If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears two distinct sounds: one is the original (or direct) sound and the other is the sound heard after reflection from the wall or hillside which is called an echo. Hard surfaces reflect sound waves. An echo is a sound that has been reflected before you hear it.  

Thus, The sound heard after reflection from a rigid obstacle (such as a cliff, a hillside, wall of a Building, etc.)is called an echo. 

Condition for formation of an echo:

An echo is heard only if the distance between the person producing the sound and the, rigid obstacle or reflector is long enough to allow the reflected sound to reach the person at least 0.1 second after the original sound is heard. The reason is that the sensation of sound persists in our ears for about 0·1 second after the exciting stimulus ceases to act. Hence, the echo is heard distinctly (separate from the original sound) if it reaches the ears at least 0.1 second after the original Sound.

To hear the echo distinctly, the reflecting surface in air should be at a minimum distance of 17 m cm the listener (why?). If the distance is less than 17 m, the reflected sound will reach the ears before the original sound dies out and so echo will not be heard. This means that in air, an echo is not heard if the reflector is at a distance less than 17 m. In such a case, the original sound just mixes up with the reflected sound. Due to repeated reflections at the reflecting surface, the sound gets prolonged. This effect is known as reverberation.

Measuring the speed of sound by an echo method:

The following worked example illustrates how echoes may be used to measure the speed of sound.

The echo method can be used for determining .the speed of sound in air. For this, sound is produced from a place at a known distance say, d at least 50 m from the reflecting surface. The time interval t in which the echo (that is the sound after reflection from the reflecting surface) reaches the place from where the sound was produced, is noted by a stop watch. Then the speed of sound is calculated by using the following relation:

The experiment is repeated several times in order to get the average value of speed of sound.