For HT and music. 80% HT usage, 20% music.
My HT room is half open to another room, i.e. L-shaped. The main portion of the HT is about 1900 cubic feet and the remaining portion of the L is another 1,100 cubic feet for a total of 3000 cubic feet. But there's a lot of stuff in the other part of the L space, so if you subtract the furniture it's probably more like 1,650 cubic feet in the main room and 750 in the remaining part of the L, 2,400 or 2,500 cubic feet total instead of 3,000 cubic feet. There are black velvet curtains separating the main HT part of the L from the other part. It is a room on the bottom floor of a four story house, but it is an old house with zero insulation in ceilings or and at least two of the walls, while the wall behind the projector screen, behind the sheetrock wall with 3.5 inch wood studs, is multiple yards of concrete, and I'm unsure about the last wall. I can treat with bass traps and acoustic absorption on pretty much any of the walls, and maybe the ceilings depending what is required for installation, but there is nothing but air and curtains to put anything in the opening to the L room.
Let's assume cost isn't an issue and we're just talking about what would be the ideal size for that shape and size of room. My goal for the bass I want is, considering I will not be able to play it at high volumes most of the time, that I want a sub that performs well from the lowest LFE you're supposed to hear in movies (not the stuff you're not supposed to hear) to the highest bass you want the sub(s) doing, and that the sub can do this at low and mid volumes not just high volumes. I don't know if there are subwoofers that can play low hz sounds at low volume or if they can need to be cranked up to high volume to create enough air for 5Hz and 10Hz, or how exactly that works.
Someone recommend two of this sub, https://www.audioholics.com/surements%20and%20analysis
. He told me that bass needs to proliferate before it sounds good and that's why a sub with 12" drivers is a goldilocks type of size for a 20-21 foot long room like mine because the subs can be set up to face the front wall and then reflect back to the seat, for a total spread about of 30 feet or more before the bass waves reach my ears, whereas the bass waves are longer on larger drivers requiring even more room to proliferate. He said he really likes 12 inch drivers as a size and would recommend them over larger sizes. Other people have told me he has no idea what he's talking about. This guy was someone who has no home theater experience, no audio engineering degree or anything, but has decades of experience setting up subwoofers for indoor concerts and proms in small rooms, as well as for video presentations of various kinds. He has looked at subwoofer setups in rooms larger than mine.
He seemed to think the idea that the HSU could be too small was laughable, but I know some movies go below 18hz for LFE and the HSU can't seem to do that without distortion although maybe only louder than I need, I'm not clear on that. I also read that some movies have LFE on the audio track that is not actually meant to eb heard (or felt), that it got left on by mistake by the sound engineer who did not have subs that could go low enough to know it was there, or something, and that some low LFE can be unhealthy to listen to and put your body on edge. I would just want my subs to know to play the good LFE, not the unhealthy LFE, so I don't know how that would work either, or if there is any solution to that other than choosing to either get all the LFE, even the unwanted, or get none of it, without a middle ground. Since I don't know that or the other aspects of this, I joined audioholics to ask the folks who do. Thanks!