The Incident Pressure Field node is a subnode to all nonreflecting boundary conditions (plane, cylindrical, spherical wave radiation, and matched boundary). From the 
Physics toolbar, add to 
Matched Boundary, 
Plane Wave Radiation, 
Spherical Wave Radiation, or 
Cylindrical Wave Radiation nodes. Three options exist for transient models: 
Plane weave (monochromatic) (the default), 
Modulated Gaussian pulse, and 
User Defined.
 
    where ω is the angular frequency and 
k is the wave vector. The function 
f is any function, for example, a sine function. This spatial and temporal dependency is a requirement for the radiation boundary condition to work properly.