The absorption and dispersion of ultrasonic waves in acetic acid

Abstract

Abstract The paper gives the results of measurements of the absorption and velocity of propagation of ultrasonic waves in acetic acid over the frequency range 0.5 to 67.5 Mc./sec. and at tem­peratures from 16 to 60° C. It is shown that a dispersion and a maximum in the value of absorption per wave-length occur, and the results confirm the existence of a relaxation pro­cess arising from the perturbation of a molecular equilibrium by the ultrasonic vibrations. The absorption coefficient, α, at a frequency, v, is represented by an equation of the form α = Bv2 + Av2 / 1 + (v/vm)2, where A, B and vm are parameters independent of v but varying with temperature, and vm is related to the relaxation time, r, by vm = 1/2πr. These parameters are evaluated for a series of temperatures. The results are discussed in the light of existing theory, and in particular the activation energy of the process is obtained from the measured variation of relaxation time with temperature. It is suggested that the relaxation mechanism is possibly connected with a perturbation of the equilibrium between single and double molecules of acetic acid. The results also indicate the existence of a further relaxation process giving a second maximum in the absorption per wave-length at some frequency greater than 67·5 Mc./sec.