line array and MTM in theory

[holger honda]

Ok I know i asking for quite a lot but could someone clarirfy to me
why the center channel have a W-t-W configuration and
W
a classic D'Appolito have a t configuration and they booth
W
are supposed to have good dispertion horisontally?
High quality PA systems have line arrays that looks like:
WtW WtW
WtW WtW
WtW WtW
WtW WtW
are also boasting this feature but now Mackie comes out with a speaker that looks like this: WW and is supposed to have
t
WW
great dispertion horisontally.
What is the cosensus here? Is woofers on the sides of the tweeter or woofers on top and bottom of the tweeter the better in terms of dispertion in the higher harmonics?
Thanks for taking your time.

[holger honda]

Seems like the coding made my little illustrations all
scrambled.
Center channel =wtw
W
t
W
= D'Appolito
WW
..t..
WW
= Mackies new design.
Maybe that makes sense...or not.

[HeatMiser]

Generally speaking a sideways M-T-M center channel does NOT have the best horizontal dispersion, manufacturers make them that way mostly because they fit under a television and look nice. In fact one of the goals of a vertical D'Appolito alignment is to minimize reflections off the floor and ceiling by reducing vertical dispersion, so you can imagine turning one on its side will do the same thing in the horizontal.

Pro sound speakers like the Mackie you mentioned have different design goals, and if they mention wide dispersion in the ad copy you should read that in the context that pro speakers are usually intended to throw the sound a long way in large venues, so their dispersion characteristics will still tend to be narrow by home audio standards.

[owdi]

MTM center channels do not have good horizontal dispersion, but they look better laying on their sides. Check out the MTM project at www.zaphaudio.com to see an example of the vertical dispersion problems of an MTM. Basicly, you need a low crossover point to avoid deep nulls in the off axis response.

A WMTW configuration, with the tweeter above the mid, can avoid these problems with a low mid to woofer crossover point.

A ribbon seems like a good solution if you want wide horizontal dispersion, but you need one that can handle a low crossover point (about 1500hz)

Dan

[Jim Griffin]

MTM (I'm talking about those with a single M driver above and below the tweeter) have essentially the same horizontal axis dispersion as the individual drivers in the arrangement. The vertical axis dispersion is impacted by the MTM configuration and essentially can have nulls in their vertical radiation pattern. that is why some listeners can hear the diminished sound as they move forward toward the speakers. The compromise in dispersion is in the vertical plane. You can mitigate the vertical axis dispersion issue by crossing over to the tweeter at a low frequency (roughly a wavelength spacing betwen the M's which places significant requirements on the tweeter's ability to work to a lower frequency than normal).

Now if you place the MTM on its side (as done by many home theater center speakers) you move the vertical axis issue to the horizontal axis for the listeners. Again, you'll have some listeners who will not hear decent sound if they sit off axis in the horizontal plane around the room. Again you can alleviate this issue by crossovering very low which demands an excellent tweeter. Often some on the side MTM center speakers are really a MT which have some bass reinforcement from a passive radiator that looks like the active midrange .

On the line arrays composed of MTM sections stacked on each other likely have a low crossover frequency which lessens any change for horizontal dispersion issues. The McIntosh recent zillion driver line array uses a low crossover frequency to achieve this.