Should I proceed? Math versus intuition....

[laptoppop]

Wow, Steve - thanks for the analysis! I think I actually understand it -- amazing!

Using 1 1/2 inch thick wood seems big and heavy - I think I'll play around with the spreadsheet and see if I can work out thinner wood -- but thanks to you, I really think I know how to proceed.

I'm probably going to go ahead and splay the slats like in Left Bank - I'll use the "average" slot width for the calculations. Maybe I'm fooling myself, but I think it will broaden the Q of the frequencies just a bit. If I have a splayed slot going from zero on one side to 1/4 inch on the other, I'll use 1/8 inch in the calculations.

I'll work out some details and post them here.

Thanks again - this was VERY VERY helpful.

-lee-

[knightfly]

Lee, you can get 170 hZ with an average depth of 5-1/8" and 3/4" boards, just use 1x6's spaced 1/16", and you're there.

I would be MUCH more concerned that you slant the fronts of your boxes toward the rear of the room than worry about varying slot widths - if you believe in varying slot widths, you should also believe in varying cavity depth - and the part that REALLY will work is the angled fronts. They will, if nothing else, help your flutter by non-parallelling the walls.

Besides, if you're going to use 1x lumber for the slats the slots will need to be small or you'll take up a lot of depth in order to get the lower frequencies. The dimensions I gave above would work for all three trap frequencies, just change the slot width to a wider value for higher frequencies. Changing slot width to .106" would give 220 hZ, and .200" would give 300 hZ. With an average depth of only 5.125", a 3" slant in 24" would go from 3.625" to 6.625", which won't eat up a lot of room width (important in a 10' wide room)

1/16" is about minimum for practical slot widths, you can get that by using washers for spacers. (You may have to tweak the cavity depth slightly based on what thickness of washers you find)

Those are my thoughts, now I'll shut up and let you do what you were going to do in the first place... Steve

[laptoppop]

"Those are my thoughts, now I'll shut up and let you do what you were going to do in the first place..."

Hey Steve, Please don't misunderstand...I'm a onry cuss sometimes, but I can be taught.

One thing I haven't made clear in all this is that I absolutely, positively WILL angle the front of the slots. Its one of the big complaints I have against just using panel traps - I need to compensate for my very rigid parallel walls.

I am still wondering about splaying the slats TOO -- but you're right - if the depth is changing/averaging out, the added effect of the splaying is probably minimal. Besides, straight across slots are easier to make! The thought of using washers to help space the slots rocks.

I keep thinking that the wood itself is resonating, but I guess its not not the wood, just the air in the cavity. If it were the wood, then 1" wood would excite more easily than 2" wood, but since its the air, I just need to make the right cavity, and 2" (OK 1 1/2") wood is fine. I'll get it through my thick head sometime. My wimpy nail gun can drive through a "1 by" board, but not a "2 by" board, so I may still use "1 by" boards just to make it more buildable for me. On the other hand, "2 by" boards are actually cheaper than "1 by" pine....hmmmmmmm.

I wonder how much the "ceiling nodes" are really contributing anymore. I've got 19 panel hangers up there, from 4'x8' sheets to 4" x 8' sheet. 6" below that, I've got a drop ceiling across the entire ceiling using 4" thick 703. Its as close as I can come to a black hole for sound.

Thanks again for your help - I really appreciate it.

-lee-

[laptoppop]

OK, with many thanks to Steve, John, etc., here's what I think I'll do. I'm probably buying the lumber and starting big time tomorrow, so this is the last chance to set me straight!

I measured the room as accurately as possible, ran it through Ethan's mode calculator and got numbers very similar to Steve's. I love it when things agree!

I'm going to use 4 24" wide resonators on each side - with slanted slots varying in depth from 3 5/8" to 6 5/8" -- average depth 5 1/8".

Here's the details for the four:
1) 1x6 slats, 1/16" slots - center frequency 169 Hz
2) 1x6 slats, 1/8" slots - center frequency 234 Hz
3) 1x6 slats, 3/16" slots - center frequency 291 Hz
4) 1x3 slats, 3/16" slots - center frequency 346 Hz

I'll be using fender washers of various widths to help space the boards as I nail them. Of course, I'm a lousy woodworker, so I'll be lucky if the resonators are reasonably close to the desired frequencies.

I still wonder about splaying some of the slats, but I'm not going to do that this round. The varying depth on the slats gives me about 7 degrees on each wall - 14 degrees total, more than the 12 degrees I've heard recommended as a minimum.

I haven't worked out the math for the corner bass absorbers yet. It seems like I'll be able to get the 110 Hz without a big problem, but 55 Hz is looking harder. I may check out panel traps centered at 55 Hz. Of course, it would be WAY cool if I built those with Barefoot's shaping/ideas. Hmmmmmmm. Still more to figure out.

In the meantime, thanks again!!!
-lee-

[knightfly]

Lee, whether you use 1" or 2" nominal lumber, I'd use screws instead of nails to fasten the slats to the frame. That way, you won't get any chance of fasteners "popping" if the wood tries to twist as it changes moisture content. If you're concerned with looks of open screws, you could countersink the screws by about 40% of the 3/4" wood and insert dowel plugs, sand smooth, and stain/varnish to taste...

If you hold each slat firmly against the shim washers and use one of the adjustable depth screw pilot bits available to drill the holes, then drive the screws (deck screws will work good) - after the screws are tight, you can pull the washers and not worry about something like a slightly loose nail allowing things to slip.

This wouldn't be much of a concern with wider gaps, but the narrower the gap the more percentage of change will be caused by a slight incremental change.

All your other comments seem fine, start makin' sawdust... Steve

[laptoppop]

Thanks again for the help! I'm kind of dreading drilling so many pilot holes, but I'll take your advice and do it the "right" way.

I guess I'll use two screws on each side of the 2 foot long 1x6 slat. Please let me know if that isn't enough. Silence implies agreement...

Thanks,
-lee-

[John Sayers]

Lee - this is how we did the slots at Left Bank.

I've angled it 7 degrees like yours.

cheers
john

[laptoppop]

Thanks, John! That kind of detail really helps.

I was thinking last night about the screws -- when I try to drill pilot holes and then screw in screws, the fabric would get all wrapped up around the drill bit and the screw. I think I'll stick with nails and rebuild/adjust as needed over time.

-lee-

[knightfly]

Lee, I generally go with three fasteners in a 6" board, one in the center and one each side- if you're using nails, I'd put the first one in the side closest to the previously fastened one, with your spacers in place, and I'd slant the nail (toenail it) slightly toward the spacer. This will make sure that the slot is the width you intended. Then do the middle nail straight in, and finally the last nail straight or slightly toenailed the opposite of the first nail. Do both ends of the board at the same time; in other words, do both ends nails closest to the previous board, then both middles, then both outer sides. This sequence should keep the slats from "walking" away from the gap you've set.

Another old carpenter trick - if you're using nails near the end of wood, a lot of times the wood will split. If you take each nail and rest its HEAD on a solid piece of steel, then lightly strike the nail on its POINT with a hammer (not hard enough to bend the nail, though) you will "blunt" the point of the nail slightly.

Now, when you drive the nail, the blunt point will "punch" out a small part of the wood instead of just trying to "spread" the grain - you have just as much holding power, but virtually NO SPLITTING of the wood. This trick works best if you orient the widest part of the "blunt" CROSSWISE of the grain.

This trick is faster than trying to drill pilot holes for the nails that are just slightly smaller than the nails themselves, and just as effective at stopping splits.

If you're using processed wood such as MDF, then there is NO grain - if you experience "breaking out", try to put the nails further from edges and/or drill pilot holes that are slightly smaller than the nails, so they'll hold but not put so much splitting pressure on the "wood"... Steve

[laptoppop]

Wow Steve, once again great detail! I'll be nailing them in just as you've outlined.

Thanks!
-lee-

[audio hobbit]

another carpentry tip.. if you use "twisty" nails ( not sure the proper name, but they are slightly spiral) that will also help to prevent the nail "creeping out". even better would be to rent an air nail nailer and use 2" finish nails with the preapplied glue. the glue heats up as the nail enters the wood and these suckers do not slip... just be sure to get the seals tight in the first place.

[barefoot]

I've been procrastinating in my response to this thread because it raises a lot of questions that could conceivably require a book to fully answer.

I still stand by my argument that alternating slat widths and varying the cavity depth will not "locally" widen the bandwith of the resonator. I know this basic principle very well in practice because the situation is directly analogous to a ported loudspeaker cabinet. A slot resonator and a ported speaker cabinet are both simply Helmholtz resonators. The resonance frequency and Q depend on the cavity volume, the port length, the total port area, and the damping. The resonance frequency and Q are independent of the cavity shape, the number of individual ports that make up the total port area, or the shape of the ports (unless the lateral port dimensions get so small that the airflow resistance significantly increases).

Remember a basic slot resonator follows all of these principles. The only reason you don't find volume and slot (port) area in the design equations is this:

The resonance frequency of a Helmholtz resonator is given by

F0 = c*sqrt[S/(V*L)]/(2*Pi)

Where

c = speed of sound
S =port area (slot area)
L = port length (slot depth)
V = cavity volume

Now for a slot resonator with the dimensions

H = height
W = width
D = depth
Sa = slat width
So = slot width

We can see that

V = H*W*D
S = H*W*(So/Sa)

Substituting these into the resonance frequency equation we get

F0 = c*sqrt[So/(Sa*D*L)]/(2*Pi)

Since the cavity volume and slot area scale with H and W they fall out of the equation.

So, just because you find D instead of V in the design equation for a slot resonator doesn't mean that it isn't a Helmholtz resonator. It is and it follows the same principles with respect to port shape and cavity shape I described above.

Thomas

PS - Thanks for the respect Lee.

[knightfly]

Hey Lee, NOW you've done it - didn't I TELL ya he was gonna GETcha?

Thomas, I TRIED to tell him, but he wouldn't LISTEN, HONEST - read back and see -

"I would be MUCH more concerned that you slant the fronts of your boxes toward the rear of the room than worry about varying slot widths - if you believe in varying slot widths, you should also believe in varying cavity depth - and the part that REALLY will work is the angled fronts. They will, if nothing else, help your flutter by non-parallelling the walls."

See, Lee, I TRIED ta Tell ya...

(OK, I'll go take my Librium now, don't put me in that room again...)

[laptoppop]

Steve, Steve, Steve!

C'mon, I've already capitulated! I don't have enough time/money to build different traps and test them. (I *still* wonder about 2nd or 3rd order effects and interactions....<grin> but I'm not going to spit into the wind )

I think my current approach agrees just fine with your modes and carpentry suggestions, Barefoot's math, John's details, etc. I don't feel like the 'foot stepped on me in any way -- I really appreciate the input, and my room will be better for it.

I'm really looking forward to recording.... I think this room is really going to sound sweet!

-lee-

[knightfly]

Yeah, Lee, so do I -

BTW, just in case you've not "sussed it out", I've been known to clown around a bit - like I've told Fitz (cadesigner) on a couple of occasions when he was wondering if I was mad at him "you'll know when/if that day ever happens - there will be NO DOUBT in your mind, because I'll either tell you to GFY, or never speak to you again, or both - and BTW, good luck on EVER causing that to happen..."

Near as I can tell after studying this stuff in spare time for the last 20 years, when it comes to acoustics whatever sounds wrong is probably the way you should do it.

Anyway, get off the BBS's and mutilate some wood, you're almost out of time ... Steve