$A$ mechanical wave (sound wave) in a gas is:

Which of the following is not a transverse wave?

$A$ car is moving with a speed of $72 \,km/h$ towards a hill. The car blows its horn at a distance of $1800 \,m$ from the hill. If the echo is heard after $10 \,s$, the speed of sound (in $m/s$) is:

$A$ source of sound of frequency $600 Hz$ is placed inside water. The speed of sound in water is $1500 m/s$ and in air is $300 m/s.$ The frequency of sound recorded by an observer who is standing in air is .... $Hz$

The velocities of sound at the same pressure in two monatomic gases of densities ${\rho _1}$ and ${\rho _2}$ are $v_1$ and $v_2$ respectively. If ${\rho _1}/{\rho _2} = 2$,then the value of $\frac{{{v_1}}}{{{v_2}}}$ is:

Explain the propagation of sound waves in:
$(i)$ air
$(ii)$ solids