Sub In The Fireplace

Sound travels at 1130 feet per sec. Thus (1130/60)= 1 wavelength for 60 Hz 1/4 of this. The reason I choose 1/4 wavelength is that it is impossible to have 2 peaked values.

The location of the fireplace sounds good. Is it brick or pre-fab? If pre-fab you could get some really bad rattles--but you would have rejected the idea by now if that were the case. If it's brick--the brick isn't going to vibrate much--so any insulation is not needed unless the sub is rattling. The only concern might be the damper and the flue. I would consider closing it off and possibly putting in a heavy sheet of plywood layered with Owens Corning on either side. You could even go to the extent of using some small leveling jacks to push the sub down (put a sheet of plywood on it too) and push the plywood up into the damper. This would really make for a "sound" connection. The fireplace likely improves the efficiency of the sub--no loss of energy (or very very little) through it--so it's all directed into the room. If the fireplace is less than 1/4 wavelength of the crossover point (4.7 feet for 60 Hz) then I can't imagine it causing any strange anamolies in the frequency spectrum. If it's greater than 1/4 wavelength you might consider measuring it's response in and out of the fireplace just as a precaution. But it sounds like it's small.

The leveling jacks are to hold the plywood and Owens corning fiber board over the flue and while it is pressing up on this--it will also be pushing the sub down--just creating a very rigid positioning of the sub. It is very important to wall off the flue. Flex has shown the problems he had, but my guess is his fireplace was bigger and didn't adhere to the 1/4 wavelength rule for his cross-over point. This would most probably yeild a major bump at probably 2 low frequencies and a very colored disappation of energy. If you looked at a waterfall plot of a sub enclosed in this manner you would likely find at least one and perhaps 2 prominent frequencies that held energy for a long time. Again--this is why it's always good to take an acoustic measurement--then you can be sure. This is a theoretical model--and I have not tried nor measured the response in a fireplace (but now a really want to and compare those greater than the 1/4 wavelength rule and those less than the 1/4 wavelength rule). If you do measure it I would be very interested in your results.