The ZET-Horn essentially consists of a piston inside a cylinder driven by an AC 3-phase motor and a joining sound horn. The piston oscillates at a certain frequency making the air inside the sound projector vibrate in resonance to produce a clean sound audible over a wide range. Higher amplitudes of the harmonics ensure best possible penetration of the normal background noise level on board. The ZET-Horn has a run-up time of less than 0.15 seconds which guarantees a pure, concentrated sound with best directional properties.