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29 CHAPTER 6 Conclusions, Recommendations, and Suggested Research Conclusions truck SPB levels could be predicted even for 50 mph. Also unexpected was the finding that the more aggressive "winter" Based on the research work completed in this project, the Dunlop tire did not provide any better correlation to the OBSI method was found to be the preferred approach for truck passby levels than the SRTT. This leads to the conclu- developing an at-the-source tire-pavement noise test proce- sion that changes in passby noise levels with pavement for dure. The CPX approach was not desired due to spectral dis- heavy trucks can be fairly well predicted on the basis of tire- tortion observed relative to the passby data, a slightly lower pavement levels alone at least for speeds of 50 mph and above. ability to correlate with overall passby levels, practical con- However, it is unclear why the levels are typically almost 10 dB cerns in the use of a CPX trailer or "facility," and the expense higher for trucks than light vehicles. of acquiring and maintaining a CPX trailer. The two methods Within the uncertainty of site-to-site variation encoun- were found, however, to correlate well with each other on an tered for non-porous pavements (Test Sites 1 through 11), the overall level basis and one could be used to reliably estimate porous Test Site 12 did not display any unique behavior. the other, particularly after some initial calibration. The spectrum shape of the OBSI levels was unique relative to Site-to-site variation was found to be a significant issue the other sites and displayed the same shape as the porous affecting the correlation between OBSI and passby measure- pavement Test Site S4. As a result, the Test Site 12 pavement ments, as observed in sound propagation measurements. In would be expected to have similar sound-absorbing proper- using passby data to quantify pavement noise performance, ties as those documented for S4. However, much of the dif- more strict requirements on measurement sites need to be ference between this pavement and the others appears to be considered for direct comparison of data from different sites. accounted for in the OBSI data. For Test Site 12, actual prop- Simultaneous OBSI and CPB measurements were found to be agation over the porous pavement was quite limited; larger an effective means of identifying and quantifying site biases. effects may be encountered for propagation over multiple OBSI data can be used to predict SPB levels for light vehi- lanes of sound-absorbing pavement. cles and heavy trucks using offsets applied to the OBSI levels. This yields predictions for an "average" site as defined by the In regard to the test tire, no overwhelming experimental sites tested in this research. Use of the offsets defined in this evidence was found to favor one tire over the other. The Dun- work should provide a reasonable estimate of passby levels lop tire produced levels closer to those measured for the light based on measured OBSI levels. For specific sites of varying vehicle statistical passby events, however, both tires tracked properties, greater variance could be expected between pre- equally well with the differences seen in the SPB data for dif- dicted and measured levels, however, a better defined "aver- ferent pavements. Both tires displayed similar sensitivity to age" site would probably not help to reduce this uncertainty. test variables in most cases. With the lack of a clear difference Consistent with the REMELs database (22), heavy trucks based on acoustic performance, the selection of the test tire were found to be about 9 dB louder than light vehicles. Un- can be made on the basis of other, non-noise related issues. expectedly, the SPB levels for trucks could be almost as accu- rately predicted from OBSI data as it could for light vehicles. Recommendations The analysis indicated that at lower speeds (i.e., 50 to 55 mph) some increase in noise level, which is not attributable to tire- Based on the findings of this research, recommendations pavement noise alone, occurs with trucks likely due to engine/ for the implementation and enhancement of the test proce- exhaust noise. However, within a standard deviation of 1 dB, dure and other recommendations are provided.