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.