[Ron Alcasid]

I've read many times on this forum how D'Appolito MTM's are designed to reduce floor and ceiling reflections by creating lobes the vertical plane. That is also the reason people do not recommend MTM as horizontal centers. Yet his original paper was titled "A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers."

http://www.aes.org/e-lib/browse.cfm?elib=11762


So is it possible to design an MTM center with good off axis response or was Dr. Joe D'Appolito just plain wrong?

 

[Dennis Murphy]



Quote:
Originally Posted by Ron Alcasid /forum/post/18167463


I've read many times on this forum how D'Appolito MTM's are designed to reduce floor and ceiling reflections by creating lobes the vertical plane. That is also the reason people do not recommend MTM as horizontal centers. Yet his original paper was titled "A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers."

http://www.aes.org/e-lib/browse.cfm?elib=11762


So is it possible to design an MTM center with good off axis response or was Dr. Joe D'Appolito just plain wrong?

This is a controversial and murky subject. But for starters, D'Appolito was addressing off-axis lobing behavior in the crossover region, which in 2-ways is usually between 2k and 3k. It's a different kind of off-axis behavior that makes horizontal MTM's problematic for HT use involving multiple listeners. On-axis, the two woofers are equal-distant from the tweeter and the listener. But as you move off-axis horizontally, the relative distance of the woofers to the listener changes. This will be more dramatic the closer you are to the speakers and the further off-axis you go. That causes destructive interferrence between the woofers, which in turn can cause a huge suck-out from around 1500 Hz all the way up to 3500 Hz. You can see what that looks like here, at the bottom of the page: http://murphyblaster.com/content.php?f=gem_mod.html


D'Appolito himself has moved away from the crossover topology he originally recommended (3rd order acoustic) to 4th order acoustic, and frankly I'm not sure what the facts are for improved lobing behavior in the crossover region.

 

[Speedskater]

Regardless of the cross-over network design, a symmetrical line array (with the drivers in a vertical row) will have a narrower vertical dispersion pattern than horizontal.

 

[Dennis Murphy]



Quote:
Originally Posted by Speedskater /forum/post/18170155


Regardless of the cross-over network design, a symmetrical line array (with the drivers in a vertical row) will have a narrower vertical dispersion pattern than horizontal.

Yes, but you have to design the line array very carefully to avoid serious comb filtering (desctructive interferrence). So there are two different concepts at work here. Controlled vertical dispersion from driver configuration and crossover design, and destructive interferrence from changes in the relative distance of the drivers as you move off axis.

 

[Speedskater]



Quote:
Originally Posted by Dennis Murphy /forum/post/18170264


Yes, but you have to design the line array very carefully to avoid serious comb filtering (desctructive interferrence). So there are two different concepts at work here. Controlled vertical dispersion from driver configuration and crossover design, and destructive interferrence from changes in the relative distance of the drivers as you move off axis.

True, very true. But a line array (minus a cross-over) will still have different vertical and horizontal dispersions.

 

[Dennis Murphy]



Quote:
Originally Posted by Speedskater /forum/post/18170297


True, very true. But a line array (minus a cross-over) will still have different vertical and horizontal dispersions.

Right--but it would have to. The relative position of the drivers doesn't change horizontally off axis, but it does vertically. So I think we're talking about the same concept.

 

[Ron Alcasid]



Quote:
Originally Posted by Dennis Murphy /forum/post/18169000


It's a different kind of off-axis behavior that makes horizontal MTM's problematic for HT use involving multiple listeners. On-axis, the two woofers are equal-distant from the tweeter and the listener. But as you move off-axis horizontally, the relative distance of the woofers to the listener changes.

Wouldn't that be applicable to any horizontal speaker, MTM and WTMW alike?

 

[Dennis Murphy]



Quote:
Originally Posted by Ron Alcasid /forum/post/18172771


Wouldn't that be applicable to any horizontal speaker, MTM and WTMW alike?

Well, if the T and M are vertically oriented, then you avoid horizontal off-axis problems with them. And if the W's are crossed low, you won't run into comb filtering problems between the woofers. There will be a dip at the W-M cross point, but it will be much less apparent than the huge midrange suckout you get with the 2 M's operating well up into shorte wave lengths.

 

[blazar]

waking up an old thread here, but how would D'Appolito speakers such as Definitive Technology DI6.5 In-wall speakers work for a height layer (auro3d configuration) of speakers around the room? These are a reasonable price and definitive tech's sound quality is never that bad from what I have owned and what I have in my ceilings.


I need a speaker in horizontal configuration due to some room constraint issues with my existing woodwork in the room. I can't have a cutout dimension any more than 8" tall and about19" wide. I have about 6" depth.


The DI6.5 LCR seem to fit perfectly but the primary issue is dispersion pattern for a height layer of speakers. I need speakers that are In-wall only for decor reasons.


Any suggestions and is my suspicion that D'Appolito MTM style configuration would be really bad for a height layer?

 

[Bill Fitzmaurice]



Quote:
Originally Posted by blazar  /t/1228503/dappolito-mtm#post_24519196


Any suggestions and is my suspicion that D'Appolito MTM style configuration would be really bad for a height layer?

There's no reason not to use them if they're placed the way MTMs are supposed to be placed, vertically. They would not work well in that application, or any application, mounted horizontally.

 

[blazar]



Quote:
Originally Posted by Bill Fitzmaurice  /t/1228503/dappolito-mtm#post_24519266


There's no reason not to use them if they're placed the way MTMs are supposed to be placed, vertically. They would not work well in that application, or any application, mounted horizontally.

Thats what i figured, they market them is an LCR but dont specifiy which direction they must be placed. Everything i read suggested horizontal is a bad idea...


I will go for a Polk 265-LS instead. It's three-way with a ring radiator tweeter and therefore it should not matter that i place it horizontal.


Unfortunately in-wall speakers cant generally be aimed much and so my off axis response is going to be affected by almost anything I buy.


My decor will defintely not allow for wall mounted speakers on swivel which I know is ideal for an Atmos arrangement.


I was debating horn loaded klipsch in-walls but I am not sure about their off-axis (probably not great) performance either when mounted high and not "aimable"

 

[craig john]



Quote:
Originally Posted by Bill Fitzmaurice  /t/1228503/dappolito-mtm#post_24519266


There's no reason not to use them if they're placed the way MTMs are supposed to be placed, vertically. They would not work well in that application, or any application, mounted horizontally.

I agree... if they're angled down towards the LP. If they're aimed straight forward, and they're mounted well above the LP, (as they would be for height channels), the listener would end up in an off-axis lobe. It's very difficult to angle down an in-wall speaker.

 

[Bill Fitzmaurice]



Quote:
Originally Posted by craig john  /t/1228503/dappolito-mtm#post_24520207


I agree... if they're angled down towards the LP.

That goes without saying, though mainly to have the listener on the tweeter axis. Way too much is made of the lobing of the midbasses, which is always shown visually with a polar plot that extends a foot or two from the baffle. At listening distances in excess of six feet or so the lobes are gone, having coalesced into a single cohesive wave. That can be seen in this applet, which shows six sources:
http://www.acoustics.salford.ac.uk/feschools/waves/diffract4.php


Lobing is a genuine concern at nearfield distances, explaining why you don't see MTM nearfield monitors.

 

[blazar]

Excellent info, thanks guys. My speakers are height's at about 8ft+ almost all the way around so maybe less of an issue.


I will try and do some aiming if possible since my speakers will be hidden by acoustic fabric.

 

[morphemes]

Lobing is a genuine concern at nearfield distances, explaining why you don't see MTM nearfield monitors.

huh, wut?
Say the midwoofers are roughly nine inchs apart & the dispersion of all drivers dictate that all the sound is going to begin to fade at roughly 10 degrees from the center axis of all your drivers (all frequencies, we're roughing it here) VERY conservative estimates, worst case scenario really... Anyway! You'd want to be, at the very least, only two feet away from this particular MTM. That is, if you sit very... very still. So, lets assume your head moves up and down within about a six inch diameter... then you'll want to sit... at least... oh about three and a half feet away... The back of your chair is great for determining where your head is on the vertical axis...

Maybe time could be an issue, but if sound moves at 300 meters per second & your head wobbles around inside a sphere with a diameter of 1 meter... for it to have any real world consequences to what your hearing... you'd have to be moving your head so fast... Like... you'd die! or at least spend the rest of your life in a wheel chair.

anyway, yeah... the main reason you don't see many nearfield MTM monitors... is because you simply don't need them. They're overkill for a single seated listener & I suppose, yes, you do need to sit a bit farther away, but it's really negligible. Oh, and how could I forget! When it comes to studio monitors, majority rules! The simple fact is, more people have plain jane TM loudspeakers & studios want to target as many people as possible, so yeah. There's that too.

 

[bluewizard]

From what I've been lead to believe, there are ways to minimize this Lobing effect. The Lobing is usually frequency related. So you simply make sure the problem drivers are not operating at the problem frequency.

This can be done with a 2.5-way or 3-way design. In a 2.5-way, the second bass driver comes in at a frequency below the problem frequency.

The same in a 3-way design. In the problem frequency range, only a single midrange driver is in operation. Below the problem frequency both bass drivers are in action.

From what I've been lead to believe, the problem frequency can be predicted by the distance between the two horizontal drivers. If those drivers are not operating at the problem frequency ... then no problem.

Steve/bluewizard

 

[Bill Fitzmaurice]

huh, wut?
Say the midwoofers are roughly nine inchs apart

If the CTC is nine inches, which is one wavelength at 1.5kHz, then you will need to crossover no higher than 1.5kHz to have no lobing. The size of the tweeter required to allow the midbasses to have nine inch CTC makes it difficult, if not impossible, to cross over that low. Realistically a crossover of at least 2.5kHz will be necessary, and crossed that high the midbasses will exhibit quite a bit of lobing. As I already explained the lobing will disappear with sufficient distance to the LP, but it would be highly unusual for that to occur within two feet. As to "for it to have any real world consequences to what your hearing... you'd have to be moving your head so fast.." this is a studio monitor, where you don't want off-axis response to have major deviations on either the horizontal or vertical plane at the typical listening distance. A LP vertical axis change of a couple of inches at close range would result in just that.

BTW, my 'source' is Joe D'Appolito. We had a few conversations when we were both contributing editors at Speaker Builder magazine.