Brownian motion in the cochlear partition.

Abstract

Estimates have been made of the Brownian-pressure fluctuations in the air. These are 18 dB below threshold (minimum audible field) for 3000 Hz. It is thus necessary to look into the ear itself to see whether thermal fluctuations limit the sensitivity of the ear. It is assumed that the first stage of amplification of acoustical energy takes place at the hair cell, and that it is the relative shear displacement between the top of the hair cell and the stereocilia that is the relevant parameter. Two estimates of Brownian fluctuation are made. The first estimate assumes a connection rigid enough to provide the greatest flow of acoustical energy to a hair cell—i.e., proper impedance match—and loose enough so that each hair cell has independent thermal displacements. The second assumes a rigid connection between the stereocilia and the tectorial membrane and thus the source of noise is the fluctuation in amplitude of the basilar membrane displacement. The impedances used in making these estimates were based on the in vivo measured impedances at the eardrum and the anatomy of the middle ear. The first estimate gives a singal-to-noise (S/N) ratio of 22 dB at threshold for 3000 Hz. The second estimate gives an (S/N) ratio of −33 dB. This calculation shows that it is necessary to assume that the stereocilia are attached to the tectorial membrane and that the hair-cell bodies are imbedded in the organ of Corti with some degree of rigidity. There is some direct experimental evidence for the assumption of rigid attachment, and there is much indirect evidence from ultrastructural studies. A critical evaluation is made of the assumptions and of the data used in the calculations.