Seandtaylor99 is dead correct. I have seen this on the lead-in track of some LPs - no audible sound, but the woofers were doing the "wubba wubba" inaudibly and grotesquely - like a fat man bellydancing!
One thing I _have_ noticed, is that once you hit a certain point, the "wubba wubbas" start to grow exponentially - this certainly implies the energy from the speakers is being picked up again and reamplified by the phono cart and pre, causing a dangerous and unattractive "wubba wubba" feedback loop. Woe unto he who step unwittingly into one of these.
There is hope... isolation can help here, as others have mentioned. I ran an experiement with a turntable on my equipment rack, on the floor, and on the floor on top of a soft pillow (ok, my cat's bed - he never uses it, typical cat.) The rack and floor setups both produced dangerous excursion on an affected record. There was some excursion with the on-the-floor-with-soft-pillow combo, but I was able to go from 9-o-clock volume to almost 12-o-clock before the "wubba-wubbas" started to grow exponentially. This was a very major improvement with isolation, and we're *still* only talking about the lead-in track to an LP side. So this really does point to some serious LF information coming off the record warp, and does not make any statement about program content causing this, whether it can or not.
I've isolated my TT (on foam computer wrist rests... nothing fancy), and my speakers are now fully horn-loaded up front, so if any "wubba-wubbas" are present, I can't tell or see them any more.
I've heard "rumble filter" used before as terminology for the filters that remove these "subsonics". In any event, gotta get rid of those wubba-wubbas.
Mike
