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 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. In these cases care is needed in interpreting the results. The Working Committee responsible for the German collaboration at DIN, the German Institute of Standardization e. V., is NA 005-10-26 AA "Lärmschutzeinrichtungen" ("Noise reducing devices") of the Building and Civil Engineering Standards Committee (NABau), acting as mirror committee of CEN/TC 226 WG 6.