June 2002

Surround Speaker Confusion -- Part One: Speaker Design

For those that thought choosing two suitable surround speakers and correctly placing them in a 5.1 system was difficult, there is now the added complexity of 6.1 and 7.1 configurations, which require the use of additional surround speakers. These systems not only have the usual left and right surrounds, but also require one or two center (sometimes called back) surround speakers.

The added complexity of 6.1 and 7.1 systems, in addition to the relative lack of information from manufacturers on implementation, has led to confusion among consumers on how best to configure them. Even amongst "experts" there has been considerable debate on the placement and design of surround loudspeakers.

This installment of Cinema Cynergy will investigate the various designs commonly used for surround speakers and some of their advantages/disadvantages. It is important to understand how these speakers are designed and function, which will in turn affect how they are utilized and placed in a surround speaker array.

Speaker designs

There are two basic designs that are commonly used for surround speakers: dipoles and direct radiators. Dipoles radiate sound in both a forward and backward direction (each side out of phase with the other), while direct-radiating designs project sound in only the forward direction.

Dipoles

Dipoles gained popularity for surround use when Lucasfilm’s THX Division first introduced their specifications. The THX certification program required dipole surrounds in order to create a diffuse sound field to the sides and behind the seating position, which would envelop the listener without being localizable. Many manufacturers now produce dedicated surround speakers that are dipolar designs (some of which are THX Certified and some of which are not).

Dipolar surround speakers are designed to be positioned so that they do not radiate sound directly at the listener. Instead, they are designed so that the listener is positioned in the null region of the speaker’s radiation pattern, which means the drivers are pointed north and south, if for example, the listener is east. The listener then receives more reflected sound than direct sound, which should result in a diffuse and enveloping sound.

Planar loudspeakers such as those from Magnepan or MartinLogan are inherently dipolar in their radiation patterns, but most purpose-built dipolar surround speakers are conventional designs with similar driver arrays mounted on opposite sides of the cabinet.

The proponents of dipole surround speakers cite the benefits of a diffuse rear sound field that envelops the listener. This, they feel, is a desirable objective in home-theater sound reproduction. Sounds that originate from off screen, and particularly from the sides and back of the room, can become distracting if they are too localizable. This effect is lessened by the use of dipole surrounds.

Another advantage of dipole surrounds is that they can usually be placed closer to the seating position. They do not need the usual distances from the speaker to the listener for the drivers to integrate with each other because they rely more heavily on reflected sound. In fact, if dipole surround speakers are placed too far away from the listener, the reflected sound may become overly diffuse and vague.

Direct radiators

Direct-radiating loudspeakers project sound in only one direction and are typically what most people think of as a conventional speaker design. These speakers usually consist of a cabinet with two or more drivers mounted on the front baffle. Most direct-radiating surround speakers are no different from direct-radiating stereo speakers and could be used in either position.

Direct-radiating speakers generally provide a more localizable sound that has greater image specificity. Because discrete digital formats such as Dolby Digital and DTS allow for split-surround effects (sounds in either the right or left surround speaker), the argument goes that direct-radiating surround speakers are required to provide proper localization and placement of these effects.

Other designs

There are surround speakers that are not true dipoles, but rather are hybrid designs with a single woofer pointed in a forward direction and two horizontally opposed tweeters pointed away (or some other similar configuration). These speakers behave much like dipoles, because the lower frequencies reproduced by the single woofer are not particularly directional, so they already seem to radiate diffusely. There are many variations on this design and Canadian-based manufacturer Axiom Audio actually produces surround speakers that radiate sound forward, backward, and up and down in a design they call a Quadpole.

Another Canadian speaker manufacturer, Mirage, even has surround speakers termed "Omnipolar." These designs are said to radiate sound in all directions, or in the case of the surround implementation of this technology, in a hemispherical pattern (a half-grapefruit shape).

Six of one, half a dozen of another

The debate between the utilization of dipoles and direct-radiating speakers for surrounds often centers on the localization of effects: dipoles being characterized as having a diffuse non-localizable sound and direct radiators having a precisely placed sound. People tend to blame or praise Lucasfilm’s THX Division for championing the use of diffuse-sounding dipolar surround speakers.

Some of the main objectives of a THX Certified system are:

• A sense of being enveloped in an acoustic environment that is appropriate to the image on the screen or nature of the music.
• An excellent timbre and spatial match to the screen.
• An absence of localizable sound sources intrusive enough to inappropriately distract the listener from the presentation on the screen.
• A sufficient sense of cardinal directionality (left, back, and right) to provide convincing special effects or musical accents.

The last two goals of THX may seem to be in conflict with each other, but actually show how the desired sound of a surround speaker array is often a compromise between creating an enveloping sound (what THX surround speakers are most known for) and the ability to localize certain directional effects.

It should be pointed out that dipoles are not so diffuse sounding that they cannot provide any localization of sound and direct radiators are not so localizable that they cannot provide any sense of surround envelopment. Both types of speakers share these characteristics -- just to varying degrees. In fact, dipoles can be made to sound more localizable by pointing the drivers toward the listener and direct radiators can be made more diffuse sounding by pointing the drivers away from the listener and reflecting the sound off of room boundaries.

The arguments for and against both types of speakers are many and often circular. For instance, discrete sounds occurring off screen are in fact distracting, but some localization of sound is desirable for split-surround effects. A diffuse and enveloping rear sound field can provide a very involving surround-sound experience, but this should not be dictated strictly by the hardware.

Basically it all comes down to a matter of personal preference. I generally prefer my surround speakers to be full-range stereo speakers that would normally be used as left and right mains, but most people would not be willing to place relatively large floorstanding speakers at the back of their rooms as I have done. However, I have heard many satisfying home-theater systems that have used less-than-full-range surround speakers that were direct radiators, dipoles, tripoles, bipoles, etc.

Next month we will look at some tips on the placement of your surround speakers to help you get the most out of your home-theater system.

...Roger Kanno
roger@hometheatersound.com

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