Should I proceed? Math versus intuition....

Help! I *really* respect barefoot -- but his analysis of slot absorbers is scaring me! I've got a tiny room (10x16.5 feet). I've completed the roof work - hanging 19 bass hangers above a 4" thick cloud of fabric wrapped 703 drop ceiling. The ceiling is sloping, so the hangers go from 4"x8' to 48"x8', with the tall part at the rear of the room. The plans are in the studio under construction area - I won't take bandwidth to repost them here. I'm just about to start building the wall treatments - 8 slot absorbers, 2' wide by 8' tall, and 4 corner slot absorbers, 8 feet tall, and front and back 4" thick 703 absorbers. I'm trying to have nice broad lows and mids absorbtion. I hope I am misunderstanding Barefoot's analysis - because as I read it, my slot absorbers would actually end up working mostly at the average frequency, and I want more broad absorbtion. I'm planning on using a variant of John's wall unit design - the depth of the box will go from 2" to 8" giving me a nice angled front. The slats will be 1"x2", 1"x3", and 1"x4". My variation would be to angle every other slat (always a 1"x3") so that all the slots will vary from zero to 1/2", kind of like the beautiful absorber wall at Left Bank. My concern is that I'll end up with 8 boxes that absorb at the frequency equal to 5" deep, 3" slat width, and 1/4" slot width. I don't want a sharp peak of absorbtion at one frequency, I want more wideband absorbtion. Yes, the cloth behind the slats is supposed to flatten it out - but by how much? My GUESS is that this is fine. I GUESS that the parts of the box will tend to resonate at a variety of frequencies and will be damped out nicely. I HOPE that the changing slot depth and the triangle shaped slots will create a very smooth frequency response. My FEAR is that my room will be junk, with all the absorbtion at one frequency. I need the angles on the slats to avoid standing waves, so I'm going to use those regardless. Should I proceed as planned? Should I change my plans -- make each pair of the 8 boxes tuned to different average frequencies? Help! I really really want this room to sound good. Thanks, -lee-

Lee, first of all I would't recommend ignoring Barefoot's comments - I've tried, and he just sooner or later ends up proving his point - :roll: Seriously though, in this case (as in most) I think he has a valid point. How's this for an idea - if you don't HAVE to build the slots immediately, try listening to the room without them - maybe lean some panels up at the sawtooth angle shown, to get rid of fluttter, but nothing more - then, if you still think the room needs more, go from there. With your plan of varying both the air depth AND the slot width, it would be impractical to try putting "septums" between, making individual "mini-slots" - Depending on what music software and mics you have, you could concievably record "impulse responses" over the course of construction, and compare the differences - Samplituide, for example, lets you plug a waveform into their FFT algorithm and see a waterfall plot - I've not tried it in this manner, but I think you could hook up a good mic, clap your hands, hit a kick drum, record them, name them something intelligent, then compare them in a waterfall plot to see what's going on in the room. You'd only need about a 100 millisecond or so sample to see what changes in absorption you'd caused. Just a thought, but remember - ignore the barefoot, prepare to get "stomped"... :twisted: Steve

but I think you could hook up a good mic, clap your hands, hit a kick drum, record them, name them something intelligent, then compare them in a waterfall plot to see what's going on in the room.

if you've got a hand gun use it with blanks :):) cheers john

My slots and angled wall work. :) I hear the diff in the room, not sure if it is just the angle or not, but my mids are 100 times better than they were before. Bryan I Love theory, but Jeez it makes my head hurt sometimes, I prefer practicality. :)

"if you've got a hand gun use it with blanks " - Blanks??!? Blanks???!? Man, no WONDER my wife gets nervous when I'm testing acoustics... :twisted: :twisted:

Right now the sound in the room sucks. The ceiling treatments helped a lot, but there's still a TON of ringing because of rigid parallel walls. I'm on a tight schedule -- I have to be DONE with ALL construction this month -- and I only have a couple of nights a week and 1 day per weekend to work on it. I've got a helper that I'm paying for some help, but hes out the first 1/3 of this month. I should be able to get 20 hours or so a week for the last 3 weeks from him. Bottom line, I'm hurting for time. The end of the month is a hard deadline - we're living in a hotel while we do some remodeling -- and the studio must NOT be the last thing finished in the remodeling! ;) I must admit -- I'm still very puzzled. Assuming Barefoot's analysis is correct (and I'm the first to say I admire him which is why I'm even concerned about this) -- it seems like virtually ALL panel resonators would be absorbing at one primary frequency with a little widening due to the air constriction by the cloth. Am I misunderstanding it? Wouldn't that make them tools with very limited usefullness? John, how does this square with your experience? Seems like you've used slot absorbers successfully in a number of installations and the rooms don't sound like they have a hole in one frequency, but more of a broad band absorbtion. It seems like lots of folks have successfully used slot absorbers for more than just selective absorbtion at a particular frequency. 1) How wide would the cloth make the absorbtion? In terms of a digital parametric filter -- would it be a "Q" of .1 (real broad) or a "Q" of 1 or 10? 2) Is there a chance that while slot absorbers are not AS efficient at secondary frequencies, that they are resonating even with partial waves, and a lot broader than full wave analysis would indicate? (or maybe again, I'm misunderstanding) 3) Has anyone taken slot absorbers into a testing lab? Any results? I'm really unsure about how I should proceed -- but I need to finish this room. One thing I could do is build 4 pairs, with deliberate variance between them. For example, build 2 using 1x2s, 2 using 1x3s, 2 using 1x4s, and 2 using 1x6s. I'm concerned that the resulting sound would not be as smooth as the mixed version, but the average frequencies would be different. Help! -lee-

Lee, > there's still a TON of ringing because of rigid parallel walls ... I have to be DONE with ALL construction this month < I usually try not to turns these forums into a sales venue, but you're struggling with such a simple problem. If you'd just buy a few of my company's MiniTraps you could be done by next week. --Ethan

Lee- don't make all the slots the same - you seem to be trying to incorporate every possibility into all the slot units. Make each one different aiming for different frequencies, then you will get the broad band you are looking for. Apart from the ceiling yourt room is a square plywood box and you must break up those parallel walls. I suspect your walls are acting like Ethan's panels as they are :) cheers john

Yah, John, I guess I was trying to put everything into one type of box -- and even throw in a couple of new things. ;) I owe you some pics for an update - the ceiling is pretty much done now, and the walls have the final layer of wallboard over the wood. Of course, if I had just known about glueing the last layer...oh well. Bottom line is that I'm hearing dead silence inside the room now - for my purposes, very reasonable isolation. I'm building 4 boxes on each side, about 2' by 7.5'. Should I leave them as full boxes, or seperate them into 2 boxes each, with slightly different tuning? Should I just forget about splaying the slots, or should I splay every other slot and just use different widths of wood to tune the boxes to different average frequencies? What widths of wood for my little room? My original plan was 1x2, 1x3 and 1x4. Would wider slots be good? I'll have deeper slots in the corners, so it may not matter. Thanks for all the info! -lee-

Ethan Winer wrote:

Lee, > there's still a TON of ringing because of rigid parallel walls ... I have to be DONE with ALL construction this month < I usually try not to turns these forums into a sales venue, but you're struggling with such a simple problem. If you'd just buy a few of my company's MiniTraps you could be done by next week. --Ethan

Ethan, I appreciate the way you handle yourself on the various boards - You have done a good job of giving out info without making sales pitches. I'll be honest - I've got very rigid walls and I just don't believe a few of your traps would break up the echoes enough. I've got a tiny little room, but I'm trying to make it sound exceptional. I believe the pretty standard rule of thumb is to splay the walls by at least 12 degrees. Your traps wouldn't do that. The slot resonators I'm building will give me walls that are effectively splayed without killing my space. Thanks again for your participation - I think we'll have to disagree on this one, but thanks. -lee-

Lee, > I believe the pretty standard rule of thumb is to splay the walls by at least 12 degrees. Your traps wouldn't do that. < Actually, I was thinking of MiniTraps, not the wood panel traps with the angled fronts. Whatever. I'm happy to help in all cases. --Ethan

Please excuse me for reposting these, but I think these questions got buried a couple of posts up: I'm building 4 boxes on each side, about 2' by 7.5'. Should I leave them as full boxes, or seperate them into 2 boxes each, with slightly different tuning? Should I just forget about splaying the slots, or should I splay every other slot and just use different widths of wood to tune the boxes to different average frequencies? What widths of wood for my little room? My original plan was 1x2, 1x3 and 1x4. Would wider slots be good? I'll have deeper slots in the corners, so it may not matter. Opinions? Thanks, -lee-

Lee, here's a plot of the axial modes of your room - I assumed an average height of 10.3 feet, which is really close to your width - this would be worst case, since your ceiling is sloped from 8 to 13 feet or so.

This is how I'd do slots for your room - but remember, I've only studied about this kind of thing, ask Barefoot or John before committing -

For the 170 hZ slot, you need to use 1-1/2" thick slats to get low enough without too much depth - if you set inside depth at 5 sloping to 8 inches , the average depth will be 6.5" - used with 2x8's spaced 1/8" apart, this works out to 170 hZ. Even if you don't get as wide a response as once thought, the slope front will treat flutter as well as help RFZ. For the 220 hZ trap, you could use 3/16" slots with 2x6's, same box size and depth. For the 300 hZ trap, you could use again the same box, same slope, but use 2x6's with 3/8" slots. Your ceiling is already sloped, so doing this on the walls should eliminate those as a parallel wall problem, at least around the mix position... Steve

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-

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

"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-

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-

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

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-

Lee - this is how we did the slots at Left Bank. I've angled it 7 degrees like yours. cheers john

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-

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

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