Endless variations

July 23, 2022
 by Paul McGowan

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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36 comments on “Endless variations”

  1. So it seems most omnidirectional speakers avoid a bipole, parallel plane chassis configuration and tend to radiate in several directions but never directly opposing within two parallel planes like bipolars do, right?

    Isn’t the rear tweeter of the FR30 in bipolar configuration or is it out of phase?

    1. jazz,
      I looked everywhere in the aspen FR30 on-line promo & couldn't find anything about them being either 'bi' or 'di' polar but I think that there is a switch on the rear panel that will allow you to disable the rear tweeter if having both in operation does not suit the listener or the room acoustics.

      Given that the IRSV's are a dipole design there's a good chance that the aspen FR30's are too...unless Paul was overruled by Chris 😉

  2. I'm guessing that if you're the kind of audiophile/enthusiast who is constantly searching
    for the 'perfect' sound from your 2 channel rig then you're probably going to investigate
    every possible & seemingly endless variation of loudspeaker &/or amplifier topology.
    Cones, domes, horns, AMTs, ribbons, monopoles, bipoles, dipoles, tripoles, etc.
    Just like Ben & Jerry's; different flavours for different folks.

    1. There are near perfect loudspeaker cabinets (Rockport, Wilson Audio, Magico, Vivid Audio), near perfect driver motors (see Børresen), near perfect drivers (ion tweeter) but there will never be a perfect multi-way passive crossover loudspeaker! The best approximation to perfection might be possible with an active loudspeaker featuring DSP correction schemes. Headphones (no cans) as from AKG (K1000) or Raal hanging freely near the pinnae can demonstrate the best degree of high fidelity. Thus in the end there is a giant diversity in individual “sound” preferences.

      1. ps,
        I like the idea of granite loudspeakers cabinets a la 'Acora Acoustics'.
        And then there's open baffle, ie. no cabinet a la 'Spatial Audio Lab'

        1. FR,
          I would always go for in-outwall (brick-walls of course, driver’s rear directly facing the garden) loudspeakers, if I ever had the chance. 🙂 An alternative option would see these in-wall speakers emitting the back sound to a cellar room. 🙂

        2. My Von Schweikerts bass modules have 77mm (almost 3") thick cabinet walls consisting of three layers: MDF, artificial stone and hard rubber, while the treble driver assembly is minimally encased, with lots of air around it.

          1. At an audio show in NYC around the turn of this century I heard some incredible Von Schweikerts speakers. They were 600 lbs. per speaker, each had an active subwoofer in its enclosure and they cost about $75K back then ( well beyond what I could afford ). I still dream about them.

    1. I auditioned Apogee Duettas decades ago. Wonderfully musical, transparent and airy.
      Within their frequency and dynamic limitations they're a truly great transducer - perfect for singer-songwriter and jazz.
      There are systems that combine bipole and dynamic. Done right, you're close to heaven.

    1. I wouldn't call it boring. I would call it logical, efficient and practical. My speakers came with dipole out-of-phase rear tweeters. I tried them at different volumes and ended up turning them off.

  3. Thanks for that Paul. I wasn’t really aware of the phase differences with bi and dipole designs. But, when I listen to how critical the speaker positions are for woofer, mid and tweeters, required axis of radiation, cabinet shapes and their characteristic frequency vs sound pressure curves, etc. etc. it sounds like there is really only one one right design. But of course there isn’t.
    Then I look at the endless designs with speakers ahead, behind, by the side of, on the top, the bottom, and sides, passive radiators, dipole and bipolar etc, not to mention the construction, I get to realise there is, apparently, no right or wrong. Just what sounds right to the designer, within their price point. And are some designed just to mainly grab attention? Fall in love with looks and overlook sonic deficiencies?

  4. I've owned a pair of Mirage OM 5 for about 20 years. I really like the sound that seems to come from a wide sweet spot. On the other hand they don't localize instruments as precisely as other speakers I've heard. Anyway they fill the room (and first floor) very evenly. Now that I'm monaural it doesn't make much difference. I'm sure a single FR 30 would sound mighty fine. Thanks for mentioning them, they seem to have gone under the audiophile radar.

  5. So I feel there are two camps. And I like both admittedly. Specificity and bass that hits you in the chest like a monopole or big spacious images with a more contiguous sound like a magnepan. I find the former ‘fun’ and the latter to be something more akin to a symphony. I really like both for each of their strengths I have gone to a Mahler symphony / gone home and played the same sacd over my Maggie 20.7s and damn it was very close. With a monopole - it gives you that jump factor - nothing i would ever hear live. But maybe I don’t care - it’s a different ‘thing’ and very fun.

  6. Imagine that a human mouth is like a midrange driver. In what circumstances would a mouth on both the front and back of the speaking (or singing) person's head be advantageous to a listener or listeners? If listeners are positioned both in front and behind the speaker or singer, then a synchronized mouth on the front and back would make sense. If listeners are only on one side, two mouths emitting the same sound would be redundant and a waste of biological resources. If volume is the issue, then a bigger mouth (lungs, larynx and orifice) or multiple mouths would be an answer (such as a choir). In nature, evolution has typically favored only one mouth, on the side of the listener. Thank goodness we don't have to feed our speaker drivers everyday like we do our mouths. And rarely do we have to tell our speakers to shut up!

    1. I meant to extend the human mouth analogy to include di-polar politicians. What comes out of their front mouth often contradicts what comes out of their rear mouth. Endless variations indeed!

    2. I like this analogy, a different and imaginative way of looking at things and kind of makes perfect sense, yet we have side firing speakers and downward firing subs, both of which are very effective.
      When I read an analogy like this I tend to visualise it literally which creates a pretty bizarre image. I also took the feeding one stage further, which was likely your intention, not nice 🙁 😉
      As for the politicians, you nailed that one.

      1. It was not my intention with the feeding joke to lead anyone in a "not nice" direction. Your one stage further was your own invention 🙂

        As for the dipolar politicians, you correctly interpreted. I was referring to their propensity to speak out of two mouths, in two directions to different audiences, one mouth 180-degrees out of phase with the other mouth.

  7. Mirage were terrible sounding speakers. The company that was more famous for bipolar speakers was Definitive Technology and they sounded pretty good for their price: bp10’s, bp8’s which I owned for my home theater in the 90’s. They probably had 15 different designs of their bipolar speaker systems and might possibly still sell them today.

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