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PostPosted: Thu Apr 19, 2018 11:12 pm 
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Location: United States - all of 'em - I'm on tour!
Hello to all.
I've spent years lurking, reading, learning from this forum but have not previously posted.

I'm seeking input about the use of spray on insulation, its efficacy or not, for use in a drum booth wall.

I'm currently touring with a broadway show, band is live on stage, with a set built barrier wall around the drums.
We had a thick layer of rock wool panels velcro'd to the drummers side of the barrier that was removed and replaced with spray on insulation.
The logic given that the spray on foam has the same absorption specs on the manufactures web site as the rock wool panels.

The drum iso wall is open studs on the drum side and a 1/4 plywood on the band side. At the height of the toms, its 1/4 plexi.
The spray foam was sprayed into the open studs but due to construction limitations, there are entire sections left with no foam at all.

As musicians, we are in the difficult position of having to explain to management why this might not have been a good idea, and to possibly offer a better solution.

Its all about knocking down a few more db's for us. We are all on in ear monitors but every db of lowered volume would help.

I realize this is the studio building forum but as I've always been impressed with both the high level of knowledge here as well as the generous helpful nature of the sharing of that knowledge, I'm hoping to be pointed to good reference material I can forward to the design team that will help clarify and improve our situation.

thanks so much in advance for your thoughts and input!


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PostPosted: Fri Apr 20, 2018 4:04 am 
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First off, realize that unless your drummer is in a sealed room, your drum sound will leak out. Spray foam/insulation has been discussed countless times on this forum and never been favoured. I have googled the matter myself and not once did I come across a reputable lab result.

IMO you should be expecting the barrier of your shield to stop some amount of high frequencies from hitting your vocal mics (some cymbal volume) and that's about it. Now, since classic drum shields made of plexiglass have nothing to dampen frequencies, all of that energy is no shooting right back into the drum mics and/or the drummers ears. Having said that, your best course of action would be to utilize the 2x4 construction of your shield as a place to put acoustic treatment. I'm talking about insulation. Real proven insulation like the 700 series Owens Corning. If transportation of insulation batts is the concern, you can easily hold it in with chicken wire and/or fabric. To further attenuate some of those high frequencies, we're all taught that mass is everything. So, get thicker plexiglass. Use thicker and heavier wood/sheathing. Make sure that any connections for chunks of your walls have a seal.

Now, as I mentioned above, any sound you stopped from shooting into the audience or your performers mics are now going to be contained inside your drummers little box. So, you have to transfer as much of that sound as you can to heat. That means you need to put as much insulation around your drummer as possible. I'd guess that pink fluffy fibreglass like OC R24 behind some OC 703 would be a good start. Place a ceiling on their box and of course treat the wall behind them. Just make sure they have some fresh air getting into their world.

Or, make the drummer mad and tell them to use an electric kit. haha

Greg


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PostPosted: Fri Apr 20, 2018 5:29 am 
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Hi there "edhamilton", and Welcome! :)

Quote:
with a set built barrier wall around the drums.
Is that a complete shell around the drummer and the kit? All sides and also a "roof" on top? If so, then that's the best possible scenario, but as Greg pointed out, even then you won't be getting dramatically reduced sound on the other side, unless you invest in a serious isolation booth. You would need what is known as a "fully-decoupled 2-leaf MSM system" around the drums. It is possible, but expensive. And HEAVY. Greg mentioned that too: the only way to really stop sound from getting from point "A" to point "B" is with a massively heavy barrier. Assuming you are limited to a single-leaf barrier, the maximum isolation (reduction in sound pressure level) that you can expect is maybe 15 to 20 dB or so, but probably more like 10 or 12 dB.

On the other hand, if the drum booth is open (just a low wall around the kit with no roof), then you can't expect even that much. Probably 5 to 10 dB, tops. Sound will simply go over the top.

Quote:
We had a thick layer of rock wool panels velcro'd to the drummers side of the barrier that was removed and replaced with spray on insulation.
Bad move! :shock: Not a good idea at all...

Quote:
The logic given that the spray on foam has the same absorption specs on the manufactures web site as the rock wool panels.
I'm not sure where you found real specs for the spray-on foam, but I certainly CAN assure you that they are nothing at all like those for mineral wool! Whoever told you that is either lying, or was looking at totally the wrong specs. The spec you need to know is something called "gas flow resistivity", which is measured in the rather obscure units of "MKS rayls" (or Pa·s·m^^−1 if you prefer, also known as N·s·m^^−3 and kg∙s−1∙m^^−2). The vast majority of insulation manufacturers don't even bother measuring that, or publishing it, because it is meaningless for the primary purpose of insulation: keeping the heat in. It's only us crazy guys that want to use the insulation for OTHER purposes who need to know that. So basically, what that "gas flow resistivity" thing actually is, is the measure of how the material resists the flow of gas (duh!). In other words, how air moves through it, which also means how SOUND moves through it, since sound is nothing but vibrating air. So this "gas flow resistivity" number tells us acousticians a lot about the acoustic properties of the insulation, and there's a useful range of values for various acoustic purposes. Generally you want something in the range of about 5,000 rayls to about 20,000 rayls (but there are some applications where numbers outside that range are needed). Those are good numbers for acoustics, as they indicate good acoustic absorption. Now, here's the thing: the "gas flow resistivity" for your spray on insulation, is infinite! :shock: Think of the biggest number you can invent, and the "gas flow resistivity" for spray foam is going to be even higher than that number The foam forms a hard, solid, perfectly airtight surface layer, and if you cut it open, inside it consists of a huge number of tiny cavities, each of which is also fully sealed, all around. Like bubbles. That implies, logically that the "gas" (air) cannot "flow" through it at all! It absolutely and totally "resits" all the "flow" of "gas". Therefore, it is of no use at all, as an acoustic absorber.

Therefore, whoever told you that it has the same properties as mineral wool, simply doesn't have a clue, and is totally wrong. Porous insulation such as mineral wool, fiberglass, cellulose, cotton, and proper acoustic foam, are all technically know as "open cell" materials, because at the microscopic level, there are tiny holes and gaps between the fibers or parts of the foam, and air can move through those gaps. But things like "foam in a can" spray-on polyurethane foam is "closed cell". Just millions of tiny bubbles of air, each trapped forever in its own little sealed cocoon, with no way for air to move through it.

Quote:
The drum iso wall is open studs on the drum side and a 1/4 plywood on the band side
That will get you about 18 dB of isolation, MAXIMUM, if you build the booth as a totally air-tight perfectly sealed box. By the time you add air vents and a door, you are down to maybe 12 dB, with luck. The drum kit can easily put out 115 dB of sound, so reducing that by 12 gives you 103 on the other side. That's LOUD! It hardly makes any impact at all on the sound. It will take the edge of the cymbals, hi-hat, and the "crack" of the snare, but it won't do anything realistic for the toms, kick, and "punch" of the snare.

Now here's the kicker: As Greg mentioned, adding insulation to that (mineral wool, fiberglass, or whatever) will not increase the isolation! That's a myth. A very COMMON myth, but still a myth. Insulation does NOT stop sound getting through. It can help to improve isolation in a proper isolation wall, yes, but by itself it does nothing to improve transmission loss (isolation).

What it DOES do, is to damp the resonance and reflections going on INSIDE the booth, so the drummer can stay reasonably happy, without being smothered in a massively powerful, crushing sound field. Because when you stop sound from getting OUT of the booth, that means it stayed IN the booth, bouncing around between those plywood walls, and killing the poor drummer. The mineral wool helps greatly with that. It absorbs those reflections, and the resonance, which makes the drummers life a little easier.

So, yes, you DO need insulation on the drummer's side of the barrier, to give the poor guy a chance at surviving, but no, it will NOT improve the isolation between the drums and the outside world.

If you use spray foam on the drummer's side of that barrier, then you don't even accomplish that! Because it has no acoustic absorption properties at all, it reflects back all of the sound, back into the booth, just like the plywood does.

Quote:
As musicians, we are in the difficult position of having to explain to management why this might not have been a good idea,
Read them the above explanation. Or summarize. But basically, yes, it was most certainly not a good idea at all! In fact, it was a rather bad idea. Whoever came up with that idea didn't do their homework very well.

Quote:
Its all about knocking down a few more db's for us. We are all on in ear monitors but every db of lowered volume would help.
Are you talking about lowered volume INSIDE the booth, or OUTSIDE the booth? If you want to lower the volume for the drummer in the booth, then by him a set of construction "ear defender" headphones, to wear over his in-ear buds:

Attachment:
peltor_eardefenders.jpg


That's the simplest solution for the drummer, inside the booth. But if you need the booth to stop sound getting OUT, then it needs to be either made much more massive (eg, add two layers of 5/8" drywall on top of the plywood: that would give you about max. 30 dB isolation instead, but realistically around 20 to 25. So your 115 dB drums inside would be down to 90 dB outside. Still loud, but less loud.
Quote:
I realize this is the studio building forum
No problem! It's very much studio-related question.

Quote:
I'm hoping to be pointed to good reference material I can forward to the design team that will help clarify and improve our situation.
OK, here's the thing. If you want to do a really decent job of cutting down the drum "bleed" to the outside world, the firstly seal up that plywood onto the studs so that it is air-tight. USe flexible bathroom or kitchen caulk to seal every last tiny gap or crack, in and on and around that plywood, especially in the joints between panels. Then put another layer of the same plywood on top of that, on the outside, and seal that too. Then buy something called "RSIC clips", that look like this:

Attachment:
RSIC-clip.gif


Those are isolation clips that you screw into the studs on the inside of the booth. Then put ordinary "hat channel" into those clips, like this:

Attachment:
rsic1-stud-enh.jpg


Then fill that space between the plywood and the hat channel with Owens Corning OC-703 insulation, and screw a layer of 5/8" drywall onto the hat channel, once again sealing everything air-tight with caulk. That can get you as much as 40 dB of isolation, if you do it right. Probably more like 30 to 35 in real-life. So your drums will be down to around 80 dB or so outside. Not exactly quiet, but MUCH quieter than you have them right now.

Of course, you also need to put a door in there, so the drummer can get out every now and then to go home, eat, sleep, and suchlike (he'd probably appreciate that... :) ), and that door needs to be heavy and sealed air-tight too. And you'd also need to build some type of simple ventilation system, so he can have fresh air pumped inside to breath, and the stale air getting sucked out (he'd probably appreciate that even more,... assuming he wants to stay alive in there....). And maybe a window, so he can see out, which creates a big problem as putting a window in there can greatly reduce isolation... but it can be done. That would be the plan, if you want high isolation. It might not be very portable, though. It will be heavy, and moving it would damage all the seals...

So, if management is serious about stopping the sound of the drums that is spilling out onto the stage, then that's the best way to do it.

OK, so to actually answer the original question: Spray foam is totally useless, and you can basically scrap that entire booth and build a new one, since that stiff is practically impossible to get off completely once it hardens.

Probably not what you were hoping to hear, but that's the sad truth.


- Stuart -


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PostPosted: Mon Apr 23, 2018 5:07 pm 
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Joined: Wed Apr 18, 2018 5:31 pm
Posts: 3
Soundman2020 wrote:
Hi there "edhamilton", and Welcome! :)

Quote:
with a set built barrier wall around the drums.
Is that a complete shell around the drummer and the kit? All sides and also a "roof" on top? If so, then that's the best possible scenario, but as Greg pointed out, even then you won't be getting dramatically reduced sound on the other side, unless you invest in a serious isolation booth. You would need what is known as a "fully-decoupled 2-leaf MSM system" around the drums. It is possible, but expensive. And HEAVY. Greg mentioned that too: the only way to really stop sound from getting from point "A" to point "B" is with a massively heavy barrier. Assuming you are limited to a single-leaf barrier, the maximum isolation (reduction in sound pressure level) that you can expect is maybe 15 to 20 dB or so, but probably more like 10 or 12 dB.

On the other hand, if the drum booth is open (just a low wall around the kit with no roof), then you can't expect even that much. Probably 5 to 10 dB, tops. Sound will simply go over the top.

Quote:
We had a thick layer of rock wool panels velcro'd to the drummers side of the barrier that was removed and replaced with spray on insulation.
Bad move! :shock: Not a good idea at all...

Quote:
The logic given that the spray on foam has the same absorption specs on the manufactures web site as the rock wool panels.
I'm not sure where you found real specs for the spray-on foam, but I certainly CAN assure you that they are nothing at all like those for mineral wool! Whoever told you that is either lying, or was looking at totally the wrong specs. The spec you need to know is something called "gas flow resistivity", which is measured in the rather obscure units of "MKS rayls" (or Pa·s·m^^−1 if you prefer, also known as N·s·m^^−3 and kg∙s−1∙m^^−2). The vast majority of insulation manufacturers don't even bother measuring that, or publishing it, because it is meaningless for the primary purpose of insulation: keeping the heat in. It's only us crazy guys that want to use the insulation for OTHER purposes who need to know that. So basically, what that "gas flow resistivity" thing actually is, is the measure of how the material resists the flow of gas (duh!). In other words, how air moves through it, which also means how SOUND moves through it, since sound is nothing but vibrating air. So this "gas flow resistivity" number tells us acousticians a lot about the acoustic properties of the insulation, and there's a useful range of values for various acoustic purposes. Generally you want something in the range of about 5,000 rayls to about 20,000 rayls (but there are some applications where numbers outside that range are needed). Those are good numbers for acoustics, as they indicate good acoustic absorption. Now, here's the thing: the "gas flow resistivity" for your spray on insulation, is infinite! :shock: Think of the biggest number you can invent, and the "gas flow resistivity" for spray foam is going to be even higher than that number The foam forms a hard, solid, perfectly airtight surface layer, and if you cut it open, inside it consists of a huge number of tiny cavities, each of which is also fully sealed, all around. Like bubbles. That implies, logically that the "gas" (air) cannot "flow" through it at all! It absolutely and totally "resits" all the "flow" of "gas". Therefore, it is of no use at all, as an acoustic absorber.

Therefore, whoever told you that it has the same properties as mineral wool, simply doesn't have a clue, and is totally wrong. Porous insulation such as mineral wool, fiberglass, cellulose, cotton, and proper acoustic foam, are all technically know as "open cell" materials, because at the microscopic level, there are tiny holes and gaps between the fibers or parts of the foam, and air can move through those gaps. But things like "foam in a can" spray-on polyurethane foam is "closed cell". Just millions of tiny bubbles of air, each trapped forever in its own little sealed cocoon, with no way for air to move through it.

Quote:
The drum iso wall is open studs on the drum side and a 1/4 plywood on the band side
That will get you about 18 dB of isolation, MAXIMUM, if you build the booth as a totally air-tight perfectly sealed box. By the time you add air vents and a door, you are down to maybe 12 dB, with luck. The drum kit can easily put out 115 dB of sound, so reducing that by 12 gives you 103 on the other side. That's LOUD! It hardly makes any impact at all on the sound. It will take the edge of the cymbals, hi-hat, and the "crack" of the snare, but it won't do anything realistic for the toms, kick, and "punch" of the snare.

Now here's the kicker: As Greg mentioned, adding insulation to that (mineral wool, fiberglass, or whatever) will not increase the isolation! That's a myth. A very COMMON myth, but still a myth. Insulation does NOT stop sound getting through. It can help to improve isolation in a proper isolation wall, yes, but by itself it does nothing to improve transmission loss (isolation).

What it DOES do, is to damp the resonance and reflections going on INSIDE the booth, so the drummer can stay reasonably happy, without being smothered in a massively powerful, crushing sound field. Because when you stop sound from getting OUT of the booth, that means it stayed IN the booth, bouncing around between those plywood walls, and killing the poor drummer. The mineral wool helps greatly with that. It absorbs those reflections, and the resonance, which makes the drummers life a little easier.

So, yes, you DO need insulation on the drummer's side of the barrier, to give the poor guy a chance at surviving, but no, it will NOT improve the isolation between the drums and the outside world.

If you use spray foam on the drummer's side of that barrier, then you don't even accomplish that! Because it has no acoustic absorption properties at all, it reflects back all of the sound, back into the booth, just like the plywood does.

Quote:
As musicians, we are in the difficult position of having to explain to management why this might not have been a good idea,
Read them the above explanation. Or summarize. But basically, yes, it was most certainly not a good idea at all! In fact, it was a rather bad idea. Whoever came up with that idea didn't do their homework very well.

Quote:
Its all about knocking down a few more db's for us. We are all on in ear monitors but every db of lowered volume would help.
Are you talking about lowered volume INSIDE the booth, or OUTSIDE the booth? If you want to lower the volume for the drummer in the booth, then by him a set of construction "ear defender" headphones, to wear over his in-ear buds:

Attachment:
peltor_eardefenders.jpg


That's the simplest solution for the drummer, inside the booth. But if you need the booth to stop sound getting OUT, then it needs to be either made much more massive (eg, add two layers of 5/8" drywall on top of the plywood: that would give you about max. 30 dB isolation instead, but realistically around 20 to 25. So your 115 dB drums inside would be down to 90 dB outside. Still loud, but less loud.
Quote:
I realize this is the studio building forum
No problem! It's very much studio-related question.

Quote:
I'm hoping to be pointed to good reference material I can forward to the design team that will help clarify and improve our situation.
OK, here's the thing. If you want to do a really decent job of cutting down the drum "bleed" to the outside world, the firstly seal up that plywood onto the studs so that it is air-tight. USe flexible bathroom or kitchen caulk to seal every last tiny gap or crack, in and on and around that plywood, especially in the joints between panels. Then put another layer of the same plywood on top of that, on the outside, and seal that too. Then buy something called "RSIC clips", that look like this:

Attachment:
RSIC-clip.gif


Those are isolation clips that you screw into the studs on the inside of the booth. Then put ordinary "hat channel" into those clips, like this:

Attachment:
rsic1-stud-enh.jpg


Then fill that space between the plywood and the hat channel with Owens Corning OC-703 insulation, and screw a layer of 5/8" drywall onto the hat channel, once again sealing everything air-tight with caulk. That can get you as much as 40 dB of isolation, if you do it right. Probably more like 30 to 35 in real-life. So your drums will be down to around 80 dB or so outside. Not exactly quiet, but MUCH quieter than you have them right now. [ SPAM LINK REMOVED - SPAMMER BANNED ]

Of course, you also need to put a door in there, so the drummer can get out every now and then to go home, eat, sleep, and suchlike (he'd probably appreciate that... :) ), and that door needs to be heavy and sealed air-tight too. And you'd also need to build some type of simple ventilation system, so he can have fresh air pumped inside to breath, and the stale air getting sucked out (he'd probably appreciate that even more,... assuming he wants to stay alive in there....). And maybe a window, so he can see out, which creates a big problem as putting a window in there can greatly reduce isolation... but it can be done. That would be the plan, if you want high isolation. It might not be very portable, though. It will be heavy, and moving it would damage all the seals...

So, if management is serious about stopping the sound of the drums that is spilling out onto the stage, then that's the best way to do it.

OK, so to actually answer the original question: Spray foam is totally useless, and you can basically scrap that entire booth and build a new one, since that stiff is practically impossible to get off completely once it hardens.

Probably not what you were hoping to hear, but that's the sad truth.


- Stuart -

Great stuart. You have provided really in-depth answer.


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PostPosted: Mon Apr 23, 2018 11:53 pm 
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Joined: Thu Apr 19, 2018 10:48 pm
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Location: United States - all of 'em - I'm on tour!
Thanks Stuart.

Your incredibly detailed answer is greatly appreciated!

To answer some of the questions -

The point of the drum "wall" is just to knock enough db's down for the musicians that are literally sitting right up against it.
The perception is that the physical impact increased when the rock wool was exchanged for spray foam.

So the goal now is just to modify it enough to help as much as possible.
Weight is an issue. (whole band on a automated moving platform)

122db was the peak readings post spray foam.
115 average in the louder songs.
(drums are the only acoustic instrument, everything else is DI so its really 122db of drum hits)

with in ear monitors offering up to 22 db in noise reduction, is it possible to have the wall knock down the level enough to not cause ear damage over a long running show?

Does anyone have a decibel calculator?
as in 122 db stage volume minus 22 db of in ear monitor protection = ??? (realize its a log problem!)

thanks again Stuart. Just awesome of you to take the time!


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PostPosted: Sun May 20, 2018 12:30 pm 
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Posts: 10792
Location: Santiago, Chile
Quote:
The perception is that the physical impact increased when the rock wool was exchanged for spray foam.
Yup. That makes perfect sense, and is exactly what I would expect.

Quote:
So the goal now is just to modify it enough to help as much as possible.
Start by digging that foam out! Then put insulation in there again, but this time use something called Owens Corning OC-703, or alternatively, OC-701. Make it thick: at least 4", 6" would be better, and 8" would be better still.

Quote:
with in ear monitors offering up to 22 db in noise reduction, is it possible to have the wall knock down the level enough to not cause ear damage over a long running show?
To a limited extent, but not completely: You are the drummer, so you are getting the direct sound form the drums, and that can easily be 115 dBC all by itself, without considering reflections from around you. Even if you could completely eliminate all reflections, you'd still be exposed to that 115 dBC direct sound field. The only way to reduce that, is to play softer... Reduce your own level by hitting less hard, and using sticks that are not as loud. You can knock another 10 dB off like that.

Workplace safety recommendations list 85 dBC as the limit for safe exposure level. Assuming 115 dBC from the drums, and allowing for the 20 dB reduction from your earplugs, that means you are still getting 95 dBC inside your ears. That's twice as loud as it should be. If you can drop the level by 10 dB using the methods above, that would get you to the "safe" level of 85 dBC.

Yes, by using good absorption on the surfaces around you, you can certainly reduce the REFLECTED level, but that does nothing to control the DIRECT level: the sound from the snare goes direct from the snare to your head, and no amount of treatment of the sides, or behind, or in front, can reduce that. It only reduces the reflected level, not the direct level.

Quote:
as in 122 db stage volume minus 22 db of in ear monitor protection = ??? (realize its a log problem!)
Actually, in this case it's not a log problem! It's simple math, because we are not subtracting sound power levels here: we are comparing sound PRESSURE levels. So if you have 120 dBC and you need to get it down to 100 dBC, then you need 20 dB reduction, and if you need to get it to 85 dBC then you need 35 dB reduction.

Think of it this way: if you set up your sound system so that it produces 110 dBC measured 3 feet away from the speaker when the console master fader is on 0 dB, then bringing down the master fader to the -20 db mark will also bring down the SPL level to 90 dBC. Pulling the fader down to -40 will give you a pleasant 70 dBC SPL level, and pulling the fader all the way back to -90 dB will bring the level down to 20 dBC SPL, which is below the ambient noise level for sure, and therefore inaudible. This is simple math, not log math.


- Stuart -

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