Triangular U Baffle

[johninCR]

Would there be any benefit to making a U Baffle woofer 3 sided instead of having 4 sides? I'm thinking that it would eliminate the need for damping material in the cavity, or are the resonances related to the length, and not the width and height?

[rick57]

If you don’t get help here, (register) and try at dipoles@topica.com

Warning, the forum culture there is not kind to the non-experts; but what have you got to lose?

Cheers

[johninCR]

I'm already on that listserve, but they seem to be die hard flat baffle or pure dipole guys although we've had some OB "wings" discussions often leaving things somewhat unresolved. I've learned a lot and validated some of my ideas. OB/dipole is still out on the fringes. Even though the science is probably understood, I think there is a lot of unexplored territory in design.

[Nuuk]

John, if you have access to scrap chipboard or similar, it seems the easiest way to answer that question would be to try it! I would guess that you would still need some stuffing!

[Rudolf]

Originally posted by johninCR
are the resonances related to the length, and not the width and height?

The resonance that has to be eliminated in the first place is the lambda/4 resonance of the depth/lenght of the U-baffle. There are (lesser) lambda/2 resonances of the width and height, but these should be well outside of the passband.
That´s what I have learned by reading Linkwitz and Kreskovskys homepages.

[johninCR]

Actually they will be in the passband because the plan is open back cabs for my 15" Selenium coaxials. I guess I'll make the sides of slightly unequal depth and open them up some instead of perpendicular to the front baffle in an attempt to eliminate the 1/4 wave resonance. That approach works with folded back wings, but in this case I'm going to end up with some really funky angles.

[GM]

........but in this case I'm going to end up with some really funky angles.

Greets!

This is no different than dealing with a room's 'slap echo', so whatever included angle = 1" of expanding offset for every 6" of 'wing' length measured perpendicular to the baffle will solve your problem without the need for stuffing or different length 'wings', though for sure you want them well damped. Double thickness MDF, sand filled core, or similar.

If the fourth side is enclosed then the same min. included angle should be used and a diffusor placed at the terminus to damp the standing waves. Foam folded/glued into a 90 deg 'V' 'plug' works well. Unfortunately, the math to calc the requisite density is literally 'Greek' to me, so I had to stack layers until I got the desired attenuation, though MJK's WS should be able to get you in the ballpark. The time I could have saved if I'd had such a powerful tool back when I was actively building..........

GM

[johninCR]

GM,

Is that a 1/6 ratio only or is greater OK? Also I remember reading that there were certain angles to avoid but the author did not elaborate on what those angles are.

Regarding damping: I was planning on using some 1" hardwood I have and putting a cross brace to the center of each of the 3 panels near the exit. Should I add another layer of something to the interior? If so, is plywood OK?

[GM]

Greets!

It's the minimum. If there's angles to avoid, I've never read anything about them, though in thinking about it....... if the 'horn' was excited in the BW where the included wall angle began controlling directivity then standing waves will develop across the terminus, so at this point the 'wings' would need to be flared out at a minimum of twice whatever the angle is, same as a CD lens. That, or provide enough absorption around the rim of the terminus to damp them.

Unless you have very long, widely spaced 'wings' though, this won't be an issue since the 'horn' won't be long enough to support the terminus' DI frequency, so if he was talking specifically about OB 'wings' I can't think of any problematic angles, or why they would be, other than those < 1/6.

You want good damping on the 'wings' (not baffle!) so there will be a rapid decaying of the reflections, ergo plywood is going in the wrong direction. Mass is where it's at, so glue something really lossy like super cheap/crumbly particleboard or that pressed cardboard insulation paneling (can't think of the trade name at the moment, my 'camera' ran out of 'film' over a decade ago) to the inside areas to offset the glue's and plywood's rigidity.

GM

[johninCR]

GM,

I don't understand why damping these panels is more important than with a box speaker. There's much less pressure and less reflections. Is it because the terminating edge doesn't have the structural support of a corner like a box has?

Since the driver has an Fs of 50hz and I don't want to apply much power below that anyway, I'm only going to go with 12-15" of depth. That will give me an effective baffle width equal to a 50-60" wide flat baffle, so I should get solid bass down to 50hz with a shallow cavity.