I have used two part polyurethane resin with about 60% by weight 320 mesh silica filler. It was very dense. The filler keeps the shrink down also. The fine mesh filler keeps the surface nice.
trans192 said:
Thanks, that's very useful and exactly the thought I'm toying with for a pair of Jordan JX92s. Minus the details: two thin aluminum skins formed into a cylindrical JR149-type shape, the void between filled with a resin. Possibly a third sheet in the middle to create a psuedo constrained-layer damped cabinet. The baffle can be 1/4" aluminum between the skins, drilled and tapped for speaker screws.
I've had the opportunity to compare two wildly different cabinet materials - 1/4" carbon steel and 3/4" pine - and though both are ridiculous the difference through the bulk of the lower midrange and down is amazing. I'm really keen to avoid wood for the Jordans and a resin sandwich construction seems appealing and relatively easy to make.
If you have two 'skins' you can also fill the void with expanded polystyrene using the aerosols that they sell at DIY stores. 
I like aluminum for the skins. I thought I was being smart and used sonotube. The tubes worked fine to make the cabinet and there were no structural issues from that stand point. The problem I encountered was in finishing. The thick layers of paper were murder to flatten out for a fine high gloss finish.
If you decide to go with the expanding foam nuuk mentions above, you should be careful not to use to thin a material for the skin, as the foam may cause it to buldge out.
For my baffle I used 2 inch think oak. It was a bear to cut the baffle holes in and the finished product, although strong, was not resonance free. the thin area between the tweeter hole and the woofer holes vibrated quite noticeably.
I would suspect that quarter inch aluminum is too thin for you baffles. If you investigate the Wilson Benesch line of speakers, they are using composite cabinets and metal baffles. I think depending on sizes of your drivers and baffles, you should be looking at something thicker than quarter inch. The WB act2 white paper suggests they use an 18mm steel/aluminum sandwich (constrained layer maybe) for their baffles.
If you decide to go with the expanding foam nuuk mentions above, you should be careful not to use to thin a material for the skin, as the foam may cause it to buldge out.
For my baffle I used 2 inch think oak. It was a bear to cut the baffle holes in and the finished product, although strong, was not resonance free. the thin area between the tweeter hole and the woofer holes vibrated quite noticeably.
I would suspect that quarter inch aluminum is too thin for you baffles. If you investigate the Wilson Benesch line of speakers, they are using composite cabinets and metal baffles. I think depending on sizes of your drivers and baffles, you should be looking at something thicker than quarter inch. The WB act2 white paper suggests they use an 18mm steel/aluminum sandwich (constrained layer maybe) for their baffles.
I work for a company that builds sunrooms and screenrooms. We have this board made from a sandwich of plastic, superdense foam, and plastic. It is extremely light weight (a 4x8 sheet weighs less than 30lbs) with a thickness of about 2.5 inches. The board seems to be very stiff and might be a neat experiment for a subwoofer enclosure. I was thinking how nice it would be not to have a 200 pound subwoofer.
For the front baffle my thinking was more along the lines of skin-damping-1/4" alu- damping-skin. I don't want too much overhang at the back of the driver. Total thickness would be closer to 1/2" of five different layers and the baffle itself wouldn't be much wider than the Jordan, 8" max. It would be trivial to cut the two surface skins in such a way as to create a non-uniform wall thickness in the resin sections, say 1/2" at the baffle, quickly expanding to 2" by half way towards the back and taperig to 1" at the rear. It would be extrodinarily stiff. In fact, if someone wanted to go to the trouble they could deform the internal skin into any desired arbitrary shape to optimize stiffness or control internal reflections.
I'm not sold on using a light material between the skins though. The cabinet walls must be stiff to avoid resonating - either material could do that - but it must also contain bass waves. I'm not convinced a light foam will do that.
BTW, one of the thoughts I had was bonding an aluminum wrap around Sonotube with polyurethane glue. Instead I think I'll use Sonotube as a jig for shaping the walls and go high mass for the resin.
I'm not sold on using a light material between the skins though. The cabinet walls must be stiff to avoid resonating - either material could do that - but it must also contain bass waves. I'm not convinced a light foam will do that.
BTW, one of the thoughts I had was bonding an aluminum wrap around Sonotube with polyurethane glue. Instead I think I'll use Sonotube as a jig for shaping the walls and go high mass for the resin.
Great advice for the moulding resin and polyurethane and thing, especially with the preparation and hazard instructions. Installation foam isn't very springy though, I wouldn't recommend it for filling any void, in fact I would suggest to get some expanded polystyrene from any place and put it in a blender until a desirable grain size is achieved and then use this in the void. It would probably have better soundproofing properties. But you probably aren't as insane as me.
To mould a perfectly round 12 inch pipe, do you think it might be possible to make a 12 inch tube from some suitable cardboard, and to link it to a motor so it turned around like a wheel, and then pour moulding resin on it so that gravity assures an even coat.? How viscous is it? If this method works wouldn't it allow you to sandwiches many layers as you want just like skins, but in a composite tube instead ?
To mould a perfectly round 12 inch pipe, do you think it might be possible to make a 12 inch tube from some suitable cardboard, and to link it to a motor so it turned around like a wheel, and then pour moulding resin on it so that gravity assures an even coat.? How viscous is it? If this method works wouldn't it allow you to sandwiches many layers as you want just like skins, but in a composite tube instead ?
VvvvvV said:
I'm guessing this isn't the blender your wife uses for margarita's.
Yes we all are sort of insane, just some more than others 😉
Cal
Good sailboats are made from rigid foam (1-2") or balsa sandwiched between outer layers of polyester or epoxy resin infused fibereglass, kevlar, and carbon fiber. The whole shebang is put in a (big!) vacuum bag to squueze out excess resin.
This makes a VERY stiff hull that also has some damping It apears. Although rigidity is a major factor on a boat, they also need some toughness to take impacts and overloads, which is why they don't use only carbon fiber. In a speaker I would think that carbon fiber is the BEST. I believe that W. Benesh agrees.
I would think that even one layer of carbon separated by an inch of foam would be impressive..
This makes a VERY stiff hull that also has some damping It apears. Although rigidity is a major factor on a boat, they also need some toughness to take impacts and overloads, which is why they don't use only carbon fiber. In a speaker I would think that carbon fiber is the BEST. I believe that W. Benesh agrees.
I would think that even one layer of carbon separated by an inch of foam would be impressive..
I'm surprised no one has mentioned concrete yet. It's a very effective speaker material and you can make cabinets by pouring liquid concrete into molds.
People say that concrete doesn't have the right acoustic properties, it makes a high-pitched sound when you tap it, I think the best speaker material probably makes a dull thud kind of sound.
Here's the plan:
1: make a cardboard tube of 12 inches diameter, set it on a motor say that it turns like a roller
2: get lots of expanded polystyrene balls and mix them in with some casting resin just so they are sticky enough, and then pour one inch of the mixture on the roller as it turns
3: use the rest of the casting resin to create another inch of pure plastic to form the main part of the casing et voila
Here's the plan:
1: make a cardboard tube of 12 inches diameter, set it on a motor say that it turns like a roller
2: get lots of expanded polystyrene balls and mix them in with some casting resin just so they are sticky enough, and then pour one inch of the mixture on the roller as it turns
3: use the rest of the casting resin to create another inch of pure plastic to form the main part of the casing et voila
In thin layers concrete is pretty flexible. With steel reinforcement even springier. I suspect about an inch thick with granular admixture of say marble chips and NO steel, it would be fine.
Inch think would be plenty heavy though!!
Inch think would be plenty heavy though!!
With respect to the idea of the rotating motor. This idea isn't as wild as it may sound - when I was investigating my resin based design I considered several approaches not just the moulded one. I called several industrial suppliers to see what I could get made for me and the cost.
The second place choice which I think would have been excellent was spun fiberglass/kevlar/carbon fiber. The deal with this approach (as I understand it) is they get prepreg fiber materials , make a mould and spin the mould to lay on the layers of matting/cloth. using this method you can get a cabinet of any reasonable wall thickness you could want.
This approach brings up an issue I should mention: you have to consider how you are going to mould the top and bottom without using a two part process (two parts moulded separately, then glued together to make a single piece), it isn't practical to mould all sides in one pour.
I chose to build the mould in such a way that I poured the top, and side walls all in one pour, and then used a heavy piece of oak for the base to cover the area of the last panel that wasn't mouded. To make assembly easier, during the moulding process I put bolts into the uncured resin so that the thread portion of the bolt protruded through my base board. This left the heads of the bolts in the resin. No T-nuts needed no inserts. The bolts are part of the cured plastic now, and they aren't going anywhere. Also, because they went through the actual holes in the actual board used to seal the base they were in perfect alignment for final assembly.
If you aren't going to do something like I did you should put some serious thought into how you are building your top and bottom panels. My experience shows these panels have the most vibration after the baffle. So extra care here is in order.
The second place choice which I think would have been excellent was spun fiberglass/kevlar/carbon fiber. The deal with this approach (as I understand it) is they get prepreg fiber materials , make a mould and spin the mould to lay on the layers of matting/cloth. using this method you can get a cabinet of any reasonable wall thickness you could want.
This approach brings up an issue I should mention: you have to consider how you are going to mould the top and bottom without using a two part process (two parts moulded separately, then glued together to make a single piece), it isn't practical to mould all sides in one pour.
I chose to build the mould in such a way that I poured the top, and side walls all in one pour, and then used a heavy piece of oak for the base to cover the area of the last panel that wasn't mouded. To make assembly easier, during the moulding process I put bolts into the uncured resin so that the thread portion of the bolt protruded through my base board. This left the heads of the bolts in the resin. No T-nuts needed no inserts. The bolts are part of the cured plastic now, and they aren't going anywhere. Also, because they went through the actual holes in the actual board used to seal the base they were in perfect alignment for final assembly.
If you aren't going to do something like I did you should put some serious thought into how you are building your top and bottom panels. My experience shows these panels have the most vibration after the baffle. So extra care here is in order.
I have to agree with RDF, if you aren't doing dedicated cabinets for each frequency range, a heavy cabinet is going to perform better over a wider range of frequencies. I loaded my polyester resin with dry, clean playground sand and they were heavy but manageable for a single person. Aside from the baffle (sigh!) my cabinet walls show very little vibration.
I like the polystyrene idea, a few 1" sheets of this with skins of carbon fiber or fiberglass on each side wouold make very stiff panels that weigh almost nothing. Think surfboard, dude 😉 THe only problem is then in joining them.
Here's where I stumble on the light-weight wall concept. Picture a completely inert room - say 20' concrete on five sides - with one wall constructed of any of the proposed methods. Put a listener on one side of the test wall and wide band noise source with substanital low frequency content on the opposite. A truck for example. Which would allow the absolute minimum pass through? I can't help but think the truck would be clearly audible through a foam-type wall, all other variables (stiffness, etc.) being equal. There's obviously much more to consider than just acoustic pass through but it's hard to see how a light cabinet could ace this test.
Celestion made some speakers out of HexCell(tm) type material (They called it AeroLam) and they were critically acclaimed, but they were small monitors and had lightweight bass to begin with..
Acoustic transmission through building walls is an area where mass is an advantage. The wall is like a very big membrane and is analysed as such - a membrane without stiffness but with mass. In the area above resonances, the mass law is a very effective predictor of transmission through walls TLo=20*Log(ws*f)-28dB where ws is the weight per square foot and f is frequency
In a speaker, the wall is much smaller (so proportionally much stiffer) and the resonant frequencies of said membrane is in the hundreds of Hz - beyond the frequency range of a subwoofer, In the area below resonance, the mass law does not hold.
If it is so important to have massive walls, what about sound that is reradiated through the cone? 😉
Acoustic transmission through building walls is an area where mass is an advantage. The wall is like a very big membrane and is analysed as such - a membrane without stiffness but with mass. In the area above resonances, the mass law is a very effective predictor of transmission through walls TLo=20*Log(ws*f)-28dB where ws is the weight per square foot and f is frequency
In a speaker, the wall is much smaller (so proportionally much stiffer) and the resonant frequencies of said membrane is in the hundreds of Hz - beyond the frequency range of a subwoofer, In the area below resonance, the mass law does not hold.
If it is so important to have massive walls, what about sound that is reradiated through the cone? 😉
Martin Collums' book High Performance Loudspeakers addressed this issue. I haven't read it in a while, so I am paraphrasing here:
Below a certain frequency - say 40 hz stiffness is the most effective method of controlling vibration. Above this frequency resistance/damping is most effective until a second break point - say 100 hz, and above that frequency range it's mass that is most effective.
I don't have the book on hand, or I would be more specific (and accurate 😉 please forgive my vagueness.
So true subs - cabinets that play infra bass, and the bottom octave are best served with ultra stiff cabinet walls. Woofers are best served with stiff well damped walls, and mid woofers should have heavy, well damped stiff walls.
I don't know if the break points I mentioned above are the correct points, but if the description above is correct I think it's fair to say that the mix of frequencies present in the bass cabinet will indicate the most efficient approach to removing vibration from the cabinet walls. VvvvV, you didn't indicate the type of cabinets you wanted to build, were you thinking of a two-way bookshelf speaker? a sub woofer? knowing what you had in mind would help direct the discussion.
Below a certain frequency - say 40 hz stiffness is the most effective method of controlling vibration. Above this frequency resistance/damping is most effective until a second break point - say 100 hz, and above that frequency range it's mass that is most effective.
I don't have the book on hand, or I would be more specific (and accurate 😉 please forgive my vagueness.
So true subs - cabinets that play infra bass, and the bottom octave are best served with ultra stiff cabinet walls. Woofers are best served with stiff well damped walls, and mid woofers should have heavy, well damped stiff walls.
I don't know if the break points I mentioned above are the correct points, but if the description above is correct I think it's fair to say that the mix of frequencies present in the bass cabinet will indicate the most efficient approach to removing vibration from the cabinet walls. VvvvV, you didn't indicate the type of cabinets you wanted to build, were you thinking of a two-way bookshelf speaker? a sub woofer? knowing what you had in mind would help direct the discussion.
It may be worth pointing out that while a number of theories have been put forward in this thread, my suggestion to use polystyrene is based on (my) experience! 😉
If practical, I prefer to 'do' rather than hypothesize. It saves quite a lot of time!
The idea that you must stop sound escaping from a loudspeaker cabinet is almost ludicrous, and is pratically unachievable. Think about it, if you had a cone that sound-proofed the speaker cabinet, it would be too heavy to move!
Agreed, there is a need to prevent the rear wave of the cone meeting the front wave and causing bass cancellation, but I know of no other reason to prevent all the sound from inside the box getting out. In fact, it may even be beneficial to have some 'escape'!
If practical, I prefer to 'do' rather than hypothesize. It saves quite a lot of time!
The idea that you must stop sound escaping from a loudspeaker cabinet is almost ludicrous, and is pratically unachievable. Think about it, if you had a cone that sound-proofed the speaker cabinet, it would be too heavy to move!
Agreed, there is a need to prevent the rear wave of the cone meeting the front wave and causing bass cancellation, but I know of no other reason to prevent all the sound from inside the box getting out. In fact, it may even be beneficial to have some 'escape'!
Some factors are being overlooked. My Jordans are in 24" x 8" square pine MLTL cabinets - thrown together for break in at a whopping $15. The cone's surface area comprises roughly 1.3% of the total system surface area. In its pistionic range the amplifier's feedback loop acts to attenuate cabinet sound coming back through the cone. Above that range, for probably all but subwoofers or atypical designs, internal stuffing has attenuated most of the rear wave. In all acoustic return through the driver in my example will be well under 1% of total system maximum potential output due to pass though. It's true that some internal cabinet sound will find its way back though the cone but I don't see how it's valid to write off the remaining potential 99% plus.
In any case, going way back to the begining of the thread, I'm in a boat on the same lake as Nuuk but rowing in the opposite direction. It was my experience with similarly alignmed cabinets of constrained layer damped 1/4" steel (TB W4-657s) against regular 3/4" pine (Jordans JX92s) which leads me to believe acoustic pass through is an important component of a cabinet's design, along with stiffness and a low resonance signature.
OT - Hey RonE, thanks for attributing that quote in your sig. It's what we said of our Engineering profs but I never knew its origins.
In any case, going way back to the begining of the thread, I'm in a boat on the same lake as Nuuk but rowing in the opposite direction. It was my experience with similarly alignmed cabinets of constrained layer damped 1/4" steel (TB W4-657s) against regular 3/4" pine (Jordans JX92s) which leads me to believe acoustic pass through is an important component of a cabinet's design, along with stiffness and a low resonance signature.
OT - Hey RonE, thanks for attributing that quote in your sig. It's what we said of our Engineering profs but I never knew its origins.
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