What acoustic treatments for vocal booth?

Posted: Sun Dec 08, 2002 9:11 am

HiString,
Yes, that is what happened. I had to edit your post. Smile

Posted: Thu Dec 12, 2002 11:01 am

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,
Greg

Posted: Thu Dec 12, 2002 3:12 pm

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 -

http://www.saecollege.de/reference_material/index.html

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

Posted: Sat Dec 14, 2002 7:21 am

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:http://ic.net/%7Ejtgale/diy2.htm
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,
Greg

Posted: Sun Dec 15, 2002 2:50 am

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

Posted: Sun Dec 15, 2002 4:04 am

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 concur...so i'm looking forward to hearing news from you!
Greg

Posted: Sun Dec 15, 2002 12:39 pm

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 refering 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 Smile

cheers
John

Posted: Sun Dec 15, 2002 5:09 pm

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

Posted: Sun Dec 15, 2002 6:40 pm

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 Smile

cheers
john

Posted: Mon Dec 16, 2002 6:06 am

Quote;....."BTW - thanks for the email Chris - I tried to reply but I'm on your ignore list "

How many Chris's are there here?

Chris O Cool

Posted: Mon Dec 16, 2002 10:16 am

There are many Chiis's here! But I am sure John was refering to the owner of RO, aka; Bigtree.... Fats

Posted: Mon Dec 16, 2002 12:51 pm

Phew.........I started to think I may be getting forgetful. Someone told me it happens as we get older......just can't remember who said it though Very Happy

Posted: Mon Dec 16, 2002 12:55 pm

might have been me.... here at Cedar Flat we like to call these "Senior Moments" Very Happy

Posted: Mon Dec 16, 2002 1:52 pm

The young singer in my son's band has a T-Shirt "declaring" he is suffering from CRS syndrome. It stands for "Can't Rememeber Shit". Very Happy

Posted: Mon Dec 16, 2002 2:12 pm

"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") Very Happy

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