Nice problem to have!
Ok, three or four things in there to think about:
1. Celotex - although excellent insulation - is a closed-cell foam, and so provides no acoustic absorption. For this reason I suspect it's the ceiling of your new room which offers the least resistance to traffic noise, since it has the least mass by far. Definitely consider using two layers of plasterboard on this surface, and making those 'soundbloc' or similar. (notes below)
2. There's nothing to choose between plasterboard on dabs, or wet plastered as a finish, except
neither will help the sound in room. Such hard /dense surfaces offer no low-frequency absorption. With block and a hard finish, you will have a very long reverb time at low frequencies and that will make the room unpleasant for music and for cinema with a lumpy/modal bass variation around the room.
Instead I strongly recommend you consider using a stud lining instead - 50-60mm studs (timber or metal, doesn't matter) spaced at least 20mm off the clockwork, with a minimum of 50mm of mineral wool of density 20-40Kg/cubic metre in between the studs (use roll or batt, but not closed-cell foam). Regular 12.5mm plasterboard lining on the room to provide a finish. 2mm Skim plaster finish is fine.
This provides a little flex, and the mass of the plasterboard with the damped cavity behind will provide really useful absorption of bass energy across the spectrum low frequencies. Rather than more words, here is a very quick simulation of your room dimensions in CARA. Look at the difference under 300Hz (don't worry above that, it becomes dominated by your furnishings - more on that below)
First - assuming wet plaster on solid walls: rev time is long, 1second or more (about the same as a tiled bathroom, typically)
Then swap four walls for the build-up I suggest:
Note the massive difference - the bass end is very well damped with well-controlled reverb time, well within the usual recommendations (between the two green lines). Nice tight controlled and extended bass will be the subjective result - and funnily enough a perception of
more bass than you'd have in the hard finished room.
Now obviously taking ~90mm off each side of the room looses you quite a lot of floor area. But given the large areas involved you only really need to do this to two or three walls to gain the help. But do include the largest, if possible (4.2 x 3m high ?). Even even just two walls will get you very close to the second result. If you can do to, preferably pick two sides in an L - that will have the most effect by having some effect on both directional modes and the tangential ones. (Note - the board ceiling to vented cavity above is already contributing a little absorption to the bass in this model - not need to think about similar treatments to this surface). Obviously you can run conduit in the void easily, too; you can get 'putty pads' for the socket backboxes that will help maintain the acoustic performance.
Finally - the detail of the execution really matters..:
- Put the ceiling up first, running the boards right up to the clockwork, and use a soft sealant bead between the two. When using two layers of board stagger the joints (and if you do two layers on a wall, always put the finish layer with the joints vertical/visible lines later)
- Install the stud lining and any other wall finishes/ wet plastered walls up to the ceiling. This stops air path between the cavities, helping both absorption and minimising incoming noise from outside. Seal the independant stud lining board to the other walls with a softish sealant to finish, too.
- If you do install a stud lining, put a parge coat on the wall. This is a thin render coat (DIY sand:cement will do, too) and its function is to seal the surface against air movement. That's good for both thermal and acoustic reasons. Doesn't have to be pretty, just complete in coverage.
- Plasterboards are not crested equal. For that ceiling for preference use a higher-density plasterboard (even with two layers)- standard 12.5mm gypsum board is about 9kg/sq.m, the soundbloc types are about 12kg/sq.m. It helps.
- Picking mineral wool for acoustic purposes it's the density that matters, not the label on the roll Remember this once you work out that anything labelled 'acoustic' often attracts a premium…
- Absorption in the midrange and up will be entirely dominated by your furniture and finishes. For clarity /reverb control) in the midrange (speech) region and into the treble, the rule of thumb that works is, ideally you need about the floor area covered in something fibrous - ie carpet on decent underlay, or at least 80% of the floor area in equivalent provision of soft finishes. Something to think about now, if you were considering a hard floor finish (timber etc).
Hope this helps / doesn't tell you how to suck too many eggs!
