Triple leaf

Tonio · Post by **Tonio** » Sat Aug 05, 2006 1:31 pm

1st dumb question from a newb.
I searched for info on the triple leaf effect and could not find what I was looking for. It may be that I could'nt grasp the literature.
Please correct me if I am misunderstood. I triple leaf construction has mass >air>mass >air>mass>air>mass.
Any information I could find is that a triple leaf constructed wall is a waste of time and money for isolation. But I am guessing that it would effect acoustical treatment(and/or response within the room) also?
If so, what exactly? Does it resonate (wall dependant) into the inner space, or is it a structural noise issue for attached rooms? Both?
Hopefully , my question isn't too vague.

Tony

knightfly · Post by **knightfly** » Sat Aug 05, 2006 3:25 pm

Actually, what you described is a QUAD leaf; triple would be mass-air-mass-air-mass (3 separate "leaves" of mass) -

Any time you use a panel type trap in a room, if there are already two leaves of mass in your walls and you place this trap close to the wall (where it must be in order to function right) you've just created a third leaf at that location. (Possibly even a fourth, if said trap has a separate back instead of using the wall as its back)

It's also true that if you build "extra" leaves into a wall, they will do pretty much the same as the panel trap; they will have their own resonance which is a function of panel mass, cavity depth, stiffness of the inner wall, air flow resistivity of whatever insulation you placed inside, plus whatever "interaction" with the original 2 leaves causes - normally a reduction in TL at any and all resonant frequencies caused by any combination of masses and air gaps.

This combination of effects is why I recommend to anyone building a room that they put at least one more layer of wallboard up than they think they need for isolation purposes; that way, the negative effect some acoustic treatments have on TL can be at least partially defrayed... Steve

Tonio · Post by **Tonio** » Sun Aug 06, 2006 5:14 am

Thanks for correcting the triple/quad issue.

So if you you install a panel trap on to a a double leaf wall, it would make a triple leaf system. Wouldn't that in a sense negate the acoustical treatment then? Or perhaps even cause a resonance? Thats my understanding from your response.

Related to my question I have a garage door @ the back wall that is probably giving me some grief. I'll get some scetches made up and post.

Hopefully I can get some assistance in getting this remedied.

T

knightfly · Post by **knightfly** » Sun Aug 06, 2006 10:04 am

So if you you install a panel trap on to a a double leaf wall, it would make a triple leaf system. Wouldn't that in a sense negate the acoustical treatment then? Or perhaps even cause a resonance?

No, a properly designed panel trap targets a specific frequency range (typically about an octave wide), and the way they work is that they are placed at a point where that frequency has maximum SPL; the high pressure (not velocity) causes the front panel to resonate (again, this resonance is controlled by panel mass and the depth of the air INSIDE the panel trap. Although panel traps don't need to be SEALED in order to work, they DO need to be sealed in order to make the formula work close enough to get the right frequency -)

When the panel resonates, it causes the air INSIDE the trap to try and move back and forth - placing rigid fiberglass batts inside the trap (but not touching the front panel) causes this moving air to be restricted in its movement thru the absorbent -

The two mechanisms most responsible for "trapping" or converting the unwanted frequency to heat are (1) flexing of the front panel at resonance requires energy, and the flexure dissipates part of the sound wave as heat. (2) INSIDE the trap, the absorbent forces the moving air to travel through all the tiny interstices of the insulation, losing even more energy as it's converted to heat.

So, the upside of all this is you lose sound energy at a frequency range you WANT to lose sound energy - the downside is, this DOES slightly weaken the wall AT THAT POINT ONLY, and it's this part that requires you to build your walls a bit better if you don't want to lose isolation while improving the sound of your room.

People generally state that sound isolation and sound conditioning are two separate things; mostly this is true, but hopefully this explains why it's not a TOTAL separation in some cases... Steve

Tonio · Post by **Tonio** » Sun Aug 06, 2006 2:07 pm

knightfly wrote:

So, the upside of all this is you lose sound energy at a frequency range you WANT to lose sound energy - the downside is, this DOES slightly weaken the wall AT THAT POINT ONLY, and it's this part that requires you to build your walls a bit better if you don't want to lose isolation while improving the sound of your room.

People generally state that sound isolation and sound conditioning are two separate things; mostly this is true, but hopefully this explains why it's not a TOTAL separation in some cases... Steve

Thank you for the in depth response Steve. I understand the design of the membrane panel trap. However your statement above is a consideration. In short, would the wall construction effectiveness(isolation or lack thereof ) hamper the acoustical treatment? e.g. The lower the TL of the wall , lowers the treatment efffect?
What I'm getting at is this: I am not concerned about isolation to a grater degree, I just want a balanced sound. I have toiled with the rear wall that is a converted garage door. It does have a false inner wall. To this wall I installed a 4x4 panel trap. Then modified to a slat resonator. Bottom line is I'm still getting a dull recording in this room. Spectral analysis renders a dip in the 80-160 hz and 300, 400 area where I'm concerned.
I did post some pics over at the other forum for reference (if you recall). I can post all relevant info if need be.

Tony

z60611 · Post by **z60611** » Fri Aug 11, 2006 10:03 am

knightfly:

this DOES slightly weaken the wall AT THAT POINT ONLY

In the same way a 6"x6" hole through the wall weakens the wall at that point only?

knightfly · Post by **knightfly** » Sat Aug 12, 2006 2:19 pm

In the same way a 6"x6" hole through the wall weakens the wall at that point only?

Not quite; a 6x6 hole would have a flat response (depending on wall thickness, if you consider any possible helmholtz action) to sub-audible frequencies, whereas the panel trap would cause the section of wall it's in front of to exhibit multi-leaf tendencies; so low frequency would tend to pass thru easier at that location in the wall, while some un-known midrange/higher frequency would actually exhibit BETTER isolation thru that portion of the wall - also because of multi-leaf properties. (I think )

And yes, this would either improve or degrade the ENTIRE ROOM; what I meant was that the particular portion of wall with the trap would be the cause, not that the rest of the room would see no effect

Sooo, was that (sorta) what you were getting at, or did I miss your point entirely?

Steve

z60611 · Post by **z60611** » Mon Aug 14, 2006 2:56 am

knightfly

You got it. That was my point exactly.

I'll rephrase, with examples.

a) at HF, the extra layers would improve the TL. But assuming this is a relatively small area (e.g. a 4'x4' panel on a 20'x8' wall), the TL of the entire wall system at HF would be barely changed (i.e. it would change from STC 63 to STC 63 -- not any difference in two digits of accuracy in a good wall ::: well STC 40 might become STC 41 in a bad wall although the math says it wouldn't but mightn't such a bad wall already be a sufficent absorber/resonator at 95hz due to it's MSM being also 95hz?)

b) at LF, the resonance would decrease the TL. Assuming the same relatively small area, the TL of the entire wall system would be perceptibly changed. e.g. If the 20'x8' wall had 40dB (good RSIC wall) of isolation at 95hz, and the resonator resonates at 95hz and that 4'x4' section of wall now has 20dB (idiot guess) of isolation at 95hz, then the entire wall system now has 30dB (calculated) of isolation at 95hz. Assuming that it doesn't resonate the entire wall (I figure it averages out).

Yes the cause is AT THAT POINT ONLY, but the whole wall system degrades -- and it's the wall system that listeners will hear on the other side.

ozymandias · Post by **ozymandias** » Mon Aug 14, 2006 6:47 am

Hey all,

If I may expand on Tonio's question -
Are triple leafs also worse when, say, the middle mass is significantly more rigid, or has significantly more mass (i.e. has a different impedance) than the outer two leafs?
For example, I understand how

drywall - 2" fiberglass - drywall - 2" fiberglass - drywall

is worse than

drywall*2 - 4" fiberglass - drywall

(trying to compare fairly - same amount of depth)
But what about something such as

drywall - 2" fiberglass - 4" concrete block - 2" fiberglass - drywall

Is that still worse?
(for the sake of comparison let's assume equal depth - the double leaf partition has 8" of fiberglass inbetween the two drywalls, or 4" fiberglass + 4" air, whichever one is better - not sure about that either

)

z60611 · Post by **z60611** » Wed Aug 16, 2006 5:51 am

I don't have any TL stats on 4" concrete block.
I do have stats down to 63hz on 6" and 8" concrete block with similar constructions to what you mention.

For example
a) PacIntl's STC63 wall has 40dB at 100hz (RAL).
b) {5/8" gypsum, 2.5" steel studs with fiberglass batts, 8" concrete block, 2.5" steel studs with fiberglass batts, 5/8" gypsum} is STC72, A LOT heavier, but also 40dB at 100hz. ("Noise Control In Buildings" by Cyril M. Harris, page 5.54, NRC)

Generally speaking (shaky belief), adding a triple leaf in the middle removes a lower frequency resonance, but adds two new resonances proportional to the new dimensions (higher frequency) and a new structural resonance. The new resonances are weaker than the old single one, but their harmonics also affect the STC region negatively. Hence this sort of thing:
http://www.bobgolds.com/WallCharts/Quad ... eafSTC.htm

More common is to ask what if we put a limp membrane in the middle -- and for that you might want to check the green glue website for thoughts about limp inside of a gypsum wall (the text more than the lab results) http://www.greengluecompany.com/greenGlue-vs-MLV.php

Eric_Desart · Post by **Eric_Desart** » Thu Aug 17, 2006 12:55 am

z60611 wrote:
Generally speaking (shaky belief), adding a triple leaf in the middle removes a lower frequency resonance, but adds two new resonances proportional to the new dimensions (higher frequency) and a new structural resonance. The new resonances are weaker than the old single one, but their harmonics also affect the STC region negatively. Hence this sort of thing:
http://www.bobgolds.com/WallCharts/Quad ... eafSTC.htm
http://www.greengluecompany.com/greenGlue-vs-MLV.php

Z,

I like all your nice references and sources ...
A not important note, and not sure if I express this correct myself, since I shouldn't refer to removing a resonance but just creating a new system:
One could say it lowers the existing resonance frequency and adds a second higher one.
Have a look here:
http://www.vibrationdata.com/animation.htm

z60611 · Post by **z60611** » Thu Aug 17, 2006 4:27 am

Eric Desart:

It's a nice visual of "Two degrees-of-freedom" which you've mentioned occasionally.

The bottom two show the effects of the same mass at two different resonant frequencies (sort of a triple leaf wall, with one perfect leaf of infinite mass).
When (e.g. "Mode 1 f1 = 60.5 Hz") the two masses move in the same direction at the same time (both at resonance) the top mass moves very far (i.e. loud).
When (e.g. "Mode 2 f2 = 146 Hz") the two masses move in opposite directions at the same time (both at resonance) the top mass moves less (i.e. relatively quieter).

Would "Response of SDOF Systems to Half-sine Pulse" be at all like the coincidence region?

Eric_Desart · Post by **Eric_Desart** » Thu Aug 17, 2006 5:19 am

Bob,

It's not sort as. It really is a triple leaf system (2DOF).
I once calculated this whole thing substituting that k (springconstant = stiffness) by an airlayer with the same k (adjusting depth cavity).

But lost it somehow. Maybe will make it again later.

Indeed it's one side an infinite mass. You can imagine this as gypsum boards mounted on cavities on a heavy brick wall.

Notice that this bottom mass and spring equals the Single degree of freedom system higher up.
This means:
1) The top animation: a gypsumboard in front of a brick wall.
2) The bottom animations; starting from exactly the same wall as 1) but putting a second wall in front of that, you get one lower and one higher resonance. Hence that's not even putting a panel in between but leaving the original wall, making him significant thicker with an additional cavity and panel.

On several occasions I searched and searched the net and books and whatever to find some good representation or animation where this infinite mass is also a normal mass. Until now without success.

The reason is simple: those simulations are often made for decoupling of machinery, which mostly is mounted on stable floors.
If you should find something better I should be glad to hear it.

But believe it or not, what you see there is calculated with exactly the same formulas than for walls.

Your last question asks too much thinking for now. Sorry ....

z60611 · Post by **z60611** » Thu Aug 17, 2006 6:11 am

Eric_Desart:

I thought the cause of the lower resonance was due to the top spring having a lower stiffness (175 N/mm) as much as the doubling of mass. (lower stiffness ~ larger wall cavity).

It is noteworthy that the original resonance frequency is gone.

Eric_Desart · Post by **Eric_Desart** » Sat Aug 19, 2006 7:17 am

Bob,

I made something to get a better feel.
I also integrated the calculation for the vibrationdata animations:
http://www.vibrationdata.com/animation.htm

You find them in:
1) Table 1 Wall 6

As you will see this leads to a rather strange wall with large cavities and very light and thin panels.
When converting this animation to walls, I interpret the mass as a mass per m2 (logical metric approach) and the spring constant (stiffness) also as a spring from 1 m2.

2) Table 2 Wall 6
To make this a more logical wall I secondly interpreted the animations as a mass and a spring valid for a square foot of wall.
Since my model is metric you see that the numbers are increased.
That's just the conversion between m² and sft.
In fact this calculation and this animation allows to visualize what happens.

Further you get 3 Tables in total.
Just study those numbers and different combinations.
It will give you a better feel.

As you can see for paneltraps one can even use this triple leaf principle to get more broadband traps, or traps tuned to more than 1 frequency.

The calculations are rather complicated.
Since one is working with more than 1 unknown, I'm not yet sure if that can be done analitically or calls for solver procedures, to design a trap to 2 predefined frequencies, without iterative procedures.

Bob, it really asks for a lot of time to make a US unit version of this.
But I think you get the picture.