Huh? Isolation stands for audio applications have routinely gotten the resonant frequency down to below 10 Hz, almost any of the usual suspects have Fr around 3 Hz or less. The Champs at Fr are my Nimbus Sub-Hertz Platform at 0.5 Hz and the Minus K, also around 0.5 Hz. Without jumping through a whole lot of hoops it’s fairly easy to obtain 3Hz. The trick is getting isolation in more than one or two directions, of which there are six, actually a lot more if you count the boatload of direction in the horizontal plane. Zee plane, boss! Zee plane! |
Not to be too picky ⛏ but it’s actually that the Earth’s crust is moving like a wave, that’s what produces the *very low frequency* up and down movement of the building, and the horizontal movement of the building and the rotational forces that try to rotate and twist the building. It’s just like someone shaking out a carpet. This is not to say other things produce seismic type vibration, you know, wind, wave action, traffic, subways, footfall, fans, etc.
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I already explained that the building and everything in it - including the cement slab - are vibrating right along with the Earth crust motion, at frequencies between 0 Hz and 20 Hz and higher. The peak frequency of Earth crust motion is around 1-3 Hz. Now, I’ll grant you, that cement slabs are not very affected by rotational forces. |
For an iso stand stiffness is highly desirable for the top plate since a stiff top plate better resists bending forces, I.e., rotational seismic forces. This is why 3” maple boards sound better - generally speaking - than 1” maple boards. They’re stiffer. It’s also why some of the big commercial iso platforms use thick granite slabs for the top plate. It’s also why I use thick granite or bluestone slabs for my stands sometimes.
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The big problem with the “heavy and massive” approach is the entire building is moving, thus everything that’s not decoupled from it is moving right along with it. The mass of the building itself cannot withstand the enormous forces produced by seismic type vibration. “Resonance Control doesn’t work for very low frequencies or is very ineffective. That’s where isolation steps up to the plate. Pretty basic stuff.
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For turntables the primary problem is the resonant frequencies of the tonearm and cartridge are very low, circa 10-12 Hz - too low for the acoustic waves from speakers, but just right for very low frequency seismic type vibration, you know, the ones coming up from the floor. Thus, the tonearm and cartridge are excited by seismic type vibration in the range 10-12Hz. So, the answer is YES, turntables should be isolated. Final answer. 🔚
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Re pneumatic isolators, it might be obvious, maybe not, but using three pneumatic isolators, If it can be done with stability, is better than using four. At least theoretically. The reason is because the total spring rate is the spring rate per isolator x the number of isolators. And since resonance frequency Fr of the system is a function of total spring rate (and mass) you will get lower Fr with three isolators, all things being equal. My original Nimbus employed only one (!) pneumatic isolator, which is how I got the Fr down below 1.0 Hz. As opposed to air bladders and bicycle inner tubes, tall and narrow pneumatic isolators have the ideal geometry.
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I don’t see rigidity in the equation F=ma. You must have better eyes 👀 than I do. The problem generally for rigid racks is they move right along with the entire building’s motion. Even something as massive as a building is moving in all 6 directions due to seismic type forces. That’s why decoupling the component is the only real solution to the low frequency vibration problem. Mass per se doesn’t save you.
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The best isolation occurs when there is a great ease of motion. Therefore, ironically and perhaps counter-intuitively, a flimsy and flexible stand would sound better than a rigid one. A rigid frame actually transfers vibration more efficiently than a rickety frame. |
The problem for two stacked iso (spring) systems is they will interfere if the Fr resonant frequencies are close to each other. My Nirvana 2 stage isolation base has two stacked heavy mass-on-spring layers, where the Fr for each stage is predetermined to not be close enough to interfere. The lower layer of springs supports a much higher load than the top layer of springs. If two spring systems are arbitrarily stacked it will be like a car going down the road with two shock absorbers per wheel. It would not be smoother, it would be very bumpy.
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Both the vibraplane and my Nimbus single airspring platform of yore employed “underslung masses” to achieve very low Fr isolation and lateral stability. The Nimbus also employed a large aux air canister to boost the effectiveness of the single relatively small air spring. An air spring on steroids. Most iso stands cannot isolate in the rotational directions, which are also important. Isolation effectiveness is proportional to the ease of motion in a particular direction.
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Symposium discovers springs. Whoa! Shut my mouth and call me corn pone! |
