A speaker that radiates in two directions out of phase.
An open-back speaker that radiates sound equally front and rear. The front and rear waves are out of phase and cancellation will occur when the wavelengths are long enough to "wrap around". The answer is a large, wide baffle or to enclose the driver creating a monopole.
A speaker which has two sets of drivers, each facing opposite directions, that are wired out of phase. This type of speaker is often used for surround sound, as they generate a non-localizable effect.