The occlusion-impedance of the ear canal and its influence on the occlusion effect and external physiological masking

Abstract

Bone conduction (BC) as the limiting factor for the sound protection rate of earplugs and earmuffs can be calculated by models. A model of this kind is part of a dummy head system, recently developed in our laboratory for objective sound attenuation measurement of sound protection devices. Based on the separation of BC into external-, middle-, and inner-ear components, the model allows determination of BC according to the actual type of occlusion. BC is mainly affected by the acoustic impedance, seen outwards from the entrance to the ear canal. We have measured this “occlusion-impedance” for different occluding devices. Sample results will be presented. The occlusion effect is, then, determined by the occlusion impedance and, also, by the relative vibrations between skull and earmuff, the latter depending on the mechanical impedance of earmuff and underlying skin as well as on the vibration modes of the skull. Further, the physiological noise spectrum, as generated in the external ear, is affected by the occlusion impedance too, thus leading to a characteristic external physiological masking, former known as the “missing 6-dB effect.” [Work partly supported by Minister für Wissenschaft und Forschung, NRW.]