The more confined the radiating angle, the higher the directivity of the wavefront. In a sense, the boundary conditions act like a waveguide, defining the radiating angle. However, this is only true if the sound source is omnidirectional to begin with, the hypothetical pulsating sphere. In practice, sound sources are not that way, and have complex directional characteristics of their own. If the sound source is already more directional than the boundary conditions it is radiating into, then it might as well be operating in freespace.