Sound waves in air are

The path difference between two particles of a sound wave is $50 \ cm$ and the phase difference between them is $1.8 \pi$. If the speed of sound in air is $340 \ ms^{-1}$, the frequency of the sound wave is (in $Hz$)

Sound waves of wavelength greater than that of audible sound are called

The temperature at which the speed of sound in air becomes double of its value at $27^\circ C$ is ... $^\circ C$

$A$ train, standing in a station-yard, blows a whistle of frequency $400\,Hz$ in still air. The wind starts blowing in the direction from the yard to the station with a speed of $10\,ms^{-1}$. What will be the frequency and speed of sound for an observer? (The speed of sound in still air is $340\,ms^{-1}$)

An engine approaches a hill with a constant speed. When it is at a distance of $0.9 \, km$,it blows a whistle whose echo is heard by the driver after $5 \, s$. If the speed of sound in air is $330 \, m/s$,then the speed of the engine is .... $m/s$.