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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)?

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Solution

## (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, $v=\frac{2x}{t}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}⇒x=\frac{vt}{2}=\frac{343×0.1}{2}=17.2\mathrm{m}$ Hence, minimum distance in the air required from a sound reflecting surface to hear an echo is 17.2 m.

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