Medical Isolation Platforms

Minus-k and Halcyonics are comparable in vertical-isolation performance and both are superior to the Vibraplane. The Minus-k systems are passive and have resonant frequencies (0.5 Hz) that are an order of magnitude lower than pneumatic isolators. Once set up, a Minus-k table needs no maintenance. In contrast, the Halcyonics units are active and require 110 V; they are also much more expensive. The extra $$ buy you somewhat better horizontal isolation compared to Minus-k; however, it's not clear that this offers much benefit in audio applications.

I have Minus-k systems in my laboratory for an atomic-force microscope and a surface-plasmon imaging apparatus. The effectiveness of the isolators can be readily measured with those instruments. What motivated me to buy Minus-k units for the lab was the obvious sonic benefit of mounting my turntable on a Minus-k BM-8 isolator at home. In my view this remains the most cost-effective upgrade I've ever done.

Jtimothya,

The BM-8 is their highest-performance isolator for loads in the 100-lb range and has the additional advantage of a low profile (about 4 in. high). I replaced the standard top plate with a 28x20x0.75-in slab of polished granite to support my VPI TNT 'table. The sum of these masses (minus the mass of the original top plate) comes just within the maximum load rating of the BM-8.

Ducatirider,

The isolator's resonant frequency is a good figure of merit for this type of analysis. All objects have a natural set of resonant frequencies; this mode structure is typically modeled in terms of mass/spring systems. The response of the mass/spring system to a periodic driving force is described by a simple mathematical expression, the Lorentzian function; this is the transmissibility curve that's seen in some manufacturers' technical literature. The Lorentzian has a peak at the resonant frequency, which becomes broader with increasing values of the damping factor (decreasing Q). At a damping factor of 1.0 the system becomes critically damped and no longer responds harmonically to the periodic driving force. With damping factors > 1 displacements of the system from mechanical equilibrium have an exponential response and the return to the equilibrium position becomes progressively slower as damping increases.

The broadening of the resonance peak that's introduced by high damping factors has the net effect of increasing transmissibility at frequencies above the resonance (relative to an undamped system). Therefore, instead of using a critically damped isolation system, it is more effective to have a weakly damped system with a resonant frequency well below that of the system you're trying to isolate.