Abstract:
The disclosed active vibration noise control device is suitable for use in cancelling out vibration noise by outputting control noise from a plurality of speakers. When a vibration noise frequency is in a dip bandwidth, the active vibration noise control device alters the step size parameters used to update the filter coefficient at at least one filter coefficient update means from among a plurality of filter coefficient update means. Thus, the filter coefficient update speed can be retarded in unstable dip bandwidths, enabling loss in silencing effect which occurs during dip characteristics to be appropriately reduced.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a technical field for actively controlling a vibration noise by using an adaptive notch filter. 
       BACKGROUND TECHNIQUE 
       [0002]    Conventionally, there is proposed an active vibration noise control device for controlling an engine sound heard in a vehicle interior by a controlled sound output from a speaker so as to decrease the engine sound at a position of passenger&#39;s ear. For example, noticing that a vibration noise in a vehicle interior is generated in synchronization with a revolution of an output axis of an engine, there is proposed a technique for canceling the noise in the vehicle interior on the basis of the revolution of the output axis of the engine by using an adaptive notch filter so that the vehicle interior becomes silent, in Patent Reference-1. 
         [0003]    By the way, in a narrow vehicle interior environment, there is a case that a deep dip of transfer characteristics from a speaker to a microphone occurs due to a sound wave interference and a reflection in a vehicle interior space. In such a frequency band that the deep dip occurs, an operation of the adaptive notch filter tends to become unstable, and a noise-canceling effect tends to decrease. 
         [0004]    For example, in Patent Reference 1, there is proposed a technique for solving the above problem. In Patent Reference 1, there is proposed a technique for switching a speaker to be used in accordance with a noise frequency by using plural speakers. Concretely, the technique verifies transfer characteristics (in other words, amplitude characteristics. The same will apply hereinafter.) of paths related to the speakers, and selects a path of speaker in which an influence of the dip is small. 
         [0005]    There are disclosed techniques related to the present invention in Patent References 2 and 3. 
       PRIOR ART REFERENCE 
     Patent Reference 
       [0006]    Patent Reference-1: International Patent Application Laid-open under No. 2007-011010 
         [0007]    Patent Reference-2: Japanese Patent Application Laid-open under No. 04-342296 
         [0008]    Patent Reference-3: Japanese Patent Application Laid-open under No. 07-230289 
       DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
       [0009]    However, by the technique disclosed in Patent Reference-1, an error signal detected by the microphone tends to increase when the speaker to be used is switched. Namely, the noise-canceling effect by the active vibration noise control device tends to decrease. This is because, since the technique uses only one adaptive notch filter, a filter coefficient of the adaptive notch filter is readapted when the speaker is switched. Therefore, the error signal tends to increase due to a discontinuity of a phase change of the filter coefficient when the speaker is switched. 
         [0010]    The techniques disclosed in Patent References 2 and 3 do not perform a control in consideration of the above dip characteristics. 
         [0011]    The present invention has been achieved in order to solve the above problem. It is an object of the present invention to provide an active vibration noise control device which can appropriately suppress a decrease in a noise-canceling effect during dip characteristics. 
       Means for Solving the Problem 
       [0012]    In the invention according to claim  1 , an active vibration noise control device for canceling a vibration noise by making plural speakers output control sounds, includes: a basic signal generating unit which generates a basic signal based on a vibration noise frequency generated by a vibration noise source; plural adaptive notch filters which generate control signals provided to each of the plural speakers by applying filter coefficients to the basic signal, in order to make the plural speakers generate the control sounds so that the vibration noise generated by the vibration noise source is canceled; a microphone which detects a cancellation error between the vibration noise and the control sound, and outputs an error signal; a reference signal generating unit which generates a reference signal from the basic signal based on transfer functions from the plural speakers to the microphone; plural filter coefficient updating units which update the filter coefficients used by each of the plural adaptive notch filters based on the error signal and the reference signal so as to minimize the error signal; and a step-size parameter changing unit which changes a step-size parameter used for updating the filter coefficient of one or more filter coefficient updating units in the plural filter coefficient updating units, when the vibration noise frequency is in such a frequency band that the dip occurs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1A to 1C  are diagrams for explaining dip characteristics. 
           [0014]      FIG. 2  shows an example of a vehicle on which an active vibration noise control device in an embodiment is mounted. 
           [0015]      FIG. 3  shows an example of transfer characteristics of paths. 
           [0016]      FIG. 4  is a configuration of an active vibration noise control device in an embodiment. 
           [0017]      FIG. 5  shows a diagram for explaining an example of a determination method of a dip band. 
           [0018]      FIG. 6  is a flow chart showing a process for changing a step-size parameter in an embodiment. 
           [0019]      FIG. 7  shows a diagram for explaining an operation and an effect by an embodiment. 
           [0020]      FIGS. 8A ,  8 B,  8 C and  8 D show other examples of transfer characteristics of paths. 
           [0021]      FIGS. 9A and 9B  show examples of impulse responses. 
           [0022]      FIG. 10  shows still other examples of transfer characteristics of paths. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    According to one aspect of the present invention, there is provided an active vibration noise control device for canceling a vibration noise by making plural speakers output control sounds, including: a basic signal generating unit which generates a basic signal based on a vibration noise frequency generated by a vibration noise source; plural adaptive notch filters which generate control signals provided to each of the plural speakers by applying filter coefficients to the basic signal, in order to make the plural speakers generate the control sounds so that the vibration noise generated by the vibration noise source is canceled; a microphone which detects a cancellation error between the vibration noise and the control sound, and outputs an error signal; a reference signal generating unit which generates a reference signal from the basic signal based on transfer functions from the plural speakers to the microphone; plural filter coefficient updating units which update the filter coefficients used by each of the plural adaptive notch filters based on the error signal and the reference signal so as to minimize the error signal; and a step-size parameter changing unit which changes a step-size parameter used for updating the filter coefficient of one or more filter coefficient updating units in the plural filter coefficient updating units, when the vibration noise frequency is in such a frequency band that the dip occurs. 
         [0024]    The above active vibration noise control device is preferably used for canceling the vibration noise (for example, vibration noise from engine) by making the plural speakers generate the control sounds. The basic signal generating unit generates the basic signal based on the vibration noise frequency generated by the vibration noise source. The adaptive notch filters are provided for the plural speakers and generate the control signals provided to the plural speakers by applying the filter coefficients to the basic signal. The microphone detects the cancellation error between the vibration noise and the control sound, and outputs the error signal. The reference signal generating unit generates the reference signal from the basic signal based on the transfer functions from the speakers to the microphone. The plural filter coefficient updating units are provided for the plural speakers and update the filter coefficients used by the plural adaptive notch filters so as to minimize the error signal. Then, the step-size parameter changing unit changes the step-size parameter used for updating the filter coefficient of one or more filter coefficient updating units in the plural filter coefficient updating units, when the vibration noise frequency is in such a frequency band that the dip occurs (hereinafter, the frequency band is referred to as “dip band”). Therefore, in an unstable dip band, it is possible to set an update rate of the filter coefficient of the filter coefficient updating unit to an appropriate rate. Hence, it becomes possible to appropriately suppress the decrease in the noise-canceling effect (in other words, a decrease in a reduction effect of the vibration noise) during the dip characteristics. 
         [0025]    In another manner of the above active vibration noise control device, when the vibration noise frequency is in the frequency band, the step-size parameter changing unit changes the step-size parameter to a value which is smaller than a basic step-size parameter used when the vibration noise frequency is not in the frequency band. 
         [0026]    According to the above manner, it is possible to delay the update rate of the filter coefficient of the filter coefficient updating unit in the dip band. Namely, it is possible to suppress an excess following of the adaptive notch filter and the filter coefficient updating unit. Therefore, it becomes possible to suppress the decrease in the noise-canceling effect during the dip characteristics more effectively. 
         [0027]    In another manner of the above active vibration noise control device, only for a speaker in the plural speakers which has such a frequency band that amplitude characteristics of the transfer functions are equal to or smaller than a predetermined value, the step-size parameter changing unit changes the step-size parameter for updating the filter coefficient used by the adaptive notch filter which generates the control signal of the speaker. 
         [0028]    According to the above manner, the step-size parameter changing unit changes the step-size parameter only for the path of the speaker in which the dip tends to occur, and does not change the step-size parameter for the path of the speaker in which the dip hardly occur. Therefore, it becomes possible to suppress a needless delay of the update of the filter coefficient. 
         [0029]    In another manner of the above active vibration noise control device, only for a speaker in the plural speakers which is arranged adjacent to the microphone, the step-size parameter changing unit changes the step-size parameter for updating the filter coefficient used by the adaptive notch filter which generates the control signal of the speaker. 
         [0030]    According to the above manner, the step-size parameter changing unit treats the speaker arranged adjacent to the microphone, as the speaker by which the dip tends to occur. Then, the step-size parameter changing unit changes the step-size parameter only for the path of the speaker which is arranged adjacent to the microphone, and does not change the step-size parameter for the path of the speaker which is not arranged adjacent to the microphone. Therefore, it becomes possible to suppress a needless delay of the update of the filter coefficient. 
         [0031]    Preferably, the above active vibration noise control device includes a dip band determining unit which determines that a predetermined frequency band is such a frequency band that the dip occurs, based on amplitude characteristics of an output sound from the speaker, and a storage unit which stores the predetermined frequency band determined by the dip band determining unit, wherein the step-size parameter changing unit uses the predetermined frequency band stored in the storage unit, as such a frequency band that the dip occurs. 
         [0032]    In a preferred example of the above active vibration noise control device, the step-size parameter changing unit sequentially compares amplitude information related to each of the transfer functions from the plural speakers to the microphone which is preliminarily stored for each frequency with a predetermined threshold value, and uses a frequency band in which the amplitude information is below the threshold value, as such a frequency band that the dip occurs. 
         [0033]    In a preferred example of the above active vibration noise control device, the step-size parameter changing unit uses a frequency band in which amplitude characteristics of the transfer functions are equal to or smaller than a predetermined value, as such a frequency band that the dip occurs. 
         [0034]    In a preferred example of the above active vibration noise control device, with regard to amplitude characteristics of the transfer functions, the step-size parameter changing unit uses a value in accordance with a difference between an amplitude in such a frequency band that the dip occurs and an amplitude in such a frequency band that the dip does not occur, as a changed value of the step-size parameter. Therefore, it is possible to change the step-size parameter to an appropriate value. Hence, it becomes possible to update the filter coefficient at an appropriate rate. 
       Embodiment 
       [0035]    Preferred embodiment of the present invention will be explained hereinafter with reference to the drawings. 
         [0036]    [Dip Characteristics] 
         [0037]    First, a description will be given of dip characteristics, with reference to  FIGS. 1A to 1C . Here, as an example, a conventional active vibration noise control device having a speaker  10  and a microphone  11  is shown in  FIG. 1A . The active vibration noise control device is mounted on a vehicle. The speaker  10  is installed on the front side in the vehicle interior, and the microphone  11  is installed on the passenger&#39;s side. 
         [0038]    The conventional active vibration noise control device makes the speaker  10  generate the control sound based on the frequency in accordance with the revolution of the engine output axis so as to actively control the vibration noise of the engine as the vibration noise source. Concretely, the active vibration noise control device feeds back the error signal detected by the microphone  11  and minimizes the error by using the adaptive notch filter so as to actively control the vibration noise. 
         [0039]      FIG. 1B  shows a result example of a process by the above conventional active vibration noise control device. Here, as an example, a result in case of using an artificial engine noise (sweep signal) is shown.  FIG. 1B  is a diagram showing a noise-canceling effect by the above conventional active vibration noise control device. In  FIG. 1B , a horizontal axis shows a frequency, and a vertical axis shows a noise-canceled amount. The noise-canceled amount becomes large on the lower side of the vertical axis. Namely, the noise-canceling effect becomes large (The same will apply hereinafter). The noise-canceled amount is an amount corresponding to an amplitude of the error signal detected by the microphone  11 . 
         [0040]      FIG. 1C  is a diagram showing transfer characteristics (amplitude characteristics) in case of using the above paths. Concretely, in  FIG. 1C , a vertical axis in an upper graph shows an amplitude of the speaker  10 , and a vertical axis in a lower graph shows a phase. Additionally, a horizontal axe in each graph shows a frequency. 
         [0041]    In a frequency band shown by a dashed area R 11  in  FIG. 1B , it can be understood that the noise-canceled amount significantly decreases. Additionally, in a frequency band shown by dashed areas R 12  and R 13  in  FIG. 1C , it can be understood that the amplitude decreases and phase characteristics unnaturally change. Namely, in the above frequency band, it can be said that a relatively large dip occurs. When the dip occurs, there is a tendency that a control signal output increases and an operation of the adaptive notch filter becomes unstable. Then, when the operation of the adaptive notch filter becomes unstable, there is a possibility that the noise increases and diverges. 
       Active Vibration Noise Control Device in Embodiment 
       [0042]    Active vibration noise control device in an embodiment performs a process for appropriately suppressing the decrease in the noise-canceling effect during the above dip characteristics. 
         [0043]    The embodiment shows such an example that an active vibration noise control device having two speakers  10 L and  10 R and a microphone  11  which are installed in the vehicle as shown in  FIG. 2 . The speakers  10 L and  10 R are installed on the front side in the vehicle interior, and the microphone  11  is installed on the passenger&#39;s side. Concretely, the speaker  10 L is installed on the front left side, and the speaker  10 R is installed on the front right side. Hereinafter, the speaker  10 L is expressed as “FL”, and the speaker  10 R is expressed as “FR”, and the microphone  11  is expressed as “E”. 
         [0044]      FIG. 3  shows transfer characteristics of paths (paths from the speakers  10 L and  10 R to the microphone  11 ) in the above configuration. 
         [0045]    In  FIG. 3 , a horizontal axis shows a frequency [Hz], and a vertical axis shows amplitude characteristics [dB/20 μPa/V]. Additionally, a solid line shows transfer characteristics of a path (FL→E) from the speaker  10 L to the microphone  11 , and a broken line shows transfer characteristics of a path (FR→E) from the speaker  10 R to the microphone  11 . 
         [0046]    As shown in  FIG. 3 , with regard to the path from the speaker  10 L to the microphone  11 , it can be understood that the significant decrease in the amplitude occurs in a frequency band shown by a dashed area R 2  (concretely, from about 55 [Hz] to 70 [Hz]). Namely, it can be said that the relatively large dip occurs. In contrast, with regard to the path from the speaker  10 R to the microphone  11 , it can be understood that the above significant decrease in the amplitude does not occur. 
         [0047]    In response to the above result, such an example that the active vibration noise control device which performs a process for dealing with the dip only for the path from the speaker  10 L to the microphone  11  is shown hereinafter. Namely, with regard to the path from the speaker  10 R to the microphone  11 , the active vibration noise control device does not perform the process for dealing with the dip. 
         [0048]      FIG. 4  shows a configuration example of an active vibration noise control device  50  in the embodiment. 
         [0049]    The active vibration noise control device  50  in the embodiment includes speakers  10 L and  10 R, a microphone  11 , a frequency detecting unit  13 , a cosine wave generating unit  14   a,  a sine wave generating unit  14   b,  adaptive notch filters  15 L and  15 R, reference signal generating units  16 L and  16 R, w-updating units  17 L and  17 R, a band determining unit  20  and a μ changing unit  21 . 
         [0050]    The active vibration noise control device  50  is mounted on the vehicle, as shown in  FIG. 2 . Concretely, the speaker  10 L and the speaker  10 R are installed on the front left side and the front right side in the vehicle interior, respectively. The microphone  11  is installed on the passenger&#39;s side. Hereinafter, with regard to the speakers  10 L and  10 R, the adaptive notch filters  15 L and  15 R, the reference signal generating units  16 L and  16 R and the w-updating units  17 L and  17 R, “L” and “R” are given to the reference numeral when it is necessary to distinguish right from left. In contrast, “L” and “R” are omitted when it is not necessary to distinguish right from left. 
         [0051]    In response to the result as shown in  FIG. 3 , only for the path from the speaker  10 L to the microphone  11 , the active vibration noise control device  50  performs the process for dealing with the dip. Concretely, the band determining unit  20  and the μ changing unit  21  for dealing with the dip are provided only on the path in which the process for generating a control signal y 1 (n) used by the speaker  10 L is performed. 
         [0052]    Here, a brief description will be given of the process for dealing with the above dip characteristics, which is performed by the active vibration noise control device  50  in the embodiment. When a frequency ω 0  of the engine pulse is within a frequency band (dip band) in which the dip occurs, the active vibration noise control device  50  changes a step-size parameter μ for updating a filter coefficient used by the adaptive notch filter  15 L which generates the control signal y 1 (n) of the speaker  10 L. Concretely, the μ changing unit  21  in the active vibration noise control device  50  changes the step-size parameter μ for updating the filter coefficient used by the w-updating unit  17 L. 
         [0053]    Specifically, when the frequency ω 0  is in the dip band, the active vibration noise control device  50  sets the step-size parameter μ to a value which is smaller than a value used when the frequency ω 0  is not in the dip band. Therefore, in the unstable dip band, it is possible to delay an update rate of the filter coefficient of the w-updating unit  17 L. Namely, it is possible to suppress an excess following of the adaptive notch filter  15 L and the w-updating unit  17 L. Hence, it becomes possible to appropriately suppress the decrease in the noise-canceling effect during the dip characteristics. 
         [0054]    Next, a concrete description will be given of the components in the active vibration noise control device  50 . The frequency detecting unit  13  is supplied with an engine pulse and detects a frequency ω 0  of the engine pulse. Then, the frequency detecting unit  13  supplies the cosine wave generating unit  14   a  and the sine wave generating unit  14   b  with a signal corresponding to the frequency ω 0 . 
         [0055]    The cosine wave generating unit  14   a  and the sine wave generating unit  14   b  generate a basic cosine wave x 0 (n) and a basic sine wave x 1 (n) which include the frequency ω 0  detected by the frequency detecting unit  13 . Concretely, as shown by equations (1) and (2), the cosine wave generating unit  14   a  and the sine wave generating unit  14   b  generate the basic cosine wave x 0 (n) and the basic sine wave x 1 (n). In the equations (1) and (2), “n” is natural number and corresponds to time (The same will apply hereinafter). Additionally, “A” indicates amplitude, and “φ” indicates an initial phase. 
         [0000]        x   0 ( n )= A  cos(ω 0   n +φ)   (1)
 
         [0000]        x   1 ( n )= A  sin(ω 0   n +φ)   (2)
 
         [0056]    Then, the cosine wave generating unit  14   a  and the sine wave generating unit  14   b  supply the adaptive notch filters  15  and the reference signal generating units  16  with basic signals corresponding to the basic cosine wave x 0 (n) and the basic sine wave x 1 (n). Thus, the cosine wave generating unit  14   a  and the sine wave generating unit  14   b  correspond to an example of the basic signal generating unit. 
         [0057]    The adaptive notch filters  15 L and  15 R perform the filter process of the basic cosine wave x 0 (n) and the basic sine wave x 1 (n), so as to generate the control signals y 1 (n) and y 0 (n) supplied to the speakers  10 L and  10 R. Concretely, the adaptive notch filter  15 L generates the control signal y 1 (n) based on the filter coefficients w 01 (n) and w 11 (n) inputted from the w-updating unit  17 L, and the adaptive notch filter  15 R generates the control signal y 2 (n) based on the filter coefficients w 02 (n) and w 12 (n) inputted from the w-updating unit  17 R. Specifically, as shown by equation (3), the adaptive notch filter  15 L adds a value obtained by multiplying the basic cosine wave x 0 (n) by the filter coefficient w 01 (n), to a value by multiplying the basic sine wave x 1 (n) by the filter coefficient w 11 (n), so as to calculate the control signal y 1 (n). Similarly, as shown by equation (4), the adaptive notch filter  15 R adds a value obtained by multiplying the basic cosine wave x 0 (n) by the filter coefficient w 02 (n), to a value by multiplying the basic sine wave x 1 (n) by the filter coefficient w 12 (n), so as to calculate the control signal y 2 (n). 
         [0000]        y   1 ( n )= w   01 ( n ) x   0 ( n )+ w   11 ( n ) x   1 ( n )   (3)
 
         [0000]        y   2 ( n )= w   02 ( n ) x   0 ( n )+ w   12 ( n ) x   1 ( n )   (4)
 
         [0058]    The speakers  10 L and  10 R generate the control sounds corresponding to the control signals y 1 (n) and y 2 (n) inputted from the adaptive notch filters  15 L and  15 R, respectively. The control sounds generated by the speakers  10 L and  10 R are transferred to the microphone  11 . Transfer functions from the speakers  10 L and  10 R to the microphone  11  are represented by “p 11 ” and “p 12 ”, respectively. The transfer functions p 11  and p 12  rec and frequency ω 0 , and depend on the sound field characteristics and the distance from the speakers  10 L and  10 R to the microphone  11 . For example, the transfer functions p 11  and p 12  are preliminarily set by a measurement in the vehicle interior. 
         [0059]    The microphone  11  detects the cancellation error between the vibration noise of the engine and the control sounds generated by the speakers  10 L and  10 R, and supplies the w-updating units  17 L and  17 R with the cancellation error as the error signal e(n). Concretely, the microphone  11  outputs the error signal e(n) in accordance with the control signals y 1 (n) and y 2 (n), the transfer functions p 11  and p 12  and the vibration noise d(n) of the engine. 
         [0060]    The reference signal generating units  16 L and  16 R generate the reference signals from the basic cosine wave x 0 (n) and the basic sine wave x 1 (n) based on the above transfer functions p 11  and p 12 , and supplies the w-updating units  17 L and  17 R with the reference signals. Concretely, the reference signal generating unit  16 L uses a real part c 01  and an imaginary part c 11  of the transfer function p 11 , and the reference signal generating unit  16 R uses a real part c 02  and an imaginary part c 12  of the transfer function p 12 . Specifically, the reference signal generating unit  16 L adds a value obtained by multiplying the basic cosine wave x 0 (n) by the real part c 01  of the transfer function p 11 , to a value obtained by multiplying the basic sine wave x 1 (n) by the imaginary part c 11  of the transfer function p 11 , and outputs a value obtained by the addition as the reference signal r 01 (n). In addition, the reference signal generating unit  16 L delays the reference signal r 01 (n) by “π/2”, and outputs the delayed signal as the reference signal r 11 (n). Similarly, the reference signal generating unit  16 R adds a value obtained by multiplying the basic cosine wave x 0 (n) by the real part c 02  of the transfer function p 12 , to a value obtained by multiplying the basic sine wave x 1 (n) by the imaginary part c 12  of the transfer function p 12 , and outputs a value obtained by the addition as the reference signal r 02 (n). In addition, the reference signal generating unit  16 R delays the reference signal r 02 (n) by “π/2”, and outputs the delayed signal as the reference signal r 12 (n). Thus, the reference signal generating units  16 L and  16 R correspond to an example of the reference signal generating unit. 
         [0061]    The w-updating units  17 L and  17 R update the filter coefficients used by the adaptive notch filters  15 L and  15 R based on the LMS (Least Mean Square) algorism, and supplies the adaptive notch filters  15 L and  15 R with the updated filter coefficients. Basically, the w-updating units  17 L and  17 R update the filter coefficients used by the adaptive notch filters  15 L and  15 R last time so as to minimize the error signal e(n), based on the error signal e(n) and the reference signals r 01 (n), r 11 (n), r 02 (n) and r 12 (n). Thus, the w-updating units  17 L and  17 R correspond to an example of the filter coefficient updating unit. 
         [0062]    The filter coefficients before the update of the w-updating unit  17 L are expressed as “w 01 (n)” and “w 11 (n)”, and the filter coefficients after the update of the w-updating unit  17 L are expressed as “w 01 (n+1)” and “w 11 (n+1)”. As shown by equations (5) and (6), the filter coefficients after the update w 01 (n+1) and w 11 (n+1) are calculated. 
         [0000]        w   01 ( n+ 1)= w   01 ( n )−μ· e ( n )· r   01 ( n )   (5)
 
         [0000]        w   11 ( n+ 1)= w   11 ( n )−μ· e ( n )· r   11 ( n )   (6)
 
         [0063]    Similarly, the filter coefficients before the update of the w-updating unit  17 R are expressed as “w 02 (n)” and “w 12 (n)”, and the filter coefficients after the update of the w-updating unit  17 R are expressed as “w 02 (n+1)” and “w 12 (n+1)”. As shown by equations (7) and (8), the filter coefficients after the update w 02 (n+1) and w 12 (n+1) are calculated. 
         [0000]        w   02 ( n+ 1)= w   02 ( n )−μ· e ( n )· r   02 ( n )   (7)
 
         [0000]        w   12 ( n+ 1)= w   12 ( n )−μ· e ( n )· r   12 ( n )   (8)
 
         [0064]    In equations (5) to (8), “μ” is a coefficient called a step-size parameter for determining a convergence speed. In other words, the step-size parameter μ is a coefficient related to an update rate of the filter coefficient. For example, a preliminarily set value is used as the step-size parameter μ. Basically, the w-updating unit  17 R uses a fixed value as the step-size parameter μ. Namely, the w-updating unit  17 R continues to use the preliminarily set value. In contrast, the w-updating unit  17 L used a changed value when the μ changing unit  21  changes the step-size parameter μ, and the w-updating unit  17 L used the preliminarily set value when the μ changing unit  21  does not change the step-size parameter μ. Hereinafter, the preliminarily set step-size parameter μ is referred to as “basic step-size parameter μ”, and the value obtained by changing the basic step-size parameter μ is referred to as “changed step-size parameter μ”. 
         [0065]    The band determining unit  20  performs the determination of the frequency ω 0  detected by the frequency detecting unit  13 . Concretely, the band determining unit  20  determines whether or not the frequency ω 0  of the engine pulse is in the dip band. Then, the band determining unit  20  supplies the μ changing unit  21  with the determination result. For example, the band determining unit  20  uses the dip band which is determined by preliminarily measuring the transfer characteristics of the paths, so as to perform the above determination. As an example, information related to the determined dip band is stored in a band table, and the band determining unit  20  refers to the table so as to perform the above determination. 
         [0066]    The μ changing unit  21  changes the basic step-size parameter μ based on the determination result of the band determining unit  20 . Concretely, the μ changing unit  21  changes the basic step-size parameter μ when the band determining unit  20  determines that the frequency ω 0  is in the dip band, and the μ changing unit  21  does not change the basic step-size parameter μ when the band determining unit  20  determines that the frequency ω 0  is not in the dip band. In this case, when the band determining unit  20  determines that the frequency ω 0  is in the dip band, the μ changing unit  21  calculates the changed step-size parameter μ′ which is smaller than the basic step-size parameter μ. In such a case that the basic step-size parameter μ is changed by the μ changing unit  21 , the changed step-size parameter μ′ is used for updating the filter coefficient of the w-updating unit  17 L. In contrast, in such a case that the basic step-size parameter μ is not changed by the μ changing unit  21 , the basic step-size parameter μ is used for updating the filter coefficient of the w-updating unit  17 L. Thus, the band determining unit  20  and the μ changing unit  21  correspond to an example of the step-size parameter changing unit. 
         [0067]    For example, the μ changing unit  21  uses a parameter (hereinafter referred to as “parameter for change α”) for changing the basic step-size parameter μ, so as to calculate the changed step-size parameter μ′. In this case, the μ changing unit  21  calculates the changed step-size parameter μ′ by using an equation “μ′=μ*α”. As an example, with regard to the amplitude characteristics of the transfer functions, the parameter for change α is set based on a difference between an amplitude in the dip band and an amplitude in the frequency band other than the dip band. Namely, the parameter for change α is set based on a degree of the decrease in the amplitude within the dip band. 
         [0068]    [Determination Method of Dip Band] 
         [0069]    Next, a description will be given of an example of a determination method of the dip band, with reference to  FIG. 5 . Here, such an example that the amplitude characteristics of the speakers  10  (in other words, the transfer characteristics of the path) are measured and the dip band is determined based on the measured amplitude characteristics will be given. 
         [0070]    In  FIG. 5 , a horizontal axis shows a frequency, and a vertical axis shows an amplitude and a value of the step-size parameter μ. Concretely, a graph A schematically shows the amplitude characteristics obtained by a measurement, and a graph B shows the step-size parameter μ. For example, the graph A corresponds to a graph schematically showing the transfer characteristics (see  FIG. 3 ) of the path from the speaker  10 L to the microphone  11 . 
         [0071]    In  FIG. 5 , an amplitude C 1  shows an average amplitude within the frequency band (for example, from 50 [Hz] to 100 [Hz]) in which the engine pulse is actively controlled, and an amplitude C 2  shows an amplitude when the deepest dip occurs. Additionally, an amplitude C 3  shows an average amplitude of the amplitude C 1  and the amplitude C 2 . In the embodiment, the frequency band in which the amplitude is equal to or smaller than the amplitude C 3  is determined as the dip band. With regard to the example shown in  FIG. 5 , a frequency band shown by a reference numeral D is determined as the dip band. The determined dip band D is stored in a storage unit such as a memory. 
         [0072]    Then, in the embodiment, the step-size parameter μ is changed in the above determined dip band D. Namely, in the embodiment, the step-size parameter μ is changed by using the dip band D stored in the storage unit. With regard to the example shown in  FIG. 5 , as shown by the graph B, the changed step-size parameter μ′ is used in the dip band D, and the basic step-size parameter μ is used in the frequency band other than the dip band D. For example, the parameter for change α [dB] is set based on a difference between the amplitude C 1  and the amplitude C 2 , and a gain of the basic step-size parameter μ is adjusted in accordance with the parameter for change α so as to obtain the changed step-size parameter μ′. As an example, when the amplitude characteristics shown in  FIG. 3  are obtained, the changed step-size parameter μ′ being one-fifth of the basic step-size parameter μ is calculated. 
         [0073]    It is not limited that the amplitude C 3  being the average of the amplitude C 1  and the amplitude C 2  is used for determining the dip band. Namely, it is not limited that the amplitude C 3  is used as a threshold value for determining the dip band. A value other than the amplitude C 3  may be used as the threshold value for determining the dip band, if the value exists between the amplitude C 1  and the amplitude C 2 . 
         [0074]    Additionally, it is not limited that the amplitude characteristics (the transfer characteristics of the path) are measured and the dip band is determined based on the measured amplitude characteristics. As another example, the dip band can be determined by using amplitude information (corresponding to information related to the amplitude characteristics) related to the transfer functions from the speakers  10  to the microphone  11  which is preliminarily stored for each frequency. Concretely, by sequentially comparing the amplitude value included in the amplitude information with a predetermined value, the frequency band in which the amplitude value is below the predetermined value can be used as the dip band. In such a case that the amplitude information related to the transfer functions is not preliminarily stored (for example, in such a case that only phase information is stored), the above method according to another example cannot be applied. 
         [0075]    Furthermore, while the above embodiment shows such an example that the fixed value is used as the changed step-size parameter μ′ (see  FIG. 5 ), the changed step-size parameter μ′ may be changed. For example, the changed step-size parameter μ′ may be changed in accordance with a frequency in the dip band. Namely, the changed step-size parameter μ′ may be changed in accordance with an amplitude value in the dip band. 
         [0076]    [Process for Changing Step-Size Parameter] 
         [0077]    Next, a description will be given of an example of a process for changing the step-size parameter in the embodiment, with reference to  FIG. 6 .  FIG. 6  is a flow chart showing the process for changing the step-size parameter in the embodiment. This process is executed by the components in the active vibration noise control device  50 , in a predetermined cycle. 
         [0078]    First, in step S 101 , the frequency detecting unit  13  in the active vibration noise control device  50  detects the frequency ω 0  of the inputted engine pulse. The frequency detecting unit  13  supplies the band determining unit  20  with the detected frequency ω 0 . Then, the process goes to step S 102 . 
         [0079]    In step S 102 , the band determining unit  20  in the active vibration noise control device  50  determines whether or not the frequency ω 0  detected by the frequency detecting unit  13  is in the dip band. For example, the band determining unit  20  uses the dip band which is preliminarily obtained by measuring the transfer characteristics of the paths. When the frequency ω 0  is in the dip band (step S 102 : Yes), the process goes to step S 103 . 
         [0080]    In step S 103 , the μ changing unit  21  in the active vibration noise control device  50  changes the basic step-size parameter μ. Concretely, the μ changing unit  21  multiplies the basic step-size parameter μ by the parameter for change α(μ′=μ*α), in order to calculate the changed step-size parameter μ′. Then, the process ends. 
         [0081]    Meanwhile, when the frequency ω 0  is not in the dip band (step S 102 : No), the process goes to step S 104 . In this case, the μ changing unit  21  does not change the basic step-size parameter μ (step S 104 ). Then, the process ends. 
       Operation and Effect of Embodiment 
       [0082]    Next, a description will be given of an example of an operation and an effect of the active vibration noise control device  50  in the embodiment. Here, the active vibration noise control device  50  in the embodiment is compared with active vibration noise control devices in first and second comparative examples. The active vibration noise control device in the first comparative example actively controls the engine pulse by only using the speaker  10 L installed on the front left side in the vehicle interior. Meanwhile, the active vibration noise control device in the second comparative example uses the speakers  10 L and  10 R which are installed on the front left side and the front right side, and switches the speaker to be used in accordance with the frequency of the engine pulse. Concretely, within the dip band, the active vibration noise control device in the second comparative example selects the speaker  10  by which the influence of the dip is small. The installation positions of the speakers  10  and the microphone  11  used in the embodiment, the first comparative example and the second comparative example are as mentioned above (see  FIG. 2 ). 
         [0083]    In  FIG. 7 , a horizontal axis shows a frequency [Hz], and a vertical axis shows a noise-canceled amount [dB]. Additionally, in  FIG. 7 , a solid line shows a noise-canceling effect by the active vibration noise control device  50  in the embodiment, and a broken line shows a noise-canceling effect by the active vibration noise control device in the first comparative example, and a dashed-dotted line shows a noise-canceling effect by the active vibration noise control device in the second comparative example. Here, results in case of using the artificial engine noise (sweep signal) from 40 [Hz] to 100 [Hz] are shown. 
         [0084]    As shown in  FIG.7 , according to the active vibration noise control device in the first comparative example, it can be understood that the decrease in the noise-canceling amount occurs in the dip band. In contrast, according to the active vibration noise control device in the second comparative example, it can be understood that the degree of the decrease in the noise-canceling amount in the dip band is smaller than that of the first comparative example. However, according to the active vibration noise control device in the second comparative example, as shown by a dashed area R 3  in  FIG. 7 , it can be understood that the decrease in the noise-canceling amount occurs. It is thought that this is caused by the switch of the speaker  10 . Concretely, it is thought that the increase in the error signal occurs due to a discontinuity of the phase change of the filter coefficient, during the switch of the speaker  10 . 
         [0085]    Meanwhile, according to the active vibration noise control device  50  in the embodiment, it can be understood that the decrease in the noise-canceling amount in the dip band is suppressed, similar to the second comparative example. Additionally, according to the active vibration noise control device  50  in the embodiment, it can be understood that the decrease in the noise-canceling amount like the second comparative example (see the dashed area R 3 ) does not occur. This is because, since the active vibration noise control device  50  in the embodiment dose not switch the speaker  10  like the second comparative example (namely, all of the speakers  10 L and  10 R constantly operate), the phase discontinuity of the filter coefficient does not occur and the unnatural increase in the error signal does not occur. 
         [0086]    Thus, according to the active vibration noise control device  50  in the embodiment, by delaying the update rate of the filter coefficient in the dip band, it is possible to appropriately suppress the decrease in the noise-canceling effect during the dip characteristics. 
         [0087]    [Modification] 
         [0088]    It is not limited that the present invention is applied to the active vibration noise control device  50  having two speakers  10 L and  10 R. Additionally, it is not limited that the present invention is applied to the active vibration noise control device  50  having one microphone  11 . Furthermore, it is not limited that the present invention is applied to the active vibration noise control device  50 , the speakers  10  and the microphone  11  of which are installed at the positions as shown in  FIG. 2 . The present invention can be applied to an active vibration noise control device having more than two speakers and/or more than one microphone, and can be applied to an active vibration noise control device, the speakers and the microphones of which are installed at various positions. 
         [0089]    The above embodiment shows such an example that the process for dealing with the dip only for the path of the speaker  10 L in the speakers  10 L and  10 R installed on the front left side and the front right side. Namely, the above embodiment shows such an example that, only for the path of the speaker  10 L, the determination as to whether or not the frequency is in the dip band is performed and the step-size parameter μ is changed when the frequency is in the dip band. Hereinafter, a concrete description will be given of a method for determining the speaker in the plural speakers for which the process for dealing with the dip is performed. 
         [0090]    As an example, the process for dealing with the dip can be performed only for the path of the speaker in the plural speakers in which the dip tends to occur. Concretely, only for the speaker in the plural speakers which has such amplitude characteristics that the amplitude characteristics of the transfer function are equal to or smaller than a predetermined value (corresponding to the threshold value used when the dip band is determined, for example), the determination as to whether or not the frequency is in the dip band is performed, and the step-size parameter μ is changed when the frequency is in the dip band. 
         [0091]    Here, by examining a cause of the occurrence of the dip characteristics, a concrete example will be given of the path of the speaker in which the dip tends to occur, with reference to  FIGS. 8A ,  8 B,  8 C and  8 D,  FIGS. 9A and 9B  and  FIG. 10 . 
         [0092]      FIGS. 8A ,  8 B,  8 C and  8 D show examples in case of installing the speakers and the microphone at different positions from the above embodiment. Here, as shown in  FIG. 8A , such an example that the speakers  10 FL,  10 FR and  10 RL are installed on the front left side, the front right side and the rear left side in the vehicle interior and the microphone  11   a  is installed on the passenger&#39;s side is shown. Additionally, as shown in  FIG. 8B , such an example that the speakers  10 FL,  10 FR and  10 RL are installed on the front left side, the front right side and the rear left side in the vehicle interior and the microphone  11   b  is installed on the driver&#39;s side is shown. Hereinafter, the speaker  10 FL is expressed as “FL”, and the speaker  10 FR is expressed as “FR”, and the speaker  10 RL is expressed as “RL”. In addition, the microphone  11   a  is expressed as “E 1 ”, and the microphone  11   b  is expressed as “E 2 ”. 
         [0093]      FIG. 8C  shows examples of the transfer characteristics of the paths shown in  FIG. 8A  (the paths from the speakers  10 FL,  10 FR and  10 RL to the microphone  11   a ). In  FIG. 8C , a horizontal axis shows a frequency [Hz], and a vertical axis shows an amplitude [dB/20 μPa/V]. Additionally, a solid line shows transfer characteristics of a path (FL→E 1 ) from the speaker  10 FL to the microphone  11   a,  and a broken line shows transfer characteristics of a path (FR→E 1 ) from the speaker  10 FR to the microphone  11   a,  and a dashed-dotted line shows transfer characteristics of a path (RL→E 1 ) from the speaker  10 RL to the microphone  11   a.    
         [0094]    In a frequency band shown by a dashed area R 41  in  FIG. 8C , with regard to the path from the speaker  10 FL to the microphone  11   a,  it can be understood that the significant decrease in the amplitude occurs. Namely, it can be said that the relatively large dip occurs. In contrast, with regard to the paths from the speakers  10 FR and  10 RL to the microphone  11   a,  it can be understood that the above significant decrease in the amplitude does not occur. 
         [0095]      FIG. 8D  shows examples of the transfer characteristics of the paths shown in  FIG. 8B  (the paths from the speakers  10 FL,  10 FR and  10 RL to the microphone  11   b ). In  FIG. 8D , a horizontal axis shows a frequency [Hz], and a vertical axis shows an amplitude [dB/20 μPa/V]. Additionally, a solid line shows transfer characteristics of a path (FL→E 2 ) from the speaker  10 FL to the microphone  11   b,  and a broken line shows transfer characteristics of a path (FR→E 2 ) from the speaker  10 FR to the microphone  11   b,  and a dashed-dotted line shows transfer characteristics of a path (RL→E 2 ) from the speaker  10 RL to the microphone  11   b.    
         [0096]    In a frequency band shown by a dashed area R 42  in  FIG. 8D , with regard to the path from the speaker  10 FR to the microphone  11   b,  it can be understood that the significant decrease in the amplitude occurs. Namely, it can be said that the relatively large dip occurs. In contrast, with regard to the paths from the speakers  10 FL and  10 RL to the microphone  11   b,  it can be understood that the above significant decrease in the amplitude does not occur. 
         [0097]    According to the results shown in  FIGS. 8C and 8D , with regard to the path of the speaker  10  which is arranged adjacent to the microphone  11 , it can be said that the relatively large dip occurs in the low frequency band. 
         [0098]      FIGS. 9A and 9B  show examples of impulse responses (time waveforms) related to the paths shown in  FIGS. 8A and 8B , respectively. In this case, an upper graph shows the impulse response related to the speaker  10 FL, and a middle graph shows the impulse response related to the speaker  10 FR, and a lower graph shows the impulse response related to the speaker  10 RL. In  FIGS. 9A and 9B , a horizontal axis shows time, and a vertical axis shows an amplitude of the impulse response. 
         [0099]    As shown by a dashed area R 51  in  FIG. 9A , with regard to the path shown in  FIG. 8A , it can be understood that a large reflected sound occurs in the path of the speaker  10 FL. Additionally, as shown by a dashed area R 52  in  FIG. 9B , with regard to the path shown in  FIG. 8B , it can be understood that a large reflected sound occurs in the path of the speaker  10 FR. 
         [0100]    According to the results shown in  FIGS. 9A and 9B , with regard to the path of the speaker  10  which is arranged adjacent to the microphone  11 , it can be said that the large reflected sound occurs. Here, a time difference between the reflected sound shown by the dashed areas R 51  and R 52  and a direct sound is about 0.008 [sec], and the time difference becomes a half wavelength at 62.5 [Hz]. Therefore, it is thought that the relatively large dip occurs at the above frequency, as shown in  FIGS. 8C and 8D . 
         [0101]      FIG. 10  shows examples of transfer characteristics of paths in case of using a different vehicle type from a vehicle type for which the measurement shown in  FIGS. 8A ,  8 B,  8 C and  8 D is performed. Here, similar to  FIG. 8A , such an example that the speakers  10 FL,  10 FR and  10 RL are installed on the front left side, the front right side and the rear left side in the vehicle interior and the microphone  11   a  is installed on the passenger&#39;s side is shown. 
         [0102]    In  FIG. 10 , a horizontal axis shows a frequency [Hz], and a vertical axis shows an amplitude [dB/20 μPa/V]. Additionally, a solid line shows transfer characteristics of the path (FL→E 1 ) from the speaker  10 FL to the microphone  11   a,  and a broken line shows transfer characteristics of the path (FR→E 1 ) from the speaker  10 FR to the microphone  11   a,  and a dashed-dotted line shows transfer characteristics of the path (RL→E 1 ) from the speaker  10 RL to the microphone  11   a.    
         [0103]    According to  FIG. 10 , with regard to the path from the speaker FL to the microphone  11   a,  it can be understood that the significant decrease in the amplitude occurs in the low frequency band. Namely, it can be said that the relatively large dip occurs. In contrast, with regard to the paths from the speakers  10 FR and  10 RL to the microphone  11   a,  it can be understood that the above significant decrease in the amplitude does not occur. 
         [0104]    It can be said that the result shown in  FIG. 10  is the same as the result shown in  FIG. 8C . Therefore, it can be said that the dip characteristics are a common tendency in the vehicle interior space. 
         [0105]    Thus, it is thought that the dip characteristics are caused by the reflected sound generated in the vehicle interior. Additionally, it is thought that the influence of the dip is large in the path of the speaker arranged adjacent to the microphone (namely, as for the speaker arranged adjacent to the microphone, the dip tends to occur), and that the influence of the dip is large in the low frequency band. Therefore, it is preferable that the process for dealing with the dip is performed only for the speaker in the plural speakers which is arranged adjacent to the microphone. Concretely, only for the speaker arranged adjacent to the microphone, it is preferable that the determination as to whether or not the frequency is in the dip band is performed, and that the step-size parameter μ is changed when the frequency is in the dip band. 
         [0106]    It is not limited that the process for dealing with the dip is performed only for a path of one speaker in the plural speakers. The process for dealing with the dip may be performed for paths (including all paths) of more than one speaker in the plural speakers. In such a case that the process for dealing with the dip is performed for paths of more than one speaker, for these speakers, the dip bands used for the band determination are set, and the changed step-size parameters μ′ (or the parameters for change α) are set. Namely, for these speakers, the different dip bands and the different changed step-size parameters μ′ are used. In this case, the dip bands and the changed step-size parameters μ′ can be determined by the same method as the above embodiment. 
         [0107]    It is not limited that the present invention is applied to the vehicle. Other than the vehicle, the present invention can be applied to various kinds of transportation such as a ship or a helicopter or an airplane. 
       INDUSTRIAL APPLICABILITY 
       [0108]    This invention is applied to closed spaces such as an interior of transportation having a vibration noise source (for example, engine), and can be used for actively controlling a vibration noise. 
       DESCRIPTION OF REFERENCE NUMBERS 
       [0109]      10 L,  10 R Speaker 
         [0110]      11  Microphone 
         [0111]      13  Frequency Detecting Unit 
         [0112]      14   a  Cosine Wave Generating Unit 
         [0113]      14   b  Sine Wave Generating Unit 
         [0114]      15 L,  15 R Adaptive Notch Filter 
         [0115]      16 L,  16 R Reference Signal Generating Unit 
         [0116]      17 L,  17 R w-Updating Unit 
         [0117]      20  Band Determining Unit 
         [0118]      21  μ Changing Unit 
         [0119]      50  Active Vibration Noise Control Device