Cavity

A deep hole machined in the face of a horn. Cavities are usually associated with cylindrical horns and are usually concentric with the stud axis. Cavities often extend to the nodal region and may be used to increase the horn gain (e.g., bell horns), to provide chip clearance during machining (e.g., radially slotted cylindrical horns), to provide clearance for a protrusion on a plastic part that is being welded, or to provide a channel through which fluid may flow or vacuum may be applied.

Also see —
Contour
Relief