If anyone produces a sound standing near the bank of a river, many of us have the experience of hearing the repetition of that sound after a while. The same case happens if we produce sound standing near a hill or a building. If a sound is produced in a big empty house, the same sound is heard just after a while. These phenomena take place due to the reflection of sound.
When a sound becomes separate from its original sound and is repeated due to reflection then this reflected sound is called echo. Simply we can say that the reflection of sound is echo.
The minimum distance of reflector
A sound that is heard persists 0.1 second in our brain. It is called the persistence period of hearing. If any new sound reaches our ear in this period we cannot hear it. Therefore for hearing an echo of any sound the distance between the source of sound and reflector must be such that the reflected wave cannot come back to the listener before a time of 0.1 second. If the velocity of sound in the air at 0°C is considered to be 332ms-I then in 0.1 sec sound can travel 332 m.
Therefore a reflector must be placed at a minimum distance of 332/2 or 16.6 m from the listener.