Field Evaluation of Reflected Noise from a Single Noise Barrier—Phase 1 (2016)

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4 C H A P T E R 1 Introduction The Problem By the end of 2010, over 180 million square feet of highway noise barriers had been constructed in the United States, of which only 2% were sound-absorbing (Ref. 1). For barriers on a single side of a highway, absorptive treatment can generally reduce the overall reflected sound level at a receptor opposite the barrier by one to two decibels. While this amount is generally considered too small to be readily perceived, state highway agencies (SHAs) have received complaints from residents on the reflective side of hard barriers after construction (see Appendix A). This research aims to quantify the sound reflected off of in-situ highway barriers compared to the sound received at similar locations without barriers by assessing the level, spectral, and sound quality differences between the two. The goal is to assess how diverse site conditions affect reflected sound. For quite some time, on many occasions and in many different states, the issue of sound reflections off a highway noise barrier to receptors on the other side of the highway has arisen. The problem arises when the community on one side of the highway qualifies for a barrier but there are residents on the other side of the highway that for one reason or another do not qualify. The problem is often exacerbated by the fact that these residents may be impacted by the highway noise yet do not meet certain feasibility or reasonableness criteria for abatement. For them, experiencing a noticeable change in the sound level caused by their neighbors receiving abatement may upset them further. There have been a number of studies, especially by the California Department of Transportation (Caltrans), to quantify 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 is that the change that these residents are experiencing 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 the addition of the reflections should be less than the 3 dB attributable to the doubling of the source energy. (In this report, the unit of dB refers to a change in level, both for unweighted sound pressure levels (designated as dBZ per the International Standards Organization (ISO)) and A-weighted sound levels (dBA)). Conventional thinking is that an increase less than 3 dB should be just barely perceptible. However, that conventional thinking only applies if the temporal aspects (i.e., time signature) and spectral content of the increased sound is similar to that of the original sound. One hypothesis 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 be different than the spectral content of the direct sound. In particular, the higher frequencies, which are more likely to be specularly reflected (as opposed to diffusely reflected) back across the road, may now stand out more in the total received sound, hence changing the character of the sound. Although the potential spectral change is not expected to change substantially the perceived annoyance of the highway noise, the combined effect of negative feelings about the highway, particularly for neighborhoods that did not qualify for a sound wall, in combination with a noticeable change in the sound character, could be sufficient for residents to perceive that the change has substantially increased the annoyance of the traffic noise. It is also possible that the path of the reflected sound back across the road may experience less attenuation than the direct sound perhaps because of the nature of the intervening terrain between the edge

5 of the near lanes of travel and the receptors on the far side of the highway. This could lead to an increase in the A-weighted sound level or changes in the spectral content that could be perceived negatively. There is also another aspect of this phenomenon that may be coming into play. It is brought to light by a comment reported by D. Barrett of the research team and C. Menge in a presentation at the 2010 summer meeting of the Transportation Research Board Committee ADC40 (Transportation-Related Noise and Vibration) (Ref. 2). The study involved a Caltrans project where sound absorption was added to a previously reflective far-side noise barrier along U.S. 101 in San Rafael, CA. A resident was quoted in the Marin Independent Journal in January 2010, “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 attenuated less than low frequency content. It 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, one hears the sound coming exactly where the vehicle is located. However, as shown in Figure 1, when a reflective far wall is introduced, not only does one hear the sound from the direct vehicle but ones hears it coming off of the far wall—and thus from a different point along the road. The relationship between the actual source and this reflected source changes as the vehicle proceeds through the area in front of the barrier. As a result, the time signature of the passby is lengthened and, in the presence of multiple vehicles, it changes the character of the normal rise and fall of the sound level of vehicle passby, and affects the one’s ability to pinpoint the direction of the sound. For curved or irregular barriers, the effect can be heightened even further due to multiple reflections. Figure 1. Plan view of the relationship between direct and reflected sound paths to a receptor across the highway from a noise barrier. This phenomenon is exactly what was observed by this project’s principal investigator on a parallel barrier project study for Ohio DOT in the Town of Silver Lake. With parallel barriers, there are potentially many reflected paths of the multiple images created by the reflections in addition to just the direct and first far-wall reflection in the single far-wall situation. As a result, when standing behind the near wall, he 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 passby, the sound level built up more quickly and dropped off more slowly when the contributions of each image were added to the noise from the actual vehicle. The effect is a sustained “shhhhh” sound behind the barrier. This and other similar observations of the sustaining of the passby noise seem quite similar to the “white noise…shhhhh” comment of the California resident. It is possible that the way in which reflections make it difficult to accurately identify the direction of the sound and raise the background level could cause humans to perceive the sound as more annoying.

6 Project Objectives and Approach Given the complicated nature of reflections off of single highway noise barriers, research is needed to assess how diverse site conditions affect the nature of the reflected sound. To address this, the objectives of this research are to: 1. 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 2. Summarize and analyze the implications of the research results for purposes of understanding the actual and perceived effects of reflected noise. To carry out these objectives, sound and other field data were collected at five locations throughout the U.S. At these locations, simultaneous measurements were made at a barrier site and at an equivalent adjacent site without a barrier under equivalent source and meteorological conditions. An analysis of this data was done by comparing: • A-weighted and unweighted 1/3 octave band (spectral) sound levels - Changes in the equivalent sound level and statistical exceedance (Ln) levels under equivalent source and meteorological conditions were analyzed. • Acoustical spectrograms - Both frequency (spectral) and temporal variations, with and without a noise barrier present, were examined visually using spectrograms, which are time histories of spectral data. This type of visualization can help reveal variations that may not be apparent when examining average overall A-weighted or 1/3 octave band sound levels for blocks of data. The variations found can help to focus data analysis and explain the effect of barrier-reflected noise. • Psychoacoustic metrics - A fundamental question regarding residents’ response to single barriers is their reported annoyance. The use of sound levels as descriptors limits the interpretation of results to energy metrics. However, other elements or complexities are incorporated into the total signal from single-barrier sites that are not present at the No Barrier sites. To the extent that these elements may be annoying residents, it is appropriate to explore and compare the received sounds using available psychoacoustic metrics. In the psychoacoustics literature, the basic metrics of Loudness, Sharpness, Roughness, and Fluctuation Strength have been combined into reliable predictors of annoyance. These metrics are straightforward to compute given a high-sample rate recording of the received sound. Therefore, it is worthwhile to examine whether these metrics can be used to identify statistically significant differences between Barrier and No Barrier sites. Chapter 2 addresses the research approach, describing the general methodology used to satisfy the project objectives. The subsequent chapters then focus on the studied locations, results, findings, applications, conclusions, recommendations and suggested future research. Appendix A is the literature review performed for this study. Appendix B is the Detailed Protocol and Results; it is a much more detailed presentation of the results of the analysis that led to the findings in this report. Appendix C is a complete catalog of the site photos for each microphone position, the meteorological station and traffic data collection station for each study location.