The use of reflected sound waves for obtaining environmental information. Certain animals, such as the bat and the porpoise, emit pulses of extremely high frequency sound (up to 50 kHz for bats, and up to 170 kHz for porpoises and whales) in narrow, intense streams. The reflection, or echo, of these sounds supplies information regarding the nature and location of objects in the environment. When an emission of varying frequency is used, such as with bats, the echoes, travelling varying distances to the ear, are heard as different frequencies in each ear, thus supplying directional information.

See: Binaural Hearing, Minimum Audible Angle, Precedence Effect, Ultrasonic.

Humans also employ this principle. Mariners have historically made use of their boats' whistles, bells, and other loud sounds, to determine their position in fog. In areas of low ambient noise, sound reflections are also useful to the blind, but in modern lo-fi environments, such reference is denied them.

Compare: Sonar, Sound Shadow. See also: Feedback, Speed of Sound.

[The boat captains] used to get their position by echo whistling. They'd give a short whistle and estimate the distance from the shoreline by the returning echo. If the echo came back from both sides at the same time they'd know that they were in the middle of the channel. They could recognize different shorelines by the different echoes - a rocky cliff, for example, would give a clear distinctive echo, whereas a sandy beach would give a more prolonged echo. They could even pick up an echo from logs.

The Vancouver Soundscape, 1974, p. 17.