The resonator surface from which the coupling stud protrudes. This surface is usually the input surface. (Transducers have no stud surface.) This surface forms the joint between the resonator (e.g., the horn) and the mating driving resonator (e.g., the booster).
Note: the stud surface is usually the rear-most surface of the resonator. However, for horns that use risers, the riser extends rearward beyond the stud surface.