ABSTRACT
An understanding of sound waves is useful to estimate things as diverse as the dynamical response timescale of a star to changing conditions or the optimal time step size to take a numerical solver for Euler's equations forward in time. The study of sound waves is therefore an important aspect of terrestrial and astrophysical fluid dynamics. A gravitational field is both responsible for an initial non-homogeneous fluid profile and has potential impact on the evolution of a fluid perturbation, for example planetary and stellar atmospheres. Based on the sound speed, the behaviour of a fluid can be divided in to two regimes: supersonic flow and subsonic flow. The chapter concentrates on plane acoustic waves that propagate in a single direction with a speed-of-sound, and focuses on the wave equation. A wave packet can be analyzed using Fourier transforms to reveal its constituent harmonics.
