National Academies Press: OpenBook

Measuring Tire-Pavement Noise at the Source (2009)

Chapter: Chapter 3 - Evaluation of Alternative Test Methods

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Suggested Citation:"Chapter 3 - Evaluation of Alternative Test Methods." National Academies of Sciences, Engineering, and Medicine. 2009. Measuring Tire-Pavement Noise at the Source. Washington, DC: The National Academies Press. doi: 10.17226/14212.
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Suggested Citation:"Chapter 3 - Evaluation of Alternative Test Methods." National Academies of Sciences, Engineering, and Medicine. 2009. Measuring Tire-Pavement Noise at the Source. Washington, DC: The National Academies Press. doi: 10.17226/14212.
×
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Suggested Citation:"Chapter 3 - Evaluation of Alternative Test Methods." National Academies of Sciences, Engineering, and Medicine. 2009. Measuring Tire-Pavement Noise at the Source. Washington, DC: The National Academies Press. doi: 10.17226/14212.
×
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Suggested Citation:"Chapter 3 - Evaluation of Alternative Test Methods." National Academies of Sciences, Engineering, and Medicine. 2009. Measuring Tire-Pavement Noise at the Source. Washington, DC: The National Academies Press. doi: 10.17226/14212.
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7Based on the results of the literature search, testing was conducted to evaluate the two candidate methods for on- board, tire-pavement noise source measurement. This testing consisted of measuring CPX and OBSI noise levels on the same tires and then conducting controlled passby measure- ments using the test tires along with three other tires of the same design mounted on the test vehicle. The findings from this testing along with other considerations leading to the selection of the OBSI method of tire-pavement noise mea- surement at the source are summarized in this section (details of the testing and analyses are discussed in Appendix B). Overview of Evaluation Testing Passby measurements were made under both cruise and coast conditions. Sound pressure levels of tire-pavement noise at the source were measured in a manner following the ISO CPX test procedure (2). Sound intensity levels were measured using the OBSI methodology employed in previous California Department of Transportation (Caltrans) studies (6). OBSI levels were also measured on the CPX trailer for comparison to those measured on the test car. In addition to these measure- ments, testing was done to examine potential propagation dif- ferences between sites. Measurements were made at five sites: four asphalt concrete (AC) pavements at the National Cen- ter for Asphalt Technology (NCAT) test track in Auburn, Alabama, and one portland cement concrete (PCC) pavement in the nearby town of Waverly, Alabama (specific information on these sites and pavements is provided in Appendix B). Three of the pavements at NCAT were acoustically hard, pro- ducing no sound absorption. Surfaces for these sections were fine texture Superpave (Section AC S5), medium texture stone mastic asphalt (Section AC S1), and Superpave with added transverse texture (Section AC W5). The fourth pavement had a porous, open-graded asphalt concrete (OGAC) pavement of coarse texture in the travel lane with propagation over an adja- cent non-porous AC lane (Section AC S4). The Waverly site (Waverly PCC) had an old PCC surface with transverse slab joints, no roadway shoulder, and propagation over an acousti- cally softer ground, providing some degree of sound absorp- tion. At the NCAT track, test speeds of 35, 45, and 60 mph were measured at all four sites, except for AC W5 where 55 mph was substituted for 60 mph due to track banking. At Waverly, only 35 and 45 mph were tested due to posted speed restrictions. An example passby measurement setup is shown in Figure 3 for the Waverly test site. Photographs showing typical CPX and OBSI measurement setups were shown in Chapter 2. Specific information regarding the test sites, test matrix, and test meth- ods is given in Appendix B. Three sets of tires were used for the testing (see Figure 4, details of the test tires are documented in Appendix B). One of these tires is the ASTM Standard Reference Test Tire (SRTT) (13), which is currently under study by the ISO Working Group 33 as a possible new standard test tire for the ISO CPX procedure. Another tire was a Dunlop SP Winter Sport M3. This tire has been used in round-robin testing conducted by tire and vehicle manufacturers and was chosen by that group as a replacement for a light truck tire due to its more aggressive tread pattern. The size of both tire types was P225/60R16. The third tire design was the Goodyear Aquatred 3 in a P205/70R15 size. This tire design has been extensively used by a number of researchers since 2000. Due to tire and wheel size incompati- bility, comparable passby measurements for the Aquatred tire could not be made. However, CPX and OBSI measurements were conducted at all of the test sites for all three tire designs to provide a linkage to the historical Aquatred data. Summary of the CPX and OBSI Test Results For the test evaluation of the CPX and OBSI methods, the overriding issue was how well the at-the-source measures cor- relate to passby data. The simplest way to compare the CPX and OBSI measurements to the passby data is to consider the C H A P T E R 3 Evaluation of Alternative Test Methods

cross-plots of overall A-weighted level and metrics generated by these plots. A typical cross-plot is shown in Figure 5 for CPX versus passby for all pavements and test speeds. From such plots, the slope and r2 of a linear regression provide some indi- cation of data scatter and deviation from an ideal constant offset (“1-to-1” fit). A 1-to-1 line (slope of 1) can also be con- structed and deviations about that line considered. These are presented in Table 1 for the non-porous AC pavements (S1, S5, and W3), for all of the AC pavements (including the porous Section S4), and for all five sites grouped together. For the first grouping of sites (left columns), there may be a slight advantage in favor of the OBSI measurements as the standard deviation about a 1-to-1 fit of the data is smaller than that of the CPX measurements. When the porous pavement Section S4 is included, the standard deviations become identi- cal (center columns). For this grouping, the only detractor for the CPX data is that the slope of linear regression deviates more from an ideal slope of 1 than do the OBSI results. For this grouping, both source measures correlate well to passby (even with the porous pavement included) as indicated by the r2 val- ues and standard and average deviations. With the scatter of the passby data being on the order of 1 to 2 dB, it is apparent that better correlation could not be expected for these sites. When the PCC site is included (right columns), this is not the case. Although the OBSI results hold some small advantage over the CPX, the r2 and standard deviations for both are not very acceptable. From the sound propagation tests, the PCC site was found to be substantially different (2 to 4 dB) than the others due to sound absorption from the acoustically soft ground at the site and/or because the pavement was slightly depressed below the grade of the adjoining ground. These 8 Figure 3. Passby measurement setup for the Waverly PCC test site. Figure 4. Photograph of the Aquatred (left), Dunlop (middle), and SRTT (right) test tires. Figure 5. Controlled vehicle passby levels versus CPX sound pressure level for all test sites and both test tires. 25 ft Direction of Travel Microphone Position y = 0.80x - 3.23 R2 = 0.79 66 68 70 72 74 76 78 80 82 84 87 89 91 93 95 97 99 101 103 105 107 CPX Sound Pressure Level, dBA Pa ss by S ou nd P re ss ur e Le ve l, dB A AC S1 AC S4 AC S5 AC W3 Waverly PCC 1-to-1 Line Linear Regression

measured by the CPX method are consistently reduced by 3 to 4 dB relative to what would be expected from the passby spec- tra or the OBSI spectra as illustrated in the example presented in Figure 7. Although this spectral distortion has only subtle influence on the correlation of overall levels, some evidence of its effect was seen in the rank ordering of tires. The spectral distortion is thought to be related to the enclosure surrounding the test tire on the CPX trailer. Using the techniques currently under consideration by ISO Working Group 33, methods for determining corrections are being developed to account for the effects of the enclosure on the CPX measurements (14). How- ever, unless trailers were totally identical, correction spectra would need to be determined for each CPX trailer. Recommendation of an At-the-Source Measurement Method The selection of the OBSI method for further development was based on both the technical issues resulting from the evaluation testing and from considerations dealing with the expense/practicality and training/expertise required for implementing either of the two methods (detailed analysis is 9 Figure 6. CPX sound pressure level versus OBSI level for all sites and both tires. Table 1. Correlation indicators for CPX and OBSI methods to passby. data indicate that an at-the-source measurement cannot be expected to account for an arbitrary range of site characteris- tics in the prediction of wayside levels. Further insight can be gained by plotting the overall CPX levels against the corresponding OBSI levels. These data sug- gest that CPX or OBSI source levels could be predicted from the other within a standard deviation of 1.1 dB when all of the test pavements are included (Figure 6). The standard deviation is reduced to 0.8 dB, however, when the porous AC Section S4 is excluded. It was noted that the two methods handle porous pavements differently. The actual passby lev- els were consistently higher than what would be predicted from the CPX to passby correlation curve, and consistently lower than what would be predicted from the OBSI to passby correlation. Thus, the CPX levels over-predict the effect of porosity on the passby levels while the OBSI levels under pre- dict it. This is likely due to the CPX sound pressure measure- ments being more affected by pavement sound absorption than the sound intensity measurement. One of the largest drawbacks for the CPX method is spectral distortion which occurs in comparison to passby and OBSI results. In general, the 1⁄3 octave band levels below 1000 Hz Sections S1, S5, W3 Sections S1, S4, S5, W3 All Sites Metric CPX OBSI CPX OBSI CPX OBSI Slope 0.94 0.96 0.87 0.94 0.80 0.87 r2 0.94 0.95 0.94 0.93 0.79 0.87 Offset, dB 21.9 23.7 21.7 24.0 22.4 24.6 Std Dev, dB 1.2 0.9 1.1 1.1 1.8 1.7 Avg. Dev, dB 1.0 0.7 0.8 0.9 1.4 1.3 86 88 90 92 94 96 98 100 102 104 106 108 88 90 92 94 96 98 100 102 104 106 108 OBSI Level, dBA CP X So un d Pr es su re L ev el , d BA AC S1 AC S4 AC S5 AC W3 Waverly PCC 1-to-1 Line Linear Regression y = 1.04x - 6.52 R2 = 0.93

presented in Appendix B). In regard to expense and practical- ity, factors such as facilities expense, instrumentation costs, labor costs, and operational issues are associated with either method. For training and expertise, trade-offs between the two methods resulted in no net advantage for either method. The OBSI method was selected for the following reasons: • Slightly better correlation between OBSI and passby data than for CPX data, • Lack of spectral distortion seen in comparing OBSI and passby data, • Expense of an enclosed trailer for CPX measurements, and • Practical issues of acquiring, validating, operating, main- taining and storing a CPX trailer. The first two reasons resulted from the evaluation testing; the last two reasons deal with the use of a CPX trailer. The option of exposed microphone CPX was considered desirable from a cost and ease of implementation point of view, but technical issues of wind noise, test vehicle reflections and noise, and operation in traffic would lead to inconsistency from one user to another. On the other hand, the issues against an OBSI approach do not appear to be significant enough to preclude its use. 10 Figure 7. Comparison of spectra for CPX, OBSI, and passby levels with 24 dB added for Dunlop tire on Section AC S5 at 45 mph. 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 1/3 Octave Band Center Frequency, Hz Ti re N oi se L ev el , d BA S5 CPX S5 OBSI S5 Pby +24 dB

Next: Chapter 4 - Evaluation of OBSI Test Parameters »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 630: Measuring Tire-Pavement Noise at the Source examines a suggested procedure for measuring tire-pavement noise at the source using the on-board sound intensity (OBSI) method.

The following appendixes to the report are available online.

Appendix A: Review of Literature

Appendix B: Test Evaluation of Candidate Methods and Recommendation for Test Procedure Development

Appendix C: Results of Test Parameter Evaluation

Appendix D: Demonstration Testing of OBSI Procedure

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