The acoustic material upon which this composition is based was digitally recorded in numerous sessions at the Pfleiderer Institute for Turbo Machinery at the Technical University in Braunschweig. Contact microphones were mounted directly on the machines, both in order to avoid cross talk from test rigs which were operating in parallel and to simultaneously record usually inaudible processes inside the machines and tubes. In the studio, the material was then modified with filters and other methods of digital sound analysis and processing. At times they were changed extensively. One of the criteria for the combination of different sounds was their spectral complementarity. Finally the material was spatialized in a virtual acoustic space (3rd order ambisonic).
Turbomachines work with rotating solid cages (blades, vanes) in gases and liquids, and often acoustically produce a uniform noise in a wide frequency range. They usually manifest a tonal component (expressing the number of blades multiplied by the speed), which is audible in a manner akin to a root or fundamental. By cavitations in fast flowing liquids, acoustically significant deviations from the uniform noise are induced. These deviations mostly have a percussive character.
The circle and the circular motion are important (indeed, essential) for the operation of turbomachines. Both the shape and the structure of this work are derived from this overarching idea of a circle, the constant flow and rotation as a key aspect of turbomachinery movement and the increase and decrease in vertical density.
Sciencentists working in the field of continuous flow machines not only work with real test rigs and real machines, but also and increasingly with computer simulations. This is reflected in the composition in that as the piece progresses the edited recordings of turbomachinery are replaced by
"acoustic models". These models were obtained from spectral analysis of the recorded sounds and then resynthesized by means of additive sound synthesis.
The spatial concept of the piece reflects the principle of rotation of turbo machinery. The 3rd order ambisonic method, realized here with the sound synthesis language Csound, makes it possible to individually place sounds in a two-or three-dimensional acoustic space and move them along discrete trajectories. In the virtual acoustic space of this composition the individual sounds are moved in circular orbits with different radii at different speeds and in different directions.
Ideally the audience is placed (whether standing or walking) in a darkened room within a circle of 8 speakers; the diameter of the circle might exceed ten meters.