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9 Table 1. Correlation indicators for CPX and OBSI methods to passby. Sections S1, S5, Sections S1, S4, S5, W3 All Sites Metric W3 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 data indicate that an at-the-source measurement cannot be measured by the CPX method are consistently reduced by 3 to expected to account for an arbitrary range of site characteris- 4 dB relative to what would be expected from the passby spec- tics in the prediction of wayside levels. tra or the OBSI spectra as illustrated in the example presented Further insight can be gained by plotting the overall CPX in Figure 7. Although this spectral distortion has only subtle levels against the corresponding OBSI levels. These data sug- influence on the correlation of overall levels, some evidence gest that CPX or OBSI source levels could be predicted from of its effect was seen in the rank ordering of tires. The spectral the other within a standard deviation of 1.1 dB when all of distortion is thought to be related to the enclosure surrounding the test pavements are included (Figure 6). The standard the test tire on the CPX trailer. Using the techniques currently deviation is reduced to 0.8 dB, however, when the porous AC under consideration by ISO Working Group 33, methods for Section S4 is excluded. It was noted that the two methods determining corrections are being developed to account for the handle porous pavements differently. The actual passby lev- effects of the enclosure on the CPX measurements (14 ). How- els were consistently higher than what would be predicted ever, unless trailers were totally identical, correction spectra from the CPX to passby correlation curve, and consistently would need to be determined for each CPX trailer. lower than what would be predicted from the OBSI to passby correlation. Thus, the CPX levels over-predict the effect of Recommendation of an porosity on the passby levels while the OBSI levels under pre- At-the-Source Measurement Method dict it. This is likely due to the CPX sound pressure measure- ments being more affected by pavement sound absorption The selection of the OBSI method for further development than the sound intensity measurement. was based on both the technical issues resulting from the One of the largest drawbacks for the CPX method is spectral evaluation testing and from considerations dealing with distortion which occurs in comparison to passby and OBSI the expense/practicality and training/expertise required for results. In general, the 1/3 octave band levels below 1000 Hz implementing either of the two methods (detailed analysis is 108 AC S1 106 y = 1.04x - 6.52 AC S4 R2 = 0.93 AC S5 CPX Sound Pressure Level, dBA 104 AC W3 102 Waverly PCC 100 1-to-1 Line Linear Regression 98 96 94 92 90 88 86 88 90 92 94 96 98 100 102 104 106 108 OBSI Level, dBA Figure 6. CPX sound pressure level versus OBSI level for all sites and both tires.
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10 98 96 S5 CPX 94 S5 OBSI 92 S5 Pby +24 dB 90 Tire Noise Level, dBA 88 86 84 82 80 78 76 74 72 70 68 66 64 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 1/3 Octave Band Center Frequency, Hz 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. presented in Appendix B). In regard to expense and practical- · Practical issues of acquiring, validating, operating, main- ity, factors such as facilities expense, instrumentation costs, taining and storing a CPX trailer. labor costs, and operational issues are associated with either method. For training and expertise, trade-offs between the The first two reasons resulted from the evaluation testing; two methods resulted in no net advantage for either method. the last two reasons deal with the use of a CPX trailer. The The OBSI method was selected for the following reasons: option of exposed microphone CPX was considered desirable from a cost and ease of implementation point of view, but · Slightly better correlation between OBSI and passby data technical issues of wind noise, test vehicle reflections and than for CPX data, noise, and operation in traffic would lead to inconsistency · Lack of spectral distortion seen in comparing OBSI and from one user to another. On the other hand, the issues passby data, against an OBSI approach do not appear to be significant · Expense of an enclosed trailer for CPX measurements, and enough to preclude its use.