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 region of the barrier.The measurement data derived from the measurements are used to evaluate the barrier efficiency in terms of noise reduction levels expressed as overall and frequency weighted values. Moreover, the noise reduction levels of the plane shape barrier derived from the measurement results are compared with the theoretical ones using the well known prediction formula suggested by Moreland and Musa. The authors believe to have presented the essential data that could be required in industrial applications, in research projects and in thesis-dissertations as well.