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We report on the optimization of a microspeaker diaphragm for widening the working frequency band between its first and second eigenfrequencies and increasing its sound pressure level (SPL). A metallic diaphragm produces a larger magnetic force and thus a higher sound pressure level; therefore a Ni-coating on a common poly ethylene naphthalene (PEN) diaphragm is appropriate. For the best diaphragm design, it is important to simultaneously optimize the diaphragm shape and the Ni-coating distribution on the top of the diaphragm. Since complete Ni-coating increases SPL but also reduces the frequency band, special attention must be paid to the Ni-coating. We solved the optimization problem by a two-step approach: standard shape optimization of a PEN diaphragm for widening the frequency band, followed by distribution optimization of the Ni material on the diaphragm. To facilitate the Ni distribution optimization, we formulated the problem as a special topology design optimization. Since the optimization requires a coupled analysis involving the electromagnetic field, mechanical vibration and sound radiation, the multiphysical system behavior should be properly modeled. After presenting the fundamental multiphysical equations, we provide shape and topology optimization procedures to find an optimal microspeaker diaphragm. We verified the validity of an optimized diaphragm configuration optimized by our method from a physical viewpoint.