Question

A man produces sound with a trumpet. A wave has a wavelength of 8.75 mm and a frequency of 40kHz. If the man receives an echo after reflecting from the wall 0.4 s later. Calculate the distance between man and the wall.

• A. 35 m
• B. 70 m
• C. 105 m
• D. 140 m
• E. 175 m

Answer 1

The correct option is B 70 m

Given : $t=0.4$ s $\nu =40$ kHz $=40000$ Hz $\lambda =8.75mm=0.00875$ $m$

$\therefore$ Speed of sound $v=\nu \lambda =40000\times 0.00875=350$ m/s

Let the distance between the man and the wall be $x$.

Thus sound wave has to travel $2x$ distance in 0.4 s so that the man hears an echo.

$\therefore$ $2x=vt$

OR $2x=350\times 0.4$ $⟹x=70$ m