Using a mix of materials is generally a good idea, so MDF or OSB over 9mm plasterboard might be a good idea (joins unaligned). You'd also be better off using 2x sets of slimmer studs, staggered (inner skin attached to one, outer skin to the other) , so that there is no direct sound path through the studwork.

Search good for Room - in - room studio design.

Had to make some acoustic enclosures for compressors once. The cover was GRP, seems to remember that it was a 600g/m^2 layup, then 50mm of a foam material, was rigid, brittle and open cell, not so different from that stuff that flower shops use but bigger 'bubbles' in it. On top of that (on the inside) was hardboard with holes in it, about 50mm diameter with 30% coverage (leaving 70% hardboard). I assumed it worked as they asked for several after the first one was made.
Not sure how it would perform in a fire, but did use Class 'O' resin.

<blockquote><cite>Posted By: tony</cite>why not build a concrete block wall? heavy = good sound deadening = heavier the better, dont forget to seal all gaps</blockquote>

Could prove difficult to move the frames around Your point about sealing is something I will be doing but sadly I have to have large holes where the materials go in and the product comes out, these will have plastic strip curtains but I know this will be a compromise.

ST
I think the acoustic foam is the same density that you are suggesting. I have read about perforated inner skins but the machine makes a lot of dust so I am worried that this will collect in the frames creating a fire risk. I have already made up most of the timber framing and the external layer of OSB has already been bought so the area that I can still change direction on are the acoustic filling and the inner lining.

Thanks for the ideas so far.

I am afraid off cuts and bits kicking about won't go far as the enclosure is 4m x 4.1m x 12m
I would rather not say too much about the process at this stage.

I have suggested that an expert in sound control should have been used at the design stage but my friend just thinks this would involve a large bill to come up with the same plan so I am a bit out of my depth unless building a few kit Hi Fi speakers 10 years ago counts

<blockquote><cite>Posted By: Timber</cite>How much sound do you need to stop? I used to do this sort of thing quite regularly. Getting a BS 4142 assessment might cost some money, but if there is a chance that someone might complain about the new noise source, this is the right thing to do.

The plant will need some sort of cooling air in or out etc. This will be the most likely source of noise breakout, so figure this bit out first. Attenuated louvres or a labarinth type attenuator would be the way to go normally.</blockquote>

Thanks Timber
I think the sound proof enclosure is not legally required so anything it does is a bonus but without it I think it will be a pretty nasty place to work. As for cooling there are two ideas being discussed.
1 Ducted air being drawn in from outside and vented back outside.
2 Maybe a Air-Con unit mounted within the enclosure

" Attenuated louvres or a labarinth type attenuator would be the way to go normally." Sorry what do mean?

As I have said I am out of my depth but they are not prepared to pay for expert advice so I am muddling through as best I can with a bit of help from my friends

Thanks Tim
I have been torn between containment and absorption. Most of the absorption products are expensive but if I use a perforated internal layer with mineral wool behind we have the problem with dust lodging in the enclosure