An echo is the sound heard after the reflection of the sound waves from a plane solid surface (hard surface). How it can be used to measure the velocity of sound in air: Stand some distance directly in front of a vertical wall and clap two hinged wooden board sharply together and listen to hear the echo coming back from the wall. Clap repeatedly and adjust the rate of clapping until each next clap coincides with the echo heard from the previous clap.
The time interval between successive claps is then equal to the time taken for sound to travel twice the distance between the clapper and the wall. After the correct rate of clapping has been achieved, use stop watch to measure the time for about 20 clap intervals. Then the average time interval between two successive claps can be got by dividing the total time interval by 20. The lateral distance between the Clapper and the wall is measured.
Let this distance be D (metres) And let the average time interval for successive clap be t, then period of a clap and its echo equals
