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12 Table 2. Test parameter matrix. mal conditions. To evaluate the variations in OBSI levels attributable to the testing parameters, each 3-pass set of Parameter Variable Values Tire Repeatability (run-to-run) 10 consecutive runs SRTT, Dunlop parameter measurements was compared to the 3-run sets of Repeatability (day) Nominal conditions each day SRTT, Dunlop baseline measurements performed at the start and comple- Probe configuration Single probe, dual probe SRTT tion of each series of tests for each parameter. Probe location, vertical ", +" vertical SRTT, Dunlop Probe location, fore/aft ", 1" fore/aft SRTT, Dunlop The microphone signals were acquired with a five channel Probe location, from tire ", -1" from tire SRTT, Dunlop commercial analog to digital converter, which also powered Test speed 2, 4 mph SRTT, Dunlop the microphones and provided signal conditioning. This unit Inflation pressure 4, 8 psi SRTT, Dunlop Load +100, +200 lbs SRTT, Dunlop was interfaced to a laptop computer that used commercial soft- Test vehicle 4 vehicles SRTT, Dunlop ware to produce first Fourier transform (FFT) narrow band and 1/3 octave band sound pressure and sound intensity levels was a 2007 Buick Lacrosse CX. At the GM DPG, a 2007 Pon- using a 5-second averaging time. The microphones were cali- tiac Grand Prix was used as the primary (baseline) test vehi- brated using a Class I precision acoustic calibrator set for 94 dB cle. The baseline tire was the Michelin/Uniroyal SRTT, with at the beginning and end of the measurement period. OBSI the Dunlop SP Winter Sport M3 tire (Dunlop) used in those quality metrics of coherence between the two microphones conditions where tire-specific results are suspected to occur comprising each probe and the difference between sound pres- due to tread pattern differences. Photographs of the two test sure and sound intensity level were monitored during data tires were provided in Chapter 3. acquisition. The actual time signals of the four microphones Measurements were conducted using the two-probe were also monitored in order to identify any data acquisition approach (15) at a baseline test speed of 60 mph and a "cold" abnormalities. tire inflation pressure of 30 psi. For the baseline condition, the probe was positioned 3 in. from the pavement surface and Meteorological Conditions 4 in. from the face of the tire, at locations opposite the lead- ing and trailing contact patch of the tire, and oriented so that Noise measurements at the MnROAD facility were con- the sensitive axis was positioned toward the tire. For evaluat- ducted on August 17, 2007, from 8:00 am until 8:15 pm. Air ing the effects of temperature, testing was not restricted to a temperatures ranged from about 60F at 8:00 am to a high of specific temperature range. about 74F at 2:00 pm and down to 66F by 8:00 pm. The sky Three vehicle passes were made for each test parameter, was clear during the early part of the testing period and then which were averaged together during post analysis. A series of became overcast in the late afternoon into the evening. repeat baseline configuration measurements was performed Over the four days of testing at the GM DPG (September at the completion of each set of tests for each parameter. In 1013, 2007) clear skies prevailed and air temperature ranged addition to the repeat baselines, 10 or more consecutive base- from 86F to 107F. Easterly winds of up to about 18 mph line passes were measured for each test tire to examine the were present on September 11th and 12th, parallel to the ori- run-to-run repeatability under the baseline configuration. entation of the test sections, resulting in almost no crosswind. These consecutive baseline measurements were assessed indi- vidually to examine the run-to-run repeatability under opti- Results of Parameter Investigation Run-to-Run Repeatability of Baseline Condition At the beginning of the testing for each tire, ten or more consecutive passes were measured to examine the run-to-run repeatability under the baseline configuration. The tests were conducted using the SRTT tire at MnROAD and the Dunlop tire at the DPG. Testing for the SRTT runs occurred over a period of about 50 min, with an air temperature varying no more than 2F. The Dunlop measurements were made over a period of about 25 min, with air temperatures varying no more than 2F. A summary of the total range (difference between maximum and minimum for all runs) in overall A-weighted sound intensity levels and 1/3 octave bands for the consecutive baseline runs is shown in Table 3 along with the Figure 8. OBSI equipment installed on DPG test vehicle. standard deviation.