Patent Publication Number: US-7907737-B2

Title: Acoustic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-317570, filed Dec. 12, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One embodiment of the invention relates to an acoustic apparatus for measuring and correcting the resonance characteristics of the outer-ear canals of a listener, to which a sound source signal is supplied from an earphone or a headphone. 
     2. Description of the Related Art 
     While listening to music through an earphone or a headphone (hereinafter, “earphone”), a listener may perceive an unnatural sound when the outer-ear canals are plugged with the earphone and resonance occurs due to interference with the sound waves reflected from the eardrums, emphasizing the sound of the resonance frequency. It is therefore desirable to measure the resonance characteristics of the outer-ear canals and to correct the resonance characteristics before the listener starts listening to music. 
     The shapes and acoustic transmission characteristic of outer-ear canals, and the physical properties and acoustic transmission characteristic of eardrums differ from person to person. Further, the resonance in either outer-ear canal changes in accordance with the type of the earphone and the state in which the earphone is held in the outer-ear canal. Hence, the resonance characteristics of the outer-ear canals must be measured and corrected for each earphone and each listener in order to achieve accurate measurement and correction of the resonance characteristics of the outer-ear canals. 
     Jpn. Pat. Appln. KOKAI Publication No. 2004-320098 describes a damping control circuit for use in earphones (see paragraph 0013). This circuit suppresses the vibration of the diaphragms of the earphone, which is pertinent to the resonance characteristics of the outer-ear canals. The damping factor becomes larger in the frequency domain of 3 to 4 kHz, where the acoustic gain of the outer-ear canals is maximal. Hence, the harmful vibration of the diaphragms of the earphone, which pertains to the resonance characteristics of the outer-ear canals, can be effectively controlled. 
     In the damping apparatus for use in earphones, which is disclosed in the above-identified publication, the damping factor becomes larger in the frequency domain of 3 to 4 kHz, where the acoustic gain of the outer-ear canals is maximal. The harmful vibration of the diaphragms of the earphone, which pertains to the resonance characteristics of the outer-ear canals, can therefore be effectively controlled. However, the resonance characteristics of the listener&#39;s outer-ear canals cannot be measured or corrected to accord with the vibration characteristic of the earphone. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A general architecture that implements the various feature of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIG. 1  is an exemplary diagram showing the concept of measuring and correcting the acoustic transmission characteristics in embodiments of the present invention; 
         FIG. 2  is an exemplary block diagram showing an exemplary configuration of an acoustic apparatus  100  according to a first embodiment of the present invention; 
         FIG. 3  is an exemplary diagram showing an exemplary unit pulse generated as a measuring signal; 
         FIG. 4  is an exemplary diagram showing the result of measuring a response signal, with the electric/acoustic transducer  20  arranged in a free sound field; 
         FIG. 5  is an exemplary diagram showing an exemplary acoustic characteristic of the electric/acoustic transducer  20  in the embodiment of the present invention; 
         FIG. 6  is an exemplary diagram showing an exemplary measuring signal in which a control signal is added to the unit pulse; 
         FIG. 7  is an exemplary diagram showing a response signal generated in response to a measuring signal with a control signal being added; 
         FIG. 8  is an exemplary flowchart explaining a process of measuring the resonance characteristic of the outer-ear canal  61  of a listener  60 ; 
         FIG. 9  is an exemplary block diagram showing an exemplary configuration of an acoustic apparatus  200  according to a second embodiment; 
         FIG. 10  is an exemplary flowchart explaining how the electric/acoustic transducer  20  performs a process of measuring the acoustic characteristic; and 
         FIG. 11  is an exemplary diagram showing an exemplary use of the acoustic apparatus according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, an acoustic apparatus comprises a measuring signal generator configured to generate a pulse; a transducer configured to convert the pulse to a sound to be output to a free sound field and to convert a characteristic vibration of the transducer to a characteristic vibration signal; a signal analysis module configured to analyze the characteristic vibration signal in order to output a physical quality representing the characteristic vibration of the transducer; a controller configured to set one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal; and a switch configured to connect the measuring signal generator to the transducer in the first state and to connect the signal analysis module to the transducer in the second state. 
     Acoustic apparatuses according to embodiments of this invention will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing the concept of measuring and correcting the acoustic transmission characteristics of the outer-ear canals, in an embodiment of the present invention. 
     A listener  60 , who is a subject of measuring, has eardrums  62 , each in one end of either outer-ear canal  61 . An electric/acoustic transducer  20  plugs the other end of the listener&#39;s outer-ear canal  61 . Devices that convert an electric signal to an acoustic signal (or sound), or vice versa, such as earphones and headphones, shall be called “electric/acoustic transducer  20 .” 
     An acoustic signal output from the electric/acoustic transducer  20  passes through the outer-ear canal  61  of the listener  60 , reaching the eardrum  62 . The acoustic signal interferes with the sound reflected from the eardrum  62  of the listener  60 , causing resonance in the outer-ear canal  61 . The electric/acoustic transducer  20  is electrically connected to an acoustic apparatus  100 . The acoustic apparatus  100  acquires the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . In accordance with the physical quantity acquired by the acoustic apparatus  100 , the sound source signal to be supplied to the listener  60  can be corrected to a predetermined characteristic (to reduce the gain at the resonance frequency). Since the left and right outer-ear canals differ in characteristic, two electric/acoustic transducers  20  for the left and right outer-ear canals are connected to the acoustic apparatus  100 . Thus, the physical quantity inherent to the resonance characteristic of the left outer-ear canal and the physical quantity inherent to the resonance characteristic of the right outer-ear canal are acquired. 
     The acoustic apparatus  100  may be connected to, or incorporated in, an external apparatus that has an audio playback function, such as a personal computer (PC), a music player or an optical-disk player. 
       FIG. 2  is a block diagram showing an exemplary configuration of the acoustic apparatus  100  according to the first embodiment. The acoustic apparatus  100  comprises a measuring signal generator  110 , a switch  120 , a response signal analysis module  130 , and a controller  140 . The acoustic apparatus  100  is electrically connected to, or formed integral with, either electric/acoustic transducer  20 . 
     A correction coefficient generator  30  and a correction filter  40  are arranged outside the acoustic apparatus  100 . The correction coefficient generator  30  generates a correction coefficient on the basis of the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 , and then sets the correction coefficient in the correction filter  40 . The correction filter  40  corrects a sound source signal output from an apparatus having an audio function. 
     As shown in  FIG. 2 , the correction coefficient generator  30  and correction filter  40  are provided outside the acoustic apparatus  100 . Nonetheless, they may be formed integral with the acoustic apparatus  100 . Alternatively, the correction coefficient generator  30  and correction filter  40  may be incorporated in an apparatus having an audio function. 
     If the acoustic apparatus  100  is incorporated in an apparatus having an audio function, the correction coefficient generator  30  and the correction filter  40  may be incorporated in the apparatus having an audio function, too. If the acoustic apparatus  100  is connected to the apparatus having an audio function, the correction coefficient generator  30  and the correction filter  40  may be incorporated in the apparatus having an audio function, or may instead be incorporated in the acoustic apparatus  100 . 
     A switch  50  is changed over to output the sound source signal to the electric/acoustic transducer  20 , either not corrected at all (in no-correction mode) or corrected by the correction filter  40  (in correction mode). The switch  50  is changed over by, for example, a control signal supplied from the control unit (processor) of the apparatus having an audio function. The control signal may be transmitted from controller  140  of the acoustic apparatus  100  if the acoustic apparatus  100  is incorporated in the apparatus having an audio function or formed integral with the correction filter  40 . 
     The electric/acoustic transducer  20  may be an earphone or headphone that plugs the end of the outer-ear canal  61  of the listener  60  as illustrated in  FIG. 1 . The unit  20  converts an electric signal to an acoustic signal and applies the acoustic signal into the outer-ear canal  61  of the listener  60 . The electric/acoustic transducer  20  also converts the acoustic signal applied into the outer-ear canal  61  of the listener  60  and reflected by the eardrum  62  of the listener  60 , to an electric signal. 
     The controller  140  has a memory or can access a memory (not shown). The controller  140  executes a program stored in the memory, controlling the other components of the acoustic apparatus  100 . The controller  140  can switch the state of the electric/acoustic transducer  20 , between the first state in which the transducer  20  converts an electric signal to an acoustic signal and the second state in which the transducer  20  converts an acoustic signal to an electric signal. 
     The controller  140  is provided in the acoustic apparatus  100  as shown in  FIG. 2 . Instead, the controller  140  may be provided outside the acoustic apparatus  100 . If the acoustic apparatus  100  is connected to an apparatus having an audio function, the controller (processor) of this apparatus may operate as a controller  140 . In this case, the controller of the apparatus having an audio function executes a predetermined program, controlling the acoustic apparatus  100 . Alternatively, the controller of the apparatus having an audio function may control the controller  140  incorporated in the acoustic apparatus  100 . 
     The measuring signal generator  110  includes a unit pulse generator  111 , a control signal generator  112 , and an adder  113 . The unit pulse generator  111  generates unit pulses to measure the resonance characteristic of the outer-ear canal  61  of the listener  60 . The control signal generator  112  generates a signal to control the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . The adder  113  adds the outputs of the unit pulse generator  111  and control signal generator  112 , outputting a measuring signal. 
     The switch  120  is changed over to receive a signal from, or outputs a signal to, the electric/acoustic transducer  20 . 
     When set to the first state, the switch  120  is switched to the measuring signal generator  110 . In this case, the switch  120  connects the output of the measuring signal generator  110  to the electric/acoustic transducer  20 . The electric/acoustic transducer  20  converts the measuring signal output from the measuring signal generator  110 , to an acoustic signal. The acoustic signal is output to the outer-ear canal  61  of the listener  60 . 
     The acoustic signal converted from the measuring signal and input to the outer-ear canal  61  of the listener  60  is reflected by the eardrum  62  of the listener  60 . The electric/acoustic transducer  20  converts the acoustic signal, so reflected, to a response signal that is an electric signal. In order to receive the response signal, the switch  120  is set to the second state. 
     On the other hand, when set to the second state, the switch  120  is switched to the input of the response signal analysis module  130 . The electric/acoustic transducer  20  collects the sound reflected by the eardrum  62  of the listener  60 , i.e., measurement object, and converts the acoustic signal to an electric signal. This electric signal is output to the response signal analysis module  130 . The response signal analysis module  130  acquires, from the response signal, the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . That is, the response signal analysis module  130  first converts the response signal, from a time domain to a frequency domain, and then detects the peak frequency and amplitude at the peak frequency, which are the physical quantities inherent to the outer-ear canal  61  of the listener  60 . The physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 , which the response signal analysis module  130  has acquired, is output to the correction coefficient generator  30 . The correction coefficient generator  30  generates a correction signal for the correction filter  40 , from the physical quantity which the response signal analysis module  130  has obtained and which is inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . The correction filter  40  decreases the gain in terms of the resonance frequency of the outer-ear canal  61  of the listener  60 , thereby correcting the frequency characteristic of the sound source signal to a flat frequency characteristic. Such calculation as performed in the correction filter  40  can be accomplished by using a parametric equalizer or a graphic equalizer. 
     If the switch  50  is changed over to the output side of the correction filter  40 , setting the correction mode, the sound source signal corrected in accordance with the resonance characteristic of the outer-ear canal  61  of the listener  60  is input to the electric/acoustic transducer  20 . That is, the correction filter  40  decreases the gain in terms of the resonance frequency, thereby correcting the frequency characteristic of the sound source signal to a flat frequency characteristic. 
     As described above, the electric/acoustic transducer  20  performs not only the earphone function (first state) of converting the electric signal coming from the switch  120  to an acoustic signal and supplying the acoustic signal to the measurement object, but also the microphone function (second state) of converting the acoustic signal coming from the measured object to an electric signal and supplying the electric signal to the switch  120 . That is, the transducer  20  utilizes the earphone function in the first state, applying a measuring signal, i.e., acoustic signal, to the outer-ear canal  61  of the listener  60 , and utilizes the microphone function in the second state, collecting the acoustic signal converted from the measuring signal and coming from the eardrum  62  of the listener  60 . 
     If the electric/acoustic transducer  20  functions as an earphone, the diaphragm of the electric/acoustic transducer  20  converts an electric signal to an acoustic signal. At this point, the diaphragm vibrates at its specific frequency. Hence, the vibration continues even after the acoustic signal has been output to the outer-ear canal  61  of the listener  60 . Assume that this vibration continues when the measuring signal is output in the first state in the process of measuring the resonance characteristic of the outer-ear canal  61  of the listener  60 . Then, the vibration at the specific frequency generates a noise component in the response signal, rendering it difficult to analyze the response signal. Such vibration of the diaphragm, at the specific frequency, should therefore be controlled. 
     How the control signal generated by the control signal generator  112  controls or suppresses the characteristic vibration will be explained. 
       FIG. 3  is a diagram showing an exemplary unit pulse generated by the unit pulse generator  111  as a measuring signal. As shown in  FIG. 3 , a pulse having an amplitude of 1 is generated at time T=0.  FIG. 4  shows an exemplary response signal generated when no control signals generated by the control signal generator  112  are added to the unit pulse.  FIG. 4  shows the result of measuring a response signal, which is obtained if the electric/acoustic transducer  20  is arranged in a free sound field (or in a space where the reflection is sufficiently small). Since the electric/acoustic transducer  20  is arranged in the free sound field, no signals should be observed. As seen from  FIG. 4 , however, the response signal has a component inherent to the characteristic vibration of the diaphragm. In order to control this characteristic vibration, the control signal generator  112  generates a control signal in this embodiment. 
     The control signal generator  112  used in this embodiment utilizes the acoustic characteristic the electric/acoustic transducer  20  has in the free sound field, in order to control or suppress the characteristic vibration.  FIG. 5  is a diagram that shows an exemplary acoustic characteristic the electric/acoustic transducer  20  has in the free sound field. 
     The acoustic characteristic shown in  FIG. 5  has been measured beforehand by arranging the electric/acoustic transducer  20  in a free sound field. This acoustic characteristic has a peak at frequency fP, which is the frequency of the characteristic vibration. The control signal generator  112  generates a control signal that has a characteristic inverse to the acoustic characteristic (or characteristic that is quasi-inverse thereto). The control signal is output to the adder  113 . The adder  113  adds the control signal to the unit pulse generated by the unit pulse generator  111 , thus generating a measuring signal. The measuring signal, thus generated, is output to the switch  120 . 
       FIG. 6  is a diagram showing an exemplary measuring signal output from the adder  113  and generated by adding the control signal to the unit pulse. In  FIG. 6 , not only a pulse having an amplitude of 1 at time T=0, but also the waveform of the control signal is illustrated. This control signal has been generated to exhibit a characteristic inverse to the acoustic characteristic shown in  FIG. 5 . A control signal of this type is output to the electric/acoustic transducer  20 , whereby the characteristic vibration shown in  FIG. 4  can be suppressed. 
       FIG. 7  shows an exemplary response signal that may be generated if a control signal is added to the unit pulse. That is,  FIG. 7  shows the result of measuring the response signal, with the electric/acoustic transducer  20  arranged in a free sound field. As shown in  FIG. 7 , no signals resulting from the characteristic vibration are observed, because the characteristic vibration of the diaphragm is suppressed. 
     An exemplary control signal generated by the control signal generator  112  will be described. The frequency component of peak frequency fP, shown in  FIG. 5  illustrating the acoustic characteristic, seems to impose a large influence on the characteristic vibration of the electric/acoustic transducer  20 . Reciprocal Tp of the peak frequency fp, i.e., Tp=1/fP, has a time dimension. The control signal generator  112  delays the unit pulse generated by the unit pulse generator  111 , by half the time Tp, thereby generating a pulse with amplitude calculated from the magnitude of the peak frequency. If the measuring signal generator  110  has long sampling intervals, time Tp/2 can hardly be measured. In this case, the unit pulse generated by the unit pulse generator  111  after the up-sampling is delayed by half the time Tp, generating a pulse having the amplitude calculated from the magnitude of the peak frequency. After a low-pass filter process, down-sampling is performed. That is, the sampling intervals are made so short that time Tp/2 may be measured, and a pulse is set for time Tp, thereby acquiring a waveform defined by the initial sampling intervals by means of a decimation filter or the like. The control signal, thus generated, is output to the adder  113 . The adder  113  adds a control signal to the unit pulse generated by the unit pulse generator  111 , generating a measuring signal. The measuring signal is output to the switch  120 . 
     The process performed in the first embodiment to measure the resonance characteristic of either outer-ear canal  61  of the listener  60  will be explained.  FIG. 8  is a flowchart explaining the process of measuring the resonance characteristic of the outer-ear canal  61  of the listener  60 . 
     To measure the resonance characteristic of the outer-ear canal  61  of the listener  60 , the electric/acoustic transducer  20  is inserted into the outer-ear canal  61  of the listener  60  (Block B 110 ). Then, the switch  120  is changed over to the side of the measuring signal generator  110  and is thereby set to the first state (Block B 111 ). The switch  120  therefore connects the output of the measuring signal generator  110  to the electric/acoustic transducer  20 . 
     The electric/acoustic transducer  20  converts the measuring signal output from the measuring signal generator  110 , which is an electric signal, to an acoustic signal. The acoustic signal is applied into the outer-ear canal  61  of the listener  60  (Block B 112 ). The measuring signal output from the measuring signal generator  110  is an electric signal that the adder  113  has generated by adding the unit pulse generated by the unit pulse generator  111  to the control signal generated by the control signal generator  112 . 
     Then, the controller  140  changes over the switch  120  to the input of the response signal analysis module  130 , causing the electric/acoustic transducer  20  to function as a microphone to collect the sound reflected from the eardrum  62  of the listener  60  who has received the acoustic signal converted from the measuring signal. The switch  120  is therefore set to the second state (Block B 113 ). 
     The sound reflected from the eardrum  62  of the listener  60 , which is an acoustic signal converted from the measuring signal, is converted by the electric/acoustic transducer  20  to a response signal that is an electric signal (Block B 114 ). The response signal is output to the response signal analysis module  130 . 
     Next, the controller  140  determines whether the measurement has been made a predetermined number of times as required (Block B 115 ). The measurement should be made, for example, several times. If the measurement has not been made several times yet (if No in Block B 115 ), the process returns to Block B 111 . The sequence of Blocks B 111  to B 114  is repeated until the measurement is made a required number of times. When the earphone is used as a microphone, the response signal may be at so low a level that it is mixed with noise, because of the insufficient sensitivity of the microphone function. In this case, the measurement may not be achieved as is desired. In view of this, the first state and the second state are switched over several times to receive the response signal a number of times, thus acquiring an average value of the response signal. The signal-to-noise ratio is thereby increased. 
     When the measurement is completed (that is, Yes in Block B 115 ), the response signal analysis module  130  finds an average value obtained by measuring the response signal several times (Block B 116 ). If the response signal is received only once (that is, if the sequence of Blocks B 111  to B 114  is performed only once), the process of Block  116  can be omitted. 
     The response signal analysis module  130  acquires the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60  (Block B 117 ). The information for correcting the resonance characteristic of the outer-ear canal  61  of the listener  60  is thus obtained. 
     The physical quantity regarding the resonance characteristic of the outer-ear canal  61  of the listener  60  obtained by the response signal analysis module  130  is output to the correction coefficient generator  30 . The correction coefficient generator  30  generates a correction coefficient for the correction filter  40 , from the physical quantity regarding the resonance characteristic of the outer-ear canal  61  of the listener  60  obtained by the response signal analysis module  130 . The correction coefficient is set in the correction filter  40 . 
     To correct the resonance characteristic of the outer-ear canal  61  of the listener  60 , the correction filter  40  corrects the sound source signal output from the apparatus having an audio function, on the basis of the resonance characteristic of the outer-ear canal  61  of the listener  60 . That is, the correction filter  40  decreases the gain at the resonance frequency of the sound source signal in accordance with the correction coefficient generated by the correction coefficient generator  30  and set in the correction filter  40 . The frequency characteristic is corrected to a flat one. The sound source signal thus filtered by the correction filter  40 , which is an electric signal, is converted by the electric/acoustic transducer  20  to an acoustic signal, because the switch  120  is set to the first state (earphone function). The acoustic signal is output to the outer-ear canal  61  of the listener  60 . 
     In the present embodiment, the measuring signal is thus supplied to the electric/acoustic transducer  20  via the switch  120  and is output to the outer-ear canal  61 . Since the control signal generated by the control signal generator  112  is added to the measuring signal, the influence of the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  can be cancelled out. The control signal generator  112  generates a control signal exhibiting a characteristic that is inverse to a characteristic of the characteristic vibration of the electric/acoustic transducer  20 . The acoustic signal converted from the measuring signal and reflected by the eardrum  62  of the listener  60  is converted by the electric/acoustic transducer  20  to a response signal that is an electric signal. The response signal is supplied to the response signal analysis module  130 . The response signal analysis module  130  acquires the physical quantity that is inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . The correction filter  30  generates a correction coefficient for the correction filter  40 , which may cancel the resonance characteristic of the outer-ear canal  61 . 
     Since the influence of the characteristic vibration of the diaphragm of either earphone is thus cancelled, the resonance characteristic of the listener&#39;s outer-ear canal can be accurately measured. A filter that cancels the peak of the resonance characteristic can therefore be provided based on the measurement result. Thus, even if resonance occurs in the listener&#39;s outer-ear canal, the peak of the resonance characteristic is canceled. This prevents the listener from hearing any unnatural sound. Moreover, the earphone is neither large nor complex in structure, because it incorporates no microphones. Without arranging a microphone near the earphone, a simple configuration can accurately cancel the resonance in the listener&#39;s outer-ear canal. Further, the characteristic of the resonance in the earphone and the outer-ear canal of each listener is measured and the correction filter that accords with the characteristic thus measured is formed. Therefore, the resonance characteristic of the outer-ear canal, which differs in accordance with the physical characteristics of the outer-ear canal and eardrum of each listener and with the state in which the earphone is inserted in the outer-ear canal, can be canceled. The characteristics of both the left ear and the right ear may be acquired, and two correction filters that accord to the characteristics, respectively, may be formed and used to cancel the characteristics of the left and right ears, which differ from each other. 
     Other embodiments of the present invention will be described. Of the components of the other embodiments, those that are identical to those of the firs embodiment are designated by the same reference numbers and will not be described in detail. 
     Second Embodiment 
       FIG. 9  is a block diagram showing an exemplary configuration of an acoustic apparatus  200  according to the second embodiment. The acoustic apparatus  200  is equivalent to the acoustic apparatus  100  according to the first embodiment. As in the first embodiment, the acoustic apparatus  200  comprises a measuring signal generator  210 , a switch  220 , a response signal analysis module  230 , and a controller  240 . The acoustic apparatus  200  is electrically connected to, or formed integral with, either electric/acoustic transducer  20 . The acoustic apparatus  200  further comprises a switch  314 . 
     A correction coefficient generator  30  and a correction filter  40  are arranged outside the acoustic apparatus  200 . The correction coefficient generator  30  generates a correction coefficient on the basis of the resonance characteristic of either outer-ear canal  61  of a listener  60 , and then sets the correction coefficient in the correction filter  40 . The correction filter  40  corrects a sound source signal output from an apparatus having an audio function. 
     As shown in  FIG. 9 , the correction coefficient generator  30  and correction filter  40  are provided outside the acoustic apparatus  100 . Nonetheless, they may be formed integral with the acoustic apparatus  200 . Alternatively, the correction coefficient generator  30  and correction filter  40  may be incorporated in an apparatus having an audio function. 
     If the acoustic apparatus  200  is incorporated in an apparatus having an audio function, the correction coefficient generator  30  and the correction filter  40  may be incorporated in the apparatus having an audio function, too. If the acoustic apparatus  200  is connected to the apparatus having an audio function, the correction coefficient generator  30  and the correction filter  40  may be incorporated in the apparatus having an audio function, or may instead be incorporated in the acoustic apparatus  200 . 
     A switch  50  is changed over to output the sound source signal to the electric/acoustic transducer  20 , either not corrected at all (in no-correction mode) or corrected by the correction filter  40  (in correction mode). The switch  50  is changed over by, for example, a control signal supplied from the controller (processor) of the apparatus having an audio function. The control signal may be transmitted from controller  240  of the acoustic apparatus  200  if the acoustic apparatus  200  is incorporated in the apparatus having an audio function or formed integral with the correction filter  40 . 
     As in the first embodiment, the electric/acoustic transducer  20  may be an earphone or headphone and has the function of converting an electric signal to an acoustic signal, and vice versa. If the mode of measuring the outer-ear canal characteristic is set, the electric/acoustic transducer  20  converts an electric signal to an acoustic signal, which is applied into the outer-ear canal  61  of the listener  60 , and converts the acoustic signal reflected from the eardrum  62  of the listener  60 , to an electric signal. If the mode of measuring the characteristic vibration is set, the electric/acoustic transducer  20  is not inserted into the outer-ear canal  61  of the listener  60  and is arranged in a free sound field. In this case, the characteristic vibration of the diaphragm is observed. 
     The controller  240  has a memory or can access a memory (not shown). The controller  240  executes a program stored in the memory, controlling the other components of the acoustic apparatus  200 . The controller  240  can set two modes, i.e., the mode of measuring the resonance characteristic of the outer-ear canal  61  of the listener  60  and the mode of measuring the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . Further, the controller  240  can switch the state of the electric/acoustic transducer  20 , between the first state in which the transducer  20  converts an electric signal to an acoustic signal and the second state in which the transducer  20  converts an acoustic signal to an electric signal. 
     The controller  240  is provided in the acoustic apparatus  200  as shown in  FIG. 9 . Instead, the controller  240  may be provided outside the acoustic apparatus  200 . If the acoustic apparatus  200  is connected to an apparatus having an audio function, the controller (processor) of this apparatus may operate as a controller  240 . In this case, the controller of the apparatus having an audio function executes a predetermined program while controlling the acoustic apparatus  200 . Alternatively, the controller of the apparatus having an audio function may control the controller  240  incorporated in the acoustic apparatus  200 . 
     The measuring signal generator  210  includes a unit pulse generator  211 , a control signal generator  214 , an adder  213 , and a switch  314 . The unit pulse generator  211  generates unit pulses to measure the resonance characteristic of the outer-ear canal  61  of the listener  60 . The control signal generator  214  generates a signal to control or suppress the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . 
     To measure the resonance characteristic of the outer-ear canal, the switch  314  is connected to the output of the adder  213 . The adder  213  adds the outputs of the unit pulse generator  211  and control signal generator  214 , generating and outputting a measuring signal. 
     On the other hand, if the characteristic vibration measurement mode is set, the switch  314  is connected to the output of the unit pulse generator  211 . The unit pulse generated by the unit pulse generator  211  is output directly to the switch  220 . 
     The switch  220  is changed over to receive a signal from, or outputs a signal to, the electric/acoustic transducer  20 . 
     When the controller  240  is set to the first state, the switch  220  is switched to the measuring signal generator  210 . In this case, the switch  220  connects the output of the measuring signal generator  210  to the electric/acoustic transducer  20 . The electric/acoustic transducer  20  converts the measuring signal output from the measuring signal generator  210 , to an acoustic signal. The acoustic signal is output to the outer-ear canal  61  of the listener  60 . 
     On the other hand, when the controller  240  is set to the second state, the switch  220  is switched to the input of the response signal analysis module  230 . In this case, the switch  220  connects the electric/acoustic transducer  20  to the response signal analysis module  230 . The electric/acoustic transducer  20  collects the sound reflected by the eardrum  62  of the listener  60 , i.e., measurement object, and converts the acoustic signal to an electric signal. This electric signal is output to the response signal analysis module  230 . 
     The response signal analysis module  230  acquires, from the response signal, the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . That is, the response signal analysis module  230  first converts the response signal, from a time domain to a frequency domain, and then detects the peak frequency and amplitude at the peak frequency, which are the physical quantities inherent to the outer-ear canal  61  of the listener  60 . 
     In the present embodiment, the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 , which the response signal analysis module  230  has acquired, can be output to either the measuring signal generator  210  or the correction coefficient generator  30 . 
     If the output of the response signal analysis module  230  is supplied to the correction coefficient generator  30 , the operation is the same as in the first embodiment. The correction coefficient generator  30  generates a correction coefficient for the correction filter  40 , on the basis of the resonance characteristic of either outer-ear canal  61  of a listener  60 , which the response signal analysis unit  230  has acquired. The correction filter  40  decreases the gain at the resonance frequency, correcting the frequency characteristic to a flat one. A parametric equalizer or a graphic equalizer can achieve the function of the correction filter  40 . The resonance characteristic of the outer-ear canal  61  of a listener  60  can be acquired in the same way as in the first embodiment, or as shown in the flowchart of  FIG. 8 . 
     On the other hand, to make the response signal analysis module  230  measure the control signal that should be output to the measuring signal generator  210 , the control signal generator  214  generates a control signal on the basis of the physical quantity inherent to the characteristic of the control signal which the response signal analysis module  230  has acquired. The response signal analysis module  230  outputs such a control signal characteristic as shown in, for example,  FIG. 5 , to the control signal generator  214 . The control signal generator  214  generates a control signal that has a characteristic inverse to that of the input control signal (or characteristic that is quasi-inverse thereto). The control signal generated by the control signal generator  214  is output to the adder  213 . The adder  213  adds the control signal to the unit pulse generated by the unit pulse generator  211 , thus generating a measuring signal. The measuring signal, thus generated, is output to the switch unit  314 . As a result, the pulse generator  211  outputs such a measuring signal as shown in  FIG. 6 . 
     As in the first embodiment, the control signal generator  214  delays the unit pulse generated by the unit pulse generator  211  by half the reciprocal Tp of the peak frequency fP, i.e., Tp=1/fP, thus generating a pulse having the amplitude calculated from the magnitude of the peak frequency. If the measuring signal generator  210  has long sampling intervals, time Tp/2 can hardly be measured. In this case, the unit pulse generated by the unit pulse generator  211  after the up-sampling is delayed by half the time Tp, generating a pulse having the amplitude calculated from the magnitude of the peak frequency. After a low-pass filter process, down-sampling is performed. That is, the sampling intervals are made so short that time Tp/2 may be measured, and a pulse is set for time Tp, thereby acquiring a waveform defined by the initial sampling intervals by means of a decimation filter or the like. The control signal, thus generated, is output to the adder  213 . The adder  213  adds a control signal to the unit pulse generated by the unit pulse generator  211 , generating a measuring signal. The measuring signal is output to the switch  220 . 
     Thus, in the second embodiment, the switch  314  is operated to measure the resonance characteristic of the outer-ear canal  61  of the listener  60  or the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . Further, a control signal can be generated on the basis of the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . Therefore, the vibration characteristic of the diaphragm of the electric/acoustic transducer  20  need not be measured beforehand. Hence, the resonance characteristic of the outer-ear canal  61  of the listener  60  can be accurately measured with any earphone available. 
     The process performed in the second embodiment for measuring the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  will be explained.  FIG. 10  is a flowchart explaining the process of measuring the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . 
     The characteristic vibration may be measured when the electric/acoustic transducer  20  is used for the first time, or every time the electric/acoustic transducer  20  is used. Alternatively, it may be measured in accordance with the instruction of the listener  60 . When to measure a characteristic of the characteristic vibration may be preset. 
     To measure the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 , the controller  240  determines whether the characteristic vibration should be measured or not (Block B 210 ). 
     If a characteristic of the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  has been acquired, or the characteristic vibration need not be measured (if No in Block B 210 ), the controller  240  terminates the process. Then, the process of measuring the resonance characteristic of the outer-ear canal  61  of the listener  60  is started. 
     If the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  should be measured (if Yes in Block B 210 ), the electric/acoustic transducer  20  is arranged in a free sound field (Block B 211 ). The controller  240  may cause the display of an apparatus having an audio function to display a message. The message prompts the listener  60  to remove the electric/acoustic transducer  20  from his or her ear so that no reflection may occur. 
     Then, the controller  240  sets the mode of measuring the characteristic vibration. The switch  314  is thereby changed over to the output side of the unit pulse generator  211 . 
     Next, the switch  220  is changed over to the measuring signal generator  210 . Therefore, the electric/acoustic transducer  20  functions as an earphone for outputting a measuring signal and the switch  220  is set to the first state (Block B 212 ). The switch  220  therefore connects the output of the measuring signal generator  210  to the electric/acoustic transducer  20 . 
     The unit pulse generated by the unit pulse generator  211  is output from the measuring signal generator  210  to the electric/acoustic transducer  20 . The electric/acoustic transducer  20  converts the unit pulse to an acoustic signal (Block B 213 ). The controller  240  changes over the switch  220  to the input side of the response signal analysis module  230 , setting the switch  220  to the second state (Block B 214 ). 
     The electric/acoustic transducer  20  converts the characteristic vibration of the diaphragm, caused by the unit pulse, to a characteristic vibration signal (response signal), which is an electric signal (Block B 215 ). The response signal is output to the response signal analysis module  230 . 
     Next, the controller  240  determines whether the measurement has been made a specific number of times as required (Block B 216 ). The measurement should be made, for example, several times. If the measurement has not been made several times (if No in Block B 216 ), the process returns to Block B 212 . The sequence of Blocks B 212  to B 215  is repeated until the measurement is made a required number of times. When the earphone is used as a microphone, the response signal may be at so low a level that it is mixed in with noise, because of the insufficient sensitivity of the microphone function. In this case, the measurement may not be achieved as is desired. In view of this, the first state and the second state can be switched over several times to receive the response signal a number of times. 
     When the measurement is completed (that is, Yes in Block B 216 ), the response signal analysis module  130  finds an average value obtained by measuring the response signal several times (Block B 217 ). If the response signal is received only once, the process of Block  217  can be dispensed with. 
     The response signal analysis module  230  acquires the physical quantity inherent to a characteristic of the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  (Block B 218 ). The physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 , which is acquired by the response signal analysis module  230 , is supplied to the control signal generator  214 . 
     The physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  is thus acquired. The control signal generator  214  can generate a control signal on the basis of the physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . 
     To measure the resonance characteristic of the outer-ear canal  61  of the listener  60 , the switch  314  is changed over to the output of the adder  213 . A process similar to the process shown in the flowchart of  FIG. 8  is performed, measuring the resonance characteristic of the outer-ear canal  61  of the listener  60 . 
     To correct the resonance characteristic of the outer-ear canal  61  of the listener  60 , the sound source signal output from the apparatus having an audio function is corrected by the correction filter  40 . The correction filter  40  decreases the gain at the resonance frequency of the sound source signal in accordance with the correction coefficient generated by the correction coefficient generator  30  and set in the correction filter  40 . The frequency characteristic is thereby corrected to a flat one. The sound source signal thus filtered by the correction filter  40 , which is an electric signal, is converted by the electric/acoustic transducer  20  to an acoustic signal, because the switch  220  is set to the first state (earphone function). The acoustic signal is output to the outer-ear canal  61  of the listener  60 . 
     Thus, in the present embodiment, the switch  314  outputs the unit pulse via the switch  220  to the electric/acoustic transducer  20  while the characteristic vibration is being measured. The unit pulse causes the diaphragm of the electric/acoustic transducer  20  to undergo characteristic vibration. The electric/acoustic transducer  20  converts the characteristic vibration to a characteristic vibration signal (response signal). The response signal is output the response signal analysis module  230 . The response signal analysis module  230  acquires the physical quantity that is inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . The physical quantity that is inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 , thus acquired, is output to the control signal generator  214 . The control signal generator  214  generates a control signal that exhibits a characteristic inverse to that of the input control signal. In the present embodiment, a physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  can be acquired, and a control signal can be generated, which accords with the physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . 
     To measure the resonance characteristic of the outer-ear canal  61  of the listener  60 , a measuring signal, to which the unit pulse has been added, is supplied from the switch  220  to the electric/acoustic transducer  20 . The electric/acoustic transducer  20  converts the measuring signal, which is an electric signal, to an acoustic signal. The acoustic signal is output to the outer-ear canal  61  of the listener  60 . The measuring signal, to which the control signal generated by the control signal generator  214  is added, can cancel the influence of the characteristic vibration of the diaphragm of the electric/acoustic transducer  20 . The electric/acoustic transducer  20  converts the acoustic signal converted from the eardrum  62  of the listener  60 , to a response signal that is an electric signal. The response signal is supplied to the response signal analysis module  230 . The response signal analysis module  230  acquires the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60 . The correction coefficient generator  30  generates a correction coefficient for the correction filter  40 , which may cancel the resonance characteristic. 
     Since the influence of the characteristic vibration of the diaphragm is canceled in either earphone, the resonance characteristic of the outer-ear canal  61  of the listener  60  can be measured accurately. A filter that cancels the peak is therefore formed in accordance with the result of measuring. Hence, the peak of the resonance characteristic can be canceled even if resonance occurs in the outer-ear canal of the listener. This prevents the listener from hearing any unnatural sound. Moreover, the earphone is neither large nor complex in structure, because it incorporates no microphones. Without arranging a microphone near the earphone, a simple configuration can accurately cancel the resonance in the listener&#39;s outer-ear canal. Further, the resonance characteristic of the outer-ear canal, which differs in accordance with the physical characteristics of the outer-ear canal and eardrum and with the state in which the earphone is inserted in the outer-ear canal, can be canceled, because the characteristic of the resonance in the earphone and the outer-ear canal is measured and the correction filter that accords with the characteristic thus measured is formed and used. 
     As described above, the process ( FIG. 10 ) of acquiring the physical quantity inherent to the characteristic vibration of the diaphragm of the electric/acoustic transducer  20  and the process ( FIG. 8 ) of measuring the physical quantity inherent to the resonance characteristic of the outer-ear canal  61  of the listener  60  are performed independently. Nonetheless, these processes may be performed continuously, one after the other. 
       FIG. 11  is a diagram showing an exemplary use of the acoustic apparatus  100  according to the first embodiment or acoustic apparatus  200  according to the second embodiment. If acoustic apparatus  100  or  200  is incorporated in an audio player  90 , the apparatus may be incorporated not in the main unit of the player  90 , but in a remote control  92  or an earphone  94 . Further, the apparatus  100  or  200  need not be incorporated, in its entirely, in the audio player  90 . Rather, the correction filter  40  may be singularly incorporated in the audio player  90 . That is, the audio player  90  may use the correction filter  40  to correct sound source signals read from a flash memory, a hard disk or the like (not shown), whereas a personal computer, for example, may generate measuring signals, may measure the resonance characteristics and may generate a correction coefficient. Alternatively, if the correction filter  40  is incorporated in the audio player  90 , a sound source signal may be first corrected and may then be stored (downloaded) into a memory or the like. 
     The resonance characteristic of the outer-ear canal  61  of the listener  60  may differ in accordance with the physical characteristics of the outer-ear canal and eardrum and with the state in which the earphone is inserted in the outer-ear canal. Therefore, the characteristic vibration of the electric/acoustic transducer  20  and the resonance characteristic of the outer-ear canal  61  of the listener  60  may be measured and corrected in the acoustic apparatus  200 , for example, very time the audio player  90  is activated, when the user operates the apparatus  200 , or upon the lapse of a time the user has preset. 
     According to an embodiment of the present invention, an acoustic apparatus comprises a measuring signal generator configured to generate a pulse; a transducer configured to convert the pulse to a sound to be output to a free sound field and to convert a characteristic vibration of the transducer to a characteristic vibration signal; a signal analysis module configured to analyze the characteristic vibration signal in order to output a physical quality representing the characteristic vibration of the transducer; a controller configured to set one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal; and a switch configured to connect the measuring signal generator to the transducer in the first state and to connect the signal analysis module to the transducer in the second state. 
     According to another embodiment of the present invention, an acoustic apparatus comprises a measuring signal generator comprising a pulse generator configured to generate a pulse, a control signal generator configured to generate a control signal, and an adder configured to add the measuring pulse and the control signal in order to generate a measuring signal; a transducer comprising a diaphragm of which characteristic vibration is controlled by the control signal and configured to convert the measuring signal to a measuring sound to be applied to a measurement object and to convert a measuring sound reflected from the measurement object to a response signal; a response signal analysis module configured to analyze the response signal in order to output a physical quantity representing an acoustic characteristic of the measurement object; a controller configured to set one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal; and a switch configured to connect the measuring signal generator to the transducer in the first state and to connect the response signal analysis module to the transducer in the second state. 
     According to another embodiment of the present invention, an acoustic apparatus comprises a measuring signal generator configured to generate a measuring pulse to be converted to a sound by a transducer in a free sound field; a signal analysis module configured to analyze a characteristic vibration of the transducer in order to output a physical quality representing the characteristic vibration of the transducer; a controller configured to set one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal; and a switch configured to connect the measuring signal generator to the transducer in the first state and to connect the signal analysis module to the transducer in the second state. 
     According to another embodiment of the present invention, an acoustic apparatus comprises a measuring signal generator comprising a pulse generator configured to generate a pulse, a control signal generator configured to generate a control signal for controlling a characteristic vibration of a transducer, and an adder configured to add the measuring pulse and the control signal in order to generate a measuring signal; a signal analysis module configured to analyze a response signal output from the transducer by converting a measuring sound corresponding to the measuring signal and reflected from a measurement object in order to output a physical quantity representing an acoustic characteristic of the measurement object; a controller configured to set one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal; and a switch configured to connect the measuring signal generator to the transducer in the first state and to connect the signal analysis module to the transducer in the second state. 
     According to another embodiment of the present invention, a method of measuring a characteristic vibration, comprises generating a pulse; converting, with a transducer, the pulse to a sound in a free sound field and converting a characteristic vibration of the transducer to a characteristic vibration signal; analyzing the characteristic vibration signal in order to output a physical quantity representing the characteristic vibration of the transducer; and setting one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal. 
     According to another embodiment of the present invention, a method of measuring an acoustic characteristic, comprises generating a pulse; generating a control signal for controlling a characteristic vibration of a transducer; outputting a measuring signal by adding the pulse and the control signal; converting, with the transducer, the measuring signal to a measuring sound and converting a measuring sound reflected from a measurement object to a response signal; analyzing the response signal in order to output a physical quantity representing an acoustic characteristic of the measurement object; and setting one of a first state in which the transducer converts an electric signal to a sound and a second state in which the transducer converts a sound to an electric signal. 
     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The various modules of the systems described herein can be implemented as software applications, hardware and/or software modules, or components on one or more computers, such as servers. While the various modules are illustrated separately, they may share some or all of the same underlying logic or code. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.