How Nature Designs Ears

Abstract

Most ears respond to the pressure component of sound, but some ears detect the oscillatory flows of the medium associated with sound. Ears generally receive sound pressure by means of an ear drum, the vibrations of which may either be guided to an inner ear (e.g., via middle ear ossicles in terrestrial vertebrates) or detected by receptor cells attaching to the ear drum (e.g., moths, grasshoppers). Most hearing animals can determine the direction to the sound source, but the cues exploited depend on whether the animal is sufficiently large to use differences in diffraction and/or in the time-of-arrival of sound at the ears. In small animals, the sound can often reach both surfaces of the ear drum, and the ears may then have directivity properties similar to those of pressure gradient microphones. While some animals such as the insect prey of echolocating bats do not need to analyse sound frequency, most animals using sounds for social communication have a capacity for frequency analysis. The mechanism of frequency analysis generally involves mechanical filters, but evidence for additional active mechanisms (mechanisms depending on metabolic energy) has been found in both vertebrates and insects.