To improve speech quality of a voice under a noise.

Plural sounding units A22 being electrostatic acoustic to transducers (capacitor earphones) and a sound reception unit B23 are placed in a cavity of a sound-proof housing 21, the units 21 are placed in a feedback loop and a level of an output of the unit B23 is adjusted by a preamplifier 24, a sample-and-hold circuit 25 samples the output, a delta modulation circuit 26 applies delta modulation to the sampled signal and an arithmetic circuit 27 calculates number of output pulses to generate a drive signal from the unit A an applies arithmetic control to detect vibration of a diaphragm of the unit B and to keep it zero at all times. On the other hand, a sound signal received from a signal terminal 28 is superimposed on the unit A drive signal and the superimposed signal is emitted into the cavity.