The German scientist Ernst Chladni was one of the pioneers of experimental acoustics. His research on different kinds of vibrations served as the basis for the scientific understanding of sound that later emerged in the 19th century.
In 1862, the German scientist Herman Helmholtz invented another important acoustic instrument, the double siren. The new instrument combined two Dove Sirens, which were positioned to face each other and coupled on the same shaft. Both sirens were also connected to the same air supply, which made it possible to produce a variety of frequencies, all of which would slide up or down the scale as the air pressure was increased or decreased.
In its simplest form the siren consists of two metal disks, each having the same number of regularly spaced concentric holes. The lower disk is mounted on the base of the siren, while the upper disk is mounted on a steel shaft just above it. As air is forced through the system, the upper disk begins to rotate. As it spins, the holes in the two disks briefly line up and then quickly close. Each time they do this, a brief puff of air is released. Each of these puffs acts like a single sound wave, and if enough of them are produced we perceive them as a continuous musical sound.
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.