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Why Your Isolation Pad Made Vibration Worse: Resonance, Natural Frequency, and Transmissibility
Bolt a soft pad under a vibrating box and the box can shake harder, not softer. The pad did not fail; it was mistuned. An isolation mount is a small spring-mass system with a resonant frequency of its own, and whether it isolates or amplifies depends on where the disturbing vibration sits relative to that resonance. This article explains natural frequency and transmissibility in plain language, the three ways a mount ends up mistuned, and how to design one that lands in the isolation region.
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Adding a pad can amplify the very vibration you set out to kill
It is one of the most counterintuitive results in mechanical design: bolt a soft pad under a shaking box and the box can shake harder, not softer. The pad did not fail; it was mistuned. An isolation mount is a tiny spring-mass system with a resonant frequency of its own, and whether it isolates or amplifies depends entirely on where the disturbing vibration sits relative to that resonance.
Get the relationship right and the pad earns its place; get it wrong and you have built an amplifier. Understanding why is the difference between specifying an isolator and guessing at one.
What natural frequency actually describes
Put a mass on a resilient pad and you have a spring and a weight. Nudge it and it bounces at a particular rate: its natural frequency. The pad's stiffness is the spring, and the supported equipment is the mass, so the two together set that frequency. Soften the pad or add mass and the natural frequency drops; stiffen the pad or remove mass and it rises. Nothing about the material alone fixes the natural frequency, which is why a grade that isolates one box can amplify on another: same pad, different mass, different tuning.
The transmissibility curve, in plain language
Transmissibility is simply the ratio of motion coming out of the mount to motion going in. It is not a flat number; it depends on how the disturbing frequency compares to the mount's natural frequency. Below a frequency ratio of roughly 1.4 (the square root of two), the mount passes vibration through or amplifies it. Right at the natural frequency, at resonance, it amplifies the most, limited only by damping.
Above that crossover, the mount finally isolates, and isolation keeps improving the further the disturbing frequency sits above the natural frequency. The whole game is to land the mount's natural frequency well below the lowest frequency you need to isolate, so you are working in the isolation region rather than the amplification region.
How a mount ends up mistuned
The pad is too stiff for the mass
The most common error is a pad chosen for its footprint or load rating without checking the resulting natural frequency. Too stiff, and the natural frequency climbs into or above the disturbing frequency, putting the mount in the amplification region. The fix is almost always a softer grade or more deflection, which lowers the natural frequency back under the disturbance.
The pad has no room to work
An isolator needs travel. A pad that is too thin, or one squeezed past its comfortable deflection by an over-torqued mount, bottoms out and behaves like a hard shim. Its effective stiffness shoots up, the natural frequency rises with it, and isolation evaporates. Specifying enough thickness and the right operating deflection is what keeps the spring soft.
The pad relaxes over time
A mount tuned perfectly at installation can drift in service if the pad takes a compression set, losing thickness and shifting its stiffness. This is exactly why low compression set is so valued in isolation grades such as PORON microcellular urethane: a pad that holds its thickness holds its tuning, where one that relaxes slowly walks the natural frequency toward the amplification region over years.
Reading a Mount That Is Not Isolating
| Symptom | Likely cause | Direction |
|---|---|---|
| Vibration worse with the pad in | Natural frequency at or above the disturbance (too stiff) | Softer grade or more deflection to lower natural frequency |
| Isolation good then degrades over months | Compression set relaxing the pad | Low-compression-set grade; verify operating deflection |
| Mount feels rock-hard under load | Pad bottomed out; too thin or over-compressed | More thickness; reduce installed deflection |
| Big resonant peak during startup or sweep | Passing through resonance with little damping | Add damping to tame the peak; accept slight high-frequency penalty |
| Quiet across the operating band | Natural frequency well below the disturbance | Working as intended; hold the tuning over life |
Directional guidance only; the actual natural frequency depends on the supported mass, the pad stiffness, and the geometry of your mount. Confirm with your own analysis.
Designing the mount to land in the isolation region
The design target is straightforward to state: place the mount's natural frequency well below the lowest frequency you need to isolate. In practice that means choosing a soft enough grade, giving it enough thickness to deflect, and accounting for the supported mass per mount. If the equipment must pass through resonance on the way up to speed, or sweep through it during a qualification profile, add damping to tame the resonant peak, accepting a small loss of isolation at the very highest frequencies as the trade.
The material supplies the resilience and the damping; the tuning comes from matching that material to the mass and the footprint.
How H-O Products helps
H-O Products does not tune your vibration system, and no converter should claim to; what we do is cut the pad your analysis calls for, accurately and repeatably.
We die-cut and waterjet PORON, silicone-foam, and cork isolation pads to thickness and footprint, back them with adhesive, and kit them by mount location, all under our ISO 9001:2015 certified quality management system with lot-coded traceability and the maker's force-deflection and compression-set data on the order. Tell us the grade and geometry, or the mass and frequency you are designing around, and we will help land the part.
Frequently asked questions
Why would an isolation pad make vibration worse?
Because it was too stiff for the supported mass, putting the mount's natural frequency at or above the disturbing frequency, where the mount amplifies rather than isolates. A mount only isolates once the disturbing frequency sits well above its natural frequency. The usual fix is a softer grade or more deflection, which lowers the natural frequency back under the disturbance.
What is transmissibility?
It is the ratio of motion coming out of a mount to motion going in. Below a frequency ratio of about 1.4 times the natural frequency the mount amplifies; at resonance it amplifies the most, limited by damping; above that crossover it isolates, improving the higher the disturbing frequency sits above the natural frequency. Designing an isolator means landing the natural frequency well into that isolation region.
How does compression set detune a mount over time?
If a pad slowly loses thickness to compression set, its stiffness shifts and the mount's natural frequency drifts, so a mount tuned at installation can walk toward the amplification region over years. That is why low-compression-set grades such as PORON microcellular urethane are valued for isolation: holding thickness means holding the tuning.
Does adding damping always help?
Damping tames the resonant peak, which matters if equipment must pass through or sweep across resonance during operation or a qualification profile, but it slightly reduces isolation at the highest frequencies. It is a trade, not a free improvement. Use it when crossing resonance is unavoidable; rely on correct tuning, a natural frequency well below the disturbance, for the steady-state isolation itself.
What does H-O need to help with an isolation pad?
The supported mass per mount and the disturbing frequency you need to isolate determine whether a pad isolates, so send those along with the available footprint and thickness. We will convert the grade and thickness your analysis calls for to your drawing; the vibration analysis and any qualification testing remain with your program, since H-O is a converter, not a test house.
Have a mount that is not isolating reviewed
Send the supported mass per mount, the footprint and thickness available, and the disturbing frequency or source RPM, and we will help land the right grade and thickness and quote converted pads to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D3574 (flexible cellular urethane, force-deflection and compression set); ASTM D395 (compression set); ASTM D2632 (resilience); ASTM D1056 (flexible cellular rubber).
- MIL-STD-810 and RTCA DO-160 (environmental and vibration test methods). Cited by designation for orientation; H-O performs no qualification testing.
- Manufacturer technical data sheets: PORON 4701 / 4790 microcellular urethane, silicone foam, cork wonder pads. Single-degree-of-freedom isolation behavior is standard vibration theory, summarized qualitatively.
This article summarizes vibration-isolation and NVH material-selection principles using the families and method designations cited on the parent application page. Comparisons are qualitative; mechanical values are per each grade's current technical data sheet, and no test results are quoted.
Test standards such as MIL-STD-810 and RTCA DO-160 are named by designation for orientation only; H-O performs no qualification testing and makes no airworthiness determinations. All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a converter, not a raw-material producer.