Isolation puts a soft spring between a vibrating machine and its surroundings so less force gets through. The physics is simple, but the intuition trips people up: a stiffer mount is not always better — often it's worse.
It is all about the frequency ratio
Every mount + mass has a natural frequency fₙ. What matters is the ratio r = f/fₙ of the disturbance frequency to fₙ. Isolation only happens when r > √2 (≈1.4). Below that you either do nothing or, right at r = 1, amplify the vibration through resonance. Find fₙ with the Natural Frequency tool.
A machine running at 20 Hz is bolted rigidly to the floor. You fit soft mounts, giving the machine a natural frequency of 18 Hz. What happens to the force reaching the floor?
Isolation does not begin until the forcing frequency is above √2 × fn. Below that the mount amplifies, and at 20/18 = 1.11 you are sitting near the resonant peak — the worst place to be. To isolate 20 Hz you need fn below 20/√2 ≈ 14 Hz, and comfortably below it. Half-soft mounts are worse than no mounts at all, which is why “we tried rubber and it got louder” is such a common story.
Try it — the transmissibility curve
Drag the ratio down through √2 ≈ 1.41 and watch the curve cross above 1: the mount starts amplifying. Everything to the left of that line is a mount making the problem worse. Then raise the damping and watch it tame the resonant peak while degrading isolation out to the right.
Soft is good — to a point
To isolate, you want fₙ well below the forcing frequency (a common target is r ≥ 3, so fₙ ≤ ⅓ of the running speed). That means a soft mount and more static deflection. Rule of thumb: more static sag under the weight = lower fₙ = better isolation — but too soft and the machine wallows and the mount bottoms out.
Damping is a trade-off
Damping tames the resonant peak (vital when a machine passes through resonance on start-up and shutdown) but worsens isolation at high frequency. Rubber mounts have useful built-in damping; steel springs need a separate damper or a snubber for the pass-through.
Getting it right
Identify the lowest forcing frequency (often 1× running speed), choose a mount whose fₙ is well below it, check the static deflection is sensible, and make sure resonance on run-up is survivable. For sensitive equipment you isolate into the base (reduce transmitted motion) — same maths, transmissibility.