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 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 standard, is strictly valid only for plane absorbers and in particular excludes devices which 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 1 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. NOTE 2 In the previous version of this European Standard dated September 1997 the basic test method was derived from EN 20354:1985. In the current version of this European Standard the basic test method is derived from EN ISO 354:2003. In the current version of the basic test method an essential change was introduced compared to the previous version, viz. the application of a correction method for changes in sound attenuation in air caused by changes in air temperature and humidity during the test. Because this correction was not applied in the previous version of the test method the results from the current version may deviate considerably from the results obtained with the previous version of the method. Specifically for test samples with a high absorption coefficient these changes may amount to 1 dB or 2 dB for the single number rating DL, either in a positive or a negative sense. The effect of the application of the correction will be most noticeable in the higher frequency bands. To avoid possible confusion between the old and the new measurement results the continued use of measurement results obtained with the previous version of this European Standard (dated September 1997) is not recommended.