This document specifies a test method for determining the intrinsic characteristics of sound diffraction of added devices installed on the top of road traffic noise reducing devices. The test method prescribes measurements of the sound pressure level at several reference points near the top edge of a road traffic 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 method specified gives the acoustic benefit over a simple barrier of the same height; however, in practice the added device can raise the height and this could provide additional screening depending on the source and receiver positions).
This document is applicable to:
— the preliminary qualification, outdoors or indoors, of added devices to be installed on road traffic noise reducing devices;
— the determination of sound diffraction index difference of added devices in actual use;
— the comparison of design specifications with actual performance data after the completion of the construction work;
— the verification of the long-term performance of added devices (with a repeated application of the method);
— the 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 100 Hz and 5 kHz. If it is not possible to get valid measurements results over the whole frequency range indicated, the results are given in the restricted frequency range and the reasons of the restriction(s) are clearly reported. A single-number rating is calculated from frequency data.
For indoor measurements, see Annex D.