NCHRP Project 1-44: Measuring Tire-Pavement Noise at the Source APPENDIX C. Results of Test Parameter Evaluation

Transcription

1 NCHRP Project 1-44: Measuring Tire-Pavement Noise at the Source APPENDIX C Results of Test Parameter Evaluation

2 Introduction As a portion of the overall research work, an examination of test variables and measurement uncertainties was conducted to provide initial guidance for users to determine the control limits needed to implement the OBSI procedure. Based on input from current OBSI users and information contained in the draft ISO CPX procedure, several pertinent variables were identified that could affect the measurement results. The sensitivity of the OBSI results to variations in the configuration of the OBSI measurement fixture, tire inflation pressure, test vehicle type, test speed, and load were measured systematically. Pavement temperature was measured and its effect on OBSI levels was analyzed for the ranges encountered in the testing. Measurements were conducted to evaluate the effects of test variables on OBSI measurement results. The test matrix is shown in Table C-1. Chapter 4 summarizes the evaluation and results of the test parameter investigation, and makes recommendations on parameter limits and controls. This Appendix includes more detailed information on the test procedure, measurement sites, and results of the investigation. Table C-1: Test Parameter Matrix Parameter Variable Values Tire Repeatability (run-to-run) 10 consecutive runs SRTT, Dunlop Repeatability (day) Nominal conditions each day SRTT, Dunlop Probe configuration Single probe, dual probe SRTT Probe location, vertical ±¼, +½ vertical SRTT, Dunlop Probe location, fore/aft ±½, ±1 fore/aft SRTT, Dunlop Probe location, from tire ±½, -1 from tire SRTT, Dunlop Test speed ±2, ±4 mph SRTT, Dunlop Inflation pressure ±4, ±8 psi SRTT, Dunlop Load +100, +200 lbs SRTT, Dunlop Test Vehicle 3 additional vehicles SRTT, Dunlop Temperature As it occurred SRTT, Dunlop Test Procedure Measurements were conducted using the two-probe approach 1 at a baseline test speed of 60 mph and a cold tire inflation pressure of 30 psi. Under the OBSI procedure, the sound intensity fixture and associated microphones are attached to and supported by the test vehicle to allow for measurement positions that are very close to the leading or trailing edge of the tire contact patch. Two phased matched condenser microphones and associated preamplifiers are attached to a plastic probe holder at a spacing of 16mm in a side-by-side configuration and fitted with a spherical windscreen for each measurement location. The probe was positioned 75mm from the pavement surface and 100mm from the face of the tire, at locations opposite the leading and trailing contact patch of the tire, and oriented so that the sensitive axis was positioned toward the tire. Data from the leading and trailing edge positions are acquired separately for the same section of pavement and then averaged together during post-analysis. Use of the dual-probe sound- C-1

3 intensity fixture allows the leading and trailing edge positions to be measured simultaneously (see Figure C-1). Figure C-1: OBSI Equipment Installed on DPG Test Vehicle The baseline test condition for each test pavement and test tire followed the measurement protocol stated in Appendix A using full-sized rental vehicles with a baseline load consisting of two people and the OBSI instrumentation. Ideally, the same test vehicle would have been used as the baseline for all of the test scenarios. However, due to the relocation of the second portion of the testing, two different baseline vehicles were used. The MnROADs test vehicle was a 2007 Buick Lacrosse CX. At the GM DPG, a Red 2007 Pontiac Grand Prix was used as the primary (baseline) test vehicle. The baseline tire was the Michelin/Uniroyal Standard Reference Test Tire (SRTT), with the Dunlop SP Winter Sport M3 tire (Dunlap) used in those conditions where tire specific results are suspected to occur due to tread pattern differences. Photographs of the test vehicles are shown in Figure C-2 and photographs of the Dunlap and SRTT tires are shown in Figure C-3. Figure C-2: Primary (Baseline) Test Vehicles for MnROADs and GM DPG MnROADs Test Vehicle, 2007 Buick Lacrosse CX DPG Test Vehicle, 2007 Pontiac Grand Prix C-2

4 Figure C-3: Photographs of Dunlop and SRTT test tires Dunlop SP Winter Sport M3 SRTT Three passes were made for each test parameter, which were averaged together during post analysis. A series of repeat baseline configuration measurements was performed at the completion of each series of tests for each parameter. In addition to the repeat baselines, ten or more consecutive baseline passes were measured for each test tire to examine the run-to-run repeatability under the baseline configuration. These consecutive baseline measurements were assessed individually to examine the run-to-run repeatability under optimal conditions. To evaluate the variations in OBSI levels attributable to the testing parameters, each 3-pass set of parameter measurements was compared to the 3- run sets of baseline measurements performed at the start and completion of each series of tests for each parameter. The microphone signals were acquired with the Bruel & Kjaer PULSE System in FFT narrow band and ⅓ octave band levels using a 5-second averaging time. The microphones were calibrated using a Larson Davis Model CAL200 acoustic calibrator set for 94 db at the beginning and end of the measurement period. M easurement Sites The initial portion of this testing was completed at Minnesota DOT s MnROAD Low Volume Road facility in Albertville, MN. Due to an extended period of rain, testing at MnROADs was limited to the SRTT tire and only a portion of the test matrix was completed. The remainder of the testing was relocated to the General Motors Desert Proving Ground (DPG) in Mesa, AZ. MnROAD Low Volume Road The MnROAD Low Volume Road, located in Albertville, MN, is a 2.5-mile closed loop that contains 20 pavement test sections. Noise measurements were conducted on August C-3

5 17, 2007 from 8:00 am until 8:15 pm. During the testing period, several of the pavement sections were under construction, resulting in the selection of adjacent pavement sections to reduce test time while allowing for the maximum range in pavement variety and noise level. Two sections were selected as test surfaces; a fine textured asphalt (AC), and a random transversely tined PCC (MnROAD sections 25 and 36). The site and test section locations are shown in Figure C-4 and photographs of the two pavement surfaces are shown in Figure C-5. Figure C-6 presents the average ⅓ octave band spectrum for each surface under baseline conditions using the SRTT test tire. Air temperatures ranged from about 60 F at 8:00 am, to a high of about 74 F at 2:00 pm, and dropped back down to 66 F by 8:00 pm. The sky was clear during early part of the testing period and then became overcast in the late afternoon into the evening. Figure C-4: MnROADs Low Volume Road Site Location Section 1: AC Section 2: PCC Figure C-5: MnROAD Low Volume Road Pavement Test Sections Section 1: Fine Textured AC Section 2: Random Transverse Tined PCC C-4

6 Figure C-6: ⅓ Octave Band OBSI Levels for Test Sections at MnROAD SRTT AC SRTT PC Sound Intensity Level, dba Frequency, Hz General Motors Desert Proving Grounds (GM DPG) During the week of September 10 th, 2007, the parameter testing continued at the DPG in Mesa, Arizona. At this facility, only two pavements could be efficiently tested, a relatively smooth AC and an exposed aggregate PCC pavement. The site and pavement section locations are shown in Figure C-7. Pictures of the two test sections used for this study are shown in Figure C-8. The average ⅓ octave band spectrums for each surface under baseline conditions using the SRTT and Dunlap test tires are shown in Figure C-9. Over the four days of testing, the weather consisted of clear skies and air temperatures ranging from 87 F to 107 F. Easterly winds of up to about 8m/s were present on the 11 th and 12 th, parallel to the orientation of the test sections, resulting in almost no crosswind on any of the measurement days. C-5

7 Figure C-7: GM DPG Site Location Section 1: AC Section 2: PCC Figure C-8: GM DPG Pavement Test Sections Section 1: Smooth AC Section 2: Exposed Aggregate PCC C-6

8 Figure C-9: ⅓ Octave Band OBSI Levels for Test Sections at GM DPG 105 Sound Intensity Level, dba SRTT AC SRTT PCC Dunlap AC Dunlap PCC Frequency, Hz 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 run-to-run repeatability of the SRTT tire was tested at MnROADs and the Dunlap tire was tested at the DPG. Testing for the SRTT runs occurred over a period of about 50 minutes, with an air temperature range of about 1 C. The Dunlap measurements were made over a period of about 25 minutes, with air temperatures within about 1 C. A summary of the total range in overall A-weighted sound intensity levels and ⅓ octave bands for the consecutive baseline runs is shown in Table C-2. The ⅓ octave band spectrums for each consecutive pass on each surface under baseline conditions are shown in Figure C-10 for the SRTT tire and in Figure C-11 for the Dunlap tire. Table C-2: Range in OBSI for Consecutive Baseline Runs SRTT - AC SRTT PCC Dunlap - AC Dunlap - PCC A-Wtd ⅓ Oct. A-Wtd ⅓ Oct. A-Wtd ⅓ Oct. A-Wtd ⅓ Oct. Range 0.8 db 0.5 to 0.8 db 0.7 to 0.6 db 0.5 to 0.7 db 0.6 to Std Dev db 0.2 to db 0.2 to db 0.1 to db 0.2 to 0.6 C-7

9 Figure C-10: ⅓ Octave Band OBSI Levels for Consecutive Passes, SRTT Tire Sound Intensity Level, dba SRTT PCC SRTT AC Frequency, Hz Figure C-11: ⅓ Octave Band OBSI Levels for Consecutive Passes, Dunlap Tire Sound Intensity Level, dba Dunlap AC Dunlap PCC Frequency, Hz C-8

10 The total range in overall A-weighted OBSI levels for the consecutive baseline runs was 0.8 db for the SRTT tire on both the AC and PCC pavements. For the Dunlap tire, the range in level was 0.6 and 0.7 db for the AC and PCC pavements, respectively. The baseline runs for this portion of the analysis were made consecutively and no changes in the fixture configuration or measurement protocol were made between runs. As a result, the variability measured for the consecutive baselines can be considered to be measurement uncertainty. Where OBSI levels under different parameter values fall within the standard deviation of the consecutive baselines, the changes in noise level cannot be reasonably attributed to changes in the given parameter and slight variation of the parameters in the testing configuration would not be anticipated to adversely affect the OBSI result, assuming testing is conducted following the standard protocol. Test Tire (SRTT vs. Dunlap) Relation of the SRTT to the Dunlap tire was examined using the baseline measurement results from the DPG, where both test tires were assessed on the same set of pavements. Because baseline measurements were conducted for each test tire over a period of several days, some variation in the baseline levels occurred, associated with temperature variations (discussed under Environmental Variables). To more readily examine the relation of the two test tires, baseline measurements were averaged for each tire on both the AC and PCC pavements. The Dunlap tire baseline measurements conducted prior to 8:30 am were not included because similar early morning measurements were not conducted with the SRTT. The ⅓ octave band levels for the SRTT and Dunlap tires on both DPG pavements are shown in Figure C-8 for the baseline condition. The Dunlap tire resulted in overall sound intensity levels that were 2.2 and 1.9 dba higher than the SRTT levels for the AC and PCC pavements, respectively, with an average difference of 2.0 dba. Higher ⅓ octave band levels occurred with the Dunlap tire for all frequencies except the 2,000 and 2,500 Hz bands, where levels with both tires were similar. These differences were lower than those measured for the Phase I test sites, which ranged from 2.3 to 3.0 dba, with the Dunlap producing higher levels than the SRTT, but were similar to the differences calculated for the Phase II passby sites, which ranged from 0.9 dba to 3.1 dba with an average difference of 2.0 dba. Environmental Variables Three-run sets of baseline configuration measurements were performed at the completion of each series of tests for each parameter. Over the day of testing at MnROADs, air temperature ranged from about 19 o C to 23.5 o C for the baseline configurations and pavement temperature varied from 26.6 o C to 41.8 o C for the AC pavement and from 25.6 o C to 37.0 o C for the PCC pavement. The temperature fluctuation throughout the day in Mesa was greater than that in Minnesota and, unlike the MnROADs testing, the baselines at the DPG were acquired over multiple days. At the DPG site during baseline measurement runs, the air temperature ranged from about 30 to 39.4 o C. The pavement temperature at the DPG varied from 37.2 o C to 61 o C for the AC pavement and from 35.2 o C to 55.2 o C for the PCC pavement over the four-day testing period. The relationship between overall A-weighted OBSI levels and the measured Air and pavement temperatures are plotted in Figures C-12 and C-13 for the SRTT and Dunlap test tires on both DPG pavements, along with a linear regression for each data set. C-9

11 Figure C-12: Relationship Between OBSI Levels and Air Temperature for DPG Pavement Sections 1.0 Difference from Average Baseline Level, dba 0.8 SRTT AC SRTT PCC Dunlap AC 0.6 Dunlap PCC Air Temperature, deg C Figure C-13: Relationship Between OBSI Levels and Pavement Temperature for DPG Pavement Sections 1.0 Difference from Average Baseline Level, dba 0.8 SRTT AC SRTT PCC Dunlap AC 0.6 Dunlap PCC Pavement Temperature, deg C C-10

12 The range in overall levels for the SRTT tire was only slightly higher than the standard deviation of the consecutive baselines; 0.5 and 0.3 db for the AC pavements, and 0.6 and 0.5 db for the PCC pavements at MnROADs and the DPG, respectively. No clear correlation between the SRTT and air/pavement temperature was indicated in the data. A slight downward trend with an increase in temperature was found, but R 2 values were very low (from 0.0 to 0.4). For the Dunlap tire at the DPG, the ranges in level between baselines were 1.1 and 1.0 db for the AC and PCC pavements, respectively, with the levels showing a decreasing trend with an increase in temperature. The results indicate a decrease of 1 db in the overall OBSI level measured with the Dunlap tire with an air temperature increase of about 10 o C or a pavement temperature increase of about 27 o C. The R 2 values for the Dunlap tire ranged from 0.76 to 0.82 for air temperature. Systematic Vehicle and Test Execution Variables Measurement parameters including probe location in the vertical and fore/aft directions, probe distance from tire sidewall, vehicle test speed, vehicle loading, and tire inflation pressure were evaluated systematically for both the SRTT and Dunlap tires as discussed in Chapter 4. Three-run sets of baseline repeats were conducted prior to and after each series of tests for each parameter. Measured differences between OBSI levels under each parameter variation and the baseline repeat measurement conducted prior to the series of tests are shown in Table C-3, as well as the calculated slope and r 2 values for each series of tests. Consistent trends in the overall A-weighted OBSI levels, indicated in bold and plotted in Figures C-14, C-15, and C-16, were found with the variation of probe location in the vertical direction, vehicle speed, and vehicle loading. In considering the results of Table C-3, it must be noted that the trends only apply within the range of the parameter variations measured in this testing. There was a consistent downward trend in noise levels as the probe location was moved further from the tirepavement contact patch in the vertical direction (about 0.4 dba decrease in noise levels per ¼ of movement). For vehicle test speed, OBSI noise levels increased with speed (by about 0.3 dba per 1 mph). Similarly, noise levels increased with an increase in the vehicle load (0.2 to 0.4 dba increase per 100 lb load increase). For 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 Dunlap test tires and the spectral characteristics of each pavement were maintained. Vehicle loading on the AC pavement and SRTT tire resulted in slightly lower increases than the PCC pavement and Dunlap tire; 0.2 dba increase per 100 lbs load for the AC pavement, as compared to 0.3 and 0.4 dba increase for the SRTT tire and Dunlap tire on the PCC pavement respectively. The loading related increases occurred primarily in the frequencies below 1000 Hz, although the Dunlap tire on the AC section also resulted in a small increase with loading in the mid to high frequencies. The ⅓ octave band levels for the Dunlap tires on both pavements are shown in Figure C-17 for the baseline vehicle load and the +200 lbs condition. C-11

13 Table C-3: Results of Systematic Vehicle and Test Execution Variable Testing Parameter Difference from Baseline Value, dba SRTT AC SRTT PCC Dunlap AC Dunlap PCC Baseline Height +1/4" Height +1/2" Height -1/4" Baseline Slope (dba/ ¼ inch) Probe Location, Vertical Distance from Tire Sidewall Vehicle Test Speed Vehicle Load Tire Inflation Pressure (Cold) Probe Location, Fore/Aft, LE Probe Location, Fore/Aft, TE *Not measured. r Baseline Inboard 1/2" * * Inboard 1" Outboard 1/2" * * Baseline Slope (dba/ ½ inch) r Baseline mph mph mph mph Baseline Slope (dba/mph) r Baseline lbs lbs Baseline Slope (dba/100lbs) r Baseline (34 psi) psi psi psi psi Slope (dba/4 psi) r Baseline Forward 1/2" Forward 1" Back 1/2" Back 1" Baseline Slope (dba/inch) r Baseline Back 1/2" Back 1" Forward 1/2" Forward 1" Baseline Slope (dba/inch) r C-12

14 Figure C-14: Relationship Between OBSI Levels and Probe Height Above Pavement 1.0 Difference from Baseline OBSI Level, dba SRTT AC SRTT PCC 0.5 Dunlap AC Baseline DunlapPCC Probe Height Above Pavement, Inches Figure C-15: Relationship Between OBSI Levels and Vehicle Test Speed 1.5 Difference from Baseline OBSI Level, dba SRTT AC 1.0 SRTT PCC Dunlap AC DunlapPCC 0.5 Baseline Vehicle Test Speed, mph C-13

15 Figure C-16: Relationship Between OBSI Levels and Added Vehicle Load 1.0 SRTT AC Difference from Baseline OBSI Level, dba 0.8 SRTT PCC Dunlap AC DunlapPCC Baseline Added Vehicle Load, lbs Figure C-17: ⅓ Octave Bands Levels Under Various Vehicle Loading, Dunlap Test Tire 105 Sound Intensity Level, dba AC - Baseline PCC - Baseline AC lbs PCC lbs Frequency, Hz C-14

16 As tire inflation pressure increases, ⅓ octave band levels below 1000 Hz decrease and levels above 1000 Hz increase, resulting in small overall changes to the sound intensity level (0 to 0.5 dba increase per 10 psi increase). These changes in level were within the repeat baseline variability. However, the frequency shifts are notable. The ⅓ octave band levels for the SRTT tires on both pavements are shown in Figure C-18 for the two extreme conditions, 26 psi and 42 psi. As indicated in this figure, increases in ⅓ octave band frequencies of up to 3.9 db occurred over the tire inflation pressure range of 16 psi for the SRTT tire. The Dunlap test tire results were similar. Figure C-18: ⅓ Octave Band Levels at Various Tire Inflation Pressures, SRTT Tire 105 Sound Intensity Level, dba AC - 42 psi PCC - 42 psi AC - 26 psi PCC - 26 psi Frequency, Hz The data did not indicate a clear correlation between OBSI levels and probe location in the fore/aft directions. A small, but consistent, downward trend in noise levels occurred as the probe location was moved further from the tire sidewall (about 0.2 dba per ½ inch of movement). Small changes in noise level due to variation of the probe distance from the tire sidewall are generally within the standard deviation for the consecutive baselines and slight variation of these parameters in the testing configuration is not anticipated to adversely affect the overall or ⅓ octave band levels. Test Vehicle At the GM DPG, a 2007 Pontiac Grand Prix was used as the primary (baseline) test vehicle and results were compared to three other vehicles, including a second 2007 Pontiac Grand Prix, a 2007 Chevrolet Impala, and a 2007 Buick Lacrosse. The same measurement system and tires were used for all vehicles. Photographs of three of the four test vehicles are shown in Figure C-19. The forth vehicle, a 2007 Buick Lacrosse C-15

17 was not photographed (see Fig. C-2 for the same vehicle model). Measured differences between OBSI levels for each test vehicle and the baseline test vehicle are shown in Table C-4. Figure C-19: Photographs of Test Vehicles 2007 Pontiac Grand Prix, Baseline Vehicle 2007 Pontiac Grand Prix NOT AVAILABLE 2007 Chevrolet Impala 2007 Buick Lacrosse Table C-4: Summary of Results of Test Vehicle Testing Test Vehicle Difference from Baseline Value, dba SRTT AC SRTT PCC Dunlap AC Dunlap PCC Baseline Pontiac Grand Prix Pontiac Grand Prix Chevrolet Impala Buick Lacrosse For the SRTT tire, the overall differences between vehicles varied by up to 0.6 db for the AC pavement and by up to 0.8 db for the PCC pavement (values shown in Table C-4 do not result in these levels due to rounding). For the Dunlap tire, the overall levels varied by up to 1.2 db for both the AC and PCC pavements. Although the differences between test vehicles exceeded the standard deviation for the consecutive baseline runs, the C-16

18 variability of the environmental conditions was also considerably higher for these runs. The differences between vehicle results can be attributed to differences in temperature (discussed under Environmental Variables), which were more apparent with the Dunlap tire. A considerable amount of time was required to change tires and vehicles between measurement sets, resulting in notable air and pavement temperature differences between vehicle sets. Measurements conducted during the morning period (prior to 9:30 am), when temperatures were 6 to 7 o C lower than during the late morning and afternoon, resulted in the highest levels. The three vehicles that were measured during midday (between 11:00 am and 4:00 pm) were within 0.5 dba for both the SRTT and Dunlap tires and the spectral characteristics measured for each vehicle were similar. It is important to note that although the test vehicle variation did not produce substantial differences in OBSI levels, the same vehicle family, measurement system, and tires were used in all cases. Fixture Configuration (Single Probe vs. Dual Probe) Single versus dual probe configurations were examined using the SRTT tire at the GM DPG site. Photographs of the single and dual probe configurations are shown in Figure C-20. The comparison of the probe configurations was made for test speeds of 45 and 60 mph. The results in terms of overall A-weighted level are shown in Figure C-21. For leading edge probe position, the levels with the dual probe are lower by 0.3 to 1.0 db. For the trailing edge, the levels are essentially equal within the range of run-to-run variation. This is consistent with that reported previously 1 in which small and inconsistent trends in level between the two probe orientations were found. The spectral shapes for both probes are very similar throughout the measured frequency range. The ⅓ octave band levels for both probe configurations using the SRTT tire on both pavements are shown in Figure C-21 for the 60 mph test speed. Figure C-20: Photographs of Single and Dual Probe Configurations Dual Probe Configuration Single Probe Configuration (from Phase 1) C-17

19 Sound Intensity Level, dba Figure C-21: Overall OBSI levels for the single and dual probe configuration Single Probe 104 Dual Probe AC PCC AC PCC AC PCC AC PCC 45 mph 60 mph 45 mph 60 mph Leading Edge Trailing Edge Figure C-22: ⅓ Octave Band Levels for Single and Dual Probes, SRTT tire at 60 mph 105 Sound Intensity Level, dba Dual Probe 60 mph 2 - Dual Probe 60 mph 1 - Single Probe 60 mph 2 - Single Probe 60 mph Frequency, Hz Data Quality Criteria During the data acquisition, the coherence between the two microphones comprising each probe (coherence) and the difference between sound pressure and sound intensity level C-18

20 (PI Index) were monitored and recorded for each ⅓ octave band. For definitions and an explanation of the development of the data quality parameters, see reference 2 2. Coherence was greater than 0.8 in all ⅓ octave bands from 400 to 4000 Hz and the PI index was less than 5 db in all ⅓ octave bands from 500 to 5000 Hz. In the 400 and 4000 Hz bands, slight decreases in coherence occasionally occurred at the DPG site when high temperatures caused equipment overloads and overheating (a total of 4 runs out of 578). These data were discarded. At the 5000 Hz band, coherence was less than 0.8 for 38% of the parameter runs. PI index values for the trailing edge position occasionally exceeded 5 db in the 400 Hz band (about 3% of the runs). Since the levels in the 400 Hz ⅓ octave band were sufficiently low so as to have minimal effect on the overall level, the 400 Hz band was not included where the PI index exceeded 5 db. An example of a typical OBSI run where data quality criteria are met is shown in Figure C-23. Figure C- 24 shows an example of a run that was discarded because data quality criteria were not met, in this case due to heat overloading of the data acquisition system. Past experience has shown that similar data quality flags occur when microphones are not properly secured onto the preamplifiers, when dirt or debris becomes lodged in the microphone windscreens, when wind conditions have adverse affects on the measurements, or when noise interference from reflections or other noise sources (i.e., a truck passing) influence the data. Figure C-23: PI-Index and Coherence for Valid OBSI Measurement (for Hz) PI-Index LE PI-Index TE Coherence LE Coherence TE PI-Index, db Coherence /3 Octave Band Frequency, Hz 0.0 C-19

21 Figure C-24: PI-Index and Coherence for OBSI During Equipment Overload PI-Index >5 db PI-Index LE PI-Index TE Coherence LE Coherence TE 1.4 PI-Index, db Coherence PI-Index <-1 db Coherence < /3 Octave Band Frequency, Hz Donavan, P., Further Development of the Sound Intensity Method of Measuring Tire Noise Performance of In-Situ Pavements, California Department of Transportation, Division of Environmental Analysis, Sacramento, CA, prepared by Illingworth & Rodkin, Inc., Petaluma, CA, January Donavan, P. and Oswald L., The Identification and Quantification of Truck Tire Noise Sources Under On Road Operating Conditions", Proceedings of Inter Noise 80, Miami, FL, Dec.1980 C-20