What is transmissibility in vibration isolation?

Transmissibility (T) is the ratio of transmitted force to the disturbing force. T 1 means the mount amplifies vibration, which happens near resonance. Effective isolation requires operating well above the natural frequency (frequency ratio r > √2 ≈ 1.414).

How do I choose the right mount stiffness?

The mount stiffness determines the system natural frequency. For effective isolation, the natural frequency should be at least 2.5–4× lower than the machine operating frequency. Lower stiffness = lower natural frequency = better isolation, but too soft causes excessive static deflection and instability.

What is a typical damping ratio for machine mounts?

Typical damping ratios: rubber mounts ζ ≈ 0.03–0.10, spring isolators ζ ≈ 0.01–0.03, air springs ζ ≈ 0.02–0.05. Higher damping reduces resonance amplification but slightly reduces high-frequency isolation efficiency.

Why does adding foundation mass help reduce vibration?

Increasing total mass (machine + foundation) lowers the natural frequency for the same stiffness, increasing the frequency ratio. This moves the system further above resonance, improving isolation. A heavier foundation is also harder to accelerate, reducing vibration amplitude directly.

What happens at resonance (frequency ratio r = 1)?

At resonance, the transmissibility peaks dramatically. With low damping (ζ = 0.05), T can exceed 10× — meaning the transmitted force is 10 times the disturbing force. This is why machines must never operate at or near their mount natural frequency. During run-up/coast-down, passing through resonance briefly is managed by increasing damping.

Free Engineering Tool

Foundation Vibration from Machine

Calculate the transmitted force from machine unbalance to foundation, transmissibility ratio, natural frequency, and vibration isolation efficiency.

Transmissibility Isolation Efficiency Natural Frequency

Results

Transmitted Force to Foundation

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Transmissibility Ratio T

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Frequency Ratio r = ω/ω_n

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Natural Frequency f_n

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Unbalance Force F_unbal

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Isolation Efficiency

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Unbalance Force

The centrifugal force generated by the machine unbalance:

Natural Frequency

The natural frequency of the machine-foundation system on mounts:

Where k is total mount stiffness (N/mm × 1000 → N/m), and m_total = m_machine + m_foundation.

Transmissibility

The ratio of transmitted force to disturbing force, including damping:

r = ω/ω_n — frequency ratio
ζ — damping ratio (dimensionless)
T < 1 — isolation is effective
T > 1 — amplification (near resonance)

Isolation Efficiency

Practical Example

Example — Fan on Concrete Foundation

Given: Machine 500 kg, Unbalance 3000 g·mm, 3000 RPM, Foundation 2000 kg, k = 5000 N/mm, ζ = 0.05

ω = 2π × 3000 / 60 = 314.16 rad/s

F_unbal = 3000 / 1e6 × 314.16² = 296.1 N

ω_n = √(5,000,000 / 2500) = 44.72 rad/s → f_n = 7.12 Hz

r = 314.16 / 44.72 = 7.02

T ≈ 0.0206, F_transmitted = 296.1 × 0.0206 = 6.1 N

Isolation efficiency = 97.9%

💡 Tip: For effective isolation, aim for a frequency ratio r > 3 (isolation efficiency > 88%). This means the natural frequency should be at least 3× lower than the operating frequency.

Foundation Vibration from Machine | Transmitted Force

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Unbalance Force

Natural Frequency

Transmissibility

Isolation Efficiency

Practical Example