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Traffic Noise Model: Frequently Asked Questions FAQs

Menu: Calculate

  1. TNM outputs "invalid" results listed in the Sound Levels results table when I model berms. What is causing this error message?

    The toe of the berm (end of slope) can bury receivers and it may not be obvious. This will cause "invalid" results for that receiver. Use the skew view to make sure the toe of the berm is not overlapping a roadway or receiver location.

  2. Does TNM allow barriers to be modeled underneath roadways on-struct (overpasses) or barriers on-struct?

    No, TNM does not. While this is a very feasible occurrence in real life, TNM does not allow barriers to go underneath structure-roadways (or barriers), and will produce "invalid" receivers every time a barrier goes underneath a roadway/barrier on-struct (instead of stopping the calculations). To avoid this error, stop your barrier just shy of the structure roadway, and then start it up again on the other side.

  3. How does TNM calculate insertion loss?

    When a barrier is present in a TNM run, TNM accounts for both the sound reduction generated by the diffraction over the barrier and ground effects for any sound that reaches the ground between the barrier and receiver (where TNM accounts for the shadow zone). The net effect of barrier diffraction, combined with the partial loss of the ground effect, is the barrier insertion loss.

    Insertion loss = barrier attenuation - (loss of ground effects)

    = barrier attenuation - (no barrier ground effects - with barrier ground effects)

    TNM does not currently incorporate the existing background level in determining the final insertion loss; TNM users must determine when it is appropriate to incorporate the background levels. The background level can be measured in the same or similar neighborhood at a distance from the roadway where the highway traffic noise is not heard. TNM's predicted "with barrier" level can be logarithmically combined with the background level (in a simple process in a separate spreadsheet) to determine the insertion loss.

    At far distances, the existing background noise is typically at higher sound levels than the received highway noise, so accounting for background noise is essential. There are cases where meteorological effects (e.g., wind speed and direction, temperature lapse or inversion) can play a major role in the received sound levels, causing the highway noise to exceed the background noise. Since TNM does not currently account for these meteorological effects, users must apply good engineering judgment when modeling receivers at far distances.

  4. How does ground type affect insertion loss?

    Here are some examples of how ground type can affect insertion loss:

    When removing absorptive ground cover between a barrier and a receiver, you are removing the loss of soft ground attenuation in the shadow zone and should therefore increase the insertion loss.

    When replacing ground cover with an acoustically more absorbent ground cover between a barrier and a receiver, you are increasing the loss of soft ground attenuation in the shadow zone and should therefore decrease the insertion loss.

  5. How do you add background noise to TNM results?

    TNM does not currently incorporate the existing background noise levels in its results. The "Existing Level", that can be input for each receiver, is only used to determine the "Increase over Existing" in the results tables. To account for background noise in TNM, it must be added in externally. First, the TNM user must determine/measure the appropriate background noise levels. In many cases, the background level can be measured in the same or similar neighborhood at a distance from the roadway where the highway traffic noise is not heard. Then, TNM's predicted levels can be logarithmically combined with the background noise level (in a simple process in a separate spreadsheet) See the equation below:

    Ladj. = 10 X log10(10(LTNM/10) + 10(LB/10))

    , where
    LTNM = TNM-generated sound pressure levels [dB(A)], and LB = Background noise level [dB(A)].

  6. I started to do a simple test run, which passes the input check and then immediately gives a "Segmentation Violation" dialog and stops the calculation. What does this mean?
    a Segmentation Violation dialog which stops the calculation. A division by zero calculation has occurred.

    There are certain TNM object placements that will cause TNM to overflow. Because TNM performs many of its geometric calculations by drawing triangles along the source-receiver path, division by zero calculations are possible if the extensions of the objects are pointing at receiver locations.

    Overflows also are more likely to occur if there is a high density of path-oriented objects (e.g., terrain lines, building rows), such as two terrain lines too close together. When deciding how close your terrain lines should be, always take line-of-sight into account. For example, cases involving noise barriers may have different elevations in front of and behind the barrier; if not modeled properly, the highest vehicle source (at a height of 3.66 m [12 ft.]) could be inadvertently blocked by the barrier or exposed to the receiver. In general, use good engineering judgment to simplify your geometry.

    If TNM outputs a segmentation violation message, don't click OK right away; make a note of which receiver caused it to crash, and try moving the location slightly, or disabling it in future calculations.

  7. Do I lose my results if the program crashes?

    No. If some of the receivers finished calculations, but the Sound Level Results table is grayed out, you can recover those already calculated receivers. Restart TNM. Re-open the run. In the Calculate menu, select Sound Levels. When TNM is finished with Input Check and starts calculations, select the Cancel button. The Sound Level Results table in the Tables menu should now be selectable.

  8. Why are my "No Barrier" and "With Barrier" sound level results the same?

    Unless you enter a non-zero number for the your barrier increment parameters, TNM will designate the particular barrier as a fixed-height barrier (e.g., Jersey barrier). To designate a design barrier, enter values in the Increment, #up, and/or #down columns.

  9. How does TNM handle terrain lines?

    Insert terrain lines wherever you wish TNM to know the general height of the terrain between source and receiver. However, please note that small changes in terrain elevations have little effect on the final sound levels. TNM's ground-smoothing algorithm will automatically "smooth" away minor elevation changes prior to sound level calculations to reduce computation time. TNM has shown some weaknesses in the diffraction algorithms, which would be invoked when computing the effects of terrain lines (This has been corrected in TNM version 2.5). Please refer to the previous terrain line FAQ, "Should I use terrain lines as often as possible?" for guidance on the use of terrain lines.

  10. I ran the official TNM test case in Version 1.0b/1.1/2.0/2.1/2.5. Why didn't I obtain the same results as the table listed in Figure 75 (Page 152) of the TNM User's Guide?

    (Applies to TNM versions 1.0b and above)
    The output in Figure 75 was calculated using TNM Version 1.0. Some of these sound levels are slightly different because of minor acoustical changes implemented in Version 1.0b, and more significant changes implemented in Version 2.5. For information regarding the acoustical changes in TNM 1.0b-2.1, please refer to the Version 1.0b release notes. If you are using TNM Versions 1.0b or higher, refer to the output table on Page 25 of the TNM 2.0 User's Guide Addendum. For information regarding the acoustical changes in TNM 2.5, please refer to the Version 2.5 release notes, and refer to the output table in the TNM 2.5 User's Guide Addendum.

  11. What does a "DB" error mean?

    (Applies to TNM versions 1.0, 1.0b, 1.1 only)
    It is a database error. The TNM database manager has an internal fix, which can be accessed in the File menu using the Cleanup Run selection.

  12. What does a "Floating Point" error mean?

    It is probably an error encountered in your geometry. Usually moving the receiver coordinates, which caused the crash, slightly will remove the error. Please send any runs with receivers invalidated to TNM Technical Support for further testing and diagnosis. A run consists of an OBJECTS.DAT and an OBJECTS.IDX file. Also provide an indication of which receiver the error occurred on and a detailed description of the error message.

  13. What does an "HPP Count Mismatch" or an "SQRT Domain" error mean?

    It is probably an error encountered in your geometry. This error usually occurs when you have a barrier where a segment is actually underneath a roadway on structure. Usually eliminating the portion of the barrier underneath the structure roadway will remove the error. Please send any runs with receivers invalidated to TNM Technical Support for further testing and diagnosis. A run consists of an OBJECTS.DAT and an OBJECTS.IDX file. Also provide an indication of which receiver the error occurred on and a detailed description of the error message.

  14. What does a "Subdivision missed a segment" error mean?

    This message was created for diagnostic purposes only. Clicking OK will continue calculations.

  15. What does an "Assertion failed - Lemis" error mean?

    Check your traffic input and make sure the speed and volume data in the columns weren't switched when entered.

Updated: 07/15/2011
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