Frequency-selective damping in the inner ear
Before 1992 it was unknown that one of the major functions of the mammalian inner ear is to selectively damp the loud frequency components of sound. Without this function we would not be able to follow normal speech, and music would be a torture rather than a pleasure.
That our ears are frequency selective amplifiers of weak sound was first suggested by Thomas Gold in 1948 and was finally proved in the 1980s. We now also know that there is a complement of low-level amplification: high-level damping.
The border between amplification and damping lies at ca 60 dB SPL, which is the typical mean sound level for normal speech. This means that weak spectral components in normal speech are amplified already in the inner ear, and strong spectral components in normal speech are damped already here.
The mechanism of frequency selective damping consists of a flexible membrane, which runs along the full length of the coiled inner ear and is set into vibrations by sound levels above 60 dB. The vibrations absorb high-level sound energy and take it out as thermal energy, which then can no longer distort or mask the sensation of weak sound components or even act as a stressor for the delicate sensory cells.
Due to the specific anatomy of this so-called basilar membrane, each sound frequency has its own place of maximal vibration. This enables the inner ear to amplify some frequencies while at the same time, but at different places, damping others.
An example: A complex sound has 70 dB at 300 Hz, 50 dB at 500 Hz, and 80 dB at 800 Hz. The weak second component is amplified, whereas the other two components are damped. Without their damping we would hear the second component much worse or not at all.
Formalization of current evidence in four figures
Braun, M. (1993) Basilar membrane tuning re-examined: frequency selective damping of high level input may be its genuine function. In: H. Duifhuis, J.W. Horst, P. van Dijk, and S.M. van Netten (Eds.), Biophysics of Hair Cell Sensory Systems, World Scientific, Singapore, p. 406.
Braun, M. (1994) Tuned hair cells for hearing, but tuned basilar membrane for overload protection: evidence from dolphins, bats, and desert rodents. Hear. Res. 78, 98-114. Abstract
Braun, M. (1996) Impediment of basilar membrane motion reduces overload protection but not threshold sensitivity: evidence from clinical and experimental hydrops. Hear. Res. 97, 1-10. Abstract
Nilsen, K.E. and Russell, I.J. (2000) The spatial and temporal representation of a tone on the guinea pig basilar membrane. Proc. Natl. Acad. Sci. USA 97, 11751-11758. Discussion, figure belonging to discussion is found here.
Kössl, M. and Vater, M. (2000) Consequences of Outer Hair Cell Damage for Otoacoustic Emissions and Audio-vocal Feedback in the Mustached Bat. J. Assoc. Res. Otolaryngol. 1, 300-314. Discussion
Historical evidence: Wever and Lawrence (1950)
Recent additional evidence:
Wada, H., Takeda, A., and Kawase, T. (2002) Timing of neural excitation in relation to basilar membrane motion in the basal region of the guinea pig cochlea during the presentation of low-frequency acoustic stimulation. Hear. Res. 165, 165-176. Abstract, Comment, and Discussion
Interesting links: Rémy Pujol