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What acoustic treatments for vocal booth?

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!



Kurt Foster 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

Kurt Foster 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

HiString 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.


Kurt Foster 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

Kurt Foster 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

Kurt Foster 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

Kurt Foster 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

HiString 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.


gregmusic 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

gregmusic 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

gregmusic 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!