This European Standard specifies the laboratory method for measuring the sound absorption of flat noise barriers or flat cladding for retaining walls or tunnels. It covers the assessment of the intrinsic sound absorption performance of roadside noise reducing devices that 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 plane absorbers; in particular, it excludes devices that act as weakly damped resonators. Some devices will depart significantly from these requirements and in these cases care is needed in interpreting the results. 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.