The inner ear amplifies quiet sounds by up to 30 dB through active feedback.

Evidence

The level dependence of the tuning curves indicates a strong nonlinearity leading, at low levels, to an additional gain of as much as 30 dB. Very careful measurements of the displacement of the basilar membrane have shown that similar effects can already be found in such displacement patterns. These data stem from animals. Taking into account some additional peripherally located effects like cubic difference tones (see Chap. 14) or otoacoustic emissions (see Sect. 3.1.4), both observed in man, it must be assumed that our peripheral hearing system operates with some active nonlinear feedback. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 235

Categorization

• XREF: This active cochlear amplifier idea links to broader neuroscience themes of peripheral sensory systems doing active signal processing, and to otoacoustic emissions research.
• DEFINE: Establishes the concept of ‘active nonlinear feedback’ in the auditory periphery as a mechanism for low-level gain.