Figure 2.

Basilar-membrane resonance. (a) A computer-generated voltage stimulus in the form of a geometric frequency sweep (top) drove an earphone that produced a constant pressure at the basilar membrane (middle) but elicited a striking resonance in the displacement of a hair bundle of an inner hair cell (bottom). (b) The frequency spectrum of the applied pressure demonstrates the linearity of the stimulus over the range of frequencies of interest. (c) The spectrum of bundle-displacement magnitude reveals a resonance peak at 850 Hz that is well fit by a Lorentzian curve (gray) with a Q₀ of 4.7 and a peak sensitivity of 1.8 μm·Pa⁻¹. (d) The response amplitude of a passive hair bundle was linear over a wide range of stimulus intensities. The measured slope in this doubly logarithmic plot (black line) is 1.03 (r² = 1.00). (e) A plot of the inverse square of the resonant frequency (ω₀) against the fluid mass in the lower compartment can be fit linearly (solid line) to obtain the stiffness of the system, in this instance 120 N·m⁻¹ (r² = 0.98). Fixation of the same preparation with formaldehyde increased the system’s stiffness to 200 N·m⁻¹ (dashed line; r² = 0.99).