Discussion Forum

A stationary source emits a sound of frequency f₀=492 Hz. The sound is reflected by a large car approaching the source with a speed of 2 ms^-1. The reflected signal is received by the source and superposed with the original. What will be the beat frequency of the resulting signal in Hz? (Given that the speed of sound in air is 330ms ^-1 and the car reflects the sound at the frequency it has received].

Answer:

Explanation:

Frequency observed at car

            $f_{1}=f_{0}(\frac{v+v_{c}}{v})$

                                                       [v= speed of sound)

Frequency of reflected souns as observed at the source.

                $f_{2}=f_{1}(\frac{v}{v-v_{C}})=f_{0}(\frac{v+v_{C}}{v-v_{C}})$

 Beat frequency=f₂ -f₀

$=f_{0}(\frac{v+v_{C}}{v-v_{C}}-1)=f_{0}(\frac{2v}{v-v_{C}})$

 =  $492\times\frac{2\times 2}{328}=6Hz$