Dustcover Blues

@atmasphere , The original Sapphire came with a dust cover. It was not an option. Later it became an option, extra charge. This design is not all that much different from the original Sapphire. The early Cosmos had a Corian plinth cover. Later they reverted to wood. As far as I know this problem has never been reported before. If I had to guess it is because few people reach bass output below 30 Hz that would excite the resonance. I have to go almost to max volume to excite it and this is with very boosted bass. Look at the curves I put up on my system page. There are two pictures of the table with a microphone sitting on the platter. "UP" is with the dust cover up (the "UP" curve) and the other with the dust cover down (the "DOWN" curve) You can see that up against the side wall the bass is way up, at least 20 dB from where it is in the center of the room. It may be worse than usual because the table is in an alcove. If you read my posts you'll get a better feeling for what is going on. If I press down hard on the cover the feedback will slowly diminish. If I open the cover it stops immediately. The reason it feeds back is the sub chassis is bouncing at 24 Hz which gets the subs going at 24 Hz which keeps the sub chassis boucing. The sub chassis resonance frequency is 2-3 Hz. The tonearm is 8-10 Hz. 

I have three cavities. The one below the sub chassis, the one above the sub chassis and the one under the dustcover. The last two communicate through the tonearm board hole and the 1/8" space around the platter. The last two communicate through the slot that goes all the way around the sub chassis. It is also about 1/8" 

I am going to build a skirt to go around the plinth to seal off the space under the turntable. I'm hoping that will stop it.  I plan on making a new plinth cover out of a more exotic wood than walnut. Depends on what I find. It is like shopping for vegetables. You buy whatever looks good.

Look at those curves again. The dust cover diminishes higher frequencies up to 10 dB. Except in the bass it is 10 dB quieter under the dustcover. Does this mean anything? Who knows? I can not tell the difference between up or down until the feedback starts. I keep it down to protect the records. Am I actually doing that? Who knows? 

You can see that up against the side wall the bass is way up

My surmise is that is your room boundary effect and may also be complicated by a standing wave (looks like you have cancellation in the center of the room). A distributed bass array might be really helpful. You'd only need a pair of subs to break up the standing wave. IIRC sounds like you already have them. If they are located by the Sound Labs  they are probably exacerbating the problem- they need to be elsewhere in the room.

Once the bass is evenly distributed you may not have this problem anymore.

I never had a dust cover for my Cosmos (it was serial number 0).

I suspect your alcove is a bass node. Have you tried putting the 'table in a different spot?

Yep.  Audiokinesis and Duke Lejeune.  Woofer array.

The question of whether the resonance has anything to do with Helmholtz is somewhat relevant, because if it were due to the Helmholtz phenomenon, then you could live on with your prior conviction about the unalloyed benefits of the dust cover.  You point out that there is a pathway for air to exit the space enclosed by the sealed dust cover and plinth, downward around the platter evidently.  But if that constitutes a Helmholtz Resonator, then sealing the space between the edge of the dust cover and the plinth ought to have changed the resonant frequency. Sealing the dust cover to the plinth changes both the area of the aperture and the net path length across the aperture, and those two parameters are in the equation for resonant frequency of a Helmholtz Resonator.  Face it; dust covers are not all good.  If you admit that, I will admit they are not all bad.

Works for me Lew. Obviously, I am having trouble involving the dust cover. It is a first for me and my music habits have not changed. Once I fix the problem the dust cover will be all benefit again:-)

@atmasphere , Actually, because there is no rear wall and I use dipole linear arrays my room is very well behaved. The turntable is adjacent to the listening position. The bass is due to the boundary effect. This happens in almost every room. The table handles this just fine with the dust cover open. I am not putting it in the middle of the room.

I already have four subwoofers soon to be 8. Although they meet the definition of a "swarm" system they are arrayed in a specific pattern for a reason. At any rate the problem is not with the subs. The problem is a resonating turntable and it only happens with a few records cranked to the max. Next weekend I will make that skirt to see if closing off the space under the turntable will stop the resonance. The dustcover is now effectively sealed. Sealing off the bottom will prevent air pressure from having immediate access to the interior of the table.

The bass is due to the boundary effect.

Yes. But it appears you have a standing wave; if you were to break that up the boundary effect might be a bit reduced (that's how it worked out in my room anyway...). From how you've written about this, it appears that if you make just a bit of an improvement the problem would be gone since it only happens with certain cuts at higher volumes. I forgot to ask- what sort of stand are you using for the turntable?