Monopole Mobility Calculator

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Calculated Properties

Rick Molloy OM (10/25/2025) • Steel-string Live: Rick Molloy OM (10/25/2025)
Monopole mobility score
--
score
1000 / √(K · Meff)
Top stiffness K
--
N/m
K = m·g / x
EFFECTIVE MASS Meff
--
g
Meff = K / (2π fu)2
Lower scores mean a stiffer and/or heavier top. Guitars scoring highly (> 20 for steel string guitars, > 30 for classicals) will likely require composite construction to survive over time.
Top stiffness, K and effective mass, Meff can be entered into the 4‑DOF solver to model your guitar’s air/top/back coupling and to indicate realistic brace/plate tuning changes when aiming at specific frequencies.

Overview & required measurements

Use this tool to translate your measurements into top stiffness K, effective modal mass Meff, and a monopole mobility score using either the classic Gore jig (Static Mode) or a two-reading mass-loading FFT method (Dynamic Mode).

Why this matters: Monopole mobility shows how freely the top pumps air at its main resonance and is a measure of how responsive a guitar is across a whole range of frequencies. It shows how much sound you can get out for how hard you pluck the strings. It doesn’t help guard against structural issues, but does show that low mass, low stiffness tops will be more responsive (provided they hold up!)

Default dataset (Static Mode): Rick Molloy OM (10/25/2025) (spruce / steel-string, m = 1.02 kg, x = 0.18 mm, fu = 153 Hz)
Overview & setup workflow

Choose your measurement path. Static Mode uses the Gore & Gilet jig: load the bridge with a known force and record the deflection. Dynamic Mode skips the dial gauge and instead uses two monopole resonance readings: unloaded and with a small added mass.

Measurements required

  • Uncoupled top monopole frequency, f0 (Hz)
  • Static Mode: bridge deflection under test load, x (mm)
  • Static Mode: test mass, m (kg)
  • Dynamic Mode: loaded frequency f1 (Hz) and added mass (g)
  • Guitar type (steel-string, classical, other)

How to use this tool

  1. Seal the sound hole with a low mass, rigid cover so the box is airtight.
  2. Mute the strings, tap the top, and record the uncoupled top frequency f0 with your FFT app.
  3. Pick a mode:
    • Static Mode — Mount the mobility jig, load it over the bridge using a known test mass (m) and measure the panel deflection (x) in millimeters.
    • Dynamic Mode — Add a small known mass at the bridge, measure the shifted monopole frequency f1 with your FFT app. No dial indicator required.
  4. Enter the mode-specific inputs. The calculator computes K, Meff, and mobility.
  5. Review the score and compare to the target ranges.

How this tool works

The calculator supports two workflows: the classic Gore & Gilet monopole mobility jig (Static Mode) and a mass-loading frequency shift method (Dynamic Mode). Both paths output stiffness K at the bridge, effective modal mass (Meff), and the standard monopole mobility score for build-to-build comparisons.

Static Mode: Force from test mass

The mass sets the applied force through the jig.

F = m · g

Static Mode: Top stiffness at the bridge

The applied force divided by the deflection x (converted to meters) gives the stiffness, K.

K = F / x

Dynamic Mode: Effective modal mass from frequency shift

Two frequency measurements and the added mass give Meff directly.

Meff = (f12 · m) / (f02 − f12)

Effective stiffness (Dynamic Mode)

Once Meff is known, stiffness follows from f0.

K = (2π f0)2 · Meff

Effective modal mass (Static Mode)

Derived from stiffness, K and the uncoupled top frequency, fu.

Meff = K / (2π fu)2

Monopole mobility score

Index used for build-to-build responsiveness comparisons. Monopole mobility has dimensions of admittance, (s/Kg)

Mobility score = 1000 / √(K · Meff)
Need help with this tool? I read every message.

If your results don’t match what you’re hearing, share your measurements and a short tap clip. I’ll take a look.

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