To perceive speech, the brain relies on inputs from sensory cells located near the top of the spiral-shaped cochlea. This low-frequency region of the inner ear is anatomically difficult to access, and it has not previously been possible to study its mechanical response to sound in intact preparations. Here, we used optical coherence tomography to image sound-evoked vibration inside the intact cochlea. We show that low-frequency sound moves a small portion of the basilar membrane, and that the motion declines in an exponential manner across the basilar membrane. Hence, the response of the hearing organ to speech-frequency sounds is different ...