AUDITORY PHYSICS - PHYSICAL PRINCIPLES IN HEARING THEORY .3.

This paper is a continuation of a series of papers dedicated to the physical aspects of hearing. In the past decade several novel properties of the cochlea (i.e., the inner ear) have been discovered. The most spectacular finding is that most normal ears constantly emit weak sounds, the spectrum of the emissions being as characteristic for each ear as the fingerprint of a person. This finding implies that the cochlea contains elements that are capable of producing acoustical energy actively. A second important fact is that experimental mechanical response curves (velocity of vibrations inside the cochlea, measured as a function of frequency) are found to be so frequency-selective that they cannot be simulated by a model of the cochlea of the type previously described. Accordingly, theories of cochlear mechanics have had to be worked out for models in which active elements are included. As long as models of this type are stable they could be taken as linear. However, experimental observations dictate that nonlinear elements must be included. Further, in unstable active models, it is necessary to add nonlinearity in order to limit the amplitude of the ensuing oscillations. The present paper describes these theoretical developments against the history of the experimental discoveries.