# (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.

(a). Echo: When the repetition of sound is occurred due to reflection of sound from the surface (e.g. wall, ceiling), it is called echo. Echo is formed when the sound is reflected from the walls and ceiling in a vacant room.

(b). In the air, at $20^o\ C$ the required distance is $17.2\ m$ from a sound-reflecting surface to hear an echo.

(c). As given, speed of the sound $=330\ m/s$

Distance between a cliff and man $d=825\ m$

Therefore, time is taken to hear the echo $=\frac{distance(2d)}{speed}$

$=\frac{2\times825\ m}{330\ m/s}$

$=\frac{1650\ m}{330\ m/s}$

$=5\ s$

Thus, he will hear the echo after $5\ s$.

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Updated on: 10-Oct-2022

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