1" birch

Let's not talk about OSB, that **** is awful.
Awful to work with, but very dense and rigid. I used it in my c-pillar wall with good results and there's a fella in Rhode Island that used it in his daily driver Accura that does 157's (DB-Drag) and 160+ USAC with a pair of 15s. Even just particle board + resin has been tested to = MDF + resin on the meter so to a large degree material isn't really as important as just adding strength/support where needed and ensuring everything is 100% air tight.
 
Awful to work with, but very dense and rigid. I used it in my c-pillar wall with good results and there's a fella in Rhode Island that used it in his daily driver Accura that does 157's (DB-Drag) and 160+ USAC with a pair of 15s. Even just particle board + resin has been tested to = MDF + resin on the meter so to a large degree material isn't really as important as just adding strength/support where needed and ensuring everything is 100% air tight.

Yea, but 2 main issues with OSB. It is ******* awful to work with, splinter city. And it's just as bad as MDF with moisture.

Plus, you basically have to paint it for anything to look decent, lol. Remember, I'm not going for meter here, so a little flex isn't going to matter all that much for me. I just know that 3/4 isn't going to survive AT ALL. My trunk baffle is already taking a beating with a single 18 on 3k.

Matt
 
And it's just as bad as MDF with moisture.
I've got a pallet made out of basically the same composition out here as part of a wall on a duck house and its in its 4th winter now holding up fine outside. Definitely nothing like MDF and really if you have enough water getting into your car to sponge up MDF you've got bigger problems. Again, the point isn't to run out and use that necessarily (it is a nightmare to work with, but I had several sheets left over from a flooring project) it's that you shouldn't need to pay 200$ a sheet for imported wood to build a sturdy and functional box. Or put another way, over-paying on fancy plywood isn't going to be your silver bullet to fix skimping out on layers and/or bracing where appropriate. It's not going to be magic and everybody underestimates how much this sort of thing will flex just about any material you attempt to use to contain the pressure.
 
I've got a pallet made out of basically the same composition out here as part of a wall on a duck house and its in its 4th winter now holding up fine outside. Definitely nothing like MDF and really if you have enough water getting into your car to sponge up MDF you've got bigger problems. Again, the point isn't to run out and use that necessarily (it is a nightmare to work with, but I had several sheets left over from a flooring project) it's that you shouldn't need to pay 200$ a sheet for imported wood to build a sturdy and functional box. Or put another way, over-paying on fancy plywood isn't going to be your silver bullet to fix skimping out on layers and/or bracing where appropriate. It's not going to be magic and everybody underestimates how much this sort of thing will flex just about any material you attempt to use to contain the pressure.

I completely agree with everything you said here. My experience with OSB(central Florida, mostly general construction situations like roofing or shed floors) is different than yours, but I do agree it has its place.

We can agree that different materials have different uses. I wouldn't use home depot special wood for my cabinets in a house. Even if the wood is sturdy, it's going to take alot of work to make it look nice. Conversely, if you are making a workshop table I wouldn't be ordering 100 board feet of mahogany for the top.

In this case, I'm using the Birch because it's void free, very stiff, looks nice, and most importantly is lighter than MDF, and the edges can be hit with a trim router to leave a nice finish. I'm already adding hundreds of pounds to my car, and here I can spend a little extra and finally save some weight while also saving myself some time making the final box look nice.

As far as flex goes, personally I don't see flex being as concerning as most people make it out to be. Yes, I agree it can be a problem but unless you are building a box out of 1/2" material you probably won't have a huge problem. Higher power requires thicker material, but eliminating every bit of flex from an enclosure is something I only see being truly beneficial in an SPL competition where a flexing panel could cost you a few tenths on the meter and put you in 3rd place.

My current baffle is 3/4" 7-layer birch from Home Depot. It's 42" wide and 28" tall and is bolted to the rear strut tower/trunk brace with 6 M8 bolts. The sub is located in the direct center of the panel. At a soft clip (about 3000w on the smart 5)on bass heavy music it flexes about 3/4 of an inch forward(the only direction it can actually flex).

That amount of flex is concerning. So going with a 1.5" thick baffle is a requirement. Doubling the panel thickness increases rigidity by roughly a factor of 2(between 2 and 2.5 depending on how you calculate and how you measure). This is a little shortcut I've used in the past with shelving and cabinet making:

  • If shelf span is reduced by one-fifth, stiffness is roughly doubled (deflection is halved).
  • If shelf span is increased by one-fourth, deflection doubles.
  • If shelf span is doubled, deflection is eight times greater.
  • If shelf thickness is doubled, deflection is reduced to one-eighth.
  • If shelf depth is doubled, deflection is cut in half.
Making the top panel 2" with the addition of a pair of Dowels I am essentially cutting the deflection by a factor of 4 when talking about a fully un-supported panel. If you take into account the front and sides of the enclosure, you further reduce the deflection by some number(I'm not doing that math, lol). 3/4" divided by 4 would give me about 1/16th of an inch + or-deflection. Double that for the addition of a second sub and I'm looking at 1/8th + or - based on napkin math applied to a current setup.

I think I'll go with 1" birch for the sides, back and bottom, add a 1/2" layer to the top for 2.5" total, and add a 1/2" layer to the front for a 1.5" total. Add in a single side to side brace in the middle of the large side panels, add in a pair of top to bottom dowels in the center of the top but offset to 1/3rd width. And then add a final dowel top to bottom between the sub mounting cutouts.

Matt
 
eliminating every bit of flex from an enclosure is something I only see being truly beneficial in an SPL competition where a flexing panel could cost you a few tenths on the meter

I think the difference may become audible (greater than 1dB) much more quickly than you think, both in output and in creating distortion. I had a reliable source explain to me the math behind predicting this sort of thing and it surprised me when he threw out some numbers about how little panel flex it takes to become audible.

If spending money for weight savings and making your working with the material easier is good enough to justify the cost for you, it's all good, you get to prioritize the cost vs. value of those features. Just so long as you don't get the idea that it's going to be magic and negate the need for more thickness and bracing where appropriate.

Bear in mind the exponential scale of deflection vs length of the panel as you go.
 
Yea, it's not cheap, total cost plus shipping is going to be about 1200ish, still working out details. But rather than 4 sheets of 3/4 it will either be 1 sheet of 1" and 1 sheet of 3/4 or 2 sheets of 1". The bottom, back, and large side panels are all going to be against the body of the car, so 3/4 would be fine, going with 1" because why not. Just need to decide if I REALLY need 2" thick top or if I can just 1" the whole thing on 5k with a few dowel braces.

Matt
Birch is really strong, it rates almost 100 higher on the janka scale +-. If my math is right, that's close to 30% when compared jointly. So a 1" piece of birch would be like a 1-5/16 piece of mdf. And Baltic is stronger than regular birch but I couldn't find it's janka scale. I would say a dowel is fine, but when in doubt double it out lol.
 
What I like to do and it will save you some $ is get a sheet of 1/2” red oak cabinet plywood at **** deep hole it's like 80$ for a full sheet of 3/4" here in tx so the 1/2" is like 40$ a sheet it's the strongest plywood with the least amount of flex that they carry (pre COVID was 35$ a sheet for the 3/4" but I digress) and grab there 3/4” red oak dowel rod they have hardly any flex I love them then what I do is I grab a gallon of fiberglass resin because I always put 2 coats inside and outside AND a 3rd " coat" in between the 2 layers of half inch and 3/4” instead of the typical wood glue and let me tell yah them subs ant gonna be able to make that box flex at all especially if you brace it the right way IE running the dowel rods from opposite sides IE from to back /left side to right side/tech
Fiberglass resin adds a little wright but gives you a smooth surface so that if you paint it you can use automotive car paint and make it look pimpin good
And that b!#=& will be practically nuclear bomb proof , I do this to all of my boxes and people that hear my boxes will use my boxes as the reference box to judge other people's/brand sub boxes they are looking to buy even years down the road , you can hear the difference between a regular box and a box with 0 flex and smooth inside and out and you bought the Rolls-Royce of plywoods so might as well seal it like a damn boat lol.

Ps if $ isn't a problem and you can get your hands on 1/2” of the same kind of plywood that the box is made out of the by all means that would be better I only mentioned the red oak because that's the best local readily available option IMO.
 
What I like to do and it will save you some $ is get a sheet of 1/2” red oak cabinet plywood at **** deep hole it's like 80$ for a full sheet of 3/4" here in tx so the 1/2" is like 40$ a sheet it's the strongest plywood with the least amount of flex that they carry (pre COVID was 35$ a sheet for the 3/4" but I digress) and grab there 3/4” red oak dowel rod they have hardly any flex I love them then what I do is I grab a gallon of fiberglass resin because I always put 2 coats inside and outside AND a 3rd " coat" in between the 2 layers of half inch and 3/4” instead of the typical wood glue and let me tell yah them subs ant gonna be able to make that box flex at all especially if you brace it the right way IE running the dowel rods from opposite sides IE from to back /left side to right side/tech
Fiberglass resin adds a little wright but gives you a smooth surface so that if you paint it you can use automotive car paint and make it look pimpin good
And that b!#=& will be practically nuclear bomb proof , I do this to all of my boxes and people that hear my boxes will use my boxes as the reference box to judge other people's/brand sub boxes they are looking to buy even years down the road , you can hear the difference between a regular box and a box with 0 flex and smooth inside and out and you bought the Rolls-Royce of plywoods so might as well seal it like a damn boat lol.

Ps if $ isn't a problem and you can get your hands on 1/2” of the same kind of plywood that the box is made out of the by all means that would be better I only mentioned the red oak because that's the best local readily available option IMO.

Since this is being assembled inside the vehicle I'm not using any resin or any kind. Fiberglass resin by itself is very weak. It adds mass, not strength per-sey. The benefit you are seeing is by layering the plywood with the resin and essentially making a composite.


I'll be bracing appropriately, and if it's flexing more than I want, I'll add some C-channel on the inside of the panels or something. There are plenty of ways to stiffen a box. The best way would be to make internal bracing out of the 1" ply. Unfortunately, I'm not interested in eating another sheet and wasting alot of material to make some braces.

Matt
 
There are plenty of ways to stiffen a box. The best way would be to make internal bracing
Yes, this can be done with dowel (cheap), or even some cheap 2x2 or 2x3 or whatever you have laying around.

Most people report gains from painting boxes with resin, but it's pretty nasty work so may or may not be worth the cost or hassle.
 
Since this is being assembled inside the vehicle I'm not using any resin or any kind. Fiberglass resin by itself is very weak. It adds mass, not strength per-sey. The benefit you are seeing is by layering the plywood with the resin and essentially making a composite.


I'll be bracing appropriately, and if it's flexing more than I want, I'll add some C-channel on the inside of the panels or something. There are plenty of ways to stiffen a box. The best way would be to make internal bracing out of the 1" ply. Unfortunately, I'm not interested in eating another sheet and wasting alot of material to make some braces.

Matt
Actually the wood soaks up the resin and the wood fibers act like fiberglass so it does add to the strength of the wood and doesn't add that much weight . The best thing to do is cross brace with red oak dowel rods but I digress I prefer compounding benefits of multiple methods of approach
 
Anyone used this stuff? It's like 20 layer full 1" thickness Baltic birch. I'm FINALLY about to get my box cut and I can't decide if I want to eat the airspace and double baffle the top(48wX30d), or if I just want to brace it with 1" dowel. The entire enclosure will be made of the 1" birch, and the top is the only panel with a size over 18x18.

Subs are 18" TC sounds pro 5100s, about 50 pounds each, power is from a Smart 5 at 2 ohm.

Trying to get the panel layout done and get a final quote so I can finally finish my build. I'll toss up a picture of the panel layout shortly.

Matt

17 layers and it's worth it. Get the Russian stuff if you can. $206 per sheet in Vegas these days.
 
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