Experiment time ( or fun with rubber )

giles117 · Post by **giles117** » Tue Oct 07, 2003 11:47 am

Hey Desart glad you chimed in, was looking forward to your insight. OITC is it.

Good to know these things, hope things are well in Belgium.

Bryan Giles

knightfly · Post by **knightfly** » Tue Oct 07, 2003 12:30 pm

Eric, thanks for posting that - I thought I had heard you say before that MTC went lower, must have mis-interpreted. I'll read through your file a few times soon as I get a chance, is it OK if I just post a link as part of a resources page I'm working on?

Again, always good to hear from you... Steve

eric · Post by **eric** » Tue Oct 07, 2003 12:57 pm

This is great. I thought I was asking a simple question and wholly mackeral the science is staggering. This is like so many things in life where a standard is just one guys way of getting his point across. Everyone else can be on a different page and who knew? I suspect that the concepts are similar but the measuring method is different ? The problem I have is that no one seems to have published anything based on what I want to try. ( or at least not anything easily found ) Sheetrock is obviously going to give a different result since it's sheetrock and not rubber. It seems that the tests are all done with sheetrock and other standard building materials. So what's a guy to do?
Anyway I am very nervous about the weight of this stuff. My plan wasn't to glue anything but to actually use laminating screws using two layers of sheetrock one on either side of the matt. Early strength test are not reassuring. The thing I get from gluing is that it may create a focused resonance? Man I am so confused. That's the price for being inexperienced and full of crazy ideas.
Eric

blas · Post by **blas** » Tue Oct 07, 2003 2:14 pm

Knightfly, I'll post later any data I can draw from using the vinyl as one of the layers, but currently (about 16 or so months into the reconstruction) I'm still too busy finishing EVERYTHING to get the studio back up and running again! The only thing that's saving my ass from so much lost revenue is/are the mobile jobs I do.
Yes, you are so right about the cost of the product. But if you're a good shopper........money can be saved buying from the right place. As the old song from the 60's goes "silence is golden", today we can also add the line "and expensive too"!

knightfly · Post by **knightfly** » Tue Oct 07, 2003 3:40 pm

"Man I am so confused. That's the price for being inexperienced and full of crazy ideas. " - Join the club - just over 20 years ago I built this room in my barn, thought I knew what I was doing - I was so bummed out when I discovered that my wife could hear what song I was playing on the piano from 100 feet away inside the house... That's what started my course of random study of all things acoustic and quiet, still at it...

If you're just talking about doing walls, I'd consider glue laminating the rubber between layers on the side toward the tracking room, making that leaf effectively one layer with a "gooey center" (love those gooey centers...) if you use 5/8" sheet rock on both sides of the rubber, the 2 layers of sheet rock and the rubber combined should lower the coincidence dip of that leaf to around 27 hZ, assuming a 5.5" air gap - if you mount that side solid to the frame and mount 2-3 different thickness of sheet rock on RC to the other side of the frame, you should get a pretty impregnable wall between your CR and Studio.

I'm not sure how you would go about doing the laminating, unless you made several "super panels" horizontally and put them up after laminating. The downside of that is the need to thorougly caulk between 4 x 8 panels to avoid hairline cracks that will ruin the TL of the wall. If you laminate in place and stagger joints it would be less likely to cause gaps, but I've never tried to do that with something like rubber. Guess you get to be a pioneer on that one...

Blas, don't worry about that right now. Your main goal of "keeping beer in the frig" needs to take priority. If you get to it in the next year or so, we'd appreciate any feedback though... Steve

eric · Post by **eric** » Wed Oct 08, 2003 3:52 am

You bring up a good point in that I was considering putting the rubber panels on the inside wall of the c.r. It would actually be easier to do it as you suggest. Since they are 4'x7' I would have to do the two part thing.
Just out of curiosity how on earth do you figure out that there would be a coincidental dip at 27hz? And of course thanks again for all the help folks this is great and much appreciated.
Eric

knightfly · Post by **knightfly** » Wed Oct 08, 2003 4:53 am

Everest gives the formula in Master Handbook of Acoustics - it's

Fo= 170/sqrt(M*D), where M=mass in lbs/square foot, D= depth of air space in inches

From here -

http://members.tripod.com/Bills_Drywall/Questions.html

I got the weight of standard drywall panels - divided by 32 for 4x8 panels gives you M in pounds per square foot

Air space depth is self-explanatory -

5/8 rock is 2.3125 psf, so 2.3125*5.5" = 12.71875; sqrt= 3.566335654, /170 and invert = 47.66 hZ.

You added a second layer of 5/8 rock, plus 65 pounds of rubber divided by 28 square feet - so now, replace the 2.3125 psf figure with 6.946 psf - that times 5.5" (for 2x6 walls, not sure what you have) = 38.203, sqrt= 6.18, divide by 170 and invert = 27.5 hZ.

That's only if you glue each layer. Otherwise, the panels need to be calculated individually. I'm not sure which would be better, gluing or not - but I wouldn't glue the panels on BOTH sides of a wall - dissimilarity is what keeps the wall from passing one frequency better than others. that's why I recommend 3 different thicknesses on a wall over 2 thicker ones... Steve

rod gervais · Post by **rod gervais** » Wed Oct 08, 2003 7:17 am

Ummm,

One thing i might want to add here.......... when i visit the site of the companies that manufacture the membrane products for this use - what i have seen for new installations is to install the membrane 1st - and the 2 layers of drywall afterwards........ the sandwich between 2 layers is only reccomended for situations with existing drywall..........

Rod

knightfly · Post by **knightfly** » Thu Oct 09, 2003 8:28 am

Rod, I'd noticed that too - I'm assuming it would improve damping similar to having the insulation touching the inner wall layer ?

I just have a hard time wanting to pay that much for something that seems like it should be much cheaper, so I tend to ignore the mass loaded vinyl more than I should. I noticed the same thing when I had to buy some heavy vinyl shower pan liner a few months ago - anybody trading in vinyl futures these days? Must be a "tunna-munney" in it considering the price... Steve

Eric_Desart · Post by **Eric_Desart** » Thu Oct 09, 2003 5:53 pm

Hello All,

Just to say this ones more:

A mass loaded vinyl is a material with high internal demping causing a higher insulation than other materials with the same mass.
However this difference is minor when compared to drywall.

Combining mass loaded vinyl with any combination of drywall layers will hardly make any difference with a drywall layer combination of the same total mass.

It increases the damping of the layer combination a bit, but for the same total mass you can be happy if you find individual frequency bands in low and mid improving with 1 dB (2 dB if EXTREMELY lucky).
On the overall insulation it is negligible. The effect is mainly to be found at the coincidence but can easily be substituted by different drywall thicknesses = adding thin board with deviating coincidence)

You can easily (in as far this is important) substitute this mass loaded vynil by an additional thin layer of drywall which will damp the coincidence (which has minor influence on music noise) as well.

In the pro world a drywall with mass loaded vinyl combined in general is seen as designed by non-acousticians and not taken seriously. The price and energy versus return on investment is as bad as can be.

It's simple:
You find a lot of TL measurements of mass loaded vinyl on the net. But they are measured as a free-hanging autonomic object.
Such sites WRONGLY (direct or indirect) suggest that those values should have anything to do with using this material in combination with a drywall.

WHAT YOU NEVER FIND:
An objective comparison of a good drywall design of the same mass, measured in comparable controlled circumstances wit a wall where the mass-loaded vinyl is used.
What should be easier to prove their (misleading suggested) point than publishing such (scientific objective) comparison.

Note:
Traditional quality roofing at room temperature has comparable to better properties than PVC.
A lot of the cheaper materials are based on mass loaded bitumen (roofing).

The advantage of vinyl versus roofing is mainly that it works better at very low outside temperatures.

PVC can't burn. As such is fire resistant. But when burned by external source the chloor in it is very poisened.

Just some thoughts.

My personal vision:
Using mass loaded vinyl in a drywall combination should be useful if:
1) you want to spare few mm.
2) You have too much money and energy

This does not mean that there aren't usefull applications but certainly not where most studio sites suggest this material should be used.

Best regards
Eric

PS: Steve, don't confuse mass-spring resonance with coincidence. I saw your calculation which is mass-spring, not coincidence. The coincidence frequency is what in US books is often refered to as the critical frequency (wavelength incidence noise equals bending waves in panel).

In general gluing layers together is wrong. It lowers this coincidence to a more critical frequency range.

knightfly · Post by **knightfly** » Thu Oct 09, 2003 7:42 pm

OK, Eric, I'm really trying to get this right but it never seems to surface all in the same post - are you saying that the Mass-spring-mass I referred to isn't ALSO lowered by gluing panels together? Does gluing ONLY affect the coincidence dip, which in gypsum wallboard is around 2-4 kHz?

I would really appreciate it if you could clarify BOTH of these in one post - What, if any, effect will gluing have on Mass-spring-mass?

Then, what, if any, effect will gluing have on Coincidence Dip?

And finally, if lowering resonance below audibility helps TL at bass frequencies, why wouldn't it work to use closer spaced studs and glued panels in ONE leaf of a wall, then 2-3 different thicknesses of screwed gypsum panels on RC (over 24" centers) on the OTHER leaf?

Maybe a better way would be if you could give an example of a really effective drywall assembly, then explain WHAT makes it good?

I'm basically just trying to "put this to bed", or in other words, settle the confusion, in a way that is simple enough for us non-acousticians to grasp -
If you can help me do this, I'll definitely owe you at least half a dozen beers

Not to mention being just plain grateful... Steve

Eric_Desart · Post by **Eric_Desart** » Thu Oct 09, 2003 11:55 pm

Hello Steve,

I know I better should find some more time to explain all different phenomena in detail on a page with graphs.

Lets first speak about the strongest phenomena of double leave systems:

1) MASS-SPRING-MASS RESONANCE

Basically the main calculation starts from the Mass-Spring-Mass resonance, let's call it further fo.
This is the most powerfull source defining the acoustic behavior.

Theoretically at straight incidence:
Assume a complete undamped theoretical system:

fo = insulation 0 dB. In practice there is damping by the connection structure edge clamping and whatever. So fo is not 0 dB but its the lowest point.

From there the TL rizes theoretically with 18 dB per octave in practice with 12 dB per octave.
Starting point of this inclining line is "fo" at the level of the mass-law of both leaves together.
So it's clear the lower this point the better since it also influences the subsequent higher frequencies, in fact you shift this inclining straight line to the left.
Improving a double leave wall is mainly trying to get this "fo" as low as possible. Al the rest is secundary.

This is defined by the combination of the 2 masses (both wall leaves) versus the stifness of the spring.

You increase insulation by:
a) increasing the masses (more layers)
b) weakening the spring (increasing cavity width)

Note that I don't speak about waves here: the cavity is much too small to ellow those long low frequency waves to develop.
The cavity acts as a pressure room, which is about comparable with sound in a room far below the lowest room modes. That's why wavelengths do not matter here the air is only a spring.

Here you see something fun:
A drywall = exactly equal to a panel bass trap.
Just flip the TL graph upside-down, substitute the TL values by absorption values and see, there you are.
The Q of this dip depends on weight versus spring ratios damping and so on.

This inclination of 12 dB/octave changes in 6 dB/octave when point 2 starts:

2) THE CAVITY RESONANCES

Exact as in a room there are cavity resonances or modes, where the first cavity resonance is located at 1/2 wavelength.
However the width of this effect causes this downward bend from 12 to 6 dB per octave to start earlier at 1/4 wavelength or even below.

Then we come to the 3th phenomena:

3) THE COINCIDENCE FREQUENCY in US called CRITICAL FREQUENCY.

This is where this previous described rize of 6 dB/oct bends down the a decline of the curve towards the coincidence frequency.

Here I copy another message including the link you know:

Look at:
http://www.domesticsoundproofing.co.uk/tloss.htm

If you want the math I suggest to get a book in the library about
acoustics.

The coincidence freq., in the states more referred to as the critical
frequency.
This is the frequency where the wavelength of the incident soundwaves
equal the bendingwaves in the panel itself.
This causes a serious transmission loss dip.
Some general things:

Doubling the thickness of a panel will lower this frequency with 1
octave.
It depends on material properties. E.g. this frequency positioning is
better for drywall than for wood, making drywall better for TL.

This frequency is angle of incidence dependent.
It is about the only frequency where the radiated noise on the
receiver side preserves the direction of the incident noise at the
source side.

Below this frequency the bending waves in the panel become shorter
than the wavelength of the incident energy.
This causes the radiation ratio of the panel to become lower than
100%.
What happens is that the air pressure toggles between the crests and
the throughs of the bending waves of the panel. As such only part of
the vibration energy is transformed in noise again.
In fact one could interpret this a bit as natural anti-noise (poetic
licence).
That's why things as drywall are such efficient insulators.

Above this frequency the bending waves in the panel become larger
than the wave length of the incident energy.
This causes the radiation ratio to become 100%. Every bit of
vibration is converted again to noise at the receiver side. The wall
act a bit as a piston.
In my language (literally translation) we call a wall with very low
coincidence a bending-stiff wall (as e.g. a concrete or brique wall),
while a wall with very high coincidence we call a bending-soft wall
(as steel plate, drywall, particle board etc.). In US one refers to
the latter as a flexible wall or panel.

Best regards
Eric

Now: If you once should find the time to play with the excel files on my site you REALLY should get more the feel for those concepts.

Now a good wall is:

1 ) a lot of layers on both sides (increased mass - the exact relative effect can be calculated with my files)

2) a cavity as large as possible (weakened spring - the exact relative effect can be calculated with the same mass-spring file)

3) If using wooden studs, disconnect them with RC on ONE SIDE ONLY. If doing on both sides and making your wall theiretically more perfect THAT'S A DISADVANTAGE, since then you loose the damping of the system causing the "fo" to become TOO DEEP, making the wall WORSE.

4) try to limit the coincidence dip a bit, but DO NOT EMPHAZISE THIS TOO MUCH, since music noise is as overall insulation hardly improved by maximizing this phenomena.

THe ways you can do that depends on the number of layers. Take panels with different thicknesses with each there own coincidence frequency. This will cause a damping of this dip by dismatching bendingwave lengths.

It's enough that you put a 6 mm or 9 mm panel in-between, on the outside or whatever.
ANY FURTHER IMPROVEMENT will NOT show in the overal TL.

I even do not believe that this special screws to connect gyproc to one another rather then to wooden studs is an advantage when one side is disconnected by RC.
It even can be a disadvantage: if this stiffens the panels too much by acting as a single leave causing the coincidence to lower again.

Best regards
Eric,

This isn't complete, but to time consuming.
Basically a good wall is a simple one, all the rest is magic.

If I should build it for myself now:

With metal studs:
Direct connection to studs
filling with something as 702 (not 703).
Number of layers and cavity depending on insulation I need.
Thickness of at least one board on both sides strongly deviating from the others e.g. a 1/4 or 3/8" panel.
Both sides symetrical or not (doesn't matter that much)

With wooden studs:
The same but one sided with RC or comparable
If using RC I should do the side without RC at a spacing 16"

If I should want to have a havier wall I use staggered studs or independent frames at ca 16" OC

Eric

PS: PLay with the acoustic selector Excel file.
You can alter the TL values from existing BORAL measurements, to imediately see what the effect is on the overal insulation.
You you can check yourself the effect of excluding or reenforcing the coincidence dip, highering or lowering the mass-spring dip etc.

[/quote]

knightfly · Post by **knightfly** » Fri Oct 10, 2003 11:04 am

Eric, thank you VERY much - that was very clear and understandable (even by ME

)

I've got all your Excel files on my laptop and hope to find the time this weekend (if the evil Bad Luck Genie leaves me alone long enough) - I'm looking forward to this, it feels like I'm poised on the brink of a breakthrough here ('bout damn time, too)

In spite of only understanding maybe 1/8 of the "why", I've gotten pretty close to the mark on best wall construction - now, hopefully I'll reach a point where I can also explain it.

Again, thanks so much for your time and patience... Steve

giles117 · Post by **giles117** » Sat Oct 11, 2003 7:02 am

I am right there with yah steve. It makes total and complete sense to me now (ok, I lied 50%) but I do understand the pressure wall explanation. And from sticking my head in the wall cavity I have built here in my studio and hearing all the Low end that was trapped. (I built my walls, cranked the PA then cut the opening for the window and listened to the diffrence. It starts to come to light for me.

I actually have about 20" of space between my control room wall and my Live room wall. But it does make sense to me. The damping effect of the space, the walls and the insul.

The test will be for me to explain it in extreme laymens terms. LOL. Happy bulding.

Bryan Giles

eric · Post by **eric** » Fri Oct 24, 2003 11:48 am

Howdy'
The walls are up and the soffits are done. So far with only the mix room walls finished I have to say the rubber thing seems to be working. Very little of anything is getting through that wall. Is it worth it? For the extra $200. I spent I don't know. As for the hastle of holding 65lb rubber sheets in one hand and trying to use the screw gun with the other, I should have called for some help!!! The speakers are in the soffits and so far they sound pretty good. The bass coming through the wall is minimal. I can't wait to get the second wall finished. Thanks all for the tips.
Eric