Why no D'Appolito?

[diesel_88]

I've read a bit about the D'Appolito/3rd order Butterworth configuration. Its "quasi-spherical" radiation pattern seems like a good thing, yet I rarely see it used in projects. How come?

[SY]

Mostly because a 4th order L-R seems to work better for that configuration. And that's what d'Appolito uses in his more recent designs.

MTMs with 4th order crossovers are quite common these days.

[diesel_88]

Originally posted by SY
Mostly because a 4th order L-R seems to work better...

Do you mean it produces a more-spherical radiation pattern, or does it have other advantages over the 3rd order Butterworth?

Also, does "4th order L-R" mean a 4th-order electrical crossover, or a network which produces a 4th order acoustic rolloff? Those terms have me a bit confused, as I see them both in project galleries.

[SY]

4th order acoustic. Sorry to be ambiguous, that sort of thing drives me crazy, too.

From what I can see, it narrows the polar pattern, reduces lobing, and is less critical of zero delay plane offset.

[Salas]

It shows worse power response vs 3rd order around crossover but it helps with the off axis M-M cancellation due to higher cut off rate.
If it ain't 90deg apart at crossover it is not D'Appolito. Its MTM. It may be preferable or not given the particular speaker though.

[diesel_88]

Sorry, need a bit of help with terms (kind of a newbie):

narrows the polar pattern

That sounds like it makes the pattern more directive rather than spreading it out evenly in all directions. Do I have that right?

reduces lobing

Produces fewer peaks and dips in the polar pattern, right?

is less critical of zero delay plane offset.

I think I lost you there. Does that mean less sensitive to the phase differences between the drivers?


Thanks!

[SY]

d88, in order:

Yes, that's exactly it, specifically in the vertical plane.

Yes, again. Some people like that sound, others don't. For me, it makes the image much "tighter" and more defined, but it does narrow the sweet spot. And as salas implied, it makes the speaker less room-critical.

"Phase" is a bit tricky. "Acoustic center" is a different way to think about it. There are some nice illustrations of this in Dickason's Loudspeaker Design Cookbook.

[LineSource]

Originally posted by diesel_88
I've read a bit about the D'Appolito/3rd order Butterworth configuration. Its "quasi-spherical" radiation pattern seems like a good thing, yet I rarely see it used in projects. How come?

John K's music and design site has some interesting data on MT and MTM polar and power response which seems to show some advantages for odd order Xover acoustic slopes for MTM.

http://www.musicanddesign.com/Power.html

"Here we make a brief comparison of several crossover when used for MTM systems. The first two figure compare a 2K Hz 3rd order Butterworth MTM and MT speaker. The MTM speaker has significantly reduced power response below the crossover frequency due to cancleation between the M drivers. The polar response, at the crossover frequency, is much smoother for the MTM, as was originally demonstrated by D'Appolito. however, we can expect symmetric dips in the polar response below the crossover frequency, again, due to the interference between the M drivers."