Help me finish my polystyrene cab build

[neal sanger]

So I saw someone on talk building a cab with fiber glass and insulation foam, and I am trying to do the same with just foam. I plan to use 10.125" of schedule 80 PVC pipe for a 31hz tuning port (the math just worked out nicely), and the checked it out with win isd and AJ vented designer MFC application and the cab resonances seem evenly distributed, but since it's polystyrene, I can't predict how it will behave, but that's not the issue. I'm planning to glue a piece of plywood with a cutout for a baffle in it to the last piece of foam board which will hold the speaker, but I can't think of a way to do this where I would be able to remove the speaker and swap it for another. anyway, if I mount the speaker directly to the foam core, I worry it will chew up the mounting points in short order due to vibration from the speaker. I have had poopy luck with T-nuts swapping drivers more than once, and once the last foam panel is glued, it will be nearly impossible to change anything without wrecking the cab. I'm a total noob and have never even built a cab before, but I just wanna finish this project because it's been sitting in my basement for a month and it's pissing off my family.

 

[fdeck]

I typically use 1/2" plywood for my DIY speakers. I've had great luck with just basic drywall screws for the speaker, and small washers. I realize it's not "professional" but I get decent uniform compression of the gasket and the screws have stayed put for years. It's easy to sneak them out and replace them without crossing the threads if you just get them started with your fingers.

For my speakers, I tend to dispense with all of the trimmings of tour-worthy gear, because the cost of that stuff adds up, and I'm just not banging my speakers around a lot.

 

[neal sanger]

I typically use 1/2" plywood for my DIY speakers. I've had great luck with just basic drywall screws for the speaker, and small washers. I realize it's not "professional" but I get decent uniform compression of the gasket and the screws have stayed put for years. It's easy to sneak them out and replace them without crossing the threads if you just get them started with your fingers.

For my speakers, I tend to dispense with all of the trimmings of tour-worthy gear, because the cost of that stuff adds up, and I'm just not banging my speakers around a lot.

This definitely won't be a gigging cab since the foam is super fryable. I guess I could just screw the baffle to the backing plywood, but I designed the cab mainly to weigh very little while being huge and having fat low end. To support that goal, I was planning to use 3/8" plywood and it would not do a good job holding screws. Perhaps I could glue on some wood blocks on the rear of the wooden backing board where baffle mounts and retain my weight savings while increasing the security of the mount.

 

[neal sanger]

that's a lot more down to earth than I was thinking.

 

[agedhorse]

Due to the compressible nature of the foam my guy feeling is that the box won't behave as a solid. I would recommend verifying the tuning frequency after it's all built up, you might find it quite different from what's predicted.

I must applaud you for your creativity

 

[neal sanger]

Thanks. I know the foam is gonna do something (hopefully not something bad) to the sound, I just have no idea what it will be. I checked it with software, but that's nearly useless. I worry that I would be unable to modify the port length once it's in there, so I'd probably be stuck with whatever it ended up at. I tried to brace the thing far more than was necessary, but this is, as you mentioned, some what uncharted territory. How would I go about testing the tuning experimentally (on the finished cab)?

 

[Downunderwonder]

You might have hit on the next big thing in cabinet making but I doubt it. The possibilities for laminating a hard ply layer onto the outside are fantastic. I suspect you are in for a learning experience but you are so far in you should definitely do it.

You should make your baffle side in the same material but with a big oversize hole for the speaker and another for the port. Laminate a 1/2" ply baffle skin onto that with the true size holes. You should definitely brace that all to the back side.

You can put your port in most of the way and check the tuning before you glue it. Better make it a little long. Trim it down to get to tuning point.

30hz is ambitious. Lots already written about that. You generally lose a lot of power handling to go that low.

 

[agedhorse]

Look up the rice test, see how close reality comes to the predicted number.

At high power, I would worry that your joints might tear apart, not the adhesive but the foam itself. At lower volumes it should hold up ok.

Gut feeling is that there will be an added resonance from a secondary spring effect, but I'm not even sure how I would go about modeling it...

 

[neal sanger]

The lower of the two cross braces is glued to the back panel(like a T), but it's impossible to see in my photo. would it be a good idea to glue the cross braces to each other with another length of foam? the stuff is very easy to manipulate and the glue has been an amazing bandaid for my novice building skills.

I was trying to think of materials that would make good laminates, but most of them were either flimsy (veneer) or heavy (melamine board) which would be antithetical to the entire point of the cab. Would definitely help with the rigidity though.

my plan for attaching the port was to use a couple pieces of foam board like giant washers around the port tubing to hold it in place. I will give the pre-glue test fit tuning length a shot. I hadn't conceived of that. I worry the tubing may buzz against the foam board if it isn't glued in place which could confuse the testing results.

As for modeling the polystyrene, I am completely at a loss as well. This is all new to me.

 

[neal sanger]

my sloppy finger jointing was filled in with copious gobs of loctite 200. It might make a stronger joint, but again, I have pretty much no idea what to expect.

 

[neal sanger]

some pics from earlier in the build process if anyone wants to see

 

[neal sanger]

IIRC, the power handling was about 150W before port velocity should cause port noise and get flubby, but I can't tell you the basis for that number since I did all of this math out like a month ago.

 

[JimChjones]

The conventional thing to do to put high load bolts into a foam core would be to replace the foam with a high density material in the way of the bolts. The obvious choice (availability) is solid wood. I made a ply/fibreboard/ply sandwich front baffle for a lightweight cab, and simply replaced the fibreboard with pieces of ply for an inch or so all round where the bolts were going to go. In your case I think I'd get some 2 * 2 square timber and cut it into cuboids the thickness of your foam and glue it in there, then cut the speaker cutout and drill it for the bolts. If you put the grain end on cedar will be strong enough and light.

I'd also be inclined to have 4mm (3/16in) ply sandwiching the baffle to make that more rigid (ie one piece front, one back). You want to transfer the big loads from that vibrating mass into the main structure as well as you possibly can. If you do this together with the solid wood pads then the speaker weight and energy is going to be as well transferred into the structure as possible, and you can just bolt the driver in with T nuts as conventional. The sandwich front and back in 3/16 ply will be way way better than just 3/8 ply on the front face.

This is something I've thought a lot about, because I've done a lot of home build high tech boat building in foam, glass and carbon. I've seriously considered an all foam sandwich cabinet using PVC foam, but the way my sums went by the time I got the outer skin strong enough to be reasonably roadworthy a substantial part of the weight saving would be gone. However if you're not planning to make it roadworthy then there ought to be some considerable savings in weight.

 

[DeltaDelta]

So I saw someone on talk building a cab with fiber glass and insulation foam, and I am trying to do the same with just foam. I plan to use 10.125" of schedule 80 PVC pipe for a 31hz tuning port (the math just worked out nicely), and the checked it out with win isd and AJ vented designer MFC application and the cab resonances seem evenly distributed, but since it's polystyrene, I can't predict how it will behave, but that's not the issue. I'm planning to glue a piece of plywood with a cutout for a baffle in it to the last piece of foam board which will hold the speaker, but I can't think of a way to do this where I would be able to remove the speaker and swap it for another. anyway, if I mount the speaker directly to the foam core, I worry it will chew up the mounting points in short order due to vibration from the speaker. I have had poopy luck with T-nuts swapping drivers more than once, and once the last foam panel is glued, it will be nearly impossible to change anything without wrecking the cab. I'm a total noob and have never even built a cab before, but I just wanna finish this project because it's been sitting in my basement for a month and it's pissing off my family.View attachment 3277723 View attachment 3277727

I too applaud you for your creativity and I must say I am very curious about the results!

Look up the rice test, see how close reality comes to the predicted number.

Quite interesting...I've seen something similar in the study of the differing vibration modes of violin soundplates.
I believe they are called Chladni patterns.
On a side note, if you do a Google search, remember to add "speakers" or the results you get will mainly be centered around...ahem..breast implants!

 

[micguy]

The key thing you're missing here (putting my Acoustical Physicist hat on for a bit) is panel stiffness. You can probably get the thing well sealed, and get the right volume, but the thing is gonna "breathe" when pressurized - the panels are going to flex quite a bit. If you're dead set on using just foam, I'd suggest adding more layers of it - maybe triple the walls. Yes, it'll be bigger, but it still won't weigh much, and it might work OK as an "around home" kind of speaker box, so long as people don't sit on it or something. Obviously, not up for life on the road.

 

[ThinCrappyTone]

Thanks. I know the foam is gonna do something (hopefully not something bad) to the sound, I just have no idea what it will be. I checked it with software, but that's nearly useless. I worry that I would be unable to modify the port length once it's in there, so I'd probably be stuck with whatever it ended up at. I tried to brace the thing far more than was necessary, but this is, as you mentioned, some what uncharted territory. How would I go about testing the tuning experimentally (on the finished cab)?

This of course begs the question... What is the best tonefoam?

 

[JimChjones]

This of course begs the question... What is the best tonefoam?

For maximum panel stiffness you'd use a sandwich of foam and other materials. A Carbon fibre/foam/carbon fibre laminate has very impressive panel stiffness for light weight. The fly in the ointment is that a minimum weight structure would dent and even get holed like crazy in normal conditions. Even a ply/foam/ply sandwich will deliver superior stiffness.

But what got me when I was considering it was the roadworthiness. I reckoned 8mm ply to be the minimum that will reasonably survive the average load out, so then the base panel I figured would be needed would be 8mm ply, 10mm foam, 3mm ply inside (thinner ply appallingly expensive). Assuming (for easy sums) about 50kg/m3 for the foam and 500kg/m3 for ply then that would give a panel weight roughly equivalent to 12mm ply. We might also guess that might give us a structure vaguely equivalent to 18mm ply. So that's a 33% weight saving. Or is it. The panels are only part of the story. I reckon that the corners and joins will be more trouble, and everywhere there are through fastenings they will need to have high density pads built in as I outlined above. Then of course all the fittings, the driver, all that will weigh the same. I reckoned you might be doing well to achieve about a 15% weight saving overall, for a huge amount of hassle, and you can probably achieve 15% over 18mm ply by clever design and bracing.

Now beyond that I wonder if there aren't implications on the sound properties - foams are, after all, sound insulating, so what effect does having a sound absorbing foam layer between the skins have compared to a solid ply skin? I wouldn't be surprised if it were advantageous, but have I the facilities to do proper testing? Nope, neither the facilities nor, frankly, the patience, but sooner or later the high end hi fi kids will have a go!

Of course if it isn't going to go on the road you could go way way lighter, esp if money is no object - maybe 200gsm carbon skin - but if it isn't going to go on the road why are you caring so much about weight anyway unless you have special circumstances?

 

[Coolhandjjl]

Appreciate the enthusiasm, ingenuity, and passion. Kudos!

There was cab builder on the Greenboy Forums (fEARful) many years back with a company called Art Of Noise. He tried to use some sort of epoxy and synthetic fiber reinforced panels to build cabs. Long and short of it, it didn't work. And he was light years ahead of where you are with the design and fabrication.

Hard to beat 1/2" Okume plywood with properly engineered bracing for the appropriate lightness/stiffness ratio. Some have gone to 3/8", but again, engineered bracing is the key.

Most DIY cab builders use 1/2" Baltic Birch plywood. Still needs proper bracing, but is the best value out there considering weight, strength and cost.

 

[mmon77]

I was trying to think of materials that would make good laminates, but most of them were either flimsy (veneer) or heavy (melamine board) which would be antithetical to the entire point of the cab. Would definitely help with the rigidity though.

My lightweight travel trailer has panels made from a sandwich of 1/4" luan ply / 2" foam / 1/4" luan ply. The stuff is surprisingly strong for how light it is. Water is it nemesis. It causes the whole sandwich to de-laminate and loose its stiffness.

My understanding is the stuff is glued and pressed under considerable pressure to form the laminate, so I'm not sure how feasible that would be to do at home.

Another +1 for your creativity though. I'm interested to see how it turns out. Good luck!

 

[neal sanger]

Well I saw tech ingredients (a particularly excellent youtuber) building speakers from transducers mounted to foam board and I was curious how it would translate to a traditional speaker cabinet. I was thinking of just surfacing the box in loctite 200 to give a little added toughness. Carbon fiber would be great, but I do not know how to work with it nor do I have any of the necessary tools. Additionally, I think the epoxy needed to seal carbon fiber would simply eat right through the foam. I can't really justify all of my design retroactively, but I'm taking notes on everyone's input.