The type of sound propagation differs depending on whether the medium is a solid or a fluid. In the case of solids, we are talking about structure-borne sound; in the case of fluids, the range relevant to the fan is referred to as airborne sound. Sound transmission in the case of structure-borne sound can take place in longitudinal waves (wave deflection in the direction of propagation) and transverse waves (wave deflection perpendicular to the direction of propagation).
Sound propagation in fluids takes place only in the form of longitudinal waves. The present form of propagation manifests itself by compression and stretching of the particles, often there is also talk of location-dependent pressure and density fluctuations. If different frequencies are present, the time dependence is added. How far the sound propagates is influenced by the absorption of the medium, as well as the diffraction, refraction and reflection at surfaces and obstacles.