Which of the following waves is used in sonography?

The frequency of a sound wave is $n$ and its velocity is $v$. If the frequency is increased to $4n$,the velocity of the wave will be

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

$A$ steel rod of length $100 \, cm$ is clamped at the middle. The frequency of the fundamental mode for the longitudinal vibrations of the rod is ..... $kHz$ (Speed of sound in steel $= 5 \, km \, s^{-1}$)

Which of the following is a longitudinal wave?

The velocity of sound is $v_s$ in air. If the density of air is increased to $4$ times,then the new velocity of sound will be