Pavement noise evaluations were conducted on 42 pavement surfaces in New Jersey using the Close Proximity Method (CPX) via the NCAT Noise Trailer. The CPX Method is a current ISO Standard that measures sound levels of the tire/pavement interface, thereby providing a method to evaluate solely the influence of pavement surface on traffic noise. The surfaces were comprised of both hot mix asphalt (HMA) and Portland cement concrete (PCC). The HMA surfaces consisted of dense-graded asphalt mixes (DGA), open-graded friction course (OGFC) with and without crumb rubber, stone-mastic asphalt (SMA), NovaChip®, and a microsurfacing slurry mix. The PCC surfaces, pavements and bridge decks, had varying surface treatments consisting of transverse tining, saw-cut tining, diamond grinding, and broom finish. The main focus of the research was to: 1) Evaluate how different pavement surfaces influence the generation of tire/pavement noise, 2) Evaluate the effect of vehicle speed on the tire/pavement generated noise, and 3) Provide guidance as to the repeatability of the CPX method and optimal test distance on the roadway to aid in maximizing testing efficiency.
Results of the testing indicated that the asphalt-based surfaces provided the lowest tire/pavement noise levels. Of the HMA surfaces tested, the OGFC mixes modified with crumb rubber provided the lowest noise levels (96.5 dB(A) at 60 mph (96.5 km/h)). However, not only were these mixes modified with crumb rubber, but they also had the finest aggregate gradation. The loudest HMA surface was a 12.5 mm SMA mix (100.5 dB(A) at 60 mph (96.5 km/h)). The PCC surfaces had the highest noise levels. Of all PCC surfaces tested, the transverse tined surface obtained the loudest noise levels (106.1 dB(A) at 60 mph (96.5 km/h)). It was found that if the PCC surface was diamond ground, the noise levels could be comparable, and sometimes lower, than typical HMA pavement surfaces. Typical noise levels of the diamond ground PCC surfaces were approximately 98.7 dB(A) at 60 mph (96.5 km/h). To evaluate the effect of vehicle speed, noise measurements were conducted at 55, 60, and 65 mph (88.5, 96.5, and 104.6 km/h). Test results within this range indicate that on average, the tire/pavement noise increases linearly and at a rate of approximately 0.18 dB(A) for every 1.0 mph (1.6 km/h). The NovaChip® mixes were less susceptible to the increase in vehicle speed (0.15 dB(A) increase for every 1.0 mph (1.6 km/h) increase), while the PCC broom finish (no treatment) surfaces were affected the greatest by vehicle speed (0.29 dB(A) increase for every 1.0 mph (1.6 km/h) increase). The CPX method was found to be repeatable, with an average standard deviation of approximately 0.13 dB(A), as long as the test distance was greater than 0.2 miles (0.32 km). This is most likely due to the sensitivity of the test method being influenced by the ability to track the identical wheel-path in successive test runs.