New off-line method for detecting spontaneous otoacoustic emissions in human subjects.

Spontaneous otoacoustic emissions were evaluated in 36 female and 40 male subjects. In agreement with the results of previous surveys, emissions were found to be more prevalent in female subjects and there was a tendency for the male subjects to have fewer emissions in their left ears. The digitization of five minute samples of ear canal signals, combined with sophisticated data analysis, produced a substantial reduction in the emission detection threshold. 588 emissions were detected in 72% of the subjects and 56% of the ears. Of the observed emissions, 18 could be identified with cubic distortion products of other emissions, and 11 could be identified as harmonic products (i.e., integral frequency multiples of other emissions). The large number of emissions detected (one subject had 32 in her right ear and 25 in her left) permitted evaluation of the pattern of separation of emissions. The average effective separation along the basilar membrane (according to the Greenwood frequency map) for adjacent emissions of all ears was 0.427 mm with interquartile values of 0.387 mm and 0.473 mm. The relationship between emission power, frequency, and full width at half maximum appears to be in agreement with the implications of a noise perturbed Van der Pol oscillator model of spontaneous emissions.