The Absorption of Ultrasonic Waves in Liquids and its Relation to Molecular Constitution

Measurements of the absorption of ultrasonic waves in water and ethyl alcohol were made by the pulse method, and are more accurate than those made by other methods. The absorption coefficient α in water was found to vary as the square of the frequency ν over a wide range of temperatures. The measurements in water were made at frequencies between 7.5 and 67.5 Mc/s. and temperatures between 0 and 95° c. The absorption decreased with increasing temperature by a factor of 8 between freezing and boiling points. The observed absorption was about three times that calculated from Stokes formula and this ratio was nearly independent of temperature. In ethyl alcohol measurements were made at 52.4 Mc/s., and at temperatures between - 50° and + 60° c. The absorption decreased by a factor of 3 in this range and observed values were about twice those given by Stokes' formula. Values of (α/ν 2 ) at 25° c. are 22.0 x 10 -17 sec 2 /cm. for water and 50.5 x 10 -17 sec 2 /cm. for ethyl alcohol. Published values of the absorption coefficients of a wide range of pure liquids and of some mixtures are reviewed, and a classification proposed. The experimental results suggest that the absorption in associated liquids is due to a different mechanism from that in non-associated liquids. In the latter the absorption is probably due to the same phenomenon as in gases: a slow exchange of energy between different degrees of freedom. The relaxation frequencies must be above 200 Mc/s. for most liquids. A simple theory is given to explain the observed variation of the absorption with concentration in mixtures of non-associated liquids. The semi-quantitative agreement of theory with experiment lends support to the relaxation hypothesis.