ABSTRACT
Normal hearing is most sensitive in the frequency region from about 2,000 to 5,000 Hz. Although the sensitivity of the ear is remarkably good, it is still a few orders of magnitude poorer than any fundamental limitation to such sensitivity that is determined by the random vibration of the molecules in the air. To understand this limitation this chapter reviews thermal fluctuation and the concept of noise. It considers the definition of “normal” hearing in more detail and reviews the factors that influence the sensitivity of the average hearing adult. The stapes is the last of three small bones connected to the eardrum and its motion is essentially that of a piston. Hence, the stapes’ linear displacement is easier to specify than the eardrum, a conical structure whose mode of movement is somewhat complicated. Noise is the sound produced if people amplify the random excursions of the diaphragm of a sensitive microphone placed in air.
