The Collaborative International Dictionary of English v.0.48:

Phoneidoscope \Pho*nei"do*scope\, n. [Phono- + Gr. e'i^dos 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.
   [1913 Webster]