TL taper in PVC

[Ang]

The bass portion of my 3-way is going to be in a 8" pvc pipe bent in a U. To taper it (in hope of getting flatter response. I was thinking of sawing off an oblique section of the pipe and gluing on a piece of mdf. Or… inserting some kind of conical object (this sounds bad) that would take up enough volume to create the taper.

Con's for sawing the pipe is that it will probably decrease the overall rigidity even if I attach a thick piece of mdf.

Con for the taper is that I'm not sure what to use... really dense foam perhaps?

Input appreciated.

[azira]

Sawing PVC probably won't come out like you expect. The material is under tension so as you make cuts, it'll tend to want to expand and you'll end up with a bigger diameter than you expected.

I'd vote for inserting a conical section into the pipe. Or, supporting your pipe at the appropriate angle and using some kind of self-leveling fill material that cures (like poly urethane or epoxy resin). Although with an 8" pipe, that might not be as practical.

[Ang]

thanks azira - didn't realize it was under tension

[planet10]

Originally posted by Ang
thanks azira - didn't realize it was under tension

Rob Sampson just screwed a wedge of wood (he used 2x4 as base material) to get a taper in his... http://www.t-linespeakers.org/projec...son/index.html ... i thot that was a good approach.



Just make sure your pipe is big enuff for the driver. (ie a 4" driver probably needs 6" pipe to get to its best)

dave

[Ang]

Thanks Dave,

Since the pipe length necessitates the u-bend, I would only be able to taper the final 2/3 of the way along the tube. Would this deviate from complete taper simulation results significantly? This is a 7" Usher in a 8" tube.

[Audio-Lego]

Ang:
I've been pondering on this problem for years now. The best solution so far is to segment the tapered insert into many short pieces 2cm in length. The insert need only be segmented through the curved sections of pipe and the straight sections can be separately fixed into the straight sections of pipe. I have found the easiest way to organize the segmented pieces is to staple and glue them individually to a nylon fabric tape. The tape serves as a flexible spine which curves around the inside curve of a U section nicely. The tape and insert segments can be affixed to the inside of the pipe with a gap-filling adhesive such as silicone seal.
The downside is that this is not a quick process. It is a major fiddle. The segmented sections where they flare apart rounding the curves probably adversely effect smoothness of the cross sectional area of the T-line.
I ultimately gave up on this idea as my premise for using pipe in the first place was to keep the build simple.
Rob Sampson

[Ang]

Hi Rob - that looks like a nice design there.... this is a bit of a challenge.

One simple thing I overlooked was I assumed I'd have a linear taper just inserting a straight board/wedge, but the constriction will actually be nonlinear since I'm transecting a cylinder.

[johninCR]

I've ripped PVC lengthwise before with a table saw and found it to be dangerous because the pipe I had pinched closed. The secret is to wedge something inside to prevent pinching. If I did it again, I'd just use an open ended hacksaw and do it by hand, since it's not difficult to cut.

If the pipe you get pinches closed when cut, then you may be able to just cut out a triangle to achieve your desired taper very cleanly, like a long funnel.

Filling 8" pipe to create your taper sounds like it will take a lot of material, but concrete might be the right thing to use for an economical solution. I'd paint the PVC with a slow curing epoxy resin designed for bonding new concrete to other materials to ensure a good bond with the pipe. Cheap and easy, like Audio-Lego prefers.

[Ang]

Does the shape of the TL chamber cross section matter?

I'm still fiddling around with this and stuffing various wedge-shaped things around the bend. If the absolute cross-sectional area is more important than the shape that makes this easier.

[OzMikeH]

To get the concrete to stay in place in the pipe: coarse thread self drilling screws. drill them in and pour the fller in around them. (you might end up with some rattles but it is a lot cheaper than the epoxy)

I'd add some fibrous material to help hold the filler together too.

Good point about the taper in a cylinder, to get a constantly reducing cross section you will need to make the surface of the concrete an S shape, not flat.

then again the variable rate cross section reduction of a flat surface in a cylinder might be a good thing. Ask a Voight pipe boffin.

Make it so it comes apart just in case any rattles develop.

Another idea if stepped edges dont matter: a bunch of broom handles glued together of various lengths.