Question

A person in front of a mountain is beating a drum at the rate of 40 per minute and hears no distinct echo. If the person moves 90 m closer to the mountain, he has to beat the drum at 60 per minute to not hear any distinct echo. The speed of sound is

(a) 320 ms⁻¹

(b) 340 ms⁻¹

(c) 360 ms⁻¹

(d) 380 ms⁻¹

Answer 1

Answer is : (c) 360 ms⁻¹

As drummer does not hear any echo this means time between two successive wavefronts is equal to time in which a wavefront reaches back to drummer.

Distance covered by sound = 2x

If v = speed of sound, then

2x/v = time interval between two successive wavefronts.

Substituting for x from Eq. (i) in Eq. (ii), we get