dear Kenjit, show us the math on the Anechoic wedge needed for 20 HZ absorption?The math of the theory has shown that it would take a 20 metre thick wedge of high density fibreglass to supress this energy.
However, at the lower frequencies, my infinity baffle in wall speaker technology could be used in conjunction.
The anechoic room technology would mainly need to be applied from say 100hz up which would reduce the required thickness considerably.