The Noise-Cancelling Headset—An Active Ear Defender

Abstract

The purpose of this device is to reduce ambient noise at the ear by using a headset to generate a sound pressure equal in magnitude and opposite in phase to the noise. This paper reports the results of pure-tone experiments. A microphone was mounted on each earphone of a conventional military headset. A two-channel system was used in which the signal from each microphone was passed through an amplifier and a phase-shifting network and was then applied to its earphone. The subject wore the headset in a pure-tone field and adjusted the amplifications and phase shifts for minimum loudness. The signal produced by each earphone was then substantially equal in magnitude and opposite in phase to the sound coming through and under its earcap. The proper amplification and phase shift are frequency-dependent and vary among subjects. Eight complete measurements were made on two subjects. The mean values of the two parameters were computed for each frequency. Calculations indicate that a device having these mean characteristics would provide a noise reduction between 100 and 1200 cps that averages 10 db more than is provided by the earcap alone.