Help needed with model to find acoustic transmission loss of a membrane.

I've been trying to 2 weeks now to get this model working with no joy so far.

It's a membrane with a mass attached to the centre. I have YM, PR & density for membrane & mass.

I need to calculate the transmission loss the membrane+mass provides to acoustic waves that pass through the membrane+mass.

I've tried to replicate an impedance tube within comsol using a long cuboid shape with the membrane-mass half way along. I've used 26 PMLs surrounding the cuboid to get rid of reflections.

I've been using intop1 & intop2 with either a plane wave radiation source on the source side wall, or a background field (the domain before the membrane-mass).

I've also read somewhere about adding face loads to the membrane-mass.

Any help in getting this model to work would be hugely beneficial,

Thanks