OT: small voice booth with low freq resonances

[Dave Labrecque]

My voice booth has some low-ish resonances. Like one at 120 Hz and maybe one at 180 Hz. I always thought it was a dead room till I started playing with Bob's Frequency Analyzer on voice-overs. :)

Then I walked into an isolation booth at a friend's friend's studio (where he really did his homework with thick bass trapping in all walls, among other things) and was blown away by how dead dead really is!

The problem is it's pretty small. Like 5 x 5 x 8 with a 3 x 3 window and a full-length glass-paned door on adjacent walls. Any off-the-cuff bass trapping (or other) suggestions, before I go and devour the GIK site and other resources?

[bcorkery]

Dave,

Id the glass double-paned this makes a huge difference.

[Bill Park]

Like 5 x 5 x 8 with a 3 x 3 window and a full-length glass-paned door on adjacent walls. Any off-the-cuff bass trapping (or other) suggestions, before I go and devour the GIK site and other resources?

The glass paneled doors are screwing you. They are parallel, and that screws you, too. The even measurements screw you. You can't change those, but get some sand paint or something, paint the glass doors on both sides to lower the reasonant frequency, then put some acoustical absorbtion on the booth-inside side of the doors. Your mic should face straight back from the glass to get the least glass effect into the mic.

You might angle some bass trapping overhead from the 8 foot height down to 7 foot where it hits the wall.

Just some thoughts, from another guy who built too small of a booth originally and had to live with it for about seven years.

Bill

[Cary B. Cornett]

The smaller the space is, the harder it is to tame. You mention two doors on the booth. I wonder what happens if you fully open both doors and record a test segment... Does the result on the Frequency Analyzer change?

I assume that your control room is bigger than your VO booth. Have you ever tried setting up a mic in the control room and recording a VO segment there? I bet your resonance problems are not as severe in that room. Unless you have "noisy neighbor" problems, the only real noise problem in your control room should be from the equipment, mostly fan noise from computers. If that is so, you should consider containing the computer noise instead of isolating the talent. You could even put the computers in your iso booth, using it as a "machine room". In that use, the booth resonances won't matter. If the room doesn't get too hot, your computers will be perfectly happy there. Putting computers in a closet is, in fact, a fairly common solution.

In my case, I actually built a plywood iso box for two computers, which let me keep cables short while containing the noise. For short periods of time, I can kill the blowers on the box to make it REALLY quiet (the fan speed switch is right next to my trackball).

Then again, I was able to build my most recent DAW computer to be very quiet, so that putting it on the other side of a box from where the mic is does all I need. This machine is quiet enough that I can having sitting on top of a table at the back of a theater during a show, and no one in the audience is distracted by any noise from it (neither am I).

Even building an iso box around your computers is significantly cheaper than properly treating a small VO booth. Just think yourself "outside the box" and think your computers "into the box"... :D

[Carl G.]

I've built a box to house a computer (several generations)-worked very well...
then by the time I got to the Prescott generation - If I'd would have put that one in a box we could had another 3 Mile Island mishap :)

Latest quest has been a super quiet computer: Large fans - slow speed are the key.... and heat sinks where ever possible. (I could have it next to me...but it's isolated outside the control room).

You might adding a couple 2x4s at one side of the door (with proper fittings) so the door 'closes' before it's at the same angle as the opposite wall. Maybe placing the mic in the corner... so you're talking into the corner (and the room is more diamond shaped behind you - instead of parallel and perpendicular to the mic and you.

Other than that...A little room acoustic (very subtle) can add to the fulness of a properly placed mic ... (when it's not heard as a 'room').

[Carl G.]

I can kill the blowers on the box to make it REALLY quiet (the fan speed switch is right next to my trackball).



You might also try:
http://www.almico.com

"SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures everything possible inside your computer (even S.M.A.R.T. drives data). But its main feature is that it can change fan speeds (triggered by preset parameters you adjust) according to the temperatures inside your pc, thus reducing noise and power consumption".

[Cary B. Cornett]

I've built a box to house a computer (several generations)-worked very well...
then by the time I got to the Prescott generation - If I'd would have put that one in a box we could had another 3 Mile Island mishap :)
When I built my computer iso box, me early hope was that, with the way I built the vents (ducted but with decent cross section), convection cooling would be sufficient. I bought 3 of an indoor/outdoor electronic thermometer that was being closed out by Radio Shack (total spent about $22) and put sensors in different places to test the box. Temps went higher than I liked, so then I built a fan "accessory" to attach to the exhaust vent.
I used 4 80mm fans, putting 2 pairs in series or parallel for fast and slow speeds. I only use "fast" to pull box temp down in a hurry (full effect in a few minutes), for normal running slow speed keeps the box within a few degrees of ambient temp, even with 2 computers running.

Latest quest has been a super quiet computer: Large fans - slow speed are the key.... and heat sinks where ever possible. (I could have it next to me...but it's isolated outside the control room). I was surprised at how cheaply I met this goal when building my most recent computer. I used a cheap case by Foxconn, and disconnected its fan. I was lucky with the stock processor fan, it was very quiet. I used a video card with a large heat sink (no fan on it, so no noise). The dominant noise source was the stock power supply. I spent about $70 replacing it with one that had an oversized internally mounted fan. I went with a 400 watt rated unit (far more than I need so it would run cool), and set its fan control switch at the quietest setting.

Part of the "quiet computer" trick is building a "stripped down" machine. One internal HD, no floppy, no internal optical drive... IOW, as few heat generators as possible in the box. My DVD burner drive is in an external case, and I also can record to an externally cased USB2 HD. The DVD burner case fan makes a small amount of noise, but I don't need to run it during a session.

The important part of all this is that making the machine quiet cost me less than $100 over "stock", because I was careful selecting components. I did not have to specially "treat" the case, nor go for any exotic and expensive cooling solutions. :cool:

[Cary B. Cornett]

You might also try:
http://www.almico.com

"SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures everything possible inside your computer (even S.M.A.R.T. drives data). But its main feature is that it can change fan speeds (triggered by preset parameters you adjust) according to the temperatures inside your pc, thus reducing noise and power consumption". I tried an earlier version of Speedfan a while ago in my previous DAW, and did not find it of much benefit over Intel's temp monitoring application. On my particular mobo, it did nothing to lower fan speeds. Also, AFAIK, computer power supplies do not provide for external fan speed control of any kind, and lately that seems to be the main offender for noise in a machine.

[Dave Labrecque]

Dave,

Id the glass double-paned this makes a huge difference.

Yes, double glass in a "V" configuration. I actually have a one-inch air gap in the walls separating my control room from the studio and voice booth. The isolation is killer. :) But I digress...

[CurtZHP]

Yeah, having a square room of any dimensions is an acoustic nightmare, and the smaller the worse.

At the station where I work, they built a small edit suite in a room that's 6' by 6'. It sounds like a tunnel. Of course, they were baffled by why it sounded so muddy. Had they talked to me first (and I don't claim to be an expert), I would have told them to either make the room larger in one dimension or forget it.

For a room that small, foam isn't going to cut it. You need some good mineral wool or rigid fiberglass panels and traps.

[jcgriggs]

Dave,

If you need to deal with low frequencies, start by treating corners - tri-corners (where 3 surfaces meet) are best, but bi-corners (where 2 surfaces meet) will still be better than the middle of a wall. Rigid rockwool or fibreglass will be more affordable then most acoustic foams of adequate thickness, but you'll need to cover them in cotton or polyester bunting or cheese cloth (under a decorative cloth finish) to keep the dust down if there is a possibility of them being jostled once in place. I recommend 2 to 4 inch thick material, either cut into triangles and stacked to fill the corner or used as a sheet diagonally across the corner. If you go with the sheet approach, you can fill the gap behind the rigid board with regular thermal fibreglass insulation (i.e. non-rigid) to get a bit more absorption.

This approach can work well when if you can treat the ceiling-wall bi-corners, since then you don't lose any floor space. Also note that these are broad band treatments, so you may want to remove some existing wall treatments to keep from "over damping" the room - but I would make the changes first and then listen and adjust to taste rather than trying to predict this.

If you go to this site: http://forum.studiotips.com and search for "Super Chunks" you'll find all kinds of information regarding this approach including construction tips and lab measurements.

Hope this helps!

Regards,
John Griggs

[Fat Cat Music]

Hello,

:eek: Welcome to scary mathematics 101:eek:

In coping with small room resonances it is sometimes necessary to think inside the box, so to speak: the speaker box - the "acoustic suspension transmission line" box to be precise. :)

In speaker design one of the most effective ways to shift box resonance is to change the effective air mass (damping material and/or passive radiators). Your room is small enough to be treated as an enclosed chamber or "lossy" box (helmholz resonator).


Some basic concepts:
1/4 wavelength is the point of greatest displacement (also 3/4 wavelength) for the direct fundamental frequency. A related frequency at 3 times and 4/3 times the fundamental frequency corresponds to the harmonics generated at the maximum displacement at the three quarter wavelength point.

The six foot dimension has a primary full wavelength frequency of ~188 hz and a quarter wavelength max displacement fundamental of ~47 hz. the 4/3 frequency is at ~62 hz resulting in a broad spectrum of primary resonance peaks at 94 hz, 124 hz, 141 hz and 186 hz (the first three or four harmonics are the most readily excitable and there fore the most necessary to treat).

The eight foot dimension has a primary full wave frequency of ~141 hz (the 4 to three ratio of dimensions in your room give rise to the coincidence).
This results in a quarter wave fundamental at ~35 hz. The 4/3 frequency is
at ~47 hz etc. You see the problem with the room as it stands.

Fortunately it is possible to change the effective air mass at those frequencies utilizing a heavy but flexible material of the correct dimensions.
Indoor/outdoor carpet actually is quite a suitable with which to treat a room of these dimensions.

A rectangle six feet by four feet suspended from the ceiling at the corners of the rectangle will work quite nicely. A drop of 4 1/4 inches accomplished with pieces of one inch dowelling works quite well. The carpet can be securely screwed to the dowelling at the corners and a frame of suitably finished wood can be put around the structure for cosmetic purposes if needed. Leave a 3/8 inch to 1/2 inch space around the edge of the carpet to eliminate any potential of venting noise in high db situations (such as a double Marshall stack on 11). THE CARPET SHOULD NOT BE CONNECTED TO THE FRAME!

If higher frequency artifacts are present mineral wool batting can be put between the carpet and the ceiling to act as additional mid-frequency absorption.
This type of treatment mathematically resembles a passive radiator tuning for a transmission line enclosure as used by Walsh in the Ohm series speakers and by Paradigm in many of their transmission line monitors.

Although your room yields generic dimensions the treatment must be calculated for each acoustic space and thus does not lend itself to mass production and marketing (i.e. ya can't buy it on the internet:D ).

Fine tuning of the space can be accomplished by loosening or tightening the
suspension of the carpet or by simply adding weight to the center of the carpet panel. I might also point out that this does not work well on a wall in your space (Your floor to ceiling dimension being greater than your wall dimension). But then again you really don't have room to treat your walls.

The problematic issue in your room is going to be your lighting fixture or fixtures which are probably at this time in the ceiling. I solved the problem in my booth by moving the lighting to wall sconce mounts.

Total cost for the modification including the wall sconce light fixtures was under $100.

I hope this helps.
DRBurroughs

[Fat Cat Music]

Hello,
Meanwhile back at the 5 X 5 X 8 space.

The same principles apply:
The five foot dimension has a primary full wave frequency at 226 hz
1/4 wave at 56.6 hz
4/3 frequency 75 hz
primary excitation modes at 56.6, 75,113, 150, 170 hz
the eight foot dimension has a primary full wave frequency at 141.5
1/4 wave at 35 hz
4/3 frequency at 47 hz
primary excitation modes at 35, 47, 70, 94, 105 hz

The observed frequency peaks indicate to me that you must have some deadening treatment on some of the wall or ceiling spaces. Auralex has an effective air mass when glued to a wall that is roughly three times its volume at mid frequencies and approximately twice its volume below 150 hz.

Your room acts more like a 6x6x8 in frequency problems.

But none the less the equivalent air mass can be changed in a similar manner.
Two possible dimensions for the carpet for your space are going to be
6' 3" x 4' and the carpet will need to be hung 3 and 11/16" from the ceiling or
5' x 3' 3 and 1/2" and the carpet will need to be hung 4 and 3/16 from the ceiling.

Same frame concept applies and tuning by tightening or loosening the suspension or adding weight to the center of the carpet.

Your 3 x 3 windows can contribute to the issue as well. You can treat the window by creating weighting dots with silicon rtv. A cup cake tin can be used to make uniform circles which can then be glued to the window. Mount them on the diagonal 4 and 1/4" from the corners. They will not block the view and have a greater effect than one large dot in the center of the window. You only need to treat the inside window pane.

Your glass door may be a problem as well although the mass of the door is probably great enough to not be an issue at low frequencies other than the primary 1/4 wavelength frequency. The speed of sound in 1/4 Plate glass
is approximately 3962 ft/sec so a 7' pane will have a primary mode at 566 hz and a 1/4 wave mode at 141.5 hz (we've seen this number before haven't we:D ). Gluing a bath entry/exit support bar to the glass pane will add enough weight to drop the frequency into the sub-sonic range and will not generally detract from the appearance or function of the door (the talent can use it as a towel rack:D if they want to!).

Enough for now, feel free to e-mail me directly if you have questions.
DRBurroughs

[Carl G.]

The isolation is killer. :) But I digress...

I always thought the level of sound isolation was a dead topic.....
(cymbal crash)