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 following cruise-throttle vehicle types:

• Automobiles: all vehicles with two axles and four tires – primarily designed to carry nine or fewer people (passenger cars, vans) or cargo (vans, light trucks) – generally with gross vehicle weight less than 4,500 kg (9,900 lb);
• Medium trucks: all cargo vehicles with two axles and six tires – generally with gross vehicle weight between 4,500 kg (9,900 lb) and 12,000 kg (26,400 lb);
• Heavy trucks: all cargo vehicles with three or more axles– generally with gross vehicle weight more than 12,000 kg (26,400 lb);
• Buses: all vehicles designed to carry more than nine passengers; and
• Motorcycles: all vehicles with two or three tires and an open-air driver/passenger compartment.

• Dense-graded asphaltic concrete (DGAC);
• Portland cement concrete (PCC);
• Open-graded asphaltic concrete (OGAC); and
• A composite pavement type consisting of data for DGAC and PCC combined.

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

• Stop signs;
• Toll booths;
• Traffic signals; and
• On-ramp start points.

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.TNM propagates sound energy, in one-third-octave bands, between highway systems and nearby receivers, Sound propagation takes the following factors into account:

• Atmospheric absorption;
• Divergence;
• Intervening ground its acoustical characteristics and its topography;
• Intervening barriers: walls, berms and their combination;
• Intervening rows of buildings; and
• Intervening areas of heavy vegetation.

During calculation, TNM perturbs (increases /decreases) intervening barrier heights up and down from their input height, to calculate for multiple heights. Then during acoustical design of selected barriers, combined with selected receivers, TNM displays sound-level results for any combination of height perturbations, where a perturbation is defined as the height increment that a noise barrier’s input height is increased (perturbed up) or decreased (perturbed down) during barrier design. It also contains an input-height check, to determine if noise barriers break the lines-of-sight between sources and receivers. In addition, it provides summary cost and benefit information for each barrier design, from user-supplied unit barrier costs and land-use information.

For selected cross sections, TNM also computes the effect of multiple reflections between parallel barriers or retaining walls that flank a roadway. The TNM user can then enter the computed parallel-barrier degradations as adjustment factors for individual receivers in TNM’s calculation of receiver sound levels.TNM computes three measures of highway traffic noise:

• L_aeq1h: hourly A-weighted equivalent sound level (1HEQ);
• L_dn: day-night average sound level (DNL); and
• L_den: Community Noise Equivalent Level (CNEL), where “den” stands for day / evening / night.

• Sound-level contours;
• Noise Reduction, i.e., insertion-loss, contours for noise barriers; and
• Level-difference contours between any two noise-barrier designs.

To aid during input and to document the resulting input and barrier designs, TNM shows the following graphical views:

• Plan views;
• Skew section;
• Perspective views, including a specialized perspective view for noise-barrier design; and
• Roadway profiles.