(1) I know this may seem a stupid question but if I have two seperate timber wall frames seperated by a small air gap (25mm) will I get the same/similar results if I put three layers of wallboard on one side and one layer on the other side as apposed to two layers on each side . The reason I ask this is that the other three of my live room walls only have one layer of wallboard so adding another layer to just one part of this system will be a waste of time. I am thinking I may be able to compesate somewhat by adding another layer to the control room wall. Not ideal but it can't be a total waste of time
(2) Do I leave the airgap between these walls as an actual airgap or should I put 703/705 in the cavity
An airgap is still considered an airgap with insulation unless it is closed cell. Have you considered Roxul AFB's? According to this forum, Roxul seems to be the insulation of choice.
you definitely need to insulate. it will increase performance SUBSTANTIALLY.
however a lot of people would just use ordinary fiberglass as it has similar performance, especially at low frequencies, the performance may even be slightly better.
make sure it is unfaced or if it is faced, put the facing next to the leaf surface. even the thin facing can create 3rd leaf effects.
unless money is no object, then by all means use 703/5.
and i think your idea of using different layers on each side is ok in principle.
. The reason I ask this is that the other three of my live room walls only have one layer of wallboard so adding another layer to just one part of this system will be a waste of time. I am thinking I may be able to compesate somewhat by adding another layer to the control room wall. Not ideal but it can't be a total waste of time 