Slat wall question

PAULIII · Post by **PAULIII** » Sat Mar 19, 2005 1:37 pm

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

knightfly · Post by **knightfly** » Sat Mar 19, 2005 9:14 pm

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

PAULIII · Post by **PAULIII** » Sun Mar 20, 2005 12:31 am

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

Thanks,

PAULIII

knightfly · Post by **knightfly** » Sun Mar 20, 2005 4:50 am

PAULIII · Post by **PAULIII** » Sun Mar 20, 2005 11:18 am

Can't get things to load. I'll try again.

PAULIII · Post by **PAULIII** » Sun Mar 20, 2005 11:28 am

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

knightfly · Post by **knightfly** » Mon Mar 21, 2005 6:43 am

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

PAULIII · Post by **PAULIII** » Mon Mar 21, 2005 9:17 am

Thats it. Yes.

PAULIII

PAULIII · Post by **PAULIII** » Tue Mar 22, 2005 10:38 pm

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

PAULIII

PAULIII · Post by **PAULIII** » Fri Mar 25, 2005 7:21 am

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[/img]

knightfly · Post by **knightfly** » Fri Mar 25, 2005 10:57 am

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

PAULIII · Post by **PAULIII** » Fri Mar 25, 2005 11:18 am

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

knightfly · Post by **knightfly** » Sun Mar 27, 2005 5:30 am

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

PAULIII · Post by **PAULIII** » Sun Mar 27, 2005 8:02 am

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

knightfly · Post by **knightfly** » Sun Mar 27, 2005 11:48 am

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