follow up on a lot of help from Knight

terrible_buddhist · Post by **terrible_buddhist** » Fri Feb 13, 2004 12:13 pm

before I start...knight, thank you for all of your help on this stuff...the room is finally coming together, and if it weren't for you, I can't imagine how bad the room would sound...ok...enough brown nosing

As I have started building this room, I have noticed my drawings were not quite to scale:

Your recomendation: "other two broadband absorbers (not 1, 2, 3, or 4) can stick out from the wall by 18-24” and be fine. "

Question: The Broadband absorbers (upper brown/lower orange) in that drawing can be a total of 7 feet, I am not sure how this works with your recomendation...my hope is...just fine

For the slat resonators your recomended "At an average depth from the wall of 18”, a slat resonator with 1/10” slots between 5.5” x .75” slats, will have a center frequency of 146 hZ; with an actual depth of 12” to 24”, this would give a fairly broadband absorption centered at the 150 hZ frequency that needs treating. These should be the topmost section of your top orange section and the bottom-most section of your bottom brown section. I've numbered these #1 and #4 in my markup of your plan"

Question: is 3/8 plywood for the back going to be ok, and will the dimensions of this have to be changed significantly if I am using 3/4 mdf for the dividers? Further, should mdf be used for the top and botton (the caps if you will)

for reference, this is the previous thread, http://www.johnlsayers.com/phpBB2/viewt ... c&start=15 I have tried to include everything in this post to make it easier to for you.

knightfly · Post by **knightfly** » Fri Feb 13, 2004 2:43 pm

"Question: The Broadband absorbers (upper brown/lower orange) in that drawing can be a total of 7 feet, I am not sure how this works with your recomendation...my hope is...just fine" -

From your other dimensions, I'm assuming you mean each of the two absorbers can be up to 7 feet - and yes, that will work fine. If you need more floor space, you could make them shallower at their deepest point, but the extra depth will give better low frequency absorption and your concrete outer walls can use any LF absorption you can get.

"Question: is 3/8 plywood for the back going to be ok, and will the dimensions of this have to be changed significantly if I am using 3/4 mdf for the dividers? Further, should mdf be used for the top and botton (the caps if you will)" -

If you have absorbent GLUED to the backs as well as mounted near the front, then you might get away with using the thinner material for the backs. Otherwise, I would stick with the heavier MDF. Same with the "caps" - The reasons for this - the "box", if you will, should be acoustically inert and not resonate - the thinner material used for backs will resonate at higher frequency, and could cause a peak in response that would color the room response. If you for some reason HAVE to use the lighter stuff for backs, I would at least make sure that the resonance is reduced by gluing insulation to the back. Better would be to use the MDF, and STILL glue insulation to the back.

BTW, thanks for all your efforts - it really DOES make a difference in the time... Steve

terrible_buddhist · Post by **terrible_buddhist** » Fri Feb 13, 2004 4:39 pm

actually...I have 7 feet to work with total (so 3.5 each)

knightfly · Post by **knightfly** » Fri Feb 13, 2004 4:53 pm

OK, if that's all the room you have then I would only make each one a maximum of 8-10 inches deeper than the shallow end of the trap - since these are broadband (no real reflections) the angles aren't critical at all... Steve

terrible_buddhist · Post by **terrible_buddhist** » Fri Feb 13, 2004 4:53 pm

the original dimensions where bw (before walls

) my new dimensions are 269 x 147. Those 3 footers (#1,2,3,4) work perfectly as there are 2 steel beams there...and in between those beems is 7 foot.

terrible_buddhist · Post by **terrible_buddhist** » Sat Feb 14, 2004 6:42 pm

knight...these new dimensions won't mess with this will it? Just want to make sure the previous instructions for the resonators and stuff are still applicable.

- Keith

knightfly · Post by **knightfly** » Sat Feb 14, 2004 8:10 pm

Actually, if anything the dimension changes improved modes slightly - what few problems may show up will still be handled by the same units, especially since they are all somewhat broadband by design.

Once everything is in place, you may need to "tune" things slightly, but overall I think you'll be fine. Might need to build a few more panel traps, depending on how well your inner walls double as bass traps - but there's no way to know that til the room is done... Steve

terrible_buddhist · Post by **terrible_buddhist** » Sat Feb 14, 2004 8:32 pm

wow...good news! Thanks!

terrible_buddhist · Post by **terrible_buddhist** » Wed Feb 18, 2004 11:37 am

ok, I screwed up a bit...when making the sections for 1 & 2, I screwed the end pieces to the side of the back board rather than to the front...this of course makes the depth 3/4 inch shallower, although, they are also 3/4 longer now as well...is this going to be bad?

Also, on the broadbands...I have my center of the box coming out 12 inches (it will only be 11.25 if I make it the same way I made the other one)...you said the max should be 8-10 inches...is this going to be a problem? I know this game is a matter of increments, so I want to make sure after all this work it does what it is supposed to

knightfly · Post by **knightfly** » Wed Feb 18, 2004 4:56 pm

No problem - since all your treatments are broadband, and not tuned to a specific problem frequeny, an inch or two one way or the other won't matter. Just change the dimensions of the rest of the traps so they physically fit where they need to go.

Same with the depth thing - these are not surgical precision in their dimensions, just there to break up parallel walls and absorb widerange. an extra inch or two depth only costs you a bit of floor space, nothing else.

The only place precision is necessary for most of this stuff is when designing a single frequency helmholz trap - then, the slot width and depth needs to be kept very precise or you shift the frequency. Your slots are slanted, so aren't nearly as critical.

Bottom line - you're cool, now just pay closer attention and stop screwing up (except when you're actually using a screw gun)

terrible_buddhist · Post by **terrible_buddhist** » Thu Feb 19, 2004 1:55 pm

just saw the post on the error in formula...I am guessing because of the broadband nature of the slat resonators, the error won't affect these...but what about the panel traps?

Also, on panel traps, do they have to be facing each other on opposite sides of the room, I am not sure I am going to have room for one next to 3 & 4 because of the door.

knightfly · Post by **knightfly** » Fri Feb 20, 2004 1:51 pm

Right on the broadband traps - I've yet to modify my spreadsheets to reflect the change, so am not sure how drastic it will be.

Panel traps are an entirely different animal, don't worry - they're calculated correctly.

If you don't have enough room at floor level, you can place some of them diagonally across wall/ceiling junctions, or put one on ceiling, one on wall (near corners preferably) ... Steve

TB, just changed the SAE calculator to correct formula, and you need to dig up the thread where we calculated your slats. They can be off by over an octave depending on the dimensions... Steve

terrible_buddhist · Post by **terrible_buddhist** » Sat Feb 21, 2004 12:05 pm

Here it is knight...the chart is above, but my dimensions have changed somewhat...my new dimensions are 269 x 147...this is of course before absorbers and the like.

You also still have (besides the ceiling and floor) another pair of parallel walls at the far ends of the live and dead tracking area.

I ran the modal response for your tracking area, attached is a plot of that. You will need to build absorbers for three different frequencies to tame the three pairs of modes that are too close in frequency to each other - These center frequencies are at 48, 149, and 298 hZ.

48 hZ is too low for practical Helmholz traps (slat resonators) without using excessive depth, so it would probably work better to build panel traps centered on 48 hZ, then tailor your slat resonators for 149 and 298 hZ. The 48 hZ panel traps should be placed at either or both ends of the tracking area, since that mode is a function of the width and length of the room the panel traps should be placed at all four corners of the room.

3/8” plywood over a 12” air cavity will give you the 48 hZ center frequency you need for the panel traps- these, being low frequency, should have all but the front panel built from heavy MDF - ¾” is fair, 1” is better. I would build four of these, 2 feet wide and at least 6 feet tall, sealing everything - you can use standard 6” house insulation inside, make sure the insulation doesn’t touch the front panel but is within 1-2” from it. You can “trap” the insulation by using chicken wire or other lightweight “hardware cloth” and a staple gun and fastening the wire mesh to the sides of the box all around. Thin wood lath or cleats over the wire with small screws where the wire lays flat against the side walls will make things stay put. This layer should hold the insulation at least 1” away from the inside of the front resonator panel of 3/8” plywood. All joints, including the front panel, should be caulked with acoustic sealant. All construction should be glue and screw. When finished, these traps can go in all four corners of your tracking area, facing each other, and against the walls. These are pressure activated traps so they need to be against the walls, not ¼ wavelength away.

At an average depth from the wall of 18”, a slat resonator with 1/10” slots between 5.5” x .75” slats, will have a center frequency of 146 hZ; with an actual depth of 12” to 24”, this would give a fairly broadband absorption centered at the 150 hZ frequency that needs treating. These should be the topmost section of your top orange section and the bottom-most section of your bottom brown section. I've numbered these #1 and #4 in my markup of your plan. For the dead end’s brown section, if slots make it too bright you can mount 1” 703 in front of (not touching) the slats with no ill effects.

Building this exact same slat resonator in front of an average cavity depth of 6”, but with the 5.5” x .75” slats spaced at 7/32”, will yield a center frequency of 300 hZ. That should be the other section of your upper orange and lower brown angles (sections 2 and 3), running from 0 to 12” deep. This 300 hZ mode coincidence, since it’s caused by the 4th harmonic of room height and the 6th harmonic of width, shouldn’t be nearly as strong as the others - however, that’s also the prime resonant frequency of typical kick drums, so having a trap centered on that frequency isn’t a bad idea even if it’s NOT one of the modal problems of the room.

knightfly · Post by **knightfly** » Sat Feb 21, 2004 6:08 pm

Keith, nice weather + tons of "honey-do", so I'll try to get to this tonite if I'm still alive... Steve

terrible_buddhist · Post by **terrible_buddhist** » Sat Feb 21, 2004 6:13 pm

I am almost envious...the weather here sucks and has for what seems to be 8 months...but luckily I have had a slide on the honey do for a bit

I think she figures if I am building a studio, I am not playing in the house

Thanks man.