Acoustic Bipole and Tripole
An acoustic bipole consists of two identical monopoles driven in phase (whereas, in a dipole, they are out of phase), separated by a short distance along the axis. As shown in the Details section, the radiated sound produces a directivity pattern , where is measured between the point of observation and the normal of the bipole and stands for the wavenumber.
The radiation field for a bipole is
where the directivity pattern can be written as
For the tripole
See Part 6 of .
 P. B. Nagy. "Acoustic Radiators." (Nov 9, 2001) www.ase.uc.edu/~pnagy/ClassNotes/AEEM7028 Ultrasonic NDE/.