A Theory of Ultrasonic Absorption in Unassociated Liquids

In many liquids the absorption of sound waves is very much larger than can be explained classically, in terms of viscosity and heat conduction. It is now fairly well established that different types of liquids behave very differently in this respect, and that the cause of the excess absorption in unassociated, polyatomic liquids like benzene and carbon disulphide lies in the incomplete excitation, by the sound wave, of the vibrational degrees of freedom. It is shown that this effect leads to mean dispersion frequencies in the range 10 9 to 10 10 c/s. at 300° K. This frequency range shows that, as in the case of gases, the efficiency of collisions in producing excitation or de-excitation of vibrational degrees of freedom is fairly low; a theoretical treatment of this effect is outlined. Detailed considerations of temperature coefficients of absorption lead to the conclusion that all the vibrational levels of a liquid do not relax at the same frequency but that high excited levels relax at relatively low frequencies. In the case of carbon disulphide a numerical estimate of these individual dispersion frequencies is made. It is also shown that the variation of absorption with concentration in binary mixtures can be described quite simply in terms of the analysis developed in this paper.