Supplemental Guidance on the Application of FHWA’s Traffic Noise Model (TNM) (2014)

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74 C H A P T E R 1 1 11.1 Introduction As sound travels from its source, the speed and direction of wind can increase or decrease the amount of sound energy that arrives at the receiver relative to a calm condition. This is primarily due to the refraction of the sound toward the ground in a downwind case or upward in the upwind case, as illustrated in Figure 49. This refraction is caused by a gradi- ent in the wind speed (which affects net sound speed), with lower wind speeds near the ground due to drag and higher air speeds above ground. In downwind propagation, the higher wind speed farther above the ground would cause a higher net sound speed farther above the ground because the speed of sound would add to the wind speed, causing downward refraction. In the upwind direction, the wind is in the oppo- site direction of the propagation and the wind speed would be subtracted from the speed of sound for the net sound speed. Thus the increase in wind speed with height above the ground would cause a lower net sound speed and upward bending of the sound path. Like wind gradients, temperature gradients cause sound speed to vary with distance from the ground. This gradi- ent in sound speed causes refraction of the sound, which increases sound levels through the reduction of ground effect or the reduction of barrier attenuation or decreases sound levels by the creation of shadow zones. Negative tem- perature gradients, where the temperature decreases with height above the ground, are referred to as lapse conditions. Positive temperature gradients, where the temperature increases with height above the ground, are referred to as inversion conditions. Sound propagation paths for the typi- cal daytime lapse condition and a temperature inversion are shown in Figure 50. TNM does not currently incorporate the effect of wind speed and direction or temperature gradients. It is the research team’s understanding that there is no plan for imple- mentation in the near future. Instead, measurements, model- ing, and an understanding of current literature can help in evaluating these effects on sound propagation. Armed with effective tools, TNM users and state highway agencies can address concerns and questions about the effect of wind and temperature effects on noise levels in communities. Further, evidence of a prevailing wind condition or a daily inversion scenario and an understanding of its effects could inform local officials and residents of long-term trends in sound levels. A synthesis of the state of practice for analyzing the effects of wind speed and direction and temperature gra- dients on sound propagation would help to establish sound- level adjustments that may be appropriate based on various parameters (such as effect in relation to varying wind speed and direction, effect in relation to varying temperature gra- dients, distance from the road, effect in relation to shield- ing objects [e.g., noise barriers], etc.) and when to apply the adjustments. 11.2 Research Approach There is an extensive literature covering meteorological effects on sound propagation, but only a limited number of studies examine the effect on highway noise in a quantita- tive way. Examples of such studies are the Caltrans I-80 Davis OGAC Pavement Noise Study,38 the Arizona Department of Transportation’s Atmospheric Effects Associated with High- way Noise Propagation,39 and the Volpe Center’s Validation of Wind and Temperature Gradients 38 Illingworth & Rodkin, Inc., I-80 Davis OGAC Pavement Noise Study 12 Year Summary Report, prepared for the California Department of Transportation Division of Environmental Analysis, May 2011. 39 Saurenman, H., J. Chambers, L. C. Sutherland, R. L. Bronsdon, and H. Forschner, Atmospheric Effects Associated with Highway Noise Prediction—Final Report 555, FHWA-AZ-05-555, prepared for the Arizona Department of Transportation, October 2005.

75 FHWA’s Traffic Noise Model (TNM): Phase 1.42, 43 These stud- ies provide measured sound levels under various meteoro- logical conditions. In addition, Part C of the Caltrans report, Additional Calibration of Traffic Noise Prediction Models,44 describes a method of empirically determining a function to adjust measurement results to a calm wind condition. These studies are discussed in greater depth in Appen- dix J, which is available on the NCHRP Project 25-34 web page at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay. asp?ProjectID=2986. A sound propagation model other than TNM can easily help determine the effects of wind speed and direction and temperature inversion or lapse conditions for highway sites of interest. (For a brief and readable comparison of how the models mentioned here deal with weather conditions and noise barrier mathematics, see Barriers to Consistent Results: the Effects of Weather.45) The SoundPLAN46 computer pro- gram was used to run test scenarios using the Nord200047 sound propagation model, which accounts for detailed mete- orological effects. This model has been selected for this evalu- ation because of its ability to compute sound levels under a variety of wind and temperature conditions and because its calculations are validated with published studies. It is dis- cussed in further detail in Appendix J. Other well-established sound propagation models such as ISO 9613-248 and the General Prediction Method49 assume a moderate downwind condition in order to replicate an equivalent long-term average. Wind and temperature effects can be entered in these models only as a user-specified decibel Copyright © 1992 by John Wiley & Sons, Inc. Figure 49. Sound propagation under downwind and upwind conditions.40 Copyright © 1992 by John Wiley & Sons, Inc. Figure 50. Sound propagation under temperature inversion and lapse conditions.41 40 Beranek, L., Vér, I., Noise and Vibration Control Engineering: Principles and Applications, John Wiley & Sons, Inc., 1992. 41 Beranek, L., Vér, I., Noise and Vibration Control Engineering: Principles and Applications, John Wiley & Sons, Inc., 1992. 42 Rochat, J. L. and G. G. Fleming, Validation of FHWA’s Traffic Noise Model® (TNM): Phase 1, DOT-VNTSC-FHWA-02-01 and FHWA-EP-02-031, Acoustics Facility, John A. Volpe National Transportation Systems Center, Cambridge MA, August 2002. 43 Rochat, J. L. and G. G. Fleming, TNM Version 2.5 Addendum to Validation of FHWA’s Traffic Noise Model® (TNM): Phase 1, DOT-VNTSC-FHWA-02-01 Ad- dendum and FHWA-EP-02-031 Addendum, Acoustics Facility, John A. Volpe National Transportation Systems Center, Cambridge MA, July 2004. 44 Hendriks, R., Additional Calibration of Traffic Noise Prediction Models— Technical Advisory, Noise TAN-03-01, California Department of Transporta- tion, Division of Environmental Analysis, Hazardous Waste, Noise & Vibrations Office, August 2003. 45 Smith, M., Barriers to Consistent Results: the Effects of Weather, paper for Acoustics 2008 Geelong, Victoria, Australia, November 2008. 46 Braunstein + Berndt GmbH, SoundPLAN® User’s Manual, January 2012 (in- cluding update information for Version 7.2—November 2012). 47 Delta, Proposal for Nordtest Method: Nord2000—Prediction of Outdoor Sound Propagation, January 2010. 48 Technical Committee ISO/TC 43, Acoustics, Subcommittee SC1, Noise, ISO 9613-2:1996(E), International Organization for Standardization, Geneva, Swit- zerland, 1996. 49 Kragh, J., B. Andersen, J. Jakobsen, Environmental Noise from Industrial Plants General Prediction Method, Danish Acoustical Laboratory, Danish Academy of Technical Sciences, Report No. 32, 1982.

76 adjustment. Although like Nord2000, the Concawe50 model can compute the effects of wind speed and direction, it was not selected because no equivalent measurement validation studies were identified for it. The Nord2000 modeling was utilized to predict sound lev- els over a range of cases. Output from the various predictions has been described in terms of potential input for adjust- ments in TNM or simply as offsets from a condition with calm winds and neutral atmosphere with no temperature gradient in order to better understand potential variation in community noise levels and field measurements. 11.3 Research Tasks A SoundPLAN model of a typical highway geometry was created to document the effect of different meteorological conditions at various receiver distances and heights. The model assumed flat ground with a four-lane (two lanes traveling in each direction) highway with a typical mix of automobiles and trucks traveling at 60 mph. A string of receivers was placed at heights of 5 ft and 15 ft at the following distances from the roadway: 50 ft, 100 ft, 200 ft, 400 ft, 800 ft, and 1,600 ft. A noise barrier (height of 17 ft) was included in some runs. Multiple runs were computed by varying the presence of the noise barrier, changing the model to assume all hard or soft ground, and by varying the presence of trucks on the roadways. Further details of the model and the Nord2000 validation with measured sound levels under various atmospheric conditions are included in Appendix J. The combinations of variables described above were run in SoundPLAN with various wind and temperature condi- tions, and the results were compared to results under calm/ neutral atmosphere conditions. Moderate upwind and down- wind conditions were modeled by assuming a wind speed of 2.5 m/s (5.6 mph) at a height of 10 m above the ground. (Wind speeds and temperature gradients are reported pri- marily in metric (SI) units in this chapter because these parameters are nearly always reported in SI units and because the modeling was conducted with SoundPLAN, which is an SI-based model.) Strong upwind and downwind conditions were modeled by assuming a wind speed of 5 m/s (11.2 mph)51. Positive temperature gradients associated with inversion conditions were modeled by assuming +0.1°C/m and +0.5°C/m. Negative temperature gradients associated with lapse conditions were modeled by assuming -0.1°C/m and -0.3°C/m.52,53,54,55 While SoundPLAN includes the implementation of the Nord2000 model, it also includes the implementation of TNM algorithms, both with and without “bug fixes” that the SoundPLAN developers have made. As a point of com- parison, the results produced using the SoundPLAN imple- mentation of Nord2000 for the test cases using calm weather conditions were also run using the TNM implementation in SoundPLAN. The comparison of the results using Nord2000 and TNM indicated that the two models provide gener- ally consistent results. There is more conformity with hard ground and with the TNM results in general with the “bug fixes” SoundPLAN TNM implementation. These small dif- ferences are expected due to the different vehicle source emis- sion levels in Nord2000 and from differences in the sound propagation algorithms. These differences are discussed fur- ther in Appendix J. It should be noted that while there are differences between the calculated sound levels in Nord2000 and TNM, the point of the study was to determine the dif- ferences between various atmospheric conditions and calm/ neutral conditions using a roadway noise source model, and this has been successfully accomplished with Nord2000. 11.4 Outcome of the Research— Effect of Wind Speed and Direction and Temperature Gradients on Highway Noise Sources Table 14 provides the results of the modeled meteorologi- cal conditions relative to calm/neutral atmosphere condi- tions. Positive numbers indicate sound levels higher than sound levels with calm/neutral conditions, and negative numbers indicate sound levels lower than those with calm/ neutral conditions. Table 14 is broken into multiple sections based on various configurations of the variables automobiles/ trucks, hard ground/soft ground, with noise barrier/without noise barrier described above. TNM users are encouraged to 50 Marsh, K. J., “The Concawe Model for Calculating the Propagation of Noise from Open-Air Industrial Plants,” Applied Acoustics Vol. 15, No. 6, November 1982, pp. 411–428. 51 Rossing, T., Springer Handbook of Acoustics, Springer Science+Business Media, LLC, New York, New York, 2007. 52 Illingworth & Rodkin, Inc., I-80 Davis OGAC Pavement Noise Study 12 Year Summary Report, prepared for the California Department of Transportation Division of Environmental Analysis, 13 May 2011. 53 Saurenman, H., J. Chambers, L. C. Sutherland, R. L. Bronsdon, and H. Forschner, Atmospheric Effects Associated with Highway Noise Prediction—Final Report 555, FHWA-AZ-05-555, prepared for the Arizona Department of Trans- portation, October 2005. 54 Ying, S., Sound Intensity Attributed to Temperature Inversion at Night, Paper presentation at Noise-Con 87, Pennsylvania State University, State College, PA, June 8–10, 1987. 55 Kasper, P., R. S. Pappa, L. R. Keefe, and L. C. Sutherland, A Study Of Air-To- Ground Sound Propagation Using An Instrumented Meteorological Tower, NASA CR-2617, Prepared for the National Aeronautics and Space Administration, October 1975.

77 Automobiles and Trucks, Hard Ground, with Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −2 −4 6 11 −1 −1 3 8 100 5 −2 −4 6 10 −1 −2 3 9 200 5 −2 −3 5 10 −1 −2 3 10 400 5 −1 −3 4 9 −1 −2 3 11 800 5 −2 −6 3 8 −1 −4 2 13 1600 5 −4 −9 5 9 −3 −11 5 17 50 15 −3 −5 7 12 −1 −2 3 7 100 15 −2 −4 6 10 −1 −2 4 9 200 15 −2 −3 4 8 −1 −2 4 10 400 15 −1 −2 3 8 −1 −2 4 12 800 15 −1 −2 3 7 −1 −3 3 14 1600 15 −1 −5 6 9 −2 −10 6 17 Automobiles and Trucks, Soft Ground, with Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −3 −5 8 12 −1 −1 4 9 100 5 −3 −5 7 11 −1 −2 4 10 200 5 −3 −5 6 11 −1 −2 4 11 400 5 −5 −8 5 11 −2 −5 4 13 800 5 −5 −9 5 11 −3 −8 4 16 1600 5 −6 −11 5 9 −5 −12 5 18 50 15 −3 −6 8 12 −1 −1 3 7 100 15 −3 −5 6 10 −1 −2 3 9 200 15 −2 −4 5 9 −1 −2 3 10 400 15 −2 −3 3 8 −1 −3 3 12 800 15 −2 −5 2 6 −2 −6 3 14 1600 15 −2 −8 3 5 −4 -13 4 17 Automobiles and Trucks, Hard Ground, without Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 0 −1 0 0 0 0 0 0 100 5 −1 −2 0 0 0 0 0 0 200 5 −2 −5 0 1 0 0 0 1 400 5 −7 −11 1 1 0 −1 0 1 800 5 −13 −19 1 2 −1 −5 1 2 1600 5 −20 -25 2 2 −4 −11 2 4 50 15 0 0 0 0 0 0 0 0 100 15 0 0 0 0 0 0 0 0 200 15 0 −1 0 0 0 0 0 0 400 15 −1 −4 1 1 0 0 0 1 800 15 −6 −11 1 1 0 −3 1 1 1600 15 −12 −18 1 2 −2 −9 1 3 Table 14. Differences in sound levels relative to calm/neutral conditions.56 56 Source: Harris Miller Miller & Hanson Inc., 2013.

78 Automobiles and Trucks, Soft Ground, without Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −2 −3 3 3 0 −1 0 2 100 5 −3 −4 6 8 0 −1 1 4 200 5 −4 −6 10 12 −1 −2 2 8 400 5 −7 −9 13 14 −2 −4 3 11 800 5 −11 −14 14 15 −4 −8 4 12 1600 5 −16 −21 14 14 −7 −11 4 13 50 15 −1 −1 1 1 0 0 0 1 100 15 −1 −3 2 2 0 −1 1 2 200 15 −3 −5 4 6 −1 −2 1 4 400 15 −5 −8 8 10 −2 −4 3 8 800 15 −8 −12 11 13 −3 −7 4 11 1600 15 −13 -16 12 13 -7 −12 5 12 Automobiles Only, Hard Ground, with Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −2 −4 7 12 −1 −1 3 9 100 5 −2 −4 6 11 −1 −2 4 10 200 5 −2 −3 6 11 −1 −2 4 11 400 5 −1 −2 4 10 −1 −2 3 13 800 5 −2 −6 3 9 −1 −4 3 14 1600 5 −5 −10 6 10 −4 −12 6 18 50 15 −3 −5 8 13 −1 −2 3 7 100 15 −3 −4 6 10 −1 −2 4 9 200 15 −2 −3 5 9 −1 −2 4 11 400 15 −1 −3 3 8 −1 −3 4 13 800 15 −1 −2 3 8 −1 −3 4 15 1600 15 −1 −6 6 9 −3 −11 6 18 Automobiles Only, Soft Ground, with Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −3 −5 8 13 −1 −1 4 10 100 5 −3 −5 7 12 −1 −2 4 10 200 5 −3 −5 7 11 −1 −2 4 11 400 5 −5 −9 6 11 −2 −5 4 13 800 5 −5 −10 5 11 −3 −8 4 16 1600 5 −7 −11 5 9 -6 −13 5 19 50 15 −4 −6 8 13 −1 −1 3 8 100 15 −3 −5 7 11 −1 −2 4 9 200 15 −-2 −4 5 10 −1 −2 3 11 400 15 −2 −3 3 9 −1 −3 3 12 800 15 −2 −5 2 6 −2 −6 3 14 1600 15 -2 −8 3 5 −4 −13 4 17 Table 14. (Continued).

79 Automobiles Only, Hard Ground, without Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 0 −1 0 0 0 0 0 0 100 5 −1 −2 0 0 0 0 0 0 200 5 −3 −6 1 1 0 0 0 1 400 5 −8 −13 1 1 0 −2 0 2 800 5 −15 −21 2 2 −1 −6 1 3 1600 5 −22 −27 2 3 −5 −11 2 5 50 15 0 0 0 0 0 0 0 0 100 15 0 −1 0 0 0 0 0 0 200 15 −1 −1 0 0 0 0 0 0 400 15 −2 −5 1 1 0 0 1 1 800 15 −7 −12 1 2 0 −4 1 2 1600 15 −13 −19 2 3 −2 −9 2 3 Automobiles Only, Soft Ground, without Noise Barrier Receiver Distance (ft) Receiver Height (ft) Sound-Level Difference (dB) Wind Condition Temperature Condition Moderate Upwind (2.5 m/s) Strong Upwind (5 m/s) Moderate Downwind (2.5 m/s) Strong Downwind (5 m/s) Weak Lapse (−0.1°C/m) Strong Lapse (−0.3°C/m) Weak Inversion (+0.1°C/m) Strong Inversion (+0.5°C/m) 50 5 −3 −4 3 4 0 −1 1 2 100 5 −4 −6 7 9 −1 −2 1 5 200 5 −6 −8 12 14 −1 −3 3 9 400 5 −8 −10 15 16 −2 −5 4 12 800 5 −12 −15 16 17 −5 −8 5 14 1600 5 −16 −21 16 16 −8 −11 6 15 50 15 −1 −1 1 1 0 0 0 1 100 15 −2 −3 2 2 0 −1 1 2 200 15 −4 −7 5 6 −1 −3 2 5 400 15 −7 −10 10 11 −2 −5 4 10 800 15 −10 −13 13 14 −4 −9 5 13 1600 15 −13 −16 14 15 −8 −12 6 14 Note. Positive numbers indicate sound levels higher than those with calm conditions. Negative numbers indicate sound levels lower than those with calm conditions. Table 14. (Continued). use the data in these tables to explain the difference in sound levels for validation purposes and for explanation of sound levels for agency and public purposes. 11.4.1 Effect of Wind Speed and Direction on Highway Noise Sources Figures 51 and 52 are sample graphs of the sound-level dif- ferences among varying wind speeds and directions and calm conditions. Both figures show the results with automobiles and trucks at a 5-ft receiver height over soft ground. Figure 51 includes a noise barrier, and Figure 52 does not. Similar figures showing the differences relative to calm conditions for all of the various combinations in Table 14 are included in Appendix J, as well as figures showing the computed hourly equivalent sound level (Leq). 11.4.1.1 Effect of Receiver Height on Results Overall, the results show that sound levels in wind and calm conditions show a similar pattern of variance at 5-ft and 15-ft receiver heights. As would be expected, the variance in the results at the two receiver heights is more pronounced at greater receiver distances and under soft ground conditions. 11.4.1.2 Effect of Noise Barrier on Results The presence of a noise barrier in the model typically had a large effect on the results. However, the difference between the results with and without a noise barrier was affected more by wind conditions than by temperature conditions.

80 -30 -20 -10 0 10 20 10 100 1000 noitidnoC dni W ,ecnereffiD leveL dnuoS -C al m (d B) Distance (feet) calm up 2.5 m/s up 5 m/s dn 2.5 m/s dn 5 m/s Figure 51. Sound-level difference with noise barrier and varying wind and calm conditions (automobiles and trucks, 5-ft receiver, and soft ground). -30 -20 -10 0 10 20 10 100 1000 noitidnoC dni W ,ecnereffiD leveL dnuoS -C al m (d B) Distance (feet) calm up 2.5 m/s up 5 m/s dn 2.5 m/s dn 5 m/s Figure 52. Sound-level difference without noise barrier and varying wind and calm conditions (automobiles and trucks, 5-ft receiver, and soft ground).

81 11.4.1.3 Effect of Ground Type on Results The type of ground (either hard or soft) in the model also had a large effect on the results. As would be expected, the dif- ferences between the hard and soft ground cases varied much more at the greater receiver distances. 11.4.1.4 Effect of Truck Percentage on Results Varying the model to include some trucks or assuming no trucks had a relatively small difference on the results. The presence of trucks in the model was not one of the most sig- nificant variables. 11.4.2 Effect of Temperature Inversion and Lapse on Highway Noise Sources Figures 53 and 54 are sample graphs of the variance in sound level at varying temperature gradients under calm con- ditions. Both figures show the results with automobiles and trucks at a 5-ft receiver over soft ground. Figure 53 includes a noise barrier and Figure 54 does not. Similar figures show- ing the differences relative to calm conditions for all of the various combinations in Table 14 are included in Appendix J, as well as figures showing the computed hourly equivalent sound level (Leq). 11.4.2.1 Effect of Receiver Height on Results Overall, the results show that sound levels in wind and calm conditions show a similar pattern of variance at 5-foot and 15-foot receiver heights. As would be expected, the differences in the results at the two receiver heights is more pronounced at the greater receiver distances, and under soft ground conditions. 11.4.2.2 Effect of Noise Barrier on Results The presence of a noise barrier in the model typically had a large effect on the results. However, the difference between the results with and without a noise barrier was affected more by wind conditions than by temperature conditions. 11.4.2.3 Effect of Ground Type on Results The type of ground (either hard or soft) in the model also had a large effect on the results. As would be expected, the dif- ferences between the hard and soft ground cases varied much more at the greater receiver distances. 11.4.2.4 Effect of Truck Percentage on Results Varying the model to include some trucks or assuming no trucks had a relatively small difference on the results. The presence of trucks in the model was not one of the most sig- nificant variables. -30 -20 -10 0 10 20 10 100 1000 noitidnoC .p meT ,ecnereffiD leveL dnuoS -C al m (d B) Distance (feet) calm +0.5 C/m +0.1 C/m -0.1 C/m -0.3 C/m Figure 53. Sound-level difference with noise barrier, varying temperature, and calm conditions (automobiles and trucks, 5-ft receiver, and soft ground).

82 11.5 Combined Effects of Wind and Temperature Gradients on Highway Noise Sources The focus of this research was primarily the separate effects of varying wind and temperature conditions on sound levels from highway noise sources. Often, wind and temperature gradients sufficient to affect sound propagation do not occur at the same time.57 However, it is possible that some moderate wind and temperature conditions may occur simultaneously. These could have the effect of being additive, or in theory, cancel each other out. For example, in the case of downwind sound propagation and a temperature inversion, the sound levels would be greater than in the case of downwind propa- gation and no temperature gradient. It was not practical to model all the various combina- tions of wind and temperature conditions and compare them. However, some combinations of moderate tempera- ture inversion and lapse conditions were modeled with var- ious wind speeds and directions. Those results are included in Appendix J. -30 -20 -10 0 10 20 10 100 1000 noitidnoC .p meT ,ecnereffiD leveL dnuoS -C al m (d B) Distance (feet) calm +0.5 C/m +0.1 C/m -0.1 C/m -0.3 C/m Figure 54. Sound-level difference without noise barrier, varying temperature, and calm conditions (automobiles and trucks, 5-ft receiver, and soft ground). 57 Rossing, T., ed., Springer Handbook of Acoustics, Springer Science+Business Media, LLC, New York, NY, 2007.