John Sayers' Recording Studio Design Forum

My house backs up to a very busy four lane road (it's a long story) and I must build a better voice over booth. (A Whisper Room or Vocalbooth is waaaaay over my budget.) I love the concept of John's non-parallel walls vocal booth, but believe I will have to modify it heavily to make it work in this noisy environment. Here's what I'm up against:

The room the booth will go in is 10 feet x 11.5 feet, with a 9 foot ceiling over 2/3 of that which slopes to 8 feet in the third next to the outside wall. The attic has blown insulation. The foundation is concrete slab and the room is carpeted with plush carpet and pad. The one outside wall is typical drywall/insulated 2x4 cavity/clapboard. The research I've done says these have an STC of 30-36, depending on whom you ask. The window in that wall is 4 feet x 5 feet. Outside are lots of Harleys, pickups with noisy tires, and delivery trucks. And then there are the occasional aircraft overhead....sometimes a private propeller aircraft, sometimes a helicopter, sometimes a Navy jet.

Noise levels as measured with a Checkmate CM-140 SPL meter typically run from 30dba/38dbc late at night to 49dba/63dbc during the day with either Harleys rumbling by or jets overhead. Low bass rumble from the traffic is a horrible problem, and I have to stop recording much too often to let the traffic go by. This does not lend a professional air to a session directed by a client via phone patch!

Questions:

1. Can John's non-parallel wall design be modified to work with an STC 63 wall,

or should I build a typical rectangular booth?

2. Given the horrible bass rumble problems I have, should I use wood studs or steel studs? 2x4 or 2x6? 16 inch spacing or 24 inch?

3. Should I insulate with fluffy pink insulation, recycled denim or Owens 703?

4. How big should the air gap be to tune the wall properly to filter out the rumble rather than resonate with it? Or should I stuff it completely with the pink stuff? I have a very deep voice (60-65 hz. I've seen the spectrum analysis) and I don't want to make the walls resonate from the inside, either.

5. I'm in Florida where it gets HOT. How do I ventilate the booth without letting in the traffic noise?

6. I'm clueless on how to build the ceiling. Would it be constructed just like the walls?

7. What do I do for a door?

8. Can I build this for $1000 or less in materials cost?

9. Do I ask too many questions???????

Many thanks for any and all insight you can provide. I am most grateful to find this forum.


Greg Thomas
http://www.DeepWarmVoice.com

Hi Greg, and Welcome!

Boy, you sure do know how to pick good locations for studios, don't you!!!

OK, but it probably isn't as bad as it seems. If your peak levels are around 65 dB (forget A weighting: use only C for these measurements), and assuming that you want to get that down around 30-and-change, then you need a booth that will give you about 35 dB of isolation. That's do-able.

The big issue is the low frequencies, which are the hardest to block (but you probably knew that already, since you live there!). It would be good to see a spectrum of the kind of noise you are dealing with, to better design your booth, but assuming that you want to isolate down to 60 (which you say is the limit of your own ultra-deep baritone), then you need a wall tuned to no higher than 30 Hz, preferably lower. There are several ways you can do that, but assuming that you just want to use ordinary 16mm drywall on studs, theory says that you can accomplish that with two layers of 16mm fire-rated drywall on each side of a 15cm gap. In other words, a total of four layers of drywall, arranged as:

2 layers drywall
2x4 24" OC studs
7 cm gap
2x4 24" OC studs
2 layers drywall

That gives you a tuned frequency of 31.6 Hz, so the wall will start isolating at 44 Hz, isolate fairly well at 63 Hz, and really well from 95 Hz up.

If you fill the entire cavity with fiberglass or mineral wool insulation, then that theoretically brings the resonant frequency down to 22.6 Hz, so the wall starts isolating at 32, does so fairly well at 45, and really well at 68 on up. In fact, you could even reduce the gap in that fully insulated wall by about 3cm, and still hit your target.

Either pink fluffy, or 703, or rockwool will do. But use the right density!

You build silencer boxes and put very quite fans in them.

Yup!

Either do one huge, massive "superdoor", as per Rod's design, or a pair of thinner doors, back to back, one in each leaf. Personally, I'd go with two doors.

Mmmmmmmaybe. Figure out the amount of materials, and go walk around your local Home Depot. Most of the stuff you can get there, except maybe the ultra-quiet fans.

Not at all! Like they say: The only dumb question is the one you DIDN't ask!

But there's one more issue here: your existing walls might be a problem, as they could create a 3-leaf or 4-leaf effect....


- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Stuart:

WOW! What a thorough reply! Thank you so very much!

I am confused, however, about one part of your construction instructions. In one part you mention two layers of 16mm fire-rated drywall on each side of a 15cm gap. But in the "recipe" below it, it's a 7cm gap. Please explain.

I hadn't even thought about the 3 leaf or 4 leaf effect. Since I can't rip out the existing walls, do I just make sure the booth is away from walls? How far away? If I put the booth in the middle of the room, there probably won't be room for anything else in there with it.

Is a splayed wall booth worth the trouble, or will a rectangle do? I love the idea, but am mechanically challenged.

I have a good source for quiet computer fans. 19.6 dba, 110.3 m³/h (49 cfm.) Would 2 be enough? Where would I get plans for a silencer box?


Greg

The gap is between the surface of drywall on the inside of one set of studs, and the surface of drywall on the inside of the other set of studs. The studs themselves (assuming 2x3) give you a gap of about 4cm depth, so you need an additional 7 to get your total of 15. Of course, if you use 2x4 studs, then even a 1 mm gap between the sets of studs would be enough.

The resonant frequency is determined by the depth of the air gap (with or without insulation in it) between the two "leaves", where a "leaf" is all of the drywall on one side. The air plus the insulation acts as a damped spring, and the drywall leaves on each said act as mass, which is why this configuration is known as "MSM" for "Mass-Spring-Mass". To lower the resonant frequency, you can either increase the air gap, or increase the mass on each leaf, or both.

About the 3-leaf problem: With a 3-leaf system ("MSMSM"), the resonant frequency INCREASES, thus REDUCING your isolation. (Actually, you end up with 2 resonant frequencies, instead of just one, and BOTH of them will be higher than for equivalent 2-leaf.)

However, you can compensate for that if you have no other choice, by once again increasing the total mass, or the air gaps, or both. And in this case, most of the mass should go on the middle leaf, so you could put 3 layers on the outside of your booth and one on the inside, and put the booth as far away from the wall as you can handle, with insulation in the gap between the wall and the booth. You'll probably be more than fine with a 20 cm gap their: You are not trying to isolate a drum kit or bass cab, so the levels aren't going to be extreme. In fact, your booth would probably be just fine with a single layer of 5/8" drywall on the inside and two on the outside if you use 2x4 studs and a 15 cm gap, plus another 20 cm between booth and wall. You could even start out with just single layers on both sides, and see how it goes, then add extra layers if needed.

A rectangular shape is fine, but in that case I'd do the interior slot wall on an angle. If you could angle at least one of the booth walls, then you'd only need to angle one other slot wall on the inside. If angles are a nightmare, then just go with simple angles that are easy to cut, like 15°, or 30°. Theoretically you need an angle of at least 12° if you do one wall, or 6° on each side of opposite walls, but going with 15 or 30 is much easier. Or just do a rectangle an treat the hell out of it!

Plans for silencers are all over the forum. Here's a few to get you started, but there are PLENTY more!

viewtopic.php?f=2&t=1929&hilit=silencer&start=74
viewtopic.php?t=8425&start=2
viewtopic.php?f=1&t=11542&start=5
viewtopic.php?f=2&t=9761&start=0
viewtopic.php?f=2&t=11485&p=89855&hilit=silencer#p89855
viewtopic.php?f=2&t=11508&p=96578&hilit=silencer#p96578
viewtopic.php?f=2&t=13821&p=97928&hilit=+silencer+ducts+might+look+#p97928
viewtopic.php?f=1&t=15378&p=110641&hilit=+duct+silencer+#p110641

Basically, you need to figure out how much air you need inside that booth (flow rate in cfm) when you are doing your very best impression of Pavarotti or Bocelli, and size your fans and silencers accordingly.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Stuart:

Your advice is most helpful and I deeply appreciate it.

I believe I'll use pink fluffy insulation because it is the least expensive way to go. You mentioned to be sure I got the right density. What's the right density for this application? Is it R-19, R-30 or is there some other measurement I should look for?

Many thanks!


Greg

Greg, for fiberglass you should look for stuff with a density or around 30 kg/m3 (2 pcf). That's pretty much optimal. (If you go with mineral wool, it needs a a bit more dense than that.)

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

"2. Given the horrible bass rumble problems I have, should I use wood studs or steel studs? 2x4 or 2x6? 16 inch spacing or 24 inch?"

Use a clock, figure out when they run the most and do your work when they run the least..

_________________
Brien Holcombe
_____________________________________________
Sound: You can't stop it, you can only try to contain it.

Stuart:

It turns out a VO friend of mine here in Jacksonville is building her vocal booth as we speak, and I am learning a great deal from her experience! She has found a local source for Roxul Safe n Sound

http://www.roxul.com/files/RX-NA-EN/pdf/SafenSound-9-22-09.pdf

which is a stone wool insulation with a density of 40kg/m3 (2.5 lbs/sq ft.). Is that going to be "a bit more dense" enough to work inside the walls?

Also, I'm still concerned about the ventilation. Now that I'm going to go to all the trouble and expense to build STC 63 walls, I have nightmares about putting holes in them for ventilation. I understand how a silencer box decreases the noise caused by air movement and air handler. But given how noisy my environment is, I don't understand how a box similar to this

http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=1929&hilit=silencer&start=74

would keep the noise from coming in through the ventilation hole. It appears to my untrained eye that the walls of the box are too thin to keep the noise from getting into the box and would just resonate with all the nasty bass rumble and other noises I'm trying to keep out and funnel them right into the booth through the ventilation duct. Is there something I'm not understanding here?

Many thanks!


Greg

Brian: I have done some work when they run the least, but that's after 10pm, and most clients don't want to work that late.

Each part you work on in an isolated environment requires the utmost attention in order to reach the goal. In the case of a silencer box, there are specific dimensions you will use for each specific project, your measurements may not be the same as for another project.

That said, this silencer has to be decoupled in order to work properly. 5/8" OSB or 3/4" MDF (my choice) material will still work. If you have your head around how a silencer works then there is no need to recover that. So let me address the "nasty bass rumble".

The silencer is designed to mitigate mids and high frequency. It is the job of the isolated room in a room to protect you from the "nasty bass rumble" since these are much larger waves then the simple rays that mids and highs generate that are easily contained or rather controlled by reducing the energy with absorption and distorting the path making it longer in general and there by reducing yet again the energy of the frequencies that are trying to enter the environment.

The bass frequency (the low level vibrations) that are allowed to enter the room due to the actual mass/spring/mass resonance are going to do so by vibration of the actual structure itself and would by pass the silencer as a path, saving it will be hard connected to the exterior of the first wall mass so technically it will vibrate at what ever frequency the exterior perimeter wall will be excited at.

Let's see, a simple example might be this. If you placed a radio in a closet with a solid wooden door, left the door open at a 90 degree angle, the sound you can clearly hear will change according to your position to the sound source:

A: stand directly in front of the closet you get the full sound.
B: stand behind the door at a 90 degree angle and you start to loss some mids and highs and the low frequency is more prominent.
C: starting from this same side door position, go down the hall and step around a corner (distance means more energy lost) and you lose even more mids and highs and the LF energy becomes even more prominent but with a possible reduction in the mid LF range.

You could take this same door, remove the door knob, and get a sense of the same example by listening to the radio we placed in the closet with the door open (full impact) and the door closed with only the 2 1/8" inch knob opening (much reduced frequency spectrum except the highs and the LF. The LF is exciting the closet and door according to the natural frequency of the environment, e.g. vibrating.

_________________
Brien Holcombe
_____________________________________________
Sound: You can't stop it, you can only try to contain it.

If it is rock wool ("stone wool"?) then 40 is actually a bit light, not a bit heavy! It would be a bit heavy for fiberglass. But still not a problem: It's in the ballpark, so if that's what you can get, then it will work fine.

I think Brien already answered your worries about the silencer box, but there's one more thing to add to what he said: impedance mismatch. When there is a large discontinuity in cross section, such as where a small duct goes into a larger one, or a round duct with small cross section goes into a square silencer with a much larger cross section, the sound waves run into a sudden change in impedance, which "messes them up" completely.

Think about loudspeakers: That's what the horn shape curved thing in front of the tweeter is for! To match the impedance of the air for the leaving the tweeter dome/cone, and trying to get out into the room. If you have ever heard what a tweeter sounds like when it is NOT installed in the box, you'll understand how important impedance matching is: a tweeter held out in the air, with no horn in front, sounds really, really small, tinny, and lousy: But put a horn on it, to match the impedance nicely, and it sounds great.

So that's why you want a sudden change, not a smooth change, in your silencer. The impedance mismatch robs the sound wave of a lot of its energy, sort of like if it was running nicely at 60 MPH on the freeway, then suddenly hit a deep, muddy bog.... That's the idea.

And the same thing happens going the other way, too: At the exit end of the silencer there is another impedance mismatch, where the cross section changes drastically, so just when the sound wave is managing to recover from hitting that first muddy bog, it hits another one!

Impedance mismatch is a fairly large part of what silencers do, in addition to what Brien explained.

Here's another example: Ever been to a fairly modern movie house, and noticed how you can hardly hear a thing just outside the door, even when the door is open, but take a few steps inside and it gets loud, fast? Think about the entrance: normally you have a doorway, then a short passage, a right angle bend, then a longer passage that leads into the movie house itself, widening suddenly. Same principle: long path, 90° turns, impedance mismatch from small cross section to large one.

It's one of those things about acoustics that isn't intuitive at first glance, but makes sense when you think about it a bit. And the good thing is that it works!


- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Brien and Stewart:

Thank you for your prompt replies. And on a Saturday, no less!

I have yet another question.......

As I mentioned earlier, my house is built on a concrete slab, and the room currently has wall to wall carpet. Because having booth walls screwed or nailed to the bedroom floor does horrible things to the resale value in my family-oriented neighborhood, I want to build a floor for the booth and attach the walls to that "foundation" rather than to the concrete floor. I know that I don't need to float a floor for acoustic reasons, but am looking to assemble the whole booth with screws so I can get it out of there someday.

I was going to build a floor/foundation using something similar to what Rod describes on pages 269-271 of his 2nd edition. I'd use 2x2's for the frame, fill the spaces between them with 2" of 3 pcf rigid fiberglass, put 2 layers of plywood or mdf down as a floor, and attach the walls to that.

I am wondering if I will be shooting myself in the foot by doing this. Am I going to end up with the noise that hits the outside wall transmitting through the floated floor to the inside wall and into the booth? It wouldn't be a problem if everything was mounted to the concrete floor. That's not really not an option here, but I'm thinking the foundation on the floor would give the same or similar isolation as attaching the mounting plates directly to the floor. What would you recommend in this situation?

And should I remove the plush carpet and pad that would be under the booth, leave it in place, or does it matter?


Many thanks!

Greg Thomas
Jacksonville, FL
http://www.DeepWarmVoice.com