Do you remember which 2K polyurethane worked for you? They appear to be
marketed for different applications with different properties such as hardness, etc.
Brushing it on seems like a better method for getting a uniform thickness vs squirting tubes of Green Glue. However, since my rows of fasteners will be up to 24" apart I do see a potential problem if the poly is too thin and I don't get good adhesion in the area midway between the rows of fasteners, particularly on the ceiling.
Excuse me but you can't because the plywood fibers lenght, orientation, quality, density and structural arrangement will destroy the equation.
I would disagree with this to a fair extent. Competent DIY folk can design and assemble good speakers. Not the very best but competitive with commercial offerings. There are of course plenty of poor DIY designs but this is more by personal choice rather than the knowledge and resources not being available to a competent hobbyist prepared to invest some time and effort.
Passive absorbers are simpler to design and manufacture than speakers. The active absorber I linked to is only modestly more complicated than a speaker due to the addition of a control system. It should be reasonably straightforward to DIY for those competent in designing and building DIY speakers.
Perhaps the biggest obstacle these days for many is an unwillingness to learn the basics about sound and vibration on which to build their personal understanding. There is a tendency to look instead to personalities rather than scientific knowledge for understanding and guidance. This would be OK if the personalities provided competent guidance but this is often, possibly usually, not the case. Without understanding the basics a reader is poorly equipped to distinguish competent advice from poor and so what is attractive and plausible tends to grow.
I don't say generalities about everything in general, except when i'm in socety and i have to say someting, competence is the ability to search and find rapidly what is usefull or not in your general understanding.
When i perform a pair of spakers, it is only because they don't exist in the market and i will defy anyone to find a more adequate solution to my constraints and personnal technical choices.
When i perform a pair of spakers, it is only because they don't exist in the market and i will defy anyone to find a more adequate solution to my constraints and personnal technical choices.
Truth be told, in-wall applications I'd be doing double layer 1/2" sheetrock with green glue and call it done. The larger scale you can apply the technique the greater the overall efficacy, especially because you need *large* surface area to effectively couple the low frequency content into the walls. A worse solution applied more completely to the room yields an overall superior end result. That was your original plan and I thought it quite sensible. You can evaluate where you stand after that is done, especially because any further modification would be after the paint is dry and furniture is placed -- all the things which will make a material difference in room behavior.
As an academic note, the moment of inertia of each respective side will play a role as well, so a stiffer 1/4" material will behave quite differently in flex (and it must flex if there is to be shear, but to what degree) than 1/2" material even if they match E*S, as you specify. The easiest and best way without getting lost in the weeds is to have identical panels around your shear layer.
Tell us how it goes!
Earl,
Thanks for sharing your insight on designing rooms / speakers for best listening experience. Besides CLD for low frequency absorption in small rooms I also found your other writings and videos on speaker / room recommendations very convincing:
-Direct frequency response of speakers should be near flat with some fall off at high frequencies.
-Minimum of aberrations in the critical 700 – 7000 Hz range
-Constant and narrow directivity speakers
- DI above 6 dB to minimize early reflections off near side walls or back
-Based on gammatone filter bank model it is desirable for first reflections to be 10 ms or more
-Horizontal directivity is by far the most important. Early reflections impede imaging whereas later reflections enhance spaciousness.
It's sad that most speaker manufacturers either don't know or don't provide this information.
I agree; your reasoning is persuasive. Stiffness data on drywall, mdf and plywood appears to be very limited. I did find a general report on wood properties that had data on several unidentified plywoods. I also found a research study on a few baltic birch plywoods that would have placed BB at the top of the range of the plywoods in the other report. This makes sense since BB uses birch in the inner plys as well and is supposed to limit voids in the inner plys.
Tim,
Thanks, I checked the Innovative Polymers website. They have about a dozen Shore A polyurethanes with a listing of quite a few different properties including gel time and viscosity. I haven't called the company for more information but the viscosities are a lot higher than a typical paint and if gel time is related to working time it might be difficult to quickly apply to a large panel.
Daniel,
Thanks for your advice. I've decided to go with two layers lightweight drywall with green glue, hat channel and isolation mounts for the walls.
For the ceiling I will be substituting 2 layers of 4 ft x 6 ft x 1/4" baltic birch for the 4 ft x 14 ft 1/2" drywall. This should improve performance on ceiling and will make it a much friendlier diy project. I will be adding some trim moldings to hide the joints and give it a shallow coffered ceiling look. Most importantly though I did a pencil drawing and my wife very much likes the looks of the proposed ceiling.
Sorry about that, my link info wasn't comprehensive. I should have looked at it more closely before posting. As noted in Post 302 of the previously linked thread, Earl mentioned using the IE-40 product.
Web page:
Innovative Polymers, Inc. | IE Series™ Urethane Liquid Plastic :: Shore A
Data:
http://innovative-polymers.com/images/specs/INNOTHANE-Classic-Shore-A-Data/IE-40A.pdf
Gel time is listed as 25 minutes. I can only make assumptions about the working time based on experience with epoxies and polyesters, but it sounds reasonable to me.
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A tree essence wood densitiy can vary of more than 50% and this density is not consistent on the entire layer, a real high quality plywood should be more costly than a high modulus carbon sheet of the same thickness IMO.
I've taken some contacts with building professionnals suppliers in the past, and was blowed by the atsounding expensiveness of the highest grade avaliable, and i don't speak about the luxury boats wood suppliers because i really think that is would be cheaper to use pure gold instead of wood.
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I build/re-furbish boats, and this is overstating it a bit. Some highly engineered sheet products for boats can cost $500 US for a 4'x8' sheet. Most of these are high stiffness low weight materials with (relatively) good moisture resistance properties.
But the overall point you are making is a good one.
It can be very difficult to spread in a short time which is why gell time is critical. I go for a longer time for more working time. I have had problems with mixing too much and not working fast enough. If you make a mistake it's best to just start over as once the material starts to set it becomes unworkable.
Most speakers are designed and sold based on what the current fashion is. And since science cannot tell us very much about fashion, design science takes a backseat to fad and fashion and instead plays to the marketing people. My design principles don't play very well to marketing - it's not what they want to hear.
Yes, i'm talking about plywood, i still have some old mahogany plywood parts (all layers are made of perfect sheets of beautiful mahogany), the best looking ones (AAA+ grades) were 10 times more expensive than the normal retail price and the most beautiful were negotiated... do you see what i mean
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