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 (less than 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 (LF) sound, we exploited a by-product of the active amplification of sound outer hair cells (OHCs) 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, LF sounds significantly affects OHCs: a 90s, 80 dB(A) LF sound induced slow, concordant and positively correlated frequency and level oscillations of spontaneous otoacoustic emissions that lasted for about 2min after LF sound offset. LF 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.
Kathrin Kugler, Lutz Wiegrebe, Benedikt Grothe, Manfred Kössl, Robert Gürkov, Eike Krause, and
German Center for Vertigo and Balance Disorders (KK, RG, EK, MD) and Department of Otorhinolaryngology, Head and Neck Surgery (RG, EK, MD), Grosshadern Medical Centre, University of Munich, Munich, Germany.
Department Biology II, University of Munich, Martinsried, Germany (KK, LW, BG).
Institute for Cell Biology and Neuroscience, Johann Wolfgang Goethe University, Frankfurt/Main, Germany (MK).
Royal Society Open Science 1: 140166. doi: 10.1098/rsos.140166 
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