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Position of the Center Channel Relative to the Main Speakers

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Right Speaker DiagramWhen choosing a center channel for your home theater, it’s important not to apply absolutes as they will only reduce your viable options.  Most people not using front projection systems or utilizing transparent screens deal with height restrictions and thus require a speaker that has a narrow vertical height.  In this case an MTM is the obvious choice. Ideally one would be advised to use identical vertically mounted speakers for the front three channels. But, even in this “ideal” setup, you still will NOT get a perfectly timbre-matched blend of the front three speakers at the sweet spot (see Figure 2), much less any other listening position. The reason is you will be directly on-axis to the center channel but not the front main speakers. Take our first example where we are sitting on-axis with our center channel 12 feet away.  This puts either front left and right speaker about 23 degrees off-axis to us assuming they are 10 feet apart and equidistant from the center channel.  Using a liberal amount of toe-in on the front channels can help this.  Using excessive toe-in may adversely affect the other seats while also limiting the width of the sweet spot.  This doesn’t preclude the fact that it’s generally sound advice to match the front three speakers, but even so understand that perfect blending is basically impossible to achieve in a real world environment. 

Editorial Note about the Origins of the Center Channel by Paul Apollonio
Back on April 27th 1933, Bell Telephone Laboratories conducted a historic experiment whereby the sound of the Philadelphia Symphony Orchestra was sent by radio and reproduced by loudspeakers hidden behind a curtain for an audience in Constitution Hall in Washington DC.  A series of scientific and technical papers were written in 1933 based on the research and development done just prior to and subsequent to this milestone experiment; and they were presented at the American Institute of Electrical Engineers convention in January 1934 in NY, NY.  These papers were presented by engineers and scientists, all working for or with Bell Telephone Laboratories who were members of either the Acoustical society of America or the AIEE.  The fundamentals of audio reproduction are well covered and discussed in this series of papers. These technical papers were compiled by Paul W. Klipsch in 1964, and made available to serious audiophiles as the reprinted "Symposium of Auditory Perspective". 

In the second of the reprinted papers, titled "Auditory Perspective - Physical factors" written by JS Steinberg and WB Snow, the forward reads "In considering the physical factors affecting it, auditory perspective is defined in this paper as being reproduction which preserves the spatial relationships of the original sounds.  Ideally, this would require an infinite number of separate microphone-to-speaker channels; practically, it is shown (in their paper) that good auditory quality can be obtained with only 2 or 3 channels." 

It is noteworthy that the system employed in Washington DC used not two, but three loudspeakers, with a center channel speaker being employed specifically to help the listeners perceive a sense of depth in the performance.  The broadcasted performance in Philadelphia was captured by three microphones, one left, one right and one center.

I mention this as it seems a good number of readers are of the opinion that the need for a center channel speaker was an idea born with the advent of Dolby 5.1.  It actually preceded it by many years.  The sense of depth was shown in the Auditory Perspective paper as requiring that third center channel, and the idea was kept alive from the 40's through the 1990's by Paul W Klipsch who often touted two Klipschorns and a Center Channel LaScala as the ultimate 3-channel High Fidelity Speaker system for the home.  That good sound requires a center channel speaker is not a new idea.  When measured by the length of our lifespans, it is a very old one. 

The Proof is in the Pudding

As an electrical engineer, I’ve been trained with the mantra that the best way to illustrate a point is through proof and measurements.   As such I grabbed an RBH Sound 661-SE/R MTM speaker and my beloved Status Acoustics Decimo two-way bookshelf speakers to pull some measurements.  The Decimo is not only the best sounding bookshelf speaker I have heard to date, but it also measures phenomenally well on and off axis.  Both speakers utilize the identical Scan Speak 9500 tweeter, and the same 6 ½” aluminum phase plug driver.  The 661-SE/R however utilizes two mid bass drivers, in an MTM orientation.  

Center Speakers  Theater Chairs

Left Pic RBH 661-SE/R & Decimo; Right Pic Mic in seated area

I positioned each speaker on a 30” stand and pulled measurements at the listening position of my front row 12 feet away which represents 0 degrees or on-axis, 3 feet shifted to the left or 14 degrees off-axis and 6 feet shifted to the left or 27 degrees off-axis.  Note the 6 foot measurement placed the microphone only 20 inches from my left sidewall.  I never position a seat this close to a sidewall, nor do I recommend it for anyone, not even the mother in-law. The microphone was placed at ear height for all of these measurements.  I pulled these 3 measurements for the MTM vertically and horizontally mounted and the two-way bookshelf vertically mounted.  I didn’t have a W(T/M)W on hand so the two-way was the closest I could come and in fact could arguably produce more ideal off-axis measurements.

Measurements 0 Degrees

1/4th Octave Frequency Response 0 Degrees Off-Axis
Blue: MTM Horizontal; Red: MTM Vertical; Green: two-way bookshelf

I had to boost the two-way speaker 5 dB to match the output of the MTM; 3 dB related to sensitivity, and another 2 dB resulting from the MTM being a lower impedance speaker (6 ohms vs the 8 ohm MTM).  This is a significant power boost required and something to consider for those wanting to use a two-way speaker as a center channel when they are already using more efficient MTM designs for their main channels. The vertically mounted MTM measured almost ruler flat from 200 Hz to 2 kHz while showing a slightly tapered response in the 3-7 kHz region.  The MTM actually measured better in the 3-7 kHz region when horizontally mounted though it did exhibit 10 dB dip centered at 450 Hz but considering it was very narrow in bandwidth, its audibility is questionable playing real program material.  The 200 Hz dip exhibited by the horizontally mounted MTM and the two-way bookshelf is likely a measurement anomaly related to the microphone position relative to the floor.  The two-way measured similarly good to the MTM vertically mounted.  So far all these speaker orientations seem to indicate measurably good results.

 Measurements 14 Degrees

1/4th Octave Frequency Response 14 Degrees Off-Axis
Blue: MTM Horizontal; Red: MTM Vertical; Green: two-way bookshelf

At 14 degrees off axis, this represents the left seat in my front row of my theater.  The MTM vertically and horizontally mounted measured similarly good.  The horizontal orientation produced a slightly reduced output in the 1kHz to 2.5kHz range which again arguably looked more linear when compared to the entire frequency spectrum.  If you compare the measurement of the horizontal MTM to the two-way, they look almost identical.  So let me get this straight, a horizontally mounted MTM in a real theater room measured this well at a real world listening position that was off axis from center?  Yes it did!

Measurements 27 Degrees

1/4th Octave Frequency Response 27 Degrees Off-Axis
Blue: MTM Horizontal; Red: MTM Vertical; Green: two-way bookshelf

At this measurement position the MTM measures very similarly whether oriented horizontally or vertically with the exception of a 5 dB boost centered around 2 kHz in vertical orientation. Arguably the MTM measured more linearly at the primary listening position when horizontally placed.  As we indicated earlier, the reduced output of the off-axis MTM is actually beneficial when the listener is placed near a sidewall.  The two-way again measured nearly identical to the vertically mounted MTM.  Hence even sitting near a sidewall we see the MTM does a good job whether placed horizontally or vertically.   It’s important to note that this is a very well executed MTM design and your mileage may vary depending on how well the manufacturer executed the crossover and chose the correct drivers to match the system. 

Choosing your Center Channel

Once you factor all of these variables into the equation, the horizontally mounted MTM design doesn’t fare so badly as some might think.  As you can see MTMs can still be very viable and often advantageous options whether placed horizontally or vertically.  Ideally we would always desire to place our MTM speakers vertically but you can still achieve good results for most home theater installs using a horizontally mounted MTM design for the center channel provided you stay within a recommended +/-30 degree listening window. 

It’s important to remember that just because you can measure an artifact, doesn’t mean it is actually audible.  The human ear-brain mechanism perceives sound in a way vastly different from how a spectrum analyzer displays microphone measured data.  Remember most of us hear with two ears separated by the physical distance of our head where as a microphone measures at a single point only.   To paraphrase  Paul Klipsch, “you can claim to hear the 15 degree phase difference at 20 kHz but your ears are thousands of degrees apart.  Which one were you listening with?” 

A good example of this very measurable but often inaudible phenomenon is comb filtering.  You can measure artifacts of two speakers playing an identical “mono” signal” but these artifacts aren’t typically audible for real program material.

Chose your center channel wisely as its importance in a multi channel theater system cannot be understated.  If possible, try out a few options that work within your budget and physical placement constraints. 

Below are some useful guidelines to consider when choosing a center channel speaker:

  • Choose a center channel that has similar output capabilities and sensitivity as your main channels.
  • It’s usually a good idea to choose a speaker from the manufacturers same product line of your main channels to ensure similar drivers and tonal characteristics “voicing”. 
  • Consider a horizontally mounted MTM design if you have a height restriction and your primary seats are in a +/- 30 degree listening window or less.
  • Consider 3 matching vertical MTM designs for the front 3 speakers if you can place the center channel behind a perforated screen.
  • Consider W(T/M)W design if your listening window exceeds the +/- 30 degree and you are height constrained to not use identically matched speakers to your mains.
  • Bottom line, chose what sounds best for your application that tonally matches your main speakers as closely as possible.
  • Don’t forget to calibrate all of the channels using the internal pink noise generator (aka. Test tones) of your A/V receiver / processor. 

MTM and W(T/M)W are the two basic center channels designs that are most popular.  There are many variants of each all with their associated strengths and weaknesses.  The best advice one could give when choosing a center channel speaker or any of the speakers in your theater room is to NOT just blindly rule out a particular type of design because someone says it theoretically cannot work.  Test them with your ears in your listening environment across your listening area to decide if they are right for you.

I'd like to thank Paul Apollonio and Shane Rich of RBH Sound for their technical insights and peer review of this article.

 

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Recent Forum Posts:

Karl W. posts on December 27, 2009 18:39
I use Mcintosh centers. Multiple drivers and very wide coverage.
highfigh posts on December 27, 2009 11:08
I haven't done any testing of MTM speakers but the benefit I have read and heard about that configuration is wider dispersion when orientated vertically. The other characteristic I heard and read about was that when speaker height was varied WRT the listening position, some issues occurred with phase cancellations. If the MTM center channel speaker is centered and the mic or listener moves 20° to one side, the distance to the mid-bass drivers will be different and this will cause comb filtering. The wavelength at 450Hz is about 4.8“ and if the difference is half of that, one driver will cancel much of what the other produces. The far mid-bass driver of a MTM center channel speaker of that size, laid horizontally, could easily be 2.4” farther from the mic that the near one.

Our hearing may be more geared for 1/4 octave resolution but if a note's fundamental or first harmonic is at 450Hz, it will be noticed. Phase cancellations are easily seen, even at 1/3 octave resolution and if it's not caused by two drivers that produce the same sound having different distance to the mic or listener's ears, it's either a crossover or first reflection issue.
AJinFLA posts on December 27, 2009 10:27
I would suggest 1/6th octave as most representative of what we would hear. Even so, the smoothed data displayed should result in clearly audible differences due to positioning.
Perhaps not so distracting when watching a movie, as one's position will be stationary, so the variance won't be obvious. But it will be there.
Our brains are pretty good at filtering/ignoring things when it isn't essential to hear them, just like it's good at adding things that aren't really there .

cheers,

AJ
buzzy posts on December 25, 2009 07:11
In discussions like the Technical Notes about dispersion of sound from loudspeaker systems by and the section after that, it would be great to have some examples with frequencies and driver sizes in them … “so at a frequency of x, a y inch speaker …” It would be good to convey at what frequencies beaming intensifies, at what point in the lower frequencies the benefits mentioned are meaningful, etc.
Swerd posts on December 24, 2009 12:49
Gene - Thanks for doing all that. It puts things into proper perspective. Like so many things where people don't fully understand the details, they interpret advice in a black or white fashion. Yes, it would be somewhat better if an MTM speaker stood vertically. But a horizontal MTM is much better than no center channel speaker, and as you showed, better (for different reasons) than a vertical MT speaker.

The best part was where you showed just how far off-axis you would be while sitting in a typical 3 cushion-wide sofa 12 feet away from the speakers. It isn't far enough off-axis to be real trouble.

Again, reality trumps conventional wisdom.

I would find it helpful if you could show those measured SPL levels in a polar plot such as below. Would that be possible?

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