The importance of external and middle ear contributions to bone conduction in man

Abstract

Differences between air conduction (a–c) and bone conduction (b–c) thresholds are commonly used to determine the presence and magnitude of a middle ear abnormality in the human audiotory system, making the tacit assumption that b–c directly stimulates the cochlea. Animal experiments have long established that b–c transmission is a complex phenomenon [J. Tonndorf, in Foundations of Modern Auditory Theory, Vol. 2 (Academic, New York, 1972)] with significant contributions from middle and external ear components. An artificial reversible middle ear abnormality may be induced in man by maintaining an air pressure in the external meatus. The shifts in a–c and b–c thresholds provide a means to evaluate the relative importance of the external and middle ear components. At the lower frequencies of 250 and 500 Hz, these components predominate; at 500 Hz there is an a–c shift of 15.9 dB accompanied by a b–c shift of 15.8 dB. These results have important implications for the interpretation of air and bone conduction thresholds.