Selection of measurement positions for measuring sound reflection of noise barriers

The number of positions for the in-situ measurement of sound reflection under direct sound field conditions according to EN 1793-5 for road traffic noise reducing devices (NRDs, e.g. noise barriers) depends on the surface structure of the device under test. The surface structure and material homogeneity may be categorized as flat and non-flat as well as homogenous and inhomogeneous. Flat and homogenous NRDs are measured with one central measurement position, whereas non-flat and or inhomogeneous devices require additional measurements points to assess the overall sound reflection properties. In the current version of the standard, NRDs are considered flat, if the depth of their surface structure is below 8.5 cm. Also, they are considered homogenous if the lateral extent of each material is at least 8.5 cm. It is discussed to reduce this criterion from 8.5 cm to 1.7 cm, as research showed significant differences on devices considered as flat between measurement positions for the sound reflection index. An extensive analysis of sound reflection measurements shows the influence of the measurement position for noise barriers with structural depths below 8.5 cm. A new strategy for selecting measurement positions is proposed, which also considers the period length of the surface structure.