This is a BIB Subwoofer from CSS. I used a 12" Dayton Audio Driver instead of the specified driver. When I first saw this sub (browsing online) it immediately struck me how similar it looks to a 16' Bourdon / Open Wood pipe. My immediate thought was that this would work well as a sub for Hauptwerk use and I was right! This design, as I built it, work GREAT down to 32Hz. It is ideal for 16' ranks and, is quite efficient, in that it moves so much air, in a very similar fashion, to the very pipe it's meant to emulate.
It is built entirely from 3/4" Baltic Birch and, aside from being quite heavy, it was a pretty easy build. The hole-y brace in the inside was a bit time consuming. I would strongly encourage its use as sub for 16' ranks in Hauptwerk organs in large spaces. I would really like to experiment with a "32 foot" version of this - if one could be made to get down to 16Hz, it would be a remarkable sub for this application. This is a horn design, but a TL variation does exist.
Some time ago I realized that the CSS website has gone down. I'm not sure why but I do still have the build plans if anyone is interested in them.