$A$ bus moving with a uniform speed of $72 \,km/h$ towards a building blows a horn of frequency $1.7 \,kHz$. If the speed of sound in air is $340 \,m/s$, what will be the frequency of the echo heard by the bus driver (in $\,kHz$)?

An observer moves towards a stationary source of sound with a velocity equal to one-fifth of the velocity of sound. The percentage change in the frequency will be $\dots \;$%.

$A$ whistle sends out $256$ waves in a second. If the whistle approaches the observer with a velocity equal to $\frac{1}{3}$ of the velocity of sound in air,calculate the number of waves per second the observer will receive.

Consider two sound sources $S_1$ and $S_2$ having the same frequency $100\,Hz$ and the observer $O$ located between them as shown in the figure. All three are moving with the same velocity in the same direction. The beat frequency heard by the observer is .... $Hz$.

$A$ person observes two moving trains. Train $A$ is reaching the station and train $B$ is leaving the station, both with an equal speed of $30 \text{ m/s}$. If both trains emit sounds with a frequency of $300 \text{ Hz}$, and the speed of sound is $330 \text{ m/s}$, what is the difference in the frequencies heard by the person (in $\text{ Hz}$)?

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 $v = 332 \; m/s$)