Vibration of the Head in a Sound Field and Its Role in Hearing by Bone Conduction

Abstract

In a sound field the head is set into forced vibrations, and the vibrations are sometimes heard by bone conduction. For clinical purposes it is important to know the magnitude of these vibrations of the head so that, in cases of partial deafness, the roles of air conduction and bone conduction can be understood. The pattern of the head vibrations and the velocity of the deformation waves traveling along the bony wall of the skull were measured. For the velocity we found c = 570 m/sec. In the course of the experiments, a new method of calibrating a vibration pick-up with a sound level meter was developed. It was found possible to construct an earphone that reduces the amount of “cross-hearing” to a very low value. With this earphone, hearing thresholds can be measured in many cases without using noise to mask the ear with the better hearing, even though the difference between the sensitivities of the two ears is greater than 40 db. It is shown that the maximal sound insulation that can be obtained with an ear plug is determined by the elastic forces acting between the skin of the ear canal and the bony wall of the ear canal.