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How Weather Affects the Noise You Hear from Highways (2018)

Chapter: Chapter 2 - Measurement and Modeling of Roadway Noise

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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
×
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Suggested Citation:"Chapter 2 - Measurement and Modeling of Roadway Noise." National Academies of Sciences, Engineering, and Medicine. 2018. How Weather Affects the Noise You Hear from Highways. Washington, DC: The National Academies Press. doi: 10.17226/25226.
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12 As part of this project, the research team conducted measurements along I-17 in Phoenix, Arizona, to quantify the effects of meteorology on highway noise. This chapter discusses how the measurements were made, the results of the measurements, and a comparison of the measure- ments to various highway noise models. Details of the data collection are found in Appendix B with detailed results in Appendix C. Measurement Methods The monitoring area, shown in Figure 8, included two locations: • No-Barrier—This was the primary monitoring location that did not have a highway noise barrier between the highway and the monitoring positions. It was south of State Road 74 and west of I-17. • Barrier—This was the secondary monitoring location, which had a highway noise barrier situated between the highway and the sound monitoring positions. It was north of West Dynamite Boulevard and west of I-17. No-Barrier Location The No-Barrier location (Figure 9) is generally flat, with either no vegetation closer in or low scrub further out. The area includes no development and the primary source of sound is either from animals (biogenic), wind, or traffic on I-17. The research team positioned microphones at 15 meters, 30 meters, 60 meters, 120 meters, 240 meters, 480 meters, and 960 meters, measured from the center of the nearest lane (Figure 10). At the 240-meter and 480-meters positions, microphones were placed at heights of 1.5 meters, 4.6 meters, and 7.6 meters. At the remaining positions, microphones were placed only at 1.5 meters. In addition to the microphones, the research team measured wind, temperature, pressure, and humidity. Figure 11 shows the locations of these measurements along with microphone positions. Barrier Location The Barrier location (Figure 12) is a small park extending from the highway. It is generally flat, with little vegetation and a gravel-like ground. Homes are situated to the north and south. The top of the highway barrier was 6.1 meters above ground and constructed of concrete. Due to the microphone positions in a slight relief of 1.5 meters, the effective height of the barrier was 7.6 meters above the microphone elevation. A frontage road parallels the highway, with a small C H A P T E R 2 Measurement and Modeling of Roadway Noise

Measurement and Modeling of Roadway Noise 13 barrier on the home side of the frontage road, but away from the part where the microphones were located. This would diminish sound from the frontage road but would not affect traffic noise coming over the barrier from the highway. A more detailed description of the location can be found in Appendix B. A reference microphone was placed 15 meters from the roadway, to the south, at a location that was not blocked by the highway noise barrier. The remaining microphones were placed at a 1.5-meter height at 60 meters, 90 meters, 120 meters, and 150 meters from the barrier (Figure 13). As indicated in Figure 14, meteorological instruments were placed 110 meters from the barrier. Meteorological Measurements The research team deployed an array of meteorological sensors to capture the on-site meteo- rological conditions throughout this measurement campaign. On the morning of March 4, 2016, Source: Google Maps and RSG Figure 8. Overview map of monitoring locations.

14 How Weather Affects the Noise You Hear from Highways Source: Wyle for NCHRP Project 25-52. a b Figure 9. Photos at the No-Barrier location (a) Near I-17 and (b) 500 meters from I-17. Source: Wyle for NCHRP Project 25-52. Figure 10. No-Barrier location, microphone positions.

Measurement and Modeling of Roadway Noise 15 Source: RSG for NCHRP Project 25-52. Figure 11. Aerial view of No-Barrier location, showing microphone and meteorological positions. Source: Wyle for NCHRP Project 25-52. Figure 12. Barrier location photo, looking from a microphone toward the highway barrier.

16 How Weather Affects the Noise You Hear from Highways Source: Wyle for NCHRP Project 25-52. Figure 13. Barrier location, microphone positions. Source: RSG for NCHRP Project 25-52. Figure 14. Aerial view of Barrier location, showing microphone and meteorological positions.

Measurement and Modeling of Roadway Noise 17 the measurement team captured several aerial photos of the measurement area via a hot-air balloon. The photo in Figure 15 is looking north toward the upper air measurement station and the No-Barrier measurement positions. Similarly, Figure 16 was taken on the same day near the No-Barrier 960-meter measurement position looking southeast toward Phoenix, Arizona. Note the density and height of the vegetation in relation to the 10-meter measurement tower. Pol- lution over Phoenix can be seen trapped against the mountains in the distance, illustrating the presence of a strong inversion. Source: NE Wind for NCHRP Project 25-52. Figure 15. Aerial view of the study area on March 4, 2016, looking north. Source: NE Wind for NCHRP Project 25-52. 960m Measurement Station Figure 16. Photo of 960-meter measurement position looking southeast toward Phoenix.

18 How Weather Affects the Noise You Hear from Highways Upper Air Measurement Station The “atmospheric boundary layer” is the part of the atmosphere adjacent to the ground and extending upward to a point where the atmosphere is no longer significantly influenced by the ground. The lowest part of the boundary layer is called the surface layer, which has a height of typically 100 meters. For the purposes of sound propagation from a highway, it is most impor- tant to quantify the meteorology in the surface layer. For this study, the research team measured boundary layer wind and temperature parameters using a Leosphere WindCube v2 Lidar system (Figure 17) and a MTP5-PE Temperature Profiler (Figure 18) at the upper air measurement station, located 1.2 kilometers east of the No-Barrier location and 5.5 kilometers north of the Barrier location. Source: NE Wind for NCHRP Project 25-52. Figure 17. Upper air measurement station with the Lidar system in the foreground sited atop ridgeline academy. Source: NE Wind for NCHRP Project 25-52. Figure 18. View of the MTP5-PE Temperature Profiler, looking south.

Measurement and Modeling of Roadway Noise 19 The Leosphere WindCube v2 Lidar system measures the Doppler shift of five infrared beams as reflected off aerosols such as dust, pollen, precipitation, and pollutants. From these beam returns, the instrument computes various wind parameters, by height, every second. For use in this study, the Lidar was configured such that it would record the horizontal wind speed, wind direction, and the northerly, easterly, and vertical wind speed vectors, at the 40-meter, 60-meter, 80-meter, 100-meter, 120-meter, 140-meter, 160-meter, 180-meter, and 200-meter heights above the ground. From these data, the research team then calculated other param- eters like wind shear (change in wind speed with height), directional veer (change in wind direction with height), and turbulence intensity (function of wind speed variance and scalar wind speed). In addition, the measurement team installed a MTP5-PE Temperature Profiler System. This technology measures the microwave thermal radiation at various angles for a series of scans. As thermal radiation changes with temperature, and thus elevation, each scan angle yields a differ- ent spectrum. These spectra are analyzed and the atmospheric temperature at varying heights above ground is calculated. For this study, the temperature profiler calculated the temperature in 5-minute samples every 25 meters from 0 meters through the 100 meters above ground, then at 50-meter intervals between 100 meters and 1,000 meters above ground level. No-Barrier Location Meteorological Equipment Tower-mounted meteorological equipment was deployed at several locations at the No-Barrier location. At the 120-meter position, a 10-meter mast was outfitted with precision ultrasonic U-V-W anemometers and aspirated thermometers at 1.5 meters and 7.6 meters above ground (Figure 19). The picture shows the lack of vegetation between the mast and the highway. At the 960-meter measurement station, an ultrasonic U-V-W anemometer was installed at the 10-meter height between February 24 and March 5, 2016 (Figure 20). All aspirated thermom- eters and ultrasonic U-V-W anemometer data were logged each second by a Campbell Scientific CR1000 logger. In addition, a RNRG Symphonie logger system was installed at the 960-meter measurement position at a height of 1 meter above ground. This system comprised a RNRG Symphonie logger, #110S thermometer, RH5X relative humidity sensor, and a BP-20 pressure Source: RSG for NCHRP Project 25-52. Aspirated Thermometer Ultrasonic U-V-W Anemometer Aspirated Thermometer Ultrasonic U-V-W Anemometer Figure 19. Image of the 120-meter meteorological and acoustical measurement position.

20 How Weather Affects the Noise You Hear from Highways sensor. The RNRG Symphonie system recorded in 10-minute intervals and was in operation from February 24 through March 7, 2016. Barrier Location Meteorological Equipment On March 5, 2016, the No-Barrier 960-meter measurement station U-V-W ultrasonic ane mo meter, CR1000 data logger, and meteorological mast were decommissioned and transferred to the Barrier location (Figure 21). The 10-meter meteorological mast was reduced to 2 meters above ground as the ground conditions at the barrier location were not conducive to anchoring guywires. To mirror the RNRG Symphonie system remaining at the 960-meter No-Barrier loca- tion, an additional RNRG Symphonie system was mounted to the Barrier location meteorological mast at a height of 1 meter above the ground, equipped with a temperature and humidity sensor. Measurement Results No-Barrier Location Figure 22 shows traffic counts every 5 minutes over the course of the monitoring, with the nighttime hours shaded in gray. The traffic counts are by vehicle type, where A are automobiles, MT are medium trucks, HT are heavy trucks, B are buses, and MC are motorcycles. As shown, during the weekdays (first two and last two days), there are distinctive morning peak (AM peak) and afternoon peak (PM peak) traffic periods. The AM peak has greater south- bound (SB) traffic and the afternoon peak hour has greater northbound (NB) traffic, but the vol- umes are approximately the same. The late-night periods have relatively little traffic compared to the daytime. The highest amount of truck traffic is midday. Source: RSG for NCHRP Project 25-52. Ultrasonic U-V-W Anemometer Symponie Temperature Pressure Humidity Sensors Figure 20. The No-Barrier location 960-meter measurement position showing the ultrasonic U-V-W anemometer and down tower symphonie temperature pressure and humidity sensors.

Measurement and Modeling of Roadway Noise 21 Source: Wyle for NCHRP Project 25-52. Symphonie Temperature Pressure Humidity Sensors Ultrasonic U-V-W Anemometer Figure 21. Image showing the Barrier location 2-meter ultrasonic U-V-W anemometer and 1-meter RNRG Symphonie system. Note the barrier present in the background. Source: RSG for NCHRP Project 25-52. Figure 22. Directional traffic volume on I-71, by vehicle type.

22 How Weather Affects the Noise You Hear from Highways Figure 23 shows the calculated sound power level of the highway. This is the sum of the sound emis- sions from all vehicles and is a function of vehicle speed, vehicle mix, and traffic volume. As shown, the sound power level varies by less than approximately 3 dB during daytime, and approximately 13 dB between the highest part of the day and lowest part of the night. Therefore, if meteorology would not affect sound levels, then one would expect the sound levels measured to vary accordingly. However, that is not the case with the actual sound measurements made in the field. Figure 24 shows the sound pressure level (as L905-min) measured at 480 meters from the highway. (The L90 is the sound level exceeded 90% of the time in a given period.) The AM peak hours have sound Source: RSG for NCHRP Project 25-52. Figure 23. Sum of I-17 vehicle sound power level (No-Barrier location). Source: RSG for NCHRP Project 25-52. Figure 24. Measured sound pressure level (5-minute L90) measured at 480 meters (1.5 meter height) at the No-Barrier location.

Measurement and Modeling of Roadway Noise 23 levels between 50 and 54 dBA and the PM peak hours have sound levels of 32 to 36 dB—an approximate 18 dB difference. This change in sound level cannot be explained by differences in traffic volume, mix, or speed as the highway sound power is roughly the same during the AM and PM peak hour. The change must therefore be mostly due to meteorological effects. As mentioned in Chapter 1, sound refraction in the atmosphere is primarily a function of the vertical sound speed gradient (i.e., how the speed of sound changes with height). This is calculated by combining the effects of the vertical temperate and wind profiles. In temperature inversions, such as those that commonly occur at night in the study area, the temperature would increase with height, creating a positive temperature-induced sound speed gradient (measured as the change of sound speed in meters per second per meter of height [m/s per m or s-1]). In downwind conditions, where the microphone is downwind of the roadway, the wind-induced sound speed gradient would generally be positive (assuming wind speed increases with height above ground). A positive sound speed gradient will result in downward refraction of the sound and a negative sound speed gradient will result in upward refraction away from the ground. Figure 25 shows the effective sound speed profile for the same period as Figure 24. As shown, the effective sound speed tends to be positive during the night and neutral or negative during the day. During the AM peak hour, the nighttime inversion is still in place, with positive sound speed profiles of 0.10 to 0.20 s-1. During the PM peak hour, the sun has heated the ground over the course of the day, creating a negative sound speed profile of -0.05 to -0.20 s-1. Given that highway sound power does not vary by more than approximately 3 dB during the day, and that the research team observed a swing upwards of 18 dB, this higher difference is explained through the change in the effective sound speed profile. Figure 26 plots the difference between the 480-meter and 15-meter position 5-minute Leq as a function of the measured 1.8- to 10-meter effective sound speed. As shown, the trendline of the data is in a characteristic S shape. The advantage of plotting the sound level difference between two microphones, and not an absolute sound level at a single microphone, is that the sound level difference is in good approximation unaffected by variations in sound emission, i.e., by Source: RSG for NCHRP Project 25-52. Figure 25. Effective sound speed profile for 1.8 to 10 meters above ground (during No-Barrier measurements).

24 How Weather Affects the Noise You Hear from Highways variations in traffic volume and speed. Therefore, the sound level difference varies primarily due to meteorological effects. Since meteorological effects are small at distance 15 meters, the sound level difference between 480 meters and 15 meters is a good measure of meteorological effects on the sound level at distance 480 meters. The scatter in the graph is due to several factors. One of the factors is that the effective sound speed difference on the horizontal axis does not represent a single atmospheric state but multiple atmospheric states (i.e., multiple combinations of wind direction, wind speed, and temperature). Of importance is also the fact that the effective sound speed only represents a single propagation direction (the direction of the line of microphones), while the other directions from other parts of the road are ignored. Despite this, the effective sound speed difference used here is a useful quantity with a considerable predictive power for the sound level. Other factors that contribute to scatter include: • Air turbulence. Turbulence due to wind shear and buoyancy will tend to increase sound levels in the shadow region. • Background noise. Though an extensive program was implemented, not all background noise could be filtered out of the data. • Spatial meteorological changes. The meteorological tower was 240 meters from this micro- phone. While representative, it is not an exact representation of the meteorology along the propagation path. • Other measurement error. There may be other small errors due to instrumentation and the temporal changes over time. For example, this compares an arithmetic average sound speed difference with an equivalent continuous average sound level, which may lead to the over- weighting of higher sound levels during any 5-minute period. • Source distance. The source is scattered over approximately 12 lanes of traffic with different lane distribution throughout the day. Source: RSG for NCHRP Project 25-52. Figure 26. Sound level difference between the 480-meter and 15-meter microphones as a function of the effective sound speed profile with third-order polynomial trendline.

Measurement and Modeling of Roadway Noise 25 Barrier Location The sound levels measured at the Barrier location behave differently than at the No-Barrier location. As shown in Figure 27, the traffic volume at the Barrier location showed a similar pat- tern to the No-Barrier location, with a distinctive diurnal pattern. Similarly, the highway sound power shown in Figure 28 is relatively constant throughout the day, within approximately 3 dB. However, the sound levels at the Barrier location (120 meters from barrier) do not substantially Source: RSG for NCHRP Project 25-52. Figure 27. Directional traffic volume on I-71, by vehicle type at the Barrier location. Source: RSG for NCHRP Project 25-52. Figure 28. Normalized sound power of the highway (Barrier location).

26 How Weather Affects the Noise You Hear from Highways change during the day. As shown in Figure 29, the measured sound levels follow along the same general pattern as the highway sound power from Figure 28. Thus, there is no indication of any substantial meteorological effect on highway noise at this position despite the sound speed pro- file going through the same diurnal pattern of nighttime temperature inversions and daytime ground heating (Figure 30). When the sound level difference between the 120-meter microphone position is plotted against the sound speed profile (Figure 31), the characteristic S-shaped curve is not apparent. However, at the 120-meter position at the No-Barrier location, the S-shaped behavior is clearly visible in the trendline. This indicates that the sound speed profile has a considerably smaller effect on the sound level behind the barrier, at least out to 150 meters. This follows with previous research showing the extent of the shadow zone behind barriers. Figure 29. Measured sound pressure level (5-minute L90) measured at 120 meters (1.5 meter height) at the Barrier location. Source: RSG for NCHRP Project 25-52. Source: RSG for NCHRP Project 25-52. Figure 30. Effective sound speed gradient for 1.8 to 10 meters above ground (during barrier measurements).

Measurement and Modeling of Roadway Noise 27 In this case, the research team concluded that sound levels at positions well inside the acoustic shadow region behind a tall barrier are less affected by meteorological effects than sound levels at unscreened positions are. Modeling Results Harmonoise Modeling TNM versions 2.5 and 3.0 assume an acoustically neutral atmosphere and neglect meteoro- logical effects other than atmospheric absorption. Actual noise levels under specific weather con- ditions will deviate from levels for a neutral atmosphere, especially as one moves further from the highway. To quantify the deviations, the research employed both a direct approach (i.e., by measuring the deviations) and a modeling approach. For the modeling approach, the research team used the Harmonoise model. Description of Harmonoise The Harmonoise model is a point-to-point sound propagation model and can be used to calculate sound propagation from a point source to a receiver. The model can be applied to traf- fic noise by dividing a road into segments. Each road segment is represented by a point source, to which the Harmonoise point-to-point model can be applied. The overall sound level at a receiver is the sum of the contributions from all the point sources. [59] This approach is similar to the road segmentation approach of TNM. The Harmonoise model was developed in the European project Harmonoise (2001 to 2004) and extended in the European project Imagine (2004 to 2006). A complete descrip- tion of the final model was published in Acta Acustica. [30] A distinguishing feature of Source: RSG for NCHRP Project 25-52. Figure 31. Comparison of sound level difference between Barrier and No-Barrier 120-meter sound level difference as a function of the effective sound speed profile with third-order polynomial trendlines.

28 How Weather Affects the Noise You Hear from Highways the Harmonoise model is that it considers meteorological effects, including atmospheric refraction, the scattering by atmospheric turbulence, and related effects on ground attenua- tion. Analytical reflection and diffraction formulas are used for the calculation of the excess attenuation. The effect of atmospheric refraction is considered through a coordinate trans- formation of the terrain profile. This means that the ground is shifted, depending on the (linearized) sound speed profile. For example, under a sound speed profile with positive gradient (such as an inversion with no wind), the ground would be curved upward (Figure 32). The effect of atmospheric turbulence is considered by a scattering contribution to the excess attenuation. Modeling in Harmonoise The research team modeled the No-Barrier and Barrier Locations in Harmonoise using the same geometry and site conditions as were used in the TNM modeling, to the extent possible. Details of the Harmonoise modeling can be found in Appendix D. In the modeling, a set of 30 different meteorological classes were defined, based on atmo- spheric stability and wind speed. It included five stability classes, S1 to S5, going from least stable (like a sunny day) to most stable (like a strong temperature inversion on a clear night). Six wind classes, from V1 to V6, go from strong downwind conditions to strong upwind conditions. Definitions of these classes are given in Appendix D. Harmonoise uses these meteorological conditions to estimate a sound speed profile. As shown in Figure 33, the tem- perature profile and vector wind speed profile can be combined to estimate a sound speed profile. The sound speed profile is then used to estimate the degree to which the sound rays would bend, as illustrated in Figure 32. Harmonoise Modeling at the No-Barrier Location Table 2 presents the results of the Harmonoise model under different meteorological condi- tions and distances from the highway. For this table, the meteorological effect is calculated as the sound level difference between the level for a refracting atmosphere and the level for a nonrefracting, or acoustically neutral, atmosphere. Consequently, positive values represent sound levels that are higher than the corresponding level in a neutral atmosphere, and nega- tive values represent sound levels that are lower than the corresponding level in a neutral atmosphere. Source: TNO for NCHRP Project 25-52. Figure 32. Illustration of how Harmonoise uses a curved ground to simulate a refracting atmosphere.

Measurement and Modeling of Roadway Noise 29 The values in Table 2 clearly illustrate the effects of wind and atmospheric stability on sound levels. • Sound levels are generally raised by downwind conditions and lowered by upwind conditions. • Sound levels are generally raised by stable conditions (S4, S5) and lowered by unstable conditions (S1, S2). The effects are largest at the largest distances in the table, since in general meteorological effects on sound propagation increase with increasing distance from the source. Figure 34 plots the meteorological effect by distance for wind vector class V3, which is under zero wind speed. As shown, the stable categories of S4 and S5 show increases in sound levels, which increase with distance, while the unstable categories show relatively large decreases in sound level starting at approximately 60 meters and decreasing in effect at and beyond 240 meters. The larg- est difference between downward refraction (S4, S5) and upward refraction (S1, S2) is 29 dB, and occurs at distance 120 meters. Similarly, Table 2 shows that the largest difference between strong downwind (V1) and strong upwind (V6) is 30 dB and occurs at distance 120 meters. This large difference originates primarily from the low value of the meteorological effect (-25 dB) for (a) (b) (c) (d) Figure 33. Illustration of how the vertical profiles of the temperature and vector wind determine the profile of effective sound speed.

30 How Weather Affects the Noise You Hear from Highways Vector Wind Class Stability Class Distance from Road (M) 15 30 60 120 240 480 960 V1 Strong Downwind S1 1 1 2 5 7 10 14 S2 1 1 2 5 7 11 12 S3 1 1 3 5 8 8 14 S4 1 1 3 5 6 7 10 S5 1 1 3 5 5 4 12 V2 Weak Downwind S1 0 -1 -4 -14 -19 -16 -9 S2 0 0 -2 -3 -2 1 2 S3 0 0 0 1 4 8 12 S4 1 1 2 4 8 7 13 S5 1 1 3 5 5 4 13 V3 Zero (Zero Wind Speed) S1 -1 -4 -18 -24 -22 -17 -9 S2 -1 -2 -16 -22 -22 -17 -9 S3 0 0 -1 -4 -8 -15 -8 S4 1 1 1 4 7 8 10 S5 1 1 3 5 4 5 12 V4 Zero (Crosswind) S1 0 -1 -2 -1 2 4 5 S2 0 -1 -2 0 3 5 9 S3 0 -1 -1 0 3 5 9 S4 0 -1 -1 1 3 5 9 S5 0 0 0 1 3 5 9 V5 Weak Upwind S1 -1 -5 -19 -24 -22 -17 -9 S2 -1 -3 -17 -23 -22 -17 -9 S3 0 -2 -15 -21 -22 -17 -9 S4 0 0 -8 -18 -20 -17 -9 S5 0 0 -1 -3 0 5 9 V6 Strong Upwind S1 -2 -10 -21 -25 -22 -17 -9 S2 -2 -9 -21 -25 -22 -17 -9 S3 -1 -8 -20 -25 -22 -17 -9 S4 -1 -8 -20 -25 -22 -17 -9 S5 -1 -7 -20 -25 -22 -17 -9 * Gray shading shows sound levels decreasing because of meteorology. Unshaded are zero or positive values, i.e., no change from neutral or an increase in sound levels. Source: TNO for NCHRP Project 25-52. Table 2. Values of the meteorological effect (effect of refraction on traffic noise level), for 30 meteorological classes and 7 distances from the road, modeled with Harmonoise (in dB).* Source: RSG for NCHRP Project 25-52. -25 -20 -15 -10 -5 0 5 10 15 20 25 10 100 1000 So un d Le ve l D iff er en ce fr om N eu tr al (d B) Distance (m) S5 S4 S3 S2 S1 Figure 34. Harmonoise, effect of stability on meteorological effect in zero wind speed (No-Barrier).

Measurement and Modeling of Roadway Noise 31 upward refraction. It will be shown later that the measurements yield a considerably smaller sound level difference between downward refraction and upward refraction (at most about 14 dB). This triggers the question whether approximations in the Harmonoise model are responsible for the difference between Harmonoise model results and measurement results. However, in Appendix D it is demonstrated that the more rigorous PE model yields results that agree with Harmonoise results for upward and downward refracting atmospheres. This suggests that Harmonoise approximations play only a limited role in the observed difference between model results and measurement results. Further investigations are needed to shed light on this issue. In Appendix D, Harmonoise runs are shown for changes in ground flow resistivity, turbulence, and percentage of trucks. These produced the following conclusions: • Ground flow resistivity. The ground flow resistivity is a function of the porosity of the ground. Porous grounds, like farm fields, have low flow resistivity and pavements have high flow resistivity. The research team varied the model from flow resistivities of 100 kPa s m-2 to 500 kPa s m-2 and found little difference in the results. The conclusion is that within that range, the effect of ground flow resistivity on the meteorological effect at this location was small. • Turbulence. The atmospheric turbulence strength is represented as the standard deviation of wind speed divided by the wind speed. Turbulence generally breaks up a shadow zone and increases the sound level in upwind or unstable conditions. The research team varied the Harmonoise turbulence parameter between moderate and high turbulence. For higher turbulence, the Harmonoise results showed considerably higher sound levels in upward- refracting conditions, as expected. For example, at 240 meters from the highway, the sound levels for the strong upwind class are 5 dB higher under higher turbulence than under mod- erate turbulence. • Vehicle mix. Vehicle mix is represented by the percentages of vehicle types in the traffic stream. Vehicle mix is important when calculating the meteorological effect because different vehicle types have different spectral characteristics. For example, heavy trucks have more low frequency sound than passenger cars. The research team varied the vehicle mix in the Harmo- noise model from 2% to 12% heavy trucks. The difference in the meteorological effect over that range was on the order of 1 dB. The research team concluded that vehicle mix, over this range, does not substantially impact the meteorological effect. Harmonoise Modeling in the Barrier Location As detailed in Appendix D, the Harmonoise model was also run for the Barrier location. The insertion loss of the Barrier was modeled to be between 17.3 dB and 19.7 dB, decreasing with distance from the barrier due to diffraction effects. The results of the modeling of the meteoro- logical effect are shown in Table 3. For upward-refracting conditions, such as with an unstable atmosphere or upwind conditions, the meteorological effect is smaller than for the No-Barrier situation. Under these conditions and distances around 100 meters, the meteorological effect for the barrier location does not drop below 9 dB, while for the No-Barrier location values as low as 25 dB were calculated. This difference can be traced to the fact that the noise barrier “disturbs” the formation of a sound shadow under upward-refracting conditions. The Nord2000 tables in NCHRP Report 791 show a similar difference between a situation without noise barrier and a situation with noise barrier. [35] Figure 35 plots the meteorological effect for just the wind class V3, zero wind speed under vary- ing meteorological conditions with the Barrier in place, from Table 3. As shown, the meteorologi- cal effect is almost nonexistent at 70 meters, and increases out to 160 meters. At 160 meters, the modeled difference between unstable (S1) and stable (S5) conditions is 17 dB.

32 How Weather Affects the Noise You Hear from Highways Vector Wind Class Stability Class Distance from Road (m) 70 100 130 160 V1 Strong Downwind S1 1 2 3 3 S2 1 2 3 4 S3 1 2 3 4 S4 1 3 4 5 S5 2 4 5 6 V2 Weak Downwind S1 -1 -2 -2 -3 S2 0 1 1 1 S3 0 2 2 3 S4 1 2 3 4 S5 1 3 5 6 V3 Zero (Zero Wind Speed) S1 -2 -5 -8 -8 S2 -2 -4 -6 -7 S3 0 0 0 0 S4 1 2 3 4 S5 1 4 6 9 V4 Zero (Crosswind Wind Direction) S1 -1 0 0 0 S2 -1 0 0 1 S3 -1 0 1 1 S4 0 1 2 2 S5 0 2 4 6 V5 Weak Upwind S1 -3 -6 -8 -8 S2 -2 -4 -6 -8 S3 -1 -3 -5 -7 S4 0 -1 -3 -4 S5 1 1 1 2 V6 Strong Upwind S1 -4 -8 -9 -8 S2 -4 -8 -9 -8 S3 -4 -7 -9 -8 S4 -3 -7 -9 -8 S5 -3 -7 -9 -8 * Gray shading shows sound levels decreasing because of meteorology. Unshaded are zero or positive values, i.e., no change from neutral or an increase in sound levels. Source: TNO for NCHRP Project 25-52. Table 3. Values of the meteorological effect, for 30 meteorological classes and 4 distances from the road, with noise barrier, modeled in Harmonoise (in dB).* -25 -20 -15 -10 -5 0 5 10 15 20 25 50 70 90 110 130 150 170 190 So un d Le ve l D iff er en ce fr om N eu tr al (d B) Distance (m) S5 S4 S3 S2 S1 Source: RSG for NCHRP Project 25-52. Figure 35. Harmonoise, effect of stability on meteorological effect in zero wind speed (with Barrier).

Measurement and Modeling of Roadway Noise 33 Comparison of Modeled to Monitored Sound Levels Comparison to “Neutral” Conditions From the standpoint of outdoor sound propagation, acoustically neutral would be a situa- tion where the vertical sound speed gradient is zero. That is, sound speed does not change with height. This can differ from a neutral atmospheric stability, such as used in air quality analyses, where temperature decreases adiabatically near the surface due to decreasing atmospheric pressure, such as found on a cloudy day or night. Most of the analyses conducted on the data collected in the field are based on a comparison of the sound levels measured at one position compared to a reference position located 15 meters from the centerline of the nearest lane. In this way, characteristics of the sound generated by the roadway can be ignored. For example, the pavement characteristics of the highway may change the amount of sound generated by vehicle passbys, but not how sound propagates from the 15-meter position to the other microphones. As a result, the research team focused on the change in sound level rather than the absolute sound level at a position. Figure 36 shows the standard deviation of the measured sound level difference from 15 meters at each microphone location. As shown, the standard deviation increases as one moves further from the highway in both Barrier and No-Barrier locations. At 120- and 240-meter positions, there are three microphone heights at the No-Barrier location, and the standard deviation increases with decreasing microphone height. The increase in standard deviation is due to the increased effect of meteorology as one moves further from the highway (i.e., the swings in sound level increase as the distance increases). These swings are due to both changes in the temperature profile and vector wind speed. Vector wind speed is the wind speed in the direction from the road to the microphone. For example, if the wind were blowing at 5 m/s from the roadway to the microphone position, then the vector wind speed would be 5 m/s. If the wind were blowing from the microphone to the roadway, then the vector wind speed would be –5 m/s. And if it were blowing along the roadway, the vector wind speed would be 0 m/s. 0 1 2 3 4 5 6 7 10 100 1000 St an da rd d ev ia tio n of s ou nd le ve l d iff er en ce re la tiv e to 1 5 m (d B) Distance (m) No-Barrier Barrier 4.6 m 4.6 m 7.6 m 7.6 m Note: Microphone height is 1.5 meters unless otherwise noted. Source: RSG for NCHRP Project 25-52. Figure 36. Standard deviation of sound level difference by distance to the highway.

34 How Weather Affects the Noise You Hear from Highways The effect that the temperature profile has on sound levels relative to the neutral condition is shown in Figure 37. In this figure, a temperature inversion is where the temperature increases with height and a lapse condition is where temperature decreases with height. As shown, inver- sions increase sound levels out to 480 meters, but this effect diminishes beyond that. In fact, at 480 meters and beyond, the worst-case condition is a weak inversion. This effect needs further investigation. On the other hand, lapse conditions result in decreasing sound level differences as one moves away from the roadway. In this case, the effect continues to increase beyond 480 meters. This is because the sound wave is bending upward, and there are no ground interactions to affect it. The effect of vector wind speed is shown in Figure 38. Downwind conditions tend to increase sound levels and upwind conditions tend to decrease sound levels. In this case, moderate upwind Source: RSG for NCHRP Project 25-52. Figure 37. Sound level difference, by temperature profile category and distance from highway. Source: RSG for NCHRP Project 25-52. Figure 38. Sound level difference, by wind speed/direction category and distance from highway.

Measurement and Modeling of Roadway Noise 35 conditions had a greater effect than strong upwind conditions. This is because intense winds can create turbulence in the air and this causes sound to scatter into the shadow zone. No data were collected in strong downwind conditions. Comparison to FHWA TNM The research team modeled the No-Barrier and Barrier locations in TNM using assumptions that represented actual field and traffic conditions at the sites. This was done to determine the departure of measurements made under nearly neutral conditions to what the FHWA TNM (Version 2.5) predicts. A digital terrain model provided elevations for TNM objects, including receivers (measure- ment locations), roadways, barriers, and ground contours. Modeled roadways were extended an adequate distance to the north and south of each location to prevent end effects and road- way widths were modeled to account for the pavement width and the near edge of the road. The I-17 pavement is an open-graded rubberized asphalt that is not a pavement type in TNM. Therefore, the models used open-graded asphalt concrete (OGAC) and the predicted sound levels were adjusted. Modeled TNM barriers include the I-17 median, jersey barriers, and rows of closely spaced residences at the Barrier location. At the No-Barrier and Barrier location, the ground was represented as loose soil. There were no completely neutral conditions (zero wind and temperature gradients) at the sites. However, there were some 5-minute periods at both the No-Barrier and Barrier locations that had close to neutral conditions where vector wind speeds were less than 1.25 m/s and the temperature gradient was less than 0.05 °C/m. The measured and predicted sounds levels at the reference microphone for three nearly neu- tral periods at the No-Barrier location and the three nearly neutral periods at the Barrier location were compared to develop an adjustment factor. This factor was applied to the predicted sound levels to account for the effects of the rubberized asphalt. This adjustment process may oversim- plify the effects of the rubberized asphalt—the effects could vary by vehicle type, distance, and frequency. However, the data needed to develop more accurate adjustments are not available and collection of the data was not included in the scope of this project. Figure 39 shows the sound levels from TNM (adjusted to account for the rubberized pavement; see Appendix D for details) and the measurement results at one nearly neutral 5-minute period. The modeled and measured levels are within 3 dB within 240 meters, but the measured levels drop more rapidly than the modeled levels beyond that. Figure 40 compares the measured and modeled levels for the Barrier location for a single nearly neutral 5-minute period. The measured sound levels are within 2 dB of the monitored sound levels. Comparison with Harmonoise Model No-Barrier Location As noted, the research team considered six vector wind classes (V1 to V6) and five stability classes (S1 to S5) for comparison to Harmonoise. The measurement results for these 30 combinations of classes are shown in Table 4. During the measurement period, 96% of the time was characterized as having clear skies. Since these stability categories are a function of the cloud cover and when it is day or night, the research team obtained few periods for stability classes that are dependent on some cloud cover. That is, 96% of our meteorological categories are in Stability Classes S1 (day) and S5 (night). With this caveat, the research team compared the measured meteorological effects to the

36 How Weather Affects the Noise You Hear from Highways Source: Bowlby & Associates for NCHRP Project 25-52. Figure 39. Comparison of measured sound level under neutral conditions (one 5-minute period near 6:00 p.m.) with TNM 2.5 model run for same conditions at the No-Barrier location. Source: Bowlby & Associates for NCHRP Project 25-52. Figure 40. Comparison of measured sound level under neutral conditions (one 5-minute period near 6:00 p.m.) with TNM 2.5 model run for same conditions at the Barrier location.

Measurement and Modeling of Roadway Noise 37 most pertinent modeling results from Table 2. The results are shown in Table 5. Positive numbers indicate higher modeled levels for that specific meteorological condition. The most systematic and dramatic deviation between the modeled and measured results is seen in the upwind conditions (V5, V6), where the model predicts significantly more attenuation at 60 meters and beyond. Note that upwind conditions during the measurements were observed approximately 25% of the time, but only approximately 5% of those periods fell into Stability Classes S2 to S4. The observed deviation between Harmonoise model results and measurement results was already discussed before. It was concluded that further investigations are needed to shed light on this issue. Part of the reason for the differences between Harmonoise results and measurement results is how day and night are defined for the purposes of stability. In this exercise, “day” was defined as the time between sunrise and sunset. However, this definition does not consider the time it takes for the sun to heat the ground and make the atmosphere unstable, or the time needed to make the atmosphere unstable after the sun sets. Figure 41 plots the sound pressure level normalized to 5,000 vehicles per hour versus the Harmonoise stability class. Normalization was done to clearly remove the effects of different vehicle volumes and mixes during each analysis period. As shown, there is a clear diurnal pattern to the meteorological effect, but the highest levels do not start until 1 to 2 hours after sunset, and continue for 1 to 2 hours after sunrise. As a result, Chapter 3 discusses other ways to consider stability when calculating meteorological effects. Vector Wind Class Stability Class Distance from Road (m) 15 30 60 120 240 480 960 V1 Strong Downwind S1 0 -1 -1 -3 S2 S3 S4 S5 V2 Weak Downwind S1 0 0 0 0 0 -1 -2 S2 S3 S4 0 2 5 8 3 -4 S5 0 1 2 4 7 6 3 V3 Zero Vector Wind (No Crosswind) S1 0 -1 -1 -1 -3 -2 -3 S2 S3 S4 -1 -1 -2 -3 S5 0 1 1 2 3 4 3 V4 Zero Vector Wind (with Crosswind) S1 0 -1 -2 -3 -6 -8 S2 S3 S4 S5 0 1 2 3 5 7 8 V5 Weak Upwind S1 0 -2 -3 -4 -6 -7 -7 S2 0 -2 -3 -3 -4 -5 S3 0 -2 -3 -3 -3 -4 S4 3 2 2 -1 S5 0 0 0 0 -1 -3 -5 V6 Strong Upwind S1 0 -3 -3 -5 -6 -4 -4 S2 0 -3 -4 -5 -3 -3 S3 0 -3 -4 -3 -3 -3 S4 1 -3 -4 -4 -2 0 S5 1 -2 -3 -3 -3 -3 Source: RSG for NCHRP Project 25-52. Table 4. Measured meteorological effect at the No-Barrier positions at receiver height = 1.5 meters (in dB).

38 How Weather Affects the Noise You Hear from Highways Vector Wind Class Stability Class Distance from Road (m) 15 30 60 120 240 480 960 V1 Strong Downwind S1 1 2 2 9 S2 S3 S4 S5 V2 Weak Downwind S1 0 -1 -4 -12 -16 -14 -8 S2 S3 S4 1 -1 -1 -2 3 16 S5 1 0 0 0 -2 -2 9 V3 Zero Vector Wind (No Crosswind) S1 -1 -3 -13 -21 -19 -16 -7 S2 S3 S4 2 1 5 9 S5 1 0 1 2 1 1 9 V4 Zero Vector Wind (with Crosswind) S1 0 0 -1 1 6 10 S2 S3 S4 S5 0 -2 -3 -3 -3 -3 0 V5 Weak Upwind S1 -1 -3 -12 -19 -16 -11 -3 S2 -1 -11 -17 -19 -13 -5 S3 0 -9 -14 -18 -14 -6 S4 -3 -9 -16 -17 S5 0 0 -1 -3 0 6 12 V6 Strong Upwind S1 -2 -6 -15 -20 -16 -14 -6 S2 -2 -14 -21 -17 -15 -7 S3 -1 -14 -20 -19 -15 -7 S4 -2 -13 -20 -18 -16 -10 S5 -2 -14 -21 -19 -15 -7 Source: RSG for NCHRP Project 25-52. Table 5. Sound level difference between Harmonoise modeling results and measurement data, No-Barrier (in dB). Source: RSG for NCHRP Project 25-52. 1 2 3 4 5 20 25 30 35 40 45 50 55 60 2/ 25 0 0: 00 2/ 25 1 2: 00 2/ 26 0 0: 00 2/ 26 1 2: 00 2/ 27 0 0: 00 2/ 27 1 2: 00 2/ 28 0 0: 00 2/ 28 1 2: 00 2/ 29 0 0: 00 2/ 29 1 2: 00 3/ 1 00 :0 0 3/ 1 12 :0 0 3/ 2 00 :0 0 3/ 2 12 :0 0 3/ 3 00 :0 0 St ab ili ty c la ss N or m al iz ed s ou nd p re ss ur e le ve l ( dB A ) Date/Time Normalized Lp Class Figure 41. Sound pressure levels normalized to 5,000 vph, at 480 meters, compared with Harmonoise stability class.

Measurement and Modeling of Roadway Noise 39 Barrier Location The Barrier location was monitored for approximately 40 hours. During this period, most of the time there were clear conditions, other than a short cloudy period with weak upwind winds. The measured meteorological effect shown in Table 6 is relatively small, ranging from -3 dB to +2 dB at 150 meters from the barrier. These effects are much smaller in magnitude compared with the Harmonoise model (Table 7). Comparison with Nord2000 The research team also compared the measured results to the Nord2000 tables presented in NCHRP Report 791. This comparison employed an identical methodology to that described for the Harmonoise modeling comparison, in that the research team used normalized sound levels to compare the deviation from the neutral condition. To generate the table and eliminate the interaction between wind and temperature conditions, the research team only considered tem- perature conditions when the wind condition was neutral and only considered wind conditions when the temperature condition was neutral. Although measurement data were more evenly distributed among the Nord2000 conditions than those used in the Harmonoise modeling, only a fraction of the data measured in the field are represented in the table (10% for the wind con- dition, 30% for the temperature condition) due to the lack of permitted interaction between temperature and wind conditions in the NCHRP Report 791 table. Also, no strong downwind periods were observed when the temperature condition was neutral. Table 8 shows the comparison of the measurement data to Table 14 from NCHRP Report 791 for Mixed Traffic, Soft Site, No Barrier. The distances were matched to the closest receiver distance Vector Wind Class Stability Class Distance from Barrier (m) 15 60 90 120 150 V1 Strong Downwind S1 – – – – – S2 – – – – – S3 – – – – – S4 – – – – – S5 – – – – – V2 Weak Downwind S1 – -1 0 -1 -1 S2 – – – – – S3 – – – – – S4 – – – – – S5 0 1 -1 2 V3 Zero Vector Wind (No Crosswind) S1 – 0 0 0 -1 S2 – – – – – S3 – – – – – S4 – – – – – S5 – 0 0 0 0 V4 Zero Vector Wind (with Crosswind) S1 – – – – – S2 – – – – – S3 – – – – – S4 – – – – – S5 – – – – – V5 Weak Upwind S1 – -1 -2 -3 -3 S2 – -1 -1 -2 -3 S3 – 0 0 -1 -2 S4 – 0 0 0 0 S5 – -1 -1 -1 -2 V6 Strong Upwind S1 – – – – – S2 – – – – – S3 – – – – – S4 – – – – – S5 – – – – – Source: RSG for NCHRP Project 25-52. Table 6. Measured meteorological effect at the Barrier positions at receiver height = 1.5 meters (in dB).

40 How Weather Affects the Noise You Hear from Highways Vector Wind Class Stability Class Distance from Barrier (m) 15 60 90 120 150 V1 Strong Downwind S1 – – – – – S2 – – – – S3 – – – – – S4 – – – – – S5 – – – – – V2 Weak Downwind S1 – -1 -1 -2 -2 S2 – – – – – S3 – – – – – S4 – – – – – S5 – 2 3 4 7 V3 Zero Vector Wind (No Crosswind) S1 – -2 -5 -8 -8 S2 – – – – – S3 – – – – – S4 – – – – – S5 – 1 4 6 9 V4 Zero Vector Wind (with Crosswind) S1 – – – – – S2 – – – – – S3 – – – – – S4 – – – – – S5 – – – – – V5 Weak Upwind S1 – -3 -5 -6 -5 S2 – -1 -3 -5 -6 S3 – -1 -3 -5 -6 S4 – 0 -1 -3 -4 S5 – 0 2 2 3 V6 Strong Upwind S1 – – – – – S2 – – – – – S3 – – – – – S4 – – – – – S5 – – – – – Source: RSG for NCHRP Project 25-52. Table 7. Sound level difference between Harmonoise modeling results and measurement data, No-Barrier (in dB). Receiver Distance (m) Receiver Height (ft) Sound Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind Strong Upwind Moderate Downwind Strong Downwind Weak Lapse Strong Lapse Weak Inversion Strong Inversion 15 5 -2 -4 3 – 0 -1 0 -2 30 5 -2 – 5 – 1 1 0 -2 60 5 -2 -4 9 – 0 0 1 -2 120 5 -4 -6 11 – 0 -1 1 -4 240 5 -6 -11 10 – 0 -3 0 -6 480 5 -9 -18 8 – -2 -5 -1 -9 120 15 -2 -5 7 – -1 -2 2 -2 240 15 -3 – 8 – 0 -3 1 -3 Source: RSG for NCHRP Project 25-52. Note: dashes indicate no data Table 8. Sound level difference between modeling results (NCHRP Report 791 Table 14 automobiles and trucks, soft ground, no barrier) and measurement data from the No-Barrier location.

Measurement and Modeling of Roadway Noise 41 specified in NCHRP Report 791 (the maximum discrepancy resulting from this comparison is 7 meters at the 480-meter location; heights are identical). As shown, there is good agreement between the measurements and predictions within 120 meters for the temperature conditions except for strong inversion. The measurement results indicate much smaller increases compared to neutral conditions than the levels modeled in NCHRP Report 791. However, the differences between the measurements and model predictions for wind conditions are much greater than temperature conditions, particularly for moderate downwind. Distances beyond 120 meters show the highest differences, especially for strong upwind, moderate upwind, and strong inver- sion conditions.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 882: How Weather Affects the Noise You Hear from Highways documents the meteorological effects on roadway noise propagation under different atmospheric conditions. Highway noise changes from day to day and hour to hour—not just because of variations in traffic volumes, vehicle mix, and speed, but also because of the weather. The report develops guidance to identify when atmospheric conditions should or should not be considered in noise analyses.

The report is accompanied a PowerPoint presentation and a tool called the AERMET sound speed profile calculator. The report also includes a brochure designed to communicate the concepts of the research to non-technical audiences. The brochure is made available in MSWord format to enable customization and the ability to insert an official logo and contact information. An Interactive Tool is also available for download. The interactive tool includes audio files that allow the user to hear differences in highway noise under various meteorological conditions.

Disclaimer: This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

Original data used to develop NCHRP Research Report 882 are available upon request. Send requests via email to Ann Hartell, ahartell@nas.edu, and include a short explanation of the intended use of the data (for example, name of research project, research sponsor, affiliation and location of research team, and general plan for publication of results).

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