Question

Two whistles A and B produces notes of frequencies $600Hz$ and $596Hz$ respectively. There is a listener at the midpoint of the line joining them. Both the whistles A, B and the listener start moving with speed $30m/s$ in the same direction. If the speed of the sound is $330m/s$, the number of beats that will be heard by the listener are:

• A. $2$
• B. $4$
• C. $6$
• D. $8$

Answer 1

The correct option is B $4$

Apparent frequency between A and the person is, $f_1=\left(\frac{v+v_o}{v+v_s}\right)f_A=f_A$

Apparent frequency between B and the person is, $f_2=\left(\frac{v-v_o}{v-v_s}\right)f_B=f_B$

$\therefore f_1-f_2=(f_A-f_B)=600-596=4Hz$