Absorption of Ultrasonic Waves in Acetic Acid

Abstract

THERE are broadly three possible causes of absorption of ultrasonic waves in liquids. The first two are viscosity¹ and conduction of heat² ; these give rise to an absorption the coefficient α of which varies as the square of the frequency ν at all frequencies^3,4. The third general cause is the perturbation by the wave of any equilibrium that may be set up in the liquid, if the new equilibrium state is attained exponentially, with a ‘relaxation' time constant. The equilibrium may be chemical⁵, or one involving the various degrees of freedom contributing to the specific heat³, or between molecules arranged in an orderly structure and others arranged at random⁶. Whatever the mechanism responsible for the relaxation time, Kneser⁷ has shown that it will contribute an additional absorption α, which is a function of the frequency ν ;. He has shown that for frequencies considerably below a relaxation frequency, α varies strictly as ν² ; considerably above this frequency it becomes constant ; the quantity reaches a maximum value at the relaxation frequency.