Spontaneous narrowband acoustic signals emitted by human ears

In the ears of many persons, a spontaneous and continuous narrowband acoustic signal can be measured with a probe microphone in the ear canal. In a sample of 32 persons with normal hearing who were tested bilaterally, an oto-acoustic emission (OAE) was detected in 22 ears of 16 persons. These signals were most often found between 1.0 and 2.0 kHz and in all cases, the sound pressure in the occluded ear canal was less than 200 mu Pa. The behavior of these emissions was investigated in a number of experiments on the author's own OAE at 1910 Hz. In one of these studies, the suppression of an OAE by an external tone was examined. It was found that a contour of constant suppression (amplitude versus frequency of external tones that produce a criterion reduction in the amplitude of the OAE) exhibits frequency selectivity like that commonly associated with cochlear frequency analysis. The existence of OAE's of cochlear origin demonstrates that the ingredients for active biomechanical processes are present in the inner ear and supports the notion that such processes increase the sensitivity and selectivity of mechanical frequency analysis in the cochlea.