Seismic noise attenuation as a function of depth below the earth's surface is described in terms of surface wave modes. The energy density of the noise, which often represents a combination of modes, is computed as the arithmetic sum of the energy densities of all contributing modes. Three models are used to determine the fractional contribution of each mode. Model A stipulates that each mode contributes equally to the energy of the noise; Model B specifies that the energy fraction contributed by each mode is dependent upon the energy in those beds in which the shear velocity of the bed exceeds the phase velocity of the mode; and Model C prescribes that all modes have equal energy density at the free surface. A comparison of observations with attenuations based on the three models reveals that Model B is the best.