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Field Evaluation of Reflected Noise from a Single Noise Barrier (2018)

Chapter: Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research

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Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Page 144
Suggested Citation:"Chapter 7 - Applications, Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Page 144

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141 3.15 kHz bands and in the 250 Hz to 500 Hz bands, although the results are inconsistent. The results are more mixed and lower in magnitude for the sound-absorbing barriers. The third type of change examined was an increase in the background sound levels. This increase is due to the lengthen- ing of the signal of a vehicle pass-by caused by the reflection of the sound off the barrier wall. The reflected sound length- ens the signal by creating an image source of the vehicle. This increase in the background sound level—apart from a similar increase in the equivalent sound level—appears to be greater closer to the road than farther away. Close to the road and absent the barrier, the pass-by signal rises and falls rather quickly. In the presence of the barrier wall, the direct and reflected paths of sound from the vehicle combine to cause the sound to be heard sooner as the vehicle approaches the pass- by point before its arrival and to last longer after the vehicle passes and recedes. As a result, the sound level does not have as much time to drop off to a background level before the enhanced sound of the next vehicle is heard (earlier than in a No-Barrier situation). The effect is to sustain the background sound at a higher level than without the reflected sound. The effect on background level appears to decrease with increas- ing distance from the road and barrier. As distance increases, the effect of other vehicles upstream and downstream from the pass-by point becomes greater. The rise in level and fall in level during an individual vehicle passage are both made smaller by the effect of distance and the increasingly impor- tant contribution of upstream and downstream vehicles to the total equivalent sound level. Moreover, the results suggest that, like the overall increase in sound levels, the increase in background sound levels is less apparent or nonexistent for sound-absorbing barriers than for sound-reflecting barriers. These three changes—level, spectral content, and back- ground level—can prompt perceived changes in sound among listeners. Where a barrier is present, the sound can be perceived as louder, different in character, or as lasting longer than the sound perceived where a barrier is not present. The extent to which any of these three parameters may change— and affect a listener—varies by distance from the road. By itself, an increase in A-weighted sound level may not be suf- ficient to be noticed or trigger a reaction; however, changes to the shape of the sound spectrum or to the duration of the signal are readily perceived. Spectrograms provide a clear visualization of both fre- quency and temporal effects that result from barrier reflec- tions. The presence of a reflective barrier causes sound levels to increase over a broad range of frequencies and causes higher sound levels to be sustained for a longer period of time. The increased sound levels include frequencies that domi- nate highway traffic noise. These observations apply to traffic and multiple vehicle types (autos, heavy trucks, and motor- cycles) that were examined on either side of the road using microphones positioned at multiple distances and heights from the roadway. Evidence exists that the barrier effect is more pronounced at farther distances from the road. It is assumed that the path-length difference between direct and reflected sound is one of the variables controlling the strength of the effect seen from barrier reflections (smaller difference = greater effect). The sound-absorbing barriers examined cause only slight changes in the sound levels, which can be difficult to discern in the standard spectrograms. For single-vehicle pass-by events, difference spectrograms allowed the research team to examine subtle differences in the spectrograms and also revealed harmonically related spectral peaks attributable to barrier reflections. These peaks occur because of a comb- filtering effect (with frequency and geometry dependent constructive and destructive interference), which can add a buzziness or raspiness to the sound. The psychoacoustic metrics applied to the audio recordings did not show reliable, positive correlation with higher annoy- ance at Barrier sites for continuous traffic. Annoyance met- rics (UBA and PA) consistently showed similar results. The CSA metric was ineffective at indicating any differences at all sites and microphone locations. Simple descriptive statistics extracted from the resulting UBA and PA time series revealed cases in which the mean values from the Barrier sites differed to an appreciable level of significance from those at the No-Barrier sites. The statistically significant differences tended to occur in the data from the higher microphones, and were more pronounced in data from microphones located at mod- erate distances from the roadway. In all cases, metrics derived from microphones located close to the roadway showed no difference between sites. To the extent that the mean values of annoyance showed significant differences, they were contra- indicative: the annoyance metrics at Barrier sites tended to have lower values than those at the No-Barrier sites. However, with lower traffic volumes, where individual pass-by events were more noticeable, PA did increase opposite the Barrier relative to the No-Barrier site. This effect is due to the weight- ing of the loudness of individual vehicle pass-bys and is con- sistent with the results of the spectrogram analysis, showing a lengthening of the sound from individual vehicles. Recommendations On the basis of this research, several recommendations can be made for state highway agency noise practitioners and researchers: • Using sound-absorbing barriers should be considered when receptors on the opposite side of the roadway have not qualified for a barrier based on an agency’s noise abate- ment feasibility and reasonableness criteria in their noise policies.

142 • For Type II noise barrier retrofit projects on existing highways and for Type I highway widening projects, true before/after measurement studies (i.e., using the same site before and after barrier installation, as opposed to using an equivalent before site) should be done when noise- sensitive receptors are present on the opposite side of the highway that do not qualify for a barrier. – If the barrier will be sound absorbing, the before mea- surements could be important evidence in demonstrat- ing that the noise environment opposite the barrier has not been made worse by the installation of the barrier. – If the barrier will be sound reflecting, the before mea- surements provide a base case for learning if the noise environment changes after the barrier installation and in what way and by how much. – Such studies can include a sociological component, sur- veying residences on both sides of the barrier regard- ing their perceptions of the highway noise environment and any changes introduced by the widening and/or the barrier. • True before/after noise measurement research studies should even be considered in areas where there are no noise-sensitive receptors on the side opposite the bar- rier, especially if a sound-reflecting barrier is to be built. Such sites eliminate the variable of possible reflections off buildings to provide a more controlled setting for study- ing the effects of distance, height, cross-section, terrain, and meteorological factors on the change in level, spec- trum shape, and background level. • To further understand barrier reflections and evaluate the benefit of sound-absorptive barriers, it is suggested that the current research using spectrograms, difference spec- trograms, and comb-filtering analysis be continued. The additional analysis could be applied to sites with absorp- tive barriers of different types and NRC values. Results will help in determining frequency ranges of sound reduction attributed to the absorption, as well as whether the absorp- tion helps to reduce sustaining of the higher noise levels seen with a reflective barrier and comb-filtering effects. • Because the derived annoyance metrics were either uncor- related with site location or were contra-indicative, their direct use (as applied in this work) cannot be recom- mended for indicating increased annoyance due to single barriers, except when applied to relatively low-volume roads. • The research team suggests that selected findings be incorporated into the National Highway Institute course NHI 142051, “Highway Traffic Noise in its Basic Concepts and Noise Barrier” lessons, using figures and spectrograms to explain concepts and high-level findings. The results also could be incorporated into the course’s “Public Involve- ment” lesson and the layperson’s guide could be included as a course reference document providing information on how to address the issue with the public. • The finding that traffic noise reflections off the nearby houses in the No-Barrier area at the I-270 location appeared to have affected the measured No-Barrier levels has implications for the impact determination process in highway project noise studies. Currently, most noise models of future conditions do not include building reflections. Yet the measured data suggest the levels can be increased by 1–2 dB because of the reflections. Such an increase could move a receptor from the “not impacted” category to the “impacted” category in a study, and thereby require consideration of noise abate- ment. Thought needs to be given to the inclusion of building reflections in traffic noise modeling. Suggested Research Further Study of Reflections Off Sound-Absorbing Barriers To further understand barrier reflections from sound- absorbing barriers and evaluate their benefit, it is suggested that the current research be continued, using the FHWA Method and spectrograms and applying the analysis to sites with absorptive barriers of different types and NRC values. Field Validation of the Single-Wall Reflections Function in FHWA TNM 3.0 Unlike FHWA TNM 2.5, the newer FHWA TNM 3.0 has a function that allows the reflections off single walls along a roadway to be computed for receivers on the opposite side of the roadway. The data collected in this study, especially for the Calm Neutral meteorological class, could be used for valida- tion testing of this component of the model. Caution should be taken about use of the data in the other meteorological classes, however, especially for data from receivers located farther back from the roadway. Time-Based Metrics Future research could further examine the data collected using time-based metrics, specifically examining the delta between the percentile metrics L10 and L90. The evidence collected by this study suggests that public annoyance can increase with increases in the average noise level or as levels become more variable. The delta L10 minus L90 is an indica- tion of how variable noise is: the greater the delta, the greater the chance of annoyance. To see if examining the delta L10 minus L90 would be worth pursuing, a few example data blocks were analyzed for the data collected at SR-71 (Chino Hills, California) and MD-5

143 (Hughesville, Maryland). This analysis showed that the L10 minus L90 delta was slightly greater (up to 1 dB) for the Barrier site compared to the No-Barrier site. Other data blocks at these two sites and data blocks from other project sites would need to be examined to confirm the trend and to make any conclusions regarding the delta value in relation to variables such as distance from the road or height above the ground. These results also would need to be compared with the literature on perceptibility and the relationship to an increase in the delta L10 minus L90. Several literature sources from the 1970s involve the Traffic Noise Index, which is based on the delta L10 minus L90. These articles and others could be examined to help determine if the greater deltas asso- ciated with barrier reflections can be tied in with adverse community perception. Examination of Time-Above Metric In addition to examining other time-based metrics, future research could examine the Time-Above (TA) metric. The TA metric represents the amount of time the sound level exceeds a threshold of interest. This metric can be applied to a noise event, such as speech interference, or to a time interval. The spectrogram data from the research discussed in this report show that traffic noise is louder and broader (in time) in the presence of an opposing barrier, which suggests that a partic- ular level could be exceeded more often in a community with an opposing barrier compared to one without. The threshold level examined using the TA metric could be associated with speech interference or related to a site-specific sound level (e.g., existing worst-hour equivalent sound level). Examin- ing the TA along with the amount of exceedance could help to explain adverse public reaction to barrier-reflected noise. Further Study Sound Quality Benefit of Sound-absorbing Barriers One-third octave band analysis indicates that sound- absorbing barriers may be reducing the comb-filtering effect of barrier reflections, which is a positive outcome. The comb- filtering effect adds a buzziness or raspiness to the sound. The research team suggests that a one-twelfth-octave (or narrower) band analysis be conducted for vehicle pass-by events for both reflective and absorptive barriers using the difference spectrograms and extraction of peaks in the spec- tra. Some of the exact frequencies of peaks and dips are being masked when one-third octave band analysis is applied, and they appear to reside in-between the one-third octave band center frequencies. Knowing the exact frequencies would help to refine comb-filtering effects for both reflective and absorp- tive barriers and help to confirm the reduction in effect due to sound absorption. Barrier Reflections Screening Tool Enhancements The Barrier Reflections Screening Tool could be enhanced in the following ways: • Expand the screening tool to include other propagation effects besides geometrical spreading, such as ground effects and shielding. Methods would have to be exam- ined for simple implementation so that the tool remains a screening tool and does not become a more complex model. Once the additional effects are included, the results from the expanded tool could be compared to the measured barrier effects in this study for validation purposes. • Extend the screening tool to calculate the results on a fre- quency basis, to help understand frequency and sound quality effects due to barrier reflections. • Formalize a version of the screening tool to calculate the effect of reflections from homes adjacent to a highway. The geometry can be set up to place a receiver between the high- way and homes. A preliminary version of this approach was used to adjusting the measurement noise levels at I-270 for purposes of validating the screening tool. Spectral Evaluation of Propagation Effects in Relation to Barrier-Reflected Noise To further investigate the characteristics of barrier reflec- tion effects closer to the ground, an analysis using FHWA TNM 3.0 could be conducted to show how frequency ranges are affected by shielding from median barriers and highway vehicles and also by ground effects. Those frequency ranges could be compared to the frequency ranges showing bar- rier effects presented in this report. This analysis may help to show that sound-reducing propagation effects help to “expose” barrier-reflected noise. Listening Trials Using PA metrics did not yield clear correlations or indi- cations regarding expected annoyance responses due to the presence of single barriers; however, the literature regarding single barriers indicates that people do perceive changes. A psychoacoustic preference study could be used to assess the ability of listeners to detect these changes. First, in-depth surveys of the residents near single-barrier sites would be conducted; second, a series of listening trials would be conducted. The surveys would need to be carefully developed and of large enough sample sizes to avoid the many confounding non-acoustic factors contributing to human annoyance. The

144 results of such surveys could be used to estimate the true extent of the problem of annoyance with single barriers. Key factors to be explored are the relations of distance, site lines, time of day, and most importantly, listener location if or when the listener is annoyed. If listeners find themselves annoyed by the noise while indoors, highly detailed metrics such as phase, directionality, and time evolution of spectral content probably are not called for because the structure itself cleanses the signals of much of this content. If residents are annoyed when outdoors, however, it is important to under- stand how parameters such as distance, direction, and inter- vening elements contribute to the perception of the sound. Listening trials would be used to provide statistical indica- tions of those elements in the single-barrier roadway noise that might contribute measurably to annoyance. From these results, a set of actionable criteria would be extracted to guide road planners. Roadway Geometry In conjunction with the above research, the effect of road- way geometry should be studied. Of particular interest is whether roadway curvature could focus sound or increase the duration of the sound at a particular receiver. For example, situations could occur in which sound to a receiver is other- wise blocked by terrain or buildings around a curve, but when a barrier is constructed, the changes create an unobstructed reflected path to that receiver. Such a change could increase the overall sound level and the length of time elevated sound levels from individual vehicle pass-bys occur.

145 References 1. FHWA (2016), “Summary of Noise Barriers Constructed by December 31, 2016.” Retrieved May 2016 from: https://www.fhwa.dot.gov/ environment/noise/noise_barriers/inventory/. 2. Menge, C. W., and D. E. Barrett (2011), “Reflections from Highway Noise Barriers and the Use of Absorptive Materials in the United States: Why Small Increases in Noise Levels May Deserve Serious Consideration,” Transportation Research Record: Journal of the Transportation Research Board, No. 2233, Transportation Research Board of the National Academies, Washington, D.C., 2011, p. 161. 3. Lee, C. S. Y., and G. G. Fleming (1996), Measurement of Highway- Related Noise, U.S. Department of Transportation Volpe Center Acoustics Facility, FHWA-PD-96-046, May 1996. Available online at: http://www.fhwa.dot.gov/environment/noise/measure/index.htm. 4. ATS Consulting (2005), Atmospheric Effects Associated with High- way Noise Propagation, Arizona Transportation Research Center, SPR 555, 2005. 5. Johansen, L. G. (2006), “Psychoacoustics and Audibility— Fundamental Aspects of the Human Hearing,” Lecture notes for the course TI-EAKU, University College of Aarhus, 2006. 6. Bilsen, F. A., and R. J. Ritsma (1970), “Some Parameters Influenc- ing the Perceptibility of Pitch,” Journal of the Acoustical Society of America, Volume 47, Number 2 (Part 2), 1970.

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Field Evaluation of Reflected Noise from a Single Noise Barrier Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 886: Field Evaluation of Reflected Noise from a Single Noise Barrier analyzes the characteristics of sound reflected from a noise barrier to the opposite side of a highway. State departments of transportation (DOTs) periodically receive complaints from residents about increases in traffic noise that residents believe are the result of noise reflected from a new noise barrier added across the roadway from them. Currently available analytical tools are limited in their ability to evaluate reflected noise and some of the subtle changes in the quality of sound that can occur when it is reflected. Therefore, it is a challenge for DOTs to determine conclusively if complaints about reflected noise are the result of actual or perceived changes in noise characteristics, and to identify locations where absorptive surface treatments could be beneficial.

The study compares reflected noise from sound-reflecting barriers and from barriers with a sound-absorptive surface. It examines both the levels and frequencies of reflected noise to better understand how reflected noise is experienced by communities.

The full report, which includes four detailed appendices, is 27 MB and may take time to download. It is accompanied by several appendices, a tool, and a guide:

A presentation file that summarizes the research also is available on the report project page.

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