Outer haircells amplify quiet sounds only, while inner haircells respond directly to loud ones.

Evidence

At higher levels, the inner haircells are directly stimulated by the shearing force produced between the hairs of the sensory cells and the tectorial membrane, in response to the local velocity of the basilar membrane. For these high levels, the outer haircells are of no importance because the large displacements drive them to saturation. At low levels, however, the inner haircells are only very slightly stimulated in a direct way. An interaction between the active outer and the inner haircells is therefore assumed to be responsible for the large dynamic range of the auditory periphery, and for the sharper frequency selectivity that arises at low levels in addition to the selectivity attributable to the linear hydromechanic system of the basilar membrane. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 236

Categorization

• DEFINE: Lays out the functional division of labor between inner and outer haircells in the cochlea.
• XREF: The dual-mechanism explanation for wide dynamic range parallels how other sensory systems split processing between low- and high-intensity regimes.