Vibration Isolation Efficiency (ASHRAE)
Whether the isolators under a fan, pump, RTU, or chiller actually cut the vibration: the isolated system's natural frequency fn = 3.13/sqrt(static deflection in inches) Hz, the disturbing frequency = rpm/60, and the transmissibility T = 1/|(f/fn)^2 - 1| gives the isolation efficiency (1 - T). A 900 rpm fan on 1 in isolators runs fn 3.13 Hz, ratio 4.8, 95% efficient. The trap the tile flags: isolation needs a frequency ratio over sqrt(2) = 1.414 - a slow 200 rpm unit on the same soft mount sits near resonance and AMPLIFIES the shaking 7x, so the fix is a stiffer isolator. Undamped ASHRAE single-DOF idealization; the rated isolator deflection and the mechanical engineer govern.
Formula and source
fn = 3.13 / sqrt(static_deflection_in) Hz (= (1/2pi) sqrt(g/deflection), g = 386.4 in/s^2); f = rpm/60; ratio = f/fn; T = 1 / |ratio^2 - 1|; efficiency = (1 - T) x 100%; isolation requires ratio > sqrt(2).
ASHRAE Handbook -- Fundamentals, Sound and Vibration chapter, the single-degree-of-freedom vibration isolator (the classical Den Hartog transmissibility relation), by name.
Audience
This tile is built for hvac and the adjacent professions in the HVAC group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.