1AbstractAttenuation of acoustic noise from automobile components is important for passenger comfort. Since the alternator is one of the major sources of noise, many manufacturers have studied the various mechanisms which generate noise within an alternator as well as the methods to reduce the noise level. This paper presents the dynamic properties of the alternator with respect to the acoustic noise during current generation, and introduces a vibration damping structure based on experimental modal analysis. Rotating magnetic forces in a magnetic circuit (stator and rotor) can excite numerous structural resonances, resulting in acoustic noise. A modal analysis performed on the major magnetic circuit of the alternator (Nippondenso Co., Ltd.) revealed that the stator has elliptic, triangular and rectangular mode shapes in the radial coordinate plane, while the rotor does not have any significant resonances in the same 0 - 3 kHz region. At each resonance, however, the axial deformation of the stator was as significant as the deformation in the radial coordinate plane. A constrained layer damping structure for the stator was designed and fabricated, based on experimental modal analysis. Damping material inserted circumferentially between the stator and its housing reduced the vibration transmissibility from the stator to the housing and attenuated acoustic noise induced by magnetic excitation. This structure reduced the major magnetic noise contribution by approximately 10db (max.) during full current generation in the 1.5 - 2.7 kHz frequency range (except at a 1.9 kHz resonance). The reduction effect on the overall noise was approximately 3 - 4 db. Various transfer function residues from the original and the damping structure were compared to show the relationship between vibration transmissibility and acoustic noise, revealing important information about acoustic noise estimation.