Phoneidoscope

Pho‐nei″do‐scope (?), n. [Phono- + Gr. εἰ̑δοσ form + -scope.] (Physics) An instrument for studying the motions of sounding bodies by optical means. It consists of a tube across the end of which is stretched a film of soap solution thin enough to give colored bands, the form and position of which are affected by sonorous vibrations.