  Frequency Response Functions (FRFs) are one of the cornerstones of musical
acoustic experimental research. They describe the way in which musical
instruments vibrate in a wide range of frequencies and are used to predict and
understand the acoustic differences between them. In the specific case of
stringed musical instruments such as violins, FRFs evaluated at the bridge are
known to capture the overall body vibration. These indicators, also called
bridge admittances, are widely used in the literature for comparative analyses.
However, due to their complex structure they are rather difficult to
quantitatively compare and study. In this manuscript we present a way to
quantify differences between FRFs, in particular violin bridge admittances,
that separates the effects in frequency, amplitude and quality factor of the
first resonance peaks characterizing the responses. This approach allows us to
define a distance between FRFs and clusterise measurements according to this
distance. We use two case studies, one based on Finite Element Analysis and
another exploiting measurements on real violins, to prove the effectiveness of
such representation. In particular, for simulated bridge admittances the
proposed distance is able to highlight the different impact of consecutive
simulation `steps' on specific vibrational properties and, for real violins,
gives a first insight on similar styles of making, as well as opposite ones.
