In terms of sound quality, mechanical stability, and cost, the moving coil speaker is exceptional to the moving iron and electrostatic speaker. The diaphragm in the speaker is crucial in reproducing the sound wave from the electrical signal input. The materials of the diaphragm should be both rigid and lightweight. However, there are few quantitative studies on the selection of diaphragm materials. Connecting the equation of mass and the deflection of a circular plate yields a performance index that maximizes Young’s Modulus and minimizes the density. A line graph in logarithmic can be generated from the index to select materials from various categories in the Ashby plot provided by the Granta CES Edupack software. The chosen materials would be simulated in COMSOL and their sound pressure level plots would be analyzed based on the numerical solution. The Ashby plot and the numerical solutions by COMSOL have indicated that paper is the best material among the selected materials with the most stable frequency response in the mid-range frequency range, a high-performance index, and a low price per unit volume.