The assessment of musical instruments may be performed using vibroacoustical parameters, enabling to quantify objectively differences between instruments. The choice of relevant quantities evaluating the mechanical and the acoustical behavior of stringed instruments is the main aim of this thesis. The present study focuses on guitars and violins: it belongs to the PAFI project (Plateforme d'Aide à la Facture Instrumentale), which aims at developing a set of tools dedicated to instrument makers, providing an assistance for luthiers in a daily practice context. For that purpose, a method enabling the estimation of mechanical properties of dynamic structures, whether musical or not, is developed. It is based on the estimation of the modal density in the mid-frequency range, by means of a subspace method, called ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques). It is also based on the mean-value theory by Skudrzyk. This method estimates the characteristic parameters of any plate-like or curved panel structures. The modal identification is proved to be robust as long as the modal overlap factor does not exceed 100 %. Applications on experimental and simulated data show that the method is able to discriminate panels by means of the values of their mass density and their Young's modulus. The method requires solely a few measurement points and the required experimental device is affordable and easily manipulable by the luthier in its workshop. Applications of the method on guitars show that its mechanical behavior in the mid-frequency range is similar to that of a flat panel. The instrument is then characterized by a few features or macroparameters corresponding to the characteristic parameters of the equivalent plate: the equivalent mass, the equivalent bending stiffness, and the mean-value of mobility. These features discriminate two population of guitars: high-quality handcrafted guitars and lower-quality guitars. A detailed study about bridge mobility of violins, as well as a study about their modal density and the spectral features of violin sounds, enable to identify features shared by these three quantities. The spectral envelops of violin sounds present a formantic aspect, and the identified formants are associated to the mean mobility maxima. The estimation of the modal density suggests that the first maximum, located at a frequency between 500 and 1000 Hz, is a consequence of the soundboard curvature. The methods are applied to a few examples of problematic frequently encountered by instrument makers and musicians.
