Opeidoscope
Opeidoscope , noun
[Greek {not transcribed}, {not transcribed}, voice + e'i^dos form + -scope.]
(Physics) An instrument, consisting of a tube having one end open and the other end covered with a thin flexible membrane to the center of which is attached a small mirror. It is used for exhibiting upon a screen, by means of rays reflected from the mirror, the vibratory motions caused by sounds produced at the open end of the tube, as by speaking or singing into it. — A. E. Dolbear