Noise reduction apparatus

1. A noise reduction apparatus comprising:

a plurality of loudspeakers that generate control sounds to cancel noises at a plurality of control points at which noise is to be reduced;

a plurality of control sound signal generating units, each of the control sound signal generating units being provided to correspond to each of the plurality of loudspeakers and generating a control sound signal to drive a corresponding loudspeaker of the plurality of loudspeakers; and

a plurality of error microphones, each of the error microphones being arranged at a corresponding control point of the plurality of control points and detecting error sound between the noise at the corresponding control point and the control sound at the corresponding control point, wherein

each of the control sound signal generating units generates the control sound signal based on the error sound detected by a predetermined error microphone of the plurality of error microphones, and

the noise reduction apparatus further comprises an input switching unit that, when a level of the noise at a predetermined position is higher than a predetermined threshold value, feeds an error audio signal at a predetermined control point of the plurality of control points or a control point that is newly set based on the plurality of control points into each of the control sound signal generating units, the number of the newly set control point being less than the number of the plurality of control points.

2. The noise reduction apparatus according to claim 1 further comprising

a noise detection unit that detects noise generated from a noise source.

3. The noise reduction apparatus according to claim 1 , wherein

the error microphone detects noise generated from a noise source.

4. The noise reduction apparatus according to claim 2 , wherein

the plurality of loudspeakers includes first and second loudspeakers,

the plurality of error microphones includes first and second error microphones arranged at first and second control points,

the plurality of control sound signal generating units includes first and second control sound signal generating units,

when a noise level detected by the noise detection unit is equal to or less than the predetermined threshold value, the input switching unit feeds an output of the first error microphone to the first control sound signal generating unit and feeds an output of the second error microphone to the second control sound signal generating unit, and

when the noise level detected by the noise detection unit is higher than the predetermined threshold value, the input switching unit selects any one of the two control points and feeds an output of an error microphone arranged at the selected control point among the first and second error microphones into both the first and second control sound signal generating units.

5. The noise reduction apparatus according to claim 2 , wherein

the plurality of loudspeakers includes first and second loudspeakers,

the plurality of error microphones includes first and second error microphones arranged at first and second control points,

the plurality of control sound signal generating units includes first and second control sound signal generating units, and

the input switching unit further includes

an average processing circuit that averages outputs of the first and second error microphones and outputs the averaged output as an output at a virtual third control point newly set at approximately a center position between the first and second control points, and

when the noise level detected by the noise detection unit is equal to or less than the predetermined threshold value, the average processing circuit feeds an output of the first error microphone into the first control sound signal generating unit and feeds an output of the second error microphone into the second control sound signal generating unit, and

when the noise level detected by the noise detection unit is higher than the predetermined threshold value, the average processing circuit feeds an output of the average processing circuit into both the first and second control sound signal generating units.

6. The noise reduction apparatus according to claim 2 , wherein

the plurality of loudspeakers includes first and second loudspeakers,

the plurality of error microphones includes first and second error microphones arranged at first and second control points,

the plurality of control sound signal generating units includes first and second control sound signal generating units, and

the input switching unit further includes

a third error microphone arranged at a third control point newly set at approximately a center position of a straight line connecting the first and second error microphones,

when the noise level detected by the noise detection unit is equal to or less than the predetermined threshold value, the input switching unit feeds an output of the first error microphone to the first control sound signal generating unit and feeds the output of the second error microphone to the second control sound signal generating unit, and

when the noise level detected by the noise detection unit is higher than the predetermined threshold value, the input switching unit feeds an output of the third error microphone into both the first and second control sound signal generating units.

7. The noise reduction apparatus according to claim 2 , wherein the input switching unit changes the predetermined threshold value depending on a frequency of noise detected by the noise detection unit.

8. The noise reduction apparatus according to claim 7 , wherein the input switching unit sets a smaller predetermined threshold value when the frequency of noise detected by the noise detection unit is lower.