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Old 2nd February 2003, 09:21 PM   #1
kneadle is offline kneadle  United States
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Default TL Design I can't build

Hello,

I designed an experimental TL enclosure, but I can't physically make it. It's a 6-inch PVC pipe bent into a uniform 540 degree spiral over the course of 54-inches (see plan attached below), so that it looks like a spring, not a nautilus. The problem is very simple: I can't figure out a way to bend six inch PVC. Nothing I've tried (and I almost died trying) does the job. It doesn't behave like I anticipated.

Here are the two theories behind the design. One is practical, the other is, well, not.

1) TL enclosures are huge when it comes to size/performance. I figure by spiraling it the volume is the same, but the space it takes up is more manageable. I was hoping, eventually, to build a box around a reasonably-sized curly-Q of PVC pipe.

2) TL enclosure-building is a battle against unwanted resonances.

Part A) In spiral theory, when you send a substance down a spiral, gravity causes the g-force to increase, and components of the substance separate. The same is theoretically true concerning waves, except gravity doesn't cause the centrifugal force, the driver does. I saw another horn-loaded enclosure that had a spiral ( http://www3.ocn.ne.jp/~hanbei/eng-inside.html ), but I have no idea what the theory is behind it. I'm thinking it's horse manure. Not that mine isn't, but still...

I think that the resonances will be more manageable since the frequencies will have separated somehow. I'll try to explain if you'd like me to make a fool of myself further.

Part B) I still think any "uniform" enclosure has the problem of standing waves, even a perfectly cylindrical enclosure. I like the idea of a tetrahedron, but that design is a nightmare once you try to measure the volume of a non-equilateral tetrahedron. It, too, is huge, in the long run. I believe that a spiraling tube creates a perpetual oblong, egg-shaped enclosure. If you look inside a tube that spirals down from the horizontal, you'll see what I'm theorizing. What is that shape at the "back" of the pipe on the plane you're standing on?

At any rate, I don't have the equipment to bend PVC pipe of this diameter (I wasn't even using the stronger schedule 40). I'll be building a variation that doesn't involve bending, but I was wondering if any kind and able soul wanted to test the design theory.

The diagram below is eventually based on a 1:1 Sd pipe for simplicity's sake. It is also two-dimensional, so I didn't bother trying to draw a real spiral. You'll have to bend it in your brain. The calculations are not ideal because a 6.5" woofer has its bass swallowed by the narrow volume at its throat. BTW, a 6.5" woofer fits PERFECTLY onto a standard PVC tee or 90. You can also surmise that this is easy to experiment with. It takes about 5 minutes to saw sewer-grade PVC with a hacksaw, so you can literally place the Tee anywhere you want with only a little work.

Thanks,

Dave

PS--anyone know where I can get 6" 90s for under $10/per in the US?
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Old 2nd February 2003, 09:23 PM   #2
kneadle is offline kneadle  United States
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here's that image.
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File Type: jpg 6to4b.jpg (34.9 KB, 4570 views)
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Old 2nd February 2003, 10:27 PM   #3
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What an imagination you have.

My quick answer to your plan is to exchange the PVC for 6" stovepipe (and/or elbows), some zip screws and a couple bags of concrete. Build the box first, and pour it up as you assemble the pipe. (zip screws are self-tapping sheet metal screws)

I don't think a 6" corrogated drainage pipe with a 10# crush rating could be bent this tight.
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Old 3rd February 2003, 01:31 AM   #4
kneadle is offline kneadle  United States
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wait...what?

Let me see if I understand:

Build my thing out of stovepipe, build a box around it all, then pour concrete in the box, using the stove pipe as a sort of mold? Is that right?

Dave
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Old 3rd February 2003, 01:34 AM   #5
JoeBob is offline JoeBob  Canada
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That would be rather interesting. although would still be large, larger than if you made a "regular" bended TL out of wood.
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Old 3rd February 2003, 02:02 AM   #6
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Try using 10 elbows?

Tim
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Old 3rd February 2003, 02:04 AM   #7
kneadle is offline kneadle  United States
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that's why I'm asking.

Although, if I were to pour a concrete box like that, I could chip away at it till it was a pretty sculpture of some sort. Nah...now that's stupid. I'll just stick to spiraling PVC.
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Old 3rd February 2003, 02:07 AM   #8
kneadle is offline kneadle  United States
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Quote:
Originally posted by Sch3mat1c
Try using 10 elbows?

Tim

That's how I originally designed it. Have you priced 6" PVC elbows lately? If I could find a bunch of 90s real cheap, I could build it. But I haven't found a plumbing supply store that sells these things at less that $10 USD a pop. That's too pricey.

Dave
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Old 3rd February 2003, 02:42 AM   #9
Brendon is offline Brendon  Canada
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Default My two cents

My feeling is to do away with the PVC and use wood of whatever type you like. If you want to prototype a concept quick and dirty, there's nothing better than sonotube and some industrial adhesive (say PL400).

If you want to reduce the line volume because TL's are big, start the line at 4 * Sd of the driver and taper it to around 0.5 to 0.75 of the driver Sd at the end. This will reduce the line length about 35 to 40%. You box will probably turn out to be a nice 'U' shape which is easy to implement.

Use Martin King's Mathcad sheets to dig deeper into all the influencing parameters.

Good luck!


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Old 3rd February 2003, 03:09 AM   #10
kneadle is offline kneadle  United States
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Brendon's suggestion set me to thinking. Can't I cut a pipe at various angles so that I can make my pipe gently angle? Am I really delusional now?

Can someone help me figure this out? I have wads of 6" PVC pipe that's just begging to be made into a speaker enclosure.

Dave
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