Description
The apparatus here, consisting of various electrically powered tuning forks and resonators, is designed for the synthesis of musical sounds and the vowel sounds of the human voice. This particular set of equipment was built by the physicist and acoustical instrument maker Rudolph Koenig as demonstration apparatus, after a design by Herman von Helmholtz.

Helmholtz demonstrated that musical tones are complex, being composed of numerous superimposed simple tones. When you play the A above middle C on the piano, you hear a clearly defined pitch. The piano string is vibrating at 440Hz, which is known as the fundamental frequency. But it also simultaneously vibrates at many other higher frequencies called upper partials. The fundamental determines the perceived pitch, but it is the particular combination of upper partials at various intensities that give musical instruments, and the human voice, their particular sound quality, or timbre.

Musical sounds, as opposed to mere noise, always display a particular mathematical relationship between the fundamental frequency and (most of) the upper partials. The partials, that is, are always frequencies that are whole number multiples of the fundamental: these are called harmonic partials. The piano string vibrating at a fundamental of 440Hz will also produce harmonic upper partials at 880Hz, 1320Hz, 1760Hz and so on. There will always be some non-harmonic partials as well—this ‘noise’ is due to the sound of the hammer striking the string, or, in the violin, the bow scraping against the string.

Helmholtz’s apparatus consists of seven tuning forks, one at a fundamental frequency and the others tuned to the first six harmonic partials. A tuning fork on its own produces very little sound, so Helmholtz placed his tuning forks in front of resonators tuned to the exact same frequencies (see also the catalogue entry for 3429 - Helmholtz Resonators). The cavities of the resonators could be opened and closed by means of a shutter, leading to louder or softer sounds. All of these shutters were to be operated by means of a series of strings and springs connected to a keyboard (missing from this collection, but see photo adjacent). The tuning forks are driven electrically using electromagnets so that they produce a constant vibration rather than dying away. Since all of the forks vibrate at whole-number multiples of the fundamental frequency they can all be powered by a single intermittent current at the frequency of the fundamental. This current is delivered by means of a master tuning fork set to the fundamental frequency (see the catalogue entry for 4376 - electrically driven tuning fork). The apparatus is first set so that all of the tuning forks are vibrating but with all the shutters closed such that no sound is produced. The experimenter may then open various shutters to varying degrees in order to produce complex sounds.

By varying the relative intensities of the harmonic partials, Helmholtz was able to synthesize sounds of various timbres and, in particular, recreate and understand the nature of the vowel sounds of human speech (see also the catalogue entry for 1362 - Stroh’s vowel sounder). Vowel sounds in speech and in singing are produced by the resonances of the vocal tract, with each vowel defined by a series of narrow resonances. When we say or sing ‘a’ (as in ‘had’), for instance, the vocal tract amplifies frequencies in the range of 800Hz, 1800Hz and 2400Hz amongst others. These resonances are called ‘formants’ and each vowel has its own defining set.
22/05/2008
Created by: Dr. Torben Rees on 22/05/2008