In the 60's, Schelleng proposed a simple model describing the mechanical behavior of violins using lumped circuits. The interest of such modeling is to reduce the complexity of musical instruments by describing them in the low frequency range using a few parameters. We propose in this study a method to globally quantify the mechanical behavior of violins on a larger frequency range, using only a few parameters. Bridge mobilities (or admittances) are important characteristics of the instrument, which describe the level of coupling between the strings and the instrument body. These admittances have complex variations versus frequency because of modal overlap. Modal density and damping factors can be computed in the mid frequency domain using adapted signal processing techniques: they can be estimated accurately from measured impulse responses by using the high-resolution ESPRIT method. An average admittance, computed by means of a moving average, is also used for highlighting the underlying tendency of the coupling level between the strings and the instrument body. Characteristic parameters of the bridge admittance are used to compare instruments and characterizations of instrument's tuning. This study is a part of the PAFI project, aiming to develop a set of tools dedicated to instrument makers.