PURPOSE: To detect the sound produced by an instrument during a dental treatment with a microphone and to generate the sound which is approximately equal in amplitude to the sound generated by this instrument and varies in phase by 180° from a speaker so as to cancel the sound generated by the instrument.

CONSTITUTION: A symbol 11 is the microphone and 12 a speaker. The microphone 11 is mounted in an arbitrarily desired point in a place where the sound produced by the microphone can be easily picked up; for example, within the instrument, the back-rest side part of a dental chair, etc. The speaker 12 is disposed as near the patient's ear as possible, such as back-rest, head-rest or non-shading lamp. The sound produced from the speaker is so controlled as to be approximately equal in the amplitude to the sound produced from the instrument and to vary by 180° in phase therefrom near the patient's ear.