Koenig’s flame analyser was, next to the sound synthesizer, one of the clearest expressions of Hermann von Helmholtz’s theory that complex sounds were made up of a spectrum of elemental or pure tones. The adjustable resonators covering a range of 65 notes from sol1 to mi5 (96–1,280 Hz), could each be rendered visible with a connection to a manometric flame capsule. The resonators were connected to a gas-filled capsule with a rubber tube. If activated, the distinctive pattern would appear in the rotating mirror.
The double siren was one of Koenig’s more popular instruments. It consisted of two “polyphonic” or “multivoiced” sirens with more than one series of holes, and was an invention of the German physicist and former teacher of Hermann von Helmholtz, Heinrich Wilhelm Dove (1803–1879). It produced several pure tones simultaneously, in musical chords, and under greater pressure. It was ideal for demonstrating interference effects (when sound waves combined to amplify or diminish each other) and combination tones. [Pantalony 2009, 184-185]
This specific array of precision tuning forks are highly-specialized experimental forks that relate directly to Koenig’s long-standing disagreement with Helmholtz on the nature of combination tones. Some of them date back to Koenig's display at the 1876 Philadelphia exposition.
The sound synthesiser was Helmholtz’s clearest instrumental expression of his theory of timbre, or sound quality. Whereas his spherical resonators dissected compound sounds (vowels or musical sounds) into elemental frequencies, the synthesiser did this by building up complex sounds from simple frequencies. In 1857 he went to the instrument maker Friedrich Fessel of Cologne to turn this idea into reality. The initial instruments used a combination of electrically driven tuning forks, resonators and piano keys to synthesise compound sounds.