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In Reply to: RE: Should we send a "representative" to Magnepan? posted by Satie on October 16, 2011 at 17:04:48
I'm thinking that the woofer panels would be too far apart, we're getting to about a wavelength separation a bit above the crossover point, at about 500 Hz. So you're going to get interference between the woofers which will cause lobing error. This is a known issue with D'Apollito-type configurations:http://www.birotechnology.com/articles/VSTWLA.html
Also, the midrange driver will be located asymmetrically wrt to the woofers. I assume you'd have everything equidistant from the listener, but I imagine that will make the lobing somewhat asymmetrical.
The question is, how audible would this be? It would really depend I think on the first and second backwave reflections more than anything else, and that would depend on placement and room geometry as well as speaker geometry and crossover slopes, plus the characteristics of the drivers themselves. So I'm not sure what it would do to the sound, I think you'd have to model it or try it to know. You might be able to tame the effect if necessary with diffusion at the reflection points.
Edits: 10/16/11Follow Ups:
I have the XO at <300 hz so I calculated it as a marginal effect, besides, despite the lobing the d'appolito design works very well.
There should not be that big a deal on the mid's offset - and if it is, you can widen the placement to return it to symmetry.
There were a few flat and centered mods to Tympani posted here in the early 2000's and I got the impression that it worked fine without any problems that were not common to the stock speaker as well.
Well, the standard D'Apollito implementation does space the drivers as tightly as possible. They also usually use the 4th order L-R, which has a pretty broad central lobe. And the axial lobes are symmetrical and vertical.
Of course, you have lobing with the standard Tympani configuration, too, and it's going to have axis tilt. But didn't you say that you had trouble crossing the ribbon high up because of the separation between the ribbon and mid? With a crossover of 300 you're going to have significant output at frequencies at which separation is greater than wavelength.
Didn't know people had tried a centered configuration. My main question was always about low frequency acoustical modulation of the ribbon tweeter and whether it wouldn't cause Doppler distortion or even failure. I don't think you could just take the mid and tweeter, put them next to one another, and remove all the space, although it would be great if you could. But I'm just speculating here, there has to be a reason why they separate the ribbon tweeters as they do -- from the perspective of blending and crossover design, you'd want them to be as close together as possible.
Yes, the high freq is a problem because of the small wavelength at 9-10khz and the narrow beam from the mids at that area. the combo provides comb filtering as you move your head - true head in a vice. Only works at LR4 - but that kills transients and adds gain stages to the XO.
Essentially, the difference in ear to driver distances should not excede the XO wavelength so that a normal head movement of +-5" should result in a change of less than 1/2 wavelength and preferably less than 1/4 in the difference of ear to driver distance from one extreme to the other - in the tympani that would be at a 10 ft listening distance roughly 10" * 8"/120" = 0.7" which is 13500/(0.7*2) about 10khz for the 1/2 wavelength criterion - ended up too close for me, and 13500/(0.7*4) = 5khz for the 1/4 wave criterion - where you can not go from positive to negative reinforcement by moving your head in a 10" range - that did work in my setup.
Taking Fc on mid LP to 4-5 khz while keeping Fc for ribbon HP at 9+ khz - XO about 6khz solved the problem because head movements have much less of an effect with the much broader mid beam in that freq range to 6khz.
I think the holes in the Tympani around the tweeter are supposed to prevent both bass induced damage and reduce doppler effects since there should be cancellation at that point so less (if not insignificant) of a doppler effect from the bass wave.
Josh, get's my vote to rep for the forum. I would definitely give more weight to his opinion than professional reviewers. Who have various conflicts of interest.
I would like him to report on the 3.7's, the Tri-Center concept, the Mini Maggies, CCR and center channel woofer, and anything else Magnepan can come up with.
Josh, I am curious does Magnepan have any conventional speakers on site to compare against. If so what brands and models?
That's an interesting question.
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