Help! Too much low frequency spill in new build.

Hi there, I recently had a couple of new practice rooms built. Unfortunately the isolation between rooms isn't great. The builders specification for the walls wasn't up to scratch so it seems and maybe construction methods were not suitable for the job. There is always going to be some spill when it comes to practice rooms but I have a little too much - particularly in the low end, as one would expect. The construction of the wall that adjoins the two rooms is as follows. Brick walls Wooden frame - affixed directly to wall - No air gap Here I think is the main problem - builders used basic loft type insulation - I requested rigid high density Rockwool insulation but builder said this would work. The plasterboard used was Knauf Soundshield - seems to me to be too thin - here is the specific board: http://www.wickes.co.uk/soundshield-boa ... vt/224657/ and the wall was plastered. I'm wondering what is the best plan of action to remedy this? Do I need to pull down the walls and reinsulate properly or is it better at this stage to just add mass to the outside of the wall? If so, what would be best to use that wouldn't cost huge money. This is not a tracking/live room for recording so we are not looking for anything like that level of isolation. Is the ceiling just as much an issue? In the room I'm really feeling the sound is coming through the common wall and the wall at the back of the rooms but I don't feel like the ceiling is as much the problem. Any suggestions are greatly appreciated.

Hi again, cbasman!

Wooden frame

There's your basic problem, in that second photo. The stud frame is clearly in direct contact with both the block wall, and the ceiling! So it is not decoupled at all. It also looks like the surface of the concrete blocks is not painted, they are not filled with sand or mortar, and there are cracks and gaps in the mortar between the blocks. There is so much that could have been done there to greatly increase the isolation. But I'd guess that the single biggest issue is that inner leaf is not decoupled from the outer leaf at all. Without that, you'll never get good isolation, no matter how much mass you add to it, or what type of insulation you use in the cavity.

Here I think is the main problem - builders used basic loft type insulation - I requested rigid high density Rockwool insulation but builder said this would work.

That probably is not making much difference. Also, you do not want high density insulation. What you need (for mineral wool type insulation) is a density of about 50 kg/m3.

The plasterboard used was Knauf Soundshield - here is the specific board: ...

That is only 12.5mm! The minimum thickness you should use if you want good isolation is 16mm. 12.5 is too thin, too flexible, and does not have good resonant characteristics in general. Finally, it seems that the inner-leaf is not sealed. There are clear gaps under the bottom of that drywall, where it meets the floor, and those should have been sealed properly with acoustic caulk. The fact that those are not sealed suggest that the sole plates of the stud frames are also not sealed. And one more big issue: you seem to have electrical plugs and lights coming THROUGH the drywall! You can't do that. If you make even one hole like that in your inner-leaf, then you destroy the isolation properties of the entire room. The inner-leaf must be a continuous, unbroken layer of mass, sealed airtight. There can be only one single penetration into the room for bringing in the electrical power, and that must be done very carefully. from that point, all of the electrical work should be done with surface-mount ducts, plugs, switches and light fittings. To be very, very honest, if that were my wall, I would take it down completely, and rebuild it from scratch. I would start by putting a thick layer of plaster/stucco over the concrete block walls, to ensure that they are well sealed and the cracks are filled, then paint that when it is dry. I would then build the new stud framing at least 3 cm away from the concrete block wall, and also not connect it to the ceiling joists above. The sole plate of that wall must be sealed to the floor with three beads of acoustic caulk (one down the middle, and one each side of that, about 2cm away). You don't show any photos of how the subfloor above you looked before the insulation went in, but that probably also needs sealing, and might even need "beefing up" with extra mass (you could use the drywall that you take off from the bad walls for that: cut it into strips that fit in between the joists, hold in place with cleats, and caulk around the edges. One or two layers done like that would be a big improvement). I would then put new ceiling joists across the top of the new stud wall frames, making certain that NOTHING touches the existing outer leaf, not even a single nail. Next I would put up one layer of 16mm OSB on all those studs and joists (while filling the wall cavity with 50 kg/m3 insulation), and finally with a layer of 16mm drywall. That's the correct way to do it, if you want good isolation. What they did there is not going to be any better than an ordinary house wall, since that is exactly how it has been built. - Stuart -

Stuart, Thanks for your very detailed answer! It explains a lot. Fundamentally, the build was flawed in that case. I'll have weigh up the options see where we stand but something will have to be done nevertheless. I appreciate your input. Cheers.

cbasman, As a matter of practice, small rooms will produce this low frequency issue. Stuart addressed all the things that need attention right now but there is going to be a build up in an untreated room due to a small size. If I might ask. If you are looking into doing this all over again would it not be better to make this a more spacious one room area rather than slicing it into smaller rooms? Either way, good luck and looking forward to your goal being "hit" :)

Space costs money! Thats the main reason I suppose. :D The rooms aren't that small. 23'x13'. After treating the room with corner bass traps, numerous ceiling and wall traps we had the rooms sounding pretty good for practice purposes.- on the inside! Thanks.