It is fascinating to me the myriad of seemingly endless variations designers apply to sound reproduction equipment.
Take the rarely seen today bipole loudspeaker. The last mass-produced version I remember was by the Canadian company, Mirage.
Let's start with a smidge of reference. Most loudspeakers are monopoles: sound comes out of one plane of the speaker box. A smaller number are dipoles: sound comes out of two planes (front and rear) and the rear plane is out of phase with the front. A bipole is like a dipole in that it too has front and rear radiating planes, but instead of being out of phase the front and rear are in phase.
Perhaps the easiest way to picture a bipole is the idea of a pulsating cylinder, though not in the same way you might think of an MBL (which literally is a pulsating sphere). In the bipole, the same woofer, midrange, and tweeter drivers that you find in the front of the speaker are duplicated on the rear of the speaker—all wired in phase.
The acoustic pattern that is created is somewhat of a figure 8.
The bipole had some advantages, like fewer sidewall issues than monopoles, but for the most part I never really found the configuration very attractive—and it had a number of the problems we associate with dipoles and open baffle speakers—sans the bass cancellation problems.
If you're curious about the Mirage speakers, there's a well written review by Tom Norton in this issue of Stereophile.
What's interesting to me about this design is that it's but one more attempt by clever people to build a speaker that differentiated itself from the pack. One more twist to an ever-evolving evolution in the art of making high end audio products.
The variations at times seem endless.
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