Comparing tests for near-grazing-incidence noise reduction and sound absorption tests for hot-mix asphalt

Sound absorption measurements have been used as a simple, cost-effective way of assessing pavement noise properties in the laboratory. However, tire-pavement noise (TPN) is emitted close to the pavement at shallow angles of incidence between the tire and a roadside receiver. Absorption properties can be used to predict oblique incidence noise properties, provided that certain assumptions are met. Near-grazing-incidence predictions of noise vary widely, and the assumptions involved may not be applicable to porous asphalt pavements. A method of directly measuring near-grazing-incidence noise reduction of hot-mix asphalt pavements was designed. It was found that the results of the proposed test could not be predicted from absorption data alone except for dense-graded pavement. For porous or thin, gap-graded pavements, the near-grazing-incidence test gave additional useful information about the acoustic performance of the pavement samples. The test can be used to supplement absorption and other laboratory tests for more accurate predictions of TPN.