## soundproofing stiffness

Post and discuss acoustic topics, Studio design, construction, and soundproofing here

### soundproofing stiffness

http://www.avsforum.com/forum/19-dedica ... st33049761

What's the opinion of the studiotips-ers?

I remember Eric talking about stiffness a bit, and Brian doing stiffness tests with 2x4/drywall variations and deciding that stiffness wasn't good.
And there's the 7.5 pounds bit, which is an unusual way of talking about it.

I have no idea how to argue this in the most efficient way possible, I just throw spaghetti at the concrete wall as I invent arguments. I'll try again. Lets involve some good old basic physics.

When the subwoofer element moves in and out the air pressure differences from 120db are tiny across a square inch. Its 20 Pa, or 0.00290075475 pounds per square inch. But across an entire drywall in the bits between each stud it adds up to meaningful numbers. 120db on a 24 by 108 inch section experiences 7.5 pounds in air pressure difference (either from peak to bottom spread across the surface or peak to centerline and then another 7.5 pounds to the bottom, I'm not sure which, I wager its the first). If its 20hz then this pressure difference gets time to act for 1/40th of a second outwards, and 1/40th of a second inward (not going into the whole thing about how it experiences lower average pressure because its a sine wave etc). The higher you go up in frequency the less time this pressure difference has time to act on the surface. Experiencing less actual movement as you go up in frequency. The excursion of the speaker elements also go down as you increase frequency, so largely the movement of the walls go down as you increase in frequency because the time the pressure difference has to act on the wall in each wave goes down. Thus lowering the amount of movement actually imparted on the wall. Total PSI the wall experiences can go up though in higher frequencies, due to the speaker being able to move more air combined per second, reaching higher db.

Then we begin going into areas where the materials themselves can transmit vibrations internally which for low frequencies largely does not matter. Whereas on high frequencies you can tap a railroad piece and hear the noise far away by putting your ear to the steel, you can't do this with lower frequencies. Partly because we're terrible at hearing low frequencies but partly because materials only want to transmit certain frequencies in this way.

This pressure difference over time and area is the reason you see largely no decrease in low frequency sound transmission by adding more and more drywall layers and rockwool/glasswool. It may be more weight, but the PSI is still the same across the same area with the same spacing between studs. That is why I suggest half space studs, which simply is to have half the spacing between studs. In regards to bass transmission, its guaranteed to lower it. Then the same 120db is just 3.75 pounds on a given surface that can move much less (movement of panel with smallest side being 24 inches can move much more than panel with shortest side being 12 inches, measured in air displaced by movement, given the proportionally halved force. You can test it by putting X weight on a 2 by 4 and then cutting it and the weight in half and measuring how much it deflects each time). Higher frequencies will probably be affected negatively by half space studs, but high frequencies can be dealt with with drywall and insulation.

But, how much does concrete get affected by pressure differences in the air? Very little. We know that with even very thin concrete, the wall can take the given PSI over 1/40th of a second with an immeasurable amount of movement because the give in the wall is simply much less than the given PSI will impart, no matter how long the PSI is given to act upon the wall. You can push against a drywall for a second and then impart pretty much the maximum amount of movement it will take without breaking, whereas if you lean towards a concrete wall the amount it will move is much much much smaller. So even if you have 1 hertz with 100 pounds of pressure differences peak to peak across the entire concrete wall, it will still move immeasurably small amounts. However, if you have 400 hertz and 120db, you'll begin throwing a lot of sound through the concrete, since you're nearing the frequencies the concrete can transmit well internally.

Hence I put forth the STC rating of assemblies that included concrete because we know the bass transmission is better with concrete than without it. There's a reason things that don't move are said to be "set in stone/concrete".

*(number from random guy on internet says 120db is 20 Pa, sue him if it isn't correct)

Excuse me for not having a degree in efficient arguing about the sound dampening effect of concrete on the internet.

EDIT: Let me flesh out that bit about the 24 inch side versus 12 inch side movement amount. Lets have a 12 by 12 inch wall section and a 24 by 24 inch. There's studs at all sides, and drywall on both sides. When a certain psi is allowed to act on the wall on one side, it imparts motion in the wall, displacing an amount of air, which pushes against the other drywall sheet. The movement is proportional to the give in the sheet, and the give is lowered by reducing the dimensions of the sheets. Thus, less volume of air is moved, less movement is imparted on the next wall. The amount of air displaced in section 24x24 is more than four times as large as the amount of air displaced in section 12x12. Note the "more than". 4 sections of 12x12 would have less surface movement combined than one 24x24 sheet. The reason being the movement imparted by air pressure on a sheet wants to resemble a part of a sphere, and they have more volume to surface area the bigger the sphere is. Impart 1x force on 24x24 area and you get a displacement volume well over 4 times the displacement volume of an area of 12x12 being subjected to x/4 force. So even disregarding the fact that materials themselves tend to move less with lower lengths, its going to move less air if we cut the size of the sheet sections. Not to mention, you'll have some more parts of the wall that has studs behind it, where it won't move at all, only imparting sound internally.

Judging by the silence in regards to my last post, I assume I have managed to make it make sense to you why concrete stops bass. Anything that recreates the abilities of concrete will work to the same effect. Tighter space between studs, stiffer materials, gluing materials together to make a thicker laminate (use epoxy for wood v wood laminate), reinforcing the material, etc. Tiles also contain bass quite well, hardwood, etc. Get creative.

On the subject of high density variable density particle board (aka HDP), I'm sure there are similar products in the states. It probably goes under different nicknames. Shop around, ask for really stiff particle boards and fiber boards.
Bob

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### Re: soundproofing stiffness

Jesus, Bob.
What is this guy's claim in understandable english and in way less words?

TL works like this:

Under the resonance freq of the construction you have stiffness control meaning the whole constuction is trying to move in phase but can't because it is suspended one way of the other by gravity, or mechanically. So here you see better TL than at resonance.

At resonance TL is minimum and mass dictates this resonance frequency. Here the construction moves in phase with frequency and has maximum excursion. Compare it with pushing a pendulum, pushing on resonance requires least power to yield max excursion.

Above resonance things get more and more out of phase and TL improves acoording to the mass law (about 5 dB/octave), compare randomly pushing a pendulum; you will sometimes enhance movement, the other time you stop it.

At a certain point you get bending wave moving IN the material, they are NOT having frequency independent speed and when this speed equals the speed of sound, you get resonance called coincidence. It's dependent on the angle of the sound entering the material.

Stiff constructions have the disadvancement they have higher res freqs than less stiff constructions of the same kind and mass, so TL will drop.
So having studs at 600mm is better than having them at 300 mm.
25 mm of MDF is stiffer than 2 x 12.5 mm NOT glued or screwed totally together, as the two plates can be moved a bit by sound causing friction and turning ound into heat. GG can amplify this behaviour. reducing coincidence.
Last edited by bert stoltenborg on Mon Mar 30, 2015 4:33 pm, edited 1 time in total.
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### Re: soundproofing stiffness

When I read on that forum either we have been talking bullshit for 15 years or they didn't pay attention and are merrily going on being ignorant and stupid.
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### Re: soundproofing stiffness

Thank's Bert.
There was too much odd about it for me to know where to start.
Regards
Bob Golds
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Bob

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### Re: soundproofing stiffness

Bert:

[ Please critique, before I post it back where the original quote came from.]

An optimum wall is one with the lowest resonance frequency and highest coincidence frequency, and largest sum TL across all frequencies.

Think of a drum. If you tension/stiffen the membrane the resonance frequency goes up, but the mass doesn't change.
Same with a wall.

"Under the resonance freq of the construction you have stiffness control meaning the whole constuction is trying to move in phase but can't because it is suspended one way of the other by gravity, or mechanically. So here you see better TL than at resonance."
Stiff constructions have the disadvantage they have higher res freqs than less stiff constructions of the same kind and mass, so TL will drop.
25 mm of MDF is stiffer than 2 x 12.5 mm NOT glued or screwed totally together, as the two plates can be moved a bit by sound causing friction and turning ound into heat. GG can amplify this behaviour. reducing coincidence.
-- Bert Stoltenborg

"The coincidence dip may be shifted up or down the frequency range by altering stiffness, boundary conditions and changing thickness of the material. For example, if two layers of material such as wallboard are glued firmly together, they behave like a single thick layer with an associated lowering of the coincidence frequency. If the layers are only held together loosely (with screws for example) so that they can slide over each other to some extent during bending motions, then the coincidence frequency does not move to lower frequencies and the friction between the layers can introduce some extra energy losses, giving higher transmission loss near coincidence."

For more on the coincidence dip see
http://www.bobgolds.com/GlueOrScrew/home.htm

Below,
a) is a given wall, and
b) is a stiffer wall

Notice how the coincidence frequency drops, and the resonance frequency goes up.

Although a few movies might have energy in the 10hz to 20hz region, they all have energy in the 50hz to 100hz region.
In the below, notice how the stiffer wall reduces TL at the common frequencies.

I believe Dennis Erskine was fond of rigid glueing drywall to the studs (to reduce potential rattle).
I don't recall Dennis recommending to glue drywall to drywall, which would have made the wall stiffer.

Brian Ravnass did some lab experiments gluing with rigid adhesives.
note: a rigidly bonded panel is 4 times stiffer than 2 non-bonded panels.

The mechanical structure (Stud spacing, panel stiffness) plays a strong role in resonance location.
As I recall, Brian Ravnass also did some experiments with having studs at 48 inches (1200mm), 24 inches (600mm), and 16 inches (400mm) and lab measured that a more flexible wall is a better wall than a stiff wall, but not a lot. In lab measurements with gypsum double stud walls, the resonance dip is locatable in the just below the lab's confidence region.

In any event, the stiffness discussion is mostly moot. Most homes will use two 13mm layers of drywall, most home theatres interested in sound proofing will use four layers of 16mm drywall, and professional installations will use concrete. Each of these has been well thought out by people more knowledgeable than I, and use materials that are the cheapest bang (or 'lack of bang') for the buck. Or to say the same thing another way, see the common constructions in books like "Noise Control In Buildings" by Harris, choose a TL you can live with, and build it as per the tested example. Or to say the same thing yet another way, due to cost if you end up using concrete to get the needed mass, its stiffness is already predetermined.
Bob

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### Re: soundproofing stiffness

Bob

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### Re: soundproofing stiffness

Seems an exellent post to me, Bob.

Eric left me a Volvo V70 filling amount of TL-data with some interesting details, if you compare some similar but not totally similar situations.
I'll post some of it, if you're interested.
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### Re: soundproofing stiffness

I did read a small piece this granroth writes.
Stiffness of a construction should not be confused with the stiffness controled region as the latter just means the construction is mechanically fixed at the boundarie and thus moves less easy than the whole construction would move if there would be no resistance at the boundary.
A speaker cone can move relatively freely until at large excursion it reaches the limit of the suspension.
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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### Re: soundproofing stiffness

bert stoltenborg wrote:Eric left me a Volvo V70 filling amount of TL-data with some interesting details, if you compare some similar but not totally similar situations.
I'll post some of it, if you're interested.

That Eric The Teacher...
I'm interested in interesting, and that which you think I should know.
Bob

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### Re: soundproofing stiffness

bert stoltenborg wrote:Seems an exellent post to me, Bob.

Merci beaucoups.

It has been posted it back where the original quote came from.
Bob

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### Re: soundproofing stiffness

http://www.avsforum.com/forum/19-dedica ... st33143473

I think there's some tricks the average house designer isn't using in Home Theatre designs. In light of the new knowledge about stiffness and 'floppiness' in terms of sound transmission, I'm drawing up something I think may be better, and perhaps cheaper.
Bob

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### Re: soundproofing stiffness

I get a new knowledge about idiots and the democratic right to vote.
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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### Re: soundproofing stiffness

http://wiki.naturalfrequency.com/wiki/S ... ansmission

The part on the stiffness controled region is a bit vague, It's almost as if the writer isn't sure.

http://www.homerecording.be/gids/isolatie
Here I am very specific about the nature of the stiffness controled mechanism and Eric totally approved of it, several times. And Eric is my reference
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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### Re: soundproofing stiffness

bert stoltenborg wrote:http://wiki.naturalfrequency.com/wiki/Sound_Transmission
The part on the stiffness controled region is a bit vague, It's almost as if the writer isn't sure.

I'd seen that one before this week.
I thought I might have quoted it, but certainly someone else did.

bert stoltenborg wrote:http://www.homerecording.be/gids/isolatie
Here I am very specific about the nature of the stiffness controled mechanism and Eric totally approved of it, several times. And Eric is my reference

I feed it through translate.google.ca and it was readable, when combined with a little knowledge on the topic.
I think you said that filling the cavity with more than 80% insulation makes the wall stiffer.
Bob

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### Re: soundproofing stiffness

Overfilling the cavity/compressing the wool will decrease the virtual volume enlargment.
As the speed of sound in wool is lower than in air the sound thinks it's in a larger enclosure and thus resonance is lower.
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### Re: soundproofing stiffness

https://www.gearslutz.com/board/studio- ... -mass.html

Good old Rod still doesn't really get it.
Are you gonna inform 'm about the mindfuck they perform on themselves, Bob?
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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### Re: soundproofing stiffness

bert stoltenborg wrote:https://www.gearslutz.com/board/studio-building-acoustics/997130-lfe-bass-containment-stiffness-not-mass.html

Good old Rod still doesn't really get it.
Are you gonna inform 'm about the mindfuck they perform on themselves, Bob?

What don't I get Bert? I seem to be missing something here........

Rod
Rod

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