The party wall is definitely a cavity wall. Brick built with a cavity of 5 centimetres. No insulation in the cavity.
OK, you should be getting reasonable isolation from that, just from the mass of the bricks. I'm assuming that there are wire ties joining the bricks across the cavity? If so, that's not such good news, but it is common practice, so you are probably stuck with it.
The joists run front to back. The ceiling is a thin layer of plasterboard (drywall?) with wood joists above and wood floorboards. There is no insulation within the ceiling at all
That's what I suspected, and it is a weak link. Depending on how much isolation you decide to go for, you might have to attack that. Ideally, you would need to take off the drywall (plasterboard), seal the flooring above air-tight (with acoustic sealant), perhaps add some mass to that subfloor in between the joists, and then add insulation plus your final inner-leaf ceiling (more on that later).
When I said in the original post, "build new walls" these are the isolated walls of the inner room. We were hoping to build two sides of a room-in-room, and leave the exterior wall and inner wall untreated. So on the plan, we would build in front of the left and top walls, leaving the right and bottom walls as is. Would this be a terrible idea? I'm rather fearing it will be. Our logic (?) being that we would spend more to do the two walls better rather than doing all four just adequately.
Once again, it depends on how much isolation you are aiming for, but if you want decent levels, then think of it this way: Sound is like water... it takes the easiest path out. If you have a room with three great walls and one lousy wall, the sound will take the "lousy" wall path out, totally ignoring the "great" walls: Extend the water analogy to a fish tank: if you want to build an aquarium, can you do it with glass on just two sides, a piece of wood on the third side, and a piece of carpet on the fourth side?
:) Even though the glass offers great "isolation" to the water, the other paths don't, so the water will leak out through the wood and carpet. And once water is out, then it is out, splashing around all over the place. Same with sound: if it gets out, then it is out in the open and can be heard all around, even on the sides with the "good" isolation. That is more true for low frequencies, but pianos certainly can put out some pretty low frequencies, way out there on the left end of the keyboard...
So that's the issue: doing two walls great and two badly is probably worse than doing all four to a mediocre level. And in fact there are six "walls", not just four: sound travels in three dimensions, so the floor and ceiling also count.
Then there is also the issue of windows and doors: You can have the best isolated room on the planet, but if you open the window, it no longer has any isolation! So airtight seals are part of the game plan for isolation. If air can get through the gaps around your doors and windows, then so can sound. To understand this, go sit in your car in heavy, noisy traffic (hopefully with some idiot playing his radio really loud!) with all your car windows closed, and the vent closed, and listen. Now open just one window a tiny crack, maybe 1 mm or so. Listen again: Notice the large difference in sound levels? Just that tiny little air gap, only a few mm square, negates the entire sum total of all the isolation offered by the entire rest of the car. All those tons of steel and glass are worth nothing next to that little tiny air gap. So sealing is critically important to obtaining good isolation. In fact, just sealing all the air gaps in your room right now would probably make a substantial difference to your isolation! This is often the least thought about solution, but the most effective.
All that is just to say that your doors and windows will need special attention, as will your electrical system: Correct me if I'm wrong, but if I recall correctly, houses in the UK are wired through metal conduit embedded in the walls, running from every switch and plug back to the main distribution panel? And conduit is, obviously, an open passage for sound: it is just air surrounded by pipe. So that needs to be dealt with too. (There are methods for handling that, simply end effectively.)
The window is on the bottom wall of the plan. The picture of the piano in place was taken through it.
OK, got it. That looks like an "operable" window, meaning that one or more of the panes can be opened. Correct? If so, that needs to be sealed up air-tight, for the same reasons as above. Liberal doses of acoustic caulk should take care of that, and maybe a couple of well placed nails or screws, plus removal of any mechanism. If you are aiming for high levels of isolation, you might also need to replace the glass with thicker laminate glass. What type/thickness of glass is in there right now? Is it double-glazed, or just single? I'm hoping you say "single, and thick"...
When in their downstairs room that directly adjoins our studio, the sound seems to come from high up in the wall. When in their upstairs room, the noise seems to come from below (obviously).
Ouch. That's what I feared. There's some form of flanking going in through the walls somewhere. Do you happen to have electrical outlets on both walls (yours and theirs), for example? Air vents? Chimneys? Something similar? But more than likely it is where the two upper floors meet in the cavity walls. I wonder of the floor joists/beams run all the way across? It would be hard to find out for sure, but it would not surprise me that you are sharing the same floor support structure, through the walls. You might need to make a small hole in your ceiling and take a look at how the joists go through: there might even be holes there.
These are using the dBa setting on the meter.
dBc would be better for this type of measurement. Yes, dBa is probably what your municipal inspectors would use, and probably what the regulations talk about, but C-weighting is more useful, since it gives you flatter response in the low frequencies, which is what you need to know about.
I was quite surprised how little extra dB was generated for a fairly clearly audible sound.
Exactly. That might be due to the A-weighting, which is less sensitive than your ears are to low frequencies.
Your readings are rather interesting, and I suspect that they would be a bit different if you switched to "C" weighting on your meter. You seem to be getting around 30 dB of isolation within your own house, and around 50 to your neighbors house, which as actually quite good, and might even meet legal requirements. But "legal" is not necessarily "neighbourly"! You should be able to improve on that a bit with your "room-in-a-room" plan, but it gets harder and harder to make a difference as you go up the scale. Decibels are logarithmic, so increasing isolation from 30 dB to 40 dB is easy: but increasing from 30 to 50 is ten times harder, since you need to block ten times more energy. Increasing from 30 to 60 is one hundred times harder, and increasing from 30 to 70 is one thousand times harder. You get the picture. I'm not trying to scare you! Just pointing out reality. You should be able to get another 10 dB of isolation, realistically, and probably more, but I would not expect to get another 30. That said, 10 dB of isolation equates to the sound level dropping by half, subjectively, and 20 dB would drop it by another half, so your neighbors would perceive the level as being about one quarter of what it is right now. That would be a realistic expectation.
Fortunately, you are starting from a very good basis, and I suspect that just dealing with your weak spots might make enough difference. So before going to the expense of the "room-in-a-room" thing, I would suggest attacking your ceiling, as that seems to be the the biggest weak point from what you say. If this doesn't get you as much result as you need and you still have to go for "room-in-a-room", then it wont be wasted effort, as you would have needed to do it any way.
Here's what I would do if that were my room:
I would start by taking off your existing thin ceiling drywall (plasterboard), and getting a good look at what you have up there, especially where the floor joists above you meet the party wall. I would pay a lot of attention to that, to ensure that things are just as snug and air-tight as possible. Maybe add plenty of mortar all around, to get mass and seal up any air gaps, cracks, poor workmanship, etc. Then paint that entire area, all over, with a good quality paint that penetrates well into both wood and brick, to ensure good air seals (especially if there is bare brick / mortar up there: brick and cement are porous, and need sealing).
I'd do the same around the full perimeter of the room, making sure that all of your walls are completely sealed up to the subfloor, so that you have a hermetic outer "leaf" around you.
Then I'd carefully seal the underside of the floor above, filling all the gaps and cracks with acoustic caulk, and also add mass to it, by cutting strips of 16mm drywall to fit in between the joists, hold it in place with cleats nailed sideways into the joists, and seal around the edges with caulk. Add a second layer of drywall like that, if you want good isolation, and if the structure can handle the load (check with a structural engineer, to be sure).
With that complete, I'd inspect every nook and cranny around the entire room, and pump oodles of caulk into any suspicious look areas. If you even suspect that there might be a crack, then seal it. And even if you are totally convinced that there is no crack, then seal it anyway, just in case...
Your electrical system needs special attention too: If you do have conduit, then cut the power, take off the cover plates on all your switches and sockets, plug the end of the conduit with a piece is insulation rolled up and forced down the pipe a little way, and put a dab of acoustic caulk over the end. That way, if you do need to do electrical work in the future you can remove the caulk and pry out the insulation, then re-seal the same way.
Next, your windows: I'd seal them as outlined above. Open the operable panes, lather caulk in place, close then, latch/nails/screw/bolt them in place so they cannot move, and remove all hardware, such as handles or external mechanisms, and caulk the openings where they were.
Doors: replace them with solid-core wooden doors, and run good quality rubber seals all around the edges, top, both sides AND BOTTOM. The gap at the bottom of the door needs to be sealed too: You can buy proper acoustic threshold seals to do that, that drop down into place when the door is close and rise up as you open it. Zero International make a number of good devices for sealing doors.
With all that done, repeat your measurements as you did above, and see how you are doing. You might have gotten where you need to go! If so, then replace your ceiling by putting Resilient Channel across the joists, filling the cavity with insulation, and carefully hanging a layer of 16mm drywall, or two layers, if you are paranoid. Done!
But if you are still not happy with the isolation, then the final step is to build that "room-in-a-room". And the good news is that all of the above has already completed your outer leaf properly, so you only need to do the inner leaf.
With a bit of luck you won't actually need to do the "room-in-a-room". Hopefully, just fixing up the things outlined above will be enough, but if not then none of that is wasted, as you would have needed to do it anyway for the "room-in-a-room".
So that would be my approach. First ensure that your existing outer-leaf is as good as it can be, and if that isn't enough, then carry on with the original plan. Either way, use the forum to document what you are doing, post photos and descriptions so we can sort of be "looking over your shoulder" as you work, and maybe spot stuff you didn't notice.
I'm looking forward to seeing how this works out!
Oh, and Merry Christmas! A couple of days late, but better late than never...
:)
- Stuart -
