National Academies Press: OpenBook

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

Chapter: Chapter 1 - Background

« Previous: Summary
Page 3
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 3
Page 4
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 4
Page 5
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 5
Page 6
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 6
Page 7
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 7
Page 8
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 8
Page 9
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 9
Page 10
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 10
Page 11
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 11

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3 Individuals who live near highways probably notice that vehicular traffic sounds change hourly and daily. Some sound changes are attributable to the number of cars and trucks on the road, but meteorology also affects how sound travels or propagates from the roadway. In large part, meteorology affects how a sound wave bends as it propagates through the atmosphere (refraction) and as it passes over obstacles (diffraction). The study of the bending of a sound wave is not new. Acoustician Charles D. Ross indicated that sound refraction events, important for military operations, have been reported since 1666 during the Four-Day Battle. [1]. During this naval battle fought between the coasts of England and Holland, sounds of the battle were heard clearly at many points throughout England, but not at intervening points. Passengers on a yacht positioned between the battle and England heard nothing. Several other examples have been recorded since that time. Guns fired at the funeral of Queen Victoria in London in 1901 were heard in Scotland, but not across a wide region in between. The German bombardment of Antwerp in World War I was heard clearly for a 30-mile radius, then beyond 60 miles from the Belgian city, but not in between. In the United States, it is reported that refrac- tion of sound caused by temperature and wind gradients had a significant impact on the outcome of several Civil War battles (Gettysburg, Gaines Mill, Fort Donelson, Seven Pines/Fair Oaks, Iuka, Perryville, Chancellorsville, and Five Forks). Many other cases have been documented; however, for traffic noise, the field of study began more recently and is still being developed. In all cases, the atmospheric phenomena that cause these occurrences are important to predict the overall noise levels at defined locations. Perhaps the greatest source of error still experienced during modeling of traffic noise is refraction of the propagating sound due to atmospheric effects. Differences in wind speed and temperature with height, wind direction, and turbulence in the air can all influence the propagation of the wave, which can cause changes in direction and a large variance in local sound levels. Prediction of the overall impact on sound propagation is difficult due to the random nature of atmospheric occurrences that are not independent but have direct effects on other propagation phenomena. For example, the angle of interaction with the ground can change due to refraction and in doing so change the ground effects. For modeling traffic noise, several methods intended to account for atmospheric effects have been applied in the past, from simple correction values for predicted sound levels to complex equations that express the effects of refraction on sound propagation. But the general practice in the United States, in a regulatory setting, has been to ignore refraction effects on traffic noise. As a result, there can be a disagreement between measured and modeled results even after a complete review and verification of input parameters. This may often be due to atmospheric effects and may result in the application of additional resources if measurements need to be conducted again. C H A P T E R 1 Background

4 How Weather Affects the Noise You Hear from Highways Another problem often encountered is the inability to accurately model traffic noise for indi- viduals further removed from the immediate vicinity of the highway, but who are more affected by meteorological effects. Measurements have shown large fluctuations (and often increased sound levels) in these cases, but modeling has been problematic. In the United States, the preferred model for highway noise analysis is the FHWA’s TNM, but the TNM model does not currently model these types of atmospheric effects [2]. TNM versions 2.5 and 3 are complete in most aspects, but they assume an “acoustically neutral” atmosphere with no temperature or wind gradients nor turbulence effects. Only atmospheric absorption, which is primarily a function of distance, temperature, and humidity, is included. The use of TNM where adverse meteorological conditions are common can result in inaccurate noise predictions. This can lead to noise complaints, and under these circumstances, SHAs are unable to accurately model the increased noise from meteorological effects and have limited tools to explain the effects to the public. This chapter addresses the following topics related to atmospheric effects on traffic noise: • A comprehensive description of the salient atmospheric phenomena. • Past research applicable to atmospheric effects on traffic noise. • Requirements for what data need to be measured as inputs for more complete modeling. • Methods of modeling sound propagation in a variable atmosphere. • Past work that lends guidance and further requirements to the current research. Introduction to Meteorological Effects on Sound Propagation Description of Effects Sound waves travelling in the atmosphere are attenuated with increasing distance from the source. The attenuation is caused by: • Geometrical spreading (the loss of energy in the wave proportional to the inverse square of the distance, or 1/r2). • Ground effects (changes in sound level caused by the ground). • Screening (diffraction) and reflection by obstacles and irregular terrain. • Atmospheric effects. General descriptions of these effects are documents [3–10]. The results of experiments performed to assess these effects are available [11–17]. In this report, the focus is on two types of atmospheric effects: • Effects caused by air absorption of sound waves (by the medium of transport, air); and • Effects caused by wind and temperature gradients in the atmosphere (refraction). Atmospheric Absorption Absorption of sound by the air is well understood and can be calculated accurately [18–22]. Air absorption of sound waves is primarily dependent on the temperature and relative humidity (RH) of the air in Figure 1 (left). (Atmospheric attenuation is also a function of pressure, but the effect is negligible at typical temperature and humidity conditions.) For practical outdoor geometries, it is often a good approximation to calculate the air absorption from an average temperature and relative humidity [57]. Air absorption effects are frequency-dependent: typical values of the attenuation caused by air absorption range from 1 dB per kilometer at 200 Hz to 100 dB per kilometer at a sound frequency of 10,000 Hz (Figure 1).

Background 5 Using a typical frequency spectrum of a heavy truck at 65 mph, the calculated A-weighted sound pressure level at 1 km away (assuming no ground attenuation and no meteorological effects) are calculated as a function of relative humidity and temperature in Figure 1 (right). As shown, the sound level is lowest at high temperatures with low relative humidity while the highest sound levels occur at either low temperature with low relative humidity or moderate temperatures with higher relative humidity. The difference between the extremes, within normal ranges, is approximately 10 dB. Variations of air absorption due to temperature and humidity at higher elevations have a minor effect in practice. Thus, only the temperature and humidity near the ground need be known to make a good first-order approximation of the attenuation of sound due to atmo- spheric absorption of highway-related noise. Effects of Wind and Temperature Profiles The effect of how wind and temperature change with height above ground (gradients) is more complex. Vertical wind and temperature gradients influence the propagation of sound in two ways: • The bending of sound rays (refraction); and • The scattering of sound waves due to atmospheric turbulence. Because these refraction and scattering effects are not included in the current version of the FHWA TNM and because they have a greater effect on sound levels than atmospheric absorp- tion, they are the focus of this study. Refraction and scattering are explained briefly in this section. Source: RSG for NCHRP Project 25-52. Figure 1. (Left) Atmospheric attenuation by frequency and relative humidity (at 20°C and standard pressure) according to ISO 9613-1 [22] and (right) truck A-weighted sound level at 1 km as a function of temperature and relative humidity.

6 How Weather Affects the Noise You Hear from Highways Figure 2 illustrates the effects of refraction. The figure shows a sound source near the ground in an atmosphere with wind directed to the right. The wind speed increases with altitude causing the sound to travel faster, and this causes a curvature of sound rays. On the downwind side of the source, rays are curved downward, and higher sound levels typically occur due to multiple ground reflections. On the upwind side, rays are curved upward. Wind now slows the speed of the sound propagation, and a reduction in sound called a “shadow region” occurs (i.e., a region where no sound rays arrive). Sound levels in the shadow region are lower, typically 10 to 15 dB lower than on the downwind side of the source, depending on the distance from the highway. Of particular note is that the phenomena occurs because the wind speed is changing with height. The sound is not being “blown” per se. The effect of atmospheric turbulence also plays a role in the shadow region: sound energy is scattered into the shadow region by turbulent wind and temperature fluctuations, which tend to increase sound levels relative to a nonturbulent atmosphere. The effect of vertical temperature gradients is like the effect of wind speed gradients with the speed of sound affected by the changing temperature with height. The speed of sound is depend- ing on the temperature of the air which is quantified as: Equation 1. Speed of sound as a function of temperature. = 20.05 0.5c T where c is the speed of sound in meters per second (m/s) and T is the temperature in degrees Kelvin (K). The sound speed increases with increasing temperature. Therefore, where there is a change of temperature with elevation, sound will always bend toward the cooler region. Figure 3 illustrates these effects and shows that a decreasing temperature with height has a similar effect as Source: TNO and RSG for NCHRP Project 25-52. Figure 2. Schematic illustration of the effect of wind on sound propagation (not to scale). Source: TNO for NCHRP Project 25-52. Figure 3. Schematic illustration of the effect of temperature profile on sound propagation (not to scale).

Background 7 Source: RSG for NCHRP Project 25-52. Figure 4. Sound rays bending with cooler air above warmer air (not to scale). upwind conditions and increasing temperature with height has a similar effect as downwind condi- tions. Figure 4 shows the bending of sound rays when cooler air is above warmer air, such as would occur under a normal temperature gradient (normal adiabatic lapse condition). Figure 5 shows the bending of sound rays where the air above is warmer than the air below, as would occur on a clear night when there is a negative lapse rate, or temperature inversion. Under a normal lapse rate of about 0.5°C per 100 meters in elevation (i.e., a decrease of 0.5°C per 100 meters), the speed of sound would decrease by approximately 0.3 meters per second per 100 meters in elevation. In terms of their influence on the speed of sound, a vertical temperature gradient of 0.1° C per meter is equivalent to a wind speed gradient of 0.06 m/s per meter. One can calculate a vertical profile of effective sound speed by combining the wind speed and temperature profiles. The effective sound speed profile is the best predictor of sound refraction. Source: RSG for NCHRP Project 25-52. Figure 5. Sound rays bending with warmer air above cooler air (not to scale).

8 How Weather Affects the Noise You Hear from Highways The performance of noise barriers is also influenced by atmospheric effects. The performance of a barrier in some situations can be reduced by downward refraction, since sound is directed downward as it travels over a barrier along downward-curved sound rays and into the shadow zone. The performance of a barrier in the shadow region is expected to be reduced (i.e., less “insertion loss”). The effect of upward refraction is more complex. A further complication is that noise barriers not only screen sound, but also have a local effect on wind speed profiles, as shown in Figure 6. Since wind is forced to flow up and over the barrier, the wind speed profiles are distorted such that at and just downstream of the barrier large wind speed gradients occur. These gradients cause enhanced downward refraction and a reduction of barrier performance. This effect is referred to as RESWING: REfraction by Screen-induced WINd speed Gradients [23]. For natural noise barriers, such as earthen berms, the RESWING effect is smaller. Due to its sloped sides, wind flow over an earthen berm is smoother than wind flow over a wall resulting in smaller wind gradients. Summary of Previous Research The effects on sound from meteorology have been observed for some time. Approaches to modeling sound propagation under various meteorological conditions have advanced over the last 35 years. One of the first modeling approaches to account for meteorology was done for CONCAWE (Conservation of Clean Air and Water in Europe) in 1981 [113]. In this approach, C.J. Manning measured sound generated (as point sources) at various heights and distances under different atmospheric stability classes. Using a statistical analysis of the resulting sound levels, Manning developed meteorological adjustments for combinations of wind speeds and stability classes. Computational approaches to outdoor sound propagation were originally developed as an outgrowth of work done on underwater acoustics. The first conference bridging the underwater and land-based models was the First International Symposium on Long Range Sound Propa- gation in Diamondhead, Mississippi, in 1981 [114]. Topics considered included the Fast Field Program, Parabolic Equation Modeling, and Ground Effects. Jim Lawther proposed a model for predicting highway traffic noise, but the incorporation of refraction was not included in the model [115]. Numerical methods, such as parabolic equation (PE) modeling, boundary element modeling (BEM), and finite difference time domain (FDTD) models provide relatively precise results if the vertical sound speed profile and horizontal ground impedance profile are known in advance. Source: TNO for NCHRP Project 25-52. Figure 6. Schematic illustration of the effect of wind on the performance of a noise barrier on the downwind side of the source.

Background 9 These methods, described in more detail in Appendices A and B, are computationally intensive and not appropriate for engineering models. That is, they require a great deal of data that is usu- ally not available without great cost; these procedures also take too long to run to make them useful to the general practitioner in a regulatory setting. European Engineering Models for Traffic Noise Two engineering models that account for meteorology, Nord2000 and Harmonoise, were developed in Europe. • Nord2000 was developed by the five Nordic countries (Denmark, Finland, Iceland, Norway, and Sweden) in 2000 [74]. The model considers meteorology through the input of an effec- tive sound speed profile. For the case of strong downward refraction, or for long propagation distances, an approximate method is used to account for the occurrence of sound rays with multiple ground reflections. For the case of strong upward refraction, an approximate method is used to account for the occurrence of an acoustic shadow region. Long-term average levels can be calculated with Nord2000 by combining levels calculated for specific atmospheric conditions with meteorological statistics. • The Harmonoise model is the result of two successive European Union (EU) research projects: Harmonoise from 2002 to 2004 and Imagine from 2005 to 2006 [75, 76]. In several ways, the Harmonoise model is a further development of the Nord2000 model, but with respect to meteorological effects, Harmonoise follows a different approach than Nord2000 does. Harmonoise makes use of a coordinate transformation that replaces a system with a refracting atmosphere over flat ground by a system with a nonrefracting atmosphere and a curved ground surface (see Chapter A-1 of Appendix A). In other words, atmospheric refraction is replaced with ground curvature. The (curved) ground surface is represented by a set of segments, and the model considers sound rays with multiple ground reflections. While the Harmonoise model is useful for complex meteorological situations, the French “Nouvelle Methode de Prevision de Bruit” (NMPB) traffic noise model follows a simpler approach toward calculating meteorological effects: long-term average sound levels are calculated as a weighted average of a downwind sound level and a sound level for a neu- tral atmosphere. In this way, only two meteorological conditions need to be known, and a weighted average is taken of the two. Approaches in the United States In the United States, highway noise prediction for federally funded highway projects is governed by FHWA regulation 23 CFR 772, Procedures for Abatement of Highway Traffic Noise and Construction Noise. Section 9 of this regulation states, in part, (a) “Any analysis required by this subpart must use the FHWA TNM, which is described in “FHWA Traffic Noise Model” Report No. FHWA-PD-96-010, including Revision No. 1, dated April 14, 2004, or any other model determined by the FHWA to be consistent with the methodology of the FHWA TNM.” [97] (d) “In predicting noise levels and assessing noise impacts, traffic characteristics that would yield the worst traffic noise impact for the design year shall be used.” [97] The TNM model is the only model approved by the federal government for use in the United States for highway traffic noise prediction for federally funded projects. By regulation, model pre- dictions and abatement decisions are based, in part, on the traffic during the worst-case one-hour period for noise. The TNM model assumes a non-refractive atmosphere—an approximation that does not take account of the change with altitude of air temperature and wind speed.

10 How Weather Affects the Noise You Hear from Highways While this is the case, several studies in the United States have evaluated the effect of meteo- rology on highway traffic noise. Roger Wayson collected a large database of measured meteo- rological and sound level variables near a highway. [33] Wayson found, through multivariant curve fitting, that upwind and downwind propagation displayed different distributions of sound levels. Wayson developed empirical formulas for estimating the attenuation due to refraction in both upwind and downwind conditions. NCHRP Report 791: Supplemental Guidance on the Application of FHWA’s Traffic Noise Model (TNM) [35] highlights several studies done in the United States to account for meteorology in highway noise. • Caltrans’ I-80 Davis OGAC Pavement Noise Study [116] – In this study, data were collected over a 12-year period at two distances and heights from the highway. The data were used to show the effect of the vector wind speed on the difference between the reference microphone near the highway and distant microphones. • Arizona Department of Transportation’s (DOT) Atmospheric Effects Associated with Highway Noise Propagation [117] – In this report, Saurenman et al. investigated complaints of rela- tively high sound levels distant from a highway in Phoenix, Arizona. They collected traffic and meteorological data over several weeks, and concluded, in part, that the higher sound levels were due to nighttime inversion, with increases of 5 to 8 dB at distances greater than 400 meters from the highway. • Volpe Center’s Validation of FHWA’s Traffic Noise Model (TNM): Phase 1 [112] – This study is a database of sound levels measured at a range of distances from highways under various ground and wind conditions. In NCHRP Report 791, a series of tables based on Nord2000 modeling is provided to deter- mine the sound level difference between the neutral and refractive atmosphere for various meteorological scenarios. Table 1 provides an example of one of the tables and Figure 7 shows the results graphically. This approach is simple to use but may lack the desired flexibility in noise analysis and accuracy to base large decisions upon. Source: NCHRP Report 791: Supplemental Guidance on the Application of FHWA’s Traffic Noise Model (TNM), Table 14. Table 1. Nord2000 differences in sound levels relative to calm/neutral conditions [35].

Background 11 Pitfalls to Avoid Based on this overview from the previous section, it is apparent that modeling has multiple pitfalls that need to be avoided, whether due to extremes or just simple oversights. The extremes occur where a model may be based on sound theoretical analysis but is too complex and computer intensive to be adapted for normal highway project use. The opposite extreme is a simplified model with a high degree of uncertainty that makes multimillion-dollar decisions precarious. Other pitfalls may come from use of simplifications that result in a model that is applicable only in certain conditions, such as downwind conditions. For a model to be successful within the highway modeling regime, it should be robust, easy to use, based on applicable physics (although simplifications may be needed), and sufficiently accurate to be trustworthy in making real-world decisions. Figure 7. Nord2000 sound level difference without noise barrier and varying wind and calm conditions (automobiles and trucks, 5-ft receiver, and soft ground) [35] Source: RSG for NCHRP Project 25-52.

Next: Chapter 2 - Measurement and Modeling of Roadway Noise »
How Weather Affects the Noise You Hear from Highways Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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).

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!