1. Historical overview

1.1 Precursors

The engineer Henri Coanda is credited with the first description of a noise reduction system based on the use of interference. In two French patents, "Procédé de protection contre les bruits" [1] , filed in 1930, and "Procédé et dispositif de protection contre les bruits" [2] , filed in 1932, he states: "... in the path of a sound wave to be eliminated, another wave (compensating wave) is brought into play, with the same characteristics as the first, but phase-shifted by one or more half-wavelengths in relation to it. The interference of these two waves produces a neutral zone, a zone of silence. A few years later, Paul Lueg filed a patent...