*3.2.2 Noise source*

Most of the indirect method-based studies use road traffic as a noise source. The ISO standard proposes that naturally occurring road noise should be used as the sound source equivalence for the "before" and "after" measurements. The use of traffic noise has the obvious advantage of representing the natural source, but also the disadvantage of describing fluctuations in traffic volume, speed, and composition that may affect the accuracy of the results.

The use of artificial noise sources is infrequent and is used only when it is not possible to use traffic noise in equivalent conditions. This artificial point source may be based on a loudspeaker that reproduces traffic noise [20] or a regulated artificial signal such as pink noise [43].
