A source of sound S of frequency 500 Hz situated between a stationary observer O and a wall W, moves towards the wall with a speed of 2 m/s. If the velocity of sound is 332 m/s, then the number of beats per second heard by the observer is (approximately)

For direct sound source is moving away from the observes so frequency heard in this casen1=nvv+vs=500332332+2=500332334HzThe other sound is echo, reaching the observer from the wall and can be regarded as coming from the image of source formed by reflection at the wall. This image is approaching the observer in the direction of sound.Hence for reflected sound, frequency heard by the observer is n2=n vv−vS=500 332332−2=500 332330HzBeats frequency =n2−n1=500×3321330−1334=6.