In many applications of acoustics in architecture, geometric techniques yield valuable results and can provide an excellent method of acoustical analysis of the proposed architectural form. Geometric acoustics assumes that people can represent outgoing spherical waves from a sound source by "rays" travelling perpendicular to the advancing wave front. Since the physical size of reflecting panels is often limited by a variety of parameters, this then also limits their low frequency sound reflection characteristics. Sound diffraction is of major importance in the design and performance of indoor and outdoor acoustical barriers. Although a wide range of sound-absorbing materials exist which provide absorption properties dependent upon frequency, composition, thickness, surface finish, and method of mounting, they can be divided into several major classifications. Porous materials are characterized by the fact that the nature of their surfaces is such that sound energy is able to enter the materials by a multitude of small holes or openings.