Fourier Synthesis
  The nineteenth century physicist would not have recognized the phrase Fourier synthesis. Instead, he would have talked about combining tones to form various vowel sounds. The apparatus at the left is "Helmholtz's large apparatus for compounding timbres of 10 harmonics ... 1,500 francs [$300]", which is item number 56 in the 1889 Koenig catalogue. 

   This apparatus is at the University of Toronto.

   The ten electrically-driven tuning forks, each facing a Helmholtz resonator tuned to the same frequency, run continuously, but produce little sound. Pressing one of the keys moves the dull black shutter away from the hole of the resonator, and the sound becomes quite loud. Rubber feet under the corners of the wooden stand keep the vibrations from reaching the baseplate that runs under all ten of the tuning fork systems. 
   This is the back view of the apparatus. The harmonic content of each of the vowels had been previously determined through the use of the  Fourier analyzer, and they could then be reproduced with the synthesizer. The relative strength of each harmonic was adjusted by pushing the keys down by various amounts, thus allowing the volume of the sound produced by each resonator to be varied. 
   The close-up at the right of some of the smaller forks shows the iron yokes used to complete the magnetic circuit through the tuning forks when the coils are energized.

   This apparatus is from the latter part of the nineteenth century and is in an excellent state of preservation. 

   The system is driven by an intermitted current provided by this master tuning fork, which oscillates at the same frequency as the lowest frequency fork. This supplies one excitation per cycle to this fork, one excitation every two cycles for the next higher fork, etc. This driving system is missing from the Toronto apparatus, but is part of the system below at Vanderbilt University. 

   A master tuning fork by Koenig, provided with a make a break contact, is in the collection at the  University of Toronto

   The Helmholtz synthesizer below is in the Garland Collection of Classic Physics Apparatus at Vanderbilt University in Nashville, Tennessee. The electrotome above is part of the apparatus, but is in a separate display. The keyboard is missing, and some of the resonator and tuning fork systems are disassembled (their bases are missing), although they could be easily put back together again on replacement bases to form a complete instrument.

   “The edge of the disk before you answers to a curve corresponding to a tone compounded of a prime and four perturbed harmonic partials. The fundamental consists of 24 waves. The first upper partial consists of 49 waves (2x24+1); the second of 75 (3x24+3); the third of 101 (4x24+5); and the fourth of 127 waves (5x24+7). The resultant curve embraces 24 waves, all different in form, and some of them very irregular in outline. When air is blown through a narrow slit against the teeth of this disk, a very disagreeable and slightly intermittent sound is the result.” 

From J. A. Zahm, Sound and Music, second edition (Chicago, A.C. McClug & Co.), pg 381

 Return to Koenig Home Page | Return to Home Page