A siren emitting sound of frequency `800 Hz ` is going away from a static listener with a speed of 30 m/s. Frequency of the sound to be heard by the listener is ( Take velocity of sound as 300 m/s )

Options

A. 733.3 Hz
B. 481.2 Hz
C. 644.8 Hz
D. 586. Hz

Answer

A

Explanation

Frequency of siren ` (u)=800 Hz `
Speed of siren ` (v_(s))=v-v_(o)=30 m//s `
Velocity of listener ` (v_(o))=0` and velocity of sound (v)=330 m/s .
From the Doppler's law , frequency heard from siren when it is going away from a listener is ` =((v-v_(o))/(v+v_(s)))xxv=((330+0)/(330+30))xx 800=733.3`Hz .