Need help designing a round enclosure

[Subless]

Now getting to more finalized volume calculations. I'll go ahead and post this stuff

in case someone who's done a sonotube style sub sees anything funky about my calculations.

Here are the specs for what's required for this particular sub woofer for a sealed enclosure.

Net volume required after the volume of everything else in the box including the driver

are taken into consideration is .35 cu feet. JL recommended for my Jeep to bump that up by 10%

if possible. So, I will shoot for .385 cu feet or 665 cubic inches of free air space after it's all said and

done but realizing if I get into some trouble with space, I could go as low as .35 cu feet which is 605 cu inches

of free airspace, so we'll say between 605 to optimal 665 cubic inches.

TARGET FREE SPACE = 605-665 cu in

This drum shell I'm looking at is 13 7/8" in diameter.

Wall thickness for an 8-ply shell is .21" or about 7/16" for both walls

So the inner diameter will be 13 7/8" - 7/16" = 13 7/16" diameter

The radius of that is approx 6.7"

To get our cu in multiplier

pi x r x r or

3.14 x 6.7 x 6.7 = abt 142 cu inches per each inch length of tube.

So, starting with a tube length of 7", I get

7 x 142 = 987 cubic inches of air space before adding on the ends

or inserting the driver.

Subtract the displacement of the driver which is .047 cu feet or 81 cubic inches

987 - 81 = 905 cu inches of free space remaining

If the two ends are 3/4" deep, then the amount of cubic inches they will take up

will be 3/4+3/4=1.5" x our multiplier above which is 141 cu inches per each inch of tube length

1.5 x 141 = 212 cu inches will be used up by the bottom solid end and the top (sort of).

The top is a combination of about 2 5/8" of 3/4" thick wood and the top part of the driver

whose space is already subtracted. But I'll just consider the top to be a solid piece of wood

keeping in mind I have some fluff I can use if necessary.

So, the total remaining free space is:

905 - 212 = 693 cu inches of free air space. The minimum required space is 605 ci to optimum which is 665 ci

So I have between 88 ci and 28 ci remaining for extra support, wiring connectors, bolds for mounting, dynamat, etc.

Seems a 7" deep drum shell should suffice which is really good news considering my worse case scenario

photos above showing a 9" deep model which takes up more space than I care for. 2" less will be fantastic.

Any flaws in my logic ?

 

[Subless]

... removed duplicate entry

 

[blazian87 10+ year member]

@Subless; I see where you're getting at now. How do you know the enclosure won't move around from that spot while you're driving? Is it snug enough to not move out of place? I would take JL's specs with a grain of salt. Having a box too big is not as bad of a problem as the box being to small. Atleast that way you can add layers for bracing to get to your target net space. Don't get caught up with all the numbers. Sealed boxes are much more forgiving so as long as you're in the ballpark, it'll sound good.

 

[Subless]

[quote name='blazian87']@Subless; I see where you're getting at now. How do you know the enclosure won't move around from that spot while you're driving? Is it snug enough to not move out of place? I would take JL's specs with a grain of salt. Having a box too big is not as bad of a problem as the box being to small. Atleast that way you can add layers for bracing to get to your target net space. Don't get caught up with all the numbers. Sealed boxes are much more forgiving so as long as you're in the ballpark, it'll sound good.[/QUOTE]

The enclosure will ultimately be bolted into place. I had read somewhere that loose subs are heavy projectiles in car accidents. If I do this project, I'll listen to the sub first before drilling any holes. Here is JL's Jeep sub - JL Audio Stealthbox® (Black, Driver's Side) Custom-fit fiberglass enclosure with one 10TW1v3-4 subwoofer — fits 2007-up Jeep Wrangler Unlimited at Crutchfield.com
using the same driver that I'm looking at. I might still just buy this one, but $600 x 2 seems like more than I want to pay. I'll have to add up what it's going to cost for me to build and decide from there. At least their product is tested and has very good reviews. I may be taking a bit of a risk for just saving two or three hundred bucks per sub. The drum shell will cost more than the Sonotube, but it is made out of layers of maple which I understand is a good choice for a sub box material. I guess at the moment I'm still deciding on how the ends will be designed and whether the drum manufacturer will do the ends or if I will. The guy I'm working with has not answered whether he can do the ends or not. I got to thinking that what might work is a 1 1/4" or 1 1/2" thick piece of mdf with a circular slot routed about 3/4" deep so that the edge of the drum shell can just slip in that slot and glued into place. I don't know if that is a difficult piece of woodworking or not. I certainly don't have the tools for it, so that cost will come into play and that's true even if I do the typical end piece that is flush inside the tube. I'll have to pay someone to cut it. I've only found a web site that sells whole sheets of this stuff and so the cost and shipping might be prohibitive if I can't find it locally. If not, then I can do what most other folks do and glue and screw two 3/4" pieces together. I just thought it would be a nice clean design to go with one thick piece with the slot.
And yes the size is really an important question. You can see from JL's product that the internal space is not very much. My model at 9" is really bigger than I want. If a 7" one could sound like JL's, I'd be in heaven. I had not thought that I would need to build in bracing other than the thick ends since this is not a very long box. I know from being a drummer that drum shells are very rigid. One thing I haven't decided on is whether to go with 10 layers instead of 8. I'm thinking I might just for peace of mind. The manufacturer seemed to feel that 8 would be sufficient though. Any idea whether those sonotubes flex unless they're reinforced with fiberglass or other materials?
Editing this - Your question about size did bring up a good question. I know the specs for the driver, but I need to find out what the net air space is for the JL Stealthbox noted above. I've sent a request to JL asking. I'm wondering if they will give me that info.