How does TNM calculate Barrier Noise Reduction?
TNM calculates barrier NR as the difference in sound level before and after installation of a barrier, where the source, ground, and atmospheric conditions have been judged as equivalent. NR accounts for both the barrier shielding effect and the loss of ground effects in the shadow zone behind the barrier. For sound that reaches the ground behind a barrier, TNM accounts for ground effects. Note that TNM’s Barrier Noise Reduction (NR) is the term used for Insertion Loss (IL). Keep in mind that TNM does not currently include background or existing sound levels in its NR calculations. You should always consider background sound levels when presenting final NR values, especially when receivers are placed a far distance from the roadway(s). Background sound levels should be logarithmically added (see equation below) to your predicted sound levels in order to obtain accurate NR values. Also, keep in mind that TNM does not currently include meteorological effects such as wind and temperature
