I was experimenting with Chris Whealy's spread sheet and came up with a design for an absorber that was appreciably effective at 50 Hz. 6" of 703 with a 6" air space between the 703 and the wall. Absorption @ 50 Hz is about 0.40 so it would have to be large - covering a significant part of wall(s).
I'm actually considering building one of these later on this year, as the back wall of a media booth that is currently experiencing a lot of bass build up in this range.
An alternative would be a resonant bass trap, for which a possibly useful design guide exists at