Corner bass resonators/absorbers - construction question

laptoppop · Post by **laptoppop** » Tue Jul 29, 2003 2:57 am

On the main page of John's site is the link to wall units. In that helpful set of pages, there are drawings of triangular corner mount bass traps that show rigid fiberglass inside at an angle to the box.

How the HECK am I supposed to mount the fiberglass in the box?

Thanks,
-lee-

knightfly · Post by **knightfly** » Tue Jul 29, 2003 3:38 am

Lee, the only corner unit I see on the Wall Units page is the slot resonator at the top of the page. It shows beveled cleats, to which the slats would be glued/screwed.

Since this isn't a panel resonator, there is no need for total hermetic seal - if you wanted to make a broadband absorber by replacing the slats with, say, 4" 705 FRK, I would make the box the same way as in the pic - then, instead of glue/screwing the slats to those cleats, I would just wrap the 705 with the cloth of choice, gluing the wrap to the FRK at the rear perimeter of the fiberglas, letting it dry, then using construction adhesive beads on the beveled cleats and gluing the wrapped FRK to the cleats. You can leave the other insulation inside, it will help absorption also.

Since most of these materials are only available in 2 x 4 foot sheets, you could improve rigidity and visual appeal by separating each 2 x 4 foot horizontal piece of fiberglas with horizontal wood dividers (placed at 2,4,6 feet) across the 45 degree front. Those could just be glued/screwed to the beveled cleats shown in the drawing.

Hope that helps... Steve

laptoppop · Post by **laptoppop** » Tue Jul 29, 2003 9:07 am

Thanks for the info!

I'm planning on building them as slot resonators as shown. If I have cloth across the front behind the slats, do I need to wrap the internal fiberglass in cloth too?

Can I just use a bead of adhesive directly to the fiberglass without a cloth wrapping, or would the cloth help give a better surface for the adhesive to adhere to?

Thanks again,
-lee-

knightfly · Post by **knightfly** » Tue Jul 29, 2003 10:09 am

Lee, the cloth is just there for looks and fiber control, except in the case of slot resonators where it's part of the acoustic path. As long as the stuff is behind the cloth, it wouldn't need to be wrapped and would most likely glue easier without the cloth... Steve

BTW, don't expect to get as wide a bandwidth out of a corner resonator as it would seem - Barefoot set us straight on that one a while back. Seems that the long wavelengths associated with bass frequencies, unless the resonator is 15-20 feet long, mean that the trap will "average out" pressures. Only way around that would be to put the slats VERTICAL, and build individual "septums" between each one so you had an array of separate, different depth Helmholz resonators...

zikaj · Post by **zikaj** » Wed Jul 30, 2003 6:22 am

should there be fiberglass on the INSIDE walls of the corner trap?

ie:
705FRK across the front
2 sides of the triangle covered with fiberglass?

JZ

knightfly · Post by **knightfly** » Wed Jul 30, 2003 8:04 am

"should there be fiberglass on the INSIDE walls of the corner trap?" -

I've not tried this both ways, but my gut feel (been wrong as much as right) is that it would un-balance the absorption. This isn't necessarily bad, depending on your needs.

My explanation - The fiberglas at the front of the trap will absorp very uniformly, lows because it's away from the wall, highs because they're there. The fiberglas at the REAR of the trap will only absorb highs/mids, because it's against the rear boundary so quarter wavelengths are short. This would cause the total effect of the trap to absorb highs most, then mids, and bass least.

That could work if you have too bright a room, but otherwise I'd probably not try it... Steve

laptoppop · Post by **laptoppop** » Thu Jul 31, 2003 3:00 am

Regarding Barefoot's analysis.... I'm a bit of a heretic. I pretty much understand his reasoning, but I believe the real-world slot absorbers will have a significantly wider "Q" than what I understand his analysis to say. I would LOVE to get some of these boxes into a real lab - but who can afford that for a little bedroom studio?

For example, I believe that the splayed slots that John used in Left Bank gave a wider smoother bandwidth than a set of slots all at the average width. I also believe that the angles from 2" deep to 8" deep in John's wall panels give a wider band of absorbtion.

I have no proof at all - this is just instinct. I could easily be totally offbase.

For my little bedroom studio, I'm going with a bunch of different slot resonators. I believe they will give me a better sound than panel absorbers.

-lee-