Two sources P and Q produce notes of frequency 660Hz each. A listener moves from P to Q with a speed of 1 ms-1. If the speed of sound is 330 ms-1, then the number of beats heard by the listener per second will be

The situation as shown in the figure.Here,  Speed of listener, vL=1 ms-1  Speed of sound, v=330 ms-1  Frequency of each source, ν=660 Hz  Apparent frequency due to P ,  v'=vv-vLv  Apparent frequency due to Q ,  v''=vv+vLv  Number of beats heard by the listener per second is  v''-v'=vv+vLv-vv-vLv =2vvLv=2×660×1330=4