Before remodeling started in January, the Queen's Room had boom and rattle when playing somewhat bassy music in the living room. The bass was sometimes even louder than in the living room itself, always louder than in the adjacent hallway, and the wall behind the closet would shake and rattle threateningly. My friend said that she wouldn't mind my playing my stereo, she is not disturbed by me playing loud music, she can even sleep through it. But I never demonstrated this for her, and I don't think this was what she had in mind. The Queen's Room seemed to attract boom and rattle like a magnet
The first remodeling rebuilt the wall on one side with two layers of 5/8 inch drywall--the thicker kind. In between those two layers were applied 4 tubes of Green Glue sound damping compound. The entire wall was rebuilt, even around the door, but only on the queen's room side. On the hall side, the drywall was merely caulked to the framing to make it rattle less, and I personally pulled off two hanging pieces of drywall that had cracked out of place (the cracked pieces popped out of one of two layers of very thin drywall the original homebuilder Rayco had used.
The result of that remodeling was that the wall became much more quiet sounding than the rest of the walls in the house. There was no rattle and considerable sound reduction in the queen's room itself. The wall inside the queen's room had a "rock of gibraltar" feel to it, deader than any interior wall I had come across. Even the hall side seemed much quieter than the other walls in the house when you knocked on it.
With all this, however, there were some disappointments. For one, you could still tap on the hall side and hear sound in the Queen's room. Despite the very damped feel, there was perfect coupling between the inside and hall side walls. Rob had added extra studs to the 24" oc framing so that the average stud was about 12 inches from the next one, and often less. All these studs turned the wall into a nice, if heavily damped, string telephone. The added solidity, however, was appreciated because now the whole house seems to creak less (though this does not appear to be a load bearing wall, it is attached to one that is).
But the biggest disappointment was that the bass boom had not disappeared. There was still a bass boom at 45 Hz and 40 Hz. A 45 Hz tone played in the living room would sound nearly as loud inside the queen's room, and much louder than in the hall right outside. Although the wall was stopping higher frequencies, it was doing basically nothing for the deep bass except delivering it to the queen's room with a slight boost, just as before, except without the rattling. I played a large set of tone and ultimately did a spectrum analysis that showed that most frequencies were being shut out of the bedroom almost totally. But just a sliver between 40 and 45Hz were not being attenuated, and in fact 45 Hz was being slightly boosted. The spectrum analyzer showed 45Hz more loud in the bedroom than at the listening position in the living room.
Now it turned out, as I had feared at the time, that Rob did the electrical wiring incorrectly. The electrician John Jones could tell this just from my description and photos of the boxes Rob left in the wall without access covers. John really wanted to fix that problem.
So I decided back in late March during the first days of the recent electrical work to go ahead and remove the hallway side of the wall, fix the electrical wiring, and also re-build that side into a fully soundproof wall as well. At the time I was also thinking of remodeling the hall as well. John Jones recommended his favorite contractor, said he was a genius, so I decided to have him do it.
John's recommended contractor Tom was slow to gain interest in the project, and wasn't interested in doing the floor part of the remodeling at all. (Now btw his son has expressed interest in doing that next month.) But he quoted a price at our first meeting and I agreed so the project went forward.
On Monday April 8, Tom took the wall apart on the hallway side. On April 9, John Jones and his son T.J. rebuilt the wall, replacing the wiring and moving the electrical boxes to the attic (which is legal). In taking apart Rob's work John found other faults which indicate it was indeed a very good idea that the electrical work was re-done. It was not only done badly it was done dangerously.
On April 10, Tom and his son rebuilt the wall with Green Glue Whisper Clips holding 25 gauge furring strips separating the framing from the new wall surface, aka decoupling, which is especially useful (and about the only useful thing) for low frequencies. Then two layers of 5/8 drywall were attached with 4 tubes of Green Glue in between the sheets. I remeasured at 45 Hz while they were taking lunch and found that there had been only a modest 2dB improvement. I had suggested triple drywall to Tom earlier, and he had seemed very skeptical, but when he came back he quickly quoted a price for one more layer of drywall and I agreed. I said I was hoping for another 2dB improvement, that would make it worthwhile. I was also hoping for a big win, that somehow 3 layers of drywall would be far better than just 2. I noticed that Whisper Clips have a "recommended load" of 3 layers of drywall and wondered if that was what they needed to get the decoupling working. I also read online posts by Ted White, a leading independent seller of Green Glue, and he says that 3 is much better than 2 layers.
After the third layer of drywall was applied, I measured another 1.5dB of improvement. It wasn't what I had hoped, but 3.5dB total improvement was something real, something to be appreciated.
Within an hour or two of all the contractors being payed, I discovered something truly strange. I closed the closet door in the queen's room, and the bass now dropped by 13dB more !!!
To be clear, I was measuring about 82dB in the living room, 78dB in the hall, and 81dB in the queen's room before the second remodel. (I never measured before the remodeling project...it was too horrible to measure.) At first it seemed like I was only getting about 3.5dB of improvement, making the level in the queen's room now 77.5dB. But now the level had dropped to 64dB.
Now I could play a very aggressively bassy album, Bass Ecstasy, in the living room, and it could be easily tolerated in the queen's room. Only when I played Bass Ecstasy loud enough to rattle the walls in the living room was it more than noticeable in the queen's room.
I consider this a big win, more than what I had realistically hoped for!
But it comes with a question. Perhaps I didn't need to rebuild the wall at all to get this result. All I needed to do was...close the closet door.
Unfortunately, I never thought of closing the closet door before, and I had never done any relevant measurements. But I have several observations that suggest that what's going on here is not simply the closet door operating by itself, but an interesting acoustic interaction. My current belief is that if I had rebuilt the hall side of the wall at least a little bit, I would not have achieved this result. I even suspect that it was specifically the 3rd layer of drywall that did the trick, though I have no evidence for that. I think that the bedroom wall is now so heavy, with 5 total sheets of drywall on it, that vibrational energy takes a different path, ending up shaking the end of the house, where the closet is, rather than the center, where the bedroom wall is.
I do know that the situation was far worse in the bedroom before the new closet was even built. I also vaguely recall playing music and walking near the new closet after it was built and thinking it was a quiet place, not the noisy place it is now. So what I have is one solid fact and one vague recollection which support my current theory. I also have a lot of pretty good general knowledge, that bass is very difficult to dampen and passes through most modern wall assemblies without much attenuation and in fact with amplification at resonance regions like 45 Hz. I also do have the measurements I made which clearly showed improvement, though smaller than hoped, as additional sheets of drywall were added. If the whole problem were that I should have simply closed the new closet door (after the new closet had been constructed...and some dampening and extra drywall already added) I should not have seen 3-4 dB of improvement from the 3 new sheets of drywall achieved. While making those measurements, I did nothing that should have affected the closet door (it's been open for weeks) if that was the sole source of improvement, or even the primary noise source.
Another theory, discounting my vague recollection of hearing less bass in the closet just after it was built, suggest that if I achieved 3.5dB of improvement by eliminating some noise source, it should have been a major contributor of that noise, approximately equal to all other sources. Then, reducing some second source of comparable level (the closet) there is no limit to how much it can fall after that, if those are the only two sources. So it couldn't be that the closet was a much more important contributor of noise originally, it could only have been roughly equal. By this theory, if I had closed the closet door before the final wall construction, I would at most have seen about a 3-6dB change. So if I achieve an 18dB change in total, at least 12dB of that change must have come from the new wall.
So depending on which theory you believe, I achieved somewhere between 3 and 18 dB change by doing the latest round of wall soundproofing, at the crucial resonant boom frequency of 45 Hz.
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