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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. 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 they are unable to 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.

Echoes are reflected off walls or hard surfaces like mountains.

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 343 m/s at a normal room temperature of about 20°C, the reflecting object 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.

Echo samples

Clean signal, followed by different versions of echo.

Problems listening to the file? See media help.

The strength of an echo is frequently measured in dB sound pressure level SPL 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.

The name "echo" comes from the Greek nymph Ηχώ from Greek mythology. According to Greek mythology Echo was a Nymph 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 to say what others had just said — hence the word "echo". Yet according to Roman mythology it was Pan, who when Echo denied his love, tore her to shreds and scattered her about the Earth leaving bits of her everywhere for us to find, or rather for us to hear, forever.

"A duck's quack doesn't echo," is a much quoted scientific myth. The truth is that a duck's quack does, in fact, echo, however it may be difficult to hear. This myth was first debunked by the Acoustics Research Centre at the University of Salford in 2003 as part of the British Association's Festival of Science. It also featured in one of the earlier episodes of the popular Discovery Channel television show "MythBusters."

• The Golkonda (a.k.a Golconda) Fort of Hyderabad, India
• The Whispering Gallery of St Paul's Cathedral, London, England
• Echo Point, the Three Sisters, Katoomba, Australia
• The Temple of Kukulcan ("El Castillo"), Chichen Itza, Mexico [1]

• Listen to duck echoes and check out the myth