Question

Explain how the 'Echo' is produced. What should be the minimum distance between the listener and the reflector to hear an echo of sound propagating with a speed $vm{s}^{-1}$?

Answer 1

The repetition of sound caused by the reflection of sound waves. For example, if we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.

The sensation of sound persists in our brain for about $0.1s$. To hear a distinct echo the time interval between the original sound and the reflected one must be at least $0.1s$.

Given,

Sound is propagating with speed, $vm/s$

Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least,

$D=speed\times time$

$D=v\times 0.1$

$D=\frac{v}{10}$

Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is,

$\frac{D}{2}=\frac{v}{20}$