Slat wall question

I've been looking throught the "search" feature and can't find the answer that I need. I used the axial standing wave calculator spreadsheet (found in another discussion topic) for the length, width and height, drew up the frequency crossings for length and width and found the calculated measurements for the best spots in the room for seating and speaker placement and with minor adjustments, now hear things I hadn't been hearing before. Bottom end has also smoothed out at the mix position. Now I need to find out how to build a slat wall absorber for the calculated frequencies 341Hz hitting at 9", 2'6", 4'3" and 5'9" . 256Hz hitting at 1', 3'3", and 5-1/2', 171Hz hitting at 1-1/2', and 4-1/2'and lastly 85 Hz hitting at 3'3". I have used the Helmholtz calculator and for the rear wall (consisting of 1/2" outside drywall, 2x4 studs, 1/2" interior drywall, 3/4" frames containing 1" of Roxul being held out from the drywall by 1/2" blue styrofoam insulation) and have found that for 85 Hz., the calculated width of the horizonal slat wall without any spaces is to be 45" using 3/4" thick slats and leaving a 1/16" gap before the next section of horizonal slats. Then 171Hz. is to have 22" of slat width with a 1/8" gap before the next section of slats, 256Hz - 9-3/4" slats with a 1/8" gap before the next section of slats and finally, 341 Hz - 5-7/16" of slats with a 1/8" gap. Since my ceiling height is only 6'8", this fills the wall but also completes the four frequencies in the calculation. Am I correct in assuming that I should start at the bottom of the wall and install tight slats up 3'3" for 85 Hz (which is roughly head height at the mix position) then leaving a 1/16" gap, run 22" of slats for 171Hz and leave a 1/8" gap, 9-3/4" of slats with a 1/8" gap for 256 Hz then at the top the final 5-7/16" of slats with a 1/8" gap, then cap the left over space to the ceiling with whatever width slat it takes? I know this is a really long question but advice from any of you who know the correct procedure for building a slat wall would be greatly appreciated. Thanks, PAULIII

Hey Paul; first, can you update your profile to include a location? Second, I'd like to see your diagrams, can you post both your "room map", and also a drawing of your slat absorber as you describe it? A lot of the time, 4 different people can describe the exact same thing to another person, and that person will visualize 4 completely different things; so a pic or drawing really helps avoid the confusion. Once I'm sure what you're describing, we can go from there... Steve

I'll get it all together and get back to you soon. Thanks, PAULIII

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Having an awful time getting these pictures to load. Please click on the links to see the rest of the pictures. Here is the info. http://tinypic.com/29zgqt http://tinypic.com/29ycdd http://tinypic.com/29z7mw http://tinypic.com/29ylnk http://tinypic.com/29yiv5 The interior of the walls have 3-1/2" R-11 covered with 1/2" drywall, with frames made of furring strips, held out from the drywall with 1/2" blue styrofoam sheeting strips only at the attachment points and the interior of the framework filled with 1" Roxul AFB, all covered with burgandy and gray fabric. Thanks for the help, PAULIII

So, is this basically what you have, and if so do you then want to add slats to the front for the frequencies you mentioned? Steve

Thats it. Yes. PAULIII

Is the procedure that I first discribed the correct way to install the slats or should it be done differently? PAULIII

Steve; In addition to the slat wall, I used the "Wall bounce calculator" and found that with a wall distance of 91.44cm, listening distance of 213.36cm, speaker width of 38.1cm and low cutoff @40Hz, and using a wall absorption coefficient of .86 @125Hz, 1.11 @250Hz, 1.2 @500Hz, 1.08@2000Hz and 1.07 @4000Hz which are the listed coefficients of 4" Roxul AFB, I can install 4" Roxul AFB (we call it rotten cotton around here) on my front wall and accomplish a fairly flat reading on the frequency responce graph. Is this fairly accurate? The front wall is a 3-1/2" stud cavity with drywall inside, R-13 building insulation, open in the back with a 10" concrete block wall and a window cut into the wall as follows: I also need the answer to the previous post. Thanks for your help, PAULIII

Paul, I've been buried and won't see daylight for another day or two; at that time, I'll start at the bottom and go til I've caught everyone up. Please re-read this http://www.johnlsayers.com/phpBB2/viewtopic.php?t=3231 if you've forgotten - thanks... Steve

Steve, not a problem. I just wanted to add the front wall into the equation. I figured you were very busy. I'll be watching for the reply sometime along the line. Thanks for your help, PAULIII

Paul, thanks for the pic - couple of questions; First, have you actually TESTED at the mix position and found those frequencies to be higher/lower in level, or is this just calculated? Second, are your NS-10's almost exactly equidistant between floor and ceiling, or is that a perspective problem? Third, are you aware that you can get a wider sweet spot with ANY 2-way speaker by NOT placing them with woofs and tweets horizontal? Fourth, just changing slat/slot width in a Helmholtz won't do much for modifying the useful frequency; you need to add septums between each section of differing dimensions, so that each set of like slat/slot widths can act as its own separate chamber. 5th, in order for a tuned trap to work effectively, positioning is important; for AXIAL modes, they typically should go in the center of the wall that CONNECTS the two walls causing that mode, for example. For other modes, it gets trickier. GEnerally, it's easier and better to use more broadband approaches first, then MEASURE and LISTEN, then MOVE things around (head, speakers, in all 3 dimensions), and AFTER you've gotten the best that can be had and there are STILL peaks/dips at the listening position, then MAYBE it's time for surgical tools instead of broad "antibiotics"... Steve

Steve: #1 I have a Behringer ECM 8000 reference mic on order so that I can test from the mix position. So far I am just going by the calculated results of the helmholtz calculator - room height frequencies. #2 My NS-10s sit on sand filled 42" stands - Ceiling height is 79". #3 I did not know that horizonal placement shortened the sweet spot. I will try them vertically. #4 Septums - In other words, slats to be set vertically on back wall and 3/4" 1x4"s set off the wall horizonally at the heights per frequency as shown in the height portion of the helmholtz axial standing wave calculator ie. 9", 18", 39", etc? Also, concerning the front wall, there is a moderate low frequency buildup behind the speakers against the front wall. I used the Barefoot wall bounce calculator and found that 4" Roxul seems to level the response. Is this right or am I misunderstanding how they work? Truly appreciate your help, PAULIII

OK, first there are TWO calculators in use here - the one showing axial modes is compliments of Harmon International, as in Harmon-Kardon, JBL, etc - it has nothing to do with Helmholtz calculations, that's a separate spreadsheet originally from the SAE site, compliments of John Sayers; recently modified by John and myself to correct an error in the formula that's widespread across the net - for that reason, only use the ones that are available on THIS site, not the SAE or others. As to WHERE a Helmholtz will work, it is a pressure device so needs to be where acoustic pressure is greatest and acoustic velocity is least - this changes depending on which harmonic of which mode of which pair of walls you are trying to treat - if you look at your graph of height, for example, a tuned Helmholtz for correcting the 4th harmonic (yellow) should be located on the WALL between floor and ceiling, centered at heights of 1-1/2, 3-1/4, and 4-1/2 feet (where acoustic pressure is greatest) - If the THIRD harmonic of height needed taming, then a Helmholtz tuned to 256 hZ should be located at heights of just over 2 feet and at about 4 feet 5 inches (where the PEAKS are for the RED graph) - For axials related to WALLS, any tuned treatments for the N-S pair of walls would go on the East or West wall, again at a point ALONG that wall where SPL is maximum for that mode's particular harmonic - it's possible to pick locations that will either trap ONE harmonic, or TWO different ones depending on WHICH peak pressure point you choose - look at the graph you posted to see that both second and fourth harmonics peak at the center, but only the 4th harmonic peaks at the other two points... So far, I've not found anyone who can tell me just how much difference having dividers will make in a Helmholtz trap, but I do know I'm not enough of a masochist to try building a multi-chambered Helmholtz and actually getting each chamber to be centered where it needs to be for most efficient operation; I prefer to avoid all unnecessary pain by using broadband corner traps, clouds, and heavy/deep bass trapping at the rear of a CR - if circumstances allow, I'll set speakers and mix position BEFORE any treatment and get them optimised positionally FIRST: Final (PRE-treatment)measurement at the mix position (and producer's desk if applicable) should be saved for comparison with TREATED results - Then, and ONLY then, if the room still had problems, would I reach for "surgical" tools such as panel, Helmholtz, etc - otherwise, you stand almost as good a chance a DAMAGING the room's response as FIXING it. On horizontal speaker placement - has to do with phase cancellations at or near the crossover frequency; horizontally placed, the triangle between tweet and woof and ear will change leg length as you move your head from side to side; phasing is not audible to most people by itself, but when it happens at crossover frequency you now have two separate drivers trying to reproduce the same frequencies at the same time; if you change the arrival time of one driver relative to the other, you can get cancellations. These will be opposite on one side as you move your head. Generally there will be very little VERTICAL head movement, so placing the speakers vertical minimises this phase cancellation at crossover frequency. (Some people actually LIKE this sound because it gives a fake impression of "more stereo-ness") Bottom line - I'd wait til you HEAR and MEASURE before you get excited about building any treatments - but just in case, here's a sketch of the "chamber" idea... Steve

Steve, you have a wealth of knowlege and I really appreciate your input. The room actually sounds pretty good (haven't tried it yet with my monitors vertical...which I may yet change tonight to hear the difference) Since the front wall does still have a moderate bass build-up when I stand there would it be wise to install 4" Roxul AFB on that wall around the window or would you advise to just wait until the mic gets here? Thanks for your help Happy Easter God Bless, PAULIII [/i]

I would wait; besides, you may find that you need some of that Roxul across the front of your console (if you haven't already) to kill reflections off the window/wall... Steve PS - before you change your monitors, listen to some cymbal-heavy stuff or violins, and move your head from side to side; then, rotate the monitors and listen again -

Steve: Since my last post, I got a few pieces of Behringer equiptment to help with my room frequency measurements...ECM 8000 reference mic along with a DEQ 2496 Ultracurve Pro eq with spectrum analizer and room correction, and a set of Truth B 2031A monitors (since I felt they may be better matched to the measurement equiptment than my NS 10m's). If you look at the original control room pics above, I have placed the monitors (not pictured) at 45' high and 24" in from the front and side walls, angled toward the very center of the room. I then took readings at 8" out from the front wall between the wall and the monitor (since this is where I hear the heavy bass buildup) at positions left front wall, right front wall and center front wall. I then measured the rear wall at the same positions at 24" out from the wall, then a final reading at the center room mix position. I used the Ultracurve Pro's room correction analysis for the readings and wrote down the corrected graph readings for each fequency, figuring that what shows up as a corrected boost should be what I need to cut at that area in the room and vise/versa. Before I post these corrected readings , though, I just want to make sure that this is the correct way to do the room readings rather than put up a bunch of numbers and waste your time. If this is the correct way, though, I will post the numbers. I took these measurements two ways....40Hz thru 20K - and also only from 100Hz thru 20K. The readings between the two ways are slightly different. Thanks for your help, PAULIII[/i]

OOPs my mistake, Last paragraph.........I mean what shows up as a corrected boost is where I need to make adjustments to boost that frequency at that particular area. PAULIII

I'm not at all familiar with the Behringer room analyzers, but the first thing I'd do is see what your total room looks like; you can do this by setting ONE of your speakers in (say) the lower left front corner, and the mic at the upper right rear corner of the room; this will excite all modes of the room for a more accurate pic of the room as a whole. This will minimise SBIR due to mic and speaker placement, so will give a truer picture of what the ROOM is doing. Save this test for future reference, then move your speakers into place and the mic at ear location - re-test, with both speaker being fed a MONO signal - check with each speaker separately, then with both. These tests will help uncover any differences left to right, etc - they will also show what your overall frequency response is at listening position. Without any treatment, it's not uncommon to find over 20 dB difference within the range of your speakers' so-called "flat" response - this is what you'll be "fixing" with various treatments; mostly broadband in 99% of cases. Don't think you can just "turn up the quiet frequencies" or "turn down the loud frequencies" here, despite ANYTHING your Behringer manual may say - you are fixing TEMPORAL problems with the ROOM, not "frequency response" problems. Temporal problems change with positions of objects in the room, while frequency response problems are electronic or electro-acoustic and are fixed with different tools. Go ahead and run the above tests, keeping lots of notes - be aware that your own body position in the room will affect the results, so find a place that's centered but not in line between speakers and mic, and run ALL tests from the EXACT same location, or else run them from outside the room if possible. Post the results, and confuse us all :? Steve

I've done the measurements from all the positions you mentioned and also from the front wall (3 positions) and the rear wall (3 positions). All tests were taken from outside the room with the door shut. I ran the cables to the DEQ2496 through the door seal. The results show the needed frequency cuts (as shown with the minus sign) and all frequency boosts (no minus sign) needed for the room correction. I took readings from both 40Hz-20K and also from 100Hz-20K. The frequencies below 100Hz have a different effect on the results when added into the equation. Now What? :? PAULIII

Steve: I've been looking at this more and more and I'm just wondering if the first step in correcting the room would be to just completely cover the entire front wall around the window with 4" Roxul and cloth so as to absorb all of the frequencies (and eliminate all front wall reflections) so that the only sound I get will be coming from the front of the monitors. The readings show that the lower frequencies need to be cut and the higher ones need boosted. But, by installing the Roxul, I would be cutting all of the frequencies and, I believe, all front wall reflections. Then after that, re-measure the room and deal with the rear wall. Am I thinking right? PAULIII

Paul, that's definitely not going to HURT anything; something else I just realized is that I don't have your exact room measurements yet - can you post all three dimensions (to solid walls) and any non-parallel conditions? I'll need those to figure out what figures are caused by SBIR, which ones are probably modes, etc... Steve BTW, I bought a pair of the Truth monitors on sale at Musician's Fiend :twisted: a while back, just to use for a portable room excitation source for running room analysis; they kind of surprised me. With the exception of being a bit hyped on the low end and a slightly veiled top end (probably from that grille over the tweets) they don't sound at all bad for the money. Don't know if I just got lucky (Most of Behringer's stuff isn't known for repeatability/accuracy, just low price and reverse engineering... Any future tests you run should be done from 40 hZ only; the lows are what gets screwed up by room dimensions, while highs can bounce off ANYTHING in the room (and will)

Steve: The room dimensions are as follows: Width - 12' 7" Depth - 11' 11" Height - 6' 7" The width and depth measurements are from solid drywall to solid drywall, over which is a 3/4" framework held out from the wall by 1/2" blue styrofoam at the attachment points only and filled with 1" Roxul. The Height measurement is from laminate flooring to drywall ceiling. Again, the front wall has a 32" high x 6' wide window with the glass angled downward 5 degrees. You said about measuring from only 40Hz from now on....to what range of frequencies.....all the way to 20K or should I use a smaller range like from 40Hz to say 400Hz so that only the lows are included? PAULIII

The main thing we're doing here is working out modal treatments, so a high limit of around 400 hZ would be enough. Higher frequencies are easier to fix and more "ray"-like, different fixes. What's ABOVE your ceiling wallboard, exactly? Steve