On the experimental evaluation of the performances of noise barrier diffracting devices

In the last decades the performances of diffracting devices to be installed on top of noise barriers have been usually estimated through numerical simulations or laboratory measurements on scale models. The recently issued European Technical Specification CEN/TS 1793-4 is the first standard that gives a methodology to measure the diffracting performances of full scale samples (in laboratory or directly at the installation site).This in situ method allows the qualification of such devices by means of a Maximum Length Sequence (MLS) technique in terms of two purposely defined indexes: DI (Diffraction Index) and DI (Diffraction Index Difference). A free field test facility was realized for this purpose at the Acoustic Laboratory of the University of Perugia. Several tests on different caps were carried out to set up the measurement methodology and to assess its accuracy. Results show that the experimental facilities and the measuring methodology are both efficient and reliable, though particular attention has to be paid to geometrical correction and to time windowing.Furthermore, the installation of this kind of devices can sometimes worsen the noise reduction performances in comparison with the simple noise barrier, due to the choice of materials and the cap and to the geometry of the element itself.The paper is completed by a review of the results obtained by other authors through numerical simulations and experimental measurements on diffracting devices.