Does an oscillating source always produce sound waves?

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

$A$ sound is produced by plucking a string in a musical instrument,then

Find the wavelength of a sound wave of frequency $4.2 \text{ MHz}$ travelling with a speed of $1.7 \text{ km/s}$.

$A$ sound wave passing through an ideal gas at $NTP$ produces a pressure change of $0.001 \ dyne/cm^2$ during adiabatic compression. The corresponding change in temperature $(\gamma = 1.5$ for the gas and atmospheric pressure is $1.013 \times 10^6 \ dyne/cm^2)$ is:

The ratio of the speed of sound in hydrogen gas to the speed of sound in oxygen gas at the same temperature is