on some movies there is content that goes down to 8hz, but for the most part, if you
do a good job nailing it down to 18hz or so, you'll get most of enjoyment.
imax and other professional cinemas tend to extend down to about 25hz or so as a reference point.
here is one that uses a full sheet of plywood for the sides. it is 4 feet tall, 8 feet wide, 2 feet deep.
should be an easy build, but it is huge and going to weigh a ton.
what you give up with the lower extension is the maximum spl around 30hz or so.
this horn has more extension than the other one, but you can see on the max spl plot that
it gives up about 5db or so around the excursion limited point (the red droop around 30hz).
125db is nothing to sneeze at though and in room, it will be higher. full reference with
all channels redirected bass is about 121db, so even if you run the bass a little on the hot
side, you should be good.
going any larger than this would be getting kind of stupid, but then again, most folks would
call a 4x8x2 horn complete insanity. it might be one of the largest that has ever been built around
here. but it is a simple design, so with lots of good bracing, i don't see any reason why you
and your pal couldn't tackle it.