Why D’Apollito arrange isn't the best for center speaker

toquemon

toquemon

Full Audioholic
I was reading the review of the CEDIA courses and one of the speakers (Tony Grimani) said that the D'apollito arrangement wasn't the best for center channel, why?. Almost all the center speakers i've seen have this arrangement. My YAMANA NS-C300 has this arrangement but with the tweeter a little bit higher than the other two drives. Anybody knows which is the best layout?
 
WmAx

WmAx

Audioholic Samurai
This is a bit of a generalization, and assume that the center channel is layed in a manner so that the speaker array is laid out horiztonally and that the midranges are operating into a bandwidth that the waveforms are small in relative size compared to the spacing of the drivers. In such a case, the horiztonal dispersion will be limited in certain bandwidths due to interdriver interferance(destructive interference); specifically the frequency band that both midranges are operating within simultaneously and the relative wavelenghts in air are small relative to the midranges' spacing. It is this same principle that causes narrowed dispersion on conventional(as opposed to bending mode drivers which are not at subject here) large diamter midrange units as frequency rises. Examing the behaviour of a horiztonally placed MTM at varying horizontal axises, the waveforms of applicable size launched from simultaneous points will eventually travel a vector path to a point where the wavefronts will begin to meet at opposing/contradictory phase. This will cause comb-filtering effect(s). As angle is increased, interference will be of a magnitude that it cancels(attenuates) signifant portions of energy. The net result on a standard horizontally placed MTM center channel will be poor polar(off axis) response in the upper midrange as the device crosses over to the tweeter. This means that the sound will not be consistant over a wide seating area.

For an in-depth analysis of this configuration, you may refer to:

Vertically Symmetric Two-Way Loudspeaker Arrays Reconsidered
Konar, M.
AES Preprint: 4186

-Chris



toquemon said:
I was reading the review of the CEDIA courses and one of the speakers (Tony Grimani) said that the D'apollito arrangement wasn't the best for center channel, why?. Almost all the center speakers i've seen have this arrangement. My YAMANA NS-C300 has this arrangement but with the tweeter a little bit higher than the other two drives. Anybody knows which is the best layout?
 
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Rip Van Woofer

Rip Van Woofer

Audioholic General
The short, simple version of what Chris is saying is: Center channel speakers are only sideways so they'll fit atop a TV, at the expense of sound quality. Sideways speakers are seldom a good idea regardless of configuration. Having drivers in a line creates interference patterns that mucks up the sound; vertically aligning the drivers places those interference patterns where they are less objectionable.

There's nothing wrong with the D'Appolito configuration (also known as MTM, for Midrange, Tweeter, Midrange) when it's vertical. Hey, an Italian designed it so it has to be good!
 
gene

gene

Audioholics Master Chief
Administrator
A speaker design based on an MTM configuration with center channel placement in mind can still work relatively well as a horizontally placed center channel by using a 4th order Linkwiitz-Riley acoustic slope crossover to integrate the mid-woofer and tweeter vs. the 3rd order slope traditionally used in DAppolito alignments . Yes, there will still be some lobing though to a lesser effect, but one must consider that given most center channels are placed either above or below a television/screen and are placed directly centered on the listening position, the dispersion characteristics of the MTM design actually work pretty well for a listening position spanning the width of a traditional couch . Most center channel speaker placements are at best a compromise.
 
WmAx

WmAx

Audioholic Samurai
gene said:
...can still work relatively well as a horizontally placed center channel by using a 4th order Linkwiitz-Riley acoustic slope crossover to integrate the mid-woofer and tweeter vs. the 3rd order slope traditionally used in DAppolito alignments . Yes, there will still be some lobing though to a lesser effect...
Clarification: The crossover topology used will have no effect throughout the passband(crossover effect limited to the crossover region)upon the phenomena(destructive driver interference) I addressed. Of course, as you point out, the horizontal MTM design will function sufficiently within their intended listening window, such as a standard width couch placed midfield in an average room. But a standard orientation(vertical) MTM or a standard MT(used vertically) would be ultimately superior over a wide listening axis as compared to an average horizontally oriented MTM.

-Chris
 
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gregz

gregz

Full Audioholic
Wow; I never considered the cancellation between the two woofers in a center channel speaker. What then would be the best design? (assuming you're stuck with a glass screen that you can't fire speakers through)

Perhaps a mid above and below the screen??
 

plhart

Audioholic
From the CEDIA Seminar Part 1 original thread

Toquemon-

The following configurations of center channels work "better". That is, they exhibit less or none of the undesirable cancellation/lobing effect that takes place with a D'Apollito in which the tweeter is exactly in the center of the cabinet and the mid-woofers are thus separated completely from each other.

The cancellation effect is usually more noticeable on mid-woofers of 5.25" and up (if both are driven all the way up to the crossover point). It is less noticeable on midwoofers of 4" and under. So, the tighter together you can get each of the mid-woofers, by squeezing the tweeter out of the center, for instance, as in example #2, the more closely the two drivers approach being a point source. Explained another way, the smaller-diameter-driver cancellations take place more closely to the speaker so the cancellations are more "resolved" by the time you get back to the listening position.

The original "benefit" of the D'Apollito configuration was that, set up vertically, the tweeter frequencies dispersed widely from left to right (in the horizontal plane) while the tweeter's frequencies were limited vertically; they were more focussed. This is why you see so many THX approved left and right speakers using the D'appolito configuration. Stood up vertically as Joe D'Appolito originally intended, the left and right mid-woofers tend to cancel each other at many frequencies as those frequencies head toward the ceiling or floor. So you don't get a strong floor or ceiling axial reflection at specific frequencies. Unfortunately, when you lay the D'Apollito on it's side as is done for the center channel, all the design's attributes become liabilities.

Here are four work-arounds that many designs currently on the market feature.

1. Three-way designs with a midrange under a tweeter and flanked by woofers work great as long as that midrange is brought down low enough into the vocal range. However, most of the faux 2" and 3" size "midranges" you see are crossed over above 1KHz so they don't necessarily answer the problem their vertical alignment (tweeter above midrange) would seem to imply.

2. Small mid-woofer drivers driven up to their 2.5KHz or so crossover can work well if they're pushed as closely together as possible, like many of the Atlantic Technology designs with the tweeter nestled in between the "V" formed by the two. AT also has the switchable proximity (TV) notch filter on many of their centers which makes for clear vocals without the false and unnatural "chestiness" added to voices.

3. A D'Apollito-appearing horizontal design which uses one well designed passive radiator and one true mid-woofer works very, very well. Boston Acoustics has used this trick for quite a while and I can tell you that in listening double blind, the Boston would always trump the more traditional double-driven mid-woofer designs. It was that much better.

4. Same can be said for 2 1/2 way designs in which one of the two driven woofers is rolled off fairly quickly , usually at around 500 Hz, while the second woofer goes up to the ~2.5KHz crossover point. Works very well.
 
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BMXTRIX

BMXTRIX

Audioholic Warlord
How about center channels using planar speakers? Is attenuation still an issue?
 
WmAx

WmAx

Audioholic Samurai
BMXTRIX said:
How about center channels using planar speakers? Is attenuation still an issue?
The issue extends to the plane on any effective radiating area(s) of any type driver that is large compared to the wavelenghts in air being dispersed. That's why, as phlart points out, as smaller midranges are used on conventional mtm to effect less distance between the total radiating areas, the effect will reduce in magnitude.

-Chris
 
toquemon

toquemon

Full Audioholic
Well, i can see this is very complicated. Would it be a difference if i put my center speaker vertically (like if it was a front speaker)?. I have a Polk Audio center speaker and it has a mid-woofer, a tweeter and a front port, what do you guys think about this design?
 
Rip Van Woofer

Rip Van Woofer

Audioholic General
Yes, standing the speaker up vertically might make a difference. Prueba!

(trying my pathetic Spanish since I see you're in Mexico...)
 
toquemon

toquemon

Full Audioholic
Muchas Gracias Sr. Rip Van Woofer. I would try standing my center speaker vetically.
 

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