The governing equations, given above in Equation 5-7 and Equation 5-6, are defined in the general scattered field formulation. Assuming that all the acoustic fields are the sum of a background (prescribed) field and the resulting scattered field, the total fields can be written as

where subscript “t” stands for total, subscript “s” for scattered, and subscript “b” for background. The scattered field is the field solved for, that is the dependent variables p, u, T, and ρ (depending on the interface). If no background acoustic field is defined (the default), the scattered field is equal to the total field. The governing equations and all boundary conditions are expressed in terms of the total fields. The background acoustic fields are defined by adding the Background Acoustic Fields feature in the LE interface or the Background Acoustic Fields feature in the LNS interface. The features have a user defined option where all fields can be entered, either as analytical expressions or defined in terms of a solution to another study or model.