1. Presentation

The propagation of elastic waves, i.e. the various possible mechanical motions in a fluid and in a solid, is studied on the basis of the general equations for the conservation of matter, momentum and energy. The differential equations that determine which motions are allowed to propagate are non-linear, and their solutions cannot be formulated analytically.

• The study is simplified when the medium is a perfect fluid in which no viscosity, thermal conduction or relaxation is involved: quantities evolve adiabatically, entropy is conserved.

Assuming small motions relative to a state of equilibrium, it is possible to linearize the equations and explain their solutions using the equation of state. The only wave capable of propagating in a perfect fluid is a longitudinal wave: a series of compressions...