Seeing a voice: Rudolph Koenig's instruments for studying vowel sounds

The human voice was one of the more elusive acoustical phenomena to study in the 19th century and therefore a crucial test of Hermann von Helmholtz's new theory of sound. This article describes the origins of instruments used to study vowel sounds: synthesizers for production, resonators for detection, and manometric flames for visual display. Instrument maker Rudolph Koenig played a leading role in transforming Helmholtz's ideas into apparatus. In particular, he was the first to make the human voice visible for research and teaching. Koenig's work reveals the rich context of science, craft traditions, experiment, demonstration culture, and commerce in his Paris workshop.