New Linkwitz "LX521" speakers..

I have tried a similar setup like the LX521 about 5 years years ago (freakingly similar in fact)
Not a surprise, actually . . . once one chooses to go narrow baffle 4-way physics defines the speaker for you. The baffle defines the polar pattern and pretty much where the crossover points are going to be, you pick drivers that fit the baffle and cover the needed range, miniDSP builds the crossover (or you can do it the old analog way), and everything else is essentially cosmetic.

There are no big surprises in the LX521 design, and only a couple minor ones . . . the specific baffle shape suggests some attention to the vertical polar that is too often ignored in other designs, and the treatment of the two custom midrange drivers in particular, but that's about it. There's going to be the usual stupid squabbling about "this driver is better than that driver", but as a class the speakers are going to all sound pretty much the same, and look pretty much the same too. It's a mature design . . .

ORION was a surprisingly successful attempt to pull it off as a 3-way, and it came amazingly close, considering the difficulty and how long ago it was done. The analog active crossover was a tour de force at the time . . . now miniDSP makes it easy for anyone to do as well, or better, and for a 4-way at that. There used to be only a few suitable drivers, now there are many to choose from. Good amplifiers used to be expensive, now anything over a dollar a Watt is money down the drain.

These are good times for building good loudspeakers . . .
 
It depends, I suppose, on how loud you listen. A 1" dome will deliver 100dB at one Meter at 1500Hz. at .5mm xmax (1mm peak to peak). If you're crossing it LR4 that means you can get 105 dB out of the system. How that will sound will depend on lots of things, including the distortion the tweeter is producing at xmax, but it does set a clear limit at which obvious distress is likely to be heard.

Most acoustic (orchestra etc.) sources are unlikely to be producing their maximum output at that frequency, so the demand from a "105dB source" is likely to be a lot less, but trumpets, trombones, saxaphones and maybe even a couple flutes might push it if you're looking for front row levels (but not at normal levels for mid-house seating). For electronic or amplified music, or if you like things "louder than life", then it's certainly possible to overdrive a tweeter crossed that low.

A 4" driver (2.6" diameter cone) with the same xmax will give about 117 dB at the same frequency, and close to 130 dB with a not-hard-to-find 2mm xmax, so if you like it loud that's clearly the way to go (if the rest of the system can keep up).


It doesn't have anything to do with playing loud, it's all about an impedance mismatch between the small dome and the air it's pushing even at whispering levels.
 
+1 :)

How about using the small midrange driver in the high freq. range, eliminating the need for tweeters, similar as in SL's Plutos?

Could cross a little lower and it would have an open back.
Better or vertical polar, it will have narrower dispersion but if it's good enough for the Pluto......

It's not that simple. At the back, it won't go much higher than 2-3 kHz due to magnet&stuff. You'll have to fill in the rest anyway. That's no problem for Pluto, where the small driver is not supposed to be a dipole, but it definitely is a problem if you're after dipole polar response / front to back symmetry.

However, IMHO derived from experiments, dipole radiation and back-to-front symmetry up to more than 5-6 kHz is not really worth the effort and I found it actually detrimental if you cannot position your dipoles really far off the front wall (I cannot more than 0.9m due to space issues). It looks like an early, high frequency reflection off a highly reflective wall does a lot more harm than good. You can try to absorb it or... just roll it off at the source (gradually).

Below 5-6 khz is another story and that's where you do need uniform polar response, otherwise the tonal balance of the speakers is wrong.
 
It doesn't have anything to do with playing loud, it's all about an impedance mismatch between the small dome and the air it's pushing even at whispering levels.

Well, I always thought the same.. Even if the millenium is a damn good tweeter, 1.4khz is just too low, there still is a lot of energy in that band, especially on electronic music, and orchestras crescendo. The cleanliness of the RD-75 in comparison was rather shocking..I do listen fairly "loud", a crescendo has to be a crescendo, otherwise I can't see the point..
 
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I like to absorb the rear energy, especially since my speakers (B&G RD-75) are dipole all the way to 20 kHz, the front and rear radiation is almost identical across the 200-20 kHz frequency range. I agree that a strong front wall reflection with early arrival is not desirable.
 
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It's not that simple. At the back, it won't go much higher than 2-3 kHz due to magnet&stuff. You'll have to fill in the rest anyway. That's no problem for Pluto, where the small driver is not supposed to be a dipole, but it definitely is a problem if you're after dipole polar response / front to back symmetry.

However, IMHO derived from experiments, dipole radiation and back-to-front symmetry up to more than 5-6 kHz is not really worth the effort and I found it actually detrimental if you cannot position your dipoles really far off the front wall (I cannot more than 0.9m due to space issues). It looks like an early, high frequency reflection off a highly reflective wall does a lot more harm than good. You can try to absorb it or... just roll it off at the source (gradually).

Below 5-6 khz is another story and that's where you do need uniform polar response, otherwise the tonal balance of the speakers is wrong.

I don't believe that high frequencies will be that attenuated at the rear. 3kHz?
I've measured much higher than that.
 
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No, there is only one primary reflection from the front wall. Where will the other one come from? All other reflections will be secondary, that is they hit two surfaces.

The side wall primary reflections will be two for each speaker of course, one from each wall.

An ETC in an un-damped room shows he primary reflections as tall peaks, while the rest is far lower in level, so its easy to "spot" the primary reflections.
 
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The nearest side wall reflection from a dipole can be reduced to almost zero if the dipole is angled so that the reflection point is 180 degrees off-axis from the speaker. The same goes for the front wall reflection. This requires a 45 degree toe-in for the dipoles.
 
Well, it all depends on speaker position and room dimensions. There can be early and strong 2nd order reflections. But that wasn't my point.
In your case you were specifically talking about the reflection point between the speakers? I'd agree. But I also found strong contralateral reflections detrimental. Strong toe-in can create these.