@atmasphere , Thanx for your input. The cabinet is not really a "stand." It is a very large built in cabinet weighing hundreds if not over 1000 lbs with the granite and records installed. Look at the picture. The only thing that will affect it is ground rumble from something like a dump truck traveling down the street. Having said this, the idea behind an isolated turnable is that it should perform well regardless of what it is planted on. In this case only frequencies below 3 Hz should be able to get to it. But, I have a situation where airborn vibration, sound is able to shake the sub chassis at 24 Hz creating a positive feedback loop. The dust cover is certainly part of the resonating system because when I open it the feedback stops. As you suggest I am sure the acoustic situation around the turntable is in part responsible for getting the sound pressure levels at 24 Hz high enough to get things started. However, with the dustcover up the table sounds great and I can thump on it with the record playing, volume wide open and you won't here a thing. I can also throw myself against the cabinet and you won't hear a thing. Try that with any stand! Yes, I have now demonstrated this to non beleivers on several occasions. I have the bruises to prove it.
Lets look just at the facts that we have so far. With very loud volumes using program with high output at low frequency's the subchassis shakes at 24 Hz. If I raise the dust cover the shaking stops immediatedly. If I press down hard on the dust cover it extinguishes over a period of a few seconds. There are two cavities above the sub chassis separated by plate that has a hole the size of the tonearm board in it along with the 1/8" space around the platter. I think it is safe to assume that air is resonating in and out of this structure shaking the sub chassis. My best guess is closing off the space under the turntable will stop the resonance by preventing pressure (or vacuum) from building up under the sub chassis. I forgot to mention that the only structure I can feel vibrating is the sub chassis and I would guess that it is shaking up and down at lease 1/32"
@rauliruegas , I owned two 12" Veldyne subs for years until the foam surrounds disintegrated. I also had the main amps rolled off at 6dB/oct. Once I had digital bass management I was set free to design whatever I wanted for subwoofers. Distortion in subwooder drivers is dependent on the motor design and the size of the excursions the subwoofer is taking. The Velodyne uses a sensor on the cone in a negative feedback loop to help control the driver at long excursions. It allows you to get more clean volume out of a smaller driver. However another path to low distortion is, do not let the driver take long excursions. You can do this by increasing the size of the driver or by going to multiple drivers. It is all about the surface area of cone you have working for you. The design of my current subwoofers was based on the thought that using four 12" subs with very heavy, ultra stiff enclosures with the mass focused in line with the driver would lead to a low distortion subwoofer line array that would match well with line array main speakers. The subwoofers are made with Corian laminated to MDF with a layer of glass microspheres in epoxy between the two. Each one weights 200 Lb. They turned out to be excellent subwoofers but I still detect some coloration due to cabinet resonance in spite of the insane construction. They also are taking longer excursions than I want at high volume. The new ones are a balanced force design with a 12" driver in both ends of a cylindrical enclosure that has 1.5 to 2" walls. They vary in thickness! Cylinders are inherently stiff structures. Only a sphere is stiffer. The forces of the opposed drivers will cancel out leaving an encosure that will not shake at all. 8 drivers will decrease excursions decreasing distortion.
1st order crossovers are not steep enough for subwoofer especially when trying to match them to ESLs. To decrease the distortion and increase the headroom of ESLs it is vital to remove from 100 Hz down from them. The subwoofers also have to match the radiation pattern of ESLs to mate well. In this case the ESLs are line sources over their entire range. You have to keep the subs out of the midrange but you want to run them up to 100 Hz which requires a slope of at least 48 dB/oct. That is 8th order! Plus, you have to match the subs the ESLs in time and phase. If you use a turntable you have to add a steep sub sonic filter or your subwoofer drivers will jump out of the enclosures at volume. The only way you can accomplish all of this without "F"ing things up is with digital bass management.
I hope to have the new subs finished by next Summer. I have to finish the wife's new kitchen on the way. I will show you the results. As you can see from the frequency response curves that I put up the current system has no trouble reaching 20 Hz with power, enough to get my turntable bouncing:-) Gotta love those subwoofers!
