Vibration Transmissibility Calculator

How it is calculated

The tool models an undamped spring mount with one degree of freedom. It first forms the frequency ratio r = fforcing ÷ fnatural, where the forcing frequency is how fast the machine shakes the mount and the natural frequency is how fast the mounted machine would ring by itself, both in hertz. Transmissibility is then TR = 1 ÷ |1 − r²|. TR is the fraction of the shaking force or motion that gets through; above 1 means amplification, below 1 means isolation. The note says the mount isolates when r is greater than √2 (about 1.414), and otherwise that there is no isolation. At exactly r = 1 (resonance) the undamped formula is unbounded, so the tool shows ∞ with a resonance warning. The natural frequency must be above zero and the forcing frequency cannot be negative. Damping is ignored, so real mounts behave differently near resonance.

Common mistakes

• Mixing units, such as the machine speed in rpm and the mount frequency in Hz. Both fields are hertz, so 1800 rpm is 30 Hz.
• Swapping the two frequencies. The ratio is forcing over natural, so reversing them gives r = 0.333 instead of 3 and the note then says there is no isolation.
• Believing that a stiffer mount always protects better. Making the mount stiffer raises the natural frequency, lowers r and can move you into amplification.
• Reading the ∞ result at resonance as a real number. When the two frequencies are equal the undamped formula is unbounded; real systems have damping and a finite but large peak.
• Using the result for a heavily damped mount. The formula leaves damping out, so for rubber or damped isolators the real transmissibility above √2 is usually higher than shown.