Must Center Channels Always Have Two Woofers?

[mtrycrafts]

I'm not sure. If I recall correctly, a D'Appolito design is actually not the optimal design to use for a center channel. It creates lobing [I apologize if this is incorrect, but I remember that I read this]. This is why many center channels have an offset tweeter.

Well, the D'Appolito design is a vertical stacked design, not the butchered adaptation of it in a horizontal setting. Hence, you are absolutely correct about the lobing. There is an interesting write up in the current issue of the T$$ I brought to everyone's attention earlier.
Another article in there discusses crossovers by that know nothing person who wrote that paper about modern components being transparent
May not want to read it though. LOL.

 

[mtrycrafts]

I know one thing, I have a Paradigm CC370 and it stinks. Huge peak in the mid-bass region. I have since moved one of my mini monitors up front on a short stand. The cc370 is a D'Appolito design, but I am not sure if that is what is causing the huge mid-bass peak. The peak is not there with the mini.

If it is a D'Appolito design, mid-tweeter-mid on its side, then that is the cause, lobing issues. But a d'Appolito on a side is not the real McCoy

 

[mtrycrafts]

Actually, the lobing is minimized with a MTM design.

-Chris

Unless it is on its side on a center speaker, then it has a horizontal lobing. The true D'appolito is a vertical design with the lobing going in the vertical axis. This is discussed in issue #106 of T$$ I posted about not long ago.

 

[mtrycrafts]

This is probably more directed for the technically inclined. Why are almost all center channel designs in a MTM (D'Appolitio) configuration. Is there any technical reasoning behind not using a single woofer and tweeter in a two-way application? Is it done simply for visual symmetry? Wmax?

Check out that issue #106 in the T$$. This is discussed. The second article discusses crossovers. It is a design decision, or rather an aesthetic design choice, not having a larger speaker on top or under a TV. Certainly not an acoustic design.

 

[mtrycrafts]

Chris,

Thanks! So if I were to align the tweeter and the woofer horizontally (TM), there should be no real negative effect, or did I misunderstand that? It should not matter what side of the woofer the tweeter would be on then either, correct? If neccessary I can align them verically (TM) I guess.

Try to turn your left and right speaker on its sides and see what you get. That is what happens with this horizontal designs. But people buy them because it fits their design not having a speaker in a vertical position above the TV, or below it where there is even less space, usually.

 

[Sheep]

hey Mtry, try ONE POST!

SheepStar

 

[WmAx]

Unless it is on its side on a center speaker, then it has a horizontal lobing. The true D'appolito is a vertical design with the lobing going in the vertical axis. This is discussed in issue #106 of T$$ I posted about not long ago.

It seems you may be confusing my discussion of lobing magnitude around the crossover frequency with destructive interferance of two drivers operating within the same bandwidth, that could also be defined as lobing technically, but I used the term lobing only in the capacity of discussing the behaviour at and around the immediate crossover frequency region, since this seems be the most common capacity that this term is used within. A MTM, at and around the crossover region, has a symmetrical lobing (in positive and negative rotations being equal, which means no polar axis tilt) behaviour and a lesser potential magnitude difference[when the appropriate crossover topology is used] between lobes in the aligned plane of drivers as compared to a similar MT design. The lobing magnitude/off axis cancellations of the two low frequency drivers operating at different phases at different angles is greater off axis in this plane, moving away from the crossover frequency. But lobing magnitude at the crossover frequency is only improved in this plane[assuming a proper D'Appolito crossover formulation]. Please refer to the AES document I provided earlier in this post: http://forums.audioholics.com/forums/showpost.php?p=127363&postcount=22

In addition, here is a relative comparision of the polar response @ crossover frequency of various MT and MTM crossovers:

Summarized points....

1. Lobing is potentially less magnitude at and around the crossover frequency for a MTM in the plane of aligned drivers.

2. Lobing is worse off axis with a MTM below the crossover frequency in the plane of the aligned drivers.

3. Since the crossover band that benefits is in most circumstances narrower than the band that is under the crossover that is negatively impacted by destructive interference, MTM appears to be a worse design in actual use unless one specifically has a technical reason to desire the inherant cancellation problems that will occur off axis, below the crossover point.

-Chris

EDIT: Added graph and clarified my use of the term lobing within this discussion.

 

[jeffsg4mac]

If it is a D'Appolito design, mid-tweeter-mid on its side, then that is the cause, lobing issues. But a d'Appolito on a side is not the real McCoy

Well that is what I suspected is going on with my cc370. It is the V.1 and has the tweeter dead center. I Have not looked at xover to see if I could figure out what they are doing with it. I might stand the thing up vertical and measure it again just to see what happens. One thing for sure, it is unusable as a center channel the way it is.

 

[jeffsg4mac]

WmAx, What do you suppose would be the effect if I stuck both my mini-monitors up front. side by side together on one stand? Would they act as one or would there be some cancelation and/or accentuation going on? I hate having an extra speaker laying around not doing anything.

 

[Buckle-meister]

The very small diameter of the midrange will allow excellent dispersion...

Is the above statement specific to the speaker in question, or can it be applied to any small diameter driver? If the latter, why do smaller drivers disperse sound better than larger?

Regards

 

[WmAx]

WmAx, What do you suppose would be the effect if I stuck both my mini-monitors up front. side by side together on one stand? Would they act as one or would there be some cancelation and/or accentuation going on? I hate having an extra speaker laying around not doing anything.

Short answer: not a good effect. Please refer to my reply to Buckle-meister, as the reason/answer will be identical.

-Chris

 

[WmAx]

If the latter, why do smaller drivers disperse sound better than larger?

Regards

The dispersion is better in a given plane when the radiation surface area in that plane is small relative to the wavelengths radiated. The LF driver interactions of MTM speakers create severe combing/cancellation off axis and an 8" driver used to radiate upper midrange frequencies will be very directive(beam) because in both of these cases the wavelengths are small in relation to the effective radiation surface area in a given plane. If the two LF drivers in a MTM were operatig in a frequency band that did not involve dispersion of waves that are short in relation to the spaced distance of the drivers, then they would not have the negative effect. In fact, a single 5" midrange driver usually has some level of cancellation when analyzed at just 3kHz off axis at angles exceeding just 15 degrees. When you launch wavefronts from multiple simultaneous points(and a single cone/surface can qualify when the frequency wavelength in air becomes small in relation to the distance from one point to another on this surface) at a spaced distance that, when you refer to the vector angle off axis, the different origin point launched waves meet at an intersecting point that the path distance difference between the two causes sufficient phase rotation difference between the two to cancel energy/null, then this is a destructive interference that limits dispersion.

-Chris

 

[Tex-amp]

The lobing effect in a good MTM CC design is a little overwought by audiophiles. In many cases if you're sitting far enough to one side or the other for lobing to come into play; you've got bigger issues like sitting directly in front of one of your L/R mains and viewing your display at quite an angle.

 

[mtrycrafts]

Here is what the article said. Maybe I am not understanding it right?

The original D'Appolito array was was made to improve the horizontal directivity of a vertically arrayed MTM system at the cost of some exacerbated lobing in the vertical plane. When you use a D'Appolito horizontally you radiate those worsened vertical lobes directly into the listening area instead of toward the ceiling and floor. Why is that good?

The graph that follows shows a mean off axis dips what appears to be the crossover points of these speakers.

 

[mtrycrafts]

" This is no issue when using the TM vertically oriented. If you use a TM vertically, then it is superior in all important parameters compared to a similar MTM layed horizontally."

That is what I wanted to hear, just confirms what I have experienced.

This is what is in that issue 106, T$$ and what you stated above

 

[mtrycrafts]

The lobing effect in a good MTM CC design is a little overwought by audiophiles. In many cases if you're sitting far enough to one side or the other for lobing to come into play; you've got bigger issues like sitting directly in front of one of your L/R mains and viewing your display at quite an angle.

Oh, but the horizontal dispersion pattern off axis is crucial in proper reproduction whether you are fixed exactly on the center line of not. This is what research by Toole et al shows, no matter if the speaker is on the right side or in the center should not make a difference. Otherwise, off axis dispersion would be of no concern to designers.

 

[chikoo]

I know one thing, I have a Paradigm CC370 and it stinks. Huge peak in the mid-bass region. I have since moved one of my mini monitors up front on a short stand. The cc370 is a D'Appolito design, but I am not sure if that is what is causing the huge mid-bass peak. The peak is not there with the mini.

mid-bass peak?

what frequency is involved?

 

[mtrycrafts]

hey Mtry, try ONE POST!

SheepStar

Good idea if there is one or two posts on a thread. With this many, do you read every one? Or just all of mine, LOL.
I don't read every one, so I might miss a single post with the best info there. This way, others will not miss it.

 

[jeffsg4mac]

This is what is in that issue 106, T$$ and what you stated above

mtrycrafts, Thank you

 

[jeffsg4mac]

mid-bass peak?

what frequency is involved?

Chikoo, not frequency but frequencies. From about 100 to 160 it is up some 15db than the rest of the spectrum. I good PEQ could help but even the manual overrides on the RXV2600 are not incremented in enough steps to fix it. Besides, a speaker with that kind of issue has a serious design flaw in my opinion.