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

Answer 1

(a) The repetition of sound caused by the reflection of sound is called echo. An echo is produced when sound is reflected from a hard surface such as a tall brick wall or a cliff.

(b) The minimum distance in air required from a sound-reflecting surface to hear an echo at 20°C is 17.2 m.

(c) We have to calculate time taken by the sound wave to reach the reflecting surface.

Given: Distance between the observer and reflecting surface d = 825 m
Velocity of sound ν = 330 m/s
We know the time taken by sound wave to reach the reflecting surface is,
t = 2d/ν
⇒ t = (2)(825)/330
⇒ t = 5 s
This is the required time.