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Vertical vs Horizontal Center Speaker Designs

by Chris Seymour July 20, 2007
Horizontal vs vertical speakers

Horizontal vs vertical speakers

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.

horizontal-MTM.jpgWith 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.

double-slit-diffraction.pngIf 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.

 

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Recent Forum Posts:

TLS Guy posts on October 20, 2009 23:33
irish, post: 637493
Thanks for the recommendations. The Beta 360 would be too large for my application although it does look nice. The KEF Q series, iQ60c, looks like it might work pretty well as it's less than 7“ tall. The speaker cabinet design is a bit different but that isn't a breaking point. How do co-axials differ from in sound or performance from a more ”traditional" design where the speakers are seperate?

If more speakers had a wider bandwidth, then there would be no crossovers and multiple speakers. Multiple speakers are a workaround for a problem, not an inherent advantage.

The point of a coaxial speaker is to keep the sound coherent. What would be ideal is to have a bass/mid cone that crossed over to the tweeter, 4 kHz, then you would avoid a crossover in the speech discrimination band. However no such animal exists at present and crossover to the tweeter in current units is in the neighborhood of 3 kHz.

In a coaxial, the cone of the woofer acts as a wave guide to the tweeter. Things a re designed such that there is usually time coherence. However because a first order crossover is just about never possible, there are phase anomalies at crossover, just like any other speaker. There is symmetrical lobing and therefore the vertical and horizontal axis response is identical. The coverage is therefore conical.

As far as drivers to choose from the most well known are KEF and Tannoy. Thiel also has a coaxial center. Pioneer also have one in their range.

However, after having auditioned KEF recently the SEAS driver is in my view far superior.

You can buy a LOKI kit that is very good value.

I use these drivers in my center speaker. The tweeter is used only in the lower driver, the upper one is an active fill driver and the tweeter not connected.



In this TL, I could not be more happy with it.
irish posts on October 20, 2009 16:54
lsiberian, post: 637476
For If you are interested in a coaxial accessories4less sells KEF speakers for a pretty cheap clip. Still you'd have to like their other offering. I think the best horizontal center I've heard in the budget range is the Beta 360 treated with rockwool and peel-n-seal

Thanks for the recommendations. The Beta 360 would be too large for my application although it does look nice. The KEF Q series, iQ60c, looks like it might work pretty well as it's less than 7“ tall. The speaker cabinet design is a bit different but that isn't a breaking point. How do co-axials differ from in sound or performance from a more ”traditional" design where the speakers are seperate?
lsiberian posts on October 20, 2009 16:12
irish, post: 637446
Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm

For If you are interested in a coaxial accessories4less sells KEF speakers for a pretty cheap clip. Still you'd have to like their other offering. I think the best horizontal center I've heard in the budget range is the Beta 360 treated with rockwool and peel-n-seal
TLS Guy posts on October 20, 2009 15:30
irish, post: 637446
Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm

The paradigm C190 is on the right lines, but I think you would have to go with an all Paradigm system, as they have a definite voicing about them, that I did not care for when I auditioned them, at least the Studio 100s
irish posts on October 20, 2009 15:13
TLS Guy, post: 637438
Yes they would. To make a good horizontal center, you need either a coaxial driver, or a three way with at least the tweeter above the mid, and preferably the mid band/pass crossover point spread 350 Hz to at least 4 kHz, like the B & W.

Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm
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