SE-2 construction question - pipe resonance in woofer holes

rmr48 · « **on:** 20 Jul 2003, 04:54 pm »

I'm making my SE-2s with a thicker baffle than called for -- in part because I have some leftover material, in part to make a laminated baffle of both mdf and baltic birch for added strength. This results in a woofer hole that is 5.75" dia by 1.625" deep. Not knowing the pipe resonance formula, I can't determine whether I must flare the interior of the holes or if I do, how deep to make the flair. I've been told that the issue is pipe resonance, and the holes must be flared so as to be no deeper than the xover point for the woofer (which point I also don't know.)

So the magic question is, how much should I flare (using a chamfer bit in my router) the inside of these holes?

Anybody know the answer or can point me to the formula and xover point for these woofers? Thanks.

rwalton · « **Reply #1 on:** 20 Jul 2003, 06:44 pm »

Hello,

I have built the SE-2s, so I know exactly what you are trying to decide. I went through the same sort of mental gymnastics myself. In the end, the best answer to me was to remove as much material as possible with the flare while leaving behind just enough material to feel good that the driver mounting screws have plenty of wood to bite into. This is a judgement call. I don't recall exactly what I used, but my cabinet baffle thickness wasn't as thick as yours either.

This will take some trial and error. Just start with a small cut and slowly increase the router cutter depth/radius. I could be wrong, but I doubt that anyone will be able to answer your question directly. While you may hear a difference between no flare at all, and the largest possible flare, I doubt you'd hear a difference between a "middle of the road" flare and the largest possible one. I know that doesn't answer your questions directly, but maybe it helps a bit.

By the way, I don't think a chamfer bit is recommended. I belive a roundover bit is preferred, but I'll let the cabinet building gurus chime in on that. Good luck!

Rick

rmr48 · « **Reply #2 on:** 20 Jul 2003, 08:00 pm »

>>In the end, the best answer to me was to remove as much material as possible with the flare while leaving behind just enough material to feel good that the driver mounting screws have plenty of wood to bite into. >>

Yeah, what I expect to do, but I guess I want to do as little as actually necessary. The chamfer bit I got from the DIY 2000 design that Phil Bamberg put out a few years ago -- his box called for 45 degree chamfer -- plus he did the flaring in scallops with small spaces of no flare, I assume, to keep full depth for screws, though this is not explained in the plans. Feels like a lot of fiddling to me, who is need of tons of help in the shop, though I am willing to do it if needed.

I made the BB/MDF sandwich as described in the North Creek Box book, in part to give the baffle better material than pure mdf for screws to bite into, and here I am removing as much of the BB backing as possible -- this is what happens when you start mix and matching design elements from various sources.

Thanks for the reassuring words. I assume you are happy with your SE-@2s?

Shamrock Audio · « **Reply #3 on:** 21 Jul 2003, 05:08 pm »

The short answer to your question is, what wavelengths are about 1.6" or shorter? They may be affected by the thicker baffle, whereas longer wavelengths may not.

1130fps/8500Hz=.13ft. .13ft*12"=1.6" Therefore, the frequencies above about 8500Hz are likely to be affected by the baffle thickness, where those below are not.

There are chamfer bits available where the guide bearing is on top of the cutter. This allows you to set the depth of the chamfer you cut from the back side of the hole easily.

rwalton · « **Reply #4 on:** 22 Jul 2003, 04:42 pm »

Quote

I assume you are happy with your SE-@2s?

Yes, quite happy!

rmr48 · « **Reply #5 on:** 22 Jul 2003, 05:18 pm »

Hey Mike -- nice to hear from you.

[Therefore, the frequencies above about 8500Hz are likely to be affected by the baffle thickness, where those below are not.]

So knowing this, how do I determine the appropriate depth of the chamfer? In the end, I'm going to want to leave, what, minimum .75" for driver screws to bite into (unless I chamfer in scallops))? I'm just wondering if I can get away with less. Or I guess I could epoxy bolts into the baffle front, using as little depth as poss. and seat the drivers with some kind of nut.

[There are chamfer bits available where the guide bearing is on top of the cutter. This allows you to set the depth of the chamfer you cut from the back side of the hole easily.]

I already bought one with the bearing is at the narrowest part of the chamfer. I also figure I'm going to have to do this in several passes, because I don't want to be routering off huge hunks of bb/mdf at a time. I'm not sure how this will work -- I envision a kind of ziggurat shape when I'm done, as the bit will be working down in steps. Even if I work my bit as closely as possible, I still will have some sanding to do to remove the ridges. That'll teach me to redo the original design.

Shamrock Audio · « **Reply #6 on:** 23 Jul 2003, 05:33 pm »

[So knowing this, how do I determine the appropriate depth of the chamfer? In the end, I'm going to want to leave, what, minimum .75" for driver screws to bite into (unless I chamfer in scallops))? I'm just wondering if I can get away with less. Or I guess I could epoxy bolts into the baffle front, using as little depth as poss. and seat the drivers with some kind of nut.]

Well, assuming that your bit is about 1" tall you should use that as your depth. Just make a couple of passes to reach the depth of the bit.

[I already bought one with the bearing is at the narrowest part of the chamfer.] Then the bearing should be in the middle of the shaft, not at the end...right?

[That'll teach me to redo the original design.]