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.
Echoes are reflected off walls or hard surfaces like mountains.
Sound travels approximately 343 meters/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be half that distance away (the sound takes half the time to get to the object and half the time to return).
In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes.
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.