Passive vibrational isolation tables offer the most vibrational noise reduction for the price. They work on the same basic principle as the suspension of a car - though the wheels move up and down rapidly as you drive over a bumpy road, the spring supporting the mass of the cab keeps passengers from feeling the vertical bounce. Unlike air tables, in which air pumped into the system acts as the spring, and active tables, which use sensors and actuators to electronically correct for positional information, passive isolators are entirely passive, as the name implies.

<figcaption> Credit: Illustration: Andrew Meehan</figcaption>
Credit: Illustration: Andrew Meehan

1. Vertical vibrations are isolated by the spring's interaction with four pairs of flexures. The weight of the instrument compresses the pre-loaded spring, floating the isolator and aligning the flexures.

2. A squeeze force from another spring, controlled by the knob, is applied to the outside...

The idea is simple, explains David Platus, president of Minus K Technology, which manufactures such systems. "A passive isolator can be a piece of cork - as long as it provides a much lower frequency than the frequency of the vibration you want to attenuate." The lower the natural frequency of your isolation system, the lower the frequencies it will be able to cancel; the bigger the gap between your system and the noise it's combating, the better the isolation. (Minus K tables operate at about 0.5 Hz, and start isolating at 0.7 Hz.) Minus K tables combine a stiff spring with a "negative stiffness mechanism," which effectively loosens the spring while maintaining its load supporting capacity. For example, if a 10-pound load would normally deflect the top of the spring downward by an inch, that same deflection might take just a single pound.

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