Precautions with Classical Absorption

When using ultrasonic absorption to study molecular relaxation processes in gases, it has been the practice of our group and many other investigators to assume that the dependence of classical absorption on the frequency/pressure ratio is given $(αλ)c1 = kf/ρV2$⁠, where k is a proportionality constant, f is the frequency, ρ is the density, and V is the velocity of sound. This assumption neglects the frequency dependence of the absorption due to thermal conductivity and results in a classical absorption that is too low at high f/p. This assumption can lead to serious error whenever classical absorption is an appreciable fraction of the total absorption.