Dipole all the way 20Hz-20kHz
I hope you already know what open baffle speakers are and what sound properties they have. My dipole speakers are just the special case of polar pattern of open baffle speakers. The aim is to have 4-way system where every driver works as a point source. This means it works around first dipole peak. So it's polar response is almost perfect dipole pattern. If an open baffle speaker has baffle size larger than the wavelength of the sound wave, then they are not dipole anymore. So the polar pattern changes with frequency increase. So you can't guarantee uniform off axis frequency response and also the same energy transmitted into your room for all frequencies. Thats why there is bare minimum of material around drivers in this system. Is this off axis response important? Floyd Toole, Sean Olive and other researches thinks yes. Preference ratings are higher for loudspeakers with more uniform polar pattern from their findings. For in room listening you are listening to direct sound and reflected sound. And psychoacoustically it is very important to have reflected sound spectrum closely related to direct sound spectrum.
Of course everybody knows that dipoles at low frequencies are inefficient and max SPL will not compete to monopole, but for small rooms it is not very important and especially in small rooms dipole bass is cleaner than monopole. It is more articulated. On Elias Pekonen site you can find good modulation transmission analysis dipole vs monopole. Which just shows where could be an explanation why the bass is so natural from dipoles.
Pic.1 shows this system dipole behaviour in free field measurement. With frequency dependant windowing applied with 1/6.9 octave resolution. At +-90 degrees the speakers emits virtually no sound. Just front and back. So some room modes and reflections are removed from the room compared to monopole.
Pic 1. Directivity index in 3D of the speakers 20Hz - 20kHz
Pic 2. Polar response of the system