5. Attenuation

The fluid studied so far was perfect: it responded instantaneously to any solicitation, whatever its frequency, with no dissipation of energy. Acoustic waves propagate without attenuation. In a real fluid, several mechanisms are responsible for the progressive and irreversible dissipation of part of the acoustic energy into heat:

• viscosity due to the relative movement of groups of adjacent particles (internal friction) or to fluid friction on walls ;

• thermal conductivity between compressed parts of the fluid, whose temperature increases, and expanded parts, whose temperature decreases;

• molecular relaxation, which, in a polyatomic fluid, involves a coupling and therefore an exchange of energy between wave vibrations and the internal movements (translation, vibration, rotation)...