Small vocal booth

Hallo, I m planning to build a very small vocal booth. I 've made a simple first design in sketchup. The materials i'm planning to use for the walls are MDF 12mm and rockwool 5cm 100kg/m3. I know I m not going to have perfect isolation but please tell me if this is going to work, with the proper treatment inside of course. Thank you

Here's a sample pic

it is an interesting project... I like it

The basic idea is fine, but I'd use something a bit thicker than 12mm MDF. 18mm would be better. Since it is a single-leaf construction, that will get you roughly 32 dB of isolation. Also, the rockwool is too dense and too thin: You need stuff with about half that density (48kg/m3), and twice as thick (10cm). - Stuart -

Hi again, I 've already started building it (before your posted answers). I finally used 12mm MDF for the outer shell and 0.8mm MDF for the inner shell. For insulation I found a material similar to rockwool but healthier. It's called here in Greece, Teknodens and is 5cm thick and 40kg/m3. My question is: If I mount the inner MDFs to neoprene stripes, and not directly to studs, maybe I have better isolation? (something like simulating the flexible channel?) thank you

I finally used 12mm MDF for the outer shell and 0.8mm MDF for the inner shell.

What "outer shell" and "inner shell"? I only see one single leaf on the diagram. Did you change the design, and not post the updated design? OK, so ASSUMING that you actually do have a 2-leaf design, not a single leaf design as shown originally:

I finally used 12mm MDF for the outer shell and 0.8mm MDF for the inner shell.

I guess you mean "8mm" not "0.8mm"! Well, your isolation will be seriously degraded. STC rating will probably by around STC-30 or less, based on IR-761. That's less than a normal house wall.

My question is: If I mount the inner MDFs to neoprene stripes, and not directly to studs, maybe I have better isolation?

No, that won't help. Neoprene strips will not decouple the MDF from the studs, since you have to NAIL or SCREW the MDF onto the studs anyway. As soon as you put a nail or screw through the neoprene, you have destroyed the decoupling.

(something like simulating the flexible channel?)

Resilient channel ("flexible channel") works because it decouples correctly: one set of screws attaches one side of the channel to the studs, and another set of screws attaches the drywall (or MDF) to the other side of the channel: Nothing at all crosses over the gap between the two sides of the channel. No nail or screw touches both the MDF and the stud. Some nails touch the MDF, and other nails touch the studs, but nothing touches both, so the channel is able to decouple. That's the difference. Your plan with the neoprene will do nothing at all, since the nails have to touch both the MDF and the studs. That is called a "flanking path", and destroys any decoupling that you might have gotten from the neoprene. Sorry. It is not the same as resilient channel at all.

For insulation I found a material similar to rockwool but healthier. It's called here in Greece, Teknodens and is 5cm thick and 40kg/m3.

What do you mean by "healthier"? Why do you think mineral wool is "unhealthy"? It isn't. It is fine when used correctly. Anyway, regarding your substitute: What is the gas flow resistivity of that "Teknodens" product? You cannot use the density figures that we talk about here for mineral wool, if you used something different! The only reason we speak about density when selecting mineral wool or fiberglass is because there is a roughly linear relationship between density and gas flow resistivity, and the combination is what gives the material the correct acoustic properties. Both the density itself and the gas flow resistivity matter. Mineral wool with a density in the range of 30 to 60 kg/m3 has roughly the right gas flow resistivity and acoustic properties. Fiberglass with a density of 10 to 35 kg/m3 has roughly the right gas flow resistivity and acoustic properties. But nobody knows the correct density and gas flow resistivity of "Teknodens", so you will have to contact the manufacturer and ask them about the gas flow resistivity and other acoustic properties of their product, in order to determine if it is any use in this application, or not. If it is closed-cell insulation, then it is useless. If it is open-cell then it will only work of the properties are correct, acoustically. The STC-30 rating that I mentioned above as the estimate for your wall, applies only if you used mineral wool with a density of 35 kg/m3 and gas flow resistivity of roughly 10,000 mks rayls/m. If you used something else, then the STC rating will most likely be even lower. That's the reason we always say that people should design everything carefully, including the selection of materials, BEFORE they start building. - Stuart -

Stuart, thank you for all your observations and comments. I'm posting a pic of my wall, hopefully to make things clear. If I got it right, you mean that if i omit the inner mdf (the 8mm) maybe I have better results? I asked the company about the material i m going to use between the mdfs as inner insulation and said that its behaviour is similar to rockwool. They 'll send me specifications maybe tomorrow Very helpful information about the flexible channel installation (i ll skip the neoprene to studs) You are absolutely right about designing first and then building, but time is pressing! Waiting for your answers, thanks again!

If I got it right, you mean that if i omit the inner mdf (the 8mm) maybe I have better results?

Not really. The problem is that all of the materials you are using are too thin, too flexible, and have too little mass. What you have there right now theoretically has a resonant frequency of about 101 Hz, so it will not isolate AT ALL in that area of the spectrum. It only starts isolating at 144 Hz, and isolates reasonably above about 217 Hz. In other words, it does not isolate for the bottom three octaves, including the low end of human vocals. Also, my previous estimate of STC-30 isolation (based on IR-761) is too high, since that assumes a 90 mm air gap, but you only have 50 mm, so your wall will isolate worse than that. Maybe around STC-25, with luck. It is impossible to estimate better than that, since all of the standard research papers and tests are never done on such thin, light weight walls. I've never seen any formal testing done on materials only 8mm thick:: Normally, the minimum thickness tested is 13 mm, since any thinner than that is already known to be of no use for acoustic isolation, so I can only guess. The solution is to use much thicker, heavier materials on both sides of the wall, and a bigger air gap. - Stuart -

Stuart, I m afraid that i could change few things at that time. Of course, if there is no option left, I have to change a lot! I m thinking of putting thicker inner walls maybe 18mm as you suggested at first. If you have to choose, which is more wise to do, to thicken the inner walls, or to have bigger air gap? (the room is already small, so I have to do one of them) One last thing, if i put on the existing inner or outer wall, another 5cm of rockwool wrapped with fabric, maybe will make things any better? thank you

I m thinking of putting thicker inner walls maybe 18mm as you suggested at first.

You could replace the 8mm inner MDF with 18 mm MDF, and also add a layer of 13mm fiber cement board on the outside. That would give you roughly double the mass. The resonant frequency would go down to 75 Hz, and the wall would isolate above 107 Hz, and isolate well from 160 Hz.

If you have to choose, which is more wise to do, to thicken the inner walls, or to have bigger air gap? (the room is already small, so I have to do one of them)

Both! :) But to get a bigger air gap you'd have to take the whole thing apart and replace the 2x2 framing with 2x4 framing: That basically mean rebuilding the entire thing. Unless...:! You could use resilient channel on the inside. Take off the 8mm, put on the RC, then put on a layer of 13mm MDF. That would give you a large improvement in the exact same space as putting a layer of 18mm on the inside. Theoretically, that could give you STC-40 or maybe a bit higher. That's reasonable for a small isolation booth.

One last thing, if i put on the existing inner or outer wall, another 5cm of rockwool wrapped with fabric, maybe will make things any better?

That won't help with isolation, but you will definitely need that on the inside anyway, for treatment! You don't need to cover all the walls with it, but at least some of them. - Stuart -

Thank you Stuart I m planning to start from the inside at the moment and see the results (replacing the 8mm). I'm afraid a bit the installation of the RC. On the edges of the MDF (attached to the RC) I just put something like green glue I guess, right? If I dont go with the RC solution, do you think an MDF thickness of 18mm (inner walls) is enough or should I go for 20mm for better results? Also, on the existing 8mm mdfs, if i add another layer of 100mm mdf with screws, the result will be the same as of having a 18mm mdf thick or worse? diz

Progress report: I m going to put the thick (19mm finally!) mdf on the inside and amplify a little the ceiling, floor and door with another 12mm layer glued to the existing mdf So, now i have 12mm (outside) 5cm gap and 19mm (inside). Do you think is going to be ok? Any help would be greatly appriecieated!