Q 11-14-025NEETJEE MainMedium
A car moving at $20$ m/s towards a wall sounds its horn of frequency $400$ Hz. The frequency of the echo heard by the driver is (speed of sound $= 340$ m/s)
Answer: (A) $450$ Hz
Concept. The wall receives sound from a moving source and re-emits it as a stationary source; the moving driver then hears it as a moving observer: $f'' = f\dfrac{v + u}{v - u}$.
Step 1. Substitute.
$$f'' = 400 \times \frac{340 + 20}{340 - 20} = 400 \times \frac{360}{320} = 450\ \text{Hz}$$
Answer. $450$ Hz (Option A).
Solution by Sreeraj P, M.Sc Physics