currently operational non-destructive testing methods for pavements have many drawbacks that need improvement. This thesis investigates the possibility of utilizing the normal incidence reflection coefficient of acoustic waves in characterizing and studying of porous granular materials. The reflection coefficient of acoustic waves carries information about the physical parameters of materials. The fouling process in ballast layer and the compaction progress in sand and road base layers are the main focus of this study. Simplified fluid equivalent models are used to characterize and study the granular porous layers. The Delany- Bazley and Johnson- Allard models which require few non-acoustical material parameters are used in this analysis. A one dimensional problem which involves a porous layer backed by an infinite impedance surface has been solved in Matlab. The results from the two models have been compared and parametric studies of non-acoustical parameters have been also done. The study concludes the possibility of a new non-contact non –destructive testing method for unbound granular layers which utilize the reflection coefficient of acoustic waves.