National Academies Press: OpenBook

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

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." 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 1 - Introduction." 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 8
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Suggested Citation:"Chapter 1 - Introduction." 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 9

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9 under equivalent source and meteorological conditions were analyzed. • Acoustical spectrograms. Both frequency (spectral) and temporal variations, with and without a noise barrier pres- ent, 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 one-third 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 regard- ing residents’ response to single barriers is their reported annoyance. Using 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 to residents, it is appropriate to explore and compare the received sounds using available psycho- acoustic 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 straight- forward to compute given a high-sample rate record- ing 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. In this report, Chapter 2 addresses the project team’s research approach, describing the general methodology used to satisfy the project objectives. The subsequent chap- ters focus on the studied locations, results, findings, appli- cations, conclusions, recommendations, and suggested future research. Appendix A contains the literature review performed for this study. Appendix B presents a more detailed discussion of the analysis and the results of the Phase 2 (sound-absorbing barriers) study. Appendix C provides a more detailed com- parison of the Phase 1 and Phase 2 results that are sum- marized in the body of this report. Appendix D provides a brief user guide for the screening tool. A stand-alone PDF copy of Appendix D, a spreadsheet file containing the Barrier Reflections Screening Tool, a customizable file containing the lay person’s guide pamphlet, and online files for Appen- dices E, F, and G, which document the detailed protocols for Phase 1 and provide photographs showing each micro- phone position, the meteorological station, and the traffic data collection site for each Phase 1 and Phase 2 measure- ment site are all available for viewing or download from the NCHRP Research Report 886 webpage. Similarly, a presenta- tion file that summarizes the research can be found on the NCHRP 25-44 project page at www.trb.org.

10 Research Approach As noted in Chapter 1, this research had the following 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. • Summarize and analyze the implications of the research results for purposes of understanding the actual and perceived effects of reflected noise. Based on its understanding of these objectives and the nature of the problem, the research team investigated the changes in the broadband A-weighted sound levels and unweighted sound pressure levels and individual one-third octave band sound pressure levels between No-Barrier and adjacent Barrier sites. In addition, the following components were examined: (1) the spectral time signature of the signals of individual pass-bys with and without the far-side barrier, and (2) the difference in psychoacoustic sound quality metrics. Research Tasks This research consisted of six tasks in Phase 1 and five tasks in Phase 2: Phase 1 Task 1. Kickoff teleconference meeting and amplified work plan development; Task 2. Completion of literature review; Task 3. Development of study location selection criteria, identification of recommended study locations, and data collection, processing, and analysis protocols; Task 4. Data collection (measurements) and analysis for the first two study locations, including preparation of an interim report and presentation of it at an interim meet- ing with the technical panel; Task 5. Measurements and analysis for the remaining loca- tions and summary and analysis of implications of all research results; and Task 6. Preparation and delivery of draft final and final report and digital (PowerPoint) presentation. Phase 2 Task 7. Kickoff teleconference meeting and amplified work plan development; Task 8. Site selection and verification; Task 9. Data collection and initial data analysis, including preparation of an interim report and presentation of it at an interim meeting with the technical panel; Task 10. Final data analysis and development of a prototype Barrier Reflections Screening Tool and layperson’s guide (pamphlet); and Task 11. Preparation and delivery of draft and final amended report and digital (PowerPoint) presentation. Protocol for Data Collection and Analysis This study used four types of data analysis: 1. The FHWA Method, based on the Indirect Measured procedure in Chapter 6 of FHWA’s Measurement of Highway-Related Noise (Lee and Fleming 1996), by which simultaneous A-weighted and one-third octave band measurements are made at the Barrier site and an “equiv- alent” No-Barrier site; 2. Spectrograms; 3. Psychoacoustic metrics (Phase 1 only); and 4. Difference spectrograms and comb-filtering analysis (Phase 2 only). C H A P T E R 2

11 A single data collection process yielded the data used in all three analyses. The data collection process is discussed first. Then the data processing steps are outlined. Finally, the data analysis methodology for each type of data is discussed. In addition to the sound level data, the data collection for the FHWA Method included recording of audio in calibrated WAV files at each microphone position. The WAV files were used in this study for processing and analysis of the spectro- grams and the psychoacoustic metrics. The FHWA Method calls for equivalence of site geometry, noise sources, and meteorological parameters. As a result, the locations were chosen such that the No-Barrier and Barrier sites were adjacent. Because the sound level and frequency spectrum produced by traffic are affected by the pavement and the roadway grade, these factors were consid- ered in site selection and were required to be equivalent in the No-Barrier and Barrier locations. Two other source characteristics that can affect the sound level and frequency spectrum are the volume and speed of the traffic. Given that one goal in site selection was to avoid interchanges or intersections between the Barrier and No-Barrier sites, and because the Barrier and No-Barrier measurements were to be made simultaneously, any potential variations in traffic volume and speed between the two sites were minimized. However, traffic volumes and speeds vary over time. Likewise, experience indicates that meteorological conditions, particularly the wind speed and direction, can change even over relatively short time peri- ods. Therefore, time periods were grouped under equivalent source and meteorological conditions. Measurement of Highway-Related Noise (Lee and Fleming 1996) recommends a minimum of three “measurement repeti- tions” per site (with a preferred number of six measurement repetitions) under equivalent source and meteorological con- ditions. The challenge is that, while in the field collecting data, it can be difficult and time-consuming to demonstrate source equivalence and meteorological equivalence in real time. As a result, the research team used the following field protocol: • Collect 4 hours of sound level data at each site, in 1-second logging intervals, to be aggregated into 1-minute periods; • During the same period, video-record the traffic and measure speeds with a laser speed gun; and • During the same period, collect wind speed, wind direc- tion, and temperature data at two heights, to be averaged in 1-minute periods. As part of the data processing protocol, 4 hours of sound level data at the Barrier and No-Barrier sites were divided into 1-minute periods. Each period represents one “measure- ment repetition” for a particular combination of source and meteorological conditions. A method of determining source equivalency between periods—based on the reference microphone sound levels and the average vehicle speeds by direction of travel— was found to work well and was adopted for the study. Basing the source equivalence determination on the measured reference microphone data allowed the maximum tolerance between two periods to be 0.3 dB or less across all the locations. The wind data were processed to determine a vector wind speed (component of wind speed perpendicular to the road) and corresponding wind class (Upwind, Calm, or Down- wind) for each period. The temperature data also were pro- cessed to determine the corresponding temperature gradient class (Lapse, Neutral, or Inversion) for each period. These data were merged with the source data and sorted to deter- mine the combined equivalent source and meteorological data period repetitions. Equipment and Sound Level Descriptors A standardized data collection equipment package was used at all sites. The package comprised the following pieces of equipment: • Six Type I sound level analyzers with one-third octave band measurement capabilities with data loggers and audio recorders; • A meteorological data collection station with precision temperature sensors and precision quality anemometers capable of measuring wind speed in three dimensions at two heights (5 ft. and 15 ft.) above the ground; • A video camera and laser speed gun; and • Accessories, including a 94-dBZ calibrator, extension cables, windscreens, microphone tripods and stands, extension poles, and guy wires. Depending on the site characteristics and goals at any of the measurement locations, the community microphones could be positioned (1) at two distinct distances from the road or (2) at two distinct heights a single distance from the road. The microphones were assigned names to distinguish their positions (see Figure 2): • When two microphones were at different heights at the same distance, at the Barrier site the lower microphone was named BarCom03 and the upper microphone BarCom04. At the No-Barrier site, the lower microphone was named NoBarCom05 and the upper microphone NoBarCom06. • When the community microphones were at different distances, at the Barrier site BarCom04 was designa- ted as the more distant microphone. At the No-Barrier site, NoBarCom06 was designated as the more distant microphone.

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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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