The sound pressure reflection coefficient is a property of the boundary, and its magnitude and phase values depend on the frequency as well as the direction of the incident wave. The Sabine absorption coefficient is the random incidence absorption coefficient deduced from the reverberation time measurement via the Sabine equation. The scattering coefficient is a rough measure of the degree of the scattered sound. The transmission loss usually increases with the material density, thickness, and frequency. A sound barrier is large structure that prevents line-of-sight sound transmission between a source and receiver. The acoustic performance of a sound barrier can be determined based on the formulae obtained by fitting the measurement data. In active sound barrier systems, the locations of the error microphones are usually on the top of the barriers, and the locations of the secondary sources are between the primary source and error microphones.