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 in a room, by the walls. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.

If so many reflections arrive at a listener that he cannot distinguish between them, the proper term is reverberation.

An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived.

When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second. Thus, since the velocity of sound is approximately 344 m per sec at a normal room temperature of about 20°C, a reflecting wall must be more than 16.2 m from the sound source at this temperature for an echo to be heard by a person at the source.

The intensity of an echo is frequently measured in dB relative to the directly transmitted wave.

Echoes may be desirable (as in sonar) or undesirable (as in telephone systems).

In computing, an echo is the printing or display of characters (a) as they are entered from an input device, (b) as instructions are executed, or (c) as retransmitted characters received from a remote terminal.

In computer graphics, an echo is the immediate notification of the current values provided by an input device to the operator at the display console.

Some information from Federal Standard 1037C and from MIL-STD-188.

Mythology

The name "echo" comes from Greek mythology. Echo was an Oread who had the job of talking incessantly to Hera, the Queen of the Gods, so that her husband, Zeus, wouldn't get caught in his numerous affairs. Hera caught on to Echo's trick and cursed her to only be able say what others had just said -- hence the word "echo".