ISO 12354-3:2017 specifies a calculation model to estimate the sound insulation or the sound pressure level difference of a façade or other external surface of a building. The calculation is based on the sound reduction index of the different elements from which the façade is constructed and it includes direct and flanking transmission. The calculation gives results which correspond approximately to the results from field measurements in accordance with ISO 16283‑3. Calculations can be carried out for frequency bands or for single number ratings. The calculation results can also be used for calculating the indoor sound pressure level due to for instance road traffic (see Annex E). ISO 12354-3:2017 describes the principles of the calculation model, lists the relevant quantities and defines its applications and restrictions.