Sub In The Fireplace

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.

I couldn't disagree more. I have a reasonably large masonry fireplace in between 2 main speakers, and the fireplace plus chimney gives a hollow boomy signature to the sound. The only solution was to wall the whole affair off with tube traps, thanks to which the speakers now have normal staging and a reasonably linear frequency balance. And that is with the speakers well out (55") from the front wall and fireplace.

Flex, difference is, his fireplace has a large sub in it and yours was trapping rear wave inside of a square hole. I'm not sure that the center of the rear wall is the best place for a sub but could be fine in some rooms, like Rives said, take a measurement.

Thanks for the responses... Rives, I guess I should have been more specific. But, yes, the fireplace is actual brick, and sits on the same foundation as the rest of the house which is a concrete slab. As far as the leveling jacks... are you suggesting this as a method to couple the sub to the concrete (in a sense)? Or simply as a way to cover the flue?

Sogood51, I wasn't sure if you were referring to me or not...
"I'm not sure that the center of the rear wall is the best place for a sub".

The sub is in a fireplace that is in the middle of a wall which one would face when listening to music.

And Flex, I see what you're saying... but I can't say that I've run into that same problem.

Sorry, should have read, center of the front wall. I need to start reading my posts over more often.