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RE: Apogees and ethics - comments

Agree that a polymer-backed ribbon can have lower mass than an all-aluminum one. But I thought you liked the sound of the Apogees better than the BG's? Or is that the top-bottom ribbon configuration rather than the material of the ribbon? (Wendell calls everything not foil a quasi-ribbon, and I guess since he invented the term he gets to -- but I still haven't come up with an unambiguous, unclumsy way to distinguish between the various varieties).

The problem with wide spacing of drivers is that not only do they become directional, but the lobes multiply until you have cauliflower pattern -- and the nulls go all the way down to zero, and in the listening area, too (though not necessarily the sweet spot). Perhaps the nulls at the frequencies covered by the tweeter are so narrow that they aren't perceptually significant? Certainly the broad quasi-ribbon tweeters exhibit similar lobing problems, but to a lesser degree, since they aren't 2" wide.

I know of some other speakers that use rear radiators as well. The idea is to maintain a polar response that's fairly constant with respect to frequency; in a conventional box, the bass is omnidirectional, the highs are cardioid, and that reduces the sense of spaciousness. So I understand why Apogee used the rear ribbons, just not why they drove them out-of-phase with the midrange backwave, creating a suckout at the crossover point. Or rather, I suspect as I said that it had to do with their desire to avoid triggering sympathetic resonances in the midrange ribbon, had the front and rear tweeters been driven in phase it would have done that, but with the two out of phase, the forces on the midrange ribbon would cancel. I'm not sure whether Arnie Nudell ran the rear EMIT's in or out of phase with the front ones, but there were fewer of them and as I recall, he aimed for a cardioid pattern because he found it imaged better. Which would mean they were phased as bipoles, but at a lower SPL (which they likely were because there are fewer of them than there are on the front).

Hadn't thought about the fact that the tweeters could be narrower, though really, the Magnepan ribbons are 1/4" and go down to 3000 Hz, and the Apogees are four times as thick, so would the Apogee tweeters have a torsion problem? I was speculating on the flux strength, Apogee had to work with magnets of limited strength, which according to I think Graz is the reason for the low impedance of some early Apogees -- they needed the current to get reasonable efficiency. Not a problem anymore.

I don't think in an MTM arrangement you'd get a dipole null at the tweeter without some kind of relief port on either side of the tweeter, not until the out-of-phase wave had traveled around the barrier, anyway, and that doesn't really happen at the speaker itself because I assume the waves haven't diffracted fully, it seems they don't until you're a couple of meters out. If you left enough space to make dipole nulls, you'd destroy the efficiency of your midrange drivers and I think couldn't use a single magnet assembly. Plus you'd start to get lobing on the mids, you really want to keep the midrange drivers very close together so they operate as one.

With woofers I suppose you could do a W-M-T-M-W arrangement, but below 80 Hz there isn't much reason to and if you left enough room for dipole cancellation at the center you'd halve your effective baffle size.

I think you're right about wanting to keep the bass freqs away from the mids. Lobing isn't a problem at those frequencies, so you only have to deal with lateral image spread -- not ideal, but not too terrible, as those things go.


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