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This document describes a test method for determining the intrinsic characteristics of sound diffraction of added devices installed on the top of traffic noise reducing devices.The test method prescribes measurements of the sound pressure level at several reference points near the top edge of a noise reducing device with and without the added device installed on its top.The effectiveness of the added device is calculated as the difference between the measured values with and without the added devices,correcting for any change in height. The test method is intended for the following applications: -- preliminary qualification, outdoors or indoors, of added devices to be installed on noise reducing devices; -- determination of sound diffraction index difference of added devices in actual use; -- comparison of design specifications with actual performance data after the completion of the construction work; -- verification of the long term performance of added devices (with a repeated application of the method). The test method can be applied both in situ and on samples purposely built to be tested using the method described here. Results are expressed as a function of frequency, in one-third octave bands between 100 Hz and 5 kHz.If it is not possible to get valid measurements results over the whole frequency range indicated, the results shall be given in the restricted frequency range and the reasons of the restriction(s) shall be clearly reported.A single-number rating is calculated from frequency data. For indoor measurements see Annex B.