Isolators — Abridged Guide
Quick-reference guide to vibration isolators — types, transmissibility, damping, and selection. For full derivations and worked examples, see the Comprehensive Guide.
Comprehensive Isolators Guide →
1.Introduction to Vibration Isolators
Vibration isolators decouple a payload from floor vibrations by introducing a compliant element between the support structure and the optical table. Four technologies serve the photonics community: elastomeric, pneumatic, negative-stiffness, and active systems.
Isolators handle floor-to-table vibration; the optical table handles tabletop damping. They are complementary — a great table on rigid legs still transmits every floor vibration.
2.Isolator Classification
Each isolator type occupies a distinct frequency–load–cost region. Elastomeric mounts are simplest but limited to higher frequencies; pneumatic springs offer load-independent natural frequency; negative-stiffness mechanisms reach sub-Hertz isolation passively; active systems use servo loops for the lowest effective frequencies.
| Type | Vertical f_n (Hz) | Air/Power | Best For |
|---|---|---|---|
| Elastomeric | 5–30 | None | Breadboards, benchtop instruments |
| Pneumatic (passive) | 2–3 | Inflate once | General optical tables |
| Pneumatic (active) | 1–1.5 | Continuous air | High-performance optical tables |
| Negative-stiffness | 0.2–0.5 | None | AFM, SEM, interferometry |
| Active electronic | 0.5–2 (eff.) | Electrical | Upper-floor labs, semiconductor fab |
Pneumatic isolators maintain a constant natural frequency regardless of load — heavier payloads increase both the air pressure and the stiffness proportionally.
3.Fundamental Dynamics
Natural Frequency
Transmissibility
r = f_d / f_n (frequency ratio), ζ = damping ratio
Isolation occurs only when r > √2, meaning the disturbing frequency must exceed the natural frequency by at least a factor of 1.414. Below this threshold, the isolator amplifies vibrations.
A 2 Hz pneumatic isolator begins isolating above 2.83 Hz. At 10 Hz (r = 5), it achieves roughly 94% isolation with ζ = 0.1.
| r = f_d / f_n | T | Isolation (%) | Isolation (dB) |
|---|---|---|---|
| 1 (resonance) | 5.0 | −400 (amplified) | +14.0 |
| √2 (crossover) | 1.0 | 0 | 0 |
| 3 | 0.116 | 88.4 | −18.7 |
| 5 | 0.059 | 94.1 | −24.6 |
| 10 | 0.015 | 98.5 | −36.5 |
4.Damping in Isolator Systems
Loss Factor to Damping Ratio
Damping suppresses the resonance peak but degrades high-frequency isolation. Optimal damping for optical isolators is ζ ≈ 0.1–0.3 — enough to control resonance without sacrificing rolloff.
High-damping elastomers (η > 0.5) settle faster after disturbances but provide less isolation at high frequencies. Choose based on whether your application needs rapid settling (high-throughput manufacturing) or maximum isolation (sensitive metrology).
5.Pneumatic Isolator Design
Air Spring Stiffness
γ = 1.4 (air), P₀ = absolute pressure, A = piston area, V = air volume
Air springs maintain constant natural frequency regardless of payload mass because both stiffness and load scale with pressure. Dual-chamber designs with laminar flow damping achieve f_n of 1–2 Hz vertical with controlled resonance peaks.
Passive pneumatic isolators need only a one-time inflation (hand pump or air line). Active auto-leveling systems require a constant air supply but maintain table height automatically as loads change.
| Class | Vertical f_n | Auto-Level | Air Supply |
|---|---|---|---|
| Passive sealed | 2.5–3.5 Hz | No | Inflate once |
| Active auto-level | 1.0–2.0 Hz | Yes (±0.15 mm) | Continuous |
| High-performance | 0.7–1.5 Hz | Yes (±0.025 mm) | Continuous |
6.Elastomeric & Mechanical Springs
Elastomeric isolators are the simplest and lowest-cost option, requiring no air or power. Their achievable natural frequencies (5–30 Hz) limit them to isolating disturbances well above the 1–10 Hz range where building vibrations concentrate.
Match Sorbothane feet to 40–60% of their rated load capacity. Underloading raises the natural frequency above the useful range; overloading causes bottoming out.
Dynamic stiffness of elastomers is 20–50% higher than static stiffness. The actual natural frequency under vibration will be higher than predicted from static load-deflection data alone.
7.Negative-Stiffness Isolators
Net Stiffness
K_S = support spring stiffness, K_N = magnitude of negative stiffness
NSM isolators achieve 0.5 Hz or lower natural frequencies without impractical static deflections by canceling most of the spring stiffness with a negative-stiffness element. They are entirely passive — no air, no electricity.
At 0.5 Hz vertical: 93% isolation at 2 Hz, 99% at 5 Hz, 99.7% at 10 Hz. This exceeds pneumatic isolator performance by 3–10× at frequencies below 10 Hz.
8.Active Vibration Isolation
Active isolators use sensors, controllers, and actuators to measure and cancel vibrations in real time. They excel at suppressing low-frequency disturbances below 5 Hz but add cost, complexity, heat generation, and failure modes.
Hybrid active-pneumatic systems offer the best practical compromise: the pneumatic stage handles static load and high-frequency rolloff; the active loop suppresses the 1–5 Hz range and the resonance peak.
9.Practical Considerations
Rigid connections (taut cables, stiff hoses, papers wedged against walls) short-circuit the isolation path. Every connection between the isolated table and the surrounding structure must be compliant.
| Error | Effect | Fix |
|---|---|---|
| Taut cables/hoses | Transmits floor vibration directly | Add service loops with slack |
| Overloaded isolator | Bottoms out on mechanical stops | Redistribute load or upgrade capacity |
| Unlevel table | One or more isolators at travel limit | Adjust leveling indicators to nominal |
| Floor resonance | Amplifies vibration at structural frequency | Inertia block or relocate |
Always run cables and hoses in a loose service loop from the table to the floor. A single taut USB cable can negate thousands of dollars of isolation hardware.
10.Isolator Selection Workflow
Start with a floor vibration survey, calculate the required transmissibility from the instrument’s sensitivity specification, determine the maximum allowable natural frequency, and match to the appropriate isolator technology.
| Application | Recommended Isolator | Typical f_n |
|---|---|---|
| General laser experiments | Passive pneumatic | 2–3 Hz |
| Interferometry, fiber alignment | Active pneumatic | 1–1.5 Hz |
| AFM, SEM, nano-indentation | Negative-stiffness | 0.5 Hz |
| Upper-floor lab, semiconductor fab | Active / hybrid | < 1 Hz |
| Benchtop instruments, student labs | Sorbothane / elastomeric | 10–20 Hz |
The simplified estimate for required natural frequency: f_n ≤ f_d × √T_required. If the result is below 1.5 Hz, look beyond pneumatic to NSM or active.
Continue Learning
The Comprehensive Guide includes 6 worked examples, 5 SVG diagrams, 3 data tables, and 10 references.