The absorption of sound in emulsions and suspensions is due to viscous and thermal transport processes occurring at the interface of the nonhomogeneities, as well as to the intrinsic absorption in the materials comprising the system. A complete description of these processes for systems of fluid particles suspended in fluid media was given by Epstein and Carhart [J. Acoust. Soc. Amer. 25, 553 (1953)]. However, subsequent investigators of liquid systems have generally neglected the thermal transport process. We show in this work that the Epstein-Carhart results provide a good description of our experimental attenuation data in emulsions, and that the thermal transport process can be the major factor in the attenuation. We have here also extended the theory further to include the attenuation in suspensions of solid particles, and good agreement is found between our theory and experimental results for aqueous suspensions of polystyrene spheres.

Data for ultrasonic absorption and sound speed are given for water, methyl alcohol, ethyl alcohol, n-propyl alcohol, n-butyl alcohol, eugenol, carbon tetrachloride, n-hexane, and toluene, for pressures ranging up to about 5000 kg/cm2.

The absorption of sound in suspensions and emulsions, which can be several orders of magnitude greater than that in the pure substances, usually has been attributed to viscous dissipation processes including relative motion, the relaxing bulk modulus of the suspended material, or scattering. A description of these processes is contained in a more general theoretical treatment, which, in addition, indicates that thermal conduction in the vicinity of the interfaces results in substantial sound attenuation. The theoretical results will be discussed for viscous, compressible, thermal-conducting media and compared to observed experimental ultrasonic absorption behavior in a number of aqueous emulsions and suspensions. [This research was supported in part by the U. S. Office of Naval Research and the National Institutes of Health.]

Ultrasonic absorption and sound speed have been measured in toluene and n-hexane in the pressure range 1–10 000 kg/cm2 at 30°C. In these Kneser-type liquids, for which the absorption may be attributed to thermal relaxational processes as well as to the dilatational and shear viscosities, various contributions to the absorption can be resolved by making specific assumptions about the ratio of the dilatational and shear viscosities. An attempt has been made to relate the thermal contribution to relevant theory.

The possibility that a thermal conduction mechanism might explain the high attenuation of ultrasound in biological systems is reported. This thermal conduction mechanism arises due to inhomogeneities in the biological materials. It is known that this thermal conduction mechanism is quite significant for small inhomogeneities (0.1 to 1 μ) at frequencies between 1 and 10 MHz .This knowledge is applied to biological materials with the understanding that these materials contain inhomogeneities of this same approximate size. It is found that this mechanism could explain the observed linear frequency dependence of the ultrasonic attenuation in many biological materials, and also that the observed high attenuation can be accounted for in this theory. It is also shown that the thermal conduction attenuation for proteins the size of hemoglobin is small compared to the experimental data.