Natural Frequency

fₙ = (1/2π)√(k/m). If you know the static sag instead of the stiffness, fₙ = (1/2π)√(g/δ) ≈ 15.76/√δ(mm). Isolation only starts above √2 × fₙ.

Spring-mass

Natural frequency
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Damped fd
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Static deflection
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From static deflection

Natural frequency
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Isolation

Transmissibility
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Single degree of freedom. A helper, not a substitute for engineering judgment.