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13 Table 3. Variation in OBSI levels for consecutive baseline runs. SRTT - AC SRTT - PCC Dunlop - AC Dunlop - PCC A-Wtd Oct. A-Wtd Oct. A-Wtd Oct. A-Wtd Oct. Range, 0.5 to 0.7 to 0.5 to 0.6 to 0.8 0.8 0.6 0.7 dB 2.5 1.8 1.2 2.0 Std Dev, 0.2 to 0.2 to 0.1 to 0.2 to 0.3 0.3 0.1 0.2 dB 0.9 0.6 0.3 0.6 The total range in overall A-weighted OBSI levels for the for each test tire over a period of several days, some variation consecutive baseline runs was 0.8 dB for the SRTT tire on both in the baseline levels occurred because of temperature varia- the AC and PCC pavements. For the Dunlop tire, the range in tions (discussed under Environmental Variables). To more level was 0.6 and 0.7 dB for the AC and PCC pavements, readily examine the differences in noise between the two test respectively. The baseline runs for this portion of the analysis tires, baseline measurements were averaged for each tire on were made consecutively and no changes in the fixture config- both the AC and PCC pavements. The Dunlop tire baseline uration or measurement protocol were made between runs. As measurements conducted prior to 8:30 am were not included a result, the difference measured for the consecutive baselines because similar early morning measurements were not con- can be considered to be measurement uncertainty. Where ducted with the SRTT. The Dunlop tire resulted in overall OBSI levels under different parameter values fall within the sound intensity levels that were 2.2 and 1.9 dB higher than the standard deviation of the consecutive baselines, the changes in SRTT levels for the AC and PCC pavements, respectively, noise level cannot be reasonably attributed to changes in the with an average difference of 2.0 dB. Higher 1/3 octave band given parameter because of this uncertainty. levels occurred with the Dunlop tire for all frequencies except the 2,000 and 2,500 Hz bands, where levels with both tires were similar. The average 1/3 octave band spectrums for each Test Tire (SRTT versus Dunlop) surface at the DPG facility under baseline conditions for both Data obtained using the SRTT and the Dunlop tires were tires are shown in Figure 9. examined using the baseline measurement results from the The differences in these measurements were somewhat DPG, where both test tires were assessed on the same set of lower than those measured previously for Test Sites S1, S4, S5, pavements. Because baseline measurements were conducted W5, and Waverly which ranged from 2.3 to 3.0 dB, with the 105 SRTT AC 100 SRTT PCC Dunlop AC 95 Dunlop PCC Sound Intensity Level, dBA 90 85 80 75 70 65 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 Frequency, Hz Figure 9. 1/3 Octave band OBSI levels for SRTT and Dunlop tires.