Question

A police car with a siren of frequency $8kHz$ is moving with uniform velocity $36km/h$ towards a tall building which reflects the sound waves. Speed of sound in air is $320m/s$. Frequency of siren heard by car driver is

• A. 8.5 $kHz$
• B. 825 $kHz$
• C. 7.25 $kHz$
• D. 7.5 $kHz$

Answer 1

The correct option is A 8.5 $kHz$


Velocity of car $=36km/h=10m/s$
Frequency received by wall $f_1=\frac{v}{v-v_c}f$
Frequency received by wall will be reflected back and now wall will behave as a fixed source and car will behave as an observer approaching the source.

Frequency heard by car driver $f_{11}=\frac{v+v_c}{v}{f}_1$
$f_{11}=\frac{v+v_c}{v-v_c}f$
$f_{11}=\frac{320+10}{320-10}\times 8000$$=\frac{330}{310}\times 8000=8500Hz$