This paper discusses briefly the nature of viscous losses and solid friction losses, both of which may cause sound waves to be attenuated as they travel through a physical medium. A simple experimental technique for determining the nature and magnitude of the loss factor in small rock samples is described, and data are given which indicate that solid friction losses are primarily responsible for the observed attenuation of the seismic waves employed in the seismic reflection method. A method of estimating the attenuation factor of earth materials from seismic reflection records is outlined and it is shown that the values so obtained are not inconsistent with the laboratory data. Frequency characteristic curves of seismic wave paths are derived on the basis of the experimental data.