Vertical vs Horizontal Center Channel Speaker Designs – An Alternate Perspective

About two years ago Chris Seymour of Seymour A/V was kind enough to write us a very informative article (Vertical vs Horizontal Center Channel Speaker Designs) comparing frequency response variations of center channel speakers mounted vertically vs horizontally.  This article dealt primarily with the assumption that the listener would be significantly off-axis from the speaker and hence the horizontally mounted MTM speaker stood (no pun intended) at a disadvantage. 

Since this articles inception, we’ve noticed it being linked in various forum discussions where people have been discouraging the use of traditional MTM speakers mounted horizontally.  Some have even gone so far as to  deem common MTM speakers to be severely flawed when placed horizontally under or above a display.  While others opted to use a two-way bookshelf speaker in-lieu of an MTM design for these very reasons.   Many manufacturers have addressed this issue by designing dedicated WTMW designs that utilize two woofers for bass frequencies, and smaller driver for the midrange vertically mounted below the tweeter.  There is little doubt that these designs offer wider off-axis performance than a comparable horizontally mounted MTM.   But how far off-axis does one have to get until the MTM design becomes too problematic?  There are no free lunches with any loudspeaker designs. Thus compromises exist with both MTM and W(T/M)W designs which are discussed in this article and should be considered when choosing the right center channel for your home theater.

Center Channel Speakers: Which Design is Best for Home Theater?

As a refresher let’s review some examples of different center channel designs.

MTM (Mid – Tweeter – Mid)
A MTM (aka. D’Appolito array) is a two-way loudspeaker system that utilizes drivers where the tweeter is centered between two woofers.  Ideally the orientation of these drivers is vertical but for center channel usage, we typically turn these designs on their side for horizontal placement to snugly fit under a display.

In theory a D’Appolito array increases the SPL in the forward direction while limiting the off axis response in the axis normal to the largest dimension.  For a floor standing speaker, this means the reflections from the ceiling and floor are reduced relative to the direct sound. The problem that can arise however is destructive interference occurring between the two midrange drivers which is more apparent when the speaker is oriented horizontally than the same speaker oriented vertically. This becomes more apparent when the listener moves across the horizontal plane of the horizontally oriented speaker.

W(T/M)W (Woofer – Tweeter – Mid – Woofer)
A W(T/M)W is a three-way loudspeaker design that utilizes a single midrange below the tweeter with one larger woofer on each side of the vertically mounted woofer/tweeter array.  This type of approach produces better horizontal off-axis performance than conventional MTMs horizontally mounted but can arguably have other performance trade-offs discussed within this article. 

The Trade-Offs

Care must be taken to not compromise system dynamics at the expense of potentially wider off-axis performance, especially if your seats are never that far off-axis enough to be of paramount concern.  It is important to counter the argument against horizontally mounted MTM designs and note that a well executed MTM can still offer a level of performance that equals or exceeds a “dedicated” (WTMW) center channel speaker design and almost always beat a single two-way bookshelf alternative using the same driver components with respect to dynamic range and pure SPL output.  While it is true that an MTM speaker has more limited dispersion characteristics when oriented horizontally vs vertically, they can still be advantageously used horizontally mounted as a dedicated center channel which we discuss below.

Just how far off-axis from the center channel are the extreme seats really?
While the measurements from our original article include frequency response at up to 40 degrees off-axis, it important to note that most center channels are rarely listened to at even 30 degrees off-axis.   In fact, 20 degrees off-axis response is quite often the worst case scenario.  

Figure 1.  off-axis angles for various seats

As a case in point, take a typical home theater room (see Figure 1) which has a center listening position that is 12 feet away from the center channel and the loudspeakers tweeters are roughly at ear height of the listeners in the seated position.  Even with three theater seats across the front row, the listening position in the left and right seats are only 3 feet away from the center point in the center chair.  In other words, the total distance we need to be concerned about for listening coverage in the front row is 6 feet wide.  If you do the math (3’ off center axis that is 12’ from the center channel) ATAN 3/12= 14 degrees. At 15 degrees off-axis even a mediocre MTM design will work acceptably well.  Let’s double the seating in that row and say the listener is now 6 feet off axis (ATAN 6/12).  The listener at either far end seat of the theater is still only 27 degrees off-axis from the center channel. 

For most multi row theaters containing multiple seats per row, you are typically further away than 12 feet from the front three speakers.  Thus the further away you are from those speakers, the less the angle of incident is to the listeners.  It is important to note that the further distance you place the listener from the loudspeaker, the more the soundfield becomes dominated by reflections than direct sound.  Since those reflections are often as much dependent upon the off axis sound of the speaker as the on-axis sound, the loudspeakers off-axis behavior can become an even more important determinant in the sound that reaches your ears.  As the listening room becomes more reflective, and/or the distance from the sound source increases, the angle of incidence to the loudspeaker becomes less important as the direct sound plays a smaller and smaller role in the sound that reaches the listener.

Let’s rework the example of being 3 feet off axis but this time from the second row which in our case is 15 feet away from the center channel when on-axis.  In this case the listener on either the far left or right seat in the second row is now only arctangent (3/15) = 11 degrees off axis. If we put two more additional seats in that back row and sit the listeners at the extreme far left or right seats, they will now be 6 feet off axis from the center channel.  The listener is now only 21 degrees off-axis. 

I encourage you to work this out in your own theater room. All you have to do is measure the distance from your central seat which is on axis from your center channel.  Then measure the distance from that position to your desired seat in that row.  Next take the arctangent of the ratio of the off center divided by the other axial distance. (See example in the prior paragraph).   The bottom line is in most cases you are not as far off-axis from the center channel as you might think.  Try measuring your own theater and see what you come up with.  If your angle exceeds 30 degrees, you may wish to consider a WTMW or two-way alternative.

What happens when you are significantly off-axis from the center channel?
A horizontally mounted MTM design which has decent 15-20 degree off-axis response could have poor coverage by the time you get to 50 degrees off-axis or more.  Of course much of this depends on the distance between acoustic centers of the drivers, the distance to the listener, and the frequency range of interest.  But let’s consider worst case here.  Imagine our theater room again where we there is 12 feet of distance from the center channel.  To get 40 degrees off axis, the extreme seat would have to be 10 feet off-axis from the center position!  If the room is only 15 feet wide, that means your extreme seats are only about 3 feet from either sidewall.  If you get that far off-axis from the center or sweet spot of the theater room sidewall reflections will start dominating the soundfield.  Not to mention you will likely be too close to a surround channel which will dominate the soundfield.  An MTM design placed horizontally can be quite beneficial with regards to limiting these reflections coming off the sidewalls in the critical 500 – 2 kHz range. We also realize a reduction of SPL in the physical space that should be dominated by the left or right speaker.  So in this case, one could argue a horizontally mounted MTM would be a preferred choice despite the fact that its horizontal off-axis performance is narrower than it’s vertically mounted equal.

Technical Notes about dispersion of sound from loudspeaker systems by Paul Apollonio
Since the speed of sound does not vary with frequency, as the frequency of sound decreases, the distance between the air pressure maximums (or minimums) increases.  This relationship is simply defined as C/F = W ; where C = Speed of Sound, F = frequency of sound, and W = wavelength (distance in space occupied by a complete cycle of sound; ambient pressure to max, to ambient, to minimum to ambient again; 0, 90, 180, 270, 360 degrees = 1 cycle).  Remember, the speed of sound is constant, and independent of the frequency.  So, when you excite the air at half the frequency, the distance between the pressure peaks doubles!  That distance is your wavelength.  Since the speaker producing this wavelength does NOT change in size, its size relative to the wavelength MUST change with frequency.  This relationship is a primary cause of directivity in loudspeakers and is very important in multiple speaker arrays, as the distance between speakers is relatively small at low frequencies, and large enough at high frequencies to stop the speakers from acoustically coupling and acting as if they were a single speaker.

Source size is a very important determinant of the radiation of the sound coming from the speaker.  As the wavelength radiated from your speaker shrinks to a size approximating half the circumference, the radiation from the speaker beams in an ever tighter pattern.  Eventually the pattern becomes scalloped and uneven, with a polar pattern that looks like multiple small balloons pressed closely together.  At low frequencies, the radiation from a speaker looks like a complete round balloon when that speaker is suspended high in the air away from reflecting surfaces.  On the floor, a large wavelength comes off the speaker like a large bubble cut in half with the flat cut against the floor.  At high enough frequencies, the speaker cone itself acts like a baffle for the radiated sound.

Like a professional line array, an MTM configuration is an array that trades off the width of the dispersion for a greater on axis SPL.  The symmetry of the array, placing the high frequency driver in the center allows the effective diameter of the radiator to be equal to the diameter of the speaker plus the distance between the acoustic centers.  As the frequencies get lower, the wavelengths get longer and larger, and the coupling between drivers becomes more complete.  So, for an MTM array with two 6 inch drivers separated by a 1 inch tweeter on a 4 inch diameter plate, the distance between the acoustic centers of the "M's" in this array is 10 inches.  The size across the long dimension is 16", while across the smaller dimension is only 6".  This causes the radiation pattern (size and shape of the balloon so to speak) is going to be different across the 16" dimension than it is across the axis which is only 6".  At the larger dimension, the speaker becomes directional at a lower frequency.  For a horizontal mounted MTM as a center channel, the reflections off the side walls will be less than they would compared to a MT speaker, since the MT speaker is going to spread the acoustic power over a wider angle through the middle frequencies.

What about dynamic range?
Dedicated center channels (WTMW) are typically 3 way designs which have their own issues. If extreme care isn’t executed in the design of both the box and crossover, they can be less phase coherent than a simpler 2-way design. W(T/M)W designs can have less axial sensitivity and dynamic range in the upper mid frequencies than a comparable MTM design. Think about it, most W(T/M)W designs utilize a single 4-5” midrange driver as opposed to an MTM using dual 6 ½” drivers.  The MTM in this case has over 2.8 times more driver surface area than the W(T/M)W design to produce the critical midrange frequencies below 2kHz.  As long as the density per unit area of speaker is constant, and the radiated frequency is not so high to preclude the speaker from a uniform radiation pattern, for every doubling of surface area, you get a 3 dB increase of sensitivity.  So theoretically, we can pick up a fair amount of output and sensitivity at certain frequencies when utilizing the increased cone surface area of the dual woofer MTM design over a single midrange W(T/M)W design. A good W(T/M)W can compensate for this somewhat by significantly bandwidth limiting the midrange driver and manipulating the speaker parameters to optimize it for a narrow range of frequencies.  The only way of knowing which design is inherently superior in this respect is to do a side by side comparison.

As an experiment, the home theater enthusiast is encouraged to try putting on a movie that has a gunshot coming through the center channel and see how it differs between a dedicated W(T/M)W center speaker and a MTM design.  The reader is also encouraged to review the related articles section below on loudspeaker power ratings.

Position of the Center Channel Relative to the Main Speakers

When 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.  

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.

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.

1/4^th 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!

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.