Isolation for Granite Surface Plate

You can do a lot better! I've used different things on top of my 4" thick granite machinists plate and the best by far is Townshend Pods. They look good too, and adjust beautifully for leveling. https://www.audiogon.com/listings/lisa8492-townshend-audio-seismic-isolation-pods-set-of-4-sizes-a-b... Pods are sized according to the weight of your turntable. You can do the math or ask John Hannant at Townshend for his recommendation.  

The main problem with granite is ringing. This imparts a hard edge to the sound. You may not notice or may even hear it as detail. Until it is gone, and then you realize how much better and even more detailed the sound becomes. I went through all of this with mine. https://systems.audiogon.com/systems/8367 These are some old pictures I need to update them. Pods are now above the granite where you see BDR Cones and Round Things in the photo. BDR is very good but the Pods are quite a bit better. 

If you can't afford Pods, Nobsound springs from Amazon are awfully good for the money. (I used them before upgrading to Pods.) You will have to play around with them to get the right number of springs, and shim or something to get the table level. They are nowhere near the league of Pods but way better than you would think for how cheap they are. The main drawback with Nobsound is the lack of damping. You will eliminate the ringing edge of granite and gain quite a bit of imaging depth and detail. But if you go to Pods they do all that too and better, with greatly improved (deeper, tighter) bass, and instrumental tone and timbre so much better it is like getting a whole new upgraded turntable.

@oldskoolmark - An approach I use to prevent ringing is to employ a "sandwich" of differing materials.

• My rack has a steel frame.
• Between the frame and the MDF shelf is a thin layer of Sorbothane on the frame struts
• between the MDF and the granite is a layer of foamed shelf liner
• my components all have brass cones to drain unwanted vibrations into the granite/foam/mdf sanwhich

See details in this link
Isolation | My Audio Alchemy (image99.net)

• The sorbothane between the rack and the MDF shelf prevents the frame from vibrating
• The foam drawer liner prevents both the shelf and the granite from vibrating and
• the different densities of the MDF and Granite combined is quite heavy
• This minimizes movement due to soundwaves of the music
• The approach , makes for a very "inert" and dense mount

I originally used an MDF plinth on top of the MDF shelf, but found using a significantly more dense material like granite for the plinth worked much better and the cones do not dig into the granite like they did with MDF, which anchored the foot to the shelf, which is something to be avoided

I have not found anything better to date

Regards - Steve

Thanks guys for the benefit of your experience and research! The sorbothane / MDF / foam shelf liner / granite slab approach appeals to me from both cost and aesthetics perspectives. 

Are all MDF boards created equal? If not, what are the relevant quality parameters?

You're welcome but to be honest you don't get the benefits unless you follow the advice. The MDF idea you seem to like is so far down the list of things that work I never even mentioned it. Tried it, crap, moved on. If you don't want the best I get it, they're expensive. That's why we recommend Nobsound. If you want to reject both of the two best ideas and move way on down the list to stuff I rejected 30 years ago, be my guest. Just don't thank me while saying thanks but no thanks. 

Seriously. You don't even know anything about MDF, but why would I explain when you've already demonstrated such a knack for rejecting solid info?

oldschoolmark -  Here are some links  that might help you understand differences between MDF
https://mdfskirtingworld.co.uk/blog/the-difference-between-mdf/
https://www.weyerhaeuser.com/woodproducts/osb-panels/medium-density-fiberboard/mdf-product-specifica...
If you are interested in what Steve proposed I'd suggest sending a pm to him versus getting caught up with someone else's needless condescension 

I know that Mr. Vandersteen uses a special felt for marble bases under his speakers, you might want to call or email him.
Better yet, go to the new Vandersteen forum on the Vandy website.
Bob

oldskoolmark

Try a "half inflated" kids scooter innertube under them, complete (air) isolation worked wonders for me back in the T/T days and now with CD transport also.
Pick your size. (Tom Gillett aka: Sam Tellig of Stereophile put me on to this decades ago)
https://www.walmart.com/search/?query=scooter%20tube&typeahead=Scooter%20tube%20

Cheers George

Granite sounds bad. So does Sorbothane. 
Get a chunk of maple and Nobsound springs. 

I cannot tell about springs as i have not tried them but believe the ones who did.
If not fancied to above esthetics a possible solution would be solid aluminium up to 1 inch thickness sitting on elastomer material covering entire surface underneath. Much better than granite.

G

@oldskoolmark - I believe you can get MDF in different densities, but it is a special order from specialist outlets.

I buy mine from Home depot - it works well

I wouldn't worry about it because using materials having different densities with some type of absorbing material between them is the important part.

For the sorbothane I would recommend a DURO rating of 40-50 because  softer products will not dampen effectively.

Regards - Steve

"half inflated" kids scooter innertube

I used something like that in the '70s when making holograms, with equipment mounted to granite plate. Worked great for isolation, I'm sure it would work for audio use.

Simple and cheap!

You could try a sheet of thin vinyl based damping material, available at 
 www.partsexpress.com 
Self adhesive.

I bought a 3" thick granite surface plate years ago soley to set up my tt project.
This thing weighs 100lbs, had to be freight shipped.

You use HDF in leu of MDF. Night and day. High density Fiber and the quality of the fiber counts... There are all kinds of MDF, You’re actually better off using thinner pieces because of the hard face on the panels.

1.5 inches of 1/2" sheeting has 6 hard faces were 3/4 has only 4.
When you sandwich 3 vs 2.. :-) HDF.. Harder than OAK some of it.. and no sonic anomalies because of natural wood..

Spring and dampening are the way TT manufactures do it. It’s best to follow suit.. No need to reinvent the wheel, with spikes and all that BS.. They just don’t work..

Remember you’re not getting rid of vibrations from the table, your isolating the table from them. Why on earth would you use spikes.. LOL or anything BUT isolation.. Just the opposite effect when you think about it.. Spikes vs suspension. Kind of a no brainer if you think about it..

BTW, George has got an old proven way to add suspension to ANYTHING, not just a TT.. The ol tire innertube.. Does work great for subs too, I used them for years on 500 lb, bass bins.. ZERO rattle in my shop.. BUT for two block.. BIG BOOM BOOM.. 130 db plus.. Helped with the waves in the neighbors pool too..

Regards

Look at Symposium Ultra Stealth for best sound granite rings like a bell.Good luck though.