Question

What is least distance to hear an echo. Explain it
fast it is for 5 marks

Answer 1

To hear an echo nicely, the sounds from the echo should be heard after 0.1 sec. This is because the notes reaching the ear, persist in the brain for about 0.1 sec. If the echo returns before 0.1 sec, then the original direct sounds overpower the echoed notes. The direct notes are at a higher volume and higher quality. So echoed sounds are lost (indistinguishable).

Sound waves have a speed of 344 m/s approx.

So the minimum distance of the reflecting surface and ear = 344 * 0.1 /2 = 17.2 m
  (in 0.1 s  the notes travel to the obstacle and return too).