Sensory systems are admirably efficient in the sense that they detect astonishingly small quantities of the information they are tailored to respond to. In vision, audition and smell, these systems rival in sensitivity the achievements of modern technology. When we speak of measuring the minimum sensitivity of the auditory system we talk about amounts of energy and amplitudes of motion which are almost impossible to grasp intuitively. Within the last decade it has been possible to measure the behavior of the mechanical part of the system at something approaching its normal operating amplitudes of around 10Å, using laser interferometer techniques (Tonndorf and Khanna, 1972). Transfer of these amplitudes to those of the basilar membrane and extrapolation from these measurements down to minimum operating levels (threshold) has essentially confirmed the original extrapolation of von Békésy that minimum basilar membrane amplitudes in the sensitive mid-frequency range of the ear are about 10-2Å. Fortunately, the signal at the eardrum which creates this basilar membrane amplitude is still 18 dB above the Brownian noise amplitude of the air at the drum; and the inherent Brownian noise of the cochlea has an energy about 22 dB down from the minimum energy required for threshold at the hair cell (Harris, 1968).
KeywordsSound Pressure Sound Pressure Level Basilar Membrane Sensitivity Curve Absolute Sensitivity
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