Hi,
My mistake, OK its the effective volume of the box not the actual volume.
Stuffings and linings increase Veff by slowing the speed of sound and as
I understand it the rate of contraction and expansion of the "medium",
whilst also adding some resistive damping to airflow.
I found out the hard way some foams are too dense and can and
does damp airflow too much when used in bulk instead of linings.
If you vary the amount of stuffing in a box Fb will vary, this is
easiest to model by saying the effective box volume is changing.
The effect on damping is more subtle, if Qtc does not vary the
way you would expect it to just by varying Vbox, I think you
would need to manually adjust some damping co-efficient.
I think some simulators do a better job of modelling linings
and stuffings than others, WinISD AFAIK is not good.
rgds, sreten.