Environmental acoustics - Effects of boundaries on outdoor sound propagation

This article is a direct follow-up to [BR 100] on the influence of the atmosphere on acoustic propagation in outdoor environments. A formidable "playground" for the environmental acoustician, the urban environment combines all the complexities associated with (i) the emission and (ii) the propagation of sound. Indeed, from the point of view of emission, acoustic situations studied in urban environments often involve numerous sound sources: human activities ("anthropophony"), the presence of animals ("biophony"), natural sound sources ("geophony"), transport noise, industrial noise, etc. However, it will not be possible (or impossible) to study all of these sources in detail. However, there will be little or no discussion of sound sources in this article, as each of them is assumed to be perfectly well known, from the frequency (spectral composition), temporal (signature, stationarity, roughness, etc.), dynamic (mobility) and geometric (directivity, multipolarity, punctuality, omnidirectionality, etc.) points of view. Except where mentioned, we will continue here to consider the case of a point source, monopolar (i.e. omnidirectional), immobile, stationary and emitting uniformly throughout the audible frequency range ("white noise" or "pink noise" in thirds of octaves). Of course, the reality is quite different; this decomposition is a view of the mind in order to be able to apprehend the problem in all its complexity (multi-source), and then the system as a whole ("sound ambience").

This article therefore looks at the effects of boundaries (soil, buildings, obstacles, etc.) on acoustic propagation, particularly in urban environments, focusing on the physical phenomena involved, irrespective of the sound sources present: reflection, diffraction and scattering by building elements and street clutter, absorption by treated or vegetated surfaces, etc. Some of these physical phenomena have already been partially addressed in the article [BR 100] , but this document focuses in greater detail on their effects in a densely built-up environment. In addition, phenomena specific to the interurban environment (e.g. ground effects) can sometimes also be very influential...