An echo occurs when a sound wave reflects back towards its source after hitting a hard surface. Although the surface may absorb some of the sound, the remaining sound that is not absorbed continues moving, creating additional echoes by bouncing off surrounding objects until the sound is completely absorbed or dissipates.
The term "time delay" refers to the time it takes for the human ear to hear the returning sound wave; it can be calculated by dividing the extra distance by the speed of sound, which is a constant of 1,125 feet per second. An echo that returns to the ear in 0.1 seconds or less is referred to as a reverberation, commonly heard in music halls.
Because the human ear cannot perceive a delay of under 0.067 seconds, the reverberation actually sounds like one continuous sound rather than the separate sounds that make up an echo. For example, a sound wave reflecting off a surface under 55 feet away returns quickly enough to sound like a single sound wave to the human ear. Conversely, a surface that is over 56 feet away provides enough distance that the human ear can perceive the gap between the original sound wave and the reflecting sound wave.