Why can isolation efficiency be negative?

Below a frequency ratio of the square root of two, transmissibility exceeds one: the mount transmits more force than a rigid connection. That is genuine physics, which is why the page never clamps the number at zero.

Why does the page ignore damping?

It uses the undamped design curve, which is what isolator manufacturers publish. Away from resonance the difference is negligible, and near resonance damping dominates, which is why that band is flagged rather than estimated.

My machine sits right at the resonance band. What now?

Move it. More deflection so the natural frequency drops below the forcing frequency, a heavier inertia base, or a change in running speed. Damping limits the peak but never makes resonance a good operating point.

Do more mounts improve isolation?

Not by themselves. More mounts spread the same weight, so each deflects less, the natural frequency rises and isolation gets worse. Deflection under load is what matters; soften mounts proportionally if you add them.

Acoustic, Noise, Vibration & NVH Products calculator

Vibration Isolation Efficiency Calculator

Predict how much of a machine's vibration its isolators keep out of the floor, before the mounts are bought. Enter deflection, running speed, harmonic order and target; the page returns isolation efficiency and the deflection that meets the target.

What this calculator does

Predict what percentage of a machine's vibration its isolators keep out of the structure, from static deflection and running speed.

Formula used

Disturbing frequency = machine speed × harmonic order ÷ 60
Mount natural frequency = 3.13 ÷ √(static deflection in inches)
Frequency ratio r = disturbing frequency ÷ natural frequency
Transmissibility = 1 ÷ |1 − r²| (undamped design curve); isolation = (1 − T) × 100
Deflection for a target = (3.13 × √(1 + 1 ÷ (1 − target)) ÷ disturbing frequency)²

Inputs explained

Static Deflection Under Load: How far the mount sinks under the equipment's operating weight.
Machine Running Speed: Speed of the rotating element that drives the vibration.
Harmonic Order of Concern: 1 for imbalance, 2 for misalignment, blade count for fans.
Target Isolation Efficiency: Isolation the specification calls for; 80 to 90% suits general machinery.

How to use the result

Best suited to choosing isolator deflection before ordering mounts, explaining why new mounts made a machine louder, turning an isolation spec into a deflection.
Near a ratio of one the undamped curve is unbounded; the real peak depends on damping this page does not model. One degree of freedom only: rocking and pitching modes are not covered. Ignores floor flexibility and flanking paths through rigid piping or conduit.

Common questions

Why can isolation efficiency be negative? Below a frequency ratio of the square root of two, transmissibility exceeds one: the mount transmits more force than a rigid connection. That is genuine physics, which is why the page never clamps the number at zero.
Why does the page ignore damping? It uses the undamped design curve, which is what isolator manufacturers publish. Away from resonance the difference is negligible, and near resonance damping dominates, which is why that band is flagged rather than estimated.
My machine sits right at the resonance band. What now? Move it. More deflection so the natural frequency drops below the forcing frequency, a heavier inertia base, or a change in running speed. Damping limits the peak but never makes resonance a good operating point.
Do more mounts improve isolation? Not by themselves. More mounts spread the same weight, so each deflects less, the natural frequency rises and isolation gets worse. Deflection under load is what matters; soften mounts proportionally if you add them.

Last reviewed 2026-10-01.