Back in the early 90's Nelson Pass developed a unique product called the Phantom Acoustics Shadow.It was basically an active bass trap of sorts that he claimed effectiveness for room modes in the 20-200hz range. It works basically by introducing a signal that is exactly 180 degrees out of phase with the room mode thus reducing it's amplitude.
According to reviews it worked as claimed but was not a commercial sucess. Nelson was kind enough to reveal enough details (even answered some direct questions)about the design for one to make a working copy (or 4).
For those interested here is a link to the patent.The Shadow utilized two 8" woofers at opposite ends of a sonotube type of enclosure.
While the dimensional ratio's of my room are very favourable and conducive to good bass response,and I do get tight detailed deep bass from my subs but I find the mid to upper bass a bit ill defined and in need of some attention.Instead of building very large corner traps I thought I would try this active approach first.I intend to build two units, one for each front corner of the room but I will stray from the Shadow design by using only a single 8" woofer per cabinet.This so as to keep them compact(approx.a cubic foot per) and WAF friendly.I have yet to decide if I will go for the mini Vandersteen look with black grille cloth sock and solid wood top and bottom or sono tube with solid wood top and bottom accent pieces?
It's actually a very simple device basically consisting of a microphone element placed in close proximity to a woofer to povide a feedback signal.The signal is then preamplified, phase inverted and high and low pass filtered. Then it is amplified by a small chip amp that then drives a woofer.
Pic#1 shows the 8" SEAS woofers I already have on hand.
Pic #2 is the chip amp kit purchased from an Asian ebay source.It uses the TDA 2030's which are slightly lower power than the TDA2040's Nelson used in the Shadow. Amp power and quality are not important factors with this unit.
Pic#3 is the tiny electret mic capsules.