The Origin of Ultrasonic Absorption in Water

The origin of the excess ultrasonic absorption in water is sought in a lag in the rearrangment of the molecules during an acoustic compression. An expression is derived for such structural absorption in a liquid, and an approximate calculation is made for the case of water which agrees reasonably well with the experimental excess absorption over the temperature range 0^ to 80^. A relaxation treatment is applied to the structural component of compression on the assumption of two molecular states of packing. The acoustic wave alters the equilibrium distribution of molecules between the two states by perturbing molecular transitions to and from the sites of closer packing through the work performed by or against it in the volume change accompanying the transition. The activation energy determining the rate of transition should be close to that for shear viscous flow and the configuration of closer packing approximately that of closest packing. The relaxation time associated with the process is found to be of the order of 10^-12 second for water. This mechanism implies an isothermal bulk viscosity coefficient for liquids, which in the case of water has a magnitude several times that of shear viscosity.