Why can isolation efficiency be negative?

Because below a frequency ratio of the square root of two, transmissibility is greater than one: the mount transmits more force to the structure than a rigid connection would. That is genuine physics, not a modelling artefact, and it is the reason this page never clamps the number at zero. Showing zero would tell an engineer the mounts are doing nothing when they are actively making the problem worse.

Why does the page ignore damping?

It uses the undamped design curve, which is what isolator manufacturers publish and what selection decisions are made against. Away from resonance the difference is negligible, and adding damping there slightly reduces predicted isolation rather than improving it. Near resonance damping dominates completely, which is exactly why this page refuses to give a confident number in that band and flags it instead.

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

Move it. The options are more deflection so the natural frequency drops below the forcing frequency, a heavier inertia base which does the same thing, or a change in running speed. Damping can limit the peak but never makes resonance a good operating point, and a machine that starts and stops frequently will pass through it every cycle regardless.

How does this relate to the Resonance Frequency Check?

They share the same physics and answer adjacent questions. The resonance check asks whether the mount's natural frequency is safely clear of the forcing frequency and reports the ratio; this page takes the same ratio and converts it into the percentage of vibration kept out, which is the number specifications are written in. Use the resonance check when screening a design and this page when quoting against a requirement.

Does 95% isolation mean the floor will feel 95% quieter?

No. It means 95% of the dynamic force at the stated frequency does not pass through the mounts. What a person feels or an instrument measures also depends on floor stiffness, the building's own resonances, and any rigid path that bypasses the isolators. A perfectly isolated machine with a hard-piped drain line can still shake a floor through the pipe.

Do more mounts improve isolation?

Not by themselves. Adding mounts spreads the same weight over more of them, so each deflects less, the natural frequency rises, and isolation gets worse. What matters is deflection under load, not the number of mounts. If you add mounts, soften them proportionally or the change is a downgrade.

Acoustic, Noise, Vibration & NVH Products calculator

Vibration Isolation Efficiency Calculator

Predict how much of a machine's vibration its isolators will actually keep out of the floor, before the mounts are bought. Enter the static deflection the mounts show under the equipment's operating weight, the machine's running speed, the harmonic order that drives the vibration, and the isolation the specification calls for. The calculator converts deflection into the mount's natural frequency, compares it with the forcing frequency, and returns the isolation efficiency along with the deflection that would meet the target. Everything follows from the ratio between those two frequencies, and the answer is often counter-intuitive: a heavy machine on stiff mounts can be worse than no mounts at all.

What this calculator does

Predict what percentage of a machine's vibration a set of isolators will keep out of the structure, from the mount's static deflection and the machine's running speed.
Use it for choosing isolator deflection before ordering mounts for a new machine, explaining why a machine on new mounts got louder rather than quieter, checking a variable-speed drive at its lowest running speed, turning a written isolation specification into a deflection a supplier can quote, screening whether an existing installation can meet a tightened requirement without a redesign.
Predict what percentage of a machine's vibration a set of isolators will keep out of the structure, from the mount's static deflection and the machine's 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 efficiency = (1 − transmissibility) × 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. Measure it in place; a catalogue figure is the deflection at rated load, not at yours.
Machine running speed: Speed of the rotating element that drives the vibration.
Harmonic order of concern: 1 for shaft imbalance, 2 for misalignment, blade or vane count for fans and pumps.
Target isolation efficiency: The isolation the specification calls for. Typical targets run 80–90% for general machinery and 95%+ for precision equipment or sensitive neighbours.

How to use the result

Best suited to choosing isolator deflection before ordering mounts for a new machine, explaining why a machine on new mounts got louder rather than quieter, checking a variable-speed drive at its lowest running speed, turning a written isolation specification into a deflection a supplier can quote, screening whether an existing installation can meet a tightened requirement without a redesign.
Near a frequency ratio of one the undamped curve is unbounded; the real peak is set by damping, which this page does not model. That region is flagged, not estimated. One degree of freedom only. Rocking, pitching and the other rigid-body modes have their own natural frequencies and are not covered. Ignores floor flexibility, which introduces a second resonance no mount choice can remove. Says nothing about flanking paths. Piping, conduit and drains routed rigidly around the isolators can carry more vibration than the mounts pass. Predicts a frequency-domain ratio, not an amplitude. It does not tell you whether the remaining vibration is acceptable, only what fraction gets through.

Common questions

Why can isolation efficiency be negative? Because below a frequency ratio of the square root of two, transmissibility is greater than one: the mount transmits more force to the structure than a rigid connection would. That is genuine physics, not a modelling artefact, and it is the reason this page never clamps the number at zero. Showing zero would tell an engineer the mounts are doing nothing when they are actively making the problem worse.
Why does the page ignore damping? It uses the undamped design curve, which is what isolator manufacturers publish and what selection decisions are made against. Away from resonance the difference is negligible, and adding damping there slightly reduces predicted isolation rather than improving it. Near resonance damping dominates completely, which is exactly why this page refuses to give a confident number in that band and flags it instead.
My machine sits right at the resonance band. What now? Move it. The options are more deflection so the natural frequency drops below the forcing frequency, a heavier inertia base which does the same thing, or a change in running speed. Damping can limit the peak but never makes resonance a good operating point, and a machine that starts and stops frequently will pass through it every cycle regardless.
How does this relate to the Resonance Frequency Check? They share the same physics and answer adjacent questions. The resonance check asks whether the mount's natural frequency is safely clear of the forcing frequency and reports the ratio; this page takes the same ratio and converts it into the percentage of vibration kept out, which is the number specifications are written in. Use the resonance check when screening a design and this page when quoting against a requirement.
Does 95% isolation mean the floor will feel 95% quieter? No. It means 95% of the dynamic force at the stated frequency does not pass through the mounts. What a person feels or an instrument measures also depends on floor stiffness, the building's own resonances, and any rigid path that bypasses the isolators. A perfectly isolated machine with a hard-piped drain line can still shake a floor through the pipe.

Last reviewed 2026-08-25.