August 2003

Surround Speakers: Dipoles and Direct Radiators

In this column, I have shied away from voicing opinions on the various types of surround speakers, choosing rather to give an objective description of some of the differences in their designs. However, hardly a week goes by without someone asking me whether it would be better to use dipole or direct-radiating surround speakers in their home-theater system. So this month I will share some of my observations on the sound of different types of surround speakers.

As with most things, I believe that different approaches, such as using dipoles or direct radiators as surround speakers, have both advantages and disadvantages. It is not so much a matter of which type of speaker is better in an absolute sense, but which is better suited to your needs, depending on your personal preferences and perhaps more importantly, how the speakers interact with your room. While I have listened to many different direct-radiating and dipole surround speakers over the years -- and some of them have even spent considerable time in my own system -- I would be hard pressed to pick a favorite as many of them were excellent speakers, but for different reasons.

Direct radiators

Direct-radiating loudspeakers, as their name implies, radiate sound in only one direction (directly at the listener). They tend to image very precisely and those that have exceptionally good imaging characteristics are said to have "pinpoint imaging." By that I mean objects such as voices, musical instruments, or sound effects occupy a very definite and precise location within the soundstage and are easily distinguishable from one another.

The argument for using direct-radiating surround speakers is that discrete digital surround formats such as Dolby Digital and DTS allow for stereo surround effects, and the precise imaging characteristics of these speakers are required to accurately position these effects in the rear of the room.

I tend to agree with this and often find that good bookshelf speakers make excellent surrounds. In fact, the amazing high-value Audition home-theater speaker system from Athena Technologies successfully used their AS-B1 bookshelf speakers as surrounds. I have also had success with using five Axiom M3Ti SE bookshelf speakers in a home-theater system. By using good bookshelf speakers as surrounds you can attain a very cohesive sound. Although the surround speakers typically do the least work of any of the speakers in a 5.1 home-theater system, you should not overlook their importance. High-value bookshelf speakers are a cost-effective way of getting great sound using direct-radiating surround speakers.

Things to consider with direct radiators

When direct-radiating surround speakers are placed to the sides of the seating position and pointed directly at the listener, I find them to be excessively conspicuous, with all the sound seeming to come directly from the speakers. To avoid this, these speakers should be positioned a foot or two above ear level. I also like to place direct radiators a bit behind the listeners and angled slightly away from them. If they are to be placed directly behind the seating position, I like to have them set back at least several feet and spaced relatively far apart, angled inwards towards the listeners.

Direct-radiating loudspeakers require adequate distance from the listener for their drivers to integrate and to sound their best. Because surround speakers are typically positioned closer to the listener than the front speakers due to space limitations, their sound can become too localized, which can be distracting. By experimenting with their height, position, and angle, they can be made less conspicuous yet still maintain good image specificity.

Dipoles

Dipole speakers radiate sound in two, usually opposite directions with the sound traveling in one direction being out of phase with the other. Dipole speakers tend to have less-precise imaging characteristics than direct-radiating designs, but they seem to present a better sense of space by spreading out the sound and presenting a larger soundstage.

This results in a diffuse rear sound field that envelops the listener in an immersive surround ambiance that can be very involving. Although the sound is diffuse, dipoles do still localize sounds, just not with the same image specificity as direct radiators.

I have had some excellent diffuse-radiating surround speakers from Paradigm, Mirage, and Axiom in my system as well as the superb Snell SR30THXs, which are part of a THX Ultra2 Certified speaker system that I currently have in for review. The Snells actually radiate sound in six different directions and still have reasonable image specificity. And if that were not enough, there is a switch that will turn them into direct-radiating speakers by turning off two of their three tweeters, so that you can tailor them to suit your room and your own tastes.

Things to consider when using dipoles

One major advantage I find with dipole surround speakers is that they do not seem to require as much distance from the listening position to sound good. They also seem to be bit less placement sensitive, which I suspect is due to their reliance on reflected sound to avoid a sharp image, which is the opposite of what we normally want from a speaker.

As with direct radiators, I prefer to place dipole surround speakers a couple of feet above ear level, but directly to the sides of the seating position so that the "null" is pointed at the listeners as intended. Placing them behind the seating position will also work as long as the drivers do not point directly at the listeners.

When positioned properly, dipole surrounds do provide an enveloping diffuse rear sound field, but still retain enough imaging to give directional cues for stereo surround effects. One trick that you can use if the sound of dipoles is too diffuse for your tastes is to angle one radiating face of each speaker slightly more towards the seating position so that the listener receives more direct sound.

A tale of two speakers

Recently I experimented by comparing a pair of Axiom M3Ti SE direct-radiating speakers with the Axiom QS8 diffuse-radiating surround speakers. As expected, the M3Ti SEs gave a more focused presentation and I thought they sounded a little more tonally accurate. However, the QS8s seemed to do a better job of enveloping me with sound that was more involving and visceral. To make a long story short, I enjoyed both sets of speakers and each had its strengths with different program material, but there was no clear-cut winner.

There are no easy answers

As you can see, there is more to choosing a surround speaker than simply deciding whether you prefer dipole or direct-radiating designs. There are even surround speakers that radiate sound in more than two directions, such as the aforementioned Axiom QS8 and Snell SR30THX. Add to this the many stereo bipole speakers that could also be used as surrounds (they radiate sound in two directions, but in phase with each other), and the number of choices becomes dizzying.

I have had excellent results using surround speakers of many different designs in my home-theater system, including direct radiating, dipole, and others. All things being equal, if pressed to choose between direct-radiating and dipole surround speakers I would probably pick direct radiators. However, in my listening room, because I am forced to place my surround speakers closer than the front speakers to the seating position, dipoles and other diffuse-radiating surround speakers often sound better than direct radiators. I like my surround speakers to have a balance of both surround envelopment and image specificity, which I have found can be achieved with both types of surround speakers by experimenting with their placement and orientation, and making them work with the room.

...Roger Kanno
roger@hometheatersound.com

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