When a listener is put in a shadow zone and "does not see" directly the source, the sound however reaches him. We can say waves reach the listener by four different ways: 1) scattering from the edges of the windows called diffraction; 2) scattering from the turbulence in the air above the window and near the external source; 3) refraction by atmospheric wind; 4) reflections in the room. If we put the listener quite close the window (1m) and assume the room be constructed by adsorbent materials, edges diffraction has a dominant effect on the pressure in the shadow region. Most of the original work of diffraction theory has been reported for light diffraction; nevertheless there is no difference between the computation for light and for sound waves, as long as both are based on the assumption of a scalar potential. One of the most valuable theorems in acoustics is the Huyghens principle that permits the forecasting of sound pressure levels by assuming every point on a vibrating plane surface may be considered as the centre of an outgoing wave. Based on this principle the diffraction edge can be regarded to be consisting of many point sources (secondary sources) which are the centres of outgoing wave whose intensity is proportional to the primary incident wave and to the path length from the source to the secondary point source same. Using this powerful tool a case study is presented where the environmental impact of a quarry plant is considered.
Degan, G.A., & Pinzari, M. (2003). Application of the Huyghens diffraction theory in forecasting the sound pressure level through windows in rooms due to external sources: Environmental impact study and design of a quarry plant. ACTA ACUSTICA UNITED WITH ACUSTICA, 89(SUPP.), S48.