Measuring Tire-Pavement Noise at the Source (2009)

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

Test Procedure Implementation and Enhancement The test procedure provided in Attachment 1 is recom- mended for adoption as a national standard. To re-enforce the application of the findings of this research, technical presenta- tions should be made to professional organizations and the pavement industry and the results communicated to interested groups. Although there was no technical preference for the SRTT, the SRTT should be specified as the primary test tire for the OBSI procedure because of its expected long-term availability; additional light passenger vehicle tire types such as winter tires should not be considered at this time. Periodically, the choice of the SRTT in regard to issues that could not be investigated in this research such as tire-to-tire variability and consistency of noise generation over time should be reviewed. Initial inves- tigation of these issues has been recently reported in other research that indicated minimal tire-to-tire variation and little difference between new and used tires (23). These preliminary findings should be verified over a longer time period. Effects of temperature were indicated to some degree in the parameter testing. However, the results were too limited to develop any trends or potential corrections to account for either air and/or pavement temperature. The effects of tem- perature should be investigated using the OBSI test proce- dure developed in this research, either through collective experience of multiple users or directed research. Similarly, tire rubber hardness due to tire aging could not be addressed in the current research and should be further documented by users of the recommended procedure. In the testing per- formed in this research, no adverse effects of ambient wind conditions were noted on the OBSI data. However, recent wind tunnel testing (19) indicated that some effect may occur under specific speed and cross wind conditions. Therefore, the effect of ambient wind speed and direction should be moni- tored in future work to determine if testing should be restricted due to wind conditions. In order to establish the expected reproducibility of OBSI measurements from one user to another, comparative “rodeo” testing should be done between users following the recom- mended procedure. This research focused on repeatability for a single user/measurement combination. In application, tire- to-tire, data acquisition system-to-system, and fixture-to- fixture variation may create a wider variance than this research indicates. As an example, in this research, a small, but con- sistent difference (−0.3 to −0.6 dB) in OBSI level was noted between data taken with the horizontal single probe and verti- cal dual probe fixtures. In comparing one user’s implementa- tion of the OBSI procedure to another, such subtle differences may combine with other differences due to tires and instru- mentation to produce user biases on the order of 1 dB. Some other variables may require further specification in the pro- cedure once differences are identified and understood. This rodeo testing should determine the reproducibility among users and the reasons for differences greater than 1 dB in over- all level. In addition, issues such as the effect of roadway curvature, roadway grade, banking, roadside reflecting surfaces, and the presence of other vehicles near the probe may be of concern but are not currently documented. For undriven tires, curva- ture, grade, and banking may not be significant issues under moderate conditions, however, they are currently undefined in terms of actual data. Strict adherence to the data quality indicators is recommended to avoid data that are influenced by the presence of sound-reflecting surfaces or nearby vehicles. Other Recommendations For SPB and CPB standard measurements, a 25-ft micro- phone distance from the center of the lane of vehicle travel should be considered. Although even at this reduced distance site-specific differences were found, the 50-ft distance intro- duced additional site-specific variation. Also, for the 25-ft dis- tance, clean passby events could be more easily acquired due to the greater signal-to-noise ratio. No adverse effects for heavy truck passby events were noted in the measurements. Current procedures for SPB should be further evaluated to identify means to minimize site-specific effects. Procedures for SPB and CPB measurements should be developed for consideration and adoption as a national standard. The results of passby measurements obtained in this work should be evaluated relative to the REMELs database to pro- vide insight into how to calibrate OBSI data to those data. Such calibration could facilitate using OBSI data in traffic noise pre- diction such as is done in the FHWA Traffic Noise Model. Suggested Research To enhance the results of this research the following topics are suggested for further research: • Investigations of the effects of temperature and wind con- ditions on OBSI measurement. This research will deter- mine temperature limits and/or correction factors as well as limits for wind conditions. • Investigations of the effect of porous (acoustically absorbing) pavement on sound propagation from the OBSI measure- ment point to wayside receiver locations and incorporation of such effects in traffic noise prediction models. • Investigations of the effects of site-specific variables on SPB and CPB measurements to recommend limits for the more important variables or otherwise site corrections. • Development of procedures for CPB and SPB for consid- eration and adoption as a national standard. 30