Todd,

There are several good books on the topic. For right now, forget about absorption in the room. Here's a general rule of thumb for acoustics and noise control: Stop it at the source, failing that - stop it in it's path, failing that - stop it in the final space.

If you are wanting to build this in a metal barn, you have some challenges ahead of you to turn out a usable recording studio inside of a metal outbuilding.

1. Metal barn = metal roof, correct? If you live in an area with any amount of participation, this is going to be a huge thorn in your side. Instead of the roof serving to stop noise from coming inside, it will become a SOURCE of noise that you have to manage whenever it rains. This makes my head hurt just thinking about what you would have to do to make this a non-issue. I would seriously consider trying for an asphalt shingle roof over OSB sheeting if that's at all an option.

2. Matt is right that you will need to do a room inside of the room. That is, you will want at least two layers of isolation (sheetrock) separating your space from the larger space of the barn. Biggest challenge here will be heating/cooling that doesn't act like a big path for airborne noise to enter the room. Pay particular attention to this, it is where you will want to spend the most time and planning and even dollars. Do this right, and you can always add more layers to the outside of the room to kill off a little bit more noise, do it wrong, and no matter how many extra sheetrock layers you do, or how many auralex panels you buy, the HVAC ductwork done wrong (typical home construction) will be a constant headache to make quiet recordings. You need to plan for much larger cross-sectional area for the HVAC, with at least 2x the typical length of what you would normally use. Why? Because you need to make a 'tortuous path' for the air, that also has absorption in the ductwork.

I would not pay attention to any resource on new home studio construction that doesn't address this topic FIRST, or at very minimum, in the top 3 things to consider. It requires the most planning AND it requires quite a bit of space to do well. For most folks, they don't think about this, but I know far too many people that have spent good money on their 'foam' without taking care of this aspect first. The Auralex page on HVAC addresses most of the sources of noise coming into your room from the HVAC system (both self generated like fan noise and air rush noise, as well as transmitted noise from adjacent spaces). http://www.acoustics101.com/hvac.asp

Again, this probably doesn't seem like the most obvious thing to think about when planning your construction, but because it's infrastructure related, it's hard to correct once installed.

Non Parallel Walls, also mentioned by Matt above is important for keeping flutter echoes and standing waves to a minimum. They don't have to be huge non-parallel differences, but again, this is a primary challenge since most folks that do construction aren't used to thinking about things this way. We want everything plumb, not at unfamiliar angles. Not so with a recording space.

Getting more familiar now - entry and exit to the room. Dispose of any idea of using a typical interior door for the room. For my construction at my last house, I used a solid wood door that the builder hung into his own construction jamb, and I specified the clearance at the floor so I could use a 'bulb threshold' seal like an exterior door, and I built my own sealing system from simply 1"x2" trim and some triple-seal weather stripping. This let me experiment with pressure on the seal and different thicknesses of weather stripping.

Already covered of sorts, but just again to re-iterate; this should be a room in a room. This also means avoiding sharing any of the metal walls with the walls of this room. Impractical, yes.

Even more familiar - sheetrock.

Some fairly simple ideas rule here, if you can plan for it:

1. If you can do staggered stud wall construction, then by all means do it. http://www.controlnoise.com/staggered-stud-wall/ The only error I see in that construction is the plate is shared. When I did my room, I designed it with two independent 2x4 plates at top and bottom and only let them touch each other at the corners. What you are trying to avoid here is the mechanical transmission of noise from the exterior sheetrock surface to the inner sheetrock surface through the studs.

2. If you can't do staggered stud, then at very minimum make sure to use two different thicknesses of sheetrock. This keeps the 'drum head' formed by the sheetrock across the studs that will have it's own natural frequency, from coupling with exactly the same natural frequency of a similarly constructed sheetrock/stud combination on the other surface. Mass is your friend here - go as thick as you can imagine, even doubling up the sheetrock on one wall if you do it.

Whenever you think of stud and sheetrock construction, you also need to consider all the electrical and HVAC pass-throughs. If you can avoid it in your planning, always make sure to stagger your electrical and other pass-throughs so that they don't share the same section of wall. Avoid cutting any holes in the sheetrock (like no recessed lighting) if you can.

You can also hang sheetrock on resilient channel to help kill mechanical transmission of noise. In my previous home, I used resilient channel to hang the ceiling drywall, as there was living space above with foot traffic. It wasn't necessary for the side walls.

So far, resilient channel is the only 'fancy' material mentioned. Everything else is basic construction technique, but with more planning (the HVAC will be somewhat more costly than normal, but not preventatively so).

Once you have all of that sorted, then you can think about absorption in the room, which can actually be accomplished very inexpensively.

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