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15 Table 4. Linear relationships between test resulting in small overall changes to the sound intensity level parameters and OBSI levels. (0 to 0.5 dB increase per 10 psi increase) as shown in Figure 11. These changes are within the repeat baseline variability. How- Parameter Linear Regression Slopes Probe Location, Vertical -0.3 to -0.4 dB per " upward movement ever, the frequency shifts are notable; a 2.4 dB increase per Vehicle Test Speed +0.2 to +0.3 dB per 1 mph increase 10 psi increase was indicated in the 1,250 Hz band for both Tire Inflation Pressure Frequency shift, see explanation below pavements; shifts in the other frequency bands were smaller. Vehicle Load +0.1 to +0.4 dB per 100 lb increase The data did not indicate a clear correlation between OBSI levels and probe location in the fore/aft directions. A small specific tires and pavement (complete results are provided in downward trend in noise levels occurred as the probe location Appendix C). There was a consistent downward trend in was moved further from the tire sidewall (about 0.2 dB per noise levels as the probe location was moved incrementally 1 /2 in. of movement). The changes in noise level due to varia- from 1/4 in. below to 1/2 in. above the standard probe location tion of the probe distance from the tire sidewall are generally in the vertical direction (about 0.4 dB decrease in noise levels within the standard deviation for the consecutive baselines per 1/4 in. of movement). For vehicle test speed, OBSI noise and slight variation of these parameters in the testing config- levels increased with speed (by about 0.3 dB per 1 mph). Sim- uration is not anticipated to affect the OBSI result (assuming ilarly, noise levels increased with an increase in the vehicle testing is conducted following the standard protocol). The load (0.2 to 0.4 dB increase per 100 lb load increase). For spectral characteristics of each pavement were maintained. probe location in the vertical direction and vehicle test speed, similar trends were indicated over both the AC and PCC pavements for both the SRTT and Dunlop test tires and the Test Vehicle spectral characteristics of each pavement were maintained. At the GM DPG, a 2007 Pontiac Grand Prix was used as the Vehicle loading resulted in slightly lower increases on the AC primary (baseline) test vehicle and results were compared to pavement (and SRTT tire) than on the PCC pavement (and three other vehicles, including a second 2007 Pontiac Grand Dunlop tire); there was a 0.2 dB increase per 100 lbs load for Prix, a 2007 Chevrolet Impala, and a 2007 Buick Lacrosse. The the AC pavement, as compared to 0.3 and 0.4 dB increases for same measurement system and tires were used for all vehicles. the PCC pavement. The loading-related increases occurred For the SRTT tire, the overall levels varied by up to 0.6 dB for primarily in the frequencies below 1,000 Hz, although a small the AC pavement and by up to 0.8 dB for the PCC pavement. increase in the mid to high frequencies occurred on the AC For the Dunlop tire, the overall levels varied by up to 1.2 dB section. for both the AC and PCC pavements. Although the differ- As tire inflation pressure increases, 1/3 octave band levels ences in level between test vehicles exceeded the standard below 1,000 Hz decrease and levels above 1,000 Hz increase, deviation for the consecutive baseline runs, the variability of 105 AC - 42 psi 100 PCC - 42 psi AC - 26 psi 95 PCC - 26 psi 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 11. 1/3 Octave band levels at various tire inflation pressures, SRTT test tire.