Spent a few hours today finding fb of basic 4cu ft box tuned to 40hz built according to JBL enclosure guide. (two 4 by 6.5inch long ports 2226 woofer)

Box has moderate bracing and thick fibreglass on back.

Comes out to 35hz according to my calculations. This is lower then I expected but is this variation usual when performing the test?

I do have a fine tuned frequency generator so can step in 1hz graduations and did roll back and fowards to find what I considered peaks on DVM.

Did plug the ports and sealed with silicone for the sealed test but a small leak would sent the result out the window?

Any tips from those who do this every month or so

Dave