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
