Question

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

• A. 8.50 kHz
• B. 9.50 kHz
• C. 7.75 kHz
• D. 7.50 kHz

Answer 1

Answer: (A)

8.50 kHz

Answer 2

$36\, km/h = 36 \times \frac{5}{18} = 10\, m/s$
Apparent frequency of sound heard by car driver (observer) reflected from the building will be
$f'=f\bigg(\frac{v+v_0}{v-v_s}\bigg)=8\bigg(\frac{320+10}{320-10}\bigg)=8.5\, kHz$
$\therefore$ Correct option is (a).