Harmonic Distortion in Cochlear Models

According to Békésy, the envelope over the train of waves traveling along the cochlear partition is asymmetrical; the distal slope being steeper than the proximal one. In experiments on cochlear models, this asymmetry was seen to increase with intensity. Introduction of artificial “eddies” of varying velocity into the model (while the intensity was kept constant) indicated that the “eddies” were responsible for the observed increase in asymmetry of the envelope. Theoretical considerations showed that the “eddies” convert the (trochoidal) particle motion within the cochlear fluids into a cycloid form of motion. The bilateral effects of this particular cycloid motion lead to the following alteration of the displacement pattern of the cochlear partition: (1) to an amplitude decrement with distance (accounting for the above asymmetry of the envelope); (2) to peak clipping; and (3) to an aysmmetry, the displacement being larger toward the tympanic than toward the vestibular scala. These theoretical results were confirmed by an observed change of the particle orbits. Their elliptical shape became “flattened,” containing both odd and even harmonics. Resolution of this distorted pattern of fluid motion along the cochlear partition was in accordance with the place principle of frequency localization.