Question

Speed of sound in moist air is $\underset{–}{}$ than speed of sound in dry air

• A. Less
• B. same as
• C. more
• D. data insufficient

Answer 1

Answer: (C)

more

Speed of Sound in air = $V=\sqrt{\frac{{\rm Y}P}{d}}$, where ${\rm Y}=\frac{Cp}{Cv}$ , P = Pressure, and d = density

The density of water vapor is less than the density of dry air at ordinary temperatures in the
ratio 0.62 : 1. Therefore, the presence of water vapor in the air lowers the density of air and
consequently increases the speed of sound in it. Hence, at a given temperature, the speed of
sound in damp air is greater than that in dry air. It can be shown by calculation that

$Vd=Vm\sqrt{\{1-\left(\frac{0.378f}{P}\right)\}}$

$V_d=$ Speed in dry air, $V_m=$ Speed in moist air, $f=$

saturation pressure of water vapour at $t$ deg.$C$

$P=$ Pressure of air.