What should you use…a bipole, dipole, monopole, tadpole? It depends…
The problem with all-encompassing generalizations is that they are often in error; even if they’re correct in some cases, they’re wrong in other cases. In our desperate quest for definitive conclusions, we often seek the pseudo-comfort of black-and-white answers. Sometimes the question is flawed, such as “What’s better; a car or a truck?” The correct or appropriate answer might depend upon whether you’re driving to the store for milk or hauling 500 lbs. of mulch. When someone asks me what the best amplifier is, I 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, yada-yada-yada. There is no “best” but there definitely is a “most appropriate.”
Other than discussing religion or politics, (or what interconnects or capacitors sound like), few topics among audio enthusiasts have caused more rhubarbs or ended more friendships than the subject of surround speakers. Should you use five (or seven) identical speakers? Should you use dipole surrounds? Bipoles? Tripoles? How about monopoles?? And what do these terms mean?
Before continuing, allow me to make the sacrilegious comment that the three front speakers (LCR) and sub(s) in a home theater system are the most important speakers. Surround speakers are also important; just less so, and yes, I actually said that. If you have a mediocre front soundstage, your 10.2 system will never sound all that good. As I stated in Optimizing Front LCR Speaker Placement, 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. You can add more surrounds later. The front speakers and subs do almost all of the heavy lifting in a home theater, and in choosing them, you need to avoid compromise as much as possible. As important as surround speakers are, they play a subordinate role in most cases. Before firing off an indignant nastygram, please note I said “in most cases.”
We’ll touch on immersive surround speaker layouts but limit most of this discussion to surrounds in a 5.1, 6.1, or 7.1 system. However, the basic premise for choosing surround speakers holds true for additional surround technologies. The general categories of speakers used to reproduce discrete digital surround sound or processed analog surround are direct-radiating (sometimes called monopoles, a term I consider an affectation), dipole, bipole, or variations/combinations.
Surround Sound Speaker Types: which are the best?
Diagram of Various Surround Speaker Types courtesy of Dr. Floyd Toole
Direct Radiating Surrounds
Direct-radiating surrounds are like your typical front speakers in that they use drivers that fire forward. Audio purists tend to prefer this setup. All of the surround information is aimed at the seating position, and reflections are not utilized, or at least not as much. Proudly proclaiming to someone that you use direct-radiating surrounds makes you as cool as if you told them you never drink blended whiskey, or you ride a single-speed bicycle, and you listen to Alter Bridge.
In this type of speaker, there are usually two arrays of drivers which face in opposite directions, and they are in phase with one another. The idea is to fire the surround information into the seating area, but not directly at the listener, to avoid hotspotting. Higher frequencies tend to “beam” or “hotspot” with a monopole, much like a flashlight does with its light. Focused, direct surround sounds, at close range, can be a distraction. Bipole surrounds actually work quite well in the right setting. Bipole type speakers are also sometimes offered as front channels too. Think of the Definitive Technology towers as an example.
A tadpole is the larval stage of the life cycle of an amphibian, and not to be confused with a dipole. Dipoles (or dipolar speakers) are similar to bipole speakers, except one array (usually the rear array) is out of phase with the front array. Dipoles require a bit more explanation. There is a forward-firing and rear-firing array, and the rear array is 180-degrees out of phase with the front. What this does is create a null (cancellation) on-axis, with a resulting level drop of 8 -12dB. The audio level of a dipole then has to be adjusted higher in calibration, and the pumped-up, off-axis reflected sound is mostly what the listeners will hear. Dipole surrounds produce a less-focused, diffuse, enveloping surround field. Reviewer Corey Greenberg is quoted as saying, years ago, dipoles “deliver a more expansive sense of ambience.” In my conversations with him, I don’t believe he thought dipoles were best in all applications, especially for music.
Left pic: RBH SX-66/r Dipole/Bipole Speaker ; Right pic: Triad on-wall Gold Dipole Speaker
There are different driver configurations for surround dipoles, complicated by the fact that when one 2-driver array is out of phase with the other 2-driver array, bass is effectively cancelled. This problem is addressed in a variety of ways by various speaker designers. Some speaker companies ignore the loss of bass, and just leave both arrays out of phase into the bass region. Some companies roll the bass off in only one of the surround speakers’ bass drivers, which works, but it reduces bass output, headroom, and power handling. Some dipoles use a single woofer and two tweeters and only run the tweeters out of phase. And still other dipoles are out of phase down to approximately 200 Hz, below which the bass drivers are shifted back in phase, for optimized fullness.
Editorial Note on Surround Speaker Considerations by Dr. Floyd Toole:
When choosing a surround speaker, it’s important to note that the goals of sound reproduction of these speakers should be similar to the rest of the speakers in the system. Acoustics should be addressed first. Reflections from the side walls are good because they add to interaural de-correlation and shouldn’t be eliminated. Reflections from front and back of the room are just noise so there is a good case for deadening them to reduce this effect. A general rule, “mix it up”: reflection, diffusion, absorption, with a greater proportion of absorption on front and rear walls.
Localization of the surround speaker is a function of the high frequencies heard by the listeners on the sides of the room. This also happens at the cinema for patrons sitting in the seats at the sides of the theater. To reduce localization, the installer can either roll off the highs, turn down the side loudspeakers or elevate the speakers so that their tweeters fire over the heads of nearby listeners. There are other options that do not include timbrally corrupted “nulls” common in dipole type speakers such as a true line source that experiences an attenuation rate of -3dB / doubling distance as opposed to a point source of -6dB/dd. This will allow ALL of the listeners in the seated area to hear a more uniform soundfield without the hotspotting typical of conventional loudspeaker topologies that listeners closest to a surround speaker often experience.
Dipole surrounds were conceived as a way to make a low-pass filtered, monophonic surround channel, more convincing in small rooms. Much was made of “diffusion” but in small “deadish” domestic rooms, there is no diffuse sound field, so the design intent of these type of speakers has more to do with marketing talk than a basis in reality. Dipoles, or any solution that does not change the propagation loss in the direct sound, are not “better,” they are simply “different” or “less bad.”
With the advent of immersive surround sound hitting the home theater marketplace the de-correlated sounds can be radiated from several loudspeakers rather than concentrated in a few. Each speaker then becomes a less localizable source. Dipoles become even less relevant and indeed counterproductive in most cases. A shift towards bipole, monopole, or line source type speakers would seem to be the more appropriate course for the bed (ear level) fixed surround channels.
So, what type of surround speaker technology is best?
No question is more polarizing (see what I did there?) than this one. I have known and worked with many industry icons who strongly believe in dipoles or strongly believe in direct-radiating surround speakers. John Dunlavy did not care for dipole surrounds, although he designed a nice pair of them for the Fosgate-Audionics THX System in the early ‘90s during my tenure. Jim Fosgate prefers direct-radiating, as does Dr. Floyd Toole. If you do your critical listening alone and in the money seat, you probably prefer monopoles. From his early days at Skywalker Ranch, Tony Grimani has always been an advocate of dipoles. Historically, the greatest proponent of surround dipoles has been Tom Holman, who was the inventor of the Lucasfilm THX sound system. Tom is more responsible than anyone for the proliferation of dipole speakers used as surrounds. These people are all formidable in the industry, and their opinions differ, to say the least. I consider myself fortunate to have known them all, and basked in the bright white light of their auras, all the while feeling inadequate.
Legacy Phantom Surround
Excerpt from Legacy website: “Consistent with the new Atmos (Dolby) standards, Phantom avoids the phasey dipole effect of the early days of home theater.”
In the early days of what was known as the Home THX program, we were taught by Tony Grimani and Tom Holman that in a typical home theater, dipole speakers best replicated the surround effect from many direct-radiating surround speakers in a much larger theater. The exceptional Stag Theater at Skywalker Ranch appeared to me in 1992 to have around twenty Boston Acoustics A70 speakers mounted high on the side and back walls for surround. (I stand corrected if anyone can identify these surrounds as something different than I remember from that era.) I know that since then, the surround speakers in the Stag Theater have changed.
What determines which surrounds I should use?
You should use what’s appropriate for your room. The end.
My general advice to people who ask what type of surrounds to use is this: The smaller your room is and the more acoustically-controlled it is, and the more seats you have, the better dipoles will work. Personally, I am not especially a fan of dipole surrounds, but I use them in both of my theater systems because the rooms are not big, there are multiple seats, and the room acoustics are well-damped. My dedicated theater is 11.5’ wide with two rows of seating with three seats in each row, and unless you’re in a middle seat, you are in proximity to a surround speaker. I have side dipoles mounted between the two rows of seating so there are still some directional cues and no one sits in the null. In my room, and in others like it, a direct-radiating surround speaker will hotspot. It may be distracting to hear very specific sounds coming from 4’ away, and in this situation, the imperfect technology of a dipole surround speaker is preferable.
In a larger room; say, 4,000 cubic feet and up; bipoles or direct-radiating speakers can work very well. If the room is larger than 5,000 cubic feet and it’s not acoustically treated, direct-radiating speakers will work better than dipoles, with distinct, exciting surround effects. If you watch movies in a moderately-sized venue and there are only one or two seats, direct-radiating speakers can provide a stunning surround field. I would recommend monopoles for a single listener who can position his speakers symmetrically and equidistant from the money seat. Also, if you have only one seat in your dedicated theater, I suggest you rent “The Lonely Guy.” And while you can really dial in a system of direct-radiating, identical speakers for one listener, dipole surrounds offer a decent, encompassing surround field for a much larger listening area. Are there tradeoffs? Sure.
For rear surrounds, I would abide by the same basic guidelines, although many people prefer direct-radiating surrounds for rear speakers. I do prefer more directional surround cues from the back of the room, and it turns out our ears don’t work as well from behind. I have seen no real consensus on rear speakers; dipole or direct; with Dolby suggesting the rear speakers be wide apart, and others advocating for a narrower placement.
What surrounds should I use for Atmos?
Dolby Atmos creates 3D, moving audio with an object-based listening experience. Audioholics has covered the new technology extensively since last year, and it requires, at the least, four additional speakers. These speakers can be either ceiling-installed, or use specific Atmos-enabled surrounds placed on stands at head level with one array of driver(s) firing up towards the ceiling to reflect the effect, and another array of direct-radiating speakers aimed at the listeners. Although Dolby seems to avoid addressing the use of dipole surrounds in conjunction with an Atmos system, I have found at least one reference on the Dolby site that states that dipoles won’t work well for Atmos. I find it problematic that the implied suggestion by Dolby is that consumers should remove their de facto dipoles and replace them with surround speakers on stands. This is not a minor issue, and it affects a large proportion of people who have home theaters which feature dipole surrounds.
Dolby Atmos 5.1.4 Speaker Layout with discrete ceiling-mounted speakers
It seems that home theater has been migrating away from dipoles recently, though, and more towards bipole surrounds. Just yesterday, I had a long conversation with the head of engineering at an American-manufactured speaker company, and he went as far as to say all their surround speakers are now shipped in a bipole configuration as a default. Dipole configuration is available as a no-cost option. This company also manufacturers Atmos speakers, and when pressed about if he thought dipoles would work in an Atmos system, he hesitated in giving me a definitive answer. He still believes dipoles may work better in a tight setup, which is what I have generally maintained.
Because Atmos is a new home technology, and the implementation is still developing, and we can expect to have some definitive answers in the future when the dust settles. Dolby does say this, however: “We designed Dolby Atmos to be backward-compatible, so it will play on both new and existing channel-based systems. You’ll always hear the optimum mix for your system, from stereo to 5.1 or 7.1.”
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 speakers you should use, whether front speakers or surrounds. 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. Rather than asking “What’s the best medicine?” or “What kind of tires should I get?” you have to consider what is the most appropriate for your specific application. There is no “best” surround speaker, but there may be a best surround speaker for you.