Patent Publication Number: US-8526627-B2

Title: Noise reduction device

Description:
TECHNICAL FIELD 
     The present invention relates to a noise reduction device and, in particular, it relates to a noise reduction device capable of actively reducing noise coming to a control point. 
     BACKGROUND ART 
     When providing information such as voice guidance and the like services to passengers sitting in seats inside an aircraft, a railcar and the like filled with loud noise, the noise in the seats becomes a problem. 
     An interior space confined within a continuous wall like that of an aircraft or a car has a kind of sealed structure, and the noise environment remains persistent for the passengers when there is a noise source either inside or outside the space. Depending on a level of the noise, it can become a factor of physical and mental stresses to the passengers, and it therefore impairs the comfort. The noise thus poses a serious problem in the quality of service especially when the service is provided for the passengers inside a cabin in an aircraft and the like. 
     In the case of an aircraft, the noise of apparatuses such as propellers and engines for generating thrust of the aircraft and the sound associated with airflow produced around the airframe moving in the aerial space such as a wind noise during the flight are main sources of the noise. Since the noise inside the cabin interferes with the voice guidance and the like services in addition to causing discomfort of the passengers, an improvement of it is strongly desired. 
     To this end, the methods hitherto used are passive damping means in most instances, such that a sound insulation material having sound absorbing properties like sound barrier material and acoustic absorbing material is disposed between the sealed structure and the noise source as the measures to reduce the noise inside the sealed cabin. A high-density barrier material and a sound-absorbing sheet are examples used as the sound barrier material and the acoustic absorbing material respectively. Any material of sound absorbing property generally has a high density, which accompanies an extra weight. The fuel consumption increases with increase in weight, and the cruising range decreases. It hence causes a decline in economical efficiency and deterioration of the function as an aircraft. There are also other aspects of functionality such as decrease in the strength as being susceptible to damages and the design of esthetical quality that are not ignorable as the structural materials. 
     To address the problems associated with the noise reduction measures using the passive damping means discussed above, there exists an idea disclosed recently of a noise reduction device capable of executing control for reduction of noise coming to a control point by selectively operating any of a fixed filter and an adaptive filter and outputting a control sound of opposite phase to that of the noise coming to the control point (known as active noise control, refer to Patent Literature 1, for example). 
     In this example, the noise reduction device comprises noise-detecting microphone  9101 , adaptive filter  9201 , control speaker  9401 , error microphone  9501  and fixed filter  9601 , as shown in  FIG. 9 , and the device executes the noise control by selecting adaptive filter  9201  when a change occurs in the condition of noise. Noise-detecting microphone  9101  detects a noise and outputs it as a noise signal to adaptive filter  9201 . Adaptive filter  9201  executes signal processing on the noise signal by using a filter coefficient and generates a control signal, and control speaker  9401  outputs a control sound toward the control point. Error microphone  9501  is placed in the control point for detecting a difference between the noise and the control sound, and output it as an error signal. Adaptive filter  9201  executes an updating process of the filter coefficient in a manner to minimize the error signal by using the Filtered-X_LMS algorism, for instance, to update the filter coefficient in response to a change in the condition of the noise, and generates a new control signal. When the filter coefficient being updated converges, the device carries out the noise control by selecting fixed filter  9601 , which is preset with the converged filter coefficient. 
     The above method may make the noise-detecting microphone and the error microphone unable to continue their normal detecting functions if there is any obstacle such as a pillow, blanket, book and the like object placed around the microphones, thereby leaving a problem of presenting a possibility of increasing the noise in the control point depending on a condition of such obstacle. Among some measures known to address this problem, there are techniques disclosed as conventional arts, one of which optimizes the control by changing the filter coefficient according to a distance to the obstacle if any, and the other suppresses an abnormal sound attributed to the obstacle by setting a threshold for the filter coefficient (refer to Patent Literatures 2 and 3, for example). 
     According to the above conventional arts, however, the devices are designed to calculate the filter coefficient continuously in real time, and hence there has remained a possibility depending on the condition of the obstacle that the noise becomes larger than that without carrying out the control for noise reduction, and it discomforts the passengers when a control sound updated according to the calculated filter coefficient is output immediately. Moreover, the above arts have not given any consideration to operation with the noise-detecting microphone covered with an obstacle, thereby making it unlikely to achieve a steady effect of the noise reduction. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: Unexamined Japanese Patent Publication No. H02-285799 
         Patent Literature 2: Unexamined Japanese Patent Publication No. H09-146559 
         Patent Literature 3: Unexamined Japanese Patent Publication No. 2007-047367 
       
    
     DISCLOSURE OF INVENTION 
     A noise reduction device of the present invention comprises a noise controller unit for generating a control sound signal to cancel out a noise, a control sound output unit for outputting a control sound according to the control sound signal from the noise controller unit, a residual sound detector for detecting a residual sound by superimposing the control sound on the noise, and an obstacle detector for detecting an obstacle in the vicinity of the residual sound detector, wherein the noise controller unit generates the control sound signal according to data from the residual sound detector and the obstacle detector. 
     By virtue of the above structure, it becomes possible to execute control of the noise reduction including such cases that obstacles exist, thereby providing the noise reduction device capable of executing the control steadily with high quality even in an environment where the obstacles exist. 
     Furthermore, the noise reduction device of the present invention comprises a noise detector for detecting a noise supplied from at least one noise source, a noise controller unit for generating a control sound signal to cancel out the noise detected by the noise detector, a control sound output unit for outputting a control sound according to the control sound signal from the noise controller unit, a residual sound detector for detecting a residual sound by superimposing the control sound on the noise supplied from the noise source, and an obstacle detector for detecting an obstacle to at least one of the noise detector and the residual sound detector, wherein the noise controller unit generates the control sound signal according to data from the noise detector, the residual sound detector and the obstacle detector. 
     It becomes possible by virtue of the above structure to detect the noise and execute control of noise reduction including such cases that obstacles exist. Thus provided is the noise reduction device capable of executing even steadier and higher quality of control under the environment where the obstacles exist. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a front view of a noise reduction device showing an example of mounted configuration according to a first exemplary embodiment of the present invention; 
         FIG. 1B  is a side view of the noise reduction device showing the example of mounted configuration according to the first exemplary embodiment of the invention; 
         FIG. 1C  is a rear view of the noise reduction device showing the example of mounted configuration according to the first exemplary embodiment of the invention; 
         FIG. 2  is a block diagram showing a basic circuit structure of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 3A  is a front view showing a main component layout to illustrate operation of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 3B  is a side view showing the main component layout to illustrate the operation of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 3C  is a rear view showing the main component layout to illustrate the operation of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 4  is a block diagram showing a main structure involved in generation of a control sound of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 5  is another block diagram showing a detailed structure involved in generation of the control sound of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 6  is a control table used for generating the control sound of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 7  is a flow chart showing operation processes of updating a filter coefficient of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 8A  is a graphic representation showing frequency characteristic of white noise produced from a control speaker and detected by an error microphone of the noise reduction device according to the first exemplary embodiment of the invention; 
         FIG. 8B  is another graphic representation showing frequency characteristic of white noise produced from the control speaker and detected by the error microphone of the noise reduction device according to the first exemplary embodiment of the invention; and 
         FIG. 9  is a block diagram showing a control method of a conventional noise reduction device. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Description is provided first of the fundamental concept of the present invention with reference to  FIGS. 1A ,  1 B,  1 C and  2 , before detailing an exemplary embodiment of the invention. 
       FIG. 1A  is a front view showing an example of seat  600  of an aircraft with passenger (i.e., a user) A sitting therein, and  FIGS. 1B and 1C  are a side view and rear view respectively.  FIG. 2  is a block diagram showing a circuit structure of a noise reduction device mounted to seat  600  shown in  FIGS. 1A to 1C . 
     As illustrated in  FIGS. 1A to 1C , noise-detecting microphones  101  to  120  are disposed to exterior sides of seat  600 . Control speakers  401  to  404  are disposed to inner surfaces of seat  600  at positions of equal height to the ears of user A. Control points are set to be at the ears of user A, and error microphones  501  and  502  are placed on the ears of user A designated as the control points, as one example. 
     Noise reduction device  100  comprises noise-detecting microphones  101  to  120 , adaptive filters  201 - 1  to  220 - 1 ,  201 - 2  to  220 - 2 ,  201 - 3  to  220 - 3  and  201 - 4  to  220 - 4 , adders  301  to  304 , control speakers  401  to  404 , and error microphones  501  and  502 , as shown in  FIG. 2 . 
     A noise detected by noise-detecting microphone  101  is output as a noise signal to adaptive filters  201 - 1  to  201 - 4 . Another noise detected by noise-detecting microphone  102  is output as another noise signal to adaptive filters  202 - 1  to  202 - 4 . In the same manner as above, noises detected by noise-detecting microphones  103  to  120  are output to their corresponding adaptive filters  203  to  220  having suffixes “- 1 ” to “- 4 ” respectively. 
     Adaptive filter  201 - 1  includes a transfer function necessary from control speaker  401  to error microphone  501  and another transfer function necessary from control speaker  401  to error microphone  502 , which are preset in advance by using the Filtered-X_LMS algorism. Adaptive filter  201 - 1  updates a filter coefficient thereof in a manner to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     Error microphones  501  and  502  are placed in the control points, so that they detects the noise coming to these control points from a noise source and control sound coming to these control points from control speakers  401  to  404 . The incoming noise and the control sound from control speakers  401  to  404  reaching these control points interfere with one another (or, superimposed) in each of error microphones  501  and  502 , and a difference between the noise and the control sound is hence detected as an error signal (i.e., residual sound signal). 
     Likewise, adaptive filter  202 - 1  includes a preset transfer function from control speaker  401  to error microphone  501  and another transfer function from control speaker  401  to error microphone  502 . Adaptive filter  202 - 1  updates its filter coefficient in a manner to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     In the same manner as above, each of other adaptive filters  203 - 1  to  220 - 1  also includes a preset transfer function from control speaker  401  to error microphone  501  and another transfer function from control speaker  401  to error microphone  502 . Adaptive filters  203 - 1  to  220 - 1  update their respective filter coefficients to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     Similarly, each of adaptive filters  201 - 2  to  220 - 2  includes a preset transfer function from control speaker  402  to error microphone  501  and another transfer function from control speaker  402  to error microphone  502 . Adaptive filters  201 - 2  to  220 - 2  update their respective filter coefficients to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     Likewise, each of adaptive filters  201 - 3  to  220 - 3  includes a preset transfer function from control speaker  403  to error microphone  501  and another transfer function from control speaker  403  to error microphone  502 . Adaptive filters  201 - 3  to  220 - 3  update their respective filter coefficients to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     Moreover, each of adaptive filters  201 - 4  to  220 - 4  includes a preset transfer function from control speaker  404  to error microphone  501  and another transfer function from control speaker  404  to error microphone  502 . Adaptive filters  201 - 4  to  220 - 4  update their respective filter coefficients to minimize individual error signals from error microphones  501  and  502  collectively by using the individually preset transfer functions. 
     Adaptive filters  201 - 1  to  220 - 1  execute signal processing on the noise signals input therein by using the updated filter coefficients, and the respective results are output as control signals to adder  301 . Adder  301  adds up the control signals from adaptive filters  201 - 1  to  220 - 1 , and outputs the result to control speaker  401 . Control speaker  401  outputs a control sound according to the control signal from adder  301  toward error microphones  501  and  502  at the control points. 
     Adaptive filters  201 - 2  to  220 - 2  execute signal processing on the noise signals input therein by using the updated filter coefficients, and the respective results are output as control signals to adder  302 . Adder  302  adds up the control signals from adaptive filters  201 - 2  to  220 - 2  and outputs the result to control speaker  402 . Control speaker  402  outputs a control sound according to the control signal from adder  302  toward error microphones  501  and  502  at the control points. 
     Adaptive filters  201 - 3  to  220 - 3  execute signal processing on the noise signals input therein by using the updated filter coefficients, and the respective results are output as control signals to adder  303 . Adder  303  adds up the control signals from adaptive filters  201 - 3  to  220 - 3  and outputs the result to control speaker  403 . Control speaker  403  outputs a control sound according to the control signal from adder  303  toward error microphones  501  and  502  at the control points. 
     Adaptive filters  201 - 4  to  220 - 4  execute signal processing on the noise signals input therein by using the updated filter coefficients, and the respective results are output as control signals to adder  304 . Adder  304  adds up the control signals from adaptive filters  201 - 4  to  220 - 4  and outputs the result to control speaker  404 . Control speaker  404  outputs a control sound according to the control signal from adder  304  toward error microphones  501  and  502  at the control points. 
     Accordingly, noise reduction device  100  mounted to seat  600  can reduce the incoming noise to the ears of user A, i.e., the control points, by virtue of the updating process of the filter coefficients discussed above. 
     In this example, it becomes possible even when noise control is carried out only with fixed filters to demonstrate the effect of noise reduction equivalent to the case, in which the noise control is done with the adaptive filters if there are practically no changes in at least one of frequency and level of the noise, or the changes remain within a certain range of variations. 
     However, this does not necessarily mean that an optimum filter coefficient can always be prepared in advance for use as a filter coefficient set to each fixed filter since the noise varies in type depending on position of the seat. It is also difficult to ensure the preset filter coefficient to remain optimum for an extended period of time even if it is initially the optimum filter coefficient, because of changes in the installation environment due to renewal of equipments, age deterioration and the like. 
     In the case of an aircraft having a large numbers of seats, for instance, it becomes necessary to work with the individual seats in order to obtain an optimum filter coefficient for each of them. Furthermore, the task of obtaining the optimum filter coefficient for each seat becomes enormous since airframe, engines, configuration of seats, etc. vary from one model to another, besides certain cases that different air carriers use different engines. 
     The present invention addresses the problems discussed above, and makes possible full use of the optimum effect of noise reduction at all times by updating the filter coefficient according to a position of the seat and surrounding environment from the value set to the fixed filter when the noise is comparatively stable. In addition, the invention can substantially reduce man-hours necessary to set the filter coefficient for each seat by enabling the updating task of filter coefficient automatically. 
     Description will be provided hereinafter of an exemplary embodiment of the present invention with reference to the drawings. 
     Exemplary Embodiment 
     Referring now to  FIG. 3A  to  FIG. 5 , description is provided of a circuit structure and operation related to updating of filter coefficients, which is the primary feature of noise reduction device  1  (hereinafter referred to simply as device  1 ) according to the first exemplary embodiment of the present invention.  FIG. 3A  is a front view showing an example of seat  2  of an aircraft with user A sitting therein, and  FIGS. 3B and 3C  are a side view and a rear view respectively.  FIG. 4  is a block diagram showing a circuit structure of device  1  mounted to seat  2 . 
     In the example shown in  FIG. 1A  to  FIG. 2 , noise reduction device  100  has been illustrated as having twenty (20) noise-detecting microphones. However, device  1  being presented here is an example having three (3) noise-detecting microphones in order to simplify the following explanation. In any instance where a number of the noise-detecting microphones is increased or decreased, it will become obvious that advantages of the present invention can be demonstrated effectively by either expanding or simplifying the structure based on the concept discussed below. 
     As shown in  FIGS. 3A to 3C , device  1  comprises three noise-detecting microphones  11  to  13  disposed to exterior sides of seat  2  to serve as noise detectors for detecting noises for the control purpose, and they output the detected incoming noises as control sound signals to control filter unit  90 . There are four control speakers  41   a  to  41   d  disposed to inner surfaces of seat  2  at positions of equal height to the ears of user A. Control points are set to be at the ears of user A. Control speakers  41   a  to  41   d  (these speakers  41   a  to  41   d  are collectively referred to hereinafter as speaker  41 ) receive a control signal generated in control filter unit  90 , and output control sounds simultaneously toward the control points. Error microphones  51   a  and  51   b  (these error microphones  51   a  and  51   b  are collectively referred to hereinafter as error microphone  51 ) are placed in positions of seat  2  close to ears of user A when seated, for example. 
     As shown next in  FIG. 4 , device  1  comprises noise-detecting microphones  11  to  13  serving as the noise detectors, control filter unit  90 , control speaker  41  representing a control sound output unit, signal storage unit  60 , filter coefficient calculation unit  70 , filter coefficient updating unit  80  and error microphone  51  serving as a residual sound detector. Here, control filter unit  90 , signal storage unit  60 , filter coefficient calculation unit  70  and filter coefficient updating unit  80  compose a noise controller unit. 
     Control filter unit  90  and control speaker  41  operate in a manner as described hereinafter. 
     Control filter unit  90  is provided with fixed filters  21  to  23  and adder  31 . A noise detected by noise-detecting microphone  11  is output as a control sound signal to fixed filter  21 . Likewise, noises detected by noise-detecting microphones  12  and  13  are output as control sound signals to fixed filters  22  and  23  respectively. 
     Fixed filter  21  executes signal processing on the control sound signal from noise-detecting microphone  11  by using a fixed filter coefficient set by filter coefficient calculation unit  70  and filter coefficient updating unit  80 , as will be described later, and outputs it as a control signal to adder  31 . The filter coefficient set to fixed filter  21  is a coefficient obtained for the purpose that the control sound output from control speaker  41  according to the generated control signal becomes opposite in phase at the control point with respect to a phase of a predetermined noise that reaches the same control point. 
     The predetermined noise means a noise generated inside an aircraft while flying at a normal cruising speed, and the filter coefficient set to each of fixed filters  21  to  23  shall be the one calculated when at least one of frequency and level of the predetermined noise reaching the control point is varying within a certain range of variations. 
     Similarly, fixed filters  22  and  23  execute signal processing on the control sound signal from noise-detecting microphones  12  and  13  by using fixed filter coefficients set respectively by filter coefficient calculation unit  70  and filter coefficient updating unit  80 , as will be described later, and output control signals to adder  31 . The filter coefficients set to fixed filters  22  and  23  also have the same characteristics as the filter coefficient of fixed filter  21 . 
     Adder  31  adds up the control signals received from fixed filters  21  to  23 , and outputs the result to control speaker  41 . Control speaker  41  outputs a control sound according to the control signal from adder  31  toward the control point. 
     Error microphone  51  detects the noise at the mounted position. Error microphone  51  used as a residual sound detector is disposed in the control point, and detects the noise coming from a noise source and the control sound coming from control speakers  41  at the control point. The incoming noise and the control sound reaching the control point interfere with each other (or, superimposed) in error microphone  51 , and a difference is detected as an error signal (i.e., residual sound signal). 
     Signal storage unit  60  records noise signals n 1  to n 3  from noise-detecting microphones  11  to  13  and error signal e 1  from error microphone  51  for a predetermined period of time into an internal memory or the like, for instance. Upon completion of the recording, signal storage unit  60  gives filter coefficient calculation unit  70  a command of starting calculation of filter coefficients. 
     In response to the command from signal storage unit  60 , filter coefficient calculation unit  70  calculates filter coefficients of fixed filters  21  to  23  provided inside control filter unit  90  by using data recorded in signal storage unit  60 . 
     Filter coefficient updating unit  80  reads the filter coefficients calculated by filter coefficient calculation unit  70  at a predetermined timing, and renews the filter coefficients set to fixed filters  21  to  23  with the filter coefficients read from the filter coefficient calculation unit  70 . 
     Description is provided next of a structure of filter coefficient calculation unit  70  and how it operates by using  FIG. 5 .  FIG. 5  is a block diagram showing an example of circuit structure of filter coefficient calculation unit  70 . 
     Filter coefficient calculation unit  70  comprises adaptive filters  7201  to  7203 , adders  7301  and  7302 , acoustic system filter  7401  and adder  7501 . 
     Noise signals n 1  to n 3  received from signal storage unit  60  are input to adaptive filters  7201  to  7203 . 
     Adaptive filters  7201  to  7203  include a transfer function (Fx 1 ) necessary from control speaker  41  to error microphone  51 , which is preset in advance by using the Filtered-X_LMS algorism. 
     Adaptive filters  7201  to  7203  execute signal processing on the noise signals n 1  to n 3  respectively input therein by using the preset filter coefficient, and the results are output as control signals to adders  7301  and  7302 . Adder  7301  adds up the control signals from adaptive filters  7201  to  7203  in the end, and outputs the result to acoustic system filter  7401 . 
     Acoustic system filter  7401  has a transfer function (Fx 1 ) from control speaker  41  to error microphone  51 , which is preset in advance, and the control signal having passed through acoustic system filter  7401  is output to adder  7501 . Adder  7501  also receives an input of error signal e 1  so that adder  7501  adds up these input signals. 
     The added signal output from adder  7501  is input as error signal E 1  into adaptive filters  7201  to  7203 , which in turn update their respective filter coefficients in a manner to minimize error signal E 1 . 
     As discussed above, noise signals n 1  to n 3  and error signal e 1  are recorded as the data in a predetermined time period (one minute, for instance), and filter coefficient calculation unit  70  has access to this data for repeated use until filter coefficients of adaptive filters  7201  to  7203  finally converge. 
     Since filter coefficient calculation unit  70  is constructed independently from control filter unit  90  that outputs the control sound via control speaker  41 , it can carry out the process without dependent on a processing speed of control filter unit  90 . Although control filter unit  90  is required to execute its process in real time to complete the process within a predetermined sampling period, filter coefficient calculation unit  70  needs not complete its process within the sampling period in real time. It is therefore free to set a series of processing time for filter coefficient calculation unit  70  relative to real time, so that a time to calculate (or converge) the filter coefficients can be shortened when the process is arranged to complete faster. For example, filter coefficient calculation unit  70  can complete the calculation quicker than that of real time by making its processing operation faster than the sampling period. On the other hand, an operating load and an amount of the operation per unit time of filter coefficient calculation unit  70  can be reduced by making its series of processing time slower than that of real time. In this case, the coefficients calculated by filter coefficient calculation unit  70  come to be same as the coefficients obtained on the basis of the sampling period even though the processing time is longer than that of real time since noise signals n 1  to n 3  and error signal e 1  are the signals sampled in real time. 
     Filter coefficient calculation unit  70  calculates the filter coefficients effective to reduce the noise coming to the control points by the updating process of the filter coefficients described above. 
     When the filter coefficients for use to update those in adaptive filters  7201  to  7203  converge, filter coefficient updating unit  80  (in  FIG. 4 ) updates the filter coefficients set to fixed filters  21  to  23  in control filter unit  90  to the converged filter coefficients at a predetermined timing. 
     The update timing may be determined in any manner, for instance, by using the same timing as the filter coefficients converge in adaptive filters  7201  to  7203 , or to update at such intervals as once every few minutes or even several days. Or, it may be appropriate to carry out the update at a timing of the first flight of the aircraft, and at subsequent timing whenever furnishings and the like equipment inside the aircraft are renewed. 
     When update of the filter coefficients in adaptive filters  7201  to  7203  converge, filter coefficient calculation unit  70  gives to signal storage unit  60  a command of recording the noise signal, the error signal and the filter coefficients in the individual adaptive filters at the time of convergence, designated as data of convergence. Subsequently, filter coefficient calculation unit  70  carries out a process of allocating the filter coefficients as being fixed filter coefficients to AF 1 , AF 2  and AF 3  of adaptive filters  7201  to  7203  by using the data of convergence stored in signal storage unit  60 , and inputs through acoustic system filter  7401  each of the noise signal and the error signal at the time of convergence to adder  7501  wherein they are added up. It may be appropriate to so configure the structure that the coefficients in fixed filters  21  to  23  in control filter unit  90  are updated to the calculated coefficients in filter coefficient updating unit  80  when the output from adder  7501  is within a range of predetermined values. 
     It is thus possible by virtue of this structure to prevent control speaker  41  from outputting an inappropriate control sound to cause discomfort to the user even if an extra time is needed for filter coefficient calculation unit  70  to calculate the filter coefficients and a condition of the noise changes in the mean time. 
     As discussed above, this device  1  calculates the optimum filter coefficients independently in filter coefficient calculation unit  70  irrespective of the real time operation of control filter unit  90 , and it is hence capable of having filter coefficient updating unit  80  update the filter coefficients preset to fixed filters  21  to  23  at the predetermined timing. 
     It can hence provide a full advantage of reducing a throughput related to calculation of the filter coefficients besides the ability of calculating the optimum filter coefficients according to position of the seat and surrounding environment. 
     In addition, the calculated filter coefficients are not applied immediately to fixed filters  21  to  23 , which avoid updating to improper filter coefficients. Accordingly, even when a trouble occurs due to an obstacle, etc. in which the filter coefficients diverge, there is never an output of any control sound resulting from the diverged filter coefficients, so as to prevent discomfort to the user. 
     As a result, device  1  can exert the optimum effect of noise reduction at all times by way of changing the filter coefficients preset to fixed filters  21  to  23  to the optimum filter coefficients according to the position of the seat and the surrounding environment. 
     Referring next to  FIG. 6  to  FIG. 8B , description is provided of a structure and operation to cope with an influence exerted upon noise-detecting microphones  11  to  13  and error microphone  51  due to any obstacle, which is the second feature of device  1 . 
     First, device  1  has a structure to cope with obstacles, comprising obstacle detectors  11   s  to  13   s ,  51   as  and  51   bs  (obstacle detectors  51   as  and  51   bs  are collectively referred to as obstacle detector  51   s ), located individually in the vicinity of each of three noise-detecting microphones  11  to  13  and two error microphones  51   a  and  51   b , such that each of the obstacle detectors is disposed next to every microphone as shown in  FIGS. 3A to 3C . The obstacle detectors may be disposed in any such manner as one each of the microphones and the obstacle detectors are mounted to a seat separately but close to each other, a microphone and an obstacle detector are integrated by embedding the obstacle detector in the microphone, a pair of microphone and obstacle detector are united and mounted to the seat as a single unit, or the like. With regard to mounting locations of the obstacle detectors, consideration is made first to identify probable obstacles that may affect detecting function of the microphones, and determination is made for locations of the obstacle detectors in relation to the microphones based on assumption of possible conditions of the identified obstacles, so as to help improve detecting effect of the obstacle detectors. Presence or absence of obstacles can be determined by a method of, for instance, generating a signal sound from control speaker  41  and finding any change in the acoustic characteristic of the signal detected by error microphone  51 . The same method is also effective for making the determination with other noise-detecting microphones  11  to  13 . In this case, the determination of obstacles can be made without giving nuisance to the user by utilizing an inaudible signal such as an ultrasonic signal. When such a signal sound is used, the above-mentioned obstacle detectors  11   s  to  13   s  and  51   s  become unnecessary. It is also possible to use a pressure sensor, infrared sensor, gravimetric sensor, camera and the like as the obstacle detector besides the one using signal sound. 
     Here, description is provided of a method of determining the presence or absence of obstacles by finding a change in the acoustic characteristic of the signal sound produced from control speaker  41  and detected by error microphone  51 .  FIGS. 8A and 8B  show frequency characteristics of white noises produced from the control speaker and detected by the error microphone. In  FIGS. 8A and 8B , the abscissa and the ordinate represent sound pressure level (dB) and frequency (Hz) in logarithmic representation, respectively. The solid lines in  FIGS. 8A and 8B  show frequency characteristics when error microphone  51  is not covered, and the dotted lines show frequency characteristics when error microphone  51  is covered. As the dotted line in  FIG. 8A  shows the frequency characteristic when error microphone  51  is covered with a hand, it is known that the sound pressure level decreases in a region of high frequency as compared with the characteristic with error microphone  51  not covered, and the sound pressure increases in a certain range due to the influence of the sound being muffled. The dotted line in  FIG. 8B  shows the frequency characteristic when error microphone  51  is covered with a pillow. It is known that the sound pressure level decreases in a wide range from the mid frequency region to the high frequency region when compared with the characteristic with error microphone  51  not covered. This is considered to be attributable to the pillow made of a soft material that absorbs efficiently the high frequency component of the signal. As is obvious from the above example, the covering object causes a substantial change in the characteristic, especially in the region of high frequency. It thus becomes possible to detect the covering object by monitoring changes in the frequency characteristic with time by means of the individual error microphones  51 . It is also obvious that similar method of detection is applicable to noise-detecting microphones  11  to  13  besides error microphones  51 . 
     Although what has been described above is the example of producing the signal sound from control speaker  41 , it is also possible to monitor changes of background noise inside the aircraft instead of the changes in the frequency characteristic of the signal sound caused by an obstacle. In this disclosed system of device  1 , a plurality of error microphones and noise-detecting microphones are disposed around the seat, and it is therefore very unlikely that any obstacle can causes the frequency characteristics monitored by all of the microphones to change at the same time. A criterion of determination can be included for this reason so that any changes in the frequency characteristics of the monitored signals, if occur simultaneously on more than half of the plurality of microphones, is judged as a change in frequency characteristic of the background noise of the aircraft instead of an emerging obstacle, thereby improving the detecting accuracy. 
     In  FIG. 4 , the signals output from obstacle detectors  11   s  to  13   s  and  51   s  are transmitted separately from the signals output from noise-detecting microphones  11  to  13  and error microphone  51 . However, the signals from noise-detecting microphones  11  to  13  may be superposed upon the signals from obstacle detectors  11   s  to  13   s  when transmitting them, as a method adoptable to integrate these transmission lines. 
     As a basic operation to cope with obstacles, “Obstruction Pattern Cross-Reference Table” (Filter Coefficient Control Table) is prepared and stored beforehand in filter coefficient memory  81  (filter coefficient storage unit) provided within filter coefficient updating unit  80 , wherein filter coefficients of the individual noise-detecting microphones  11  to  13  are allocated according to data of the noises detected by these noise-detecting microphones  11  to  13  under the above-described condition of predetermined noise corresponding to obstruction patterns classified based on presence and absence of the obstacles to noise-detecting microphones  11  to  13 . In the example of this device  1  comprising three noise-detecting microphones  11  to  13 , there exist eight (8) obstruction patterns. 
       FIG. 6  shows an example of the obstruction pattern cross-reference table (cross-reference storage unit) stored in filter coefficient memory  81  of device  1 . This table includes filter coefficients F 11 , F 12 , F 21  - - - and F 73  for fixed filters  21  to  23  when noise reduction is executed according to the data detected by noise-detecting microphones  11  to  13  and error microphone  51  corresponding to obstruction patterns  1  through  8  under the predetermined noise condition. The filter coefficients to be set to individual fixed filters  21  to  23  are allocated to the obstruction patterns  1  through  8 . For example, F 11  denotes a filter coefficient of fixed filter  21  corresponding to obstruction pattern  1 , and F 73  denotes a filter coefficient of fixed filter  23  corresponding to obstruction pattern  7 . 
     The basic operation of device  1  related to the updating process of filter coefficients has already been described. 
     Fixed filters  21  to  23  ( FIG. 4 ) operate with fixed filter coefficients, which are read from filter coefficient memory  81  and set to the individual fixed filters, and these filter coefficients in filter coefficient memory  81  are updated periodically in a manner to minimize the output of error microphone  51 . Calculation of the filter coefficients is executed in off-line mode, in that the noise signals detected by noise-detecting microphones  11  to  13  and the residual sound signal detected by error microphone  51  are stored (or recorded) in signal storage unit  60  and the filter coefficients are calculated by taking a predetermined time. However, these basic operations are limited depending on the presence or absence of obstacles to noise-detecting microphones  11  to  13  and error microphone  51 . Description provided next pertains to an operation when device  1  detects an obstacle. 
     At the time of updating the filter coefficients, filter coefficient calculation unit  70  checks a result of detection of any obstacle to noise-detecting microphones  11  to  13  and error microphone  51  according to detection data received from obstacle detectors  11   s  to - 13   s  and  51   s . If an obstacle appears to exist at error microphone  51 , it verifies presence or absence of the obstacle according to the detection data of noise-detecting microphones  11  to  13 , but the filter coefficients are not updated when there is no sign of changes in the obstruction patterns of noise-detecting microphones  11  to  13 . Or, when there is a change in the obstruction patterns of noise-detecting microphones  11  to  13 , filter coefficient calculation unit  70  reads out the filter coefficients of the individual noise-detecting microphones corresponding to the related obstruction pattern by referring to the obstruction pattern stored in filter coefficient memory  81 , and updates the filter coefficients by sending a command of update to filter coefficient updating unit  80 . If error microphone  51  is covered (when there is an obstacle) and no corresponding obstruction pattern is available, then filter coefficient calculation unit  70  stops the operation of noise reduction. 
     When there is no obstacle to error microphone  51 , on the other hand, filter coefficient calculation unit  70  calculates a new filter coefficient in off-line mode, and compares a difference between a residual sound signal calculated by using the new filter coefficient and another residual sound signal calculated according to the filter coefficient before being updated. Filter coefficient calculation unit  70  then updates the filter coefficient stored in filter coefficient memory when an error based on the new filter coefficient is smaller than the error prior to the update. 
     In a process to deal with an obstacle that prevents the detection partially, there is a method available such as locating a position of the obstacle by measuring a distance of the obstacle from the noise-detecting microphone with infrared rays, and adding a correction to the obstruction pattern cross-reference table (filter coefficient control table). 
       FIG. 7  is a flow chart showing operation processes of updating the filter coefficients of this device  1 . As shown in  FIG. 7 , the update process of the filter coefficients begins as needed at the timing of periodic update of the filter coefficients or when an obstacle is detected, for example. In step S 1 , filter coefficient calculation unit  70  starts detecting an obstacle to the error microphone by checking detection data of obstacle detector  51   s  placed in the vicinity of error microphone  51 . 
     In step S 2 , determination is made of presence or absence of an obstacle to error microphone  51  according to obstacle detection data received from obstacle detector  51   s  by referring to the criteria provided beforehand. The process goes on to step S 3  when determined of no obstacle to error microphone  51 . 
     In step S 3 , data such as residual sound data of error microphone  51  and other noise data are stored (recorded) in signal storage unit  60  for a predetermined time period. 
     In step S 4 , optimum filter coefficient is calculated in filter coefficient calculation unit  70  according to the noise data of the predetermined time period stored in signal storage unit  60 . 
     In the next step S 5 , detection is carried out of an obstacle to noise-detecting microphones  11  to  13  according to obstacle detection data received from obstacle detectors  11   s  to  13   s . In step S 6 , a result of obstacle detection is compared with obstruction patterns by referring to the obstruction pattern cross-reference table ( FIG. 6 ) stored beforehand in filter coefficient updating unit  80  to determine based on the criteria provided beforehand as to whether the result conforms to any of the predetermined patterns tabulated in the obstruction pattern cross-reference table. The process goes to step S 7  when the result of determination is conformity to one of the predetermined patterns (when it is “YES” in step S 6 ). 
     In step S 7 , comparison is made of the effect of noise reduction between the new filter coefficient calculated in step S 4  and the existing filter coefficient. The new filter coefficient is cancelled if the existing filter coefficient has a higher effect (step S 8 ), and the filter coefficient updating process ends. When the new filter coefficient has a higher effect, on the other hand, the filter coefficient of the applicable fixed filters  21  to  23  is updated (step S 10 ) after the new filter coefficient is stored in filter coefficient memory  81  of filter coefficient updating unit  80  (step S 9 ), and the filter coefficient updating process ends. In another case if the result of determination in step S 6  shows no conformity to any of the prescribed patterns (including “NO” in step S 6 ), the process goes to step S 9 , and the new filter coefficient is stored as a new pattern in filter coefficient memory  81  of filter coefficient updating unit  80 . 
     In the determination in step S 2  as to the presence or absence of an obstacle to error microphone  51 , if the result shows presence of an obstacle to error microphone  51 , the process goes to step S 11 , wherein detection is carried out of any obstacle to noise-detecting microphones  11  to  13  according to obstacle detection data received from obstacle detectors  11   s  to  13   s . In step S 12 , a result of obstacle detection is compared with obstruction patterns by referring to the obstruction pattern cross-reference table to determine as to whether the result conforms to any of the predetermined patterns tabulated in the obstruction pattern cross-reference table. The process then goes to step S 13  when the result of determination is conformity to one of the predetermined patterns (when it is “YES” in step S 12 ). 
     Afterwards, the filter coefficient is read from the obstruction pattern cross-reference table in step S 13 , the filter coefficient of the applicable fixed filters  21  to  23  is updated (step S 10 ), and the filter coefficient updating process ends. 
     On the other hand, when the result of determination in step S 12  shows no conformity to any of the prescribed patterns (including “NO” in step S 12 ), the process goes to step S 14 , wherein the operation of noise reduction is terminated, and the filter coefficient updating process is ended. 
     According to the exemplary embodiment of this invention, as described above, noise reduction device  1  is provided with obstacle detectors near noise-detecting microphones  11  to  13  and error microphone  51 , stores a noise control condition when there is no obstacle, and updates filter coefficients of fixed filters  21  to  23  in control filter unit  90  by using noise control data, thereby making it capable of suppressing increase of the noise attributed to at least an inappropriate filter coefficient updated during presence of any obstacles. The invention can thus provide the noise reduction device of high quality and useful for application to an aircraft, a railcar and the like. 
     In the above exemplary embodiment, although description has been provided of the method of suspending updating process of the coefficients by prohibiting recording of the noise data, there are also other methods available to suspend updating of the coefficients. Some of the examples include a method of suspending update of the coefficients while continuing the recording, a method of assigning a “zero” weight when updating of the coefficients, as well as other suitable methods, as are obviously known. 
     With regard to the method of detecting obstacles, many means are available besides the method of detecting signal sound, such as a pressure sensor, infrared sensor, gravimetric sensor, camera and the like so long as they are effective for detection of obstacles.