Here's my thinking on this. One of the main reason a person goes thru the trouble of developing a room with in a room is to develop high isolation...right?

To that end everything you do from the exterior structure to the interior structure you have to incorporate a decoupling mechanism on each and every item that goes from the exterior structure to the interior structure, right?

So your including rigid pvc as a trunk line is already bordering on the side of short curcuiting the high isolation you are building towards.

If this rigid pipe is already in place then you must decouple this pipe from the interior structure. Or if it is easier to do, decouple it from the exterior structure.

It should not be there to begin with and ideally it would be decoupled from both the exterior and the interior structures as it is a path for vibration and that defeats, little by little, everything done to this point.

As to the the boxes..whatever Rod has drawn up use it. The only way you will cripple your air system is if you build these boxes 6 inches by 6 inchs....meaning a very small box.

If the box is as big as the air in flow or bigger which is more likely, then it will be fine.

Lining the boxes typically takes a usual product used in hvac development meaning, you are attempting to keep the noise levels down from room to room and from outside of the hvac trunks and feeders and not introduce fibers, or rather, anything that can aggrevate the respritory system.

Does that help?

Thanks for your thoughts Brien

I agree with everything you've written. I have a design whereby the duct is attached to the ceiling of the inner room and only bridges the inner and outer room once, and that is for 25mm around the circumference of the duct. I have deliberated about it a lot but could not reduce this anymore unless I didn't seal the outer room and I think maintaining an air tight seal around the inner room should take priority (I may be wrong on this)
I hope that the silencers are effective enough that they will reduce the transmission sufficiently that there will be minimal sound travelling in the ducts that can be transmitted at this point where the two rooms and the duct touch......

Somehow the duct has to leave the inner room and pass out of the outer room so it must unfortunately connect the two, I couldn't think of any way around this?
I may also install air con and the same problem will again. I'm assuming this must be a fairly common problem? In Rod's book the door frames are also connected in one of the designs so which I understand this goes against the principal I understood that sometimes things (flanking) cannot be avoided?

With regard to lining materials in the UK air con is not that common in domestic properties, the duct lining products associated that I've read about in US publications I haven't been able to source.

Thanks again

The idea when making an hvac connection from exterior to interior is "flexable" ducting...that eleminates a multitude of sin in respect to a hard connection.

Understood. I have done this at the point where they touch.

Back when I had my commercial studio - it had already been a studio on the past, and built pretty well for ISO and acoustics - although I had the control room re-designed from the ground up, because it had been originally built back at a time -late 70's - when the "small and dark control room / huge performance area" thing was all the rage ... I ended up tripling the size of the control room ...
And I remember the contractor ended up using a "flexible " duct system, IIRC it was similar to what you'd commonly find in people's basements for clothes dryer vents, just bigger and insulated on the outside.
Now, whether it was decoupled completely from the two walls (int/ext) I don't know.
I know I never had any issues with heat or AC making any noise that was noticeable...then again, the HVAC system in place then wasn't really all that noisy to begin with.
I can say that I've worked in studios in the past that were very well-isolated acoustically, but that had little-to-no ventilation, and it did manifest itself in fatigue, lack of creativity, and even short tempers. I know it sounds silly, but I'm pretty sure the lack of ventilation had a part to play in those things.
FWIW