Measurement of Acoustical and Mechanical Properties of Porous Road Surfaces and Tire and Road Noise

The absorption coefficients of dense and porous road surfaces have been measured in the laboratory with core samples with 4- and 6-in.-diameter impedance tubes and with an impedance tube mounted vertically in situ on the pavement surfaces. The 6-in. tube allows the absorption of a large core sample surface to be determined, but only up to a frequency of about 1,250 Hz. The 4-in. tube allows the absorption coefficient to be determined up to a frequency of about 1,950 Hz. The two different-diameter impedance tubes also were mounted vertically on some of the same pavement types, and the absorption coefficients of these pavement types were measured in this way too. The measured peak sound absorption coefficients of the fine- and coarse-mix aggregate porous surfaces suggest that the first peak frequency and peak absorption coefficient magnitude are only slightly different for the two types of porous surfaces. Because the fine-mix aggregate porous surface is smoother, it is preferred because it should result in less tire-tread impact noise and thus lower overall tire noise than the coarse aggregate surface. A porous surface between 1.5 and 2.0 in. thick is recommended for the type of porous surface examined, if a peak absorption frequency of about 1,000 Hz is desired, so as to be most effective at reducing the Interstate highway noise of automobiles.