Helmholtz's apparatus for the synthesis of sound: an electrical 'talking machine'

This ingenious device, designed by Herman von Helmholtz XR  (1821-1894), was the very first sound synthesizer: a tool for studying and artificially recreating musical tones and the sounds of human speech.

Helmholtz's synthesizer
Image 1 Helmholtz's apparatus for the synthesis of sound, made by Rudolph Koenig. Image © Thomas B. Greenslade Jr.
Parts of synthesizer
Image 2 Parts of Helmholtz's apparatus for the synthesis of sound, made by Rudolph Koenig, late 19th century. Image © the Whipple Museum (Wh.1312).
Part of synthesizer
Image 3 Detail of part of Helmholtz's apparatus for the synthesis of sound, made by Rudolph Koenig, late 19th century. Image © the Whipple Museum (Wh.1312).

Background

Suppose I sing the word 'car' and then on the same note sing 'we'. The two vowel sounds will be similar in so far as they have the same pitch G , yet they have a clearly distinct sound quality, or timbre G . What is it that accounts for this difference, and the timbres G  of musical sounds in general? Helmholtz set out to answer this very question in the mid 19th century, building on the work of the Dutch scientist Franz Donders (1818-1889).

Complex tones

Helmholtz showed that the timbre G  of musical notes, and vowel sounds, is a result of their complexity: just as seemingly-pure white light actually contains all the colors of the rainbow, clearly defined musical notes are composed of many different tones. If you play the A above middle C on an organ, for example, the sound you hear has a clearly defined "fundamental" pitch G  of 440Hz G  . But the sound does not only contain a simple "fundamental" vibration at 440Hz G , but also a "harmonic series" of whole number multiples of this frequency G  called "overtones" (i.e., 880Hz G , 1320Hz, 1760Hz, etc.). Helmholtz proved, using his synthesizer, that it is this combination of overtones at varying levels of intensity that give musical tones, and vowel sounds, their particular sound quality, or timbre G .

How the synthesizer works

Helmholtz's apparatus uses tuning forks, renowned for their very pure tone, to generate a fundamental frequency G  and the first six overtones which may then be combined in varying proportions. The tuning forks are made to vibrate using electromagnets and the sound of each fork may be amplified by means of a Helmholtz resonator with adjustable shutter operated mechanically by a keyboard.

By varying the relative intensities of the overtones, Helmholtz was able to simulate sounds of various timbres G  and, in particular, recreate and understand the nature of the vowel sounds of human speech and singing. Vowel sounds are created by the resonances G  of the vocal tract, with each vowel defined by two or three resonant frequencies G  known as formants. When we say or sing 'a' (as in 'had'), for instance, the vocal tract amplifies frequencies G  close to 800Hz G , 1800Hz and 2400Hz amongst others. When we require a different vowel sound, the muscles of the throat and mouth change the shape of the vocal tract, producing a different set of resonances G .

» Read more about Helmholtz

» Read more about Helmholtz resonators

» EXPLORE: Launch Helmholtz Interactive (opens in new browser window)

Torben Rees

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 16 April 2014]

 
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