John Sayers’ Recording Studio Design ForumAn archive of johnlsayers.com, 2003–2022

MDF, Plasterboard and aiming for a realistic STC for Drums

Started by Johnrg on 8 May 2012. 3 replies.

Originally posted at johnlsayers.com, topic 17568.

Johnrg
Hello All, I,m back after a long break and just about to start the design and construction process of my second studio. I have a couple of quick questions to start the ball rolling: (1) Here in New Zealand MDF (Medium Density Fibreboad) is alot cheaper than gypsm board. The plasterboard producer here is basically a monopoly where there are lots of MDF producers. For example: 24mm MDF 15.2kg/m2 density $28.88per sheet 16mm PlasterBoard 13.7kg/m2 density $57.38per sheet Apart from the fact that plasterboard is a little easier to cut, fix etc is there any reason not to use this for walls and ceilings? From these numbers as you can see the density is better and its half the price. (2) What would you guys consider the minimum STC/TL to aim for to enable rock drums to be recorded and played in a residential neighbour? My old studio had a STC of about 58db from the live room to the outside and this was no where near enough to use the studio for drum tracking whenever I wanted. What is considered a minimum STC/TL aimming point for commercial designers in the area. 70db? Thanks JohnG
Soundman2020
MDF and plasterboard actually have very similar densities (both around 700 to 800 kg/m3), so from that point of view, either will work for isolation. There might be minor difference in rigidity (stiffness) that could affect low frequency isolation, but probably nothing to worry about. On the numbers issue: STC is a lousy system for measuring studio isolation in general, and even worse for drums, since it does not even consider the low end of the audio spectrum. STC is fine for talking about isolation in the mid range, such as speech isolation between two bedrooms or two offices, but not much use for music. Especially for music with heavy bass content. In addition, STC isn't a good indicator anyway: a room rated at STC-58 might be great for drums, or it might be terrible. Two walls that both get a rating of STC-58 can sound very, very different in reality, in terms of how well they isolate, simply due to the method used to calculate STC. There could be large differences in some frequency bands, perhaps around 10 dB, that would be hidden in the "defects" and the "adjusting" of the data to fit the curve. Transmission Loss (TL) is a much better method for measuring studio isolation. So, let's talk about TL from here on, not STC. The question about "how much is enough" is somewhat relative. A drum kit can easily put out 115 dB, so that's not a mystery. The mystery part is "how quiet do you have to be?". For some people, getting their drums down to 70 dB outside might be more than enough. For others, not even reducing it to 40 dB would be acceptable. First and foremost is the legal issue: What level do you have to hit to keep the cops away? Get a copy of you local municipal noise regulations, and see what level you are legally required to comply with. The details vary from place to place. Most likely it will be higher during the day, lower at night, and it should also tell you where the level is to be measured (might be the property line, might be the location of the receiver), as well as how to measure it (A weighting, C weighting, long or short response, peak or average or Leq, etc.). Hopefully it will be A weighting, long response, average: that favors drums greatly. C weighting, short response, and peak would be bad news. C weighting is more sensitive to low frequencies (read "kick"), and both short response times and peak metering are sensitive to percussive sounds (read "snare" and "toms"). However, some regulations also have some "fine print" in there, that overrides the numbers, and adds a phrase to the effect that all "objectionable" noises is also not allowed, regardless of the measured level, or some wording along those lines. If you have a clause like that, then it doesn't matter what numbers you get, since pretty much anything could be considered "objectionable"! But beyond the strictly legal issue, is the issue of your neighbors: How close are they? How are the houses oriented with respect to each other? What part of their house faces your house? If your drum room door is 3 feet across an open lawn from their newborn baby's nursery window, that's a bit different from your drum room facing their garage 100 feet away and with a huge wall in between. So it would be nice if you could just say that 73.8 dB of TL is what you need, but that just isn't realistic. There are many factors to consider. There's even the issue of your drum kit itself, and your playing style. And another biggie, is the room itself: How is it built? How much can it be modified? Is the floor a concrete slab on grade, or a wooden deck raised above the ground? Etc. Your best bet is to get a sound level meter, and measure how loud YOU are playing YOUR drums in YOUR room (just as it is right now), and also measure the ambient levels at potential problem locations, at the same time of day as you plan to play. The difference between those two levels will give you a rough idea how much isolation you need. With that number in hand, and knowing that your construction method will involve lots of MDF, you can then plan how to achieve that level of isolation using those materials. In other words, you can calculate how may layers of MDF you'll need on each leaf, and how much of a gap you need between the leaves, how much cavity fill you need, what type, whether you'll need to add constrained layer damping, etc. That said, 70 dB TL is probably a good target to aim for. That's not easy to do, but is feasible. The biggest issues won't be the walls themselves, but rather things like doors and windows, plus the HVAC and electrical systems. Those will be the critical weak points, if you are aiming for high levels of isolation.
From these numbers as you can see the density is better
Actually, in your case it isn't better! In reality, the density of the MDF is lower, which is surprising. The numbers you quote are surface density (mass per unit area), not true density (mass per unit volume): Surface density of 15.2kg/m2 with a thickness of 24mm is 633 kg/m3. Surface density of 13.7 kg/m2 with a thickness of 16mm is 856 kg/m3. So your drywall (plasterboard) is about 135% more dense than the MDF. That sort of narrows the price gap a bit, but even so, kg for kg the MDF still wins. However, it's curious that the density of the MDF is so low. You'd normally expect that to be over 700, hopefully closer to 800. Your drywall is also a bit more dense than normal, so I'm guessing that it is fire-rated drywall? That would explain the higher density (which is good!). And that brings up another point: fire. Building with MDF on wood studs might be considered as more of a fire hazard than with drywall, so that might affect your building permits, permitted construction methods, insurance premiums, etc. Worth looking into! - Stuart -
Johnrg
Hi Stuart, Thanks so much for the long reply. All very good points you have raised. I agree, once in the new room I'll set up the kit and rip into it with wife on the noise meter outside and see what we get. Perhaps a Marshall Gtar amp and bass head as well. The standards here are 49db during the day and 42 db at night. My neareast neighbours are about 70 meters away but the house with my sleepin kiddies is only five meters away so probably more of a concern. I checked the MDF data and it's 750kg/m3, so as you say close to the fire rated plasterboard. I think when I build I'll use both with perhaps the inside layer as plasterboard to finish better. Thanks again for your time and I'll keep you posted. Regards, JohnG