$A$ source of sound is moving with a constant velocity of $20\, m/s$ emitting a note of frequency $1000\, Hz.$ The ratio of frequencies observed by a stationary observer while the source is approaching him and after it crosses him will be (Speed of sound $v = 340\, m/s$).

Two engines pass each other moving in opposite directions with a uniform speed of $30\,m/s$. One of them is blowing a whistle of frequency $540\,Hz$. Calculate the frequency heard by the driver of the second engine before they pass each other. (Speed of sound is $330\,m/s$).

The observer is moving with velocity $v_0$ towards the stationary source of sound and then after crossing moves away from the source with velocity $v_0$. Assume that the medium through which the sound waves travel is at rest. If $v$ is the velocity of sound and $n$ is the frequency emitted by the source,then the difference between apparent frequencies heard by the observer is:

$A$ car travelling at a speed of $54 \ km/h$ towards a wall sounds a horn of frequency $400 \ Hz$. The difference in the frequencies of two sounds, one received directly from the car and the other reflected from the wall, noticed by a person standing between the car and the wall is (speed of sound in air is $335 \ m/s$):

$A$ siren emitting a sound of frequency $800 \, Hz$ moves away from an observer towards a cliff at a speed of $15 \, m s^{-1}$. The frequency of sound that the observer hears in the echo reflected from the cliff is .... $Hz$ (Take velocity of sound in air $= 330 \, m s^{-1}$)

$A$ train is moving with a constant speed along a large circular track. The engine of the train emits a sound of frequency $f$. What is the frequency heard by the guard at the rear end of the train?