The material studied in this paper is a thermoplastic matrix (polypropylene) with short glass fiber reinforcement (SFRP). Complex geometry parts are cheaply produced in a process called injection molding, thus it is commonly used in numerous applications. Abe et al. [1] characterized the flow pattern of fiber reinforced polypropylene with ultrasonic C- Scans. Matsushige et al. [2] applied single pulse spectroscopy to polymeric materials. They studied the effect of glass fiber diameter and content on the ultrasonic frequency spectrum and interpreted their results according to the different scattering regimes (πD/λ ≪ 1, πD/λ, ≈ 1, πD/λ ≫ 1; D: size of the scatterer, λ: wavelength). Newell et al. [3] determined elastic constants of an approximately orthotropic cellulose fiber reinforced polypropylene by velocity measurements in the 0.5 to 5 MHz range. They observed that the dispersion curves level off at frequencies above 1 MHz, which is consistent with a relaxation time constant of the order of a few microseconds. Chu and Rokhlin [4] used longitudinal and transverse velocity measurements to characterize the fiber/matrix interphase in ceramic composites.