This paper presents an initial state of the art about the most important techniques for reducing noise in an electrical machine. The FEM 3D modal analysis (realized with the software Ansys 11.0) is then proposed to study and reduce the acoustical power radiated by a brushless machine and avoid any resonance phenomena. The method is able to analyze the impact of some design parameters on all the resonance frequencies for reducing the total sound power and gives some guidelines to their choice.