Hi Steve, and Welcome!
As I said it’s mostly acoustic bands that we record and even if they have a full kit the drummer isn’t really pounding it so they’re probably around the 90db range ( that’s a guess as I haven’t actually measured a live kit).
I think you are being just a bit optimistic there! Playing drums as quietly as 90 dB is really hard to do: most drummers I know couldn't keep it that quiet, even if their lives depended on it! Maybe soft jazz played with wire brushes, very gently...
Realistically, I'd suspect at least 100 to 110 would be a more accurate level, and 115 dB is common for a drum kit played hard. Or even more...
But as I say, if I get a pumping band in with a heavy hitter and amplified bass I’m guessing we get into the 100-110db range!
That's also on the low side: I'd say closer to 120.
Also, I'd suggest that you switch to measuring "C" weighting and slow response to get a more realistic reading. "A" weighting is fine for low levels, and is what most noise regulations specify (which is good, since it is less sensitive to bass frequencies!
). but for typical band levels, "C" weighting is far more realistic.
So maybe I’m paranoid about the isolation factor,
Being "paranoid" about isolation is a good thing, in my book!
OK, so assuming the worse: 115 dB inside, and you need 40 outside. That works out to 75 dB of isolation, which is a pretty tall order. Doable, but not easy: fortunately, you probably only need that "under 40 dB" level by the time the sound reaches the neighbor's house, and fortunately sound decays at 3 to 6 dB per doubling of distance (depending on the surroundings), so assuming that you set a goal of 35 dB at your neighbor's wall, 20m-25m away, and worse-case drop off at only 3 dB per distance doubling, if you can get your level down to around 50 dB at one meter from the wall, then you are in the ball-park (theoretically you'd get down to 35 dB at 32m, but allow a little "fudge factor", and I'm being very conservative with those estimates).
So 50 dB outside, and 115 dB inside is 65 dB TL. Much better. There are several ways of doing that.
Building stage: It's a free-standing construction and at the moment I’ve got the slab ( external dimension 9.3m x 6.4m/30.5ftx21.1ft). External walls are brick 110mm (4.3inches) wide with piers so it’s self supporting. Wall height is 2.7m (8.8ft) with a cathedral ceiling height around 3.3 m (10.8ft).
Great! That's an excellent basic shell to start with. Slab on grade with brick outer leaf bodes well for high levels of isolation. That alone should get you into the high 40's in terms of transmission loss.
Roof is zincalume (sheet metal)
Darn! OK, scratch the above: that takes you down to maybe low 20's, best case. Especially if it is mostly in drums and heavy bass.
... as it’s the only real option for me construction wise.
Does that roof already exist? From the way you phrase that, it sounds like you don't have a roof yet. So why is it the only option? Don't they sell 2x6's, plywood, and asphalt tiles in Australia any more?
I’m hoping I can get it constructed for $20-$25K (not inc gear) but I won’t set that as a hard limit as I’d rather have a better sounding room than a cheap one.
That is good news, and also bodes well for good isolation, and good acoustics. A realistic budget is the real starting point. We occasionally get folks dropping in here who want to build the next Abbey Road in their cube-shaped garage for US$ 50... (It's hard to explain to them that they are several zeros short on their budget...) But you are on the right track: So just take your best guess, multiply by any random number between 2 and 99, add in the national budget of a few small countries, and you should be about right with a realistic estimate!
My main concern at the moment is about the roof in terms of isolation. I’ve been reading through as many posts about the subject as I can find, but I still feel like I could use some pointing in the right direction.
Exactly. The roof is the weak link, from the point of view of isolation. As you already know, the only way to stop sound for a reasonable cost is with mass (lots of it), damping (truckloads of it), and decoupling (total, absolute and complete). Thin metal sheeting on a light framework qualifies as "none of the above". I'd really suggest that the roof strategy needs a re-think, if your goal is to achieve high isolation.
You need to build an entire shell that gets you 65 dB of transmission loss in all directions, and for all building elements. That means not just the walls and floor, but also the roof, doors, windows, HVAC system, electrical system, and everything else. In practical terms, that means that every part of your outer leaf needs to match the surface density of your brick walls. Brick has a density of around 1800-2800 kg/m3. Call it 2300, for simplicity. That works out to roughly 250 kg/m2 surface density (your wall is 110 mm thick). So every square meter of your outer shell should weigh 250 kilograms, theoretically. As you can guess, a square meter of tin roof weighs only a very tiny fraction of that. Even worse, it is very flexible, and will resonate mightily, given even half a chance. (Ever head the sound of heavy rain on a tin roof?...)
OK, so that density is exaggerating a bit: You'd only need such high density if you were going to rely purely on mass law. If you go to two-leaf MSM construction, then you can drop that by a factor of ten for each leaf, and get even better isolation. But we are still talking about surface densities of around 25 kg/m2 on each leaf. Your tin roof is still not on the radar!
MDF, drywall and plywood come in somewhere around 600 to 800 kg/m3, so you need two layers of 16mm plywood on your roof to get to that level of surface density.
The other option you could consider, if that tin roof already exists and you don't want to re-build it, is to do a (gasp!
) triple-leaf roof. In other words, build two leaves below the tin: In that case, you'd have to make the middle leaf three layers of 16mm "something", and the final leaf (your room ceiling) could be two layers. You'd be compensating for the third leaf with the extra mass on the middle leaf, plus larger air gaps. But you'd still need to damp that tin with something.
In a free-standing construction like this, do I need to achieve as much isolation with the external roof as I do with the external walls?
Yep! Especially if your concern is neighbors many meters away, combined with lots of bass energy (kick, toms, snare, bass guitar, keyboards, etc.).
do I need to effectively have as much mass on the exterior roof as I do with the exterior walls?
Not necessarily as much
mass, as I pointed out above: rather, you need as much
isolation, which can be achieved in other ways than mass alone.
Of course, if you were planning to have only a single leaf (no "room in a room"), then yes, you would need to have the same mass on the roof. Also the windows (how much does 2 inch thick glass cost?), and the doors (how much do bank vault doors cost?).
But regardless of the construction method, you still need to have the same level of
isolation designed in to every square centimeter of your studio. If you design the walls to give you 60 dB if isolation, but your roof only gives you 30, then your entire total isolation is 30, and you wasted a lot of money on building your walls to 60.
Think of it this way (you might have seen this before: I often use the analogy here): Sound is like water, and your room is like an aquarium (only upside-down: the roof of your room is like the floor of the aquarium). If you want your aquarium to hold water, then you cannot achieve that by putting glass on all four walls but only using paper for the floor! Nor cardboard, carpet, and old pillow, or scrunched up newspaper. The only thing that will work, is glass. Anything else will not hold water. Likewise, if you put glass on the aquarium floor, cardboard on one side, a piece of wood on another, paper on a third, and carpet on the forth, it ain't gonna work! Each of those "holds water" to a certain extent, but all of the water is going to gush out through the weakest side: the paper. And once the water is out, well then it is all over the place: splattering everywhere. It doesn't just exit through the paper and carry on traveling that way in a straight line! rather, it splurts out all over.
Same with your room: If you do your walls and roof to different levels, then sound will just take the easy path out: through the roof. And once it is out, then it is out all over, not just straight up. the difference with sound is that different frequencies will "splatter" in different directions: the highs will, indeed, mostly go straight up and only bother the birds. But the lows will wrap around the building, and spread out in all directions with equal intensity, just like water pouring out of the aquarium onto the floor: the puddle expands equally in all directions.
So with a weak roof, your drums and bass will easily be heard over at your neighbor's place.
Add to the above the facts that low frequency sound is the hardest to stop anyway, and travels the longest distance with the least attenuation, and you see the problem.
Have you ever walked past a night club, disco, or hotel ballroom where there's some loud music going on? What do you hear? "Booom-Booom-Booom". You hear the bass, not the singers. You hear the kick and snare, not the crash and ride. You hear the low growl of the electric guitars, not their high shriek. And it doesn't really matter where you are in relation to that club/disco: you can hear it all over, not just outside the front door: it still sounds loud 20 meters down the street.
Low frequency sound is always hard to isolate, and the only way of doing it successfully is with an "all or nothing" fully enclosing shell, that isolates to the same level all around. Including the roof.
So I'd really consider things you could do to improve that roof. Possibly replace it, or alternatively just plan to use other methods to compensate for the deficiency that it represents.
My $0.02.
- Stuart -
)However, now that we’ve bought our own place it’s time to set up a dedicated studio.
.