D'Apollito speaker on side incorrect? (340)

[Jonnyozero3]

Regarding this article...

http://www.audioholics.com/techtips/...libration3.php

...which near the middle of the page contends in bold "that the most common two-way center channel speaker layout, that of a D’Apollito on its side is incorrect."

Has anyone else seen this (It must have been discussed before somewhere)? I'm going to make an assumption that the CMT-340C was designed to have a relatively even radiation pattern to counter the basic effect of the D'Apollito layout, but I dunno - could I get better results by buying a 340-main and using that under my projector screen? (That would place the tweeter a bit low I would think though....). Any thoughts?

[DavidD]

This issue is frequently discussed. I don't think the problem is as bad as that article would have you believe. A response from David Fabrikant to this general issue can be found here:

Lobing

[Lee Bailey]

In the maroon colored slides above that sentence, it plainly states:

Do not place speakers horizontally unless properly designed for it.

In which case, the 340C is designed for it.

You do have to AIM the center channel at your sweet spot though, no matter if it is above or below your screen. I use a laser level to get my L/C/R speakers aimed correctly.

[Eddie Horton]

The 340C remains great sounding even a good bit off-axis, but 5 identical speakers is always ideal. The argument could be made that the D'Appolito array is a compromise, and could be made effectively. What people forget is that unless you have the bucks to get a front projector and an expensive micro-perf screen, then another identical speaker for the center is also a compromise that probably 90% of the HT public can't make. I have no complaints with my 340C.

[chas]

Thanks for that link DavidD...some great info from Mr. Fabrikant in that thread.

I've been researching speakers for my next system. I've been fairly satisfied with my current speakers other than the center. I have one of those unfortunate rooms with off-axis seating, and the original matching center that I purchased with my current system (a popular budget brand) suffered from extreme lobing in the off-axis areas. Dialog was often unintelligible in the off-axis seating areas. I've since replaced it with a bookshelf, and this solved the lobing issue but the big upright bookshelf on my television is certainly an eyesore (especially with the door stops propping up the back to angle it down!).

The time and effort that went into the design of the 340c is what has drawn me to look at Ascend speakers, but in the back of my mind I always wondered why David didn't go with a 3-way center. That thread answered my question!

[bikeman]

...which near the middle of the page contends in bold "that the most common two-way center channel speaker layout, that of a D’Apollito on its side is incorrect."

I don't believe that the 340 is a D'Apollito design. As I understand it, Professor D'Apollito came up with a design that help correct some of the problems with a basic MTM design. I'm in over my head on this subject. Anyone have a better understanding of on this? Hint, hint.

David

[Lee Bailey]

MTM is a D'Apollito design. Making the proper enclosure and crossover for it is what the particular speaker designer is challenged to perform.

[bikeman]

MTM is a D'Apollito design. Making the proper enclosure and crossover for it is what the particular speaker designer is challenged to perform.

Yes, it is an MTM design. But it's different from the way most MTM's are done.

Here's an explanation provided to me by a colleague of Dr. D'Apollito. According to him, the D'Apollito design improves upon the basic MTM design. Again, I don't know my butt from left field on this subject. Here goes:


Consider the situation in a horizontal MTM system, where:
d = the distance between the center of the drivers
D is the distance from the speaker to the listening plane, and d is
perpendicular to D
s1 refers to driver 1
s2 refers to driver 2


for any point P along the listening plane, the intensity of the sound
coming from the two drivers, s1 and s2 will vary due to the different path
lengths from each speaker driver to the point P. The maximum intensity will
occur where the distance between P and s1 = the distance between P and s2.
For expediency let's call this point Pmax. For other locations of P, located
a distance of x from the centerline where Pmax occurs, the sound from one
driver will have traveled a distance of delta farther than that from the
other driver. Let the wavelength be L. For distances where delta=0, 1L, 2L,
3L... there will be reinforcement of the waves, creating a relative maximum,
with the maximum value of intensity at delta=0. Minima will occur at
distances where delta=L/2, 3L/2, 5L/2...


As the distance from the listening plane to the speaker system D is much
larger than the seperation distance between the drivers x (at least for the
conventional MTM layout center channel driver):


(delta/d)=(x/D) (Sorry for the lack of a figure, it's hard to do in ascii,
I refers to intensity of the sound field.


The intensity of the sound field at each point Px is expressed by:


Ipx= I(at Pmax) * cosine^2 ((pi*x*d)/(L*D)


which varies sinusoidally, with decreasing amplitude for each relative
maxima the further off axis from the centerline you get.


Maxima occur when x=n*((L*D)/d) , where n=1,2,3...
Minima occur when the value n is n/2, again where n=1,2,3


In terms you might understand, with two drivers that are not located at the
same place, but rather a spacing d apart, off axis the path lengths from
each driver vary, the driver that is closer always has a shorter pathlength
than the one farther away as you move off axis (if you move to the right,
the right driver always has a shorter pathlength than the left driver). This
causes the sound waves to interfere with each other as they move in and out
of phase with each other as the distance off axis increases. When they are
180 degrees out of phase they cancel each other out, when in phase they
reinforce each other, resulting in, why!, who'da thunk it, an interference
pattern and "picket fencing." That is, it gets louder and softer at
different positions off axis, which does not occur with a traditional TW
configuration (well, it occurs in the vertical axis over the freq range
where the woofer and tweeter overlap, but not horizontally, nor does it
occur horizontally if you use an MTM vertically the way they were originally
intended).


In the "real world" where drivers radiate from a surface that is not a point
radiator, this effect is blurred somewhat, but it still exists. The effect
is that the minima do not go to zero intensity, and the difference between
maxima and minima in intensity is not as large, and the number of maxima and
minima are reduced. Again, it still exists, unless you know of a MTM
speaker where the two larger drivers don't operate over any of the same
frequencies, but then that wouldn't really be a MTM would it? You also
get interference producing intensity variations over the range between each
larger driver and the tweeter where they overlap, usually about an octave to
a 3 octaves, depending on the slope of the crossover used, higher slopes
will produce less interference, or at least interference over a much smaller
range of frequencies.


At this point it should be obvious that this approach is NOT ideal for the
construction of a center speaker. In practice, careful selection of the
driver size, their spacing, and the frequency range they operate over can
reduce this, but it is still there. If it's well designed the average
listener will probably not notice it, particularly if the room acoustics
aren't that great (which is common). An attentive, trained listener will
notice it if they pay attention to it, or sit particularly far off axis,
though most find it possible to ignore with a good design. It is still there
however, a product of physical law, and while the average listener might not
be astute enough to hear it (and good for them! They're probably happier
than people who can) it is measureable in situ.



That's it.


David

[Eddie Horton]

Damn, David........I was trying to wrap my feeble little brain around all that and I think I pulled something.

[Lee Bailey]

Strange, I've not seen an MTM design that doesn't say it is not a D'Apollito arrangement. But there is definitely more information in that note than I'd want to know. If I followed what it said, then the 340C is not the type of speaker I should be using for a center at all, but a 340M in a vertical orientation, or a CBM170?