„This instrument was used in an Ohio high school and probably dates from the late 19th or early 20th centuries. The replication of classic experiments was a common way to teach science at this time, and Chladni’s figures were considered to be both instructive and beautiful. 

See also Helmholtz resonator

Picture: Max Planck Institute for the History of Science

This is a set of 16 Helmholtz resonators. Made from sections of brass that were spun on a lathe, they are wonderfully light and easy to hold. Helmholtz designed them to demonstrate his theory that all vowel and musical sounds are composed of combinations of simple, pure notes (Helmholtz’s “Theory of Timbre”). He correctly observed that musical sounds, particularly the higher tones, are often perceived as a single mass of sound.

In 1894, Olaus Henrici (1840-1918) of London devised a harmonic analyzer for determining the fundamental and harmonic components of complex sound waves. It consists of multiple pulleys and glass spheres, called rolling-sphere integrators, connected to measuring dials. The image of a curve (for example, a phonodeik tracing of a sound wave) is placed under the device. The user moves a mechanical stylus along the curve’s path, tracing out the wave form.

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.