John Sayers' Recording Studio Design Forum

Folks,

I'm hoping Brian Ravnaas, John Sayers, Rod Gervais, or any other isolation experts here can help me to understand something.

As many of you know, on my personal web site I have THIS article from Electronic Musician magazine showing plans to build wood panel membrane bass traps. It has been suggested that this type of bass trap can reduce isolation between rooms because the front panel of the trap creates a triple leaf system with the existing wall. The back of the trap is the interior room wall, and the trap's front panel adds a third "leaf" inside the room.

It seems to me this will not reduce isolation between rooms because the third leaf does not reduce the spacing between the existing two leaves. In all of the two- versus three-leaf comparisons I've seen, they show the same fixed distance with either two leaves or a third added between. Clearly, adding a wood panel trap inside the room will not reduce the wall's interior spacing, so it's not clear why having an extra leaf attached outside the existing wall will reduce isolation. But this is outside my area of expertise so I'm asking the guys who really understand the theory behind this stuff.

Thanks!

--Ethan

hey everybody, i'm really glad to get a chance to post, this forum is always talked about but i've never been a member.

Ethan asked me to comment on this thread, so i joined.


The basic query is: is a triple leaf as bad when its deeper than the comparable double leaf... Its risky to answer questions concretely without testing them first so i dug for some data.

In IR811 the NRC has a ceiling that is OSB/joists/drywall screwed directly, with R19 in the cavity. And the same wall with RC and drywall added to form a triple leaf. The triple leaf in this case is not worse, but also not much better. (if you build a double leaf with the exact same amount of materials, the performance gains are massive). This data both points out the importance of not building triple leaves, and supports Ethans suggestion that adding to a wall to make a triple leaf isn't necessarily bad.


That is just one set of data, and i will look for more, and post them if/when they come to mind. Some (many, actually) exampes of triple leaves are found in IR-586, which studies concrete block with added drywall. In that case, the triple leaf has a distinct positive/negative effect (dpeending on frequency), but with lighter walls where the added mass may be significant, the negative effect may not be as pronounced.

I'll add more if i can, i hope that's at least a bit interesting.

now to add to that, the situation above has a mass-spring resonance from the triple leaf that falls about at the same frequency as the mechanical resonance of the reference ceiling. ... two resonances in series at about the same frequency aren't always worse than 1, and it is certainly possible to get much worse results from adding the triple leaf than what was had above (which was actually an improvement).

So, while clearly double leaf is the way to go, this data supports the thought that adding a 3rd leaf isn't necessarily bad. Still, i would recommend caution when messing with walls... as once you start venturing into the realm of the untested you're sort of asking for trouble (as you don't know what's going to happen).

i'll look for more data.

Hi Brian:

I added another .gif to the bottom of (scroll down a bit)
http://www.bobgolds.com/WallCharts/Quad ... eafSTC.htm

sorry to triple-post.


One thing(s) to think about in the case of adding a panel trap to a wall are these:

-the panel traps resonance freq depends on the mass both in front of and behind it, so the trap might react differently to being put on a lightweight wall -vs- a massive concrete floor

-the panel trap won't probably be 100% of the area of the wall. And as such, the negative effect of the trap won't be as bad (if a negative effect occurs) as if it was over the whole area of the wall. An open door isn't as bad as no wall at all (but its still very very bad, of course, from a sound isolation standpoint).

I once entered a spreadsheet at studiotips that lets one calculate the effect of a (for example) poor quality door of XX area in a wall of YY area, that could be used to estimate the overall potential effect of a panel trap in a wall.

hey, Z

you might change the 2-layers-each-side to the 16" OC example, as the triple leaf was 16" OC.

but that's a great graph and very illustrative. if you read the IR761 text, it says those inner panels weren't attached to any studs... if they had been attached, the results would have been alot more disastrous.

i have some old owens corning data about this that you can have if you like, but it only goes to 125hz.

Brian:

I made a minor edit to that new .gif, to add a Staggared Wood Stud wall.

Also done.

Brian,

Excellent. Thanks very much for your detailed replies! And thanks to Bob "Z" too.

> This data both points out the importance of not building triple leaves, and supports Ethans suggestion that adding to a wall to make a triple leaf isn't necessarily bad. <

That's the only way it made sense to me too. But since I don't have a good grasp on the underlying theory, all I could go by was a "common sense" analysis of what might be happening. I tend to see this stuff in elecrical terms, so a two-leaf system is like a passive filter made from one inductor and one capacitor. Adding another leaf inside the wall puts two capacitors of half the value in series where there had been one capacitor twice as large. That obviously reduces the "blocking" effect. Or something like that. So I could sort of see how adding it inside harms isolation.

> the panel traps resonance freq depends on the mass both in front of and behind it, so the trap might react differently to being put on a lightweight wall -vs- a massive concrete floor <

Agreed. The low-bass trap in my plans has a center frequency of about 100 Hz. It's hard to know for sure, and when I measured a portable version I made at IBM's acoustics lab it peaked at 100 Hz. But reverb room absorption measures at only third octaves, and that hides a lot of detail. None the less, this low-bass panel trap uses 1/4 inch plywood for the front surface, so that's still pretty "far away" from the mass and self-resonance of even thin (1/2 inch) sheet rock.

> the panel trap won't probably be 100% of the area of the wall. <

Well, it could be a significant part of it. I learned how to build these traps in the late 1970s from the acoustic consultants I hired when I built a large pro studio. These traps are installed as a "system" that goes all around the room. However, the trap types alternate between low-bass, fiberglass-only, high-bass, fiberglass-only, low-bass, and so forth. So 2/3 of the wall (and optionally ceiling) could potentially have wood panel traps. I have to think the high-bass type with only 1/8 inch plywood affects this even less though.

Thanks again. If you find any further data about adding a third leaf outside the existing wall please let us know.

--Ethan

Just to enter a link to the original thread:
http://www.johnlsayers.com/phpBB2/viewt ... 8355#48355

Ethan,

I think you didn't ask a question here but just were looking for a confirmation.
Brian is mainly active at AVS where you also are very active. Then you ask Brian to come here, who specially needs to register.

Brian's reply is the typical but rightful careful scientist's reply:



How triple leaf systems behave in practice can be very unpredictable sometimes looking strange. Brian carefully states: "isn't necessarily bad". He did not say: it can't harm TL.
That making the total outside measures of a tripple leaf system larger logically will be less damaging than dividing an existing space, but that's still no automatic synonym with it can't harm TL.

You TUNE your traps (logically) specific to dominant audio frequencies, and they are designed to maximize resonance.

I read between the lines that you want public confirmation that panel traps can't diminish insulation.

For double leaf systems they do. How much depends on the mass ratios but it doesn't matter that the existing wall is a very heavy wall or not.
For triple leaf systems, as said they can behave very unpredictable, but by definition creating a higher resonance will influence TL.

This all depends of a lot of factors. And the relative significance in a project also depends.


Here you start from the assumption of an existing wall assuming you just add something, without altering the existing wall. Well whether you work with a double or a triple leaf system you alter the properties of the existing wall, because you create a new system.


From all those rightful and careful question marks in Brian's replies, you seem to interpret this as a confirmation that panel traps can't hurt TL.


Even just a gypsum board in front of a heavy brick wall will alter the TL properties significantly of that wall.

In the tables I entered and very good analyzed, and presented by Bob you can see the influences on the MSM, showing you it creates different systems.
Triple leaf systems can behave strange, but you can't conclude anything more from this than the valuable but very careful comments as expressed by Brian.
Which certainly does not support the almost (free translation): "You see, I thought so too, common sense ....."
In your common sense, you saw this as something added to an existing situation which you interpret about as some constant.
The tables I entered make clear that adding something to an existing wall (whether single or double) alters the system, based on strict MSM calculations (ignoring other factors as mounting, damping etc.).
http://www.johnlsayers.com/phpBB2/viewt ... ght=#48870
In how far this all translates to real live circumstances should be tested.

And important is that you tune a trap ALWAYS to dominant disturbing audio frequencies.
Bob replied rightfully in the other thread that his reply/presentation was relevant to your question.
http://www.johnlsayers.com/phpBB2/viewt ... ght=#49275

It should indeed be very interesting to see representative real live measurements. That suggestion of Brian is very nice, gentle and welcome.

A very relevant but simple experiment -

http://forum.studiotips.com/viewtopic.php?t=1198

FWIW... Steve

Knightfly:

Cool memory.

Yup; 1 gigabyte, to be precise

(It's called [ctrl]b in Mozilla, lets you search for stuff you've stupidly filed in the wrong folder... )

Hi again everybody. Just a couple of notes, if i may:

Ethan, you are sure welcome for my contributions, and i'm grateful that you thought to invite me.

Eric, your comments and bolding of "necessarily" are very fitting and prudent. Certainly the only honest answer i could give is "i don't know for sure".

Imagine a 4' x 8' sheet of drywall or plywood freely floating in the air. If you struck that sheet of plywood, it would ring a bit like a bell. you could hear the reaosnance slowly die down.

Now go to a wall near you - a boring old conventional 2x4 wall - and do the same thing. Strike the wall and listen. You won't hear a ringing slowly die down. The reason for this is that lots of little things add damping to walls. The freely floating sheet has very little damping, but a wall has losses introduced by screws and friction and rubbing and minute air leaks and so forth. Probably 10x higher damping for the wall than the free sheet is conservative. Still not great damping, but alot better than the free sheet.

Now imagine a panel trap on a wall. Its closer to the free floating panel than to the wall for damping, i'd guess. I know the insulation adds some damping, but i don't know how much.


That isn't meant as anything other than an observation that a panel trap may have higher potential for severity-of-resonance than a typical wall. If the damping of a real wall was as low as raw drywall, TL at the low-freq resonance would be very nearly zero, regardless of mass.

In about 2 weeks i will be in a transmission loss test lab with some floor/ceiling installations.

I am open to suggestions of any kind, but i just propose this:

i'll test a wall (which would have to be tested anyway)
-then screw a 4' x 8' frame with weather stripping on the back to the wall
-add plywood and insulation to make a simulated panel trap
-test again

And report the unofficial (its more expensive to get an official test) results in this thread.

Seem reasonable?


My questions would be what thickness of plywood and what depth of trap and what type of insulation. And it will have to be 16", 32", or 48" wide.

Brian Ravnaas:

Without insulation the peak will be higher, and probably more easily detectable.

What are the modal resonances of the test room? We could target a frequency that will ring.

What's the exact wall construction you have in mind? (e.g. {2G16, SWS with insulation, 2G16}).


{No plans to 'build' it the way Paul described (have someone stand there and hold the thing)?}