$A$ boy is walking away from a wall towards an observer at a speed of $1\, m/s$ and blows a whistle whose frequency is $680\, Hz$. The number of beats heard by the observer per second is (Velocity of sound in air $= 340\, m/s$).

When the source of sound moves away from a stationary listener,we obtain $f_{L} < f_{S}$. Why?

$A$ source of sound emits waves with frequency $f \, Hz$ and speed $V \, m/s$. Two observers move away from this source in opposite directions,each with a speed $0.2V$ relative to the source. The ratio of frequencies heard by the two observers will be

Two trains,one coming towards and another going away from an observer,both at $4 \; m/s$,produce a whistle simultaneously of frequency $300 \; Hz$. Find the number of beats produced. (Assume the speed of sound $v = 332 \; m/s$)

$A$ source of sound and a listener are approaching each other with a speed of $40 \, m/s$. The apparent frequency of the note produced by the source is $400 \, cps$. Then,its true frequency (in $cps$) is (velocity of sound in air $= 360 \, m/s$).

$A$ reflector is moving with $20 \,m/s$ towards a stationary source of sound. If the source is producing sound waves of $160 \,Hz$, then the wavelength of the reflected wave is (speed of sound in air is $340 \,m/s$).