For the technique, light, stiff, and well damped are properties that make Baltic Birch (or similar many ply plywood) one of the very best practical materials.
Steel, aluminum can be made to work. Never been able to get anywhere near the same results with MDF. HDF laminated both sides with plastic laminate is OK, but sooo much work.
dave
BB does appear to be the best all around choice I agree. I am doing a sandwich of 3/8 BB with Green Glue in the middle in my current project. I will brace is as much as possible but it is a mini monitor so won't require as much for stiffness. I am going to try to isolate the driver with a Sorbothane gasket but that is easy to undue if needed. I will also add some rigid fiberglass in two densities (not to damp the walls).
I actually read every post in this thread, of course I may have forgtten some of the info since it is rather long.
Now I'm a "prove it to myself" kind of person and believe in using a bit of science and math as a starting point to a design. What I found in this thread was lots of opinions with only a bit of proof. So I searched for as much info as I could find on the governing equations for deflection (the equation posted early in this thread was incorrect), composite plate design, resonant frequencies and critical frequencies and for material properties for the range of materials discussed so far.
I've proven (on paper) for myself that BB plywood is a better choice than MDF in that the properties are more consistent, the space between bracing can be larger and I know how much sound energy will be absorbed at a frequency range from 100 Hz to 4000 Hz. I do not like the rather large range of MDF properties I have found so far and while people have stated that MDF disspates more sound energy to heat than BB plywood I can find no data to support that. I suppose one could design the box using the worst properties but then the space beteen bracing gets a bit small.
Dave,
Can you remind me what frequency range and dB levels your method applies to?
I cannot fathom how a speaker achieving 106.5 dB (my design upper limit) at 25 Hz (my proposed woofers, 18 Hz for the sub) will have no audible sound exiting through the enclosure walls no matter how they are braced and no matter what realistic material is used. My data shows that the transmission loss for 6 inches of concrete is 39 dB at 100 Hz. If the speaker can generate 106.5 dB into the air wouldn't it also generate 106.5 dB into the box? If so wouldn't the 6" concrete walls would be radiating sound at 77.5 dB at 100 Hz? That's still a lot of sound, causes hearing damage if you are exposed to it for a long time according to :
Sound pressure - Wikipedia, the free encyclopedia
I think that is is great idea under 2 circumstances :
1) you're not sure what the properties of the material are
2) you need to deal with a large range of frequencies such as in a full range speaker
Sorbothane may be a good choice. Had a look at their web site and it can damp out low frequency vibration better than anything else I've run across so far. I downloaded the design software and the guides and will use that to determine how thick the material has to be to be effective. Thanks for the pointer.
It is great for us new ones to be able to learn from more experienced people.
So far I learned that I will using BB plywood for at least 2 of the layers of a composite panel and bracing about every 4 inches for the subwoofers and woofers. The other layers, ...... will keep pondering and calculating.
So far I learned that I will using BB plywood for at least 2 of the layers of a composite panel and bracing about every 4 inches for the subwoofers and woofers. The other layers, ...... will keep pondering and calculating.
lol mixed my numbrs
lol i have to admit i mixed up my numbers...MDF IS weaker in tension and compressive breaking stress, by a margin of 10-20% compared with super quality ply...although i still think MDF is stiffer, just ultimately weaker...after all birch ply really is very springy. I think this springiness is a major factor in its high tensile stress specs, although i wouldnt have though a springy elastic material wouldve been good at NOT oscillating
otherwisde i mightve taken the glass filled nylon 66 welded together route....
whether springyness is an advantage is another debate altogether. i personally think it is the ONLY place where ply fails, other than the relative hardship of routing cleanly in the material. In these 2 ways only then i think its safe to say MDF is better...maybe a MDF-ply-MDF sandwhich would be a good comprimise, reducing the amount of routed cuts id need to use.
lol i have to admit i mixed up my numbers...MDF IS weaker in tension and compressive breaking stress, by a margin of 10-20% compared with super quality ply...although i still think MDF is stiffer, just ultimately weaker...after all birch ply really is very springy. I think this springiness is a major factor in its high tensile stress specs, although i wouldnt have though a springy elastic material wouldve been good at NOT oscillating
otherwisde i mightve taken the glass filled nylon 66 welded together route....whether springyness is an advantage is another debate altogether. i personally think it is the ONLY place where ply fails, other than the relative hardship of routing cleanly in the material. In these 2 ways only then i think its safe to say MDF is better...maybe a MDF-ply-MDF sandwhich would be a good comprimise, reducing the amount of routed cuts id need to use.
Springy is bad? (if it is springier) That would mean that the resonances (and ALL materials resonante) will have a higher Q (if excited) and therefore less audible.
I believe that to get the panel to transmit you have to get it resonanting. That is pretty easy to avoid in a sub, as it is really easy to push panel resonances well above the units bandwidth, in which case they will not be excited and the panel will not transmit. The FR systems i build take more finese.
Again... after 35+ years of making loudspeakers i will not use MDF (even thou we get it essentially for free!). It is a poor choice of material for building hifi speaker cabinets out of.
dave
higher q ... um I doubt that, or not much higher than MDF at any rate. concrete is very rigid when used as a material, but has a very high q resonance, which prouces longer spurious decay at the point of excitation.
whether that makes it less audible as a net result is debatable, i honestly dont know.
I know there have been studies into q and audibilty, but i dont think they fulled encapsulate the occurances within loudspeaker cabinets.
And despite material..i dont think ive ever heard a horn that didnt sound resonant, or OB, dipole that i didnt think sounded resonant. boxes resonate sure, and i can hear that too. this is partly what delayed my entrance into designing a TQWP design for myself. this ripple, even though small in relation to output, is very audible i find. That is unless it happens to be the inverse of my room modes! since i plan on moving out onf a small apartment i couldnt really consider designing for room modes so i have to go the route of flat anechoic, and maybe use the deq 2496 to smooth out any lumps in bass that occur due to the room being 20'x 25' l shaped and 12' wide at narrowest point! i tthink the room is a can of worms lol
horns intrigue me. the likes of your or scotts designs interest alot, but at the same time, i dont know why anyone would want to put FR speakers into a minimally lagged compression chamber and vent it- no offence meant- besides increasing output. I wouldve thought a sealed or more absorbent box would do the mids and highs of such drivers more justice. but then i dont think ive seen any FR graphs of a horn in ages, so maybe im just ignorant!
To be fair, I have NEVER used ply, personally would really LIKE to, am discouraged by the difficulty in routing, and lack of decent supply blah blah. PLy when coated in epoxy i hear is particularly rigid. which lead me on to thinking about epoxy/phenolic fabric board as used for HT insulation, mega stiff, resiliant, but also maybe over dense.
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Not debatable.
1/ Toole has shown research that shows higher Q resonances are less audible
2/ the area under and the shape of the curve of a high Q resonance means that sustained energy over a narrow bandwidth needs to be pumped into it to get the panel to resonant. Not likely with music providing the exciting energy.
dave
I just want to make we are talking about the same stiffness.
Stiffness - Wikipedia, the free encyclopedia
k = (A * E) / L
where
k is stiffness
A is cross-sectional area
E is elastic (Young's) modulus
L is length
If we compare two materials and A and L are the same for both, k is directly proportional to E.
From the materials data I've found on he web, MDF has a E ranging from 250,000 psi (poor quality cheap stuff) to 480,000 psi (high quality expensive stuff).
Plywood ranges from 900,000 psi (cheap stuff) to 1,700,000 psi for the thin and many layer stuff such as BB plywood.
This show that for stiffness even the worst plywood is better than the same thickness of the best MDF.
Dave,
This is the only thing that I find hard to believe. What you are saying is that if I built a wall out of 1/2 inch BB plywood with stiff braces every 4 inch in both the vertical and horizontal directions, it will transmit no sound below let's say 2000 Hz? So I could sound insolate a home theater room for noise below 2000 Hz using just this approach? The data I have (see page 22) :
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr43.pdf
shows that 1/2 inch of plywood (resonant frequency of between 132 Hz and 182 Hz for a 12" span 96" long such as a typical wall) blocks between 15 dB 28 dB of sound at 125 Hz and 4000 Hz respectively.
I'd suggest that a panel at resonance becomes a speaker and generates sound whereas a panel far from resonance is a barrier to sound and the effectiveness of that barrier varies with frequency.
either way, i wouldnt use less than 19mm ply or 25mm MDF for a box, since the thickness is the most important factor
i was sure that young modulus was a coefficient of elasticity not stiffness, since some materials are plastic and some elastic. some materials are very plastic but yet stiff, and vice versa. MDF is more plastic in nature and deformed more easily, ply is more elastic and also less deformable. thus i wouldve thought hardness would be the better measure of stiffness, albeit hard=brittle, even so, we arent going to push materials to their brittle breaking points by vibration alone, are we?
wouldnt a diamond box, (lol yes i know silly idea) be the ultimate in high Q but easily damped and stiff box material, a la accuton diamond tweeters?
tell me if im wrong, im here just questioning things, as i believe a good scientist should, and im open to being corrected lol...have no fear
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Thanx for that data. It will be useful as i work to complete converting this thread into an article.
Do you have any citations for that data?
dave
I have done essentially that. But, being lazy, with stiffness going up with the cube of the thickness, going to 18 mm & using a bit more finese on the braces is how we usually go.
Yes. Exactly.
When a panel goes into resonance it essentially becomes transparent to the sound inside the box.
dave
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