Values of the nonlinearity constant “D,” which are instrumental in conversion of the acoustic phonons to thermal phonons, are determined in this paper for copper in the temperature range from 300° to 80°K, using experimental data of third-order elastic constants of this material. The previously predicted temperature-dependent behavior of this constant D, which found experimental support partially at room and liquid-nitrogen temperatures only in Ge, has been thus tested for the first time on experimental basis even for a metallic crystal. The results are found to be in agreement with the previous prediction. Moreover, the temperature-dependent curve for phonon viscosity component of hypersonic attenuation can be recalculated by using our values of D. When this is done, a better agreement is obtained than that obtained previously between the theoretical and corresponding experimental curves of hypersonic attenuation as a function of temperature for longitudinal wave propagation in the (100) direction.