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RE: Floor vibration vs. Airborne sound

Posted by mr.bear (A) on June 20, 2017 at 12:01:59

In Reply to: RE: Vibration Isolation Technical Article- Interesting posted by beach cruiser on June 20, 2017 at 03:12:03:

I've been noodling on this a few years......35 or so, as a structural and manufacturing engineer, and there are a few ideas that seem basic.

Isolating floor vibration: Mass is a good idea- like the mass of a heavy slab. But that mass has to be suspended on soft, flexible springs. By soft I mean the mass must result in substantial deflection of the springs, like a couple inches! That much deflection is needed to give you a very low resonant frequency, which needs to be low to result in isolation from floor vibrations.

Floor vibrations seems to center around 5Hz or so. To get any degree of isolation requires the mass-spring isolator have a resonant frequency below say 3Hz. For the curious, the resonant frequency Fn = (1/2*pi)*sqrt(g/deflection),, where g= acceleration of gravity (386 in/sec.sec). Use consistent units.

One commercial HiFi product that applied this principal effectively was VPI's aftermarket plinths for the Denon DP-75/80 turntables. The oak base section housed a 50-lb. (!) steel/plastic laminated slab on big springs that I measured deflection of around 2". Worked!

There are the airborne sources of vibration to consider, but they're a unicorn of a different color. You have several ways the soundfield in your room affects the turntable:

One aspect of airborne pickup is direct excitation of the components of the table or the surface of the LP. Dustcovers seem to be a good source to pick up airborne sound, as do light shelves. How this affects your particular turntable is highly variable ns depends on the isolation features of the table: suspensions, feet, etc) and the will of the gods.

The result of airborne pickup is generation of "surface acoustic waves" (Rayleigh Vibration) in the plinth, platter and LP. This touches on elastodynamics and is beyond the comprehension of all but about 13 Americans getting PHDs, but the idea is that a surface excited by sound pressure responds with waves in the solid medium. A simplified analogy is if you've ever stood next to a snare drum (not being played) when the orchestra is loud- the drum will respond and you hear the snares rattle. When this occurs in your turntable it's a bad thing. These waves bounce around, resonate, get polarized (Love's Waves) and may get to the cartridge. The energy is stored to a degree that relates to the depth and internal damping of the surface being bombarded. That's why you see turntables with heavy stone-like material for the plinth- it "sinks" SAWs, reducing pickup by the cart.

These effects are subtle and hard to trace. Some of the resonances add "euphonic" colorations and may become quite likeable. I think of them as friends... The nuclear option to avoid airborne vibration would be to put your speakers in another room, with the turntable preferably in the basement! An 'appeal to extremes...' eh?

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Schiit Audio

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