Experimental Investigation of Grazing-Incidence Sound Propagation Over Free-standing Absorbers

Sound propagation at near-grazing incidence in close proximity to air–porous absorber boundaries remains an open experimental challenge. Previous investigations have reported apparent variations in sound speed near such interfaces, often based on analyses of the total sound field. In this study, sound propagation is examined using spatially resolved microphone array measurements combined with sound field separation. Two complementary analysis approaches are employed: a time-domain method based on crosscorrelation-derived time-difference-of-arrival (TDOA) estimates, and a frequency-domain method relying on phase differences between selected microphone pairs to infer local propagation directions. Measurements are conducted above horizontally oriented porous absorbers of varying type and length, with corresponding free-field reference measurements. The effects of incidence angle, measurement height, and frequency are investigated. While analyses of the total sound field reproduce apparent propagation speed variations reported in earlier studies, results obtained from the isolated direct sound field indicate that the propagation speed of the directly arriving wavefront remains constant, with no evidence of physical wave bending toward the boundary. In addition, measured reflection coefficients are compared with semi-empirical predictions to further validate the experimental approach.