This book intends to give a brief review of the literature concerning the barrier efficiency and thereafter to present the results of an experimental study on the barrier efficiency, which is carried out by performing measurements in the acoustical environment of a semi anechoic chamber. The acoustical measurements have been performed in the acoustic shadow region regarding the three variations of a barrier prototype. (plane shape, absorbent plane shape and L shape) 288 measurements were performed in 54 receiver positions arranged in vertical and horizontal planes in the acoustic shadow...