Example 1 · level 1
A child shouts near a cliff in the Western Ghats and hears the echo after 0.5 s. If sound travels at 346 m/s, how far (in m) is the cliff?
Show solution
- 2d = 346 × 0.5 = 173 m.
- d = 86.5 m.
Answer: 86.5 m
CLASS 9 SCIENCE · SOUND
Echo: distance of a reflecting surface
An echo travels to the surface and back, so 2d = v × t, i.e. d = vt/2. To hear a distinct echo the sound must return after at least 0.1 s.
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Worked examples
Example 2 · level 2
A ship's SONAR sends a pulse that returns from the seabed after 6 s. Speed of sound in sea water is 1500 m/s. Find the depth of the sea (in m).
Show solution
- 2d = 1500 × 6 = 9000 m
- d = 4500 m.
Answer: 4,500 m
Example 3 · level 3
Sound persists in our ear for about 0.1 s. If the speed of sound is 344 m/s, find the minimum distance (in m) of a wall for a distinct echo (correct to 2 decimal places).
Show solution
- 2d = 344 × 0.1 = 34.4 m
- d = 17.2 m.
Answer: 17.2 m