semiconductor technology, function blocks may, of course, be integrated using such a technology. 
     INDUSTRIAL APPLICABILITY 
     The noise control device according to the present invention is applicable in a headphone apparatus which is capable of, even in the case where processing is performed by a single arithmetic processing circuit, producing a sufficient noise reduction effect without causing an increase in an input/output delay at a control section, and also applicable in a headphone apparatus or the like which has a music playback function.