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10Aug2008
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How to Choose a Center Channel

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Mick_Jagger

Maintaining the location of voices

By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.


A center channel speaker is not there to improve the clarity of movie dialogue. It is used to keep voices located at the TV screen for listeners seated off to one side. Without the center speaker, they would hear voices coming from the main speaker closest to them.


Dead_parrot_dialog

Coverage angle

Thus, the most important aspects in choosing a center speaker is that it produces a uniform tone balance and clarity when you sit anywhere from left-to-right. It will not have to produce much low bass or high treble, but mostly voices.

However, most every center-channel speaker has been designed to first satisfy marketing's need for a slender, horizontal package. In this package, one often sees two woofer/midrange drivers with a tweeter in between. You will hear that this layout of drivers does not offer proper tone balance and clarity at either end of a sofa, let alone to the sides of a room.

For the listener to one side, the tone balance will be muffled because one of those woofer/midranges is much further away than the other woofer/midrange. The time delay between the two will cancel out the upper voice range tones. Voices, especially female voices, will become very difficult to understand since so little diction will be heard. Although magazine measurements always show this, we think they should be more vocal about this problem.

Aperture_HD

Therefore, always look for a center speaker where its tweeter is directly above one woofer/midrange. This layout will maintain the same distance to your ears even when you move to the side. This is how our Aperture model is designed.


Sonic performance

There is nothing 'magic' about a center-speaker's design. It is still a speaker that should sound good on all music and voices. You should be paying for clarity and a natural tone balance across that wide coverage angle. Its distortion must be very low, so complex soundtracks are never harsh (which especially helps when playing older DVDs).

Aperture_HD_2

On a budget, good results are obtained with just a small bookshelf speaker placed vertically below the TV. Which is, by the way, what upsets marketing, because they believe no consumer will place a regular vertical speaker down there, even if it is best for the sound.


Placement issues

A center channel speaker spends its life on a shelf or built into a wall. Much of its sound will thus be immediately reflected from all the surfaces located very close to it. This hurts clarity, since you are not hearing clean, direct sound. Put the speaker on an open shelf, with space all around it.

We developed a way to sharply limit our Aperture center speaker's output directly to the sides and vertically, so that it talks much less to the surfaces near it. We do not see this addressed in other center speakers. Should the center speaker be shielded, magnetically? Not for any flat screen or rear-projection set, but only for placement close to a standard TV set that still uses a CRT tube for its picture. On those, a speaker's magnetic field distorts the picture and color, perhaps leading to permanent damage. A 'normal' speaker can be placed no closer than about 10" (25cm) to a regular television. To check for picture distortion, first examine the edge of the picture for any off-color tint. Our center speaker is shielded.

Installed


Setting your control center

Follow all the directions in your home-theater control center's Owner's Guide, paying attention to setting the center-speaker's time delay. The center speaker is usually closer to you, by tape measure, than the left and right speakers. One adds time delay to the center signal, 'moving the center speaker back' until it is the same 'acoustic distance' away.

The time delay is measured in milliseconds (msec, or thousandths of a second), and 1 msec is needed for every 13.5" (32cm) of distance. For example, if the center speaker is 24" (60cm) closer to you than the left and right speakers, then set the time delay to 2 milliseconds. Many have found an additional 1-3 msec of time delay sounds better, as it moves voices back enough to curve the soundstage nicely, with voices also less 'in one's face.'


Blending

While the control center will give you some test noise to check for equal loudness, further fine-tune the center speaker's loudness on music videos and major motion pictures. Allow at least 100 hours of playing time for that center speaker to fully break-in.


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