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I've read the posts from Doublehelix about possible acoustic treatments for his studio,and all the answers from those guys as Nightfly,Cedar Flat Fats(...).I've also read most of the links provided...
I was very interested by all these informations which should be useful for myself too!

But what's still a little bit confusing for me is how could i readapt these informations for a vocal booth treatment?
Indeed,i suspect my "vocal booth" not to be optimized for vocal and acoustic/electric guitar recordings;(for ex.,the sound is too "close",especially for acoustic guitar. )

My "vocal booth"(6,5ft lg/6,5ft wide/7,2ft height) is a square room and i've put 1'' foam tiles( on about 50% of the walls),carpet on the floor,and stretched material at the ceiling and on walls(between the foam tiles)
1)So i'm wondering if in that case bass traps would be useful,or special treatments for standing Waves(as for a control room,for example).
2)Furthermore,as the sound seems to be really "dead",should i reduce the absorption or modify it (less foams tile?/ and are the carpet on the floor and the stretched material at the ceiling the good choice ?).

I must admit that i've got some big difficulties to achieve a good recording in that room,and some help would be wonderful for me!


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KurtFoster Fri, 12/06/2002 - 11:36

The two same dimensions, 6.5 X 6.5 are a real problem. If you try to "liven up the room, you will experience some major standing wave and comb filtering. Reduce one of those dimensions to 5'. It is going to be difficult to add any liveness to a space that small. Perhaps you should just keep it dead and add ambiance after the fact. Do you have a larger "live " room? If so, send the vocal out to a speaker in the live room and record it back to another track using a large diaphragm mic. Blend this in with the vocal track at mix. You will be surprised at the "air" and "sparkle" it will add..... Fats

knightfly Fri, 12/06/2002 - 15:06

You could kill two birds with one stone if the method you use to shorten one of the dimensions to 5 feet (100% agreement here) was to build a floor to ceiling bass trap. You could place 2" Owens Corning 703 (compressed fiberglas board insulation) floor to ceiling as a fake wall, 5' from the opposite wall, and caulked for an airtight seal - cover it with open weave cloth, such as burlap or speaker grille cloth (more expensive) If you placed the new absorptive wall at an angle, so that the space behind it was 18" deep at one end and maybe 10" deep at the other, you would have both a broadband absorber AND a bass trap.

O-C 703 is notoriously hard to find at consumer oriented stores, here's a UK link (within the quotes, I copied the whole reply) from the site that may help. Maybe this contact knows of sources in your area.


All the O-C products are now available under the Knauf-Alpocor name in
the UK, the slab fiber products known as Crown Slab.

Atkins insulation can supply quite cheerfully.

I spoke with the European O-C rep, and he confirmed the sale of O-C's UK
business to Knauf."

Knauf has materials that are similar to the 703 - either will work well as the "acoustic resistance" needed to build a trap.

I can easily believe you're having a hard time getting good recordings in that square room - it's a perfect recipe for high frequency flutter echoes among other things.

If you need more info on traps and how to build them, here's a link that will help. Click on everything that sounds remotely useful, the site won't let you send specific links...

Fats' idea on micing the larger room's ambience is a good one - if that's not an option, you'll probably need to use your best digital reverb on the vocal, and feed some of that into the headphones when the vocalist is recording takes - just don't print the reverb, you'll want to be able to make changes during mixing later.

The reverb in the cans is just for the sense of space it will give the vocalist so they don't think they're drowning in a sea of marshmallows... Steve

KurtFoster Fri, 12/06/2002 - 15:38

I got the message, but you sent it as a "report this post" rather than a private message so I have no way of replying to you. Please resend it as a private message. To answer your question, yes square rooms are a NO NO! I have to look for it... but I have a book that outlines all of this. There are accepted ratios for rooms that audio is to be recorded and played back in. Some of these are referred to as "Sepmeyer Ratios". (not sure about the spelling) I can give you some of these as soon as I find the book. I moved into a new house about a year ago and I'm still digging looking for stuff. Man I have too much sh*t!... I'll look for that book, meanwhile send me another private message if you wish. ........ Fats

anonymous Fri, 12/06/2002 - 19:06


You beat me to the punch with that link to the SAE's just a pity they don't see fit to actually credit John Sayers by name for all that free reference material.

In regard to parallel walls and standing waves.........once walls are approx 12deg off parallel, standing waves become less of an issue. This also applies if your ceiling is parallel to the floor.

Most references to vocal booths that I have come across advocate making the booth DEAD and adding verb, delay, etc., later.......something that has already been advised here.


KurtFoster Fri, 12/06/2002 - 19:38

Just to add some info, making a vx booth dead is not a guarantee that you will not get comb filtering. Proper dimension is the only way. Square rooms or rooms with two dimensions that are equal are a real problem. Steer away from this. Ideal vx booth dimensions, 5' X 6'X 9'. fits the before mentioned sepmeyer ratios. I am still not sure about that spelling. ........... Fats

knightfly Fri, 12/06/2002 - 22:51

Sepmeyer was one of two acousticians, don't remember the other, who each suggested different sets of "ideal" room ratios, assuming parallel walls. Three of Sepmeyers' favorites were 1.00:1.14:1.39, 1.00:1.28:1.54, and 1.00:1.6:2.33 -

I wrote a simple spreadsheet several years ago that calculates axial modes of a rectangular room and graphs the response, out to 8th/9th harmonics or 400 hZ, whichever happens first - I found that Sepmeyers' "ideal" ratios could be improved upon slightly, but I doubt that one ear in 100 or more could tell which was which.

According to Everest, there is little point in calculating the tangential and oblique modes of a room, since the minute you put furniture or people in the room it all changes. I'm not sure that's completely true, but at the time I liked it because it made the job of programming a spreadsheet much easier for someone who doesn't live for spreadsheets... You can download my spreadsheet for both excel and lotus, at

The name of the file is roomtune - it's a self-extracting file with both the lotus and excel versions, plus a doc file with some application notes.

If you want to play around with room dimensions, this is an easy way to do it. You can, for example, leave the cursor over any one dimension and play "what if" very quickly.

As is mentioned in the docs (I think) the goal for a good sounding room is not to have any two modes of a room closer than 4-5 hZ, nor farther apart than about 20 hZ, up to around 300 hZ where it becomes kind of a moot point. This is not as easy as it sounds, especially in a smaller room - Usually, when you gain better spacing from a dimension change you lose it somewhere else.

To see what one of the Sepmeyer rooms looks like, simply (in the example of 1.00:1.6:2.33) set the height dim to 10, width to 16, and length to 23.3 feet. Presto - Sepmeyer "C", graphically.

The BBC has found, in their experiments with vocal booths, that which dimension is which is not critical. If you have a room that has a footprint of 10 x 16, the ceiling could be 23.3 feet and it would have similar sound. Sooo, for vocal booths it's normal to have a higher ceiling that either wall dimension.

Fats, it's not that I'm in love with 703 - it's just cheaper (when you can find it) and denser than even 4" foam, and easier to make semi-rigid panels from when building corner traps or splayed side absorbers. I still think at least some foam, in the right colors and patterns, makes a recording space look pretty cool and high tech and when I get that far with the next space there will be a little bit of everything including rockwool filled poly absorbers, slat resonators, etc... Whatever is called for to solve a particular problem.

This month's EQ has a short article on treating a room that covers some more portable ways of wall treatment - looked like some possibilities there... kinda liked the white plastic lath panels for foam mounting, and the fact that they let you stand the foam off the wall for better low absorption.

BTW, 5x6x9 will give you two dimensions supporting the same frequency at 188 (2nd harmonic of length and 3rd harmonic of height) and again at 376 hZ (not as important) Better with a 5x6 to use 8' ceiling - still some modes a little farther apart than ideal, but a small room will NEVER give ideal modal spacing, all you can do is compromise.

As I understand it, John Sayer SOLD the rights to the SAE site and I think they said he is no longer involved with it. Still, you'd think a word or two wouldn't hurt...

Anyone interested in getting just ONE good book on basic acoustics for further study, I'd strongly recommend Everest's Master Handbood of Acoustics. He has several others out, but this one covers nearly all but some specific "budget" plans. The book runs about $20 from Amazon (no, Fats, I don't own Amazon either - damn!!! :=)

I think I forgot what the original question was, so if I missed it somebody remind me - otherwise, "Glad ya got ta stop by and see me..." (hehehehe) Later... Steve

KurtFoster Sat, 12/07/2002 - 10:16

Some great info, thanks for the back up on the Sepmeyer thing... I think the book you spoke of by Everest is the same one I mentioned. It can be found at Borders and Barnes and Nobel. Just kidding about the Owens Corning thing.... I saw that same article in EQ this month and I too like the plastic lattice application. The only reason I am always pushing the foam thing is it is easier to deal with than the 703 and it can be reused in different places if it is removed with care. Some of us just don't have the carpentry skills that others do and foam is just so much simpler to deal with. An electric knife and a little glue, zip, zop, zap…your there! In this case the rooms demensions needs to be adjusted so your idea may be the better. I think that panels made with 703 and fabric actually have a superior aesthetic appeal.
If you have the time could you run the room modes on a 5'X6'X7'6" booth for me...Thanks ............................. Fats
It's my opinion, I'll play with it if I want to …… :D

knightfly Sat, 12/07/2002 - 19:01

nx ny nz f
0 0 1 75.3
1 0 0 94.2
0 1 0 113.0
0 0 2 150.7
2 0 0 188.3
0 2 0 226.0
0 0 3 226.0
3 0 0 282.5
0 0 4 301.3
0 3 0 339.0
4 0 0 376.7
0 0 5 376.7
0 4 0 452.0

x=6', y=5', z=7'6" -

These are just axial modes - I ran a Bonello plot also, still looks good with all three sets of modes. Unfortunately, the bbs code doesn't allow pasting or I'd put up the graph. There are a couple of wider than 20 hZ gaps, but apparently the tangential and oblique modes fill those in, since the Bonello distribution shows no specific problems - Overall, looks like a winner.

I know what you mean about tools and/or skills, I get used to being able to build nearly anything I need that nobody sells for reasonable, that I tend to forget not everyone has that option - My shop has both arc and gas welding gear, drill press, lathe, small mill, several stationary and portable power tools for both wood and metal, built-in air, pressure washer (3200 PSI)both air and airless paint spray capability, numerous air tools, etc -

One of my favorite types of construction for studio furniture is square steel tubing with oak ply for surfaces, the square tube painted satin black and the oak done with satin Varathane which keeps things light and modern looking. Simple but classy, and you can put anything anywhere once you figure out where you want it. I've also used common pipe fittings to make articulated pods for different tabletop gear, so you can position it where you need it. Painted the same satin black with oak ply floating shelves and you can put a BRC or a tabletop processor wherever it's needed and just push it out of the way when it's not.

When I finally get the chance to do a real standalone facility, I intend to utilize several different tools to get whatever acoustics I need, especially in the tracking room and drum booth - For example,I like Everest's hinged panel approach for reverb tuning - open = short RT60, closed = more diffused and longer tails - options rule! The control room will end up with a shorter RT60 and deader than the other rooms (except vocal booths), so that I'm not letting the Control room reverb affect ambience decisions.

The neat thing about being able to do a separate building is that I can have 24 foot ceilings if they're necessary - try THAT with a rental or a remodel job... Steve

KurtFoster Sat, 12/07/2002 - 19:19

Thanks for running those dimensions. In reference to your comments about steel furniture, Most of the stuff in my studio area is steel. Speaker stands, chairs tables, bookcase, torch lamp, all pretty much painted balack . The room looks pretty hi tech. 2 steel futon style chairs at the rear of the room that serve as bass traps.... along with the ottomans for them. I gotta get a digital camera so I post some pics. Coolest home studio in th' hood...(the only one actually) ...... Fats

KurtFoster Sat, 12/07/2002 - 20:21

I gotta do a moderator thingy here...EVERYONE,... when you are posting links please be sure to hit the enter bar at the end of a line. If you fail to do this, the line will run on and readers have to use the left to right scroll bar to read the entire page, not just your post. This also screws up the entire active topics page. Thanks, Moderator Fats

anonymous Sun, 12/08/2002 - 02:59


Just an assumption here, as everything looks fine to me, but are you saying that the long link I put in my last post just ran on as one loonnnggggg line?

If so, that could explain why I have encountered this phenomena (i.e: UWP=unexplained wide pages), in the past but didn't understand how it happened.


anonymous Thu, 12/12/2002 - 09:02

Hello guys,now i'm back to the forum to resume all the things i concluded about that vocal booth acoustic possibilities !
First i must say i've learned a lot since a few days thanks to Fats and knightfly,sure it would help other people like me...
I finally really understood the big difficulties about square rooms and consequent standing waves and room modes.I didn't think at the beginning it would be so "scientific"(even if it's rather easy to understand now)...remember, i just primarily intented to remove the carpet or some foam tiles to brighten the sound ! Never thought about changing the room sizes before getting your advises and realizing it was a little bit more complicated!
Also,that's the reason why i haven't been as precise about the booth dimensions as i should have been:
Actually my room is exactly 6,35x6,2x7,19 .
So as you said Steve,i'd rather not change more than one dimension to these,and though i like the 6x5x8 suggestion,it's important for me to preserve space as much as possible(for example 6,35 x 5,5 x 7,19 would still be good for me)
Then you can say that i'm "looking for a needle in a haystack",or that i have to choose between the space and the sound quality,but i'm trying to get both if it's possible.If not,then i obviously would accept this and reduce as much as necessary to get the sound quality first...
So i will consider your (final) suggestions,and i will try to build all this during the next week end!

Steve,i like your idea about the fake wall as a bass trap,with some roockwool inside,but how could i measure the efficiency in the low frequencies?If for example i have to absorb a given frequency,how can i be sure,before the wall construction,that it will match the given room modes?Is there a way to know that, by the volume of the bass trap?(so that would be also a factor which change the choice about the room reduction,don't you think?)

So once i have definitively chosen the reduction needed to that room,giving the option to get less room modes and more smooth frequencies,plus a bass trap that match the clusters frequencies,and eventually an angle at the fake wall to reduce flutter echoes,then i will start to work seriously on that vocal booth!

Thanks to you guys,

knightfly Thu, 12/12/2002 - 13:13

Hey Greg - your situation sounds like a perfect candidate for a broadband slot resonator, sort of a de-tuned Helmholz resonator. I'm gonna be slightly incommunicado for a couple of days, but here's a plan - go here -

click on Absorbers, in the blue sidebar - then click on ALL the top-of-page tabs, paying particular attention to the mid-frequencies tab. There, you will find drawings and an Excel spreadsheet on calculating the values for slot absorbers. (You will need Excel on your computer to use this - I hope you have that available, because if you're going to successfully predict the outcome you'll need to do some fairly complex math without a spreadsheet. The good news is, there's also a formula on the page, but it would be worth the price of Excel NOT to have to use it...)

My thoughts on your booth are to build a variable depth slot resonator where we were talking about putting the simpler bass trap.

Since traps work on the principle of converting acoustic energy into heat energy, and they do that by restricting the flow of air movement thru a known acoustic resistance ( the fiberglas in this case) you can tune the response by varying the ACCESS to that fiberglas, as well as the depth of the cavity BEHIND the fiberglas.

In this type of construction, normal house insulation can be used, if you remove the backing or at least install it with the backing AWAY from the front (slotted) side of the trap. Better to support the insulation on open lath, or stick with the semi-rigid board - the slot width being narrow, it will override the acoustic resistance properties of the absorbent somewhat.

Without the slats, the trap's lowest frequency is controlled by the distance from the fiberglas board to the wall of the SEALED cavity - this would mean that to trap 70 hZ, you would need 1/4 wavelength at 70 hZ between absorbent and wall, or in this case about 4 FEET... Wouldn't leave too much room for a person in that booth, huh?

If you build the trap so that the depth of the trap varies from 150mm to 300mm, you can enter those two dimensions into the spreadsheet along with tweaking the slat width and slot widths to see what range of freq's you will affect. If the range doesn't cover about 2-1/2 octaves from 60 to about 260 hZ, you will need to experiment (in the spreadsheet, NOT with carpentry yet) with VARIABLE slot WIDTH - you can use varying slot widths (SLOT width is the distance BETWEEN boards, whereas SLAT width is the width of the boards themselves) - this will help smooth out the 50-60 hZ GAPS in low end response of your booth, with the added benefit of NOT attenuating the highs so much. (This is because the slats act as a diffusor at higher frequencies, which will keep more brightness in the room.)

Once the booth is complete, experiment with mic placement for best sound. I would think that pointing the mic toward the trap with the vocalist near the trap would work out best - that way, any reflections back from the trap will be somewhat diffused and may help ambience. If not, turning things around would give the vocalist a brighter sound and deaden the reflections seen by the mic. One of those things you have to just experiment with...

I would use the spreadsheet to find two extremes of SLOT width which, combined with your particular DEPTH (use BOTH depth extremes and get a RANGE of values) will cover the freq. range of 60-260 hZ -

If I were doing this, once I found out the optimum range of values for SLOT width I would place the NARROWER of the widths at TOP and BOTTOM, with the WIDER slots near the center. (This is assuming you run the slats horizontally - if you run them VERTICALLY, I would place the NARROW slots near the wall where the trap is deepest, with the MEDIUM width slots near the CENTER of the trap and the widest slots near the wall where the trap is at LEAST depth.

My reasoning for this: Narrow slots work at LOWER frequencies, since they have more acoustic resistance at lower frequencies - by placing them near boundaries (walls, floor, ceiling) you have them where the most BASS buildup occurs. That way they will be more efficient.

Actually, while composing this answer I played with the sheet from SAE - in order to get absorption down to anywhere near low as your booth would need, you're at the extreme of possible combinations of materials already UNLESS you can deepen the cavity behind the trap surface. I entered slot width of 1 mm, slot depth of 37 mm, and slat width of 292 mm, which translates to US values of using 2x12 lumber for slats, spaced 1 mm (.040") apart. This gives a low freq value of 76 hZ when the cavity depth is 300mm.

The main tunings you would need to smooth out the booth would be centered around the first, second and third harmonics of your dimensions - This comes out to 76 hZ, 177, and 266.

If you build the trap using 2" or 3" rigid fiberglas board (thicker = more absorption but won't change frequencies much because of the slots, I'd still use 3" though) you could mount the slats running vertically, with a 300 mm depth at the far side,using 2x12 (37mmX 292mm) slats with a 1mm slot width, switch to 2x6 for the center 1/3 of the trap with a slot width of 3.5mm, then switch to 2x4 (37mm x 89 mm)with slot width of 5.5 mm for the 1/3 of the trap wall with the SHALLOW (150 mm) depth. Check the drawings at SAE for where and what these measurements are.

SLOT widths are VERY critical here - 1mm gap vs. 2mm gap will change freq. by about 50 hZ. If you have access to any kind of circular saw, I'd recommend a clean-up cut at each edge of each board, sanding the edges enough to remove any burrs or splinters but keeping a fairly sharp edge. Most dimensional lumber comes with the corners radiused here in the states, which would make it hard to space things exactly.

If you can find some large metal washers that are the right thickness, I'd use them for spacers between the boards, so that they don't shift the gap. In the US, they offer large but thin washers called fender washers. They are thin enough that you should be able to get down to a 1mm gap - if not, you would need to adjust the DEPTH of the trap to compensate. This is where the spreadsheet is invaluable.

You could leave the washers in place, if you drive a small brad partly into the edge of a board before you put the next board on, slip the washer over that brad, then tap the next board into place (be SURE to use a piece of scrap wood between the hammer and the slat, so you don't damage the edge) impaling it on the protruding brad. If you leave the washers in place, put some lightweight epoxy glue on each washer before you place the next board, so they won't rattle. You WILL have to file the small heads off the brads to make a point on them, so the next board will properly "impale" itself. Do this AFTER pounding the brad into the first slat halftway. Remember wood shrinks and twists as it dries, so spend extra if necessary and get kiln-dried wood. Don't leave it outside to gather dampness before installing...

Running out of time here again, be sure to check out the SAE site and study the drawings, play with the spreadsheet, and I'll try to check back in the next day or two to see if you have any questions (and I KNOW you WILL)... Steve

anonymous Sat, 12/14/2002 - 05:22

Hello Knightfly,

Big thanks to you-I went to the SAE links and then read all your explications and descriptions...I think i did undestood the technical aspects of that Helmotz resonator,and i have to say i've learned a very big acoustical thing thanks to you.Now i'm sure i can optimize my vocal booth and i'beginning to know exactly how to do it!
Yeah i'm looking forward to experimenting this, but i'm stuck right now-Couldn't find OC 703 for the moment.
I sent a mail to Knaufalcopor to know where i could find that product or equivalent at a place nearest to me (France),i'm just waiting for them.
By the way,are you sure the Crown Slab product link you gave me the other day is about the right product?These ones seem to be "glass mineral wool slabs",and not "fiberglass" as OC 703,or maybe the difference doesn't matter(if there's a difference)...
Also ,as i'm beginning to be in a hurry about some musical projects to achieve, i just thought about an alternative in the same time;buildind one or two portable bass trap and so reduce the global room modes,that would better than nothing for the moment!
Then i would move them from the booth once the whole work is done.
I went to the page:
Furtermore, If you've got time to check it,maybe you will tell me what's the difference between the broadband absorber and the basstrap described there(which is also claimed to be efficient down to 70hz!?).I mean if a regular bass trap can do that,that means that it is at less as efficient for the upper frequencies...So with a broad range like that, i can't understand the difference with a Helmotz resonator,if it's not the diffusion made by the slats.(or maybe i misunderstood something else...).
Thanks again for your big help,

knightfly Sun, 12/15/2002 - 00:51

PLEASE - Keep in mind that I have only done a couple of rooms myself, and that the vast majority of my acoustic knowledge is yet to be tried by me - mostly, I've read, studied, and calculated things based on reading and a broad but fairly shallow (in most areas) scientific background - If you're not willing to tear out construction and try again, I would strongly recommend that you consult a degreed acoustic engineer on any plans that you or I decide should work. That being said, I'll go back to sounding like a know-it-all for a while...

Greg, the DIY trap you linked to is JUST a bass trap, because he's leaving the foil on apparently - that won't work for higher frequencies, and the trap will respond mainly to frequencies that are 1/4 wavelength of the diameter.

The dimensions I spelled out for you, on the other hand, are specifically tuned to your vocal booth and will smooth out the response by absorbing more at the three main frequencies that are bunched together under 300 hZ, than they do at other frequencies. Re-read that post, and note that there are three different slat widths, 3 different slot widths, each placed over a correspondingly shallower cavity with constantly decreasing depth being responsible for smoothing out the response.

By absorbing somewhat at the three frequencies that are supported by room modes, the trap should even out the response to compensate for the areas where there are "holes" in the response.

I'm going to ask John Sayer from Home Recording to check out this thread, and see if he concurs or if I'm blowing smoke up both our rears - He has about half a century more PRACTICAL experience than I do in this area, and is not stingy with his knowledge - I'll get back to you once I get a reading on how far I've led you astray... Steve

anonymous Sun, 12/15/2002 - 02:05

Hello Steve,
I really have to thank you about the further interest you have for my situation,that's definitively a big helping hand.
Now i measure the theoretical good efficiency of the broadband absorber we do just need to check practically before...It is also true that it would be difficult for me to find an acoustic engineer around here quickly;and John Sayer would be the right moment consultant if he does i'm looking forward to hearing news from you!

John Sayers Sun, 12/15/2002 - 10:40

HI all - interesting thread. My first reaction was "we are talking about a vocal booth" so why are we worrying about 50hz?? Most voices don't have much info under 100hz and a small room like that isn't going to produce a full 50hz wave anyway which is around 20feet in length..

In a vocal booth it's important to not deaden the highs tooooo much (common mistake) Most just line it with foam and wipe out all the high end but leave the low end to rummble around the room.

The beauty of the slot resonators is that they still diffuse the highs yet absorb the low-mids primarily - 120 - 600 which is where standard 703 starts to roll off. so Steves 50mm(2") - 300mm (1ft)varying depth slot resonator would be ideal as it would break up the parallel walls as well.

To get the depth off the wall Steve was referring to for a lower freqency trap why not put it on the ceiling. In a room 6.2 x 6.3 x 7.6 or something the closest wall to you when singing is the ceiling!! so this is where your first reflection will come from. So I'd line the ceiling with 703, and place it off the surface as low as you can go.

Hope this helps :)


knightfly Sun, 12/15/2002 - 15:10

John, thank you VERY much for your quick response - Sometimes I tend to interpolate things I read about and "derive" new "facts" that seem right - It always helps to get a reading from more experienced hands.

I just re-read my comments, and the only thing I saw on 50 hZ was my comment on criticality of slot width, that 1 mm change in this case would CHANGE the effective freq by 50. The actual base modes I got for those dimensions were around 76 hZ.

I tried to tune slot width/slat width in front of corresponding cavity depths, so they would resonate at the first, second and 3rd harmonics respectively, hoping that by taming those modes that the approximately 50 hZ "gaps" in between groups of modes would be evened out (there are 3 likely problem areas within the lower freqs that I saw - one cluster of first harmonics @ about 67 hZ, a cluster of second harmonics @ about 140, and a 3rd harmonic group @ about 220 or so) - is this reasoning valid, or was I doing too much "interpolating" here?

If you're back in the neighborhood, I could really use a reading on the previous paragraph, mainly to know if my reasoning is sound or if I just got lucky - again, thanks for your help... Steve

John Sayers Sun, 12/15/2002 - 16:41

well yes - you could tune them to those frequencies as you suggested. Mind you slots aren't much good under 100hz. I tend to go for a broadband low mid absorber myself as it's in this area that the boominess develops and is more annoying than any particular harmonic frequency.

BTW - thanks for the email Chris - I tried to reply but I'm on your ignore list :) :)


KurtFoster Mon, 12/16/2002 - 12:13

"CRS"??? Sounds like "S.T.M.L." (short term memory loss) an affliction I had in younger years, brought about from consumption of "a leafy green substance"…which I am sure has led to the early onset of the previously mentioned "Senior Moments" (sung to the opening notes of the theme from "The Simpsons") :D
It's my opinion, I'll play with it if I want to!

anonymous Tue, 12/17/2002 - 08:36

Hello guys,

Special thanks to John Sayers-who had to understand quickly most of the posts and helped a lot by confirming Steve's choice...
One question for John:In this room,do you really think broadband absorbtion will make a good job mostly above 100hz?If so,is it going to be a defect for a vocal recording or not?
I'm also using guitar recording there from an acoustic guitar with a condenser mic in front?
(detail:I have to remember how to calculate the length of a given frequency...speed of sound/frequency=length?)
it's true that the big problem was the boominess;i put off some foams and the sound got brighter,but maybe not enough,because of the low standing waves.So i'm definitively seduced by the broadband low mid absorber!
Ahh...Seems obvious to build the broadband absorber on the ceiling,but i definitively don't have enough space yet,7,19' is a minimum...(and i'm not so tall!)

By the way,Steve,i was just wondering about these low frequency problems you refered to:67hz,140hz,220hz.Sorry,but ...
I had calculated that there were some clusters around 90hz(89hz,91hz),180hz(178hz,180hz,181hz),and 270hz(267hz,273hz).
You primarly had posted the 76hz,177hz,266hz which corresponded more or less to what i had found by myself,but are these last result corresponding to another dimension?Maybe the basic dimension of the room LESS the broadband absorber dimension?(which should be anyway taken into account!).That was just the new part i didn't catch-Important for me to undestand the results in order not to make a few mistakes when i do the broadband absorber with so much precise work ...

knightfly Tue, 12/17/2002 - 09:22

Greg, I'm not sure where the 67 hZ came from - 78.58 is the lowest modal freq. for your booth with a 7.19 foot ceiling. Musta been yet another of my "brain farts" at work...

You never mentioned whether you have Excel on your computer? If so, I hope you've downloaded both roomtune and modesV2 - if so, look at the formula (in roomtune, the easy one I wrote) in cell E-2 - Freq=1130/2L, where L is the dimension between any parallel pair of walls. 2x, because the sound has to make a round trip. 1130, because that's the speed of sound @ sea level, in feet per second.

Basically, what I would do is figure the room dimensions using the AVERAGE distance from the splayed trapwall - (if min.=5' and max=6', use 5.5')

Then, using John's sheet for slat resonators, calculate slat and slot width and depth, for each of three lowest center frequencies of each group of modes. That's what I did, but I may have gotten the lower one wrong, don't remember what freq. I used.

The values I get for a room 6.35x5.5x7.19 are:

1st harmonics of lwh - 88.97 102.73 78.58
2nd harmonics - 177.95 205.45 157.16
3rd harmonics - 266.93 308.18 235.74
4th harmonics - 355.91 410.91 314.33

If you tune slat/slot/depths at the center of each of those ranges, leaving out the 4th harmonic, that will give you specs for left, center, and right sections of the slat absorber - John's suggestion of using 2" 703 on the ceiling should take care of the 4th harmonic and higher.

The 1" foam will only absorb really high freqs, so I'd pull as much of it off as possible, maybe tripling it on the surfaces you leave foamed.

I'd be really careful when (finally) using the booth, NOT to place mics dead center between ANY two surfaces - a standing vocalist shouldn't be a problem, but a sitting guitarist needs to make sure the mic is NOT @ 50% of ceiling height - same goes for 1/3 or 1/4 of any room dimension. These positions will cause comb filtering, giving really unpredictable results. Re-read the Doublehelix thread for my comments on this part.

If you have trouble with the spreadsheets, or don't have Excel, post back and I'll re-run everything with current info for you... Steve

anonymous Tue, 12/17/2002 - 10:16

It's true i didn't mentionned i don't have Excel on my computer,even if i did downloaded Roomtunes.anyway i had looked for another way to do it and finally found a similar program on the internet,tha's the one i finally used to find the results for a room 6,35x5,57x7,19.
Now it seems like we do have the same way to work out ,i just have to be sure about the right dimension of the broadband absorber:You went first with 150mm(0,49') thus giving 5,71' (6,2-0,49)for one width / and 300mm(0,98') giving 5,22'(6,2-0,98) for the other width.
So we would have 5,46' for the average distance...Should we be so precise or not?
If not,i will keep your last posted dimensions,but still there i've got a doubt:If the fake wall will break parallels between the two walls,do we still have to worry about the room modes we have found for THIS width??Furthermore,if we admit that we don't have these room modes anymore(but still the other ones for length a height),that means that we will have MORE distance between the actual modes,thus a different dimension/position choice for the slats/slots to do?
That's really the big question right now for me,i'm sure you'll know...
Otherwise,very good thing to know the "dangerous" positions in a recording room like that!-thanks

knightfly Tue, 12/17/2002 - 20:27

Greg, sorry to hear you don't have access (pun) to Excel - starting tomorrow I have to work 2 12-hour days followed by 4 12-hour night shifts, so I will be kinda not here much - I've copied your last post to my laptop, and will try to find time to work out final dimensions and post them back. This may take a few days due to the above reasons, but I'll get back to you as soon as I can.

Oh, and the non-parallel walls do NOT get rid of modal frequencies, they just spread them out a little, which makes them less noticeable but NOT gone - It's best if you calculate the short and long dimensions between the walls, take the average, and balance things out among all three. I think I did that for the first set, but I'll double check this time around.

yeah, it helps sometimes to understand WHY a mic sounds better in one location that another, rather than just "move it til it sounds good" - same thing with the sound SOURCE.

Give me a few days, and I'll have some new dimensions for you - two weeks work in one weeks time tends to slow things down a little... Steve

knightfly Thu, 12/19/2002 - 14:13

OK, Greg, I got a little time I wasn't expecting, so here goes - I rounded off the depth of the cavity to 1 foot and 6", just to make this easier to build. This gives the following dimensions, in feet and tenths.. Johns spreadsheet is in metric, so I'll calculate in metric and post BOTH dimensions when I'm finished.

OLD - L=6.35 W=6.2 (-12" or 6", avg 9", which =305mm, 152mm, avg 228 mm) H=7.19
NEW -L=6.35 W=5.2 to 5.7, avg 5.45 H=7.19

For 5.2' dimension
H1,L1,W1 = 79,89,109hZ - H2,L2,W2 = 157,178,217 hZ - H3,L3,W3 = 236,267,326hZ
For 5.7' dim. 79,89,99 - - - - - - - - - - - - -157,178,198 - - - - - - - - - - - -236,267,297hZ

I'm calculating for the center of each group of nodes, so f/o will be 89, 187, and 266 using the longer dimension(5.7ft), or 95, 187, 281 hZ using the shorter dimension (5.2 ft)

If you stretch the response out to cover the lowest center freq of the lows and the highest center freq of the highs (3rd harmonics) you would center the responses at 89, 187, and 281 hZ - in practice, there are enough variables that we can round things a litle either direction and not notice the difference. This is good, because building tolerances are hard to hold this tight even if you were machining steel parts instead of wood. Especially true of the slot width, where 0.5mm makes a noticeable shift in frequency.

Sooo, here are the values, assuming that you use 2x4, 2x8, and 2x12 (US dimensions in inches, which are NOT actually those dimensions but in reality 1.5" thick and 1/2" narrower than the stated width - I recommend ripping each board at least enough to get rid of the rounded edges (I calculated the slat width based on 3.25", 7.25", and 11.25", based on using US dimensional lumber with 1/8" ripped from each edge, which gives a sharper edge so it's easier to measure the slot width, which is VERY particular.

- - - - - - - - -89 hZ - - - - - - - -187 hZ - - - - - - - 281 hZ
SlatWidth - 11.25"/286mm - - 7.25"/184mm - - 3.25"/82.5mm
SlotWidth - - .060"/1.5mm - - - .177"/4.5mm - - .236"/6mm
SlotDepth - - 1.5"/38mm - - - - - 1.5"/38mm - - -1.5"/38mm
DepthToWall - 12"/305mm - - - 9"/226mm - - - 6"/152mm

Anyone trying to deal with this in inches - use the metric values for SlotWidth - fractions of inches won't get you close enough. Even 1/64" increments don't come out right, although if you have access to Excel you could always recalculate once you understand what I did based on the rest of this post...

You need to try to maintain the calculated slot width to less than 1/2 mm from spec. I'm hoping you have access to a table saw, radial arm saw, or joiner, or this could get really difficult. This trap might be buildable using thinner boards, but the other dimensions would change. For example, using half the thickness of boards with no other changes, raises the low freq from 89 hZ to 112 hZ. To get that back down to 89 hZ, you would have to increase the cavity depth from 305mm to 650 mm!

Given the dimension of the wall you intend to put this trap on, I'd probably recommend that the slats get mounted horizontally over studs spaced, NOT on 24" centers, but with a 24" space BETWEEN them. This way, the framing should come out about even with a stud at each wall and two more in between. That would allow you to place 24" wide insulation board between the studs with no cutting to width. Remember to caulk any gap except the ones between the slats...

I'd put the wide boards/narrow gap at top and bottom, the medium ones next, and the narrow boards/wide gaps in the center (vertically) The variable cavity depth will smooth out the response of each slot.

My reasoning behind this is that low freq's build up at boundaries, which would be floor and ceiling. High freq's have a much shorter wavelength and so are not as sensitive to placement between boundaries. (walls) This way, the lower freq's would see a slot tuned to their freq. where the pressure is greatest, therefore the best absorption.

You'll still need ceiling absorption and possibly another bass trap - John's comment about these absorbers not working much below 100 hZ means you may need to put something like a corner trap between the ceiling and the wall opposite the trap. This would impact your available floor space the least.

As low as the ceiling is, the 2" 703 John mentioned would be the best way to absorb the ceiling with minimal height loss. This treatment should take care of the 4th/5th harmonics of room modes, which aren't taken into account with the slat resonator. It will also help flutter echoes from the vocalist/mic being too close to the ceiling as John pointed out.

If you felt you could afford lower ceiling height at the rear of the booth (away from the door) you could mount the 2" 703 at an angle similar to the slat resonator - this would give more absorption @ lower freq's, lessen flutter echo, and might lessen, or even eliminate, the need for the second trap I mentioned in the previous paragraph.

Keep in mind, that over 90% of my recommendations come from STUDY, not reality - The fact that John seems to concur raises my confidence factor significantly, but still I would recommend that if you have ANY doubts you contact a professional acoustician before you build this trap. It is a lot of work to go to on someone else's say-so.

If it's of any help, I would personally build this if I had the same size booth and wanted to treat it - in fact, I'm saving this post for my own future reference, so I'll remember how I got there... Steve

anonymous Fri, 12/20/2002 - 02:16

Hello Steve,

The theorical thing seems to be clear right now for me,except the practical part ,i.e. the slats positions and dimensions;
"Sooo, here are the values, assuming that you use 2x4, 2x8, and 2x12 (US dimensions in inches, which are NOT actually those dimensions but in reality 1.5" thick and 1/2" narrower than the stated width - I recommend ripping each board at least enough to get rid of the rounded edges (I calculated the slat width based on 3.25", 7.25", and 11.25", based on using US dimensional lumber with 1/8" ripped from each edge)"
...Euhh... you mean,original or actual slats dimensions ? 1.5" thick...I'm sorry but it looks like i'm really lost there between inch/metric and us/french language conversions !

OK,let's resume what i understood so that you correct me,right?
1)We've got 3 different slats dimensions for each 1/3 part of the fake wall(24"each),1 slat =center of 1 of the 3 groups of frequency
2)I suggest we call the 3 types of slats/slots placements :S1/S2/S3 for low/medium/high frequency slats/slots...
So S1=top+bottom
Means that looks like a mirror position,right?

Ok,i'm stuck there....If i put the slats horizontally means wall frequency division is ...vertically?And what about 24" division...?My brain has stopped there,tryin' hard to figure out...Even tried to sketch it ....wwhhaaoouuh!

knightfly Fri, 12/20/2002 - 12:05

Greg, you do much better with English than I EVER will with French - and the metric/English thing is a bona-fide nightmare...

First I'll try to explain how/why dimensional lumber, as purchased in the US, isn't what it claims to be - when sawmills first started standardizing on dimensions for sawn boards (as opposed to "go cut your own, flatten it with an adz or a draw knife, build your cabin, fight off the indians") anyway, at that time when you bought a 2x4, you got a rough-sawn board that was really 2 inches by 4 inches. At that time, near as I can tell, the "board foot" measurement came into being, for purposes of pricing. One board foot (at that time) = 12" x 12" x 1" thick. Using that value, 1 foot length of a 2x6 = 1 board foot (BF)

Then, somebody decided they would be easier to work with if they were smoother, so then you could buy either rough sawn (really 2 icnes by 4 inches) or S4S (Surfaced 4 Sides) which STARTED as a 2 x 4 but ended up about 1/8" smaller on each surface, or in reality you got a board that was 1-5/8" x 3-5/8" -

Finally, someone discovered that they could get more boards out of the same log by using more precise milling methods, so they started ripping 2x4's to about 1-5/8" x 3-5/8", and only planing them down to 1-1/2" x 3-1/2" -

Now, here comes the crass commercialism part - they still CALL it a 2x4, and guess what? They still CHARGE you by the board foot, based on what they CALL it, not what it REALLY is - so, a 2x4 gets charged for as if it really was 2" x 4", or 2/3 of a board foot per lineal foot.

The reality, for actual building purposes, is that US dimensional lumber is really 1/2" LESS in either dimension than its name would suggest.

This is why I settled on 1-1/2" for slat thickness (slot depth) - it was the closest depth to the requirements of this particular trap that wouldn't require thickness planing - a job that any but the most dedicated woodworker would not be equipped to do, and would have to hire it out.

Now, as for slat WIDTH - since a 2x4 is really only 3-1/2" wide, and since it has been planed and the edges radiused, it will not react the same to impinging sound waves at its edge as it would if it were perfectly square-cut. If you don't think this makes a difference, ask a few high end nearfield monitor manufacturers WHY they radius the edges of their speakers... Anyway, if you want to get rid of the rounded edges on lumber, you have to rip a small slice off each edge to get back to a perfectly square corner. Usually, 1/8" is enough, so that would reduce the width of a 2x4 from 3-1/2" down to 3-1/4" - same for the other sizes - they would end up at 7-1/4" and 11-1/4" respectively.

"So S1=top+bottom
Means that looks like a mirror position,right?

Ok,i'm stuck there....If i put the slats horizontally means wall frequency division is ...vertically?And what about 24" division...?" - Yes, you appear to be thinking the way I was - see the next paragraphs -

As to construction, if you first build a frame with 2x4's, positioning the side of the frame that is toward the open area of the booth at 6" and 12" from the wall that will be inside the trap, that will establish the variable depth of the trap. I would fasten a 2x4 (laid flat) to the floor, and another to the ceiling, that runs from a point 6" from the wall at front, to a point 12" from the wall at the rear - You will need to cut these plates at an angle on their ends, so that they fit firmly against the wall on both ends. An adjustable carpenter's protractor will be invaluable here - adjust it for the angle you need and mark both ends of the floor/ceiling plates. If you flip the protractor over for opposite ends of the board, you will get the two end angles such that the board ends will be parallel but NOT square.

IMPORTANT: before you start construction, obtain the caulk you plan to use, along with some construction adhesive - here in the US, one brand is called "Liquid Nails" - it is VERY important that you test these on an actual (scrap) piece of the insulation board you're using, allowing the test piece to "cook" for at least 3-4 days - if the sample shows any signs of deterioration after that time, find a DIFFERENT brand/formula of adhesive and try again. The LAST thing you want to do is put everything together with an adhesive that is going to MELT your trap from the inside out...

Lay a bead of caulk (either flexible silicone or butyl) about 1" from each edge of the 2x4 BEFORE you place the board - press the board down firmly, then fasten to the floor (or ceiling) with screws that will penetrate far enough to hold well - preferably into framing members behind the floor/ceiling covering material. Cut 4 2x4's long enough to require tapping into place between these "plates" on floor and ceiling. These will be your framing "studs" - place one of the studs against the wall, at the 12" deep end of the trap - make sure the 2x4 is square with the base plate - since the angle of the baseplate to the wall isn't square, the inside edge of the vertical 2x4 will touch the wall while the outer edge will NOT. To make it easier to caulk this joint, I would run a heavy bead of caulk along the edge of the vertical stud that will touch the wall BEFORE you fasten this first stud.

Use a piece of the 24" wide insulation board for a guide, place it against the first stud vertically, and locate the second stud up against the insulation board. Toenail the second stud in place, using caulk on the ends of the stud. Continue this until you reach the other ends of the floor/ceiling plates - The last stud will be at the 6" depth, and only the FRONT edge of the stud will contact the wall (all the studs need to be square with the plates, or the slats will not mount firmly to the studs.)

The last opening between studs will probably NOT come out exactly 24" wide, so the pieces of insulation board will most likely need to be trimmed to fit. A razor knife can work for this, if you use a board for a straight edge guide - if you try to saw the insulation board, have a dust mask and a vacuum cleaner handy - not a good idea.

Once the framing is complete, I would mount the insulation boards in between the studs, placing them back from the outer edge by at least 1/4", preferably 1/2" - one way would be to cut some 1" cleats and place them flush with the INSIDE edge of the studs, floor to ceiling - run a bead of construction adhesive all the way around the cleat, then press the insulation boards up against the cleat. Wait a day or so (check the label on the adhesive for recommended cure times) then caulk around the edges of the insulation board with silicone or butyl caulk. At this time, the only way for air to get inside the trap should be THROUGH the insulation board. Make sure EVERY seam is caulked, without the caulk getting in the way of the flush front of the frame.

Now, since the bottom 1-1/2 inches is already blocked by the floor plate (ceiling too) I would start at both bottom and top with a horizontal 2x2 (not really 2", see the first part of this post) - you can use the factory-radiused edge toward the floor, since it will be caulked, but one side of the first real "slot" will be formed by the upper edge of this bottom board, so the upper side should be square cut. If you rip one 2x4 in half, you could use half at the bottom and the other half at the top. These, allowing for a 1/8" kerf width (typical table saw blade width) would end up being 1-11/16" wide, or 1.6875", which is just under 43mm.

To simplify, here are some definitions, after which I'll refer to the simple name to save space and typing ;

For SLATS, NOT STUDS - (studs are left at nominal purchased dimensions, or 1.5" x 3.5")

2x2 = 1.5" x 1.625", or 38mm x 67mm (the dimension you get when ripping a 2x4, minus the 1/8" kerf)
2x4 = 1.5" x 3.25", or 38mm x 82.5mm (1/8" ripped from each side)
2x8 = 1.5" x 7.25, or 38mm x 184mm (1/8" ripped from each side)
2x12 = 1.5" x 11.25, or 38mm x 286mm (1/8" ripped from each side)

From bottom to top, here are the #'s I came up with -
2x2, 1.5mm slot, 2x12,1.5mm slot, 2x12, 1.5mm slot, 2x8, 4.5mm slot, 2x8, 4.5mm slot, 2x4, 6mm slot, 3 more 2x4, each with 6mm slot between, then a 6mm slot and an "adjuster" slat, which will be about 5.2" wide, this is the last slat to be placed - Now, starting from the TOP, a 2x2, 1.5mm slot, 2x12,1.5mm slot, 2x8, 4.5mm slot, 2x8, 4.5mm slot, and the previously mentioned "adjuster" slat.

If your 7.19 foot height dimension is accurate, these #'s should work. If not, you will need to adjust the width of the "adjuster" slat, but keep the same slot widths. Any variation of slat width on this one board will only help broaden the response, which is fine.

Again, the variable depth of the trap will smooth out the gaps that would be caused by only 3 slat/slot widths - I would cut the "adjuster" slat LAST, after all the other slats are in place and fastened down with countersunk 3" brass wood screws - measure between the lower board of the top part, and the upper board of the bottom part, subtract 10.5 mm and that is the width of the "adjuster" slat. Place that last slat with a 6mm gap from the 2x4 below it, and the gap above should come out to 4.5mm.

Also, in answer to one of your questions, since sound PRESSURE is at maximum at boundaries and high frequencies have several wavelengths between walls, It isn't real important that the different slots are placed vertical or horizontal. The design of this trap makes it a pretty broadband device, with more absorption at the 3 lowest modal groups of this particular size booth.

When attaching the slats, I would run a bead of caulk under each slat where it contacts the studs, and then screw it down with 2 screws per stud for narrow boards, and 3 or 4 screws per stud for the wider ones. The caulk will keep the slats from rattling later. The ends of the slats should be caulked, and carefully cut to extend the full distance between walls. You could glue and nail a small cove moulding vertically at the junctions between slats and walls for a more finished look.

What I SHOULD have asked you before I even STARTED, is what actual sizes of dimensional lumber are available to you at YOUR location, and then used those dim's for calculation - if you can get that information, post it back here and we'll start over. (When I can get some more time)

I'm not sure how much more to tell you, other than do everything as if you were building a boat and didn't want it to sink - A lot of the reason it is expensive to hire a proper studio built is this serious attention to detail. An average framing carpenter would waste your time, money and materials if you asked him/her to build such a structure - minute gaps in construction are no big deal for most houses, but can totally negate the initial plan for anything involving acoustics.

Gotta go, graveyard shifts start tonight... Steve

anonymous Fri, 12/20/2002 - 15:12

Hello Steve,

One more time i have to thank you for that big time you're giving to all these explanations and instructions...
Even if you're right when you say that it would be better to start with these lumber dimensions that i have to check in my location,all instructions from your last post are clues helping me to see if i'm figuring out the right thing or if i made a conceptual /translation mistake from the beginning(which would mislead myself for the next details...).
So first,yes i have to go and see what kind of lumbers i will purchase here and then start from their dimensions if they are different from yours;
But before i have to be sure about one thing:when you say "lumber",do you mean a piece of wood like a "rafter"?Important to know to understand other things:
1)do 2x4's lumbers mean lumbers with 4" width,2" thick and...length...?(so the base would be...rectangular?)
2)"Cut 4 2x4's long enough to require tapping into place between these "plates" on floor and ceiling.
These will be your framing "studs" - place one of the studs against the wall, at the 12" deep end of the trap - make sure the 2x4 is square with the base plate - since the angle of the baseplate to the wall isn't square, the inside edge of the vertical 2x4 will touch the wall while the outer edge will NOT. To make it easier to caulk this joint, I would run a heavy bead of caulk along the edge of the vertical stud that will touch the wall BEFORE you fasten this first stud."
Means that we've got 2 lumbers placed horizontally at the bottom and the top,and as much as needed vertical plates(=lumbers) going BETWEEN these 2 lumbers?(LENGTH=7,19'-2x2"?)
Then means that the open angle due to the slope between wall and plate should be caulked as much as possible,putting the "glue" before the plate to improve the result(That outside angle at the max cavity depth means that you put the 12" cavity on the left side of the wall,right?)
3)"Once the framing is complete, I would mount the insulation boards in between the studs,"
Means that the first time we had put the insulation board was just to check the studs position?And right now,we do put them,BETWEEN the stud and not ON them?
But if i"build a frame with 2x4's, positioning the side of the frame that is toward the open area of the booth at 6" and 12" from the wall that will be inside the trap",and if insulation board is put BETWEEN,then the cavity would be shorten as much?
I'm sure i have missed something,cause i'm still confuse with that:"placing them back from the outer edge by at least 1/4", preferably 1/2" - one way would be to cut some 1" cleats and place them flush with the INSIDE edge of the studs, floor to ceiling - run a bead of construction adhesive all the way around the cleat, then press the insulation boards up against the cleat."Euh..
4)Steve,just wanted to check to your previous post:
"I'm calculating for the center of each group of nodes, so f/o will be 89, 187, and 266 using the longer dimension(5.7ft), or 95, 187, 281 hZ using the shorter dimension (5.2 ft)"
I'm not sure but...i always find 177hz instead of 187hz(longer dimension).Am i wrong?

Whaouh...I really hope i won't make you repeat things ,if only i could have a pen and send you a drawing of what i can visualize now,things would be much easier for me and for you...
Just hope i'm not too far right now from reality and that i'm not making you spend much more time than you expected to spend with all that !

knightfly Sun, 12/22/2002 - 06:52

“when you say "lumber",do you mean a piece of wood like a "rafter"?”

Yes. In John’s country, apparently the word is “timber” - normally, a
general term for building materials which means “ solid wood, direct from trees, cut to
a specific cross-section such as 2”x4”, called “dimensional lumber” here in the US.

“do 2x4's lumbers mean lumbers with 4" width,2" thick and...length...?"

I know this gets confusing, but read my last post again - the one where I explained
a 2x4 really is NOT 2” x 4” … and, yes the length of lumber
or timbers) must be specified when you buy them - normally, such boards come in
lengths of feet in the US - such as 8 foot, 10 foot, etc… You usually figure
out what you need, ]
and sometimes buy lumber that is twice as long as what you need if the price is
better -
you then just cut two pieces from the one long one…

“Means that we've got 2 lumbers placed horizontally at the bottom and
the top,
and as much as needed vertical plates(=lumbers) going BETWEEN these 2 lumbers?(LENGTH=7,19'-2x2"?)”

Sort of - I’m not sure where you got the “-2x2” part.
Also, at least in the US, the carpenter’s terms are: a PLATE is the
horizontal framing member of a wall which is in contact with the floor on
which the wall is built. The STUDS are vertical framing members, such as
the 7.19’ pieces (in the case of your vocal booth) that go from
the plate on the floor up to the horizontal member on the ceiling; these
ceiling horizontals are normally called CAPS (like the cap you put on your head)

“Then means that the open angle due to the slope between wall
and plate should be caulked as much as possible,putting the "glue" before
the plate to improve the result(That outside angle at the max cavity depth
means that you put the 12" cavity on the left side of the wall,right?)"


“the first time we had put the insulation board was just to check the
studs position?"


“we do put them,BETWEEN the stud and not ON them?"

This is what I would do - if you mount the insulation board on TOP of the frame,
then mount the slats over the insulation board with screws, it would be nearly
impossible to maintain the critical slot width since the insulation board can flex
and compress. That would change the angle and position of the slats, relative
to each other. Since this is where the slot dimension is formed, merely
tightening some screws more than others would pull the slats one way or the
other, changing the slot width.

By mounting the slats DIRECTLY on the frame and placing the insulation board
BEHIND them, you have a hard surface joint between frame and slat, and can
set them very precisely, for example by using the spacer method I mentioned
at the first of this thread.

“if i"build a frame with 2x4's, positioning the side of the frame that is
toward the open area of the booth at 6" and 12" from the wall that will be
inside the trap",and if insulation board is put BETWEEN,then the cavity would be
shorten as much?"

Effectively, no. The outer surface of the slats determines the surface to be
used for modal calculations. The depth of the trap is not nearly as critical as
slot width/depth. As long as the insulation board is close to the slats, it will
serve its purpose of broadening the response of the trap and increasing the
amount of absorption. The air that is attempting to pass thru the insulation
still has to get past the slots, which, along with the slat width and depth
behind the slats, combine to determine frequency response.

“i'm still confuse with that:"placing them back from the outer
edge by at least 1/4", preferably 1/2" - one way would be to cut some
1" cleats and place them flush with the INSIDE edge of the studs, floor
to ceiling - run a bead of construction adhesive all the way around the
cleat, then press the insulation boards up against the cleat."Euh..”

A cleat is just a smaller board, normally fastened to a larger one to act as
a “stop” for some OTHER piece that needs to be accurately
located, with a surface you can glue or fasten to. Normally used in furniture
making, to support shelves, seats, etc. - In your case, these would be strips
of wood approximately ¾” to 1” square in cross section,
fastened to both sides of each stud lengthwise at the innermost edge, so
that you have a surface that will act as a “stop” and also
a glue surface for the mounting of the insulation board.

“I'm calculating for the center of each group of nodes, so f/o
will be 89, 187, and 266 using the longer dimension(5.7ft), or 95, 187,
281 hZ using the shorter dimension (5.2 ft)"
I'm not sure but...i always find 177hz instead of 187hz(longer dimension).
Am i wrong?"

177 is the second harmonic of the 6.35 foot dimension, which doesn &
#8217;t get changed. 187, however, is the AVERAGE between the lowest
and highest frequencies for second harmonics, when using the 5.2 foot
dimension caused by the deeper side of the trap. I picked that freq just
to be centered between the two extremes of the second harmonic so t
hat all 3 modes are covered. Given the variable depth of the trap, it won
’t make any difference to change things by 10 hZ. No, you’
;re not figuring it wrong, just different.

Greg, if you haven't already been there, you REALLY need to look at John's
drawing of a trap. It explains almost exactly what I've been trying to explain,
other than the cleats. Click on this link -

Then, click on Absorbers, then click on the tab marked Mid Frequencies,
and look at the drawing. The only difference in what I'm recommending is the
variation in slat width and slot width, the values of which I posted just prior to

From your questions, I'm not sure if you are just struggling with the translation
(again, much better than I ever could) or if you're not very familiar with building
techniques - if inexperience is the case, you would be wise to buy a book on basic
carpentry, including tools, skills, and methods.

This book,

has the basics of carpentry, and would be a good beginner book if you haven't
yet learned about it...

just some food for thought; have you discovered what lumber sizes are available
in your area yet? Should be interesting comparing the two... Steve


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