When an ultrasonic wave propagates in a solid with a low concentration of voids the attenuation coefficient is proportional to the scattering cross section of each void multiplied by the number of voids [Gubernatis and Domany, Rev. Prog. QNDE2A, 833 (1983)]. Exact calculation was carried out for the scattering cross section of elastic waves as a function ofkafrom spherical voids in several solids. A turning point between low and highkavalues of the scattering cross section provides a turning point atka= 1. This turning point depends only on the ratio of transverse to longitudinal velocity in the solids—can be used to calculate both the pore radius and their concentration. Experimental method was developed to measure attenuation coefficient as function of frequency using ultrasonic spectroscopy system. The theoretical model was successfully tested experimentally for several aluminum cast materials containing porosity up to 6% and with average radius for 10–150μ. [This work was supported by the Center of NDE operated by Ames Laboratory for the Air Force Wright Aeronautical Materials Laboratory.]