In
audio signal processing and
acoustics, an
echo (plural
echoes) is a
reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the
speed of sound. The word
echo derives from the
Greek ἠχώ (
ēchō), itself from ἦχος (
ēchos), "sound".
Echo in the folk story of Greek is a mountain nymph who's ability to speak was cursed, only able to repeat the last words anyone spoke to her. Animals that use echoes are cetaceans (dolphins and whales) and bats.