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.