ˈfōnəˌdīk noun
( -s )
Etymology: phon- + -deik (from Greek deiknynai to show) — more at diction
: an instrument for making photographic records of sound waves in air by means of a tiny mirror that is oscillated in a rotary manner by sound waves agitating a glass diaphragm and that by its reflections of a ray of light traces corresponding paths on a moving film