Opinions on Surround Speakers – Bipole, Dipole or Monopole?
The problem with all-encompassing generalizations is that even if they’re correct in some cases, they’re wrong in other cases. In our desperate quest for definitive conclusions, we often seek for black-and-white answers.
When someone asks us what the best amplifier is, we have to ask them what speakers they’re using, what their budget is, how big their room is, how far away from the speakers do they sit, what are the room acoustics like, how loud do they like to listen, what music do they listen to, etc. There is no “best” but there definitely is a “most appropriate.”
Few topics among audio enthusiasts have ended more friendships than the subject of surround speakers. (Ok, not quite. But still….) Should you use five (or seven) identical speakers? Should you use dipole surrounds? Bipoles? How about monopoles? And what do these terms mean?
As much as surround sound excites us, allow me to point out that the three front speakers (Left-Centre-Right) and subwoofer(s) in a home theater system are the most important speakers. They do the bulk of the work.
Surround speakers are also important; just less so. There, I said it.
If you have a mediocre front soundstage, your 11.2 system will never sound all that good. Always purchase the best front speakers (and subs) you can buy. If that means you end up with “only” a 5.1 system, that’s fine. There is always tomorrow to add more surrounds.
We’ll focus on surrounds in a 5.1 or 7.1 system in this blog, as this is still by far the most popular option with Kiwi’s today. However, it is good to note that the basic ideas for choosing surround speakers also apply to additional surround technologies.
The general categories of speakers used to reproduce discrete digital surround sound are monopoles, dipoles or bipoles (or a combination).
1. Monopoles are like your typical front (or bookshelf) speakers in that they use drivers that fire forward. Audiophiles tend to prefer this setup (see later). All of the surround information is aimed at the seating position, and reflections are not utilized, or at least not as much.
2. Bipoles have drivers on two faces, opposite each other and they are ‘in phase’ with one another. The idea is to fire the surround information into the seating area, but not localized. Bipole surrounds work quite well in the right setting.
On a side note: every now and then bipole type speakers are sometimes offered as front channels too like the Definitive Technology towers that have just been announced for instance.
3. Dipoles look similar to bipole speakers, except one face (usually the rear array) is ‘out of phase’ with the front face. Dipoles require a bit more explanation: the rear array is 180-degrees out of phase with the front array. This means that when one side’s drivers are pushing out, the other side’s drivers are pulling in. What this does is create a cancellation on-axis, which results in a drop of 8 -12dB. The audio level of a dipole therefore has to be adjusted higher, and the pumped-up, off-axis reflected sound is mostly what the listeners will hear.
Dipole surrounds produce a less-focused, diffuse, surround field and are known to have weaker bass response.
So, what type of surround speaker technology is best?
There are many different opinions on this question.
A lot depends on your set-up, your seating position, your room acoustics and listening preferences.
Monopoles fire sound directly out toward the listeners’ ears. The pattern in which they disperse sound is clear and focused, producing a directional sound effect.
This places the listener in a sound-field in which the direct sound is more prominent, which can be of some benefit in listening to multi-channel music.
In a movie situation, surround speakers are intended to reproduce ambient effects like explosions, gunfights, the hum of a spaceship, or crickets at night so they sound like they are coming from everywhere in the room — not just where your speakers are located. When monopoles are use as surround speakers for movies, the surround effects lose their three dimensional and diffuse impact.
The smaller the room and the more acoustically controlled it is, the more monopoles will ‘hotspot’ (become localized), especially if you are in the proximity of one of the surround speakers (which is hard to avoid in a small room). If a large room is not acoustically treated, monopoles will actually work quite well, with distinct, exciting surround effects.
Dipole speakers create a more open space of sound without the listener being able to pinpoint the source of the sound.
The ‘null-zone’ (cancellation zone) created by dipole speakers means that the sound coming straight toward the listener’s ears is effectively dampened, and instead the listener hears virtually nothing but reflections from the room boundaries. This means that instead of perceiving sounds as coming from the speaker itself, the result is a diffuse sound-field.
The dipole speakers biggest critique is that they were invented in the old analog and matrixed surround sound days. Nowadays, most digital surround audio codecs are discrete with five or more channels and the diffuse sound-field of dipole speakers destroys this advantage. We tend to agree with that position.
Because of this, it seems that lately there is a movement away from dipoles and more towards bipole surrounds in the home theatre world. There are still plenty of die-hard dipole fans around though and in the right home theatre (small sized, with multiple seats and acoustically well-controlled), they definitely still have a place.
Bipole speakers by design provide the best of both worlds: a monopole’s clarity and focus and a dipole’s spaciousness.
Bipole surround speakers use speaker drivers aimed towards the front and back of the room to achieve a diffuse sound-field like that created by the multiple surround speakers in movie theaters, without loss of bass.
However, because bipole speakers are relying on this bouncing off the front- and back wall, the result is affected by your room’s acoustics more than with the use of a monopole speakers. You are basically very much at the mercy of the room.
On a final note, it is also good to know that dipole speakers are designed to have a null area in front of them and the listener is intended to sit within that null for the proper ambient effect.
Bipole speakers, on the other hand, have no null area, and are much more flexible in their placement.
All of these factors will play a role in the decision of which option works best for your personal situation.
So What’s the Bottom Line?
The takeaway from all this should be that the room and the surround technology you’ve embraced will dictate what surround speakers you should use. And then there is always personal taste. Your seating configuration and room acoustics will also influence your decision. The interface between the speakers and the room is crucial, and the choice of speaker must be dependent upon all the variables. There is no “best” surround speaker, but there may be a best surround speaker for you.
SVS, Klipsch, Jamo and Definitive Technology are offering options for bipole and dipole speakers.
Image from ‘Integracoustics’.