2. Propagation of mechanical shaking

The study of acoustic wave propagation in a non-viscous fluid is (as we said in paragraph 1 ) a priori simpler than in a solid, since its mechanical behavior is defined by a single elastic modulus and the acoustic wave is represented by a scalar quantity: the acoustic pressure. But the fact that not every fluid molecule can be tracked in its motion means that Euler's representation and the convection terms involved render the equations non-linear. The more direct Lagrangian description applies...