2. Qualitative description of the thermal evolution of gas particles

2.1 General equations

An acoustic wave is the result of a variation in pressure p around the value of the mean pressure p ₀ in a fluid; its propagation takes place without any overall displacement of matter. In fact, it can be seen that elastic elements, i.e. fluid particles, oscillate locally under the effect of pressure oscillation, but without overall displacement, and that this disturbance travels in the form of waves that progress from the center of emission. Acoustic waves thus travel in solids, liquids and gases alike.

The equations of acoustics are deduced from the general equations of fluid mechanics: equations of conservation of mass, balance of momentum, conservation of energy, to which we add the equation...