Firstly, well done for creating such a well thought out and great sounding piece of software.
I was wondering if you've left the absorption stats for Ceilings and Floors at 0, or have you given them some roughly accurate material.
Secondly, How to attain accurate material absorption factors.
If we take the properties of Air out of the equation, would these stats...
(Full absorption is 1. Full reflection is 0)

Floor materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz

carpet 0.01 0.02 0.06 0.15 0.25 0.45
and this graph...
http://img706.imageshack.us/img706/9382/absorptionratiotolossin.png
give you some kind of accurate relationship between material absorption and decibel loss.
(I believe this would work so long as the 'Frequency Damping' on materials; other than **AIR**, are not calculated over 75m, and so long as 50% energy loss equals 6dB as I thought when creating the graph)
Impulse Modeler does not model ceiling and floor reflections, so there is no way to specify any characteristics of these reflections.
Judging from that graph, all you need is to transform dB values to linear scale *1 minus* where (as you've noted) 6 dB equals 0.5, 12 equals 0.75.
Thanks for the info Aleksey.
Since there are no ceilings or floors, would you suggest slightly backing off the absorption sliders on each wall material to compensate.
I've just read another thread about this issue and these stats are suggested:
0.5 = 3dB
0.75 = 6dB
0.9 = 10dB
0.99 = 20dB
0.999 = 30dB
0.9999 = 40dB
Would this graph or the previous one be more accurate in relation to the way you've implemented material absorption?
If those stats sound well you may use them.
Of course, to compensate absence of floor and ceiling you could further increase the absorption coefficient.
One last thing...
In the latest version of IM, is the 'Frequency Damping' of the **AIR** material calculated over 75m? (I think you wrote this in an older thread)
Also, is the 'Frequency Damping' (for all other materials) the loss in decibels as soon as the rays have hit the surface? So with **AIR** set to 0, the decibel loss would stay constant over any distance.
EDIT: Thanks again for your help Aleksey.
Yes, "air" damping is specified "per 75 meters". If you set it to 0, no damping will take place.
Frequency damping is specified "per reflection".
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