DIY Acoustic Panels

Don't waste your time. It won't do anything either way. The fireplace is most likely not at the early reflection point. Find the early reflection points using the mirror method and dampen these with acoustic tiles. 

I was going to do a 4” thick PVC frame with recycled cotton baffle insulation. Initially thinking that a closed back might be better at least with airflow and sometimes I get water down my chimney when it rains. I ordered some burlap fabric to cover. I do think maybe open back may work to allow some of those low frequencies to pass. I guess like Miller says maybe trial and error. I could always cover later.

Oh I was forgetting being a fireplace of course there's a chimney! Get a copy of Robert Harley's Complete Guide To High End Audio it has a chapter on acoustics and bass traps. Your chimney is in effect a great big port and can be tuned to trap whatever bass frequency you want by simply changing the size and shape of whatever you use across the fireplace and chimney openings.

Its all simple physics relating to the wavelengths of the frequencies you want to tune for. Pretty sure there are tables and description in the book. Been a while, bought it 20-30 years ago. Or you could probably look it up on-line. 

millercarbon, if that is true all you have to do is adjust the damper but it would depend on the length of the chimney. 

Has anyone used "Duck Canvas" for DIY Panels? Is it not transparent enough?

millercarbon, if that is true all you have to do is adjust the damper but it would depend on the length of the chimney.

Right. Bass trap design is closely related or similar to speaker porting. In both cases its a function volume, length and resistance.

With speakers its cabinet volume and port diameter and length. Same with the tube trap. Main reason its usually free standing tubes is simple convenience. Could also be a closet or holes in walls or ceilings, or a fireplace.

Usually we think of sound as waves but for this I find it more useful to think of air as a spring. In this metaphor what you're doing with the sound and air is just like what a shock absorber does with bumps and oil.

Bass compresses the air and it wants to spring back as much as it was compressed. Which is what happens when it hits a wall. Unless there's a hole like a fireplace in the wall. Then some of the compressed air goes and expands into the hole. The air is still compressed and still springs back only now it has the extra resistance of the hole. And the tube, ie chimney. 

The longer and the more open it is the deeper the frequency and the more it damps. This is why its so easy to get solid deep bass in a car- cars are small and sealed. Open the doors and windows, give the pressure somewhere to dissipate, there goes the bass.

Same with the room. 

Again, Harley has a whole chapter on this in his book..

Its like 30 years since I first read that book. Wasn't a first edition even then. So many years this has all been known. Still amazes me how much it covers. Should be required reading. You want to build a system? You want to post? Here. Read this first.