John Sayers' Recording Studio Design Forum Archive

Hi my name is Hendrik. Long time reader, first time poster. I am hoping that I am posting enough information to allow the quesiton to be answered. The design is not complete at this time and the lease will not be signed until next week earliest.

I am in Boston, MA and the job that is driving this question is in Charlestown on the 7th floor of a large commercial building. The exterior of the building is brick and the large open space on each floor has been subdivided into 1000-4000 sqft spaces with steel framed 5/8" gypsum partition walls. There doesn't appear to be any insulation in the air gap. The client's space is about 1400 sqft. Both the ceiling and the floor are poured concrete held up with concrete columns. The height of the ceiling is almost exactly 10 feet.

We do not have permission to demo the outer perimeter wall and it is a two leaf assembly. This wall does not have enough isolation by a long shot.

The question is this: How far apart do two wall assemblies have to be before they are no longer considered part of the same wall assembly? For instance, if you build a standard stud wall with 5/8" gypsum on both sides and insulation between, you have a 2 leaf wall. How far away does the next wall have to be before the volume is big enough between the two walls so that the air doesn't spring-couple the two walls together?

I would like to design a hallway that goes around the perimeter of the rental space to separate the perimeter wall from the internal isolation walls. How wide does the hallway need to be? Doors must be at least 3 feet, passage ways must be at least 5 feet according to the code I was able to find. Is 5 feet enough?

Thanks for everyone's help!

Hendrik

Hi Hendrik, and welcome to the forum!

It sounds like you have an interesting challenge there. Isolating a room on an upper floor is never easy. It would help to know how much isolation your client needs, and what is being isolated. Airborne noise is relative easy to isolate, compared to impact noise.

For instance, if you build a standard stud wall with 5/8" gypsum on both sides and insulation between, you have a 2 leaf wall.

If that's a single-stud wall with drywall directly nailed to each side, then technically its a "coupled 2-leaf", which acts more like a single leaf in many aspects, but also like a two-leaf in other aspects.

How far away does the next wall have to be before the volume is big enough between the two walls so that the air doesn't spring-couple the two walls together?

The answer is: "It depends". The question is actually more about the difference between a two-leaf wall and a four-leaf wall, and since at least one of them is a coupled 2-leaf, it gets more complicated. Basically, all of them are resonant systems, and they only couple at and around the resonant frequency. At all other frequencies, they isolate (or rather, starting at 1.414 times the resonant frequency, they start to isolate). The resonant frequency depends on the total mass on each of the leaves, and on the depth of the air gap between each pair of leaves, and on the damping in the cavity between each pair of leaves.

For one single 2-leaf wall the math is fairly simple, and there is only one fundamental resonance. For a 3-leaf it's a bit more complicated and there are two fundamental frequencies, both of which are higher than for a two leaf wall with the same total mass and thickness. With a 4-leaf wall it gets more complex still as there are several resonances going on, and ALL of them are higher than for the equivalent 2-leaf and the equivalent 3-leaf.

So basically your question boils down to: "What distance do you need, given the mass of each leaf, and the damping, to provide a low enough set of resonant frequencies?". And that in turn leads to the obvious question: "What do you mean by 'low enough frequency'?". If you only need to isolate typical office sounds, such as conversations, phones ringing, office equipment, HVAC noise, and suchlike, then you would probably choose something like 125 Hz as your lower frequency, and you would therefore set 60 Hz as the target resonant frequency, and your job of isolating is not so hard. But if you are talking about a recording studio for rock bands, then you have kick drums down at 80 Hz, and keyboard and guitars going lower than that, then a 6-string bass going all the way down below 30 Hz, so now you need a target resonance of 15 Hz or less, which is not so easy at all. That's nearly three octaves lower!

Then there's the final remaining question: "At that lowest frequency, how much isolation do you need? (in decibels)". If you figure that you need 30 dB of isolation at 125 Hz, it's no big deal. Easily doable. But if you need 30 dB at 30 Hz, that's an entirely different ball game.

So there's no simple answer to your question, unfortunately. There are too many unknown variables. If you could provide more information, then that would help.

However, having said all that, with a 5 foot air gap between the two walls you are very likely fine, provided that your isolation wall actually is built as a proper fully decoupled 2-leaf wall, and not a typical "drywall on single studs" wall.

But adding more doubts to the mix: I'm not convinced that walls are your problem! You say that the existing coupled 2-leaf wall is not giving you any useful isolation at all, yet it should be giving you somewhere in the region of 35 to 40 dB, so the fact that you aren't even getting that suggests that there's a more basic issue, such as sound getting through the HVAC system (very, very likely), or through inadequate doors that are not up to the same isolation level as the walls (also very likely), or simply because the wall is not correctly sealed to the floor and ceiling (likely), and possibly also flanking issues (probable). So before designing the actual isolation system that you need for your client, it might be an idea to check on those other aspects first. If there is flanking going on through the building structure for example, then you'll still have a sound leakage problem, no matter how well you build your walls. And if it is the HVAC system, then you need to design silencers for those. Etc.

I wish I could give you a simple answer to your question, but not much in acoustic isolation is simple! There's usually more to it than meets the eye (or the ear).

If you can provide more information on your client's needs, and on the building itself, then it should be possible to come up with a more useful answer.


- Stuart -

Hi Stuart and thanks so much for getting to my post so quickly! I have been a fan for years! It's pretty exciting to be typing at you.

I will go through and answer as many questions as I can.

Soundman2020 wrote: It would help to know how much isolation your client needs, and what is being isolated. Airborne noise is relative easy to isolate, compared to impact noise.

"At that lowest frequency, how much isolation do you need? (in decibels)"

The answer to How Much Isolation is always As Much As Possible in my experience. I have never worked on a studio that was in the country though.

The building and the area is industrial and is actually pretty noisy during the day. There are a couple of studios in the building complex already and some rehearsal spaces. The noise generated by the studio is not going to be the problem as much as blocking out outside sounds. So we don't need to worry much about the low B strings or 808/303's irritating the neighbors!

The good news with this is that most of the industrial noise doesn't seem to be very low frequency. I have not done a NC rating study, though. I noticed a little bit of 100 Hz-ish sounds when the elevator operates which is separated from the studio space by a standard stud wall with a double layer of gypsum on the inside, a hallway, a corner and then another standard studded gypsum walls.

The majority of the work is going to be commercial recording probably of rock bands, some sound design and VO/ADR stuff. Probably vocals and acoustic guitar are going to be the quietest instruments. If we can get at least 55-60 dB of isolation in the vocal range and above and 40 dB down to maybe 100 Hz I would be pretty happy. I will try to make a small iso room as quiet as possible for the voice and acoustic instrument stuff. But this will be on inner part of the studio and not on the perimeter.

I'm not convinced that walls are your problem! You say that the existing coupled 2-leaf wall is not giving you any useful isolation at all, yet it should be giving you somewhere in the region of 35 to 40 dB, so the fact that you aren't even getting that suggests that there's a more basic issue, such as sound getting through the HVAC system (very, very likely), or through inadequate doors that are not up to the same isolation level as the walls (also very likely), or simply because the wall is not correctly sealed to the floor and ceiling (likely), and possibly also flanking issues (probable).

The doors and windows are the weakest links absolutely. The walls probably are giving about 35 dB of isolation. It's just not enough. There are some air gaps in the walls, definitely air going through the door cracks and the windows are basically open. There is no HVAC right now, so the HVAC will be isolated from the rest of the building. Hurray! But there's no HVAC so the HVAC will need to be purchased and installed. Boo! 35 dB of isolation is a little more than half of what I would like to get in the control room and iso. All the air holes need to be sealed and there is an expansion joint in the sealing that needs to be filled with sealant as well. There are some holes in the brick that will need to be caulking.

My plan was to seal everything and then add a layer of gypsum and QuietGlue to the inside of the perimeter wall. The windows will need to be removed and filled and the doors will need gasketing. The doors will need enclosed entryways and another isolating door.

The impact noise from above is something I am worried about, but I didn't notice any impact noise during my two visits. No high heels, anvil hammering, or anything else that was obvious. I am pretty sure that the ceiling is a solid reinforced poured concrete floor (1 leaf) so I am planning on a single leaf ceiling for the studio. The ceiling is 10 feet so we have a little room to play with. 4 layers of sheet rock could be an option. (I certainly hope not though...)

Thanks so much for your help and wisdom!

Best,

Hendrik

So we don't need to worry much about the low B strings or 808/303's irritating the neighbors!

Sp how about the neighbors low B strings annoying YOU? Isolation is a two way street... So whatever is loud in the rest of the building (especially if it is structure-borne, ie "flanking") is what you should be worried about. And you really should get a sound level meter to check that.

I noticed a little bit of 100 Hz-ish sounds when the elevator operates which is separated from the studio space by a standard stud wall with a double layer of gypsum on the inside, a hallway, a corner and then another standard studded gypsum walls.

Sounds like structure borne flanking noise to me... No amount of walls of insulation will stop that, since it is in the building structure itself.

I will try to make a small iso room as quiet as possible for the voice and acoustic instrument stuff. But this will be on inner part of the studio and not on the perimeter.

If it is resting on the floor, then the noise will still be inside, since we are talking about structure-borne noise. And 100 Hz mechanical vibration implies 200 Hz harmonics, as well as 400 Hz, 800 Hz... Certainly in the vocal range. So I would suggest thinking about methods of isolating the booth floor and inner leaf. I would also suggest trying to determine exactly what noises are in the building structure.

There are some air gaps in the walls, definitely air going through the door cracks and the windows are basically open.

Open windows implies practically no isolation at all! It's not surprising that things are pretty bad in there...

35 dB of isolation is a little more than half of what I would like to get in the control room and iso.

Welll, that's a rather relative "half" we are talking about! Sure, the number is half what you need (30 dB vs 60 dB), but the power involved is vastly greater than twice as much. Decibels are on a logarithmic scale, so going from 30 dB to 40 dB means there is ten times as much power. 30 to 50 is 100 times as much power. And 30 dB to 60 dB is one thousand times the power. So your wall needs to be one thousand times as good at blocking sound as the wall you have now...

All the air holes need to be sealed and there is an expansion joint in the sealing that needs to be filled with sealant as well. There are some holes in the brick that will need to be caulking.

Very much so! Tiny air gaps are probably the biggest issue in isolating existing structures. And sealing everything air-tight is probably the biggest single thing you can do to improve isolation.

My plan was to seal everything and then add a layer of gypsum and QuietGlue to the inside of the perimeter wall.

I would suggest Green Glue, rather than Quiet Glue, and I would also suggest not bothering to much with that for the existing walls, but rather saving it for your new walls.

The impact noise from above is something I am worried about, but I didn't notice any impact noise during my two visits. No high heels, anvil hammering, or anything else that was obvious.

Except the elevator... What about water noise, in the pipes in the building? HVAC noise (fans, chillers, pumps, etc.) Doors opening and closing? Traffic outside?

I am pretty sure that the ceiling is a solid reinforced poured concrete floor (1 leaf) so I am planning on a single leaf ceiling for the studio. The ceiling is 10 feet so we have a little room to play with. 4 layers of sheet rock could be an option. (I certainly hope not though...)

I think you mean "one extra leaf", not "single leaf. The concrete floor up there is already one leaf, so if you want a single leaf ceiling, then you already have it!

Your additional leaf ceiling must rest ONLY on the new inner-leaf walls: no contact at all with the existing structure. Two layers of 5/8" drywall with GG in between will probably be enough, but I would still size the joists big enough to be able to add an extra layer if necessary.

Hope that helps!


- Stuart -

Unfortunately, the project is no more. As is often the case, when comparing the cost of building a new studio to renting studio space over several years, the budget would not balance. Very sad face. The good news is that the client was awesome about the whole thing. So now I don't need to worry about the isolation!

Soundman2020 wrote:how about the neighbors low B strings annoying YOU? Isolation is a two way street... So whatever is loud in the rest of the building (especially if it is structure-borne, ie "flanking") is what you should be worried about. And you really should get a sound level meter to check that.

I did test with a meter at my last visit, but the airborne sound masked out the quieter sounds like the structure-borne flanking sounds. My ears could hear the lower frequencies, but my meter is Type II and doesn't do measurements by frequency bands. I usually use my ARTA rig for NC stuff and calibrate my test mic to the meter's calibrator.

Soundman2020 wrote:Welll, that's a rather relative "half" we are talking about! Sure, the number is half what you need (30 dB vs 60 dB), but the power involved is vastly greater than twice as much. Decibels are on a logarithmic scale, so going from 30 dB to 40 dB means there is ten times as much power. 30 to 50 is 100 times as much power. And 30 dB to 60 dB is one thousand times the power. So your wall needs to be one thousand times as good at blocking sound as the wall you have now...

It definitely is the bigger half! And it's still half empty! The reason we have a logarithmic scale is so we can say things like "half as much isolation," and so we don't have to say things like 1000 times the power.

Soundman2020 wrote:I would suggest Green Glue, rather than Quiet Glue, and I would also suggest not bothering to much with that for the existing walls, but rather saving it for your new walls.

I have used QuietGlue a number of times and I have been pretty happy with the results. I know that there is a battle of the test results between the two companies, but I know know anyone personally who has tried both and found that one was better than the other. It certainly is cheaper! I probably should check out the controversy on this forum of GG vs. QG.

Soundman2020 wrote:I think you mean "one extra leaf", not "single leaf. The concrete floor up there is already one leaf, so if you want a single leaf ceiling, then you already have it!

I should have said that I only need to make one leaf, because the other one is already there!

Your additional leaf ceiling must rest ONLY on the new inner-leaf walls: no contact at all with the existing structure. Two layers of 5/8" drywall with GG in between will probably be enough, but I would still size the joists big enough to be able to add an extra layer if necessary.

Absolutely overbuild the joists! After having to reinforce joists that are too puny with lamination, I now always overbuild the joists. The material cost increase is minimal and the labor is pretty much the same.

Anyway, thanks so much for your help and support!

Hendrik