The automatic phonograph was used to imitate five different vowels. Five discs, mounted on a rotating axle, are cut with grooves resembling the sound waves associated with various vowel sounds. When the axle rotates one of the discs rubs against a thin metal rod transmitting the ensuing vibrations to a circular diaphragm. This results in an audible sound being produced. The metal rod and diaphragm are able to slide between the five discs depending on which vowel sound is required (the machine is inscribed with 'oo', 'o', 'ah', 'a', 'e').
Invention
Hermann von Helmholtz (1821-1894) argued that vowels are compound tones, defined by the combination of non-harmonic resonances known as formants. The nature of the vowel depends on the relative intensities of these different formants, which in turn depend of the positioning of the vocal tract. The grooves in the discs of this machine are cut in such a way as to cause the vibrations of the diaphragm to reflect the combination of frequencies found in vowel sounds, resulting in the production of a similar sound.
Stroh built four talking machines in early 1878, the third of which was this phonograph. The second machine etched the sound waves of the formants of vowel sounds onto the brass discs used in the phonograph, and the fourth played discs with complete words such as 'mama' and 'papa'. One year earlier, Thomas Alva Edison (1847-1931) had made his first phonograph, a device for recording and replaying sound that also used the idea of spiral discs. However, Stroh's research focused more on the scientific understanding of the composition of sound rather than Edison's more popular appeal.
