Measurement and Modeling of a Resonator Guitar [pdf]

Resonator guitars are acoustic instruments which have one or more spun metal cones embedded in the top plate, with strings driving the cone directly through a bridge. They were originally designed to be louder than traditional acoustic guitars and are often played with a metal slide. The vibrational characteristics of resonator guitars having a single inverted-cone are studied as the basis for a synthesis model. The small-signal input admittance is obtained using an impact hammer and laser Doppler vibrometer. As well, sinusoidal sweeps are made using a modal shaker at various driving amplitude levels. The shaker measurements show that some of the modes exhibit nonlinear characteristics. These measurements are used to design body resonator filters with time-varying resonant modes for a digital waveguide model of the resonator guitar.


The measurement setup of the resonator with the vibration shaker is shown below. The shaker was used as force input while a laser vibrometer was used to measure the resulting surface velocity.



The following figure shows the admittance of the resonator guitar when the shaker was driven at different amplitudes.



Below is a diagram of the waveguide synthesis which has two different modal filter banks, one of which has the mode frequencies varying with the string velocity level.



This figure shows the mode frequency of the five time-varying modes during a pluck.

Audio Examples

Note, the difference between the linear and nonlinear plucks is very subtle and can mostly be heard as some slight beating in the low frequencies at the beginning of the plucks.

Recorded Low G Pluck
Synthesized Low G Pluck (linear)
Synthesized Low G Pluck, amplitude = 0.5 mm (nonlinear)
Synthesized Low G Pluck, amplitude = 2 mm (nonlinear)
Synthesized Low G Pluck, amplitude = 8 mm (nonlinear)
Synthesized Pluck at 128.3 Hz (linear)
Synthesized Pluck at 128.3 Hz, amplitude = 0.5 mm (nonlinear)
Synthesized Pluck at 128.3 Hz, amplitude = 2 mm (nonlinear)
Synthesized Pluck at 128.3 Hz, amplitude = 8 mm (nonlinear)