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MDF Sandwich - CDL recipes for suspended TT shelf isolation?

master_fo

master_fo

Active Member
Hi, I am a happy owner of Empire 598. Last week I also became happy owner of Rythmik F-12GSE sub. That thing is fantastic and has real infrasonic capability! Which, unfortunately, leads to feedback issues on lower volumes than I used to have with Paradigm sub (where I could cut low end with essentially brick digital filter). So even when I turn Rythmic subsonic filter on and use 20Hz rolloff with high damping, the feedback resonance starts just 6-12 dB higher than my normal (~80) listening volume. My sub sits on granite fireplace foundation - not much more to do about that side. There are also footfall issues that get amplified by sub.
So I decided to add some isolation to the TT shelf in my rack (regular VTI spiked rack on hardwood floor, standing on cork/rubber blocks + dense wood tile). The top shelf sits on 1/8" sorbothane strips.

I have a couple of spare VTI shelves so I have decided to use one of them to make top MDF shelf into a sandwich. I have applied Dynamat to both and now am going to experiment how to glue them together, possibly with more layers in between.
What would you guys suggest to bond 2 dynamatted MDF boards together, based on your experience with suspended tables?

- another layer of Dynamat with aluminum taken off (essentially butyl to stick to both sides);
- hard rubber ?
- neoprene ?
- sorbothane? (not very practical, as it either have to be not full surface bond, or it will probably be too thick for the load - also kinda expensive)
- dense foam like in SubDude?
- felt ?
- cork ?
- a thicker aluminum sheet ?
- something else ?

Wall shelf is not an option.

What kind of glue to use for materials other than butyl :
- silicone sealer?
- "nano" mounting tape ?
- "Green Glue" (for cork or other porous layers) ?
- something else ?
 
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Turn on your amplifier and turntable. Set the amplifier volume to the point where blowing on the stylus produces audible noise through the amp/speakers. Now yell at the tone arm.

That should demonstrate the effect of room sonics directly affecting the tone arm and cartridge/stylus. You could suspend the table with "Star-Wars" type anti-gravity clamps (floating freely in space) and still get acoustic feedback. It may not be the turntable shelf transmitting the room noise.
 
Watthour said:
Turn on your amplifier and turntable. Set the amplifier volume to the point where blowing on the stylus produces audible noise through the amp/speakers. Now yell at the tone arm.

That should demonstrate the effect of room sonics directly affecting the tone arm and cartridge/stylus. You could suspend the table with "Star-Wars" type anti-gravity clamps (floating freely in space) and still get acoustic feedback. It may not be the turntable shelf transmitting the room noise.
Click to expand...

Sometimes it's the table itself picking up the airborne acoustics as was my situation. The Rock is pretty good at dealing with this on its own, but as the OP my "twin" subs were causing an issue due to table location.

Moving the table wasn't an option, sooooo........

Drastic measures were needed so I took them:D
10lbs of lead sinkers and some Plastidip
20160828_201125.jpg

20160829_180200.jpg

before I had issues at anything above 10:00 on the volume knob, after at 3:00 listening to The Doors.
Screenshot_20200414-224417.png

no more issues
 
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Obviously the plinth is the first or second largest "receiver" in the equation, with the platen being the other major component. Both can become sympathetic fairly easily, but the "business" end of the table (arm and cartridge) can also be affected.
 
Watthour said:
Obviously the plinth is the first or second largest "receiver" in the equation, with the platen being the other major component. Both can become sympathetic fairly easily, but the "business" end of the table (arm and cartridge) can also be affected.
Click to expand...
Yes that may be especially the case for suspended turntable. I can knock on the plinth full force with very little audible feedback. With that in mind, I actually checked if adding silicone feet to my SUT will make any difference (and I use very thin DYI XLR cables that go through the hole in my TT shelf down to SUT, to make sure they do not touch anything else and do not damp suspended tonearm. It did help a little bit.
 
Grenadeslio said:
Sometimes it's the table itself picking up the airborne acoustics as was my situation. The Rock is pretty good at dealing with this on its own, but as the OP my "twin" subs were causing an issue due to table location.

Moving the table wasn't an option, sooooo........

Drastic measures were needed so I took them:D
10lbs of lead sinkers and some Plastidip
View attachment 2370489

View attachment 2370492

before I had issues at anything above 10:00 on the volume knob, now at 3:00 listening to The Doors.
View attachment 2370496

no more issues
Click to expand...
Nice pic! It's a good question how to properly measure (at least comparatively) real feedback issue. What is the best way to do it?
I would imagine, for final measurement, playing test sweep on 3-4 loudness level and comparing distortion between then (and with playing digital sweep at the same levels) would give some meaningful numbers.
For rough experiments, I am using a) stethoscope on the shelf surface and stomping on the floor, and b) putting the stylus on still record and checking how far I can boost volume before the resonance begins.
 
Update: no it's not airborne (at least most of it).
Tried putting the TT on SubDude platform on top of the original shelf - feedback almost gone. I suppose dense foam works (top of SubDude is also MDF AFAIK), and SubDude goes back under the sub. Will experiment with thick felt and neoprene, too.
 
master_fo said:
Update: no it's not airborne (at least most of it).
Tried putting the TT on SubDude platform on top of the original shelf - feedback almost gone. I suppose dense foam works (top of SubDude is also MDF AFAIK), and SubDude goes back under the sub. Will experiment with thick felt and neoprene, too.
Click to expand...

The rack is possibly picking up the airborne and then transmitting it, if concrete floors then most likely.
 
Grenadeslio said:
The rack is possibly picking up the airborne and then transmitting it, if concrete floors then most likely.
Click to expand...
I have hardwood floors, so it picks up structural vibrations :)
Returning to the measuring question: I used an app called VibrationAnalysis today (free IOS version), and it does show you a nice live (well, delayed) spectral graph of vibrations of the surface your phone sits on.
What I learned:
- my VTI rack actually amplifies floor vibrations! It's rather small scale on free app, but there is no question something in between 5-10 dBA are being added to the peak of 20Hz hump that follows myself stamping on the floor (between the phone laying down on the floor and then on each of the 4 shelves the amplitude in Z direction only grows. Not just that, but a whole new peak at 5Hz in direction X (between the stomp and the rack) starts rising and on TT shelf (4th) it becomes higher than the original 20Hz peak! Now that horizontal 5Hz vibration is responsible for skipping! Totally explainable by the way springy floors work.
 
Lavane said:
What about a subsonic filter?
Click to expand...
I am using subsonic filter on the sub. There is no other in my system - yes I should add another one to my phono stage - because my Open Baffle mains (Emerald Physics 4.7EP) are also getting into the resonance game - their 5-20Hz may not be that audible, but there certainly is a lot of woofer moving going on!
 
master_fo said:
I have hardwood floors, so it picks up structural vibrations :)
Returning to the measuring question: I used an app called VibrationAnalysis today (free IOS version), and it does show you a nice live (well, delayed) spectral graph of vibrations of the surface your phone sits on.
What I learned:
- my VTI rack actually amplifies floor vibrations! It's rather small scale on free app, but there is no question something in between 5-10 dBA are being added to the peak of 20Hz hump that follows myself stamping on the floor (between the phone laying down on the floor and then on each of the 4 shelves the amplitude in Z direction only grows. Not just that, but a whole new peak at 5Hz in direction X (between the stomp and the rack) starts rising and on TT shelf (4th) it becomes higher than the original 20Hz peak! Now that horizontal 5Hz vibration is responsible for skipping! Totally explainable by the way springy floors work.
Click to expand...

ya, suspended tables especially have issues with suspended wood floors, meaning without an underlying concrete slab. One of the best methods to mitigate the issue is a wall shelf that you've already nixed.

Barring that, finding an area where the floor is the most rigid, near adjoining walls possibly, over a load bearing wall if on the second floor, etc
 
OK after putting Rythmik sub back on SubDude (somehow I thought it's not needed on granite slab - I was wrong), and making an MDF sandwich the crisis is mitigated! For now, I am using sorbothane both between the stand and lower TT shelf and between the shells - I think the sandwich looks pretty good.
It was rather eye-opening to actually go and use load calculator at Sorbothane website (https://www.sorbothane.com/Data/Sites/31/edg/vibration-calculator.html) - I somehow went with that 'make it so that it compresses about 20%' rule while it was wildly-wildly inaccurate for low end!
For the combined 40 pounds of double damped shelves and TT, it actually came to 4 squares 0.2"x0.2"x0.19" of 50 Duro sorbothane to make it damp 20hz properly and 10hz 'somehow', and it still looks like 5Hz are being aplified by it! If I increase the size to 0.3" by 0.3" it would amplify both 20Hz and 5Hz. So no wonder that still does less isolation in 5-10 Hz area than the thin foam was. And no wonder about 100x amount of sorbothane I applied earlier did nothing at best.
IMG_6334.jpgIMG_6335.jpg
 

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master_fo said:
I am using subsonic filter on the sub. There is no other in my system - yes I should add another one to my phono stage - because my Open Baffle mains (Emerald Physics 4.7EP) are also getting into the resonance game - their 5-20Hz may not be that audible, but there certainly is a lot of woofer moving going on!
Click to expand...
That's where I want to add one, to my phono stage.
 
And this is an illustration on how I deal with phono cable that goes straight in the bottom of suspended tonearm :)
 

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Grenadeslio said:
ya, suspended tables especially have issues with suspended wood floors, meaning without an underlying concrete slab. One of the best methods to mitigate the issue is a wall shelf that you've already nixed.

Barring that, finding an area where the floor is the most rigid, near adjoining walls possibly, over a load bearing wall if on the second floor, etc
Click to expand...
Right. It's already there, close to the outside wall. I have tries left ant right - no luck, and my OB speakers need symmetry behind them anyway. Maybe a few more inches closer to the wall (which is a window wall) can make a difference, will try that.
 
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