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š” Voice from a cliff
A distant hard surface throws your sound back — but only if it is far enough away to arrive after a tenth of a second.
What is an echo?
- An echo is a sound heard again after it reflects off a distant hard surface.
- To hear it separately, the reflected sound must arrive at least 0.1 s after the original.
- The minimum distance to the surface is about 17 m (sound travels 34 m to and back in 0.1 s at 340 m/s).
Example 10.5 — You clap in an empty
Echo heard after 0.5 s, \(v=340\ \text{m/s}\). Sound goes to the wall and back, so distance \(=\dfrac{v\times t}{2}=\dfrac{340\times 0.5}{2}=85\ \text{m}\).
NCERT Question 4 — In a room, the reflected
In a room, the reflected sound reaches the ear 0.05 s after its production. Will it produce an echo or reverberation? Justify your answer.
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Important Definitions
- Reflection of sound — the bouncing back of sound from a hard surface, following the laws of reflection.
- Echo — a reflected sound heard again after a gap of at least 0.1 s.
