High resolution Brillouin spectroscopy has been used in a new way to study the influence of different substrates on the elastic properties of Al(x)Ga(1-x)N piezoelectric films (about 1 mum thickness). These films are interesting for their versatile applications in different fields as is the case of SAW based filters and UV detectors. Films of oriented PVDF/TrFE deposited on Si substrates have been also subject of investigation. This semi-crystalline polymer material is a well known ferroelectric and the quality of its ferroelectric properties depends on the degree of orientation. The elastic constants are very sensitive probes for structural changes, which are of fundamental importance to understand the mechanism of the ferroelectric properties degradation and so for the technological applications of these films.