Flow-generated sound sources
Digital aeroacoustics: sound sources and their radiation

The propagation of sound in a fluid is usually described by linearizing Euler's equations (conservation of mass and momentum), as disturbances are generally very small compared to mean quantities. In fact, a high sound level of 94 dB translates into pressure fluctuations p with an effective value of 1 Pa, i.e. only 1/100,000 of atmospheric pressure. The associated vibratory velocity u is worth around 2.5 mm/s because p/u is of the order of ρ₀a, where ρ₀ is the average density and a is the celerity of sound (ρ₀a ≈ 420 kg.m^–2.s^–1 in air); u is well below the velocities associated with turbulence within the fluid.

This classical linear acoustic reasoning leads to the wave equation