740x300 Lab Slide

Songwriter Series VAOM-04 as tested


D.P. Hess
Mechanical Engineering
University of South Florida
Tampa, Florida

November 2012

Vibration tests are performed to assess the transmission of vibration from the bridge to the headstock for a Voyage-Air VAOM-04 guitar shown in Figure 1. This guitar features a unique hinged-neck for convenient transportation. Vibration measurements are taken with the guitar neck extended and secured following manufacturer specifications.

Figure 1 Voyage-Air VAOM-04 guitar

The test set-up is shown in Figures 2 and 3. The guitar is placed on a pad with the neck supported on foam between the nut and ninth fret to level the instrument horizontally. The strings are damped with foam at the seventh fret.

Figure 2 Test set-up

Figure 3 Angle view of test set-up

 A small piezoelectric impact hammer is used to apply an impact force to the center of the flat on the bass or left side of the bridge as shown in Figures 4 and 5.

Figure 4 Impact hammer and top plate accelerometer

Figure 5 Impact hammer and top plate accelerometer closeup

Two miniature accelerometers attached with wax are used to measure the vibration response from the impact hammer. One is attached 1 cm in front of the bridge at the sixth string as shown in Figures 4 and 5. The other one is attached between the third and fourth string tuning pegs on front of the headstock as shown in Figures 6 and 7.

Figure 6 Headstock accelerometer

Figure 7 Top view of headstock accelerometer

A dynamic signal analyzer is used to record the force input and acceleration responses as well as to calculate the frequency responses for each acceleration response and the force input. The frequency response data is presented in Figure 8. The blue curve is the frequency response of the top plate acceleration and hammer force input. The green curve is the frequency response of the headstock acceleration and hammer force input.
The top plate frequency response (blue curve) clearly reveals the two dominant low frequency modes with peaks at 105 and 203.5 Hz as well as the Helmholtz resonance at the valley at 132.5 Hz. The magnitude of the top plate response is higher than the headstock response over most of the frequency range shown. The difference between these response magnitudes provides an indication of the amount of vibration (between 70 and 270 Hz) transmitted to the headstock compared to the top plate.

Figure 8 Frequency response data from the Voyage-Air VAOM-04 hinged neck guitar

Two common designs used today to attach the neck to the body of an acoustic guitar are the dovetail joint design and the bolt-on design. Frequency response data has been collected from guitars with both designs. Sample data are shown in Figures 9 and 10.

The frequency response data in Figures 8, 9 and 10 show some similarities common to acoustic guitars, namely, two dominant low-frequency modes identified by the peaks in the top plate frequency response curves.

The difference between the top plate and headstock frequency responses is comparable for the hinged, dovetail and bolt-on designs. This data suggests the Voyage-Air hinge neck design transmits low-frequency vibration at least as well as the sample dovetail and bolt-on neck designs.

Figure 9 Frequency response data from a 000 size guitar with a dovetail joint neck


Figure 10 Frequency response data from a dreadnaught guitar with a bolt-on neck





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