They were either responding to warps or low frequency information that was well below the point of vented resonance or your phono section was leaking DC voltage. None of those are good situations. Sean
The above answer is correct. The reason you don't see this with the tapes is that they don't go below 20HZ. You are seeing arm/cartridge resonance because the effective mass of your arm is too high for the compliance of your cartridge. This sub-sonic "signal" is actually using up a lot of your amplifier's power reserve. You could reduce this by buying a lower compliance cartridge or, if it doesn't bother you and you don't notice it, just live with it.
Ditto Sean. This is why some (esp. older) integrateds/pre's have a "low freq filter" or something similar (they all are hi-pass filters attenuating the signal from ~ 20 Hz and below). If you're sure your cartridge is OK, maybe your rig has this feature - use it. If not, you may want to add such a filter, although its proper implementation is yet another entire story.
Could it also be low frequency feedback from the speakers to the turntable ? If your turntable is not well isolated from the speakers you'll get this problem (I did a few years back). If your speakers are spiked into a wooden floor (either directly or through a stand) and if your turntable is on a rack or piece of furniture that is also sat on the same wooden floor then I bet you've got feedback.
I solved it two ways : either wall mount the turntable, or place the turntable on an isolation platform. I made my own isolation platform from a concrete slab and a thick wedge of foam. In either case the "wobbling woofers" were completely cured.
The above answers are also good reasons for your situation, but if your setup fits my description then try a simple isolation platform to see if it makes a difference.
blkadr ... my woofers used to wobble even when the record was just cued up and the track hadn't started. I think that subsonic feedback can occur just due to warping of records. For the wobbling to be visible as wobbling (and not a blur) the frequency will be well below normally audible frequencies. so I'm not sure the amount of bass on the track in question is relevant.
That's not saying that my diagnosis is correct, just that you don't have to be playing a bassy recording in order to notice it.
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.
Man is this contagious? I think I have it too!! I just set up a table (not too well mind you).
I was using a JA Michell phono stage and it wasn't marked left/right so I guessed which in/outs to connect to. It was making horrid sound as the volume increased and then went away briefly when I turned down the volume. When I raised the volume, it happened again... over and over and over. Annoying.
Just now, I switched the connectors around on the phono stage and the wubba-wubba stopped... But I have a small hum from the phono now.
Geez... is this ever easy?
Thanks to all for the responses. Mike's description of the exponential phenomenon is spot on. I ran some comparisons tonight and the dramatic looking woofer-wubbing occurs only at rock-approved levels or slightly below; equally with the dustcover on, up or off; equally between music cuts or during them; and not at all, regardless of volume setting, with the stylus out of the groove, the platter spinning and the tone arm cued up. The stylus rides flat on the test LP's so I doubt warping is the issue, unless you consider a record groove to be a warp of the vinyl blank.
I never noticed the wubba effect with the grills in place. Though I have new admiration for their wubba hiding properties, I'm leaving them off anyway.
The whole rig's fidelity to the suggested subsonic signature even with no recorded signal present makes me wonder: is subsonic woofer wubbing inherent by nature in the LP playback process?