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Thanks for your reply. I have a LOT to learn.
No problem! That's what the forum is for! Don't feel bad about not knowing stuff: acoustics is a huge subject, and many parts of it are not intuitive, because sound does not actually behave the way we expect it to, or think it should! It takes a while to get your head around some of the concepts.
- We want to build a room in a room. It should have it's own ceiling so nothing requiring a structural engineer will be hanging from the TJIs.
OK, but you should still check with one about how much weight you are adding to the sub-floor above you. There's a design limit to bot the dead load and live load, and since that is the floor of another room, it already has some loading on it. As you add more drywall between the joists, you are adding more mass, which increases the dead load. You do need to make sure you are not overloading it, even if your final ceiling will not be suspended from that.
- The floor is a concrete slab on grade. I was planning on carpeting that.
Concrete slab on grade is excellent. Your best friend. But putting carpet on it would be a bad idea. Carpet is a pretty good way of trashing your room acoustics, since it does the exact opposite of what small rooms need.
Carpet absorbs high frequencies very well, mids to a certain extent but randomly and rising with respect to frequency, then does absolutely nothing at all to low frequencies. That's the opposite of what a small room needs. All small rooms need huge amounts of low frequency absorption, some in the mid-range but less and less as frequency rises, with little to none in the high end. Carpet makes your room sound dull, boomy, thuddy, muddy, lifeless, etc.
Secondly, it is on the
floor (duh!), which means it destroys the reflections from the floor that your brain relies on to build an "acoustic picture" of the room. All your life, wherever you go, your ears are exactly the same height above the floor, and your brain is very, very accustomed to figuring out the acoustic signature of the room based on the reflections it hears from the floor. If you sit down, your brain recognizes that, and adjusts it's "image" of the room accordingly. It does not use the ceiling or the walls for that, because the distance from your ears to the walls and ceiling changes all the time, many times per second as you walk around, so the "signature" is not constant or consistent. Ceilings are different heights, and when you walk outdoors, there is no ceiling at all! But there is still a floor, and it is still the same distance from your ears as every other floor.
If you have carpet on the floor, your brain no longer has any reflections to use for this.
So forget the carpet. You'll find it really hard to have a good acoustic setup in a room with thick carpet on the floor. It messes up your psycho-acoustic perception of the speaker locations, as well as your ability to determine directionality, so you'll never get an accurate sense of the real sound-stage, and never have an accurate stereo image. Carpet is pretty good at messing with spatial perception.
Have you ever noticed that world-class control rooms practically never have carpet on the floors in the front half of the room? And ditto for pro live rooms / rehearsal rooms? Never any carpet on the floors. There's a reason for that. if you want your room to be the best it can be, do what the pros do, and leave it out.
- I would like to have between 85 - 100dB of isolation if I can afford it.
If you have a few million dollars on hand, then you can afford it!
The very best isolated studio on the planet is arguably Galaxy Studios, in Belgium. They hired the best acousticians in the world, it took them many years and millions of dollars to design and build that place. Basically, each room in the studio is a massively thick concrete bunker that is floated on huge heavy-duty steel springs along with neoprene pads. They get just a fraction over 100 dB of isolation...
So 100 dB of isolation is a fine goal to have, as long as your pockets are VERY deep. But it's out of reach for pretty much all home studios.
Let me put this in perspective for you: the decibel scale is logarithmic: each time you go up ten points on the scale, that implies you need to block ten times more acoustic energy. A typical stud-framed wall in a house with a sheet of drywall on each side will get you maybe 30 dB of isolation (assuming it is well built). To get 40 dB of isolation, you need to block TEN TIMES as much energy, 50 dB of isolation implies you need to block ONE HUNDRED TIMES as much as the standard wall (10 x 10), 60 dB of isolation means you need to block ONE THOUSAND TIMES (10 x 10 x 10) as much, 70 dB is TEN THOUSAND TIMES, 80 dB is ONE HUNDRED THOUSAND times as much, 90 dB is ONE MILLION TIMES as much, and 100 dB is TEN MILLION TIMES as much isolation as a typical house wall. Thus, Galaxy studios gets about ten million times more isolation than a typical house wall.
OK, so it's pretty clear that even if you want 100 dB of isolation, it's unlikely you will get that. Even 80 dB of isolation is beyond the reach of the majority of home studios. Most home studio builders are satisfied to to get 40-something dB of isolation, very pleased to get 50-something dB, and ecstatically-jumping-up-and-down-yelling-and-screaming happy if the manage to achieve 60-soemthing dB of isolation. The practical limit for a home studio is around 70 dB, which is roughly the flanking limit for the concrete slab on grade. To get more than that, you would have to isolate the slab-and-foundations for the studio from the slab-and-foundations for the rest of the building. You can get up into the 80's like that, with a bit of like. Going beyond that calls for the extremes of floating floors, floating walls, and at the very top end of the scale, floating concrete bunkers.
So, that leads back to the question? Why do you think you need 80 to 100 dB of isolation? What do you plan to do in there that needs such a high level of isolation?
I'd suggest re-thinking your isolation needs, and doing some actual testing with a good sound level meter, to determine how loud things will actually be in your studio, and how quote you have to be outside the studio.
- I would like to keep the budget around $5000.00 if I can.
To be very honest, that isn't realistic. From what I can see, your total room area seems to be around 400 ft2, and it looks like you'll be using about 300 of that for the studio. So, 5000/300 = US$ 16 and a few cents per square foot. You mentioned that you wanted to install carpet: the installed cost of decent carpet is around 5 to 6 dollars per square foot, so JUST the carpeting ALONE would have eaten up one third of your entire budget. Drywall installed cost runs to around 3 to 5 dollars per square foot (even for just moderate isolation), so there goes one third of your remaining budget just for the drywall on the ceiling, without even considering the joists or insulation for that, nor anything to do with the walls, or the electrical system, or the HVAC system, or the doors, or windows, or seals....
So I'd suggest that you need to re-think your budget. For high isolation, a more realistic budget would be around US$ 50 per square foot. A better way to get a realistic figure, is to call around a few local contractors and ask for their standard rate per square foot for finishing an unfinished basement as a living room or bedroom. Add 30% to that, and you'll be close.
- The drywall is attached to the subfloor with green glue and drywall screws.
Take the screws out, fill the holes where the screws were with caulk, and use cleats around the edges of the drywall to hold it in place. With small thin strips like that, you cannot screw the drywall into the sub-floor, since that causes them to act as one single slid block, and prevents the Green Glue from working. Green Glue acts as a Constrained Layer Damping compound, and it works by damping the bending waves that run along the drywall. By screwing the drywall into the sub-floor, you are preventing the drywall from moving independently of the sub-floor, and thus it cannot act as a CLD. There are also studios that show a single solid block (such as several layers of drywall glued together) has a LOWER coincidence dip frequency than the same bunch of drywall with no glue, and therefore has WORSE isolation in the mid range. Also, Green Glue is NOT glue! (despite the name). It is an acoustic compound, not an adhesive, and it cannot be used to stick things together.
- I replaced the round metal ductwork with insulated flex duct. The main trunkline is wrapped with insulation (there is still some to do). I thought I could extend the duct down to the room with flexible connections and seal the penetrations for the grilles.
Not if you want high isolation! Or even decent isolation. Flex duct has very low mass, so it does not stop sound getting through. A duct is basically a huge gaping hole in your wall, through which sound will pour wonderfully, and escape into the outside world. For high isolation, ever single place where a duct penetrates a wall leaf, you need a silencer box. You can use flex duct to link those boxes together, but not to penetrate the leaf. The box itself must penetrate the leaf, with some type of massive "sleeve", to which the duct is attached.
As you can see from the pictures, the room is not very far along at this point.
Excellent! Then there's plenty of opportunity to do it right! This is good news.
At this point I have several 5-gallon buckets of green glue, putty pads for the electrical outlets and switches and the isolation clips.
You will not be needing the iso clips, so you can send those back and get a refund. The reason why you will not be needing them, is because you already said that your inner-leaf ceiling will rest only on the inner-leaf walls, and therefore will not need any further decoupling.
and took some pictures so you can get an idea what the room looks like at this point.
You have the typical situation where you have non-movable "stuff" in some of the joist bays above you, such as HVAC ducts, plumbing, and electrical. Your only option there is to board those up with plenty of mass, and seal them all air-tight. In some cases, that is as easy as putting a couple of sheets of MDF or drywall on the bottom of the joists, with caulk to seal, but in other places you will have to build framing to go around those "things", then put the drywall / MDF on the framing, and seal that. You need to end up with a surface facing you, all around, that is completely free from any penetrations, holes, cracks, ducts, pipes, wires, etc. Just mass with insulation. Once you have achieved that then you can build your actually inner-leaf room within that "shell" that you have created. In some cases, that framing+beefing up is going to eat a long way into your headroom: you'll have to decide how you want to handle that. You could lower the entire inner-leaf ceiling, or you could angle the inner-leaf ceiling to be lower down around those areas and higher up at the other end of the room, or you could build the inner-leaf ceiling to follow the shape of all of that.
- Stuart -
