I should have known that using the preposition "behind" would get me into some sort of embarassing situation..
Sick, indeed. I went outside while Bassotronics was playing. Momma mia! The 11Hz and 16Hz bass notes could be felt as pressure on the ears and even were somewhat audible.
After all was done, I spent the next hour cleaning up the dust that had fallen from the ceiling, and picking up things that had fallen off numerous shelves.
I don't know what it is about a session like that, but the next day, I slept 15 hours. These sessions always have that effect on me.
In other news, that JBL 2403 I bought on eBay last month, and replaced the diaphragm on, well, I learned that it's the wrong diaphragm, but that there's a way to make it work. The pole piece on the 2403 is a paper thickness or two shorter than on the 2404, which the diaphragm from Sound Speaker Repair is intended as a direct drop in replacement.
Their bulletin says that you have to put masking tape on the face of the pole piece and then the inner ring will sit at the correct height. Failure to do this results in very low output because the diaphragm is being stretched tawt and is not free to vibrate normally. That explains why I couldn't get the highs out of the center channel so sound quite right. It also brings to memory the fact that I did find it a bit odd that the center ring was dragging on the phase plug as I screwed it back on after installation.
So Monday's project is to remove the screen, remove that center tweeter and redo the diaphragm install, adding the shim as required in the bulletin I just read. When I purchased these spare diaphragms in 2006, the invoice said it fit 2403, 2404, and a bunch of similar models. No mention that you have to do tricks to make it work properly in 2403 units.
In preparation for that, I installed sliding bolt action locks on the top of the screen frame, and removed the screw that was holding the screen to L brackets. Now I can slide the locks and remove the screen in seconds, without any tools. The locks slide in with positive action, and I've angled them downwards a bit, so gravity keeps them locked, so they can't shake loose.
Monday's list of modifications include adding a felt strip to the top of the screen wall frame. This will be to prevent the screen frame from rattling at low frequencies, which, under pure sine tone conditions, is audible. I also could find this rattle by hitting the top of the frame with my hand. A more difficult modification involves putting felt strips on the two center support rails inside the screen frame. Since the back is covered with light blocker (black Spandex and polyester), this will be quite a bit trickier to achieve. I may have to pull about a hundred staples along one edge, reach in and apply glue and felt strips with great care. Then I can re-tention the screen, which loosened up a bit after a few blast sessions and get out any minor waviness that have formed near the bottom.
I spent this evening adding mass to the wall behind the speakers. This wall is wood frame, dividing the furnace/laundry room from the theater. I put up some fireproof asbestos cement paneling, which is very heavy, about 100lbs a sheet, on the furnace side of the wall, from 48 year old stock I had in the back yard. This was supposed to be done before the furnace and hot water heater were installed, but things didn't work out as planned. Doing this after the fact proved to be like squeezing an elephant (me) through the eye of a needle. I had to contort myself to get in between a maze of pipes, electrical cables, oil lines, the tank and furnace and associated expansion tanks. A delicate job. I got one full 4x8 sheet up and screwed to the wall. Just pounding on the wall, it's obvious it's added stiffness and mass. That may help increase the low frequency gain on the theater side of the wall. And, of course, help with isolation of noise. Plus its fire-resistant, which is a nice thing to have on the furnace side. If I can reduce the leakage of bass from theater into furnace room, it should be quieter upstairs in the bedrooms above that area too, which is good for the inlaws.
