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TNM V2.5 Update Sheet

Technical Manual: Part 1

This Technical Manual is for the Federal Highway Administration's Traffic Noise Model (FHWA TNM®), Version 1.0 -- the Federal Highway Administration's computer program for highway traffic noise prediction and analysis. Subsequent Technical Manual insert sheets address the changes implemented in FHWA TNM®, Version 2.5 A companion User's Guide describes how to use TNM [Anderson 1998]. In addition, a companion technical report documents the vehicle noise-emissions data base [Fleming 1995], and a companion, multi-phase validation report and associated addendums compare TNM performance against measured data [Rochat 2002].

Overview of TNM: TNM computes highway traffic noise at nearby receivers and aids in the design of highway noise barriers. As sources of noise, it includes 1994-1995 noise emission levels for the following cruise-throttle vehicle types:

Noise emission levels consist of A-weighted sound levels, one-third octave-band spectra, and subsource-height strengths for the following pavement types:

In addition, TNM includes full-throttle noise emission levels for vehicles on upgrades and vehicles accelerating away from the following traffic-control devices:

TNM combines these full-throttle noise emission levels with its internal speed computations to account for the full effect (noise emissions plus speed) of roadway grades and traffic-control devices.

Acknowledgments (TNM V2.5 UPDATE SHEET)

FHWA TNM® was developed in part by:

U.S. Department of Transportation
Federal Highway Administration

Robert Armstrong, Steven Ronning, Howard Jongedyk.

U.S. Department of Transportation
John A. Volpe National Transportation Systems Center, Acoustics Facility

Overall management, emission-data design/measurement/analysis, propagation-path development, program testing, User's Guide, Technical Manual, TNM Trainer CD-ROM:

Gregg Fleming, Amanda Rapoza, Cynthia Lee, David Read, Paul Gerbi, Christopher Roof, Antonio Godfrey, Shamir Patel, Judith Rochat, Eric Boeker, Michael Lau, Tom Kincaid, Clay Reherman, Andrew Malwitz, Benjamin Pinkus.

Harris Miller Miller & Hanson Inc.

Technical management, emission-analysis design, functional requirements, conceptual program design, acoustical algorithms, design/development/testing of acoustical code and vertical geometry, User's Guide, Technical Manual:

Grant Anderson, Christopher Menge, Christopher Rossano, Christopher Bajdek, Thomas Breen, Douglas Barrett, William Robert.

Foliage Software Systems, Inc.

Program design/specification/development/testing, development of horizontal geometry and interfaces, program documentation:

Ronald Rubbico, George Plourde, Paul Huffman, Christopher Bowe, Nathan Legvold.

Special contributors:

Vanderbilt University: William Bowlby -- emission-data design/measurement/analysis, vehicle speeds.4
Bowlby & Associates, Inc.: William Bowlby -- TNM Trainer CD-ROM and program testing; Geoffrey Pratt - program testing.
Serac Technology Group, Inc.: Theodore Patrick -- TNM Trainer CD-ROM
University of Central Florida: Roger Wayson -- emission-data design/measurement/analysis.
Florida Department of Transportation: Win Lindeman -- Funding and management of subsource-height study.
Florida Atlantic University: Stewart Glegg, Robert Coulson -- subsource height measurements.5
Maryland State Highway Administration: Kenneth Polcak -- emission data.
Ohio University: Lloyd Herman -- emission data.
Environmental Acoustics, Inc.: Harvey Knauer - program testing.
Emission-data state agencies: California, Connecticut, Florida, Kentucky, Maryland, Massachusetts, Michigan, New Jersey, Tennessee.

Design and Review Panel:

Domenick Billera, James Byers, Rudy Hendriks, Harvey Knauer, Win Lindeman, William McColl, Kenneth Polcak.

National Pooled-Fund Contributing States:

Arizona, California, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, New Jersey, New York, North Carolina, Ohio, Oregon, Pennsylvania, Tennessee, Texas, Utah, Virginia, Washington and Wisconsin.

The development work of Harris Miller Miller & Hanson, Foliage Software Systems, Vanderbilt University and the University of Central Florida was conducted in part under contract to Foster-Miller, Inc. Vanderbilt University and the University of Central Florida were also under contract to the Volpe Center.

1.2 Free Field Levels (TNM V2.5 UPDATE SHEET)

Characteristics of the free-field noise level computations include:

More details on the computation of vehicle speeds are given in Section 2.2 and Appendix B of this manual; details on the computation of free field levels are given in Section 2.3 and Appendix C.

1.3 Shielding and Ground Effects (TNM V2.5 UPDATE SHEET)

The TNM incorporates state-of-the-art sound propagation and shielding algorithms. These algorithms are based on fairly recent research on sound propagation over ground of different types, atmospheric absorption, and the shielding effects of barriers, berms, ground, buildings, and trees. The TNM does not account for atmospheric effects such as varying wind speed or direction or temperature gradients. The TNM propagation algorithms assume neutral atmospheric conditions. Characteristics of the propagation algorithms include:

More details on the computation of shielding and ground effects are given in Sections 2.4, 2.5, 2.6, and Appendix D of this manual.

1.4 High-level Flow Chart (TNM V2.5 UPDATE SHEET)

This section presents a flow chart to outline the overall flow of the TNM during sound level calculation. It is presented as Figure 1.

1.5 Parallel Barrier Analysis(TNM V2.5 UPDATE SHEET)

A two-dimensional multiple-reflections module has been included within the TNM for computing the degradation of barrier performance due to the presence of a reflective barrier on the opposite side of the roadway. The results from this module are generalized by the user to modify the TNM's results where multiple reflections exist. The module is most effective in computing the effects of sound-absorbing material on the surfaces of barriers or retaining walls. More details on the parallel barrier module are given in Appendix E of this manual.

An additional field study was undertaken to determine the effective source heights of various vehicles [Coulson 1996]. This study assigned two "sub-source" heights to each vehicle type. They are 0 meters (0 feet) and 1.5 meters (5 feet) above the pavement for all vehicles except heavy trucks, where the upper source is 3.66 meters (12 feet) above the pavement. The study also determined the ratio of sound energy distributed at the lower and upper heights as a function of frequency, vehicle type, and throttle condition (cruising or full throttle). Table 1 shows the percentage of total emission sound energy distributed to the upper source height at the low frequencies and at the high frequencies. In the middle frequency range, between 500 and 2000 Hz, the sound energy distribution is presented in Appendix A, including curves showing the sound energy split by frequency for each vehicle type.

Table 1. Sound Energy Distribution Between Sub- source heights3
Vehicle Type Operating Condition Percentage of Total Sound Energy at Upper Sub- source height: 1.5m (5 ft), except 3.66m (12 ft) for HT At Low Frequencies (500 Hz and below) Percentage of Total Sound Energy at Upper Sub- source height: 1.5m (5 ft), except 3.66m (12 ft) for HT At High Frequencies (2000 Hz and above)
Autos Cruise or Full Throttle 27% 2%
Medium Trucks & Buses Cruise 36% 6%
Medium Trucks & Buses Full Throttle 37% 11%
Heavy Trucks Cruise 57% 46%
Heavy Trucks Full Throttle 57% 48%
Motorcycles Cruise or Full Throttle 28% 2%

Further detail about the energy distribution is presented in Appendix A, Section A.4.4

2.2 Vehicle Speed Computation (TNM V2.5 UPDATE SHEET)

The TNM computes adjusted speeds based on the user input speeds, roadway grade, and traffic control devices. For level or down-grade roadways, TNM uses the speeds assigned to the roadway by the user (the "input speed"). For heavy trucks (only) on upgrades equal to 1.5 percent or more, TNM reduces the input speeds. The speeds are reduced depending on the steepness and length of the upgrade in accordance with speed-distance curves similar to those published for geometric design by the American Association of State Highway and Transportation Officials [AASHTO 1990 and TRB 1985]. The TNM speed-distance curves were calibrated to the speeds measured during the emission level noise measurement program. Appendix B describes the details of these computations and gives examples.

The TNM allows the user to enter the following traffic-control devices: traffic signals, stop signs, toll booths, and on-ramp start points. The reason for these devices is to allow a more precise modeling of vehicle speeds and emission levels under these interrupted-flow conditions. TNM will compute speeds all along any roadways with traffic control devices. These devices abruptly reduce speeds to the device's "speed constraint," for the device's "

3Note: The values in this table for autos, medium trucks, buses and motorcycles have been corrected; they were previously the ratio of upper to lower subsource heights, rather than the precentage of total sound energy at the upper subsource height. For heavy trucks, 20% more energy has been shifted to the upper subsource height.

Table 6. Constants for subsource-height split.
Vehicle type, i; Pavement type, p Full
Constants, For a user-defined vehicle, use the TNM equivalent vehicle to choose the relevant table row for these five constants
X         X X X X X X 0.373239 0.976378 -13.195596 39.491299 -2.583128
  X       X X X X X   0.579261 0.871354 -177.249214 558.980283 -0.026532
  X       X X X X   X 0.566933 0.93352 -25.497631 80.239979 -0.234435
    X     X X X X X   1.330000 0.08000 -204.84400 592.56800 -159.34400
    X     X X X X   X 0.850000 -0.33000 163.021000 -492.45100 -58.00500
      X   X X X X X   0.579261 0.871354 -177.249214 558.980283 -0.026532
      X   X X X X   X 0.563097 0.928086 -31.517739 99.099777 -0.263459
        X X X X X X   0.391352 0.978407 -19.278172 60.404841 -0.614295
        X X X X X   X 0.391352 0.978407 -19.278172 60.404841 -0.614295

A.4.2 User-defined vehicles (TNM V2.5 UPDATE SHEET)

For a user-defined vehicle, TNM substitutes the subsource heights for the built-in vehicle that the user designates as most similar. Table 6 mentions this substitution in the appropriate column heading.

A.5 Vertical Subsources, Free Field (TNM V2.5 UPDATE SHEET)

Next TNM eliminates the ground effects within these measured vehicle emissions. To do this, it multiplies each measured vertical subsource emission by the values in Table 7.


Equation 8

This equation calculates each vehicle types measured energy-mean emission spectrum, as if the vehicles passed by during measurements at 15 meters without any intervening ground type. Essentially, this equation eliminates the ground effects from the measured vehicle emissions.

The subscripts, ff, stand for free field. Physically, this last equation represents each vehicle type's measured energy-mean emission spectrum, as if the vehicles passed by during measurements at 15 meters (50 feet) without any intervening ground (that is, free field).

Table 7. Multiplier, m, for each built-in subsource height.
Freq (Hz) 50 63 80 100 125 160 200 250 315 400 500 630
Multiplier m, Height: 3.66m 0.30 0.32 0.36 0.44 0.52 0.69 0.95 1.78 1.00 0.32 0.40 0.25
Multiplier m, Height: 1.5m 0.26 0.27 0.27 0.28 0.30 0.33 0.38 0.48 0.62 0.79 1.12 1.58
Multiplier m, Height: zero 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.20
Freq (Hz) 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000
Multiplier m, Height: 3.66m 0.25 0.25 0.25 0.25 0.32 0.56 1.00 1.00 1.00 1.00 1.00 1.00
Multiplier m, Height: 1.55m 0.40 0.50 0.32 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Multiplier m, Height: zero 0.25 0.25 0.22 0.20 0.25 0.27 0.34 0.42 0.47 0.52 0.59 0.67

These values were derived by using propagation algorithms of TNM to determine the effect of the (absorptive) ground present during the emission-level measurements.

A.6 Plots of All Noise Emissions (TNM V2.5 UPDATE SHEET)

Figure 6 shows A-weighted sound-level emissions for TNM's built-in vehicle types, for average pavement and cruise throttle. The following figures plot all noise emissions, separately by vehicle type and throttle condition (cruise or full):

D.4.4 Diffraction function (TNM V2.5 UPDATE SHEET)

The complete diffraction term is defined by the following function:

Equation 30

The complete diffraction term function is very complex with many other factors, including the propagation path length shown in figure 61.

where L is defined as the propagation path length. (In Figure 61, which shows the diffraction geometry, L = r0 + r)

A source, barrier, and receiver are shown with the left and right faces labeled, along with the diffraction point, image source, and several other angles.
Figure 61. Diffraction geometry

D is multiplied by a sign function that is positive when the receiver is in the dark zone and negative when the receiver is in the bright zone. To adjust the diffraction field to make it consistent with empirical results, D is also multiplied by an adjustment factor, A. A is currently set to 1.2. The factor Q is included to account for the surface impedances at the diffracting edge (see Section D.4.5). This results in the following equation:

Equation 31

The diffraction term is adjusted to be consistent with empirical results by multiplying an adjustment factor, A, and a Q factor to account for surface impedances. A sign function makes the diffraction term negative when the receiver is in a bright zone and positive when in the dark zone.

Chi function:The chi (χ) function is used to pass information about the diffracting geometry to the Fresnelfunction. It takes into account the distances from the diffraction point for the effective source and the receiver, the angle formed about the diffraction point, and the top angle of the obstruction causing the diffraction. The χ function has the following formula


This appendix provides a comparison of TNM 1.0 results to measurements and to the model results of others. Comparisons are made to five different data sets, three of which involved point-source geometry, and the remaining two involved in-situ measurements of barrier performance along actual highways. The first comparison is with Embleton's model for reflection from ground of finite impedance [Embleton 1983]. The second is to measurements by Parkin and Scholes over grassland [Parkin 1965], the third is to measurements of a noise barrier by Scholes, also over grassland [Scholes 1971]. The fourth and fifth are to measurements of noise barrier performance at two different highway locations by Hendriks and Fleming, respectively [Hendriks 1991] [Fleming 1992]. Overall, the agreement with measurements is found to be very satisfactory.

Comparisons of results from later versions of TNM to measurements are presented in the TNM validation report [Rochat 2002]. An addendum to this report reviewing the performance of TNM 2.5 will be published in 2004.

G.1 Ground Reflection Model

The TNM's model for reflection coefficients is based on the approach of Chessell [Chessell 1977], which incorporates the single-parameter ground-impedance model first proposed by Delany and Bazley [Delany 1970]. Embleton, Piercy and Daigle further developed the model and conducted measurements to determine empirically the relationship between ground type and effective flow resistivity (EFR) [Embleton 1983]. Figures 71 through 74 present a comparison of the TNM model with Embleton's model for Embleton's published geometry and four values of EFR. The geometry was: source height = 0.31 meters (1.0 feet); receiver height = 1.22 meters (4.0 feet); source-to-receiver distance = 15.2 meters (50 feet). The values of EFR span the range from very soft ground (powder snow, EFR = 10 cgs Rayls) to hard ground (10,000 cgs Rayls).

Plotted in the figures are values of the "ground effect" in dB, which represents the difference between the free-field (no-ground) condition and the condition with the ground. At low frequencies, the ground adds up to 6 dB, due to pressure doubling. In the middle frequencies and over soft ground (EFR = 100 to 500) the fairly broadband "ground-effect dip" exhibits significant reductions in sound level due to destructive interference.

G.2 Measurements Over Grassland

The TNM's reflection model is compared with very carefully conducted measurements of sound propagation over grassland by Parkin and Scholes [Parkin 1965]. The atmospheric conditions for the measurements were a normal temperature gradient (no strong lapse or inversion) and zero vector wind (no components in the source-to-receiver direction). The ground surface at the site, called "Hatfield," was grass up to 5 centimeters (2 inches) high covering silty soil. The ground was especially flat, within ± 0.3 meters (1 foot) for more than 500 meters (1500 feet). The source was a jet engine at a height of 1.8 meters (6.0 feet) and the microphone heights were all 1.5 meters (5.0 feet) above the ground. One-third octave band sound level measurements were made at the following distances: 35 m (114 ft), 62 m (202 ft), 110 m (360 ft), 195 m (640 ft) and 348 m (1140 ft).


Menge 1991 Menge, C. W., G. Anderson, T. Breen, C. Bajdek, A. Hass. Noise Analysis Technical Report: Brooklyn-Queens Expressway, Queens Boulevard to Grand Central Parkway. Report No. 290800. Lexington, MA: Harris Miller Miller & Hanson Inc.,April 1991.

Moulton 1990 Moulton, C.L. Air Force Procedure for Predicting Aircraft Noise Around Airbases: Noise Exposure Model (NOISEMAP), User's Manual. Report No. AAMRL-TR-90-011. Wright-Patterson Air Force Base, OH: U.S. Air Force, February 1990.

Olmstead 1996 Olmstead, Jeffrey R., et. Al. Integrated Noise Model (INM) Version 5.1 User's Guide. Report No. FAA-AEE-96-02. Washington, DC: Federal Aviation Administration, December 1996.

Parkin 1965 Parkin, P. H. and W. E. Scholes, "The Horizontal Propagation of Sound from a Jet Engine Close to the Ground, at Hatfield," J. Sound Vib., vol. 2, no. 4, pp. 353-374, 1965.

Rochat 2002 Rochat, J. L. and G. G. Fleming. Validation of FHWA's Traffic Noise Model® (TNM): Phase 1. Report No. FHWA-EP-02-031 and DOT-VNTSC-FHWA-02-01. Cambridge, MA: John A. Volpe National Transportation Systems Center, Acoustics Facility, August 2002.

Scholes 1971 Scholes, W. E., A. C. Salvidge, and J. W. Sargent, "Field Performance of a Noise Barrier," J. Sound Vib., vol. 16, pp. 627-642, 1971.

TRB 1985 Transportation Research Board. Highway Capacity Manual. Special Report 209. Washington DC: National Research Council, 1985.

Updated: 07/15/2011
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