Acoustics of Circumaural Earphones

Abstract

When an electro-acoustic transducer is added to a circumaural ear defender the requirements of minimum sound transmission and maximum earphone sensitivity are partially incompatible. The transmission and response of simple (single cavity) and compound (two cavity) systems have been determined by an analog method. The preferred system is a rigid structure with resistive coupling between the two cavities. This system has maximum effective coupling volume at low frequencies thus providing maximum noise attenuation and a smaller effective volume at high frequencies affording increased earphone response. When the cavities contain suitable porous materials and are coupled by an annular slit, the two cavity system becomes a lossy waveguide. Analysis of hemispherical and plane wave acoustical models indicates an effective volume which is constant up to a transition frequency and decreases steadily with further increase in frequency. The theory is supported by measurements on an experimental earphone having complementary transducer and cavity characteristics leading to constant response over a wide-frequency range. Probe microphone data show that above 1 kc the real ear response of circumaural earphones is generally greater than the flat-plate coupler response.