A Variable in the Sensitivity to Bone-Conducted Sounds. ∗

It is well known that the perceptive apparatus essential to hearing is located in the internal ear which is filled with liquid. It is also known that air vibrations are not effectively conducted into liquid directly on account of the impedance differences in the two media. Weber proposed that the middle ear apparatus had the function of a resistance matching transformer which drove, as it were, the air vibrations into the liquid and in this manner conferred great sensitivity in the reactions of the perceptive apparatus to air-conducted sounds. He believed that the tremble in the innermost ossicle, the stapes, located in the oval window, produced a mass shifting in the labyrinth liquid which could come about only through compensation motions at the second window in the internal ear, the round window. This mass shifting of the liquid is thought to result in transverse vibrations of the membranous partition between the two windows and these transverse vibrations, it is thought, in turn constitute the physical activation essential to audition. Curious-ly enough, the interpretation is founded on two assumptions which are physical impossibilities and the described displacements, which form the basis for all theories of hearing, have never been experimentally demonstrated.

It was known in the sixteenth century that the internal ear also reacted to bone-conducted vibrations and that these bone vibrations passed into the internal ear directly through the investing bony wall. Inasmuch as the bone vibrations are heard qualitatively, the same as air vibrations, it followed that the mechanical reactions in the internal ear to both sound sources were similar and a correlation was effected in the following manner: The bone vibrations produced expansions or contractions in the investing wall, which resulted in pulsating pressure changes in the enclosed liquid.