[Probes]

About to build a shed/garage sized shed/garage.. going to do it out of solid block and stick a simple rafter flat roof on. Need to try and get it as sound proof as possible.. ive heard layering plaster board up to an inch thick is a good way, but i need to fix shit to the walls, like benches and shelves. Does anyone know any other ways of dampening walls/roofs. Cheers.   

[slackline]

You planning on setting up something similar to this....

[Jaspersharpe]

[GCW]

May be easier just to apply, Probes, rather than trying to branch out as a freelance.

[Oldmanmatt]

Sound proofing is a matter of "mass layers". Alternating between (put simply) dense and less dense layers.
Anything which breaches those layers is a "Noise short".
A floating wall/ceiling will be required.
This should not be rigidly mounted to the structure. Self supporting is best, using vibration mounts to isolate, second best. You can (we have) made such mounts by gluing 20mm of medium hardness rubber between two plywood sheets. Remember to lag the wall (prior to cladding) with Rockwool or similar, 45kg/Mcubed density is typical.
If you can coat that with a dense sheet (2mm lead is best but expensive, perforated aluminium is good, calcium silicate is cheap but brittle (fire proof too, aka furnace board).
Typically, where we have mounting issues with the lining, we design the framing of the lining to have reinforcing where required or
at least backing sheets secured to the frame (not the shell!), 20mm Ply is usually enough for a large plasma TV for example.
Typing on the phone, so I'll reread later to see if this makes any sense.

[Oldmanmatt]

Ok, got time to pick up the pad...
Remember to wrap the rafters with the rockwool, not just lay it in between. Unless you are rehearsing AC/DC in there 50mm thickness should be enough.
Usually, using the metal stud work for a lining works out cheaper than timber. Though you will need a pop rivet gun...
If the stud work is lightly touching the rockwool lining of the block work, you should end up with~ 50mm air gap behind the gypsum board. That is important for sound proofing. You shouldn't need anymore than 15mm gypsum (plaster) board.
There are glue up pins on the market for fixing the Rockwool sheets to the block work.
Remember also, if you don't soundproof the floor, you may as well not bother with the rest.
~20mm of rubber works well, or a floating floor of 40x40 timber and ~18 mm (ish) ply works well. Floating floor is cheaper, just rest the timber grid on a few patches of rubber.

Uh,

I'm assuming this isn't for serial killing purposes?

And, if you don't do the doors and windows, then, again; everything else is wasted...

[Serpico]

We need a few more details about what you're soundproofing for; if it's just for muffling screams/cries for help, it'll need one approach, if it's as a workshop, rehearsal room or recording space it'll need a different approach, even what you intend to record in it is a factor - I'd take a different approach for a good sounding drum room than I would for just tracking/mastering and close recording. I say 'I', I'm not an acoustic architect - I'd just look on t'interweb.
If in doubt - build it thick and heavy, and put shitloads of rockwool in the voids.
Mass stops bass but you're talking hundreds of kgs even for a small space, so it's all dependent on what you want to achieve as to what's practical to do.

[Bubba]

You need to ask the expert:

[Probes]

Errrr  oops, maybe I should have said its for a workshop..  but it needs to be pretty damn sound proof, my router at full pelt and the bench saw is probably the equivalent to smashing the (backdoors out of a darwen bird) hell out of a drum kit. A bit of leakage could be ok but its gonna be in a super quite housing estate n be used at 'unsociable' times! 
The layer plywood sounds ok? I can fix to it then aswell. I was thinking a double glazed door for access?

[Probes]

... so its going to be high pitch stuff and not a lot of bnaging really... so thinking about it, its fuck all like a drum kit.

[horn]

Build it out of strawbales?

[Jim]

have a read of this for starters. plenty of other threads there as well

[Oldmanmatt]

For workshop, use my suggestion but forget the gypsum board and go for ply. If you double glaze and it still doesn't cut it, add a thin sheet of acrylic/Perspex on foam mounting tape (leave a 1mm gap between tape strips at the corner to let it "breathe"), inside (5mm ish). You'll need to replace that mounting tape once a year or so, but it's effective.
The mass layer (the lead etc mentioned earlier) is fantastic if you can find/afford it.
Don't underestimate the pluses of a heavy set of curtains over doors and windows.
Acoustic mass layers are readily available from marine suppliers (for lining engine compartments etc) and fairly cheap (for a given value of cheap..), so if the previous doesn't quiet it enough, you can always add a mass layer over the inside.
I'm not an acoustic architect either.
But I used to design sound and fire proof bulkheads for ships and offshore installations.
I still do consulting work on cutting down transmitted noise from installed machinery.
I've tried to convert expensive materials into readily available materials which should have similar properties (ish).
Oh yes, before you fix the ply to the studding, put a layer of foam insulating tape between the stud and ply.
There are a huge number of things you can do to improve it. One thing we did (on the type 23 frigates which had to be dead quiet for stealth reasons) is put rubber feet on tables, chairs, work benches, pillar drills etc etc.
Really, it's just a case of looking around to see where the sound might get out and then put something in the way to absorb the energy (I find it helps to think of it as energy not sound and try to find ways to "use it up").
You will have to compromise some where, otherwise you'll end up with an airtight box and will suffocate...
It should be nice and warm though...

[Ruarl]

(Final paragraph is a "quick and dirty" idea. Earlier stuff is more in-depth discussion)

I haven't read all the stuff below, but I saw "perforated aluminium" and thought I should say something. I worked as an acoustic consultant for a couple of years, and designed flats, offices, schools, and even a wee concert hall.

The other posters have the basic ideas: layers of stuff. You don't need anything fancy for what you're doing. plywood or plasterboard layered with mineral fibre is fine. Using two layers of ply/plasterboard together is slightly better than one. Doubling the layers (so from outside to in: block/mineral fibre/ply/mineral fibre/ply) is a bit better still.

However, if you leave the door open, or if it isn't sealed properly, all of this is pointless. Similarly, if the roof is just built on top of the walls, with a plaster skim, noise will escape that way, and your effort on your walls in wasted again. Fixing workbenches through the ply/mineral fibre layers to the block will also be a weakness, particularly if you have a noise/vibration generating device (Say, a pillar drill) rigidly fixed to the bench. Better to have free-standing tools and workbenches if you can. If you really need to fix to a blockwork wall, consider two skins of blockwork with mineral fibre insulation in between. This will also be pretty toasty warm.

The floor will be a bit tricky. As with the ceiling, if you just plonk the walls onto a concrete slab, then noise will be transmitted out via the slab. You might need to build some kind of access floor.

As someone else has already pointed out, if you can absorb the sound within the room this is as effective as preventing it from escaping in the first place. You don't need anything fancy for acoustic absorption. There are designs for "modular boxes" in this pdf, as well as a load of other information (quite technical):
www.bbc.co.uk/rd/pubs/...acoustics/bbc_guideacousticpractice.pdf

Now, you've got a nice airtight box which no (appreciable) sound escapes from. You fire up your router, and chuck a load of mdf-dust up into the air. Now you need ventilation. Open your door/window, and all this work is pointless. Acoustically controlled ventilation is more complicated. You'll need some kind of air-shifting device, and lined ductwork, possibly an attenuator too.

So obviously, this is all quite complicated. You might try building something much simpler: two skins of blockwork with mineral fibre in the cavity. For the roof mineral fibre in the rafters, with a suspended plasterboard/ply ceiling. Seal this to the blockwork with mastik. Minimise the number of light fittings an suchlike that come through. Seal these with mastik too. Use a door that is fully rebated with bulb seals and a drop seal. Now put in as much absorption as you can - modular boxes, matresses, duvets (not eggboxes.) Job's a good 'un!

[Probes]

   brilliant. Thats some proper advice!

Think ive got a bit of a plan now... trying to fit it in a tight budget. Single skin blockwork on its own small footing, batterned up then lined with insulation, 18mm ply fixed to the batterns but with some sort of hard foam about 10mm thick sandwiched in the fix.
Flat roof out of 18mm ply with 3inch rafter insulated and maybe a 9mm ply ceiling hung some how or again fixed but with foam fixed to the rafts. Leave a gap all the way round the edge of the ceiling and mastic shut.
Floor im thinking, chippings and then a thick ply frame job lay on top, again not touching the walls. Work bench free standing, which can fix to the floor, but kept free from the walls. Any shelves, cupboards I can just screw to the plywall I guess.
Vent I can stick a 4 inch extract in the wall that points out away from the houses out towards fields, dust freally isnt too much of a problem, whilst working so I might see how it goes with out the vent... but this is only thing unsure of.. how much leakage it'll cause, but im guess if it directed away it shouldnt be too bad?
Door get asecond hand double glazed job and line it with underlay or something.
On the ceiling i remember when i was in a recording studio they had 6 inch deep planks hanging vertically covering the whole ceiling as a sound trap? is this worth doing? Not sure how else to absorb sound though, as anything material like a mattress foam etc is just going to get 'shit' up with dust.

.... Im hopeful can get this sorted now... just need to learn how to lay bricks! 

[Oldmanmatt]

Nah, just put a silencer box on the vent exhaust.
Make it in ply. Line it inside with rockwool.

[Ruarl]

  brilliant. Thats some proper advice!

Think ive got a bit of a plan now... <elaborates on plan>
On the ceiling i remember when i was in a recording studio they had 6 inch deep planks hanging vertically covering the whole ceiling as a sound trap? is this worth doing? Not sure how else to absorb sound though, as anything material like a mattress foam etc is just going to get 'shit' up with dust.

.... Im hopeful can get this sorted now... just need to learn how to lay bricks! 

Plan sounds good. Those planks weren't an absorber, they were a diffuser. You don't need one of them.

Mineral fibre covered in something light but imperforate would serve well enough as an absorber, and not get too gummed up. I'm thinking perhaps paper, or cling film (Just don't squeeze it up tight.) You could put this wrapped mineral fibre in boxes fixed to your ply walls. The boxes should be shallow, and the face into the room should have lots and lots of holes. I think we use hardboard with loads of holes drilled in. Time consuming, but it'll save your neighbours, and your, hearing.

Putting the vent and door on the furthest side from your neighbours is a good idea. As is the muffler box. If it sounds worse with the muffler box than without, you've probably hit a resonance. Build a new one about 1.2 times bigger and try again.