There are some common misconceptions about a true D´Appolito
configuration. There are some criteria to be meet as MTM drivers can´t be spaced wide apart, crossover slope should be third order etc. Crossover frequency between midrange drivers and tweeter must be low enough if I recall right.
http://en.wikipedia.org/wiki/Joseph_D'Appolito
configuration. There are some criteria to be meet as MTM drivers can´t be spaced wide apart, crossover slope should be third order etc. Crossover frequency between midrange drivers and tweeter must be low enough if I recall right.
http://en.wikipedia.org/wiki/Joseph_D'Appolito
sumsound said:
I think its generally accepted that this should be close to the case regardless of driver alignment. None the less, yes he recommends keeping it to 1/4 wave length center to center. I.e. 1/4 wavelength between MT and TM, but not MM, as that would be roughly half the wavelength instead. However not edge to edge, it is center to center. In fact, the disadvantage of a vertical array with multiple drivers covering the same range is that there acoustic center is half way between the two, much farther away, rather than closer. This would mean that for a normal TMM design, the crossover point would have to be very low to have good power response. The other solution would be a 2.5 way design, which I think is more common with that setup anyway.
The more I study and play with this the less I understand the argument against MTM. It's main disadvantage is that the "tradeoff" made when controlling vertical dispersion is that the vertical listening window is decreased. However, for most of us, that is the least important of the listening windows, and really the one that needs the most control.
Correct me if I'm wrong here, but I believe I have read or been told before that two drivers placed inline with each other will have an acoustic center that is roughly half way between the two drivers. Wouldn't that mean in an MTM the acoustic center would be at the tweeter. This would seem like a very ideal situation in trying to create a point source.
SY said:
Yeah, 1st order x-over D'A MTM may not be a spectacular idea. SY hit it on the spot.
4th order is preferred, 3rd is possible.
My guess is that imaging and sound stage largely will depend on the room placement and room acoustics. My personal impression from MTMs that I've build is that they are superior to 2-ways in both, imaging and sound stage. But it could be the rooms that I listened in.
I have all the drivers to build the Swan system featured in Speaker Builder some years back, but I can't bring myself to saw off the mount of the Dynaudio tweeters like the plan says. They weren't too expensive back then, but I think they go for a pretty penny now a days. I'm afraid if I mount the MTM drivers further apart, the performance won't be as good. OTOH, they don't make any sound at all in their cardboard boxes. 
sumsound said:
Thanks, but I never said that I had decided to build such a thing, I asked it the concept of a conventional 4-way with double the drivers in various parallel / layout configurations was feasible. I never heard of a "Di Apolito" speaker prior to posting yesterday.
As for how far the crossover slope requirements for such a design, either 2, 3 or 4 way can be extrapolated from the relatively straightforward requirements of a conventional 2, 3 or 4 way speaker, I have no idea.
Could anyone recommend a good, comprehensive text on designing Di Apolito speakers that I could get stuck into after covering the basics?
Cheers,
Glen
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