Active noise reduction system

Abstract

An electro-acoustical feedback system (Fig. 6) which actively and passively attenuates ambient noise, uses shunt control network (60) for compensation. This network comprises several filters (61, 62, 63) parallel with the acoustical path (T16). The shunt control network dominates the loop response at low and at high frequency extremes and so stabilizes the feedback performance. A secondary feedback loop (59 and Fig. 4) is used in conjunction with a bilateral transducer (MT2) to reduce the effect of speaker diaphragm resonance and to modify in other ways the phase and amplitude response of the speaker. A bilateral transducer (MT2) serves simultaneously as a speaker to generate sound and as a microphone to sense sound in a controlled volume. An in-the-ear implementation of the electro-acoustical feedback system (Fig. 2) is shown. Forward acoustical seal is made between earplug flange (38) and the wall (42) of the ear canal (36). This isolates a small controlled volume aiding active and passive attenuation. The application of the in-the-ear active noise reduction is in hearing protection, with or without communication capability. The improvement in intelligibility in noisy environment makes this type of device attractive for vehicular communication and in noisy industrial locations. Specialized applications include electronic stethoscopes and hearing aids.