Bass traps in ceiling cavities

Hey guys, I've just started construction on post production / mixing studio up on the Gold Coast and am wondering about the best way to proceed with my ceiling. Studio fit out has a budget of $15K (not including gear) and the floor is in and the wall frames have been installed. The studio is out the back of a school auditorium with a large NEXO line array PA system (so isolating from bass noise is important) The existing room shares one wall with the auditorium, and above the existing room is empty ceiling space for air con etc. I've done a rubber mounted floating floor on the concrete slab for each of the studio rooms, but am wondering how to do my bass traps. I'm keen to have an angled ceiling to avoid it being parallel with the floor and was hoping to use any cavities as potential bass traps. So my questions are... 1. Which out of the 2 side view diagrams would be more effective in terms of bass absorption? 2. What kind of absorber would you suggest? (Acoustic hangers, Heaps of rock wool? etc) Thanks guys and Merry Christmas! Sam

Hi there, I'm not an expert but are your wall studs already built? If so it looks like the outside one has a leaf on each side of the studs then you have an interior stud with a leaf, which makes 3 leaf's, this is always advised against as it will give you less isolation than only having 2 leaf's. Can you not strip the middle leaf of the outside stud?

A little puzzled why I ended up posting the same thing 3 times :?

Hi Rougejackal, thanks for your reply. To be honest, this is something i may have overlooked. Some of the new studio studs have been installed but i still have access to the middle leaf so it could be removed. The wall thickness of the existing wall is about 100mm and the new wall (including the air gap) is 108mm. I also don't have the option to add a second layer of sheet rock to the outer most leaf. Can anyone advise if there will be a substantial benefit in removing the middle leaf? Cheers, Sam

would really help to see pix of the actual areas in question, along with a diagram clearly showing the current walls, materials, dimensions. As mentioned, anytime a 3 or more leaf issue comes up on the forum, it is important to address this while you can. Give the mods clear details so that they can advise properly.

what thickness are the boards on the studs? If 15mm or better is it possible to take of the middle leaf and use as your first inner layer on your inside studs to save yourself some money? You have posted that you need isolation from bass noise and have also mentioned that you cannot alter the outside leaf, you will need to decide how much isolation you need before thinking about treatments in the room, as treatments in the room will not help your isolation for your neighbours outside of the room. As I previously posted I am not an expert so I will leave it to one of the mods to have a chat but will read your outcome with interest, merry christmas Sam Carl :D

"Can anyone advise if there will be a substantial benefit in removing the middle leaf? " Yes. The common term is "three leaf assembly" or like in your case a coupled three leaf, same thing different effect still poor performance. What happens is what you have in a mass/spring/mass (typically known as a double barrier assembly) an air cavity, one air cavity. This air acts like the "spring" (think of a shock absorber) which when sonic pressure is placed on the one side the air interacts with the opposite side. Basically "slowing" the penetration down of sound according to the resonance frequency of the assembly. That just means that the frequency level is raised somewhat rather than reduced. The mechanical aspects of the assembly, e.g. the panels and the framing help to add the rigidity, the insulation helps to damping the panels (reduce the interaction of the sound) from creating even more noise once introduced into the interior of the cavity i.e. the spring element. A typical three leaf has 2(two) air cavities/springs. The typical three leaf is much like your current situation in that you have the two springs that are smaller than the air cavity would be if it were only one space. So the multiple springs not only creates an acoustic hole that would not have otherwise been there but also cuts the ability of the assembly to isolate the frequencies it would have been better at isolating if it were one spring. You could open up the air space(s) in a three leaf and get it to perform better but at what cost? Real estate is one, additional material/labor is the other. So it is common to recommend to remove interior existing barriers or be prepared to open up the air space to a couple of feet or better.

Thanks Carl, RJHollins, and Brien. Really appreciate your insight. I've attached a to scale cross section of the wall. At this stage, only some of my new studs have gone up so i still have access to the middle leaf. I can see from what you've said, and from searching the net, the 3 leaf design is not going to work well. Q. Do i need to remove the entire middle leaf, or would putting enough holes in it be sufficient? Also as mentioned, because the room is basically an old store room inside of a school auditorium complex, the outermost leaf of my walls are either a classroom, or part of the auditorium's 30m wide rear wall. because of this use, I don't have the option to add additional layers of sheetrock to this leaf. Q. Do i need to try and make the outermost leaf as air tight as possible? I've managed to seal the middle leaf but the outer most leaf has heaps of penetrations from the class room and auditorium. Thanks again guys, so appreciate your expertise on this. Happy holidays. Sam

I think i just found a solution for bolstering the outer leaf

Soundman2020 wrote:

In any event, you do not need to demolish the entire wall: careful removal of the sheet rock from the side of the wall that will face the cavity, is all that is required. Said "carefully-removed-sheetrock" can then be used to beef up the other side of that wall, from within, between the studs. Then you can proceed to build the inner leaf, on its own studs. And if you ever DO need to demolish your entire studio (a major and rather expensive undertaking in any case) due to moving to a new location, then all you need to do is put new sheetrock back on those original studs right where the original sheetrock was, then tape, mud and paint. Presto! Right back the way it was. Not a huge expense, and not even very complicated, compared with demolishing the rest of the studio first. - Stuart -

So can anyone shed some light on my original question regarding the ceiling cavity designs? :-D Sam