Dustcover Blues

If you cut off your subwoofers does the problem persist?

@mijostyn I have not but frankly never directly compared. I’ve had the Linn for years and no issue. Others prior perhaps. I should mention table is in front of and to the left of speakers and there is also a sub about 5 feet away firing in its direction. The basement of my current house is by far the best room structurally I have ever had my hifi in.  Much tougher if not at foundation level. 

@lewm , disappears instantly with the subs turned off and the Soundlabs run full range. But I can not run the Soundlabs that loud alone without pushing them into distortion and rapping the stators. They also can not radiate bass that low anyway at even moderate levels. I cross them out at 120 Hz and the reduction in distortion is distinctly audible, even by millercarbon. I do have to try putting Townsend springs under them. I'll cut holes in the ceiling next week. Kidding aside, at 100 dB they cruise along like most systems at 85 dB. My subs as a single unit will produce useful output right down to 10 Hz. The rumble filter cuts them off at 18 Hz where, as you can see they are still going strong.

@mapman , I do not think your Linn is suspended which is why it does so much better on a solid floor. Try playing with the dustcover up and down and tell us what you think. As you can see by the curves I posted there is no question that dust covers attenuate sound getting to the tonearm. In my case up to 10 dB which is 1/2 the volume.  

Man! You really like to turn it up. Most people that come into my listening room where the 845PXs are located (otherwise known as "our living room") complain to me that I listen too loud. My sound labs probably go lower in the bass region than yours do without subwoofers, because of greater square area of radiating surface, but I do realize that with subwoofers you’re probably getting deeper base than I do. on the other hand I am hearing wonderful continuous bass down as low as I think there is any important music.

You once had an issue due to your room frequency response correction paraphernalia.  Could it be that the same circuitry is boosting the bass in the problematic region, thereby either exacerbating the problem or actually causing it?  Also, if the problem is caused by the Helmholtz Resonator mechanism, wouldn't you expect that shutting down the subwoofers and running the ESLs full range would not so much cure the problem (as you observed) but only reduce the intensity and clarity (because now the ESLs are straining to deal with the still present spurious LF input).  The Atma-sphere amplifiers can certainly get down that low and lower.  The Helmholtz hypothesis is happening at the turntable, so shutting down the subwoofs per se should not "cure" the problem. (By the way, if you are crossing over at 120Hz, I would call them "woofers", not subwoofers, but that's semantics. I think of a true subwoofer as a woofer that comes in at below 50-60Hz, to choose an arbitrary cut-off.)  In my Beveridge system, I cross over to my home-made TL woofers at 80Hz, with an 18db/octave slope.  Have you entertained the notion that the bass energy dumped into the room at high SPLs on bass-heavy passages is feeding back to the dust cover itself, maybe setting it into motion which affects the cartridge stability in the groove? As you know, bass frequencies are encoded largely by horizontal motion of the stylus/cantilever.

Partial retraction.  OK, I see that if it were the movement of air in the room, caused by your woofers, that was inducing the Helmholtz effect, then shutting down the woofers would possibly reduce the SPLs sufficiently to ameliorate the Helmholtz resonance of your dust cover/TT.  I guess I just find it hard to believe that your subwoofers blow around enough air to do that, but I have not been there, as you have.  Still, I would take a look at your room frequency response correction devices at the suspect frequencies.

Your microphone test for proving the utility of a dust cover.  Isn't that highly dependent upon the room, the speakers, and the proximity of the TT to the speaker?  In other words, you could prove it for yourself and your system, but not universally for all situations. I'd move my turntables before employing a dust cover to shield the tonearm/cartridge from SPLs.  In addition, what do we know about the importance of the effect you observe, in terms of ultimate fidelity?