The problem of determining soil properties in the dynamic range of loading is discussed. Analytical solutions for two models, closely representing some of the systems that have been used to study the dynamic properties of soils, are presented, where the Kelvin-Voigt model (viscous damping) has been assumed to represent the material. Although sand is a complex material and no simple model will completely describe its behavior under all loading conditions, the Kelvin-Voigt model can be a useful tool for describing the behavior of dry sand subjected to small amplitude sinusoidal vibration, over a large frequency range. Experimental results, obtained from steady state and free vibration and static torsion tests, have been presented to show that for shear strain amplitudes in the order of 10-6 to 10-4 and for confining pressures between 500 and 3,000 psf and for frequencies less than 600 cps, the viscosity should be assumed to decrease with frequency such that the ratio, viscosity x frequency / shear modulus, is constant with frequency, in order to use this model. Values of this ratio for dry sand are given.