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27 100 SPB - Predicted Based on SRTT SPB - Predicted Based on Dunlop 95 SPB - Measured Raw Data SPB - Measured Normalized Data Noise Level, dBA 90 85 80 75 70 4 6 7 8 9 1 2 3 5 10 11 12 AC AC AC AC AC C C C C C C AC PC PC PC PC PC PC Figure 24. Predicted SPB based on SRTT and Dunlop tires and measured heavy truck SPB levels at 60 mph and 50 ft--raw and normalized. set between measured OBSI level and predicted SPB level is the truck SPB results, no evidence was found to suggest that the same and only the expected accuracy varies. The standard devi- more aggressive tread of the Dunlop tire would better repre- ations in Table 12 again indicate that SPB levels can be pre- sent truck SPB variation with pavement than would the SRTT. dicted from the OBSI data with virtually the same level of Since the two tires performed equally well in producing pre- confidence for both light vehicle and heavy trucks at a distance dicted SPB levels for both light vehicles and heavy trucks, the of 25 ft. decision on which test tire to be used in the OBSI procedure can be based on other, non-noise related issues (e.g., long-term availability). Test Tires As noted in Figures 21 through 24, SPB levels predicted from Summary OBSI using the SRTT and Dunlop tires are almost identical when the appropriate offset is used. From Table 11, the offsets In order to demonstrate the ability of the recommended for the Dunlop tire are 1.8 to 2.0 dB greater than for the SRTT, OBSI test procedure to characterize the noise performance of with the Dunlop producing higher noise levels. These tire dif- in-service pavements, an extensive measurement program was ferences are consistent with those measured for the OBSI completed. This program included the measurement of 1,343 parameter testing described in Chapter 4. The plot of passby light vehicle passby events and 539 heavy truck passby events level versus vehicle speed provided in Appendix D indicates at 12 sites and pavements in the states of Iowa and California, that the Dunlop tire typically produced higher passby levels and measurements of controlled test vehicle passby events and than the SRTT and generally approximated the levels of the OBSI. By comparing the CPB and OBSI data, significant site- light vehicle SPB more closely than the SRTT. In regard to to-site variation of up to 4.2 dB was identified. Site variation was Table 12. Offsets for predicting SPB from OBSI with expected standard deviations. Offsets (to be subtracted Site Average Specific Site Application from OBSI level), dB Standard Standard SRTT OBSI Dunlop OBSI Deviation, dB Deviation, dB Light Vehicles at 25 ft 21.8 23.8 0.8 1.3 Heavy Trucks at 25 ft 12.9 14.9 0.8 1.4 Light Vehicles at 50 ft 28.3 30.3 0.8 2.1 Heavy Trucks at 50 ft 19.2 21.0 1.0 1.8
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28 also indicated by average differences of up to 3.7 dB between and heavy trucks, respectively. Although the SRTT produced the 25-ft and 50-ft passby levels cross the various sites. The site lower noise levels than the Dunlop test tire, essentially, no dif- information from the CPB and OBSI data was used to normal- ferences were found when using the data to predict SPB data. ize the SPB to OBSI data and establish a 1-to-1 relationship Some indication of noise sources other than tire-pavement between them for each microphone distance and vehicle type. was found for heavy trucks, particularly at lower speeds (below It was then demonstrated that these relationships could be used 60 mph) where engine/exhaust noise are expected to become to effectively predict SPB results based on OBSI data for the more pronounced. However, the ability to predict SPB levels average of sites included in the field testing with a standard for heavy trucks from OBSI data alone was almost equal to deviation of 0.8 dB for both light vehicles and heavy trucks at that for light vehicles. Also, no issues were discovered that a distance of 25 ft from the roadway. For 50 ft, a standard would limit the use of the 25-ft microphone distance for heavy deviation of 0.8 and 1.0 dB was maintained for light vehicles trucks.