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16 the environmental conditions was also considerably higher for Table 5. Recommended parameter limits. these runs. The differences between vehicle results can be attrib- Parameter Recommended Criteria (Limit) uted to differences in temperature (discussed in this chapter Run to Run Repeatability, Within 1 dB under Environmental Variables), which were more apparent Overall A-Wtd OBSI level with the Dunlop tire. A considerable amount of time was Run to Run Repeatability, Within 2 dB Octave Band Levels required to change tires and vehicles between measurement Probe Location, Vertical 3 " above pavement sets, resulting in notable air and pavement temperature dif- Vehicle Test speed 60 1 mph ferences between vehicle sets. Measurements conducted Tire Inflation Pressure (Cold) 30 2 psi Vehicle Load 100 lbs during the morning period (prior to 9:30 am), when tem- Probe Location, Fore/Aft Leading/Trailing edge " peratures were 10.8F to 12.6F lower than during the late Probe Distance from Tire Sidewall 4 " morning and afternoon, resulted in the highest levels. The Coherence > 0.8 for frequencies below 4,000 Hz PI Index < 5 dB for data reported as valid tests conducted during midday (between 11:00 am and 4:00 pm) using three vehicles yielded results within 0.5 dBA for both the SRTT and Dunlop tires and similar spectral characteristics. slight decreases in coherence occurred at the DPG site in 4 Although the test vehicle variation did not produce substantial out of 578 runs when high temperatures caused equipment differences in OBSI levels, the same vehicle family, measure- overloads and overheating (these data were discarded). Above ment system, and tires were used. Differences resulting from a 4,000 Hz, coherence is typically lower due to limitations in the wider range of vehicle types, OBSI measurement equipment, finite difference approximation used in the algorithm for and multiple test tires of the designs, were not evaluated. determining sound intensity (15). At the 5,000 Hz band, coher- ence was less than 0.8 for 38% of the parameter runs. The PI index is also used as a data quality check. Generally, if the PI Fixture Configuration (Single Probe versus index is above 5 dB, the measurement is contaminated by flow Dual Probe) noise (14). In the parameter testing, the PI index was less than 5 dB in all 1/3 octave bands from 500 to 5,000 Hz. PI index Single versus dual probe configurations were examined using values for the trailing edge position occasionally exceeded 5 dB the SRTT tire at the GM DPG site (photographs of the probe in the 400 Hz band (about 3% of the runs). Because the levels configurations are included in Appendix C). The comparison in the 400 Hz 1/3 octave band were sufficiently low so as to have of the probe configurations was made for test speeds of 45 and minimal effect on the overall level, the 400 Hz band was not 60 mph. At 45 mph, the dual probe produced levels that were included if the PI index exceeded 5 dB. 0.1 dB to 0.5 dB lower for both pavements, while at 60 mph, the dual probe levels were 0.1 to 1.0 dB lower. These typically small and varied differences in level are consistent with those reported Recommendations on previously (14 ). The spectral shapes for both probes were very Parameter Limits similar throughout the measured frequency range. Based on the results of this research, parameter limits listed in Table 5 for the run-to-run variation, variation of probe Data Quality Criteria location in the vertical direction, vehicle speed, tire inflation During the data acquisition, the coherence between the sig- pressure, and vehicle loading are recommended. Reasonable nals from the two microphones comprising each probe and the variations in some of the testing parameters including loca- difference between sound pressure and sound intensity level tion of the probe in the fore/aft direction and probe distance (PI Index) were monitored and recorded for each 1/3 octave from the tire sidewall would not be anticipated to adversely band. Coherence is a measure of the linear dependency of two affect the OBSI results. Parameter limits on these less sensi- signals with a value of 0 being no dependency, and a value of 1 tive variables and on the data quality criteria are based on the being perfect linear dependence (16 ). Mathematically, it is the results of this study, as well as general experience in conduct- magnitude of the cross-spectrum between two signals squared ing these field measurements. divided the product of the auto-spectrum of both signals. For Sufficient data on the effects of environmental variables on sound intensity measurements made in flow such the OBSI OBSI levels are not available to set limits at this time. Measure- measurements, it is generally found that the data are contami- ment, monitoring, and documentation of air temperature, nated with flow noise when the coherence falls below 0.8 (15). pavement temperature, wind speed and direction, and pave- With only a few exceptions, the coherence was greater than ment dampness, as indicated in the standard protocol, may 0.8 in all 1/3 octave bands from 400 to 4,000 Hz during the help researchers to establish these variables in time over a larger parameter measurements. In the 400 and 4,000 Hz bands, data set.