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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?
Follow Ups:
Thanks for the reply.
I once used the nuclear opinion you mentioned, speakers in front room, turntable and amp in another. Not super handy, but the kenwood table I used at the time would howl at the sound levels some enjoyed, so a solution was found. It had a resin mixed with stone powder base, but the suspension was up to you. It was replaced, and easily bettered.
I don't have the technical chops to do a lot of productive explorations, unless you want to count on random experiments and dumb luck for your results. So I play follow the leader, reading and selecting methods that seem reasonable. Vibrations and waveform understandings are interesting to me.
I saw the slabs pictured, and thought I should mention I also had some , in case you had attempted to used them in turntable supports.
IN reading, folks say the material, marble, excites too easily, and since you are a guy with resonant frequency equations easily at hand, I figured you might have a successful way to use them . I suppose I could weigh them and figure with the math you provided. PRobably won't happen , the spring deflection would be hard to control, for a DIY result that wouldn't need a lab coat and tongs to play a record.
I once read a story by an audio reviewer, who, in seeking a quit place to listen to music, thought he had found the answer. An old english cottage built over a giant rock, the exposed portion was exploited to use as ledge and hearthstone for the fireplace. He figured the buried rock was at least fifty tons , probably more. The perfect mass isolation platform for his turntable. All he was looking for was a quiet electrical situation out in the country, and figured he got really lucky. Until he bought the place and found that a local rock quarry used the country road, but only at night, so people aren't troubled. The big rock picked up the rumble from the road a mlle and a half away and microphoned it right into his turntable. The guy didn't have many options, this was back in the fifties, hi fi was new, he was building the hobby. .
When you eliminate vibration from vinyl playback, the improvement is quite noticeable and very desirable. My most recent vinyl recordings have been made using 5.6-MHz DSD, which is exceedingly accurate and transparent. I monitor my recording sessions with headphones so my speakers remain off. This pretty much eliminates acoustic and mechanical feedback from the recordings. Then, when I play the recordings at high volume through my speakers, the sound quality is awesome. The music sounds noticeably cleaner than playing vinyl directly at high volume.
Best regards,
John Elison
That does seem like the best electrical strategy to fight vibration in playback, separate the signal generators from external excitement. Pretty cool , and clean.
Myself , I would have just grabbed another record and listened to twice as much stuff in the same time. Once the music starts, I have a high tolerance for added dreck , a trick my mind is used too, , having grown up with a consul changer in the front room , full of 78's and the playback needle that god gave you, why would you ever need to get a new one? Plus, it could flip over to 33 1/3.
Your first statement about vibration in vinyl is so very true, and hit me hard when I got a quieter table. All of a sudden stuff was flying around the room with some rock records I had owned from adolescence . The soundstage was an entirely different illusion, going from speakers are not evident , to, I can listen to directions in the hall, if an instrumental section or solo catches my attention.
Makes it easy to understand why there is a market for super turntables the size of an arm chair, with cost like scientific equipment. Not that I would ever need or want one, but I can see the game from my cheap seats, finally, as I assembled stuff over time.
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