echo

Repetition of a sound wave, or of a radar or sonar signal, by reflection from a hard surface such as a wall or building. By accurately measuring the time taken for an echo to return to the transmitter, and by knowing the speed of a radar signal (the speed of light) or a sonar signal (the speed of sound in water), it is possible to calculate the range of the object causing the echo (echolocation).

The speed of sound can be measured using an echo. The time taken for the sound to be reflected back over a known distance is recorded. The speed of sound can then be calculated using the formula: speed = distance/time.

For an echo to occur, the wavelength of the incident sound must be smaller than the size of the object. Sound obeys the same rules of reflection as for light. If the wavelength of the sound is the same as or greater than the size of the object, then diffraction takes place.

A similar technique is used in echo sounders to estimate the depth of water under a ship's keel or the depth of a school of fish.

Echo

In Greek mythology, a mountain nymph personifying disembodied sound. According to Ovid's Metamorphoses, Hera deprived Echo of her speech, except for the repetition of another's last words, after her chatter had kept the goddess from catching faithless Zeus with the nymphs. After being rejected by Narcissus, she wasted away until only her voice remained.

In another tradition, as described by the Greek writer Longus of the 3rd century AD, Echo refused the advances of Pan, the god of flocks and herds, who had her torn to pieces by frenzied shepherds. Earth concealed her remains, which continued to sing in imitation of other sounds.