[mayo29]

A monopole and a direct radiator are not the same thing. Most dipoles and bipoles ARE direct radiators. And there are monoples that AREN'T direct radiators. Monopolar describes the direction in which a driver is pointing; direct radiating decribes the manner in which a driver is coupled to the air.

A direct radiator is a speaker in which the diaphragm acts directly against the air without an intervening device. Examples of non direct radiating speakers are horns and bandpass boxes. An example of a non direct radiating monopole is a Klipschorn. Or an Altec VOT.

Some speakers are a mixture of horn loaded and direct radiating such as an Altec Model 19 which uses a direct radiating woofer and horn loaded treble. Another example would be a full range driver like a Lowther loaded to a back horn; frequencies above about 200hz are radiated directly from the cone while frequencies below 200hz come mainly from the horn.

The only non direct radiating dipolar syatem I've seen is a DIY EV horn system a friend built which has horns pointing front and rear and wired out of polarity from each other.