📏 Required Static Deflection
📊 Achieved Isolation Efficiency
Sizing Checks
Full Calculation Table
| Parameter | Value |
|---|
How to Size a Vibration Isolator for HVAC Equipment
Enter the equipment's operating speed in RPM, which determines the forcing (disturbing) frequency.
Choose pad, rubber mount, or spring isolator based on the level of isolation efficiency needed.
Enter the percentage of vibration transmission you want to eliminate, typically 90 percent or higher for sensitive installations.
Click Calculate to determine the required static deflection and natural frequency for the selected isolator to achieve the target efficiency.
Isolator Type Selection by Equipment Speed
| Isolator Type | Typical Static Deflection Range | Best Suited For |
|---|---|---|
| Elastomeric pad | 1–5 mm | High-speed equipment above 1800 RPM, light loads |
| Rubber mount | 5–12 mm | Medium-speed equipment 600–1800 RPM |
| Spring isolator | 15–75+ mm | Low-speed equipment below 600 RPM, cooling towers, large fans |
General guideline ranges. Confirm final isolator selection against manufacturer load-deflection data and the specific equipment weight distribution.
Sizing Vibration Isolators for HVAC Equipment
Rotating HVAC equipment such as fans, pumps, compressors, and cooling towers generates vibration at its operating speed, which can transmit through rigid equipment mounts into the building structure and re-radiate as noise in spaces far from the original equipment location, sometimes on entirely different floors. Vibration isolators, whether elastomeric pads, rubber mounts, or springs, interrupt this transmission path by supporting the equipment on a resilient element rather than a rigid connection.
Natural Frequency and the Frequency Ratio
Every isolator-and-equipment combination has a natural frequency, determined by the static deflection the isolator experiences under the equipment's weight: fn = 15.76/√δ, where fn is natural frequency in Hz and δ is static deflection in millimetres (a form of this relationship follows directly from the physics of a simple spring-mass system). The isolation performance depends on the ratio between the equipment's forcing frequency, determined by its operating speed, and this natural frequency. A frequency ratio below approximately 1.4 provides no isolation benefit and can actually amplify vibration through resonance; meaningful isolation only begins above this ratio and improves progressively as the ratio increases further. This selection procedure follows the approach set out in the Sound and Vibration chapter of the ASHRAE Handbook, Fundamentals volume.
Transmissibility and Isolation Efficiency
Transmissibility is the fraction of vibration force that passes through the isolator to the supporting structure, calculated from the frequency ratio using the standard transmissibility formula for an undamped single-degree-of-freedom system. Isolation efficiency, the more commonly specified design parameter, is simply 100% minus transmissibility expressed as a percentage. This calculator works backward from a target isolation efficiency to determine the frequency ratio, and therefore the natural frequency and required static deflection, needed to achieve that target for the given equipment operating speed. Once an isolator type is selected, how it is restrained against lateral movement and seismic loading is a separate design step, covered by SMACNA's technical standards, including its Seismic Restraint Manual for mechanical systems.
Why Low-Speed Equipment Needs More Deflection
Because the required frequency ratio for good isolation is roughly fixed regardless of equipment speed, achieving that ratio at a low operating speed requires a correspondingly low isolator natural frequency, which in turn requires large static deflection. This is why large, slow-turning equipment such as a cooling tower, selected with a cooling tower calculator, and large centrifugal fans, often operating below 600 RPM, typically require spring isolators capable of 25 millimetres or more of static deflection, while smaller, faster equipment such as a circulating pump, sized separately with a pump sizing calculator, can achieve the same isolation efficiency with a rubber mount deflecting only a few millimetres.
Vibration Isolation Is Not a Substitute for Airborne Noise Control
Vibration isolation addresses structure-borne transmission specifically; it does not reduce airborne noise radiating directly from the equipment surface into the immediate mechanical room or adjacent space. A complete noise control approach typically requires both vibration isolation for structure-borne concerns and separate airborne treatment, such as the equipment sound level calculator and acoustic insulation calculator for direct radiated and lagging-related noise, and the sound transmission calculator for wall or floor isolation if the mechanical room itself needs to be acoustically separated from occupied spaces.
Frequently Asked Questions
Static deflection, the amount an isolator compresses under the weight of the supported equipment, directly determines the isolator's natural frequency, and the ratio between the equipment's operating frequency and the isolator's natural frequency determines isolation efficiency. A larger static deflection produces a lower natural frequency, resulting in a greater frequency ratio for a given equipment speed and higher isolation efficiency. This is why low-speed equipment generally requires isolators with larger static deflection, such as springs, to achieve good isolation, while higher-speed equipment can often achieve adequate isolation with smaller-deflection rubber or pad-type isolators.
When the forcing frequency is close to the isolator's natural frequency, the system operates near resonance, where the isolator actually amplifies vibration transmission rather than reducing it, potentially transmitting more vibration than if no isolator were present. Effective isolation only begins once the frequency ratio exceeds approximately 1.4, and meaningful efficiency, typically 80% or higher, generally requires a ratio of 3 or more. This is why isolator selection must always be checked against the specific equipment operating speed using a calculation like this one, rather than assuming any isolator provides isolation regardless of frequency relationship.
Spring isolators are generally needed when equipment operates at low speed, such as large slow-turning fans or cooling towers below roughly 600 RPM, since achieving an adequate frequency ratio at low speeds requires the large static deflection only springs can practically provide. Rubber or elastomeric pad isolators are typically adequate for higher-speed equipment above roughly 1200 RPM, where smaller static deflection already produces a sufficient frequency ratio. Selecting spring isolators for high-speed equipment that doesn't need them adds unnecessary cost, while selecting rubber isolators for low-speed equipment likely fails to achieve adequate isolation. Use this calculator to confirm the correct isolator type for your specific equipment speed.
Vibration isolation primarily addresses structure-borne noise and vibration, transmitted through the building structure from equipment mounting points, which can re-radiate as noise in remote spaces. It does not directly address airborne noise radiated from the equipment surface into the immediate space, which requires separate treatment. A complete noise control strategy typically requires both vibration isolation and separate airborne treatment; use the equipment sound level calculator and acoustic insulation calculator to address the airborne component alongside this isolator sizing.
Related Noise & Acoustics Tools
Airborne noise from equipment
Pipe and duct lagging
Wall and floor isolation
Fan-generated noise estimate
Combine multiple sources
Target levels by space type