Office of Planning, Environment, & Realty (HEP)

# Traffic Noise Model: Frequently Asked Questions FAQs

1. Can I use the Parallel Barrier analysis module to calculate single wall reflections?

Some TNM users have simulated single barrier reflections, by making one of the parallel barriers very small (i.e.: 0.3 meters, 1 ft.). Use the results from this parallel barrier calculation and apply it as an adjustment factor. This process has not been validated.

2. Can TNM model more than 2 parallel barriers?

Yes, it can be modeled as a single cross section in the Parallel Barriers module. However, keep in mind that when a parallel barrier section contains two separate vertical surfaces offset on the same side of a road (i.e. a retaining wall near the edge-of-pavement and a barrier at the right-of-way), (1) TNM parallel-barrier accuracy is degraded somewhat for receivers on that same side of the roadway (TNM may under-compute or over-compute the noise increase), and (2) TNM may under-compute the noise increase for receivers on the opposite side of the roadway. Please refer to the diagram below:

3. How often should I analyze parallel barriers if the cross-section shape changes along the roadway?

If the cross-section shape changes along the roadway, analyze parallel barriers every 100 meters (328 ft.). For close-by receivers, adopt results of the nearest analysis. For receivers farther back, dB-average (see equation below) the results from several analyzed sections.

Equation for dB averaging (also known as "energy averaging"):

Lave. = 10 X log10([10(L1/10) + 10(L2/10) +...+ 10(Ln/10)]/n)

4. When should I analyze my parallel barriers?

Research has shown that the magnitude of the performance degradation associated with parallel reflective noise barriers is linked to the ratio of the separation (width) between the barriers and the average height of the barriers. Definitely analyze parallel barriers when the cross-section's width-to-height ratio (W:H) is less than 10:1. When the ratio is between 10:1 and 20:1, you may still want to analyze the cross-section with TNM. If the ratio is greater than 20:1, you do not necessarily have to analyze the cross-section. Such a calculation will yield inconsequential sound-level increases. Please refer to the Parallel Barriers Menu section on Page 103 of the TNM Users Guide for more information.

5. Can I model inclined barriers or barriers with an angled lip in TNM?

Yes, you can model inclined barriers in TNM's Parallel Barrier Module. The Parallel Barriers feature allows the user to model inclined barrier bases, angled barrier lips or even overall tilted barriers by editing the Parallel Barriers->Cross Section, either graphically or in the input dialog box. Once the cross section is designed, run Parallel Barriers->Calculate and apply the calculated adjustments to the appropriate receivers. TNM users should use caution and good engineering judgment when implementing these adjustment factors, because (1) the Parallel Barrier Module only accounts for 2 dimensional sound propagation, so receivers outside of the selected cross section will not be represented by the adjustments; and (2) the use of angled barriers in TNM's Parallel Barrier Module has not been validated against field measurements.

6. How do I draw a customized cross-section in the Parallel Barriers module, instead of using the cross section TNM builds?

Click on the Add Surface button (refer to Cross Section on Page 105 of the TNM Users Guide) and create the surface starting from the left diffracting edge and continue to the right diffracting edge, all as one input object. Be sure to create enough segments needed to define the cross section within +/- 0.3 meters (1 ft.), including super elevation and other section details.

7. What are some things I should consider when calculating Parallel Barriers Cross Sections?

Note that the initial heights of the parallel barriers are based on the Input Heights that you entered in the Barrier input dialog box. If you are doing a parallel barrier analysis for other heights, you will need to adjust the heights either graphically or in the Parallel Cross Section input dialog box. It is incorrect to assume that the calculated multiple reflections sound level increase will be the same for two different sets of barrier heights.

8. Where should I place additional roadways in an existing parallel barrier analysis?

When you snap new roadways to a cross section, sometimes their bottom point ends up slightly below the roadway cross-section. Ensure all additional roadways are slightly above the Parallel Barriers cross-section line. Otherwise, tire noise will emanate below the roadway's surface and none of it will contribute to the computation.

9. Do I need to make sure I cross through receivers when creating my parallel barrier section cut?

No. Receivers that are included in your section cut will appear in the barrier analysis. If they are not included, you can still input analysis locations (parallel barrier receivers) later on.

10. Do changes to the Parallel Barriers module input data invalidate results?

Making changes to Parallel Barriers input data after calculations does not invalidate previous parallel barrier results. Instead, the results table (Parallel Analysis Location Table) displays "results must be calculated if any objects are modified." Users should recalculate after a change is made to the input data to ensure that the barrier results reflect current changes.

Note* When using Remember As to save a new parallel barrier design, be sure you are using a different design name or TNM will overwrite your previous design without warning. You must also File, Save or nothing will be saved on disk.

Updated: 7/15/2011
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