Echo

An echo is the repetition of an original sound, heard again a short time later, caused by that sound reflecting off some rigid surface at a distance from its source. Shout in a hilly region, or into a large empty hall, and you are likely to hear your own voice come back to you as a distinct, separate echo.

Why doesn't every reflecting surface produce a noticeable echo? The human brain retains (keeps 'hearing') a sound sensation for roughly 0.1 second after the sound itself has stopped. For a listener to perceive the REFLECTED sound as a genuinely SEPARATE, distinct echo -- rather than the reflection simply blending into the tail end of the original sound -- the reflected sound must arrive back at least 0.1 s after it left. At 22°C, the speed of sound in air is about 344 m/s, so in 0.1 s sound covers a distance of $\text{distance}=\text{speed}\times\text{time}=344\times0.1=34.4$ m -- but this is the ROUND-TRIP distance (source to wall and back), so the reflecting surface itself must be at least HALF that, i.e. about 17.2 m, away f …