Vertical vs Horizontal Center Speaker Designs

The center channel’s job is a tough one. The consensus is that around 75 percent of a movie’s content is routed to the center channel loudspeaker. Yet, the design criteria for center channels traditionally require that it fit as stealthily as possible around that big-box television, or that huge sheet of projection screen. The sound can’t go through your glass TV screen and projection screens are usually not acoustically transparent. Ideally, the sound should come from behind the image, through the screen as it does in the movie theaters. But while there are new options with acoustically transparent projection screens, this article will focus on the more traditional problem of what compromises result from the different approaches to center channel design.

With their height constrained, center channels still need to reproduce all that mostly-vocal content with dynamics and clarity, down to 80 Hz or lower until the subwoofer takes over the heavy lifting. Most center channels try to be five to eight inches tall, which results in their midrange drivers being restricted to four, five, or six inches in diameter. To get four to six inch drivers to dynamically reproduce 80 Hz starts getting very expensive, if not practically impossible for minimizing distortion and achieving the targeted maximum sound pressure level. The common approach is to then double up the midrange drivers, splitting the workload into two. This results in a speaker that’s wide, not very tall, and as we will see, often compromised with respect to redundant driver wave interference. Throughout this article we’ll refer to these designs as they are horizontally designed by their driver types. The most common design is a midrange-tweeter-midrange or MTM as we’ll call it.

If anyone likes you enough to watch a movie with you, the center channel must reproduce all that content smoothly and predictably across all your seats. If you’re sitting perfectly in front of the center channel, having multiple drivers of the same type in a horizontal configuration can do the job just fine. But if you move slightly off-axis, or as any of the other seats will realize, having horizontally-aligned redundant drivers will cause some frequencies to be canceled and some to be reinforced. This phenomenon is called wave interference and you can read more about a double-slit experiment with light (or any other physical media that behaves in waves) here. The subtracting and adding of various frequencies at various angles can result in audible shifting in the speaker’s sound across the room. Not only does the off-axis frequency response suffer, but timing and phase response follow. Off-axis, MTM speakers can often sound hollow but the comb filtering, or lobing effect, can also shift the imaging away from the middle as a “phasy” sound. There’s a good reason why one-piece surround speakers use a lot of identical drivers (up to 40 - wow). The wave interference in those cases is used as a tool of good, not evil. To compensate for the lack of intelligibility (of the audio, not the script), people typically turn their volumes up which then can then result in some domestic tension among spouses, children and neighbors.

Floor speakers with multiple vertical redundant drivers will also have wave interference, but vertical variation in frequency response is much less of a problem than horizontal variation. In fact, the more identical drivers a loudspeaker has, the more it behaves like a line source instead of a point source. Line sources radiate in a more cylindrical pattern, which is advantageous if it is vertically oriented, as line sources interact less with the floor and ceiling. But a cylindrical radiation pattern is a disadvantage if you arrange the redundant drivers horizontally. The speaker will then interact more with the floor and ceiling, and suffer poorer response horizontally across the room.

We’re going to look at several design options, and zero in on what effects result from having multiple redundant drivers aligned horizontally. We’ll compare MTM designs with their bookshelf brethrens, see how designers can reduce the wave interference to a very minor effect, as well as explore some less common ideas to drive home the point and provide emotional drama. Improving your center channel performance can dramatically improve your overall sound, and as we’ll see can be done better with less money.

Test Setup and Methodology

To focus on the off-axis frequency variation in different speaker designs, I needed to take many measurements across different angles, map and analyze their results. In these tests I rotate the speaker instead of moving the microphone, as I don’t want to measure the acoustical differences across the room, another huge source of frequency variation as you move through the peaks and nulls of room modes and reflections. The walls have their first reflection points treated with 4 inch thick absorption and are at least eight feet away from the tripod where the speakers will rest. The Infinity IRS Epsilon I use as a center channel is about two feet behind the speaker. The red dot in some pictures is from the laser pointer to assure I have the calibrated Behringer ECM8000 microphone perfectly aimed, which is placed 11 feet from the speaker.

I played pink noise through each speaker and mapped its frequency response every five degrees from zero to 40 degrees off axis with 1/24 octave resolution. I’m assuming that the response is symmetrical, so I only measured one side for the analysis. The results below 80 Hz, a common and typically good crossover frequency to your subwoofer, and above 20 kHz were discarded. I’ll show the frequency maps of the speakers across their entire 80 to 20k bandwidth, but will focus on the frequency range of redundant drivers and measure their frequency variation as we vary the angle. All frequency responses were normalized for their zero-degree measurement. This article doesn’t care about and won’t show the absolute frequency response; you’ll select the sound quality of the speaker based on your budget and personal preferences. Instead, I just wanted to know what the change is from The Captain’s Chair to the other typically unlucky listeners. None of the speakers will have their grills attached, both to maintain some degree of anonymity but to also keep the photos and measurements as clear as possible.