embrace and extend
So you are taking the step of extending or converting a loft for audio or AV purposes and are wondering what you should consider in doing the work. WIth regard to acoustic properties there are two sides to the same coin.
First off, isolation: keeping the noise in, in a domestic setting, out of consideration for others. Mass is your friend, accept no substitutes. Masonry walls (no surprise in the UK), and as much mass as possible in the separating wall(s) and floor. Blockwork internal partitions - dense blockwork, not the aerated stuff; as heavy a floor construction as possible, meaning concrete slab or beam-and-block. This is your first line of defense, and easier and cheaper than lightweight or dry construction techniques providing equivalent isolation. That said with suitable attention to detailing, for example the use of staggered-stud and resilient mounting details, excellent results can be had with these alternatives.
Where new external walls are to be built, remember insulation in cavities has a much smaller effect than even a small amount of mass so don't worry about that; but the standard mineral-wool types are well suited to this purpose anyway and will help damp sound transmisson around the cavities. Neither should you overlook the ceiling; a double layer of plasterboard here, with the usual insulation laid directly on top, will help quell the most usual failure point - acoustic coupling through the roof void. More generally, the actual construction needs to be detailed to minimise flanking transmission paths as far as possible, though many are unavoidable.
Much of the quality of the whole actually comes down to the skill of the builder/contractor and vigilant site inspection. In the past I've had very expensive acoustic partitioning b&(*&^ed-up by contractors who treat it like any other plasterboard job. It's not; the devil is in the details.
Still on isolation, you need to pay attention to the in-air paths. Once you have a lossy shell, you can undo the good work with even small holes. Lip seals to the communicating door to the AV room are a must and will make a surprising difference; the door itself should be solid core, ie heavy, and not the usual cardboard-core rubbish sold in the UK for use as internal doors.
The second issue is the acoustics of the space itself. In a adomestic setting these really are adjustable and can't actually be settled or finetuned until you have all the furniture and kit setup - domestic rooms are dominated, in terms of acoustic response, by the fittings and furnishings. Conversions for other purposes - recording studios, bars etc - are a little outside the scope of this site, but the same principals hold. One item which will make a gross difference is your choice of floor covering, which you might be thinking about early. The large area presented by a carpet-covered floor provides a huge amount of absorbent surface affecting the critical midrange sound, and with a suitable underlay reduces impact noise for free. Reflective surfaces - tile, wooden flooring etc - throws this away, requiring that you will almost certainly need to add more absorbent surfaces in the room and may need to address impact noise (though probably not over if the new room is over a utility room/garage).
None of the above necessarily implies added cost; it all comes down to an eye for detail. If you have engaged one, tell your architect everything you can about the intended use(s) and your concerns - you're paying, so exploit their technical knowledge and you'll then be more likely to get a pleasing outcome.
Further reading: track down a copy of The Master Handbook of Acoustics, 4th ed. : F. Alton Everest: TAB Books (McGraw-Hill) 2000.
© the twisted pair 2000