Why are my woofers pumping?

Some of you have suggested that acoustic feedback may have played a role for my "woofing" or woofer pumping.

This is relevant not least in a four speaker system, like mine, where some drivers are pointed towards the back wall  (with the record player).

I tested a bit more, playing the new 2 x 45 rpm Jefferson Airplane: Volunteers. Volume: loud. This is a very good remaster, quality pressing, low vinyl noise. I hear this on the grooves between the tracks. When the tracks / music played, I used my stetoscope on top of the different parts of the player.

The results were interesting, more varied than I would have thought. The top of the maglev feet had next to no feedback, the sound was dead. The plinth, and also the motors, had more feedback, although I did not hear clearly "wrong" or exaggerated sound. I tried to "hold on" to the plinth, firmly, as suggested in the thread above. It made no difference.

I would like to test the platter and the record clamp, but since this rotates, I can’t do it with a stetoscope. Anyone knows of a phone app to test for rumble or noise, laying on the platter?

Since this thread is getting over and finished - the woofer pumping has stopped - I will only update if I find some clue, what the problem was. So far, I have not been able to recreate it. Maybe it was caused by a troubled spindle - not quite in place - or other drive problems, as suggested by vpi and others, above. Obviously, the slight leeway between the spindle housing and the plinth, which I need to tighten, may play a role, but in practice I don't think so, since due to the 10 kg platter weight, it settles quite solid anyway, and I cannot reproduce pumping by pushing the platter (or plinth).

I note an interesting thing, checking the player, and cleaning the belts, pulleys and platter. The Hanss uses two very silent motors, not vibrating much. But when I touch the top pulleys, slightly with fingertips, the vibrations are more notable. Sometimes it seems as if the motors develop a "trouble" position with the pulleys straining and vibrating. I can reduce the vibrations just by moving them a tiny bit. This is a behavior I recognize from my VPI HW-19. The motor / pulley does not quite "stall", but a combination of energies has made it stressed. Anyone who recognizes this pattern?

@o_holter The studder you are seeing in the spindle is typical af asynchronous motors and not a malfunction.  Download and read the manual for the VPI HW-19 MK 4.  There is a paragraph that explains this.

If your woofer pumping returns do not use a filter.  You would just be covering up the problem instead of solving it.

in the meantime, enjoy the music.

Thank you vpi. Always something new to learn.

Some more testing. The woofer pumping problem is still there, maybe a bit reduced, but not fully solved (sorry for too general statements above - I thought it was solved). It is very dependent on the record - some LPs have more, some have less. I am investigating whether a filter could be an idea, using a laptop and a sound card to see what goes on in the low hz region (through the program Audacity).