We introduce an interactive interface for the custom designof metallophones. The shape of each plate must be determined in the design process so that the metallophone willproduce the proper tone when struck with a mallet. Unfortunately, the relationship between plate shape and tone iscomplex, which makes it difficult to design plates with arbitrary shapes. Our system addresses this problem by runninga concurrent numerical eigenanalysis during interactive geometry editing. It continuously presents a predicted tone tothe user with both visual and audio feedback, thus makingit possible to design a plate with any desired shape and tone.We developed this system to demonstrate the effectivenessof integrating real-time finite element method analysis intogeometric editing to facilitate the design of custom-mademusical instruments. An informal study demonstrated theability of technically unsophisticated user to apply the system to complex metallophone design.