Low-frequency sound affects active micromechanics in the human inner ear

Noise-induced hearing loss is one of the most common auditory pathologies, resulting from overstimulation of the human cochlea, an exquisitely sensitive micromechanical device. At very low frequencies (<250 Hz), however, the sensitivity of human hearing, and therefore the perceived loudness is poor. The perceived loudness is mediated by the inner hair cells of the cochlea, which are driven very inadequately at low frequencies. To assess the impact of low-frequency sound, we exploited a by-product of the active amplification of sound outer hair cells perform, so-called spontaneous otoacoustic emissions. These are faint sounds produced by the inner ear that can be used to detect changes of cochlear physiology. We show that a short exposure to perceptually unobtrusive, low-frequency sounds significantly affects outer hair cells: A 90 s, 80 dB(A) low-frequency sound induced slow, concordant and positively correlated frequency- and level oscillations of spontaneous otoacoustic emissions which lasted for about two minutes after low-frequency sound offset. Low-frequency sounds, contrary to their unobtrusive perception, strongly stimulate the human cochlea and affect amplification processes in the most sensitive and important frequency range of human hearing.