The origin of acoustic distortion in the cochlea.

Abstract

Two-tone stimulation of the ear is a powerful tool for investigating cochlear function and is gaining increasing acceptance as an audiological test. The intermodulation distortion generated by two tones includes numerous cubic components. The behavior of cubic lower sidebands has been studied by measuring them with a microphone in human and rodent ear canals as the stimulus frequency was changed at various stimulus levels. It was found that distortion produced by low level stimuli (level of f2 at or below 40 dB SPL) is due to activity at the f2 region on the cochlear partition, and that the sound measured in the ear canal has passed through an additional cochlear filter (possibly the tectorial membrane). The picture may be complicated in the human ear by added energy from the distortion product frequency region. At high stimulus levels, the response is more difficult to interpret. There may be distinct sources of 2f1−f2 caused by secondary modulation of f1 by f2−f1 (f2−f1 belongs in the quadratic, rather than the cubic family). This interaction would also give f2 (thus enhancing or suppressing the f2 level, depending on phase relationship) and upper sidebands, such as 2f2−f1.