Ultrasonic attenuation in copper and copper-aluminum samples is measured as a function of frequency and average grain size. At low frequencies the attenuation scales quadratically with the frequency and linearly with average grain size. In addition, there is a large difference in attenuation between powder metallurgy samples and cast-and-wrought samples, although their qualitative behaviors are similar both in terms of frequency dependence and grain size dependence. Such difference and the discrepancy with the existing theory may point to mechanisms of scattering by grain boundaries that are not included in the current theoretical model.