The Helmholtz Sound or Vowel Synthesizer was used to combine timbres of 10 harmonics to form the vowel sounds A, E, I, O, and U. Driven by an intermittent electrical current, the tuning forks were made to vibrate using electromagnets. The forks would generate a fundamental frequency and overtones which could then be combined. The keyboard controlled a series of round shutters which covered the aperture of each resonator. When one pressed a key, that shutter would move, allowing the waves from the tuning fork to enter the resonator and produce a tone; the intensity could be adjusted by sliding the resonator closer to or farther from the fork.
With this device, Helmholtz showed that musical notes are composed of many different tones, and that the timbre of vowel sounds and musical notes is a result of their complexity. The device clearly demonstrated that the musical note not only contains "a simple 'fundamental' vibration... but also a 'harmonic series' of whole number multiples of this frequency called 'overtones'. Helmholtz proved, using this synthesizer, that it is the combination of overtones at varying levels of intensity that give musical tones, and vowel sounds, their particular sounds quality, or timbre." (Rees) Specimens of this device are extremely rare, with only one similar but smaller apparatus located in a US institution that we know of — we have not seen any others as large or finely made as this one. Max Kohl of Chemnitz is perhaps one of the most famous scientific instrument makers of the late 19th and 20th centuries. His work was distinguished by its exacting craftsmanship, and high quality materials.
For a detailed description of how to operate this apparatus see: Appendix VIII "Practical Directions for Performing the Experiments on the Composition of Vowels" in: Hermann von Helmholtz. On the Sensations of Tone as the Physiological Basis for the Theory of Music. London: Longmann, Greens & Co, 1885; David Pantalony. Altered Sensations: Rudolph Koenig's Acoustical Workshop in Nineteenth-Century Paris. New York: Springer Science, 2009; Torben Rees. "'Helmholtz's apparatus for the synthesis of sound: an electrical 'talking machine.''' Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2010 [http://www.hps.cam.ac.uk/whipple/explore/acoustics/hermanvonhelmholtz/helmholtzssynthesizer/, accessed 23 October 2017]; Max Kohl Price List No. 50, Vols. II and III. Physical Apparatus. Vol. II. Chemnitz, Germany: Max Kohl, 1909, item 53,586, p. 460.
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