In this paper, topology optimization of gearbox to reduce the radiated noise is studied based on the analysis of modal acoustic contribution and panel acoustic contribution. Firstly, the bearing dynamic loads are obtained by solving the dynamic equations of gear system. Secondly, the vibration of gearbox is calculated using FEM and the radiated noise is simulated using BEM by taking these bearing dynamic loads as excitations. Thirdly, the panel having larger contribution to the sound pressure level (SPL) at a specific field point is found by panel acoustic contribution analysis (PACA), and this panel is taken as design domain. The mode order with larger contribution is determined by modal acoustic contribution analysis (MACA), and making corresponding natural frequency becomes far away from excited frequency is taken as a constraint. Finally, the topology optimization of gearbox is completed using SIMP method, and the ribs are arranged according to the optimization results. The results show that the equivalent sound pressure level at objective field point can be reduced obviously by using this method.Copyright © 2015 by ASME