legacycentral bbs

Recent discussion (elsewhere) has lit a fire under my ass to work on the sound system for the hot rod. I miss having a little bit of bass in my car, but the weight troubles me. My old box was really heavy, so I've sworn off MDF. This leaves me in a bit of a predicament, since few materials hold up to the abuse of a decent subwoofer and keep the pounds down.

I've seen plenty of fiberglass boxes built to look damn sexy, but they end up being almost as heavy as their MDF counterparts. The main problem in their construction is the many layers of glass it takes to achieve the rigidity necessary for a sub enclosure. I plan to challenge that methodology.

The key in my approach is sandwich construction. The further one can separate the layers of a composite structure, the stiffer it will be. As little as 1/4" of separation can mean as much as five times the stiffness of the same two layers of a composite. In the picture below, we made samples with 10 oz plain weave carbon fiber. 10 oz is rather heavy carbon and difficult to work with. Anyways, the picture below shows the samples we made. The sample without a core took 25 lb to bottom out the test rig at 1". The second sample with a 1/8" core took 75 lb to yield, and the last sample with 1/4" core took 125 lb to yield. I'll be using a 3/8" core.







I'll be using 3/8" balsa as a core, 7 oz plain-weave E-glass for a base inside and out, and then 5.7 oz plain weave carbon for the outside. I wanted a 2x2 twill weave for the carbon, but Express Composites in Minneapolis was out of it and I didn't want to wait. I'll just have to radius the outside edges for the plain weave.

All said and done, the materials came to about $120.

I also stopped at my dad's house to use his router to make a 3/8" ring for mounting the sub. Balsa would struggle to hold screws, so a 1" MDF ring for mounting is necessary.








I decided to go with a basic sealed box, even though I prefer the sound of a vented enclosure. Making the box sturdy enough to introduce a vent is not something I want to get into the first time around, so I decided to leave it out. When I figure out how to vacuum bag something like this, I might build a vented box.


The subwoofer is an Alpine Type R from about eight years ago. It is a single 4 ohm voice coil sub that worked well with the Rockford 250a2 amp I had at the time. That amplifier has since bit the dust.











This is the old box. It has been a parts shelf for six years, and spent four years in my trunk before that. The outside is getting a little beat, but it still works. 16 pieces of MDF comprise the box. The vent is built into the box with the proper cross-sectional area and length. The window was a hair-brained idea since you can't see it with the box in a car. It sounded like a good idea when I was 20 years old, though. Anyways, the box is a pig and I want nothing to do with it in my hotrod.










For the rest of the system, I have on hand (by accident) four Alpine Type S 6.5" full range speakers. I plan on powering them with a four channel Xtant amp I already have. I need a head unit still, and I've been thinking about an old Alpine unit that has decent pre-amp outputs and is simple. I don't need a fancy display or anything like that. Just something reliable and three sets of decent pre-outs. I'm also looking for a sub amp in 250w RMS range.

You should just do infinite baffle. No box to deal with. Mount them in the rear deck. But you need to seal off the cabin from the trunk so you would still face some problems. I have never done one but always wanted to. I doubt the rear seat itself would isolate the waves from the front of the speakers from the waves off the back of the speakers so you would most likely have to built a wall to seal off the trunk. This could possibly create a lot of weight depending on how you made the wall.

The rear deck is another issue. You would probably need to reinforce it to keep it from flexing and also sound deaden it to keep the rattles at bay. You could also mount the subs in the actual wall as many people do but I like the rear deck better. I'm not a bass head so I don't need 2 12" subs to satisfy me. A couple 8" subs or 10" subs are plenty. I have a small hatchback metro with one 10" sub that is fine for my taste.

I think infinite baffle is more for saving space for those guys who run big subs and don't want a giant box taking up all their trunk space. That and I hear it sounds pretty good if done correctly.

Ehh....Ifinite baffle setups are not going to give you the sound quality that a good box will.

Very cool stuff Dan!

I'm interested to see how things turn out. I've got an idea of something you could try to test the rigidity of the box vs. a similar MDF box.

Build the box, build a plate out of MDF or something rigid to mount in the speaker location. Hook a fitting to an air compressor (regulated air of course). Slowly bring up the pressure and measure deflection of the walls of the box.

You'd need to setup the test rig pretty accurately to measure the deflections accurately.

I'm honestly not even sure what kind of pressure is built up in a sealed sub box. That might be the first thing to try.

I don't know....that may be way over the scope of what you're looking to do

Legacy777 wrote:Ehh....Ifinite baffle setups are not going to give you the sound quality that a good box will.

I disagree. If you use a speaker designed for infinite baffle and you isolated the front wave from the back wave of the speaker it would sound great.

I guess my experience with IB hasn't been all that great.

A buddy in high school had a set of pioneer IB subs. He wanted to put them in his nova. I used an air grinder to cut a hole in the back seat pan to the trunk, and then he took a piece of MDF to screw the speakers in, and then screwed that to the sheet metal pan behind the rear seat.

Totally ghetto fabulous! It bumped....and that was what he was looking for.

So yeah....that's my IB experience

I'm not going to do an infinite baffle setup. I have the sub that I have, and the next one I buy will have a neodymium magnet to save weight. Also, I'm not willing to cut a big hole in the rear deck. I want a box that is removable, and the second iteration will be a vented box, since I prefer that sound.




Josh, I could do that fairly easily, but not as precise as we would like. I could simply use a straight edge to check deflection and measure it relative to a corner.

I'm sure a vented box would see much less relative pressure (positive and negative), but the logistics of building one to the specs I feel comfortable with are not something I want to get into right now. Sealed boxed are just so simple to build.

Yeah, very true. If you decide to do any testing, let us know how it comes out.

Now I'm curious to what sort of pressures are created inside a sealed enclosure.

I did some quick calculations on a possible pressure. I was kind of surprised, really. A new Alpine Type R sub has a maximum excursion of .72". This means the volume only changes by roughly 62 in^3. In a 1 ft^3 box, the ideal gas law says that it would only end up at 16.4 psia. Not much over atmospheric pressure at sea level.


I think the real question will be how well it handles being throttled by the sub itself. The forces the motor structure places on the box and the resonances could be the crux of the box's durability.


I intend to add more material around the mounting area of the sub and overlap the reinforcement fibers at the corners (seams) of the core.

One thing to think about is that I only run the sub at 250w RMS. We're not talking about a 1000w monster or anything like that.

Yeah, you're probably pretty close with the pressures.

If you get or have an SPL meter you might be able to check for the box resonance. It might not give you a completely accurate representation since it's not in an anechoic room, but I don't think you'd need to go that nit picky

hmm this is an interesting thought.

my 15+ yrs experiance in sub box building is telling me this may not work all that well but I'm interested in what you end up with.

A couple things though, I would worry more about longevity of the reversing + to - psi more than a sustained psi. flex can cause havok on thin materials. I've seen a single type r 12 crack and split a spare tire well in a Dodge Intrepid.

Try using a mesh with kevlar in it instead of straight carbon fiber if the composite your working on doesnt work to well.

Also another tip use either some thick backstrapping within the main frame structure of the box and resin it into the sides or use a thin chicken coop type mesh wire into your composite to help with the flex.


just my 2 cents.

once this is made send us month to 2 month updates on the box performance once the woofer has broken in.

__TT__ wrote:hmm this is an interesting thought.

my 15+ yrs experiance in sub box building is telling me this may not work all that well but I'm interested in what you end up with.

A couple things though, I would worry more about longevity of the reversing + to - psi more than a sustained psi. flex can cause havok on thin materials. I've seen a single type r 12 crack and split a spare tire well in a Dodge Intrepid.

Try using a mesh with kevlar in it instead of straight carbon fiber if the composite your working on doesnt work to well.

Also another tip use either some thick backstrapping within the main frame structure of the box and resin it into the sides or use a thin chicken coop type mesh wire into your composite to help with the flex.


just my 2 cents.

once this is made send us month to 2 month updates on the box performance once the woofer has broken in.

I appreciate the heads up, for sure. This is more of an experiment than anything else. I've built dozens of highly calculated MDF boxes that worked extremely well, but never anything that tried to cut 2/3 the weight. Like I said earlier, we'll see how this first attempt holds up to the sub itself. I've done some homework, but only time and testing will tell.


I thought about Aramid (Kevlar) as opposed to glass, but I was talked out of it at the time. I would have had to buy a $50 pair of scissors to simply cut the aramid to use it. Not something I wanted to get into in my attempt at a box like this.



I don't have pictures yet, but the scrimmed balsa took a ton of resin off the bat, and needs more yet before I even cut it and pin-nail parts together. Its looking like I should have used a closed cell PVC foam for a core. Either way, it should be be lighter than MDF. Only testing will tell though.

The carbon will only be the finish layer of the box. The inside and outside will be 10 oz E glass with heavy reinforcements at the joints. The guy at Express Composites gave me a bunch of heavy glass tape for that purpose. I'll rough everything up heavily of course.

I might see if I can vacuum bag the final layup. That would really help in the strength to wieght department.

Pics in a day or two. I'm at the liberty of my school's composites lab.

It's looking like I need to try a different core material other than scrimmed endgrain balsa. So far the large panel has taken 1100 ml of resin. One side still needs more so it isn't dry. If it wasn't scrimmed (the slits for flexibility), it would take a lot less resin. Just filling in the voids is wasting a lot of resin and adding a lot of weight.

I might wait a little bit and start over with a closed-cell foam and use aramid instead of glass. We'll see.

The Chinese bathroom scale says five pounds on that panel. That could be +- 3 lb though.

I've always been curious how a honeycomb core sub box would perform. I know for a fact that closed cell foam becomes extremely rigid when used as a core; we used it as core for our FSAE car to build rigid panels. At 3/8", it took our resident "large" guy to have any appreciable deflection