Question

A person standing between the two vertical cliff produces a sound. Two successive echoes are heard at 4 s and 6 s. Calculate the distance between the cliffs :

(Speed of sound in air $=320m{s}^{-1}$)

• A. 1600 m
• B. 800 m
• C. 400 m
• D. 1200 m

Answer 1

The correct option is A 1600 m

Distance traveled by the sound from the $1^{st}$ cliff is calculated with the formula:
Distance traveled $=$ velocity of sound $\times$ time taken

That is, $320\times 4$ $=1280m$.

This is twice the minimum distance between a source of sound (man) and the reflector ($1^{st}$ cliff) as it is reflected sound.
Distance traveled by the sound from the $2^{nd}$ cliff is calculated with the formula:
Distance traveled $=$ velocity of sound $\times$ time taken

That is, $320\times 6=1920m$.

This is twice the minimum distance between a source of sound (man) and the reflector ($2^{nd}$ cliff) as it is reflected sound.
Hence, the distance between the 2 cliffs is given as: $\frac{1280}{2}+\frac{1920}{2}=640+960=1600m$