i humbly add the axiom audio LFR series. these are floorstanding front/rear bipole designs... the effect isn't towards surround sound but ridiculously wide sweet spot and room filling sound for larger spaces.
Imo bipolars are more practical than omnis for three reasons:
First, given placement several feet out into the room, the contribution of the bipolar's rear-firing energy ONLY increases the energy in the later-onset reflections. It will have no more early reflections than a conventional speaker, but will have significantly increased later-arriving reverberant energy, which imo is desirable. The reduced sidewall-directed energy (relative to an omni) allows bipolars to perform well in narrower rooms than omnis.
Second, since bipolars are somewhat directional, they can be aimed. And when they are toed-in, the rear-firing drivers are toed-out, which correspondingly increases their reflection path lengths (which is desirable).
Third, bipoles inherently have a bit higher direct-to-reflected energy ratio than omnis, and imo in most rooms bipolars are closer to "ideal" in this regard.
Bipolars share this desirable trait with omnis: Their reverberant sound is spectrally very similar to their direct sound. In fact the designer could theoretically even tweak the response of the rear-firing drivers to better accomplish this.
Out of all the loudspeakers tested at the NRC in Canada during Floyd Toole's tenure there, his first choice was the bipolar Mirage M1 for his large home listening room (a combination living room/dining room with a somewhat high reverberation time of about 1/2 second). I'd like to quote from the third edition of his book, page 190. Those of you who dismiss all subjective impressions as inherently untrustworthy, fire up your keyboards:
"Over the years, a parade of loudspeakers went through that room, and all disappointed. The room was an unforgiving critic of loudspeakers in which the direct and reflected sounds exhibited different spectra, and conventional forward-firing loudspeakers drew attention to themselves... Then, in 1989, a new loudspeaker came on the scene: The almost omnidirectional , bidirectional-in-phase "bipolar" Mirage M1. They performed well in double-blind listening tests in the small NRC room, and also in this large one.
They simply "became" the orchestra. " [emphasis Duke's]
Note that dipole speakers share many of the same attributes of a good bipole, only with reduced early same-side-wall interaction (which imo is desirable).
I have been involved with bipoles, dipoles, and other polydirectionals for over thirty years, along with monopole loudspeakers. Some years ago I wrote an article for an online magazine about a bipolar design concept I was using. At the time I didn't realize how close this was to what Mirage was doing in the M-1, as I had never seen the back side of an M-1 with the grille removed... the major difference turned out to be that my patterns were narrower:
The Controlled-Pattern Offset Bipole Loudspeaker (hifizine.com)
