At which sound-wave condition is absorption by a relaxing gas greatest?

In the study of relaxation phenomena, sound absorption by a relaxing gas is greatest when the angular frequency of the sound wave is equal to the reciprocal of the system's relaxation time.1

When a sound wave—which acts as a pressure wave with accompanying temperature fluctuations—propagates through a gas, the chemical equilibrium or energy distribution within the gas attempts to adjust to these changing conditions. At intermediate frequencies, the system's response lags behind the pressure variations, leading to a measurable relaxation effect. The maximum absorption occurs specifically at the point where the frequency of the oscillation matches the rate at which the system can re-establish equilibrium.21