Dustcover Blues

put the dust cover in the recyclable basket for pickup

Leaving aside our differences regarding to use or not to use a dust cover, I am thinking about your hypothesis that right now your turntable/cover are together acting according to the principle of a Helmholtz Resonator.  The idea does "resonate", but so far as I can tell from my reading, a Helmholtz Resonator requires air (or whatever fluid is resonating) to be put into motion.  Energy has to be put into the system for that to happen.  In your conception, what is the source of energy?  I would offer an alternative hypothesis, for good or ill. Could it be that energy being emitted by the cartridge body, the same energy that is heard as "cartridge noise" is bouncing around under the cover which is by chance tuned to resonate at a low bass frequency, which is why it is excited only when you play certain LPs with low bass frequencies and at a certain SPL above some threshold that causes the whole thing to get excited and re-enforce itself via positive feedback?  That is what I meant when I remarked that you are experiencing a problem that I associate with dust covers per se.  I put this out there for discussion, not to antagonize or criticize. In my own experience, when I played LPs with dust cover down, inevitably the dust cover itself could be felt to be vibrating, I always thought sympathetically with cartridge vibrations.

The dust cover is there to prevent dust build up on the equipment, not the record. Remove it when playing a record.  

Curious problem. I hear no difference in sound whether the dust cover is up or down on my TD 166 mk2 Thoreans. 

Audioguy, very incorrect. The dust cover was initially intended to keep dust and polution off of everything including the record. Why do records get noisier after repeated play? Groove wear does not make noise. It makes distortion. Why do people feel the need to spend a lot of money cleaning their records? I can afford any record cleaner on the market but don't feel the urge to buy one. I use a electrostatic loudspeakers and would think dirty records would bother me as much as anyone else. I am also very fastidious. 

Brandon2, that is what most people with a dust cover will tell you. I hear slightly better focus with the dustcover down at louder volumes. I would be the first to tell you this might be psychological. At low volumes I can not hear a difference either.

@lewm , Several reasons why I do not think that tracking noise is the problem. My cartridge produces very little tracking noise. You can barely hear it with your ear up to the cartridge and it is only very high frequenies that you hear. Pressing down on the dustcover and sealing off the opening at the bottom of the dustcover stops the feedback. Neither of these would affect tracking noise. It starts with loud low fequency sound and that is were the energy initially comes from. You are dealing with very long wavelengths, 30 feet plus. As the  pressure increases around the dustcover air rushes into the slot at the bottom lifting the cover up then as the low pressure part of the wave passes, air rushes out from under the dustcover dropping it. The same frequency is reproduced and you get a positive feedback loop. The big question in my mind is why is the tonearm/cartridge picking this up? It is over an octave away from the resonance point. Is air moving in and out from under the dust cover actually moving the tonearm. There is a venturi effect. The counterweight might be acting as a sail and it is always close to the edge of the dustcover wear velocities would still be high. 

@mapman , Do you hear a difference with the dust cover up or down?

The microphone is now positioned on the Sota's platter and we are getting warmed up to run these curves.