The difference between the apparent frequency of a source of sound as perceived by an observer during its approach and recession is $2\%$ of the natural frequency of the source. If the velocity of sound in air is $300 \, m/s$,the velocity of the source is ... $m/s$ (Given that velocity of source < velocity of sound).

$A$ motorcyclist is approaching a large wall with a velocity of $90\,km/hr$. $A$ car is chasing him with a velocity of $108\,km/hr$. If the car sounds a horn at $30\,Hz,$ the beat frequency heard by the motorcyclist is . . . . . . $Hz$. Take the velocity of sound $= 330\,m/s$.

Three musicians experiment with the Doppler effect. Musician $A$ rides in a car at a speed $u$ directly away from musician $B$ who is stationary. Musician $C$ rides in a car directly toward $B$ and travels at the same speed as $A$. Musician $A$ plays a note at frequency $f_A$ on his trumpet. $B$ hears the note,adjusts his trumpet,and plays the same note he heard. Choose the incorrect statement.

An engine moving towards a wall with a velocity $50 \ m/s$ emits a note of $1.2 \ kHz$. The speed of sound in air is $350 \ m/s$. The frequency of the note after reflection from the wall as heard by the driver of the engine is: (in $kHz$)

$A$ highway truck has two horns $A$ and $B$. When sounded together, the driver records $50$ beats in $10$ seconds. With horn $B$ blowing and the truck moving towards a wall at a speed of $10 \,m/s$, the driver notices a beat frequency of $5 \,Hz$ with the echo. When the frequency of $A$ is decreased, the beat frequency with the two horns sounded together increases. Calculate the frequency of horn $A$. (Speed of sound in air $= 330 \,m/s$) (in $\,Hz$)

Two cars $A$ and $B$ are moving in the same direction with speeds $36 \, km/hr$ and $54 \, km/hr$ respectively. Car $B$ is ahead of $A$. If $A$ sounds a horn of frequency $1000 \, Hz$ and the speed of sound in air is $340 \, m/s$,the frequency of sound received by the driver of car $B$ is ..... $Hz$.