What minimum distance should the reflecting surface be for an echo to be heard?
17.2 m
An echo is heard only when the reflected sound returns after the ear can distinguish it from the original sound, requiring a minimum time gap of about 0.1 second.
Using the speed of sound in air (about 344 m/s), the minimum distance is \(\dfrac{344\times0.1}{2} \approx 17.2\) m, since the sound travels to the surface and back.
Hence, the minimum distance for an echo to be heard is 17.2 m.
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