I should have done this long ago. The photo above was made after fixing the bookcase rattling by moving bookcases away from leaning on wall and removing junk on the left bookcase so it could be filled nicely with books and magazines, almost all the ones that were originally there. (I never got around to photographing the bookcase before the change, it was almost unimaginably stuffed with random junk.)
The two big bookcases at the rear of the living room were rattling on deep bass tracks. I'd noticed the rattling, and assumed it was coming from the walls. But the walls usually were not doing the rattling, I discovered on Sunday. It was the two big bookcases at the rear of the living room, or something on them.
I started clearing off some of the bricabrac at the top of the center bookcase, including my Fisher MPX-100 unit which I still haven't gotten around to testing. Then I noticed a serious issue. Both bookcases were actually leaning against the rear wall of the room. That leaning meant that any wall vibration (and there is plenty of that) would shake the bookcase and all of its contents. The bookcases are very heavy, but some of the nick nacks were light and flimsy, like a still-in-box Barbie Ferrari (I'm both a model car collector and a doll collector, and a red Ferrari is the iconic car of the iconic doll). To mitigate bad karma, I also had a remote control Ford Focus on top of that. BTW, I'm taking the Ferrari down for now, and it's been replaced by a removed-from-box remote control Tesla model car (barely visible on the top at the left edge of left bookcase in front of a white doll).
The corner bookcase would be very tough to clear out because it's jammed in partly behind my keyboard table. But I managed to get it off the wall mainly by shifting the magazines in the bottom two rows toward the front. That got it about 5mm away from the wall at the very tip. Fortunately I think that's enough to handle even the worst vibrations of wall and bookcase. Or at least it shouldn't happen often.
The left bookcase, which was not just packed but stuffed with books, VHS, cassettes, junk, magazines, etc., was unloaded down to the bottom two rows, then shifted out from the wall by an inch. As I was doing this, I was rocking the bookcase and trying to determine where it would be absolutely free of rocking back to the wall. I think now I moved the bookcase out too far, it is more than an inch out further out than the other bookcase. In fairness, however, I would move the other bookcase out another half inch if I could, and the center bookcase needs to be almost this far out to make the electrical outlet near the floor accessible. I don't plan to change it now, but I think I moved it out about 1/2 inch more than necessary. I think when I added back all the books, and moved the magazines out to the edge, the bookcase leaned forward more than I was planning.
Keeping the bookcases from leaning against the back wall fixed the #1 rattle problem in the room. Rattles take away greatly from the sense of dynamic range. Of course when you fix one rattle, other softer rattles become apparent. Another one that needs fixing is the door of the air handler for my HVAC system, which rattles on certain loud bass notes.
Now it has been written (by an acoustical treatment maker) that bookcases do not make for very good sound absorption or diffraction, which most rooms need a lot of. That may be true (and I'll think about it some more later) but when a bookcase has a rattle, it's like negative sound absorption. Very negative to the listening experience. Getting back to No Effect is a big step forward from that.
To be free of rattles, a bookcase should not be leaning on or touching anything but the floor. Otherwise, when the surface it is touching moves because of acoustical vibration, and/or the bookcase itself moves because of acoustical vibration and it moves away from the surface, there may be moments where the pressure momentarily releases, and you get a rattle between the two. Also, the surface may cause the bookcase to shake a bit, causing stuff, particularly the kind of flimsy stuff I used to have on top of my bookcase, rattle.
Now that neither bookcase is touching a wall, it is incredible how much the wall vibrates during deep modal bass notes, but the bookcase seems perfectly still. The bookcase panels may themselves have some resonances, but they would be at higher frequencies that don't so much cause rattling, and the bookcase is made of particle board and heavy paper, which are very lossy and vibration damping materials.
If I have increased the sound level that would cause loud and annoying rattling by 10dB, it is much like (or perhaps even better than) increasing my system dynamic range by 10dB, or almost like reducing modal resonances by that much (which is very hard to do).
I discovered the need for this when testing 1 vs 2 plugged ports in my right subwoofer. I decided NOT to bother with measurements, but to go ahead and make the change to 1 port because it is well known to be better (see earlier posts), but why not listen? As I was listening to the 1-plugged-port case, which makes bass slightly louder down to about 15Hz, I noticed the back-of-room rattles. I got the idea (though not confirmed) that I might be hearing worse rattling with 1-plugged-port than with 2-plugged-ports. At the same time, I also thought I could hear the improved dynamic range of 1-plugged-port. So I thought to myself, why not see if I can fix the rattle?
Now actually it seems to me that a bookcase can have some desireable bass trapping qualities (if not as good as engineered bass trap). To get this, you first need to abolish rattles, because if it rattles it is worse than useless acoustically. Then, the books, should be pulled out to the edge, to get them as far away from the wall as possible. Heavy books or magazines should also fill as much of the frontal area as possible. Behind the books, there will be trapped air, and behind that, my bookcases feature a lossy paper back, which serves a bit like a panel or membrane resonator. The trapped air behind the books is squeezed in and out by vibrations of the panel, and the released energy is lost moving the books and bookcase panels. This does in fact operate like a bass trap. An engineered bass trap would likely use fiberglass in front instead of books to do the lossy absorbing, but would otherwise be similar.
Actually, the bottom two shelves of my bookcases are filled with very heavy paper Stereophile and The Absolute Sound magazines...probably better than books for a bookcase acoustical damper.