The human eardrum is astonishingly sensitive to sound. At the threshold of hearing, it vibrates with an amplitude of mere picometers. That’s on a par with the membrane’s spontaneous oscillation from the thermal motion of air molecules bouncing off it and water molecules sloshing around inside the cochlea, the ear’s frequency analyzer and amplifier (see Physics Today, April 2008, page 26). At the upper limit of hearing—near the pain threshold from a roaring jet engine, for instance—the eardrum vibrates through just tens of microns. For the 0- to 120-dB sound-pressure levels we normally experience, the range of vibratory motion spans six orders of magnitude.
Although typically less than 100 µm thick, the eardrum, otherwise known as a tympanic membrane, also serves as a protective barrier to the outside world. And its linkage to the chain of bones in the middle ear forms a mechanical lever that transmits sound...
