This European Standard describes a test method for determining the intrinsic characteristics of sound diffraction of added devices installed on the top of railway noise barriers. The test method prescribes measurements of the sound pressure level at several reference points near the top edge of a noise barrier with and without the added device installed on its top. The intrinsic 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. In other words, the method described here gives the acoustic benefit of changing the shape and materials of the top edge over a simple barrier of the same height. This is an intrinsic characteristic of the added device, provided that the source and receiver positions are standardized. In practice, when the added device is placed over an existing barrier, it raises the height and this provides additional screening, depending on the source and receiver positions; this additional screening is not considered in this European Standard. The test method is intended for the following applications: preliminary qualification, outdoors or indoors, of added devices to be installed on noise barriers; determination of the sound diffraction index difference of added devices in actual use; comparison of design specifications of an added device 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); interactive design process of new products, including the formulation of installation manuals. 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 100Hz and 5kHz. 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 for the restriction(s) shall be clearly reported. A single-number rating is calculated from frequency data. For indoor measurements, see AnnexA.