Flanking noise through shared brick walls

Hello all, I've been trying to locate some information or even actual numbers on potential flanking noise between two rooms through a shared brick wall. I plan to build two rehearsal rooms beside each other. My original plan was this: -Build each room independently using steel stud construction and two layers of 5/8 sheetrock on walls and ceilings -double doors as per Rod's book -one foot air gap between the frames -floor would be shared but as a concrete slab at grade, I do not anticipate unacceptable flanking At some point, I started wondering about the effects of letting the rooms share the exterior building wall on one side, which is brick and is around two foot thick. I wondered if I did this and merely ran the partition wall up to the exterior wall and caulked the small gap, as in my diagram, would the shared, extremely thick brick wall produce significant flanking? Searching online hasn't produced much hard information in this respect (although F Alton Everest has some figures on attenuation of longitudinal waves through brick and other materials which are a little scary sounding in the abstract). I know that flanking is a crucial issue in any build where sound isolation is needed and decoupling is the much preferred method, but I'm wondering if an extremely thick brick wall such as this would significantly lower the flanking in the same way that my concrete floor at grade does. ok, it's not being further dampened by any dirt on the other side, but I'm wondering if its sheer mass reduces flanking to a point where it's not much worse than the floor? This is kind of a thought experiment at this point for me, I don't need to save money on materials or anything. I would definitely appreciate the extra square feet that using the existing wall gives, but I can also live without them. So I'm wondering if anyone has an opinion on this topic or can point me to some hard information? Or is it just a case that there are so many variables in a case like this that it's impossible to predict the level of flanking that might occur? I should say that I'm purely interested in flanking between the two rooms as neighbors are not an issue at all. Thanks in advance for any replies.

Why don't you just add a MS/GB wall in front of the wall. This way you'll also end up with a 2-leaf (double wall) construction which is good for isolation. ;)

You say it's an outside wall, right? With the nasty, noisy outside world on the other side? Regardless of flanking, I think I'd still put up a complete inner leaf, for that reason alone. A single leaf is a lousy isolator, and it is governed purely by mass law. So unless it is REALLY massive, high density, and very stiff, then I think I'd still do the full two-leaf thing. My $0.02. - Stuart -

Soundman2020 wrote:

You say it's an outside wall, right? With the nasty, noisy outside world on the other side? Regardless of flanking, I think I'd still put up a complete inner leaf, for that reason alone. A single leaf is a lousy isolator, and it is governed purely by mass law. So unless it is REALLY massive, high density, and very stiff, then I think I'd still do the full two-leaf thing.

I was trying to get people to disregard sound on the other side of the wall as a factor, to be honest. The location is perfect in the respect there is nothing and noone nearby to bother outside and nothing outside to bother the people inside. And the exterior wall is pretty damn big, I assume it's dense, if you consider brick to be dense. So leaving that factor aside what would your answer be?

Ro wrote:

Why don't you just add a MS/GB wall in front of the wall.

I don't know what that stands for and it has too few characters to search for, can you elaborate?

The location is perfect in the respect there is nothing and noone nearby to bother outside and nothing outside to bother the people inside. And the exterior wall is pretty damn big, I assume it's dense, if you consider brick to be dense.

So it never rains where your building is? The wind never blows? Nobody will ever arrive or leave in a car / truck / motorbike / horse? No airplanes will ever fly overhead? No animals will ever make a noise? I wish I could live in such a remote, noiseless location! :) And this is in New York? Seriously, here are some real numbers for you to consider: A single layer unrendered brick wall will give you an STC rating of around 39. Plaster it generously on both sides, and you should get an STC of around 40 - 42: You say that your wall is two layers of brick, but it is governed purely by mass law so the extra mass from the second layer only gains you 6 db (theoretically): So you have maybe 46 or so dB of isolation (maximum!) to the outside world with your wall (bare with me... I know you don't think isolation to the outside is an issue, but humor me for now...) (NOTE: It isn't really correct to just add 6 db to the STC rating for the doubled mass, since in reality you won't get that much due to the way STC is calculated, but I'm just giving you ball park figures here for illustration.) So for argument's sake, call the isolation of your brick wall STC = 45. In contrast, your double-stud plus two-layers-of-drywall-on-each-side plus gap-filled-with- thick-insulation wall with a one foot gap, should be getting you around 63 - 65, at least, probably better. So now consider: Even totally disregarding flanking through the wall itself, sound going out through your brick wall from one room is only attenuated around 45db, and low frequencies particularly will be less than that. So, that "escaped" sound is now outside in the real world, where the low frequencies will easily wrap around and come back in through the rather poor 45 STC rating of the wall in your other room.... How much gets back in, I really can't say, but some will. If you get someone to beat a kick drum in your bedroom, close the door and go listen in the nest room, how loud do you hear it? Now move away by one more room, and close the door. How loud do you hear it now? That will give you a rough idea of the amount of extra reduction you get from a second wall (OK, that isn't really a valid illustration, but you get the idea.) If you have windows in those walls, then all bets are off, as the isolation will be even worse: windows are weak points. So, I guess the basic question is: Why would you bother building an STC 65 wall between your two rooms, when you have an STC 45 wall to the outside, and another one coming back in? (And no, STC 45 plus STC 45 does not equal STC 90!). The point is, I think you'd be wasting money by building a good wall between the rooms, but allowing a path through two lousy walls. And that's without even considering flanking! Here's a test for your flanking: have someone stand at one end of the wall, and tap on it gently with a hammer. Walk right to the other end of the wall, put one ear up really close to the wall, and stick your finger in your other ear: can you hear the hammer taps transmitted trough the wall? There's your answer. Maybe the acoustic experts here (I ain't one!) can answer your flanking question much better than I can, but personally, I don't see the point in building a great wall when there is a path around it. If space is an issue, then my suggestion would be to build two inner leaf walls up against that brick wall as "inside-out" walls, with just a 2 or 3 inch insulation-filled air gap or something like that. It will only take about three or four inches off your floor space, but it will be a full MSM wall, and will get you MUCH better isolation, well over 50 STC for sure, and probably around 60. But that's just my in-expert opinion. Take it for what it's worth! - Stuart -

Flanking through the brick wall will affect the isolation between the two rooms unless the brick wall is really thick and really stiff. I I would just build the extra walls as the cost is only a little more. You may not need double doors on each room.