DaveMCC,
That formula offered by Jeff Szymanski (and BTW- it was blessed to work with Jeff for 6+ years, I knicknamed him "Acoustic Savant" which duly stuck)...is not the right application for your possible solution. Too tough to get needed absorption.
Ironicaly, I just built a Helmholtz Resonator for a different application entirely at that same freq range (actually is 42 Hz right now and it will actually create resonance in a space rather than trap when all's done). It's cool. When talking about a riser or something in that volume, applying this application could up the science effectively.
Back to the point: a perforated panel Helmholtz trap is going to be the best bet in that freq range.
Again, though...passive traps take that mystery out of the effort.
Hope this helps a bit.
Regards,
Jeff H