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

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3 • Sound levels are higher and spectral content changes at a position between the barrier and the road, compared to the No-Barrier site, as evidenced at I-24 and SR-71. • The background sound pressure level is elevated in the presence of the noise barrier at the reference microphone position between the barrier and the road. • Even at the reference microphone position atop the barrier, the sound pressure level can be slightly higher than at the equivalent No-Barrier position, as evidenced at I-90. However, little difference was seen at MD-5. • Near the edge of the road for the lower-height microphones, the Barrier levels are roughly the same as the No-Barrier levels, being slightly higher in the very-low-frequency bands, as evidenced at SR-71. • Near the edge of the road for the lower-height microphones, there is some evidence of an increase in the background A-weighted sound level (on the order of 1 dB to 1.5 dB at one community microphone), as seen at SR-71. • Farther back from the road, but still within 100 ft., the Barrier microphone sound levels are higher than No-Barrier levels by 0.5 to 1.5 dB. Here, the spectrum is further changed in some of the frequency bands between 250 Hz and 630 Hz and in some of the bands at and above 1 kHz, as evidenced at I-24, I-90, and MD-5. • Farther back from the road, but still within 100 ft., the background level increases in the bands from 630 Hz through 3.15 kHz, as evidenced at I-90 and MD-5, but not I-24. • At 400 ft. from the road, the Barrier broadband levels are typically 1 dB to 4 dB higher than at the No-Barrier site, as evidenced at SR-71. Spectral levels are 2 dB to 4 dB higher than at No-Barrier sites, except for likely ground effects in the 100 Hz to 250 Hz bands. • At 400 ft. from the road, all the Ln descriptors were higher at the Barrier site, not just the background levels, as evidenced at SR-71. • The increase in levels due to reflections decreased by 1 dB to 2 dB going from a lower- height microphone to a higher microphone, as evidenced at I-24, I-90, and MD-5. • No effect on sound level differences was observed as a function of traffic volume, as evidenced at all microphone pairs at all locations. • Slight differences exist in the sound level differences for different meteorological classes; however, no clear trends exist, as evidenced at I-90, I-24, and MD-5. Data collected at greater distances from the road might tell more. Spectrograms were generated for pass-by events involving individual vehicles and groups of vehicles, and for samples of highway traffic at each measurement site. The spectrograms compare data collected at the Barrier site and at an equivalent position at the No-Barrier site. This comparison allows for a visual examination of the effect of barrier-reflected noise. The Phase 1 sound-reflecting barrier spectrogram data reveal that the presence of a reflec- tive noise 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 dominate highway traffic noise. These observations apply to vehicles traveling on either side of the road, for a range of distances from the road and heights above the road, and for the vehicle types examined (autos, heavy trucks, and motor- cycles). An additional observation is that there is evidence that the barrier effect is more pronounced at distances farther from the road. (In this report, differences in the sound environment between the No-Barrier and Barrier sites are referred to as the barrier effect.) 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 (a smaller difference results in a greater effect). At each site, the signal from each sound level meter’s microphone was digitally recorded. These audio recordings were filtered and postprocessed to extract basic psychoacoustic

4 metrics as a function of time. In turn, these metrics were combined into three different psychoacoustic measures: unbiased annoyance (UBA), psychoacoustic annoyance (PA), and category scale of annoyance (CSA). Descriptive statistics for the annoyance metrics were compiled for each site. The statistics were investigated as indicators of whether the received sound from Barrier sites would be significantly different from those at No-Barrier sites. The psychoacoustic assessment yielded these findings: • UBA and PA yield similar results, whereas CSA does not yield useful indications; • Annoyance metrics show differences between Barrier and No-Barrier sites at moderate distances, but the results are contra-indicative; and • Annoyance metrics are less effective in heavy, constant traffic, but show differences in lighter traffic with separated pass-bys. Phase 2: Findings, Sound-Absorbing Barriers The following findings were developed regarding the Phase 2 (sound-absorbing) barriers in the context of comparisons to the Phase 1 (sound-reflecting barrier) results: • Differences in broadband levels at the reference microphones between the road and the barrier are seen for the sound-absorbing barriers (with noise reduction coefficients [NRC] of 0.80). • Broadband levels averaged slightly higher opposite the I-70 sound-absorbing barrier com- pared to the No-Barrier site for both microphone heights, similar to the sound-reflecting barriers. • Broadband levels averaged slightly higher opposite the I-75 sound-absorbing barrier compared to the No-Barrier site for both microphone distances. The reflection effect is less than at the sound-reflecting barriers at the farther distances. • One-third octave band levels for equivalent Leq (5 min.) periods were slightly higher opposite the I-70 sound-absorbing barrier compared to the No-Barrier site. This effect is less at the higher microphone. The differences found are similar to those found at the sound-reflecting barriers. A change in ground effects could be the cause of the larger mid- frequency difference at the lower microphone. • One-third octave band levels are slightly higher opposite the I-75 sound-absorbing bar- rier compared to the No-Barrier site for the microphones at different distances from I-75. The differences at the larger distance are less than the differences at the sound-reflecting barriers. • Broadband L90 and L99 statistical descriptors show that the background levels at both microphone heights across from the I-70 sound-absorbing barrier are not elevated above the Leq, unlike at the sound-reflecting barriers. • Broadband L90 and L99 statistical descriptors show that background levels at both micro- phone distances across from the I-75 sound-absorbing barrier are not elevated above Leq, unlike at the sound-reflecting barriers. • The one-third octave band L90 and L99 descriptors mostly support the conclusion that the background noise is not elevated due to reflections opposite the sound-absorbing barriers, unlike at the sound-reflecting barriers. • Finding suitable equivalent No-Barrier sites for studying both sound-absorbing and sound-reflecting barriers is difficult because of many factors, including reflections off other objects, differences in terrain, and localized noise sources.

5 • Compared to spectrograms for reflective barriers, where reflection effects are readily apparent, spectrograms for absorptive barriers reveal little indication of reflection effects. • Difference spectrograms revealed harmonically related peaks caused by comb filtering, which can be attributed to the barrier and can be perceived as the sound being buzzy or raspy. Absorptive barriers may reduce the comb-filtering effect. • The Barrier Reflections Screening Tool developed by the research team provides a con- servative estimate of the barrier-reflected effect and is appropriate for use in screening for potential adverse effects due to a reflective or absorptive noise barrier on the opposite side of a highway. Applications, Recommendations, and Future Research The research team found several applications of this research. The immediate applica- tion is for traffic noise analysis and abatement practitioners: traffic sound levels and sound characteristics for receptors across from a proposed single reflective noise barrier can change after the installation of the barrier. This understanding can support the appropriate speci- fication of sound-absorbing surfaces on these single barriers, especially for highway widen- ings where the roadway already exists. The finding that traffic noise reflections off nearby houses at the I-270 location appeared to have affected the measured levels has implications for the impact determination process in highway project noise studies. Currently, most modeling does not include building reflections. Thought needs to be given to the inclusion of building reflections in traffic noise modeling. The study found spectrograms to be useful in identifying the change in sound character- istics due to barrier reflections in both the time and frequency scales. Spectrograms and dif- ference spectrograms could help policy makers provide guidance on when it may be effective to use sound-absorbing barriers and help show the public what an absorptive barrier could provide. The study also found that psychoacoustic metrics of annoyance are not effective for looking at subtle changes in constant traffic, but they are helpful to provide insights into increased annoyance at lower traffic levels. The Barrier Reflections Screening Tool developed by the research team can be useful in estimating the effect of barrier reflections for projects with communities opposite a noise barrier. In addition, the layperson’s guide can be helpful in communications with the public, to help communities understand the effect of barrier reflections. Downloadable files of the following materials are available by clicking the links on the NCHRP Research Report 886 webpage at www.trb.org: • This report (including appendices A, B, C, and D); • Appendix D: Using the Barrier Reflections Screening Tool (online copy); • Appendix E: NCHRP Project 25-44, Phase 1 (Sound-Reflecting Barriers)—Detailed Protocols and Results; • Appendix F: Photographs from NCHRP Project 25-44, Phase 1 (Sound-Reflecting Barriers) Measurement Sites; • Appendix G: Photographs from NCHRP Project 25-44, Phase 2 (Sound-Absorbing Barriers) Measurement Sites; • The Barrier Reflections Screening Tool; and • “Reflected Sound from Highway Noise Barriers” (the layperson’s guide). A presentation file that summarizes the research also is available for viewing or download from the NCHRP 25-44 project page.

6 Finally, the research team suggests several recommendations and considerations for future research: • Further study reflections off sound-absorbing barriers, examining a broad range of absorptive barrier types and NRC values; • Use the measurement dataset to validate the single-wall reflections component of the new FHWA Traffic Noise Model, Version 3.0 (TNM 3.0); • Conduct before/after studies to assess the effect of the barrier in situ, using techniques developed in this project and others, as needed; • Incorporate some of these findings into various courses on highway traffic noise; • Evaluate time-based and time-above metrics to help understand other drivers of adverse community perception; • Further study the sound quality benefit of sound-absorbing barriers; a narrow-band analysis should be done to confirm that sound-absorptive barriers reduce comb-filtering effects; • Enhance the Barrier Reflections Screening Tool in one or more of the following ways: (1) include other propagation effects, (2) extend the tool to one-third octave band calculations, and (3) formalize a version of the tool that allows examination of reflections from homes; • Use the FHWA TNM 3.0 software to investigate the barrier reflection effects closer to the ground to show that sound-reducing propagation effects help to “expose” barrier- reflected noise; • Conduct listening trials to assess human reaction to reflected noise directly; and • Assess the effect of roadway geometry, such as curves, on the perception of sound from single-barrier reflections.

7 Introduction The Problem From 1963 through 2013, nearly 3,000 linear miles of high- way noise barriers had been constructed in the United States. Of these, at least 7% were sound-absorbing barriers. In con- trast, during the last 3 years of that period (2011–2013), sound absorption was used on at least 30% of the constructed barriers (FHWA 2016). For barriers on a single side of a highway, absorptive treat- ment can generally reduce the overall reflected sound levels at a receptor opposite the barrier by 1 to 2 decibels (dB). Although this amount is generally considered too small to be readily perceived, state highway agencies have received complaints from residents on the reflective side of hard bar- riers after construction (see Appendix A). This research seeks to quantify the sound reflected off in situ highway barriers compared to the sound received at similar locations without barriers by assessing the level, spectral, and sound quality dif- ferences between the two. The goal is to assess how diverse site conditions affect reflected sound. The issue of sound reflections off a highway noise barrier to receptors on the other side of the highway has arisen on many occasions, and in many different states, for some time. The problem arises when the community on one side of the highway qualifies for a barrier but residents on the other side of the highway do not qualify for noise abatement. If these residents are impacted by the highway noise but do not meet certain feasibility or reasonableness criteria for abatement, experiencing a noticeable change in the sound level caused by their neighbors receiving abatement may upset them further and make the problem worse. Several studies, especially those conducted by the Califor- nia Department of Transportation (Caltrans), have quanti- fied the problem (Appendix A). Most of these studies have considered the change in the A-weighted sound level in the community opposite the barrier. The difficulty with this approach is that changes that these residents experience may not be related to a simple increase in the overall A-weighted sound level. Assuming unobstructed propagation paths for the direct and reflected sound paths, physics says that the increase in the total sound level due to adding the reflections should be less than the 3 dB attributable to the doubling of the source energy. (In this report, the unit dB refers to a change in level, both for unweighted sound pressure levels, which are designated as dBZ per the International Standards Organi- zation, and A-weighted sound levels, which are designated as dBA.) Conventional thinking is that an increase less than 3 dB should be just barely perceptible. That conventional think- ing only applies, however, if the temporal aspects (i.e., time signature) and spectral content of the increased sound resemble that of the original sound. One hypothesis tested in this research is that the noticeability and annoyance caused by the reflections might be due to other factors. One example is that the spectral content of the reflected sound may differ from the spectral content of the direct sound. In particular, the higher frequencies are more likely to be specularly reflected (as opposed to diffusely reflected) back across the road. These higher frequencies may stand out more in the total received sound, changing the character of the sound. Given possible existing negative feelings about the highway among residents who did not qualify for a sound wall, a change in the sound character could be suffi- cient for those residents to experience increased annoyance from the traffic noise. The path of the reflected sound back across the road also may experience less attenuation than the path of the direct sound. Differences could be caused by the nature of the inter- vening terrain between the edge of the near lanes of travel and the receptors on the far side of the highway. Variations in the terrain could produce an increase in the A-weighted sound level or changes in the spectral content that could be perceived negatively. C H A P T E R 1

8 Another aspect of this phenomenon may be a factor that was noted in a study of a Caltrans project where sound absorption was added to a previously reflective far-side noise barrier along U.S. 101 in San Rafael, California (Menge and Barrett 2011). A resident observed, “It’s a significant change. . . . The white noise that you hear is gone. What’s missing is the ‘shhhhh.’” This comment supports the concept that higher frequency spectral content is enhanced by the barrier reflections, or at least is attenuated less than low-frequency content. The com- ment also suggests the potential effect of the reflected sound on the overall time history or time signature of the total received sound. When a single vehicle passes by in the absence of a far-wall barrier, the sound that is perceived originates from the vehicle’s location. When a reflective far wall is intro- duced, however, a receptor perceives not only the sound com- ing directly from the vehicle, but also the sound reflected off the far wall, which comes from a different point along the road (see Figure 1). The relationship between the actual (direct) source and the reflected source changes as the vehicle pro- ceeds through the area in front of the barrier. As a result, the time signature of the pass-by is lengthened. When multiple vehicles are present, the character of the normal rise and fall of the sound level of the vehicle pass-by also changes, affecting receptors’ ability to pinpoint the direction of the sound. For curved, irregular, or parallel barriers, this effect can be further heightened due to multiple reflections. This project’s principal investigator observed this phenom- enon on a parallel barrier project study for the Ohio Depart- ment of Transportation (DOT) in the Town of Silver Lake. Parallel barriers can potentially create many reflected sound paths. When a direct sound and first far-wall reflection reach the near wall, they can reflect back toward the far wall, then reflect back again, with each iteration of the reflections gener- ating new sound paths. Standing behind the near wall at the Ohio project, the investigator could not easily point to a single vehicle’s position on the roadway even if that vehicle was the only vehicle in the “canyon” between the two barriers. Instead of the traditional rise and fall of sound level during a vehicle pass-by, the sound level built up more quickly and dropped off more slowly when the contributions of each sound reflec- tion were added to the noise from the actual vehicle. Behind the barrier, the effect of the multiple reflections of the pass- by noise was a sustained “shhhhh” sound that resembled the “white noise . . . shhhhh” description of the California resi- dent. This similarity, plus other similar observations, suggest that the way reflections raise the level of background noise and make it difficult to identify sound direction accurately could cause people to perceive the overall sound as more annoying. Project Objectives and Approach Given the complicated nature of reflections off single highway noise barriers, research is needed to assess how diverse site conditions affect the nature of the reflected sound. To address this need, NCHRP Project 25-44, “Field Evaluation of Reflected Noise from a Single Noise Barrier,” had the following research objectives: • Determine the spectral noise level characteristics of the overall noise in the presence of a single reflective noise barrier for positions on the opposite side of a roadway through the collection of field measurements from diverse sites; and • Summarize and analyze the implications of the research results for purposes of understanding the actual and per- ceived effects of reflected noise. To fulfill these objectives, sound and other field data were collected at five locations dispersed throughout the United States. At these locations, simultaneous measurements were made at a Barrier site and at an adjacent No-Barrier site under equivalent source and meteorological conditions. An analysis of the resulting data was done by comparing the following metrics: • A-weighted and unweighted one-third octave band (spectral) sound levels. Changes in the equivalent sound level and statistical exceedance (Ln) descriptor levels Receptor Figure 1. Plan view of the relationship between direct and reflected sound paths to a receptor across the highway from a noise barrier.