Acoustic techniques are well suited to determining the effective dynamic values of elastic and viscous coefficients for rubber‐like materials. The values of these coefficients vary with frequency, and the effective dynamic modulus is, in general, larger than the modulus determined from stress‐strain curves. Because acoustic attenuation for waves involving shear is large in rubber‐like materials, the elastic and viscous coefficients must be computed from rigorous relations involving both sound phase velocity and attenuation rather than from the approximate relations which are often used. The necessary relations for computing Young's modulus and the associated viscosity coefficient from acoustic data are presented. An experimental method is described which has been used to measure the sound phase velocity and attenuation of longitudinal waves in thin narrow samples in the frequency range from one to 26 kilocycles. A vibration generator excites one end of a sample held under small tension, while a receiving element moves along the strip. Records of sound intensity and of phase as a function of distance are plotted automatically, and acoustic attenuation and phase velocity are determined from the records. The r.m.s. error in attenuation measurements ranges from 6 to 12 percent over the frequency range. The r.m.s. error in velocity measurements ranges from 2 to 9 percent. Sample acoustic data and the resulting elastic and viscous coefficients are given for a carbon‐black buna‐N cement.