Question

A car is moving towards a vertical wall with $10m{s}^{-1}$. The driver blows a horn of frequency 400 Hz. The frequency heard by the drivers is :
($V_{sound}=340m{s}^{-1}$)

• A. 400 Hz
• B. 424 Hz
• C. 430 Hz
• D. none

Answer 1

The correct option is B 424 Hz

Frequency heard by wall
where car is source and wall is object
$f_{wall}=f_{d1}\frac{V_S}{V_S-V_C}$ (i)
now the reflected sound heard by driver
in this case wall is source and car is object
$f_{d2}=f_{wall}\frac{V_S+V_C}{V_S}$ (ii)
Using equation (i) and(ii)
$f_{d2}=f_{d1}\frac{V_S+V_C}{V_S-V_C}$
$f_{d2}=400\frac{340+10}{340-10}=400\times \frac{350}{330}=424Hz$