Table 1.

Common boundary conditions for solving the Helmholtz equation in linear models.

Boundary Condition	Description
$P=0$(5)	This condition is used for infinitely soft (free) boundaries.
$\mathrm{\nabla }P·\mathit{n}=0$(6)	This condition is used for infinitely hard (fixed) boundaries. Where n is a normal vector pointing outward.
$P=P_0$(7)	This condition is commonly applied at the surface of the emitting ultrasonic transducer.