A duck's quack actually does echo, although the echo may be difficult to hear for humans under some circumstances. The cast of the television series MythBusters perform experiments to verify or debunk urban legends, old wives' tales, and the like.

Initially unable to get either duck to quack, they began chattering when paired. Initially, no echo could be found, so the team moved to an anechoic chamber for comparison. When examined by an audio expert, it was found that the echo was "swallowed" by the original quack, due to the very similar acoustic structure between the quack and the echo. Because of this, it may be difficult to tell where the quack ends and the echo begins, both having similar waveforms on an oscilloscope and blending together in a way that makes them difficult to distinguish. In the same way, human hearing may not perceive the difference between a duck's quack and its echo.

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.

If so many reflections then 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 and privacy fences.