Absorption of Ultrasonic Waves in Acetic Acid


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 frequencies3,4. 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.

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PINKERTON, J. Absorption of Ultrasonic Waves in Acetic Acid. Nature 162, 106–107 (1948) doi:10.1038/162106a0

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