This part of ISO 1996 describes how sound pressure levels intended as a basis for assessing environmental noise can be determined by direct measurement and by extrapolation of measurement results by means of calculation. This International Standard is primarily intended to be used outdoors but some guidance is given for indoor measurements as well. It is flexible and to a large extent the user determines the measurement effort and, accordingly, the measurement uncertainty, which has to be determined and reported in each case. Thus no limits for allowable maximum uncertainty are set up. Often the measurement results have to be combined with calculations to correct for reference operating or propagation conditions different from those during the actual measurement. This International Standard can be applied on all kinds of environmental noise sources, such as road and rail traffic noise, aircraft noise and industrial noise.