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sc human voices of nearly identical timbre.We additionally perceive cantly altering the passive mechanical properties that transmittions within the cochlea (Rhode, 1971; Le Page and Johnstone, 1980) as well as the discovery that healthy ears produce sounds (Kemp, 1978)—so-called otoacoustic emissions—have estab-form of spontaneous hair-bundle oscillations and as negative stiffness that can increase a bundle’s response to low-amplitude mechanical stimulation (Martin et al., 2000, 2003). Active hair-lished that the inner ear possesses an active amplification mechanism. bundle motility also contributes to nonlinear amplification in an in vitro preparation of the mammalian cochlea (Chan andsounds of vastly differing intensities, enabling us to discern the strumming of nylon strings on a classical guitar playing in concert with a full orchestra. It is remarkable that the ear can achieve such sensitivity despite the viscous damping that impedes the oscillation of structures within the cochlea. Indeed, the frequency resolution of human hearing inferred from psycho-physics is too great to be explained by passive resonance (Gold, 1948). Measurements of amplified, compressive vibra-energy to the characteristic place. Selectively perturbing amplification requires an understanding of the underlying active process in outer hair cells. Experiments involving isolated hair cells have identified two force-generating mechanisms. Themechanoreceptive hair bundles of many tetra-pods are capable of generating forces that can be entrained by an external stimulus (Martin and Hudspeth, 1999; Kennedy et al., 2003, 2005). These forces have been observed in thea frequency span of three decades allows us to distinguish This approach reduces amplification, but at the cost of signifi-Articl