空気伝導形心音マイクロホンにおける周囲騒音の伝達経路について

抄録

Phonocardiographic microphones are more sensitive to room noise as compared with the contact type phonocardiographic transducers. However, the leakage path of room noise to the sensitive element of the microphone is unknown as yet, and it has been supposed that the room noise reached the microphone element by penetrating through small gaps between microphone attachment and chest wall surface.
In this paper, the leakage path of room noise is discussed in detail. As the results of many investigations, following facts have been confirmed : The noise does not penetrate through the gaps between microphone attachment and chest wall surface but it vibrates the chest wall surface, introduces the sound pressure in the air cavity of the microphone and produces an open-circuit voltage across the microphone terminals. Furthermore, the chest wall surface covered by the microphone attachment is vibrated by the room noise in relatively high amplitude in the range of 50 to 500 Hz owing to the resonance of the system consisting of the chest wall surface, air cavity and microphone itself. To diminish these effects, it is necessary to minimize the volume of the cavity as far as possible (less than 1 cm³). Because this is very difficult, the experiments were made by using a microphone with an equivalent volume of 3. 5 cm³. As a result, the effect of room noise could be decreased by about 10 dB at frequencies under 200 Hz.