Hello. I'm a newbie here. BBC sound recordist trained, way back. Presently trying to get that elusive 21Hz organ pedal into my 17x8 room. 30 to 60 is adequate and fairly even, from two 8-inch ABRs driven by a 100 watt car amp with LPF. Intuition tells me that fitting an open speaker sub into a hole in the wall from the garage next door may work - pressurising the room. Would it?
At a previous small house, I put a 12 inch speaker in a chimney plus an eight inch through a wall hole, and together they gave me satisfactory 20. Solo, much less good.
At an even earlier house, I built the same 12 inch Goodmans (old!) into the rear of a blockboard bookcase, with folded chamber totalling 24 feet. That was wonderful - more than enough pedal power in a 25x25 ft space, and painfully loud at 50. The amp was 25 watt valve. From what's been said so helpfully and so far on this forum, my flappy suspended ceiling may well have acted as the desired bass trap. I knew not what I did.