Acoustic isolation for turntables – to couple or decouple?…

This is a "sprung" turntable
This is a “sprung” turntable

This is a trick question since the answer is often…  both!

Ok, so most audiophiles know that the “big bad wolf” of turntables can be acoustic feedback.  Simply put, this is where the speakers vibrate the floor, walls, air (and hence dust platter) and that vibration gets feed back to the stylus and amplified then sent back to the speakers where it causes more vibrations to be amplified, and so it goes.  You get the idea, the classic positive feedback loop.

This is the “big bad wolf” since if it runs out of control, which undampened positive feedback loops do by definition, it has the potential to destroy speakers and even amplifiers.  So how to you test for this in your all analog vinyl signal path?  It’s really simple.  Place the needle on a record that isn’t spinning and turn up the volume (very carefully!) until you start to hear a howl then immediately back it off from there.  That’s your acoustic feedback threshold.  If this is well above your normal listening levels or not there at all, great you have nothing to worry about.  If it’s within your listening range, you’ve got some isolation work to do.

So most already know that turntables deal with acoustic vibration in one of two ways: acoustic suspension or “sprung” turntable platforms or very heavy and solid plinths that use the physics of “mass loading” to deal with it.

 

On thing has become obvious, my system rack which consists of an teak shelving unit open on the front and back, is about the worst case scenario for this “component”. Rather than isolate it serves to amplify rumble. This is the type of thing that isn’t glaringly obvious but rather a welcome absence when it is gone.   My solution will most likely be a wall mounted shelf as virtually no rumble can pass from my concrete slab to my wood framing.  A very inexpensive solution available to me since I’m on the ground floor of a dwelling constructed as slab on grade.

 

“Generally, mass coupling works better for heavy components, and decoupling for lightweight components”, p. 406

I’m lucky since my speakers are sitting right on top of my ground floor slab contributing to fantastic bass.  My challenge is the opposite, to decouple my turntable.

You have several challenges in that department. Biggest is being on the second floor and on top of resonating floor joists. Many people in this situation cross block and other renovations underneath to address this if they are above a basement where the framing is easily accessible.

In your case you need to endeavor to couple your MLs to the sub floor. You have a nearly worst case scenario right now if you are still running them without any feet, not even the ones provided by ML. This is a very important aspect of the speaker design and I’m hoping it has a lot to do with the lacking bass in your MLs…

Granite tombstones wouldn’t be over kill given your room challenges, but any very heavy object will serve to couple to the floor, then just put your MLs, and maybe your power amps, on top of that.  It’s either that or spike your wood floors…  Not. Spikes on a penny will help, but i suspect its a half measure. Try it first, it’s cheap 🙂

 

Actually, mounting the shelf to the wall went very well.  Too well.  My first instinct was to hang it off the drywall and thereby be more isolated from the studs and therefore the floor where my speakers stand.  So I got some drywall anchors rated for 80 pounds and started down that road.  Hence the two unused holes in the wall you see in the photo, anchors actually, I just left them there.

It became immediately apparent that the dry wall anchors and brackets couldn’t handle the weight of my Denon and the two very heavy wood slabs I’ll be using as an isolation platform: 30 + 9 +9 = 48 pounds.

No worries… I go to Home Depot and get some uber heavy duty shelf support brackets, secure them 3 inches into my studs with 3 x  5/8” lag bolts per bracket, then set the wood platform and Denon on that.  Rock solid.  I could stand on one of those brackets and it wouldn’t sag or pull out from the wall one mm.

So I grab a record (APP I Robot), do my routine, then set the needle down in eager anticipation.  The first thing I notice is what’s not there… No more surface noise.  Gone, dead quiet as the needle touches down and I’m thinking, “Wow… this is gonna be good”.

Then, as the music kicks in I get immediate acoustic feedback.  I had a hand on the volume control and it was a good thing as acoustic feedback forms a positive feedback loop that can rapidly run out of control and start destroying speakers and power amps.  I’m like…  “bummer” as I play with the PFB loop and notice exactly where it starts to run wild and I turn it back down.

So, I almost directly coupled my Denon TT to my speakers through the floor (the Kefs are obviously very well coupled, a very good thing) to my studs to my shelf to my TT.  My teak rack may be creating other vibrations, but it is very effectively sheltering my TT from the “big bad wolf”, runaway PFB.  Oh well, live and learn.  I was planning for my TT isolation method to be three fold – shelf combined with first stage isolation (rubber isolators and a bike tube between the very solid and heavy wood shelves) then finally direct coupling with spikes of the TT plinth to the upper wood shelf, to take the TTs internal vibrations away from the needle.

Stage one, the shelf, clearly isn’t working, by itself anyway.  gotta wait for my spikes, isolation pads, and bike tube to show up before I experiment more.  Until then, it’s back to listening to pretty amazing analog sound quality, with a little cabinet resonance in the mix.  It’s subtle… something you notice more by virtue of it’s absence than its presence.

 

Spikes are clearly acoustical couplers (that’s why you want them on the bottom of your speakers), so why would I want to couple the upper platform to my TT you may ask?…  Well, by doing so I effectively increase the mass of my TT plinth by 9 pounds whilst the entire thing is isolated.  And, by virtue of their design they act as mechanical diodes (one way valves) to remove any mechanical vibrations from my TT motor away from my stylus (page 406).

The acoustic adventures continue…

More later…

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