Out of curiosity, how is the "bomb shelter" ventilated?
I've read that to insulate, it is best to have at least 2 layers of plasterboard, the inner being double the thickness of the outer, with an air gap between. Is this right?
No. Plasterboard has nothing to do with insulation, although plasterboard does have a low thermal insulation rating. Designing a free standing building for ISOLATION, without wasting resources, requires understanding of a few principles, of which SOUND TRANSMISSION LOSS is one.
Sound waves travel two ways. Via the air, and structurally. Make an enclosure AIRTIGHT and you keep sound from propagating through air leaks. Make the enclosure massive enough, or decouple it from the outside world, and you reduce transmission via the structure. Easier said than done though, as low frequency transmission loss is very difficult to achieve, as every structural element such as floors, walls and ceilings, have enherent low frequency resonance that transmits easily.
A combination of the two is usually the method of choice. However, without knowing what you are trying to target as far as SPL levels, or what your environmental noise levels are, it's impossible to tell you how to design these elements successfully. Even if you knew, correct specification of these assemblies is difficult at best, impossible for the amateur. Guesswork usually translates to failure and a waste of resources. You can actually make things worse under some circumstances, such as building a FOUR LEAF system.
I will say this though. The best bang for the buck construction for isolation is a MASS AIR MASS system, comprised of TWO leafs of mass, seperated by ONE hermetically sealed AIR GAP, as deep as possible, . A standard residential wall is a M-A-M assembly. However, you must understand that there are litterally hundreds of different M-A-M assembly types. Most have been TESTED and RATED for transmission loss, usually STC, which is a speech
transmission oriented rating, and fire, which is why Architects, acousticians, studio designers etc, use these Rated assemblies. Why build a wall rated for a 55db TL at 20 hz, when in reality, you only need one rated for 45db? But that is only an example. Look at this. Here are a few different types of wall assemblies. Note that the last two IMPROVE the rating, even though a leaf or two of drywall has been REMOVED
Also, these are INTERIOR partitions. Exterior walls/roofs, would have exterior type materials on one leaf. Ceiling/roof TWO leaf assemblies would have to match the LF TL of walls/floors, so it would not become a weak link, which is another animal we need to discuss. But it is beyond the scope of one post to do so. Yet I must mention, that floors are also in this scenario, especially if you have close proximity neighbors, you like to mix at 110 db at 2 in the morning, or have other sound generation problems to solve. Some people go to great lengths to have their cake and eat it too, including floating massive concrete slabs on isolators, 7-12 drwall layered leafs, HVAC silencers, etc etc.
Thats why I say, without knowing the particulars of your situation, its impossible to suggest a direction. Even then, under some circumstances, professional consultation is advisable. Especially if you don't have the resources to waste on mistakes or halfassed guesswork.
fitZ
