Active noise reduction device and active noise reduction method

1. An active noise reduction device for reducing a noise, the active noise reduction device being configured to be used with a reference signal source, a secondary noise source, and an error signal source, wherein the reference signal source outputs a reference signal having a correlation with the noise, the secondary noise source generates a secondary noise corresponding to a secondary noise signal, the error signal source outputs an error signal corresponding to a residual sound caused by interference between the secondary noise and the noise,

said active noise reduction device comprising a signal-processing device which includes:

a first input port being configured to receive the reference signal;

a second input port being configured to receive the error signal;

an output port being configured to output the secondary noise signal;

an adaptive filter configured to output the secondary noise signal based on the reference signal;

a simulated acoustic transfer characteristic filter configured to correct the reference signal with a simulated acoustic transfer characteristic that simulates an acoustic transfer characteristic from the output port to the second input port so as to output a filtered reference signal;

a least-mean-square operation unit configured to update a filter coefficient of the adaptive filter by using the error signal, the filtered reference signal, and a step-size parameter; and

a .mu.-adjustment unit configured to determine the step-size parameter, and wherein the .mu.-adjustment unit is configured to:

calculate a representative input value corresponding to amplitude of at least one signal of the reference signal, the filtered reference signal, and the error signal;

store a standard representative input value and a predetermined standard step-size parameter, the standard representative input value being a representative input value when the amplitude of the at least one signal of the reference signal, the filtered reference signal, and the error signal is predetermined amplitude, the predetermined standard step-size parameter being a value of the step-size parameter to which the filter coefficient converges when the representative input value is the standard representative input value; and

calculate the step-size parameter by multiplying the standard step-size parameter by a ratio of the standard representative input value to the representative input value.

2. The active noise reduction device according to claim 1 , wherein the standard representative input value corresponds to a maximum value of the amplitude of the at least one signal of the reference signal, the filtered reference signal, and the error signal.

3. The active noise reduction device according to claim 1 , wherein the standard step-size parameter takes a maximum value of the step-size parameter to which the filter coefficient converges when the representative input value is the standard representative input value.

4. The active noise reduction device according to claim 1 , wherein at least one value of an upper limit value and a lower limit value of a coefficient by which the standard step-size parameter is multiplied is set.

5. The active noise reduction device according to claim 4 ,

wherein the coefficient is a digital value expressed in a register of the signal-processing device having a fixed-point format, and

wherein the μ-adjustment unit changes a decimal point position of the coefficient determines to set the at least one value of the upper limit value and the lower limit value of the coefficient.

6. The active noise reduction device according to claim 1 ,

wherein the active noise reduction device is configured to be mounted in a movable body having a space,

wherein the noise is generated in the space,

wherein the secondary noise source generates the secondary noise in the space, and

wherein the residual sound is generated in the space.

7. An active noise reduction device active for reducing a noise, the noise reduction device being configured to be used with a secondary noise source and an error signal source, wherein the secondary noise source generates a secondary noise corresponding to a secondary noise signal, and the error signal source outputs an error signal corresponding to a residual sound caused by interference between the secondary noise and the noise,

said active noise reduction device comprising a signal-processing device which includes:

an input port being configured to receive the error signal;

an output port being configured to output the secondary noise signal;

a reference signal generator configured to output a reference signal based on the error signal;

an adaptive filter configured to output the secondary noise signal based on the reference signal;

a simulated acoustic transfer characteristic filter configured to correct the reference signal with a simulated acoustic transfer characteristic that simulates an acoustic transfer characteristic from the output port to the input port so as to output a filtered reference signal;

a least-mean-square operation unit configured to update a filter coefficient of the adaptive filter by using the error signal, the filtered reference signal, and a step-size parameter; and

a .mu.-adjustment unit configured to determine the step-size parameter, and wherein the .mu.-adjustment unit is configured to:

calculate a representative input value corresponding to amplitude of at least one signal of the reference signal, the filtered reference signal, and the error signal;

store a standard representative input value and a predetermined standard step-size parameter, the standard representative input value being a representative input value when the amplitude of the at least one signal of the reference signal, the filtered reference signal, and the error signal is predetermined amplitude, the predetermined standard step-size parameter being a value of the step-size parameter to which the filter coefficient converges when the representative input value is the standard representative input value; and

calculate the step-size parameter by multiplying the standard step-size parameter by a ratio of the standard representative input value to the representative input value.

8. The active noise reduction device according to claim 7 , wherein the standard representative input value corresponds to a maximum value of the amplitude of the at least one signal of the reference signal, the filtered reference signal, and the error signal.

9. The active noise reduction device according to claim 7 , wherein the reference signal generator outputs the error signal as the reference signal.

10. An active noise reduction device for reducing a noise, the active noise reduction device being configured to be used with a secondary noise source and an error signal source, wherein the secondary noise source generates a secondary noise corresponding to a secondary noise signal, and the error signal source outputs an error signal corresponding to a residual sound caused by interference between the secondary noise and the noise,

said active noise reduction device comprising a signal-processing device which includes:

an input port being configured to receive the error signal;

an output port being configured to output the secondary noise signal;

an adaptive filter configured to output the secondary noise signal based on the error signal;

a simulated acoustic transfer characteristic filter configured to correct the error signal with a simulated acoustic transfer characteristic that simulates an acoustic transfer characteristic from the output port to the input port so as to output a filtered error signal;

a least-mean-square operation unit configured to update a filter coefficient of the adaptive filter by using the error signal, the filtered error signal, and a step-size parameter; and

a .mu.-adjustment unit configured to determine the step-size parameter, and wherein the .mu.-adjustment unit is configured to:

calculate a representative input value corresponding to amplitude of at least one signal of the error signal and the filtered error signal;

store a standard representative input value and a predetermined standard step-size parameter, the standard representative input value being a representative input value when the amplitude of the at least one signal of the error signal and the filtered error signal is predetermined amplitude, the predetermined standard step-size parameter being a value of the step-size parameter to which the filter coefficient converges when the representative input value is the standard representative input value; and

calculate the step-size parameter by multiplying the standard step-size parameter by a ratio of the standard representative input value to the representative input value.

11. The active noise reduction device according to claim 10 , wherein the standard representative input value corresponds to a maximum value of the amplitude of the at least one signal of the error signal and the filtered error signal.