John Sayers' Recording Studio Design Forum Archive

Greetings,

I'm a long time lurker but first time question asker here - finally have boots on the ground in a space that I need a little acoustical guidance on. I've spent a lot of time studying this stuff online, read Rod Gervais' book several times over, and learned a whole lot from all of you here, as well as on the Gearslutz forums, AVS. Hopefully my pondering reflects some of the knowledge you've been so generous to share with us.

The space in question is a recently built room which extends off the exterior brick wall of a late 19th-century Brooklyn brownstone into the backyard, underneath a deck that provides yard access to the folks upstairs. It was built on a concrete grade beam supported by 4' deep columns, framed with Doug fir 2x6s for the walls and roof, 16" on center. The 4x4 posts that support the yard side of the deck are incorporated into the framing, but the roof is independent of the deck otherwise (they are connected to the brick wall by separate ledgers and there is a 5 or 6" gap between the bottom of the deck and the roof). The entire structure is sheathed in 1/2" OSB - the walls are wrapped in Tyvek housewrap and sided in 5/8" T&G common cedar running vertically, fastened to the sheathing through 1/4" furring strips. The low-pitched roof is covered with Grace Ice and Water Shield, and then clad with 26-gauge galvanized steel sheets. The concrete columns on the house side were set about 3" away from the (field stone) foundation and this gap was then backfilled with soil - there is no rebar connection between the house and the grade beam. The wall framing sits directly on a PT sill and is connected to the brick wall with tapcon screws. The floor is secured to the inside perimeter of the grade beam foundation with threaded rod and epoxy, and built out of PT 2x6s 16" on center. Subfloor is 3/4" ply. Beneath the floor is a crawlspace of sorts, more like a "reachspace" - approximately 12" below the bottom of the floor joists is a thick PVC pond liner that acts as a vapor barrier. A 4" PVC pipe runs from outside under the grade beam, through this space and into the basement through an approx. 2' notch in the foundation. There are two identical windows in one of the yard-side corners of the room. They are typical NYC aluminum replacement windows, except they are laminate glass. They measure 26" wide by 48" high - the actual glass area is 20" square per sash.

The interior dimensions of the room are 100" from the face of the brick to the studs opposite, 89" wide and 95" from the subfloor to the bottom of the ceiling joists rising to 99" on the brick side. The brick wall is 3 wythes thick, 12" in total. There are two rough door openings in the brick, one 36" and one 24", both 84" high. The subfloor of the backyard room extends through these openings over the foundation and is supported by framing scabbed on to the original floor joist which runs just next to the brick wall. The smaller 24" opening is framed out for a small closet.

Here's the situation: the couple who live upstairs own the building - they have two young children and a puppy who create a pretty serious ruckus nearly every morning and then again when they get home from school, and sometimes all day on the weekends. The barking and other airborne noise are not such a big deal to me (I've lived in noisier places in my 20 years in New York). But the impact sound is maddening - the running and jumping and dropped silverware jolts me awake even if I'm sleeping with earplugs in. When they renovated the place they just stripped the floors to the original (creaky) tongue and groove subfloor planks and painted them - my ceilings are just under 8' high and consist of 1/2" ultralight attached directly to the joists except in one room where the original plaster ceiling remains. The owners and I have been friends for a long time and this is actually the second place we've lived in together - they're aware that the situation is far less than ideal and we're hoping to address it with some floor treatments at some point in the future when it's logistically possible. But for now, I'm planning to finish out this back room as an acoustic sanctuary. We have a good agreement whereby my rent is locked-in for life and I'm free to make improvements. We're planning on staying in this building for a long time. They've also bequeathed me the basement space to use as I please, more on that shortly.

So: The primary goal is to achieve as much isolation from the noise upstairs as I can for $2k - $3k. This would be for materials only - I am a skilled and experienced carpenter, having worked for my father building houses in western Massachusetts from a young age until I moved away. So I'd be doing the work myself. The secondary goal, if I'm going to be making a quiet(er) space, is to be able to do some solo music practice in there as well (I'm a multi-instrumentalist). Thirdly, as I mentioned above I have access to the basement and I am entertaining the idea of building a studio down there - this little room is an opportunity to get a sense of the time and cost of studio-specific construction, and if it turns out less effective that I had planned for, to learn from mistakes on a much smaller scale.

I've done some measurements with an SPL meter - it was set to C-weighting and fast response, which I'm not entirely sure was the correct setting . . . anyway, I've seen the noise hit 90 dB SPL in the morning (some kind of gymnastic routine off the dining room table I think). I also took some readings playing various instruments in the room, from a distance of about 3', same meter settings - results ranged from 80 dB (low C on a flute) through mid-90s for violin and trumpet with harmon mute to 102 un-muted trumpet around ff and 105 overblowing an alto sax. I tried to take some measurements from outside the room in the yard but it was a real windy day and there were lots of planes passing over so I don't really trust the results. Music-wise, I'm not expecting to be able to wail on the trumpet or sax in the middle of the night but if I can achieve an STC in the mid 40s than I think I could work with that.

Here's what I (think I may have) figured out thus far: The outer leaf of the 2x6 walls have a surface mass of about 2.5 lb/sf (OSB + cedar siding). The roof is 2.6 lb/sf (OSB + Grace membrane + galvanized steel). These are based on samples of material I weighed myself - I looked up the glass, based on several sources it seems to be 2.4 lb/sf, but I'm not sure if it'd be different for laminated glass. Nor am I sure how to factor for the window frame. The plywood subfloor is in the neighborhood of 2.2 lb/sf. So fairly even until you get to the brick - I weighed about 10 different loose bricks kicking around the basement and calculated the surface weight of the brick wall conservatively at 120 lbs/sf.

Here's basically what I have in mind: add 5/8" drywall / backer rod to all stud / joist bays, caulk that and all cracks in the framing (seal it up). Fill the stud / joist bays with R-19 fiberglass insulation. Beef up the inside of the closet with extra drywall (I would also address ventilation somehow via the closet space). Add resilient channel to the walls and ceiling and two layers of 5/8" drywall to that (will have an engineer verify loads). Lay 1" 703 rigid fiberglass on the floor, then a finished floor of some kind. Install a super door a la Rod Gervais (maybe foregoing the lead), and hinging window plugs that follow a similar design.

Finally, question time:

Aside from a general "is my reasoning sound?", the big question mark is what to do in front of / on the face of the brick inside the room. I understand the mass / air / mass 2 - leaf concept, but when are the leaves so disproportionate in mass and / or the air gap small enough that it doesn't make much of a difference? The room is already very small and I'd be shrinking it further with RC and double layers of drywall. If I build, say, a 2x3 wall 1" in front of the brick in order to create an air gap / second leaf, I'll lose a significant amount of space. If I install wood furring strips on the brick and then RC on that and then drywall, I'd only achieve a 1" or so air gap. In the Master Handbook of Acoustics it states that "a small spacing in a double-pane window, perhaps less than 1", can yield lower STC than a single-pane window." I'm fairly certain that in terms of airborne sound, the brick wall is well and beyond the strongest link - but since impact sound is the main culprit, I've stood with one ear pressed up against the brick and the other plugged, trying to determine if it's transmitting any significant structure-born sound, and haven't been able to discern anything. Is there any critical gain to be had but creating a second leaf in front of the brick? I'm aware of the fact that, due to the two openings, the brick accounts for only slightly more than half the total area of that wall. I've read through the section "Composite Partitions" in chapter 16 of the Master Handbook of Acoustics, though I haven't tried to apply the math to my situation yet. Perhaps the answer is partially there.

Secondly, as far as the windows are concerned, is there a reasonably accurate method to test their performance with an SPL meter to give me a sense of how beefy the plugs would need to be? And since they are (I believe) double-pane, if I add window plugs, I'd have a 3 leaf assembly - but that seems un-avoidable. I remember reading about some small benefit of using different thicknesses of glass for control-room windows, to vary the coincidence frequencies. Could that be applied to any effect in this case?

Thirdly, the framing of the closet is still open from the inside - if I were to add a closet door, would I be avoiding a 3-leaf system by adding drywall to the outer leaf inside the closet in the same fashion as the stud bays of the exterior walls? That is, the closet itself would be the air gap (around 2'). It doesn't matter to me aesthetically so whatever is most effective.

Finally, the floor - there is one point of structural connection with the interior (old) floor framing just on the interior side of the brick wall, through the door opening. The floor towards the back of the house is a bit spongy - the backyard slopes towards the house and I think a century of rain has caused the foundation to settle a bit. This is also directly below the kitchen upstairs, which is where most of the action goes down. So I'm a bit paranoid about picking up vibrations through this one small connection point. Will the 703 help? Or would gypcrete be better? I have a bundle of really nice reclaimed solid oak flooring I'd like to use up - I'm sure it's inferior with regard to airborne sound, but the floor system is above a large air gap above dirt, essentially, so can I get away with it if I'm diligent about caulking any air gaps in the subfloor?

I hope all that was somewhat clear - I attached a drawing with dims of the brick wall (should show the floor going through the opening) and a photo of the opening (which does show the floor passing through) and a photo of the exterior of the room.

Hats off if you found the patience to read through the whole thing and many many thanks in advance for any and all guidance!

Best
Eben

Hi there Eben, and Welcome! Sorry that I only responded to your thread now, but I lost a lot of open threads when my old computer died a while back, and I'm only finding them again now.

So: The primary goal is to achieve as much isolation from the noise upstairs as I can for $2k - $3k.

To be very honest, I would start by buying them some carpet! That's the best possible way of eliminating impact noise. Buy them a nice thick carpet with good underlay for all of their flooring.

I've done some measurements with an SPL meter - it was set to C-weighting and fast response, which I'm not entirely sure was the correct setting . . .

Yes! That is, indeed, correct. "C" weighting is much more sensitive to the low end than "A", and is a closer match to human perception of loud music.

For impact noise "fast" is appropriate, since you do want to capture the transient peaks. If you were concerned about music, then "Slow" would be the right setting to use.

but if I can achieve an STC in the mid 40s than I think I could work with that.

Forget STC. It is no use at all for telling you how well your studio will be isolated. STC was never meant to measure such things. Here's an excerpt from the actual ASTM test procedure (E413) that explains the use of STC.

“These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling alleys, power transformers, musical instruments, many music systems and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.”

It's a common misconception that you can use STC ratings to decide if a particular wall, window, door, or building material will be of any use in a studio. As you can see above, in the statement from the people who designed the STC rating system and the method for calculating it, STC is simply not applicable.

Here's how it works:

To determine the STC rating for a wall, door, window, or whatever, you start by measuring the actual transmission loss at 16 specific frequencies between 125 Hz and 4kHz. You do not measure anything above or below that range, and you do not measure anything in between those 16 points. Just those 16, and nothing else. Then you plot those 16 points on a graph, and do some fudging and nudging with the numbers and the curve, until it fits in below one of the standard STC curves. Then you read off the number of that specific curve, and that number is your STC rating. There is no relationship to real-world decibels: it is just the index number of the reference curve that is closest to your curve.

When you measure the isolation of a studio wall, you want to be sure that it is isolating ALL frequencies, across the entire spectrum from 20 Hz up to 20,000 Hz, not just 16 specific points that somebody chose 50 years ago, because he thought they were a good representation of human speech. STC does not take into account the bottom two and a half octaves of the musical spectrum (nothing below 125Hz), nor does it take into account the top two and a quarter octaves (nothing above 4k). Of the ten octaves that our hearing range covers, STC ignores five of them (or nearly five). So STC tells you nothing useful about how well a wall, door or window will work in a studio. The ONLY way to determine that, is by look at the Transmission Loss curve for it, or by estimating with a sound level meter set to "C" weighting (or even "Z"), and slow response, then measuring the levels on each side. That will give you a true indication of the number of decibels that the wall/door/window is blocking, across the full audible range.

Consider this: It is quite possible to have a door rated at STC-30 that does not provide even 20 decibels of actual isolation, and I can build you a wall rated at STC-20 that provides much better than 30 dB of isolation. There simply is no relationship between STC rating and the ability of a barrier to stop full-spectrum sound, such as trumpets, saxophones, and violins. STC was never designed for that, and cannot be used for that.

Then there's the issue of installation. You can buy a door that really does provide 40 dB of isolation, but unless you install it correctly, it will not provide that level! If you install it in a wall that provides only 20 dB, then the total isolation of that wall+door is 20 dB: isolation is only as good as the worst part. Even if you put a door rated at 90 dB in that wall, it would STILL only give you 20 dB. The total is only as good as the weakest part of the system.

So forget STC as a useful indicator, and just use the actual TL graphs to judge if a wall, door, window, floor, roof, or whatever will meet your needs.

Sorry about the long rant, but STC is so misunderstood and so misused that it is worth clarifying.

So, I'll assume that you are saying that you'd be happy to get 40 dB of isolation (transmission loss), implying that your 105 dB sax will come through at about 65 dBC outside, and your 90 dB violin will come through at about 50 dBC. Is that correct? That's still pretty loud outside. Still rather audible!

Lay 1" 703 rigid fiberglass on the floor, then a finished floor of some kind.

You skipped a layer! You can't lay finish flooring directly on insulation: You need a decently solid subfloor under it. Put down a layer of 3/4" OSB or plywood on your 703, or perhaps even two layers of 5/8" screwed and glued. Then put your finish flooring on top of that. And be sure to leave gaps all around the edge, so that the subfloor and final floor does not touch the walls. Backer rod and caulk goes in that gap on the subfloor.

Here's basically what I have in mind:

It's a god plan, and clearly you have done your homework!

Aside from a general "is my reasoning sound?", the big question mark is what to do in front of / on the face of the brick inside the room. I understand the mass / air / mass 2 - leaf concept, but when are the leaves so disproportionate in mass and / or the air gap small enough that it doesn't make much of a difference?

That wall is a single-leaf, so it is subject to Mass Law alone. The other walls are partly decoupled, and not subject to mass law alone. So do the math, and figure out how much each of those walls is providing, in terms of TL:

Empirical real-world overall Mass Law transmission loss equation:

TL = 14.5 log Ms + 23 dB (where: Ms = Surface Mass in lb/ft2 )

Empirical real-world overall Mass Law transmission loss equation:

TL = 14.5 log (120) + 23 dB
TL = (14.5 * 2.079 ) + 23 dB
TL = 53.14 dB

Allowing for imperfections and the Murphy factor, that wall is giving you about 50 dB of isolation (but not STC-50, which is very different! ) I very much doubt that the partly decoupled RC walls will be giving you that much. Probably more like 45... with luck... on a good day... with favorable winds...

In fact, for high mass walls, that simplified version of Mass Law equation is not accurate, and the brick wall is probably giving you closer to 60 dB.

I'd say that there's no need to do another leaf in front of that.

HOWEVER! There will be flanking at the point where the two leaves of the side walls meet the single leaf of the brick wall. I would try to minimize that by surface grinding a flat, wide, smooth groove down the brick face, where the drywall will meet that, sealing the surface, leaving a gap between the drywall and the groove, then caulking that abundantly. That will keep the side wall drywall from flanking into the brick, and vice versa.

I would also strongly suggest that you go over that wall very carefully and fill all gaps, cracks and holes in the surface on both sides, then seal both sides carefully with a good quality masonry sealant.

Secondly, as far as the windows are concerned, is there a reasonably accurate method to test their performance with an SPL meter to give me a sense of how beefy the plugs would need to be?

No, but there's math and theory to predict that!

First, take out whatever glass you have in there right now, sell it, and replace it with some proper laminated glass of the correct thickness such that it will match the surface density of your walls, or a bit higher. You say your walls are around 2.5 psf, so you'll need glass that comes in at around the same number. The density of glass is very roughly 2500 kg/m3, which is about 150 PCF, which is about 12 pcf per inch thickness, so you'd need laminated glass that is 0.375 inches thick. That's 3/8". I would go up one step, and get laminated glass that is 1/4+1/4 with an acoustic PVB inter-layer.

Then you'll need to build your plug to match the same density as the wall, or a little higher.

And since they are (I believe) double-pane, if I add window plugs, I'd have a 3 leaf assembly - but that seems un-avoidable.

It's only unavoidable if you don't replace the existing glass!

To be very honest, what I would do if that were my place, is to rip out the entire existing frame, with the glass, sell it on e-bay, and build a new, deeper frame that allows me to have two panes of that laminated glass, separated by as large an air gap as I can possibly fit in. I'd decouple both panes from the frame, and decouple the inner-leaf drywall (the stuff on RC) from the new frame by leaving a gap, then caulking it abundantly.

I remember reading about some small benefit of using different thicknesses of glass for control-room windows, to vary the coincidence frequencies. Could that be applied to any effect in this case?

You could do that, yes. In which case I'd go for 5/16" + 1/4" in one of those.

The floor towards the back of the house is a bit spongy -

Get that fixed first, before you do anything else!

I think a century of rain has caused the foundation to settle a bit.

Time to call in the structural engineer.... Don't do anything until he takes a look at that, and tells you how to fix it. You can't add any extra load to that until he gives you the OK to do so.

Will the 703 help?

IT will help a bit, yes, but it does not constitute a properly floated floor. You need to read this: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

The floor seems to be the weak point of the entire studio....

- Stuart -