Question

(a) What is an echo? How is echo formed?

(b) What is the minimum distance in air required from a sound reflecting surface to hear an echo (at ${20}^o$ C)?

(c) A man standing 825 metres away from a cliff (steep rock) fires a gun. After how long will he hear its echo? The speed of sound in air is $330m/s$.

Answer 1

(a) The repetition of sound due to reflection is called echo. It is formed if the reflection of sound is heard after 0.1 s.

(b) At 20° , speed of sound = 344 m/s

Time for echo to reach = 0.1 s

Minimum distance = $v\times \frac{t}{2}=344\times \frac{0.1}{2}$= 17.2m

(c) distance d = 825m

speed V = 330 m/s

t = $\frac{2d}{v}=2\times \frac{825}{330}$ =5s