Acoustic theory is a scientific field that relates to the description of sound waves. It derives from fluid dynamics. See acoustics for the engineering approach. For sound waves of any magnitude of a disturbance in velocity, pressure, and density we have In the case that the fluctuations in velocity, density, and pressure are small, we can approximate these as In the case that the velocity is irrotational, we then have the acoustic wave equation that describes the system: Where we have