How does humidity affect sound waves?

Answer 1

The speed that sound waves propagate through a gas follows the equation: v = sqrt(gamma*R*T)/M), where gamma is the adiabatic index, which, for air, is about 1.4, and is barely (if at all (I'm no expert, just a physics major)) related to the density of air. R is the Universal Gas Constant (~8.31 J/(mol*K)), T is temperature (K), and M is the molecular weight (kg/mol).

Humid air is actually less dense than dry air. This is due to the fact that dry air is primarily composed of N2 and O2 molecules, which have a molecular mass of around 28 and 32 Atomic Mass units, respectively. A molecule of water has a mass of only 18 atomic mass units. Therefore, a mole of dry air molecules weighs more than a mole of relatively humid air molecules.

Because the molar mass (M) is less for humid air, the denominator inside of the above-mentioned velocity function is reduced, and the numerator effectively stays constant. Therefore, the fraction becomes larger, and the speed of sound increases as molar mass decreases.

The amplitude of sound waves stays larger for longer in humid air. This is because energy is needed in order to move the molecules in the air and propagate the sound. Less dense air (humid air) has less mass to be moved in order to propagate the sound, and therefore less energy goes into propagating the sound and more energy can remain in the form of the amplitude of the wave.

Summary: The speed of sound waves and the amplitude of sound waves through humid air is greater because humid air is actually less dense than dry air.