An ultrasonic technique previously developed for studying dewetting and cumulative internal damage in filled polymers, such as solid rocket propellents, has been improved. The previous theoretical treatment is here expanded to include internal vacuoles of general spheroidal (rather than spherical) shape. Experimental measurements of sound speed and attenuation in a solid propellant material are utilized together with the modified theoretical model to calculate the internal damage parameters of effective vacuole size and number density as functions of applied uniaxial tensile strain. Results obtained from the model near the point of material failure are in excellent agreement with those provided by independent microscopic observations made on several rupture surfaces of propellant samples stressed to failure.