PURPOSE:To prevent the oscillating phenomenon of a controller from being generated, by providing a means which subtracts a transfer function equivalent to a transfer characteristic between a signal outputted from an added sound source, and the signal inputted to a sound wave detecting means, from the signal inputted to the sound wave detecting means. CONSTITUTION:Filled sound detected by a microphone 10 is inputted to a BPF14 through a subtractor 24, and a pre-amplifier 12, and only the signal to be reduced as the filled sound is taken out. The output is inputted to a control circuit 20 as a rectangular wave 26S through a saturation amplifier 26. The control circuit 20 generates a rectangular wave 20S having a phase difference between sound generated from a speaker 22, and the input signal of the microphone 10, and also, outputs the signal having a voltage level corresponding to the volume of the sound generated from the speaker 22. Those signals are inputted to the speaker 22, and simultaneously, are sent out to a transfer function setting means 15. At the means 15, a prescribed transfer function is set, and the output signal of a phase shifter 18 is subtracted from the output signal of the microphone 10 by the subtractor 24. In such a way, only a filled sound signal is inputted to the control circuit 20.
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