vented fibreglass enclosures

neomagus00

yeah, so how would you do this? assuming you didn't want it to look like butt, of course (i.e. a 4-foot piece of pvc sticking out of a 'glass box)... i'm wanting to glass my sub into a vented box in a corner of my trunk over the summer (for several reasons, delineated below), but I can't for the life of me figure out a good way to wind a long port around inside a 'glass box, and still be sure that the diameter of the port is constant all the way through...

ooh... a thought... since a port does its thing by restricting airflow (which is why a larger port must also be longer), why don't i use a small port inside, like 3" diameter, to keep the length way down, but then have a huge flare to the outside of the box, to keep port noise down... see, the only reason i need a 12" x 3" port is to keep the air in the port below approximately the speed of light (actually, you want it below like 5% of the speed of sound, but up to 8% is okay... with a smaller port than this ~6" diameter equivalent, the speed of the air rises above 10%, which will cause horrific port noise)... so i'm thinking, if i tune it to my 31.45 Hz with a much smaller port, like 3" or 4", i can keep the vent down to a sane length, but by flaring the outside like a tuba (and nearly to that scale, at least along the long axis), i can keep the noise down...

from the little bit of horn research i've done, i don't think that horn-loading effects will be apparent with this setup, which is a good thing...

finally, my reasons:
1) my audio has taken over my trunk, which i sorta need
2) i can't put my top down, the box is too big
3) i want better sound, and the current box is too small (much too small - the vented box i'm planning will get me anywhere from 1 to 9 more decibels, from 20-80 Hz, and at smaller excursions to boot!)
4) i like the look of a 'glass enclosure much better than mdf

ideas? comments? references? abuse?

thanks!

exalted512

my box will be mostly fiberglass with a mdf port...which is how most are. Or design your own port and as long as you know the volume of the port then youre good to go.

but what i would do is just build a mdf port, or get some pvc pipe and the little 90º pvc brackets with a big **** flange on the end
-Cody

Thom

Or use a flex port and mold the mouth of it to the box itself. Then the flex port can be trimmed to length and attached to the mouth. Plus it's easier since it can bend inside the box, a piece of PVC long enough to be a port can be a pain to fit in most boxes.

neomagus00

exalted512 wrote:

my box will be mostly fiberglass with a mdf port...which is how most are. Or design your own port and as long as you know the volume of the port then youre good to go.

but what i would do is just build a mdf port, or get some pvc pipe and the little 90º pvc brackets with a big **** flange on the end
-Cody

the problem with that is that any reasonable vent must be a slot port to get the required surface area, and therefore must be around 3 feet long... bending that much slot-port into a funkily-shaped 'glass enclosure could get pretty exciting... ooh... but i just came up with an idea that could do it...

the flex-port thing - i'll look into it, because it'd be nice to just lay a big coil of bendy stuff inside the box, and build a big flare onto the end of the tube... we shall see...

i'll put a couple drawings / pictures up here if i can make them pretty enough for public release...

neomagus00

So, this is my idea for bending a slot port into my trunk. Naturally, the drawing is wickedly off-scale, to the point that the dimension A is actually nearly two feet, whilst the other stuff that isn't A is like 12 inches. With this knowledge, the picture should become (more) clear.

The objective of this project is to get the sub out of my way, and still provide it with adequate air volume. I can accomplish both goals by glassing it to the bulge at the side of the rear of my trunk. This bulge is schematically represented here as a neat little box, but in reality it's a bendy, twisty mess of sheet metal covered by carpeting. In any case, the meat of the idea is conveyed in the drawing, just without fluff such as the existence of another layer above what's drawn, which when included only serves to confuse, and in real life only provides me with volume, and not anything fun or inspirational.

The plan of attack: I mount the sub offset from the wall by an appropriate amount to achieve the proper volume, then run a trio of, say, 1/2" MDF boards down the side of the trunk, where it's around three feet deep. These provide for a very long folded slot-port, as well as an easy mechanism for flaring the end of said port; simply sand the bejeesus out of the outside board, to round it into a perfect parabola, and fill in the final corner (with caulk, I'm guessing), marked with “much flare”, to curve the air rather than bending it. A light coating of acoustic foam inside the box to suppress resonance, and the project is complete. Addition of an amp rack of some description is likely, and should be trivial.

Problems:
There are many that I can foresee. The most troublesome of these is port noise. 1300 watts behind a sub with just under 3” of travel moves a hell of a lot of air, and I'm hoping that the flaring on the slot port will be able to cope – if not, I'll probably have to fabricate pieces of MDF that will smooth the transition even further, which will look strange.

On the fabrication side, there is the same problem of pressure, and how to contain it. I'm thinking an even dozen layers of thick fibreglass should suffice; more than that and fibreglass will have ceased to be a useful material, less and I think I risk blowing apart the box.

Another fabrication problem is how to keep the slots perfectly aligned. Normally, this would be trivial – simply sandwich the 3 MDF pieces between two more thin strips of MDF, and screw the whole shebang together. But no, this has to be difficult, and so my trunk has decided that it doesn't want to be level on the bottom. I'm thinking that I do the standard solution to keep the slots aligned, but then how do I keep the assembly attached? Do I rely on the fibreglass-to-MDF transition to keep the slot assembly cantilevered off the floor? Or do I drop some sort of malleable material from the bottom of the slot assembly to the trunk floor, and let it form itself into place?

Of course, the position of the box blocks some fairly important pieces that need to be accessible to service the car, and so it must be removable. This has me fairly stumped. It needs to be held with a force on the order of those found inside black holes – keep in mind that the whole thing will weigh as much as your typical third-grader. I thought about attaching carriage bolts to the car, then passing them through the box, and so attaching the thing means removing the sub, bolting down two or three bolts, then re-inserting the sub. This wouldn't bother me – it won't be a common operation – except for two things. One, air leakage around the bolts would be a big concern. Two, actually attaching the carriage bolts to the car – lacking a way to get at the backside of the bolt – would be an entertainingly difficult challenge. Any suggestions? I'm trying to avoid external brackets, because they are ugly, but I will use them if I must.

I've reached a one-page essay, so I think I'll stop there before I scare everyone away for good.

PoweredByDodge

... this is why i don't port things -- too complicated.

i like thom's idea though.

as long as the tube you use is rigid enough to maintain its shape during woofer motion and under cabinet pressure, yet still be flexy enough to wind inside the box, go for it.

neomagus00

eew, don't be so boring... i have three months to do this, and i intend to make it complicated enough that i'll NEED all three months to do it!

I actually think i'll run with the slot-port idea for a while, make some accurate measurements of my trunk, see where it goes... i've got some time before i can begin...