John Sayers' Recording Studio Design Forum Archive

2 layers of 16mm = 32mm and 3 layers of 12.5mm = 37.5mm.
Surely this is more mass than a double 16mm wall?

Why will it not have a lower RF?

Can you possibly explain this with numbers?

Gulp! Kind of putting me on the spot!

OK, the basic issue is that the leaf acts both as a whole (one single mass) but each part also acts individually. So although the total mass is greater, it is still made up of several thinner, lighter and more flexible pieces, each of which will react to its own physical properties. "Thin" means lower surface density means higher resonant frequency (for the individual panels). "Flexible" means less resistance to bending waves, means it vibrates easier at more frequencies. "Low mass" is self explanatory. So the panels do still act by themselves, as well as acting together as a leaf. For example, there is a possibility that the panels can vibrate individually, "rattling" against each other, especially if they happen to resonate at different harmonics. Then, being thin and flexible, there is a greater chance of having tiny air cavities trapped between layers, and tiny air cavities are not good.... Then there are also the issues of bending waves and coincidence dips and shear and acoustic impedance and the different equations for thin panels and thick panels and critical frequencies, and a lot of other fancy-sounding stuff that I don't understand too well... But even something simple like the coincidence dip is much deeper for thinner panels than it is for thick panels.

Numbers.... Hmmmm... hard to find, but Presto! Here's some numbers from tests done by Thermafiber. (Check their website.) The tests were actually done on a series of walls to show how their product improves performance, but the table also happens to show the raw data of improvement by using the SAME product in two different thicknesses of drywall:

125 250 500 1k 2k 4k
1/2-in. 0.5 2.6 1.0 1.3 3.1 2.6
5/8-in. 0.1 0.2 1.2 0.0 1.6 2.6

Ok, that's not really a conclusive laboratory proof, but it does indicate something: The only place where 1/2" drywall is better, is in the region from 1kHz to 4Khz, but at such high frequencies it really doesn't matter, because the wall is isolating so well anyway! Isolation problems are always in the LOW end of the spectrum, where 5/8 (16mm) is always better than 1/2 (12mm).

If you want to get a better handle on the theory, then the best I can think of is the good old Wyle report (WR 73-5R), from way back in 1973, but still very valid, and very easy to understand, for the basics on MSM isolation.

(PS. No, gluing thin panels together to try to make thick panels doesn't work either, and that is even harder to understand!)

- Stuart -

OK I've got that report, I'll have to look into it a little deeper.

Funny you mention glueing panels, and in another post you suggested cutting panel to beef up the wall between the studs.

Well I was recently on a site somewhere, where it was suggested to use scrap drywall pieces and glue them to the panel between the studs. Then he suggested using scraps that were different in each bin (the scraps don't have to completely cover the whole area) in order to adjust the resonance in such a way that each bin would be slightly different. This would skew the resonance which would normally be the same in each resonant chamber (i.e. drywall fixed to frame and sealed) because the wall is uniformly built.

It's late, I hope you can follow what I was trying to say.

Funny you mention glueing panels, and in another post you suggested cutting panel to beef up the wall between the studs.

Sorry, I don't see the connection? Why is cutting drywall to fit between the studs similar to gluing panels? Confused...

Well I was recently on a site somewhere, where it was suggested to use scrap drywall pieces and glue them to the panel between the studs.

Ahhh! Now I see the connection! .... But BAD IDEA!!!! The guy doesn't seem to understand the concepts here...!

Then he suggested using scraps that were different in each bin (the scraps don't have to completely cover the whole area)

Umm... yeah, actually they DO have to cover the entire wall! And they have to be carefully sealed in place... Plus, using different density bits in different places WEAKENS the isolation, and does not IMPROVE it. Sheesh! And GLUING bits together???? I mean, come ON!!! Whew!

in order to adjust the resonance in such a way that each bin would be slightly different.

Ummm... no it wouldn't! The net effect would just be to weaken the isolation of the entire wall. The wall is only as good as its weakest part. If you leave an empty patch on it with only one layer, then that's how good the entire wall is! Sounds like this guy thinks you can build an aquarium using cardboard instead of glass, as long as you stick more bits of cardboard at random on top of it.... !

This would skew the resonance which would normally be the same in each resonant chamber (i.e. drywall fixed to frame and sealed) because the wall is uniformly built.

Why would you want to do THAT??? You WANT the resonance of the entire wall to be the same!!! The entire concept of an MSM wall is to get the overall resonant frequency down as low as possible. If you do anything at all to the wall that changes the resonance of one part of the wall, then you are NOT minimizing the frequency! You are HARMING things, not HELPING things... Hoo boy....

It's late, I hope you can follow what I was trying to say.

Yup, I follow, but that poor fellow's logic is rather misguided, to say the least! That's the same kind of guy who most likely recommends sticking carpet on the ceiling and egg-crates on the walls! WOW!

Thanks for giving me a good chuckle! It's nice to know that acoustic mythology is still alive and well on the internet, and continuing to feed garbage to those who are only too willing to consume it... (The worst part is, when someone is done building his place like that, he'll probably think it is great! Simply because he doesn't have a clue, and has no point of reference as to what a REALLY great studio is like.... Thus the myth perpetuates: "Well Joe Bloggs did it that way, and it turned out great!"... yeah right...

Sigh!


- Stuart -

This was the link I referred to.
http://www.hometheaterpeople.com/projec ... wroomID=24

Thanks Brian!

The drywall is 5/8" thick on the ceiling & 1/2" thick on the walls.

That alone speaks volumes...

glued left - over drywall pieces to the backside of the 1/2" drywall in the stud cavities to lower the resonant frequency of the wall panels. The amount of the scrap drywall should not be the same in every stud cavity because we don't want all the wall panels to resonate at the same frequency.

It sounds like they don't even WANT isolation here, and are using the walls for treatment? Randomly tuned panel traps? uhhh... okkkkkayyyy...

We actually use two center speakers, one above and one below the screen wall.

And I'm sure that worked out great, too!

Oh well....


- Stuart -

OK, I've found a different product which numerous people here (locally) have suggested I use (including my house architect), so perhaps you can tell me if I'm good to go with this product.

It's called Fermacell and for a 15mm thickness weighs 18 kg/m² compared to normal drywall (data from Knauf) which has a weight of 12.2 kg/m² for an 18mm thickness (14.4 for 18mm special firewall) and only 12kg for 15mm.

I can attest to it being much heavier, having tried lifting it and the normal drywall.

I also finally found some useful wood sizes for the framing (Spruce or Fir):
construction grade timber:
40mm x 60mm @ 1.50/m
and 50mm x 50mm @ 1.58/m
or planed timber:
55mm x 75mm @ 5.18/m
and 55mm x 55mm @ 4.36/m

The cheaper construction lumber won't give me a very decent depth for treatment at 60mm do you think I should go for the more expensive for the extra 15mm?

Hey Brian, I'm in France so know Fermacell well. It's used in construction for outside applications as it's completely rot-proof. If I remember well it has a mass of 1300 kg/m3 compared to about 550 kg/m3 for good quality drywall (Placoplatre or Knauf). Thing is it costs a bomb so I ended up going for 2 layers of 13mm drywall sandwiching green glue.
Make sure you don't buy the low-end plasterboard (marked only CE, which you can find at Brico Depot and other DIY chains) but the good quality one (again marked CE but also bearing an NF stamp or whatever it is in Switzerland). Building code (at least in France) makes the use of the latter one compulsory for ceilings and commercial walls - reason being that it's denser, hence heavier, which is what you want.

Right, typically a thicker material and fire rated.

for a 15mm thickness weighs 18 kg/m² compared to normal drywall (data from Knauf) which has a weight of 12.2 kg/m² for an 18mm thickness

Sounds like good stuff! With that kind of mass, you should be fine.

I also finally found some useful wood sizes for the framing (Spruce or Fir):

All of those dimensions seem to be bit on the thin side, and not really "standard" sizes at all. I would go with at least 2x4, which is something like 40 x 90 mm for the walls, and something much beefier for the ceiling, maybe 2x6 or 2x8, but you should someone who knows what they are talking about to give you the exact dimensions, based on the load, span and type of wood.

- Stuart -

"maybe 2x6 or 2x8, but you should someone who knows what they are talking about to give you the exact dimensions, based on the load, span and type of wood."


2X6, this will (southern pine) span 13 feet(ish from outside to outside of framing dimensions) with all other things under consideration, being taking into account.

For your 9 foot span, there is no reason to get any larger than that.

SoundMan wrote:not really "standard" sizes at all.

It's useless to argue about standards here (in Switzerland). Someone once told me, upon my discovering that something or other was available, "We're not behind the moon you know!"
Well they might as well be sometimes.

XSpace wrote:For your 9 foot span

I think you missed in an earlier post where I explained I wont be hanging a full ceiling, as the existing construction is already two leaves, tests indicate I currently have no noticeable propagation issues to the room above.

I tend to be somewhat wary of “experts” here (in Switzerland) – I had an ‘acoustic consultant’ come by and have a look and as I described my intentions with isolation walls he seemed to become somewhat hesitant. Although now retired, it seems he built up his company selling wall treatments consisting of 10mm thick open-cell foam covered with fabric – basically carpeting for walls. Needless to say he realized I wasn’t about to become a customer.
Remember this is Switzerland – where the Placebo was invented!

However, I had a discussion the other day with a chap who is involved with the (rapidly growing industry) construction of home theatres. He admits he’s not an acoustician, but talking with him he seemed to know a thing or two.

So he had a look and I showed him my plans and intentions. And we put things into perspective.
First, it is a cellar room, and not a living space, so apart from any propagation into any living spaces, the permitted emission levels are not as stringent, and the existing walls are well within, and even exceed building code requirements.

Secondly, the adjoining cellar rooms are being used for storage, so people will rarely ever be in them. I explained this is true but as these are classified as multipurpose rooms, that could change. The guy in the room across the hall from me watches DVDs in 5.1 in his room. (I can’t hear any trace of sound in my cellar, but in the hall the emissions from his room are similar to the levels I had during my tests (see earlier post). Another guy at the other end of the building has set up an office in his.

Third, this is not a commercial studio and will not be used for tracking live bands, furthermore, it is quite small and the Db levels will probably rarely exceed 80 tops.

We discussed the walls and ceiling and he said that this was a common problem in cellar installations, that the height is a limiting factor. I asked if he thought I should take down the insulation and make the walls full height. He said too (as is oft described on this forum) that the correct way would be to construct a proper isolation ceiling fully separate from the existing – but agreed, not in this case, because of the third leaf effect plus the fact that it is already very low. He said that taking the walls up to the concrete could conceivably exacerbate propagation to the apartment above which currently is not a problem. He mentioned that nearly every job they do there are always compromises that have to be made.

He found my idea of creating a partial ceiling or lip very intriguing, but said I should take the lip all the way to the brick wall and try to create a seal with neoprene or silicone for the air space between the walls. He highly recommended using Fermacell over Gyproc, explaining that it far exceeds normal plasterboard in all respects, fireproofing, water resistance, holding screws (even with shelves), and can also be used on floors.
He is of the opinion that I just use a single layer of 15mm. A second layer would not necessarily bring more isolation because of the construction, so may not be worth the extra cost. The money would be better spent on treatment inside the room which he said would be the lion’s share of the entire project especially considering the room’s small volume. Good absorption within the room will also help reduce emissions.

He’s very keen on hearing how it turns out particularly with regard to the Lip. He might even try using the technique on some of their builds.

I’m now a little unsure about whether I should use 1 or 2 layers on the walls.
Any further opinions?

BriHar wrote:

XSpace wrote:For your 9 foot span

I think you missed in an earlier post where I explained I wont be hanging a full ceiling, as the existing construction is already two leaves, tests indicate I currently have no noticeable propagation issues to the room above.

Your current picture suggests that you are indeed going to be framing for a ceiling. And if the existing structure is already 2 leaves, this presents a third leaf.

from a earlier post wrote:The ceiling is 250mm reinforced concrete to which 80mm dressed, rigid glass wool panels have been affixed for general reverberation control (this is applied throughout the entire cellar i.e. all rooms). Above this there is a 20mm layer of thermal insulation (kind of like styroboard with aluminium foil on each side) then a 20mm acoustic insulation layer (footstep), PE Foil (vapour barrier?) then 80mm cement poured over the heating pipes (floor heating), finally a 10mm layer of fine cement (screed?), then either parquet or ceramic tiling. This is then the floor of an apartment above.

I simply intend to create about a 1/2 meter perimeter around the top of the walls.

from a earlier post wrote:After seeing the ceiling insulation which has been applied, I am concerned that my isolation walls should go right up to the concrete to establish an airtight cavity and true MAM construct. It appears I can’t take the insulation down apparently because of building codes or certification (Minergie) so, for the isolation level I’m likely to need (the walls are supposed to have an STC rating of around 50 as they are), and as I’m not doing ‘room in room’ I’m going to make the following compromise.

The main considerations are the breach in the dead air space between the walls, and in the critical corner boundary.
I will take the Isolation wall up to the Insulation, leaving just enough space to attach 30 – 50cm wide MDF or gyproc sheets to form a lip or partial ceiling at the top of the wall.

These panels will be attached and sealed to the top of the isolation wall and will fit snug against the insulation panels affixed to the ceiling (also perhaps with acoustic caulk or acoustic weather-stripping between). This construction should hinder the sound flanking directly over the top of the wall, re-establishing the integrity of the corner acoustically, and increase the insulation path to the outer wall improving the seal breach in the dead air space (perhaps also acting as a kind of ½ meter deep trap).

Keep in mind this a compromise and is not an ersatz for RIR isolation nor deemed as effective as full floor to ceiling construction.

Well I've ordered some materials so I can begin work perhaps in a week.

I ordered 60mmx80mm lumber for the main framing and 60mmx60mm for some of the additional construction.

I was rather indecisive about using a single layer of fermacell as the guy I talked with suggested would be enough, so I bought enough to do 2 layers anyway. Cost is not so much a factor for me but I figure it'll be a heckofalot more trouble trying to add a second layer later so...

For inbetween the walls i ordered Rockwool WLG 040 half-rigid 60mm panels. I have to wait until next week to get confirmation they can deliver this otherwise I'm looking at Isover Isocomfort 035 which is a horrendous price compared to the rockwool. The older type of fluffy insulation (paper or foil backed) is pretty much impossible to get here now and is even more expensive than the semirigid. The other possibility is Rockwool Floorrock WLG035 which is only 20mm thick or Isover PBM2 80mm, which again is much more expensive but I'll probably get that over the floorrock which is actually mean't for footfall under floors.