Monopole mobility score
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score
1000 / √(K · Meff)
Calculated Properties
Top stiffness K
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N/m
K = m·g / x
EFFECTIVE MASS Meff
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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 Gore-style monopole mobility jig readings into top stiffness K, effective modal mass Meff, and then calculate a guitar's monopole mobility value, so you can compare guitars on a consistent basis.
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 does not help guard against structural issues, but does show that low mass, low stiffness tops will be more responsive, provided they hold up.
Default dataset: Rick Molloy OM (10/25/2025) (spruce / steel-string, m = 1.02 kg, x = 0.18 mm, fu = 153 Hz)
Overview & required measurements
Seal the sound hole, mute the strings, tap the top to find the frequency fu of the uncoupled monopole resonance. Then, using the monopole mobility jig, load the bridge with a known force and record its deflection x.
Measurements required
- Uncoupled top monopole frequency, fu (Hz)
- Bridge deflection under test load, x (mm)
- Test mass, m (kg)
How to use this tool
- Seal the sound hole with a low mass, rigid cover so the box is airtight.
- Mute the strings, tap the top, and record the uncoupled top frequency fu with your FFT app.
- Mount the mobility jig. Load it over the bridge using a known test mass (m) and measure the panel deflection (x) in millimeters.
- Enter fu, x, and m. The calculator computes K, Meff, and mobility.
- Review the score and compare it with your own build records.
How this tool works
This tool is based on the Gore & Gilet monopole mobility jig (Vol. 1, Appendix AI 3). The calculator converts your jig measurements into top stiffness at the bridge (K), effective modal mass (Meff), and the standard monopole mobility score.
Force from test mass
The mass sets the applied force through the jig.
F = m · g
Top stiffness at the bridge
The applied force divided by the deflection x, converted to meters, gives the stiffness K.
K = F / x
Effective modal mass
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.
Mobility score = 1000 / √(K · Meff)