Horizontal Center Channel Speakers and Why They Should Be Avoided

[Big Daddy]

CENTER CHANNEL SPEAKERS
Prepared by Big Daddy

The center channel speaker is perhaps the most important speaker in a home theater setup. Close to 75% of the soundtrack is directed to the center channel. Ideally, the sound should come from behind the screen like in a movie theater. Unfortunately, the design criteria for a center channel speaker requires that it fits under or above the TV or projector screen.

The primary job of a center channel speaker is to reproduce the midrange frequencies of the human voice in a multi-channel encoded movie. Chances are the center speaker isn't able to produce enough midrange or the midrange has some dispersion problems. One of the most common complaints about center channel speakers is that dialogue gets drowned out when the action or musical score gets loud.

A lot of these problems are associated with the design of the center channel speaker. With their height constrained, center channel speakers must be able to produce sound down to at least 80Hz where the subwoofer takes over. To design a speaker that is only a few inches tall to go down to 80Hz or lower without distortion and good sound level requires careful design and expense. The most common approach is to use two midrange drivers to divide the bass and midrange frequencies and a single tweeter for the upper frequencies. Almost all center-channel speakers have an MTM (Midrange, Tweeter, Midrange) orientation or sometimes called D'Appolito array.

PROBLEMS WITH HORIZONTALLY ORIENTED CENTER CHANNEL SPEAKERS

MTMs were designed to be vertically oriented because they have very good horizontal dispersion and less than ideal vertical dispersion.

When a center channel speaker that uses MTM orientation is laid on its side, you will get good vertical dispersion and a very poor and narrow horizontal dispersion. Although both should be wide, horizontal dispersion perhaps is more significant than vertical dispersion. Off-axis horizontal response is the sound that the listener hears while moving around the speaker.

Worst Design
Source: Seymour AV

Remember, if a speaker is operated horizontally, the vertical dispersion becomes the horizontal dispersion and vice versa.

If you sit exactly in the middle in front of the speaker, you will probably hear good sound. If you sit slightly off-axis (as little as 30° off the primary listening position), you will realize that some of the frequencies are canceled (mostly in the midrange frequencies) and some are reinforced.

In addition, you may experience that imaging has shifted away from the middle and that the speaker sounds hollow and this completely defeats the purpose of a center speaker.

WAVE INTERFERENCE (COMB FILTERING AND LOBING)

Sound systems are broad band and contain many different frequencies. When two sound waves are combined together, they will form a resultant wave that we hear. At a given point of observation, the combination of the two waves will result in some in-phase (synchronous) frequencies that are added constructively and cancellation of some out-of-phase (asynchronous) frequencies. Between these two extremes there are many intermediate cases. This can produce uneven sound coverage over an audience area.

This phenomenon is the result of wave interference. The adding and subtracting of various frequencies at various angles cause lobing and comb filtering. When similar independent waves are combined, the result can be either constructive or destructive interference, depending on whether the waves are in phase or out of phase. This interference occurs when the waves have the same or nearly the same frequency. Constructive interference will enhance sound. Destructive interference will weaken sound. If two identical waves are 180 degrees out of phase, they will cancel out. Whether the interference is constructive or destructive, the individual waves continue to exist separately. The interference itself is merely the effect of the waves together at one point in space.

Source: Headwize.com


Two waves in phase and two waves 180° out of phase.
Source: Wikipedia

The animation in the following shows two waves traveling in the same direction. The phase difference between the two waves varies with time so that we see constructive interference when maximum points are aligned (peak) and destructive interference when minimum and maximum points are aligned (null). This illustrates the problem with horizontal center channel speakers and also why it is important to adjust the subwoofer’s phase control with respect to the main speakers so that we obtain maximum output.

Animation courtesy of Dr. Dan Russell, Kettering University

Interference of two circular waves can also be seen in the following diagram. The frequencies decrease from top to bottom and the center of the two waves (the distance between drivers) increases from the left to the right). As time progresses, the wave fronts will move outwards from, but the dark regions (destructive interference) stay fixed.

Source: Wikipedia

In general, there are four problems with MTM speakers:

Beaming: A narrowing of dispersion at higher frequencies. This is the problem of all speakers.

Lobing: A series of severe fluctuations in level as you move in front of the speaker. For most common center speakers, lobing begins around 10 degrees and by 25 degrees is quite strong. Lobing error, or the dispersion of treble sounds from a different axis than midrange or bass frequencies, create a sound imbalance that varies from side to side. Equalization cannot help correct this problem.

Single Source, Single Frequency



Two Sources, Single Frequency



Two Sources, Single Frequency (Larger Picture)
Source: Live Sound International

The following animations demonstrate lobing in two and three dimensional space.

2 Dimensional and 3 Dimensional Lobing
Source: Integracoustics.com

Comb Filtering: This is caused across your listening positions by having the same frequencies reproduced by multiple drivers. When you are dealing with a horizontally placed MTM speaker, an increase in sound output is obtained on the main axis of the array, while at some points off the main axis the driver array creates a cancellation at varying wavelengths (frequencies). This phenomenon is known as comb filtering which leads to another phenomenon of loudspeaker arrays called lobing.

Demonstration of Comb Filtering
Source: Live Sound International

In the diagram above, the frequency response is not represented by a straight horizontal line, but by a series of peaks and nulls. This results in tonal coloration of the sound.

Poor Imaging: The human auditory system uses arrival time differences of sound between the two ears to determine where it is coming from. When there is no interference, the sound will arrive to both ears at the same time and level, so the brain images the sound source to be directly in the middle in front of you. With phase interference and comb filtering, poor imaging will result.

Low frequency and high frequency sound waves have different radiation patterns. As a result low frequency drivers (midrange woofers) and high frequency drivers (tweeters) have intrinsically different radiation patterns. At the crossover frequency point, the directivity of the two drivers changes. Interference between the two drivers at this point causes a dip in the level around the crossover frequency, with a peak above it. This will result in muddiness of the sound and movement of sound stage.

In the following diagram positive and negative wave forms are represented by red and green respectively. The dark areas are places where there is no displacement. The similar waves from two sources interfere with one another. When the two waves are in phase, they add to make stronger waves, shown by the alternate red and green blobs. When the waves are out of phase, they will cancel each other, shown by the dark lines .

Source: Brantacan


POSITIVE APPLICATIONS OF PHASE INTERFERENCE

Phase interference can be used to do useful things like reduce the off-axis sensitivity of a microphone (directional microphones), noise cancelling headphones, or design speakers with multiple driver arrays.

In a similar way, phase interference can be used to increase the sound level from the main speakers in an auditorium to the audience and reduce the spill of sound onto the stage.

The wave interference can be used as a tool to improve the sound and imaging of a speaker by including multiple drivers in a line array. That is the reason why Yamaha’s highly rated sound-bar speaker has 40 drivers.

LOW FREQUENCY AND HIGH FREQUENCY RADIATION PATTERNS

The radiation pattern of all speakers varies according to the frequency of the sound they produce. Lower bass frequencies have much wider off-axis response and higher frequencies have a much narrower radiation pattern. Speakers with wider dispersion will most likely sound better to the listener in the primary position and to listeners who are sitting off-axis on the sides. Also, a wide dispersion keeps sound more even when you are sitting or standing.

This can be observed by standing behind a speaker. You will notice that there is a significant drop in higher frequencies, but no change in the bass response. The higher frequencies only radiate in front of the speaker and bass frequencies tend to be non-directional.

The following diagram demostrates that high frequency sound waves have a narrower off-axis response and low frequency sound waves have a much wider off-axis response.

Source: Integracoustics

An ideal speaker system will have even dispersion at all frequencies. However, conventional speakers are unable to distribute sound equally because of horizontal dispersion patterns in addition to side wall and floor/ceiling reflections.

SOLUTION

If the MTM center speaker is not optimized for horizontal orientation, it likely will exhibit lobing and off-axis combing filtering effects. Vertical floor speakers with multiple drivers will also have wave interference, but vertical variation in frequency response is much less of a problem than horizontal variation. Vertically oriented speaker interact less with the floor and the ceiling. Horizontally oriented speakers will interact more with the floor and ceiling, and suffer from a poorer off-axis response.


Different Center Channel Designs:

Advantages and disadvantages of each design is disccussed in the following article: Pros & Cons of Various Center Channel Designs.

1. Two-way speakers make excellent center-channel choices, often at much lower cost. The optimal center-channel speaker choice will usually be a good-quality, vertically deployed 6.5" two-way speaker.

2. Place the horizontal center channel speaker vertically.

Bad



Better

3. Placing the tweeter above the horizontal line of the midrange drivers such that an approximate 45 degree angle is formed will lessen horizontal lobing.

4. Gently curving a speaker can help in covering a wider area. Radically curving the driver arrays may introduce problems.

5. A lower crossover frequency point, higher order crossover network, or other designs that have better off-axis performance can minimize the effect of wave interference.

CONCLUSIONS

Let us summarizes some of the the problems that may occur with standard MTM horizontal center channel speakers.

• Increase in sound at certain frequencies and cancellation at other frequencies.
• Poor off-axis response and narrowing of sweet spot.
• Poorer vertical response as you stand up or sit on a higher chair.
• Horizontal lobing with hot and cold spots.
• Coloration of the tone.
• Poor comprehension and intelligibility of speech.
• Lack of music clarity.
• Poor imaging and its shift from the middle.

You do not have to limit yourself to a traditional horizontal MTM center speaker. You have several options:

• Turn your center speaker vertically for improved frequency response performance across your home theater.
• Use a two-way speaker for the center channel.
• Buy a center channel speaker that has drivers that are not on the same horizontal axis.
• Buy a slightly curved center channel speaker.
• The best and final solution is to use three identical vertical speakers in the front.

Best Option
Source: Seymour AV

To get the best performance out of the center channel, use a speaker that is identical to the two main speakers. If that is not possible, look for bookshelf speakers that have identical drivers as the mains, center speakers that have a vertical arrangement of their tweeters and midrange drivers, a lower crossover frequency points, higher order crossover networks, or other designs that have better off-axis performance and can minimize the effect of wave interference.

ADDITIONAL INFORMATION AND REFERENCES

Interference (wave propagation) - Wikipedia, the free encyclopedia
http://theaudiocritic.com/plog/index...Id=28&blogId=1
Interference
Seymour AV | Center Stage screens
Comcast
Pros & Cons of Various Center Channel Designs
Vertical vs Horizontal Center Speaker Designs — Reviews and News from Audioholics
http://www.audioholics.com/education...hannel-speaker
http://www.audioholics.com/education...hannel-designs
Live Sound International | Tech Topic: Practical Realities of Phase Interference
Can Line Arrays Form Cylindrical Waves? A Line Array Theory Q & A
http://www.mccormicksnet.com/techgui...usic_sound.pdf
http://www.shure.com/stellent/groups...c_sound_ea.pdf
Sound & Communications - Audio
Planar theory
Acoustics and Vibration Animations
Interference (wave propagation) - Wikipedia, the free encyclopedia
HeadWize - Technical Paper: The Elements of Musical Perception by HeadWize
Mono Compatibility and Phase Cancellation - Tutorials
Speaker Articles (Alphabetical Sort) - eCoustics.com
Loudspeakers, Loudspeaker Technology

[Big Daddy]

Chris Seymour of Audioholics performed a series of tests on common M-T-M speakers. He placed the speakers horizontally and vertically and measurements of their on-axis and off-axis frequency performance. He reached the conclusion that almost all of them perform better in vertical orientation. Even a relatively inexpensive 2-way bookshelf speaker had better off-axis results than most center channel speakers. Here is a short summary.

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. 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.


$250 MTM Horizontally Oriented Measurements

The first speaker we’ll look at is a very common MTM design, available from a big box retailer where you can reportedly get some best buys.

The horn-loaded tweeter crosses over to the dual 5.25 inch midrange drivers at 2400 Hz. This means that while we will see the performance of the tweeter’s off-axis response, we will focus our analysis in the bandwidth of 80 to 2400 Hz.

What we can see from the chart is that the two midrange drivers exhibit significant wave cancellation from around 800 Hz to where the crossover kicks in at 2400 Hz. Some of the variation near the crossover point is from all three drivers playing the same frequencies. A higher-order crossover would reduce this problem, but just placing the tweeter above the midrange would fix it completely as we’ll see later on.

If we take a slice of one frequency from the 1/24 octave measurements we can plot out the frequency response of the loudspeaker and get a visual representation of lobing. The lobing effect is from the appearance on a polar map of the peaks and valleys of the frequency response. In a real polar graph, it looks like lobes, or flower petals.

$250 MTM Vertically Oriented Measurements

What would happen if we take that MTM speaker and turned it on its side?

The horn tweeter isn’t as happy, exhibiting worse off-axis response from 4 k to 10 kHz, but you can see the frequency response from the midrange drivers shows much less wave interference.

If we look at the same slice at 1244 Hz to compare its lobing to the horizontal orientation, you can see from the below chart that for the specific frequency it was successfully eliminated.

This company’s horn tweeter should be used in the orientation as it was designed, but if you were keen on using their speakers, how would you avoid the wave interference that the horizontally aligned MTM center channel had? One solution is to completely avoid their center channel and use a bookshelf speaker


$600 MTM Horizontally Oriented Measurements

This is a higher-end M-T-M speaker.

You can see that the interference between the two midrange drivers is audible. In the upper midrange we can see over -6 dB of cancellation and almost 4 dB of wave reinforcement. You can also see the tweeter has audible off-axis attenuation, but it’s important to emphasize that these tests only reflect variation from the on-axis response and how well we can avoid wave interference.

$600 MTM Vertically Oriented Measurements

In order to easily test if the drivers or cabinet are primarily to blame for the off-axis response, or if the MTM configuration itself is the limiting aspect, we then rotated the speaker vertically and ran it through another set of measurements

you can see that the wave interference from the two midrange drivers was virtually eliminated. The off-axis response is smooth and consistent up to 40 degrees off axis. The lower treble from the tweeter has even improved, likely due to it now having a narrow horizontal baffle and reduced cabinet diffraction. In this orientation, this speaker would do a terrific job at maintaining intelligibility and clarity across your room. This

$115 Bookshelf Speaker Measurements

A bookshelf speaker from the same horn speaker company was used. The price was $230 a pair or $115 per speaker.

Below is the chart showing the frequency response deviation from on-axis to 40 degrees off-axis. The chart shows the same off-axis roughness of the tweeter, but there may be other reasons why you would otherwise like the speaker’s sound. Importantly to this endeavor, the midrange doesn’t show any significant change in frequency response at different angles.

The 1/6 octave chart below shows that the bookshelf speaker performs very well off-axis by only having one midrange driver.

[bluseminole]

Another excellent and informative guide! Keep 'em coming! We all appreciate the time and effort you put into educating those of us who are somewhat less enlightened.

[Big Daddy]

Originally Posted by bluseminole

Another excellent and informative guide! Keep 'em coming! We all appreciate the time and effort you put into educating those of us who are somewhat less enlightened.

THANKS. WILL YOU PAY FOR MY LEGAL FEES WHEN MY WIFE FILES FOR DIVORCE?

[got rice]

BigDaddy, I can't thank you enough for taking the time out to give us this inciteful information. I've read all of your threads and I have to say they are very well written, and thought out. The way you break things down and disect the topic makes it easy for reading.
Well done. Keep it up!!!

P.S. Thank your wife for us as well.

[crazyBLUE]

Thank's for the PM Big Daddy ! Even thought it was 2:00 am when I read this The second time I read, it made more sense !
I decided to try the vertical placement for the center ! I can't stand the center on it's side w/stand I have so I put A tower there ! If this sounds better in my environment , I may have to go buy 1 more BOSTON tower for my center !!




This will be A 3 day weekend for me W/A bunch of 5.1 movie watching !

THANK'S Big Daddy !!

[aramis109]

Hah, I'm so breaking this rule. I currently have a Monitor 40 bookshelf on its side as a front until my new front comes in. Talk about a reduced sweet spot! Aesthetically it being on the side is something I have to do. The Def Tech CLR 2002 is meant to go either way, so I'll be curious to mess with it but in the end it'll be on its side.

There's really no reason that speaker companies can't make the center channel with a wider horizontal dispersion when laid sideways- curving the cabinet, or turning the speakers out a bit.

[drummerboy_2002]

Thanks for the info. I was unaware of the problems with arrays, but it makes sense. I do have some questions though:

1. If playing the same frequencies (let's say your source is mono), would your fronts exhibit the same problem (a horizontal array playing the same note)?

2. You say raising the tweeter above the horizontal plain would help. I don't get this one. The crossover distributes the highs to the tweets and the mids/lows to the mids (yes, there's some overlap). I don't see how it makes any difference where the tweeter is, as long as it's at ear level. The problem should remain entirely with the two mids.

Also, wouldn't one solution be using a center with a 2.5 way crossover? IE, sending the highs to the tweets, the mids to only one mid, and the lows to both mids. That way the mids only have one source, so no reinforcement/cancelation. The lows should display fewer issues so using both mids shouldn't be as big a deal.

Let me know if any of this makes sense.

[drummerboy_2002]

After thinking about for a bit I think I understand the problem a bit more. While same frequencies played in an array can cancel or reinforce (even double the magnitude) each other at certain angles, multiples of those frequencies can cancel or reinforce individual waves of its multiple. IE, a 100Hz signal could reduce or reinforce every other wave in a 200Hz signal. Thinking a bit more, this situation can happen not just on the array plane, but every plane not perpendicular to the array. That's it, I'm done. My head hurts.

[Riff Magnum]

I thought that most of the newer horizontal center channel speakers have a crossover that allows one of the woofers to operate during lower frequencies, and the other to provide the midrange. This keeps the two main woofers from ever pushing the same frequencies at the same time, thus reducing lobing and cancellation.
With my DLP's tricky vertical viewing angles i don't think i'd ever be able to mount my tv high enough to allow a vertical floorstanding center channel to sit underneath.

[Corpboy]

I think I only understood maybe 10% of what all that said.

Let me ask this: if a horizontal center speaker is so bad, why oh why do I see so many of them specifically built to be oriented horizontally? It seem so self-defeating in nature.

[tlmaclennan]

From what I can tell, most center speakers overcome the problems with option #3:

3. Placing the tweeter above the horizontal line of the midrange drivers such that an approximate 45 degree angle is formed will lessen horizontal lobing.

Though that's not to say all center speakers are designed this way. I'm just curious if it is actually recommended to go out and buy three of the same floorstanding speakers to be used as Front Left, Right, and Center speakers. I'm sure that this woouldn't be neccesary for the more high-end speaker lines, as their centers are probably designed well and matched to the fronts. But if I were to go with a low-end Polk line, say the Monitor 50s, would it be beneficial just to buy 7 of the towers to complete a 7.1 setup?

[Drew664]

Originally Posted by Corpboy

I think I only understood maybe 10% of what all that said.

Let me ask this: if a horizontal center speaker is so bad, why oh why do I see so many of them specifically built to be oriented horizontally? It seem so self-defeating in nature.

I know what you mean!

I would have to think the horizontal nature of these speakers are simply due to the placement - directly under your viewing source. A "vertically challenged" speaker works best for most viewing set ups.

Also, Big Daddy - great info. I had no idea. How audible is the difference though?

[drummerboy_2002]

Ok, thought some more (ouch). While a verticle speaker with well defined crossovers (all different ranges) would produce the fewest issues, it might drop the imaging too far below the screen. I think for most of use, an MTM center with a 2.5 way crossover will do it for most of us.

Edit: Forget the crap earlier about frequency multiples. The speakers only play one frequency at any given time, so only the tweet, or mids would be playing, not both. Yep, I still think the 2.5 way is the sensible way to go.

[statikcat]

Nice post but.. I don't think most people have room below the TV for a verticle center.. due to the TV stand, etc. What are we supposed to do?

[jomari]

Originally Posted by drummerboy_2002

After thinking about for a bit I think I understand the problem a bit more. While same frequencies played in an array can cancel or reinforce (even double the magnitude) each other at certain angles, multiples of those frequencies can cancel or reinforce individual waves of its multiple. IE, a 100Hz signal could reduce or reinforce every other wave in a 200Hz signal. Thinking a bit more, this situation can happen not just on the array plane, but every plane not perpendicular to the array. That's it, I'm done. My head hurts.

as mentioned in the post, its more evident in the horizontal plane and less on a vertical one. cancellation normally occurred when you have both midranges operating at the same frequency, giving you a higher chance of cancellation instead of reinforcement of the said frequency.

Originally Posted by Riff Magnum

I thought that most of the newer horizontal center channel speakers have a crossover that allows one of the woofers to operate during lower frequencies, and the other to provide the midrange. This keeps the two main woofers from ever pushing the same frequencies at the same time, thus reducing lobing and cancellation.
With my DLP's tricky vertical viewing angles i don't think i'd ever be able to mount my tv high enough to allow a vertical floorstanding center channel to sit underneath.

with the newer horizontal center channels, most companies have 'addressed' this concern, and have developed means to avoid lobing issues found in center channels. are they accurate to a certain aspect? sure. but physics is physics - we have to learn to distinguish which is truly done to dissipate the said lobing issue. some companies have addressed it in a way where we have one of the midranges to work certain frequencies, while the other would do the opposite, avoiding them to use both and cancel each other out.

Originally Posted by Corpboy

I think I only understood maybe 10% of what all that said.

Let me ask this: if a horizontal center speaker is so bad, why oh why do I see so many of them specifically built to be oriented horizontally? It seem so self-defeating in nature.

most of the center channel speakers were built back in the 90s in a more 'commercial aspect' instead of the technical course we are used to. they marketed these speakers to address a slowly growing trend back then, and have it in a convenient layout that can be placed both above or below the television/display unit. i wouldnt quickly call it self defeating in nature. there are only a few out there that would admit to purchase a vertical center vs a horizontally speaker. in essence ita aesthetics vs function.

[aramis109]

Originally Posted by statikcat

Nice post but.. I don't think most people have room below the TV for a verticle center.. due to the TV stand, etc. What are we supposed to do?

1. Put your horizontal center at approximately ear level while seated.
2. Point towards center.
3. Sit in sweet spot.
4. Enjoy movie.

While I have noticed some loss in off-axis previous to this post, its just not enough of an issue (I rarely sit there) for me to make a change. My family has never complained about it. I'm the only freak about audio in the house.

[jomari]

Originally Posted by aramis109

1. Put your horizontal center at approximately ear level while seated.
2. Point towards center.
3. Sit in sweet spot.
4. Enjoy movie.

While I have noticed some loss in off-axis previous to this post, its just not enough of an issue (I rarely sit there) for me to make a change. My family has never complained about it. I'm the only freak about audio in the house.

as mentioned by aramis, but let me take step 2 er... another step further.

in order to have a better soundstage created in your fronts, IDEALLY you would like to have the drivers aligned altogether in the same plane to, giving you a better panning experience, and less localization present.

imagine watching a jet scream from your left speaker, then suddenly go higher up (due to your center speaker placed on top of your telly), then swoop again in the right speaker.

again, its an ideology, rarely available for everyone.

[Go Blue]

What if your center is designed like this?http://www.infinitysystems.com/home/...USA&Country=US

I would think with this type of design, it would not be a good idea to place vertical, correct?

[prerich]

Originally Posted by crazyBLUE

Thank's for the PM Big Daddy ! Even thought it was 2:00 am when I read this The second time I read, it made more sense !
I decided to try the vertical placement for the center ! I can't stand the center on it's side w/stand I have so I put A tower there ! If this sounds better in my environment , I may have to go buy 1 more BOSTON tower for my center !!



This will be A 3 day weekend for me W/A bunch of 5.1 movie watching !

THANK'S Big Daddy !!

Circuit City just had one of your Bostons for sale - one only - for 179!!!! I didn't know you where in the market! You may have missed this deal !!!

Big Daddy - you know I have preached vertical center channels for the longest - thank you for putting into words my reasoning behind it !!!!