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This document describes a test method for measuring a quantity representative of the intrinsic characteristics of airborne sound insulation for traffic noise reducing devices: the sound insulation index. The test method is intended for the following applications: - determination of the intrinsic characteristics of airborne sound insulation of noise reducing devices to be installed along roads, to be measured either in situ or in laboratory conditions; - determination of the in situ intrinsic characteristics of airborne sound insulation of noise reducing 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 noise reducing devices (with a repeated application of the method); - interactive design process of new products, including the formulation of installation manuals. The test method is not intended for the determination of the intrinsic characteristics of airborne sound insulation of noise reducing devices to be installed in reverberant conditions, e.g. inside tunnels or deep trenches or under covers. Results are expressed as a function of frequency in one-third octave bands, where possible, between 100 Hz and 5 kHz. If it is not possible to get valid measurement results over the whole frequency range indicated, the results will be given in a restricted frequency range and the reasons for the restriction(s) will be clearly reported.