This standard focuses on propagation under metereological conditions that are favorable to noise propagation (downwind or under temperature inversion), but is also able to predict a long-term average A-weighted sound pressure encompassing levels for a wide variety of meteorological conditions (see Meteoeffekte (ISO-9613)).
The second edition of ISO-9613-2 [ISO-9613-2:2024] [available from SLIP'25] introduces significant updates to the calculation. Key changes concern the calculation of barrier attenuation, ground effects, meteorological effects, forest attenuation, and more. In particular, the new edition updates the treatment of downward-curved rays under favorable propagation conditions in barrier-attenuation calculations. For receivers behind a barrier, this change can cause considerable differences in calculations (higher immissions). See also Einstellungen ISO-9613 (Berechnungsoptionen).
Subdivision of extended sources in SLIP'25. At calculation time, extended sources (line and area sources) are subdivided into small portions, each represented by a set of incoherent point-sources, the density of which (sampling density) is adapted to balance accuracy and efficiency. Sampling density increases with proximity to the receiver and around rays at/near the start or end of obstructions caused by obstacles. In particular, near the start and end of a gap between obstacles, the algorithm increases sampling density when a source-portion transitions between being obstructed and being visible from the receiver's position (as the contribution is likely to vary considerably in these transition zones). This adaptive-sampling method provides results that closely approximate those obtained using the projection method required by the standard in all practically relevant scenarios. For more information, see Extended sources (line and area sources).