Dustcover Blues

@lewm , thank you Lewm. It is a pleasure instructing you:-)

As I mentioned before I dropped the crossover frquency from 120 to 100 when I got the Sound Labs as they have so much more surface area than my old Acoustats. Given you have even more you could try a little lower maybe 80 Hz but I would never go lower than that. If you have a digital crossover you can cross up higher and still keep the sub out of your midrange with steeper slopes. There is no comparison between digital and analog bass management. They are totally different worlds. If you replaced the crossover in the Beverages with a digital one you could cross up a little higher using a much steeper slope without phase issues. Would be fun to try. 

Lew, the speed of a driver will dictate it's upper frequency limit. Most subwoofer drivers, even the 18" ones are capable of reaching 1000 Hz before cone breakup.The electromagnetic forces generated by ESLs are several orders of magnetude weaker than a modern subwoofer motor. It is not a speed or transient response issue. The problem is matching the subwoofer up to a dipole ESL. The very muddy midrange that a sub produces clashes with the incredibly detailed midrange of the ESL. If you turn off the main speakers and just listen to most subwoofers you will clearly hear some midrange leaking through. This is made even worse by timing errors. This is why early on we were keeping crossover points so low, to keep the sub out of the midrange. The trade off was decreasing distortion in the ESL. The higher you cross, the less distortion you get out of the ESL. It was digital bass management that finally solved this problem.

I still have some issues with the subs I have now. I think there is some distortion added by the enclosures and although they will go very loud I think they could do it with less distortion from the drivers themselves. I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. 

There are different forms of helmholtz resonators. With the dust cover closed you have a sealed system with a large opening at the bottom (under the plinth) leading to a constriction at the opening for the tonearm board into the blind cavity under the dustcover. This is complicated by the springloaded platform hanging underneath. Regardless, as you suggest,  I have a resonating system and I need to figure out how to make it a non resonant system. My next approach will be to make a tight fitting skirt that closes off the bottom of the plinth so that air cannot enter from below. I'll use cheap poplar just to see if it works. If it works I will make a new plinth cover using the old one as a jig. 

You definitely have hackles. 

Mike

Whether its Helmholz or not isn't important. Resonance is.

This isn't the first time I've seen this. Since most high end machines aren't meant to be used with the dust cover down (if they even have one) the dust cover may well not have been included in the design of the turntable insofar as its resonance control methods are concerned.

IOW its the luck of the draw that it works or not.

If you simply grab the dust cover with both hands and try to squeeze it can you make the oscillation stop?

@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.