Distortion-Product Otoacoustic Emission Measured Below 300 Hz in Normal-Hearing Human Subjects

Physiological noise levels in the human ear canal often exceed naturally low levels of otoacoustic emissions (OAEs) near the threshold of hearing. Low-frequency noise, and electronic filtering to cope with it, has effectively limited the study of OAE to frequencies above about 500 Hz. Presently, a custom-built low-frequency acoustic probe was put to use in 21 normal-hearing human subjects (of 34 recruited). Distortion-product otoacoustic emission (DPOAE) was measured in the enclosed ear canal volume as the response to two simultaneously presented tones with frequencies f ₁ and f ₂. The stimulus-frequency ratio f ₂/f ₁ was varied systematically to find the "optimal" ratio evoking the largest level at 2 f ₁-f ₂ frequencies 87.9, 176, and 264 Hz. No reference data exist in this frequency region. Results show that DPOAE exists down to at least 87.9 Hz, maintaining the bell-shaped dependence on the f ₂/f ₁ ratio known from higher frequencies. Toward low frequencies, however, the bell broadens and the optimal ratio increases proportionally to the bandwidth of an auditory filter as defined by the equivalent rectangular bandwidth. The DPOAE phase rotates monotonously as a function of the stimulus ratio, and its slope trend supports the notion of a lack of scaling symmetry in the apex of the cochlea.