Figure 13:
A comparison of admittances for two different violins (from Woodhouse (2014)).
Excitation and response applied at the bridge, in up to 3 dimensions.
Ideally, the excitation is applied with a light hammer that will not mass load the structure and that has an embedded force transducer to report the actual force applied.
Excitation of very light objects with a shaker is likely to lead to mass loading.
The resulting vibrations of the structure are ideally measured with a non-contact system, such as a laser-doppler vibrometer. However, it is also possible to use very light-weight accelerometers (0.2 or 0.3 grams).
Provides information about resonance peak frequencies and magnitudes, as seen by the strings.
It is important to consider the way an instrument is mounted, either to avoid undesirable vibrations from the mount itself or in an effort to mimic the conditions of the instrument when played (or the conditions of a model that it will be compared to).
Several measurements are typically averaged to suppress noise but it is necessary to monitor the coherence of multiple measurements and have the ability to discard measurements that are inconsistent.
For string instrument bodies, there is no known “best” peak frequency and magnitude values, though “signature” modes tend to be present in certain frequency ranges.
It is not possible to say with certainty what the mode shapes are for any particular admittance peak. For that, it is necessary to use EMA, holographic interferometry or chladni pattern techniques.
