Question

(a) What is an echo? How is echo formed?
(b) What is the minimum distance in the air required from a sound reflecting surface to hear an echo (at 20° C)?

Answer 1

(a) An echo is basically a reflection of a sound wave. When a sound wave gets reflected by a hard surface and comes back to the listener with a little delay, then the reflected sound heard by the listener is known as the echo of the original sound wave.

An echo is formed when a sound wave reflects back towards its source after reflecting from a hard surface.

(b) The velocity of the sound at 20° C is 343 m/s.

Let the minimum distance in the air required to perceive an echo is 'x'

Minimum time required to perceive a sound by human, t = 0.1 s

Then,

$$\begin{gathered} v=\frac{2x}{t} \\ \Rightarrow x=\frac{vt}{2}=\frac{343\times 0.1}{2}=17.2\mathrm{m} \end{gathered}$$

Hence, minimum distance in the air required from a sound reflecting surface to hear an echo is 17.2 m.