This Technical Specification specifies methods for the determination of insertion loss of outdoor noise barriers intended to shield railway noise. It specifies detailed procedures for in situ measurement of barrier insertion loss including microphone positions, source conditions and acoustic environments of the measurement sites.This Technical Specification allows one to measure the insertion loss of a given noise barrier at a given site including given meteorological conditions. It does not make it possible to compare insertion loss values of an equivalent barrier at a different site. It can be used for comparing insertion loss values of different types of barriers at the same site under given meteorological conditions by the "direct method".This Technical Specification gives a method for determining insertion loss:a)from the level difference before and after the installation of noise barriers (the "direct method") and when this is not possible because a barrier has already been installed;b)using an indirect method to estimate the sound pressure levels before installation of the barrier by measurement at another site which has been judged to be equivalent.For equivalent sites, a close match is required in source characteristics, microphone locations, terrain profiles ground surface characteristics, surrounding artificial structures and meteorological conditions. This Technical Specification prescribes principles for ensuring that sufficiently equivalent conditions are maintained between "before" and "after" cases to permit certain, reliable and repeatable determination of barrier insertion loss.This Technical Specification does not cover the determination of the intrinsic acoustic characteristics of the barrier, for example the sound insulation index and the sound absorption coefficient.The equivalent continuous A-weighted sound pressure level and one-third-octave band sound pressure level are used as noise descriptors.This Technical Specification can be used for routine determination of barrier performance or for engineering or diagnostic evaluation. It can be used in situations where the barrier is to be installed or has already been installed.