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This European Standard specifies the laboratory method for measuring the sound absorption of flat noise barriers or flat claddings for retaining walls or tunnels. It covers the assessment of the intrinsic sound absorption performance of noise barriers and related devices acting on airborne sound propagation designed for railways which can reasonably be assembled inside the testing facility described in EN ISO 354. The test method in EN ISO 354, referred to in this European Standard, is strictly valid only for flat absorbers and in particular excludes devices which act as slightly damped resonators. Some devices will depart significantly from these requirements and in these cases care is needed in interpreting the results. All noise reducing devices that differ from noise barriers and related devices acting on airborne sound propagation, e.g. devices for attenuation of ground borne vibration and on board devices are out of the scope of this European Standard. NOTE The test method in EN ISO 354 is based on measurements in a reverberation room where diffuse sound field conditions prevail. As a uniformly applicable method for the determination of the sound absorptive performance of noise reducing devices under free field conditions is still under development, the measurement results according to this European Standard are temporarily considered relevant for application on noise reducing devices in reverberant as well as in free field conditions.