Frequency range of the audible sounds is

Two sound sources produce waves at a certain temperature in air with wavelengths $50 \text{ cm}$ and $50.5 \text{ cm}$ respectively. The frequencies of the sources differ by $6 \text{ Hz}$. The velocity of sound in air at the same temperature is (in $\text{ m/s}$)

The wavelength of sound waves in hydrogen gas corresponding to the lower limit of audibility is ........ $m$ (speed of sound in hydrogen gas is about $1350 \, m/s$).

What is the phase difference between the displacement wave and the pressure wave in a sound wave?

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

Does an oscillating source always produce sound waves?