Michel C. Berengier
L.C.P.C., Ctr. de Nantes, BP 19, 44340 Bouguenais, France
I.N.R.E.T.S., 69676 Bron Cedex, France
Gilles A. Daigle
Michael R. Stinson
Inst. for Microstruct. Sci., Natl. Res. Council, Ottawa, ON K1A 0R6, Canada
For the last 6 years, new porous road structures (drainage asphalt or previous macadam) have been studied in several countries, particularly in Europe. In addition to their drainage qualities (i.e., elimination of splash and spray), these porous structures lead to reduced noise levels from traffic. To study further their acoustical performance, measurements of propagation over real road surfaces have been made, for grazing and oblique incidence conditions. The experimental results are compared to theoretical predictions based on two different models for the surface impedance: One model is a microstructural model that requires airflow resistivity, porosity, tortuosity, and shape factor; the other is a phenomenological approach that has been extended to include energy dissipation through thermal conductivity effects. Additionally, these impedance models are compared to direct measurements of the acoustical properties, e.g., absorption coefficient and impedance.