Suppression of ambient noise in bone-conduction microphones for speech communication
George J. Sebesta, Alan Hofer, Richard W. Carlisle · The Journal of the Acoustical Society of America · 1976
A microphone sensitive to vibrations of the head is useful in applications where it is necessary to leave the face of the user unencumbered. Such a transducer operates on the inertia principle, with the inner and outer surfaces operating in phase. It has some sensitivity as an air-conduction microphone, and when used in free space it has a figure-eight response pattern. To suppress airborne noise and to leave it sensitive exclusively to bone-conduction vibrations impinging on the inner surface, it is necessary to convert the transducer into a cardioid-type device. The inner surface is to be sensitive to vibrations, and airborne sound is to be neutralized as much as possible. In a microphone described in this paper, the transducer is suspended by a highly compliant member in a housing which attenuates airborne sound, with optimum exclusion of high frequencies. Low-frequency airborne sound is neutralized by impressing it equally upon the inner and outer surfaces of the transducer. The over-all suppression of noise permits good speech transmission in a noise field up to 85 dB. By the use of an outer acoustic enclosure this has been experimentally increased to 115 dB.