Directivity of a transducer
Introduction to electroacoustics - Electroacoustic transduction

Any vibrating body radiates a fraction of its energy into the surrounding space. This radiation does not take place with a uniform distribution of energy in all directions, although the propagation medium is usually homogeneous and isotropic.

Sources are said to have radiation directivity.

This directivity is due to reflection, diffraction and interference phenomena. The higher the frequency, the more pronounced the directivity.

Electroacoustic sources are almost always characterized by axisymmetric directivity.

The isolated loudspeaker generally behaves like a dipole, while the loudspeaker integrated in a large planar baffle has radiation characteristics...