I think the best Extremis application would be a small sealed subwoofer with added equalization.

I was hoping that someone out there has heard the driver. From what I've read, it has a pretty good sound - but I've heard nothing about anyone using them in an array. I know that the inductance is .05mH - so it has potential for greatness from that standpoint.

Thinking about the benefits that an array offers - the extra excursion might not be used. But it is there if you need it (eight of them would have a maximum potential of around 126dB - what would you possibly use for a tweeter with THAT?)

On the subject of tweeter - generally regarded as 'best bang for the buck' (PT2?)? How about the 'upgrade' from there (NeoCD2.0?)?

I've used the PT2's but I like the Fountek much better. The PT2 is limited in how low it will cross as well as sensitivity.

What about sensitivity for PT2 vs. Fountek (real world) - what numbers are you talking about (is it 94db vs 98db or am I remembering incorrectly?)

The PT2 is rated at 94db but because of the contouring needed you actually lose a couple of db's. The Fountek is a true 98db and will work with a much wider variety of woofers.

A 3rd option would be a line of ND20FB (rear mount PE tweeter). C-T-C spacing would be 1.5", so comb effects would start around 9040Hz (which is close to the 'relaxed' 10k sited in your whitepaper). 40 per array would be ~$160 per array. With this driver, the xover would need to be the highest of the three options (due to fs of the tweeter). I'm not sure about the xover for the two situations above - could the PT2 be crossed lower (needing a steeper slope?) since there are eight working together (vs. a single PT2)?

How would the efficiency of 40 dome drivers increase in an array? Is it the same as mid-bass/woofers? I know that ribbon/planar drivers have little vertical dispersion, so they don't increase efficiency like dome drivers when in stacked in an array. My theory is that the drivers will have increased efficiency at all frequencies below 9040Hz in the case of the ND20FBs (wavelength = C-T-C spacing).

I've read and re-read your whitepaper, and I understand most of what it is saying. I'm just trying to get some of the hypothetical situations straight in my head before moving ahead.

I think that you mean my Near Field Line Array white paper. Rick and I have worked together in the past. He is the professional speaker builder while I'm the part-time sometimes university professor and tinkerer into line array theory.

Of your tweeter choices, the PT2 line will work but ribbon tweeters will take things to the next level. A few worries with the PT2's is to get their sensitivity to blend with the woofer line's output.

The small Neo dome tweeters will work with limitations because of their arrayed sensitivity falling as you go upward in freqeuncy. Their sound fields will provide directivity (increasing sensitivity) but as you go above a wavelength c-t-c distance, their arrayed gain will decrease so you may have to compensate for their performance.

Jim

So… which is better a true line array with PT2s, or a quasi-array with two Fountek ribbons per array (I could add more later…)?

You verified my understanding about using dome drivers – they will benefit with increased sensitivity (directivity), but only at or below a wavelength c-t-c distance. In the case of drivers with 1.5” c-t-c that would be just over 9kHz – the tweeter line would fall off above that frequency. This drop caused by comb effects, if I understand correctly.

The issue with comb effects would become an issue at a lower frequency with larger c-t-c spacing. Is it possible to predict the roll-off of the upper end based on number of drivers, c-t-c spacing, line length, etc? Is it best to just build it and measure?

I’ve seen different arrays that use multiple large flange dome tweeters spaced accordingly (4.5” for example). If you have 12 of these in an array, you’d have to mitigate the roll-off starting at around 3kHz.. One example is at zalytron.com their Axon 812 array has eight 6.5” drivers and 12 tweeters spaced 4.5” apart (there are different configurations on that site as well). I’ve been trying to understand how this system could be 98dB @ 2.83v when a single tweeter is only 90dB.

If you can add more Fountek ribbons later, I would vote for that approach vs. a full line of the PT2's. If you can not add additional tweeters, then I would suggest the PT2's.

Regarding comb line effects: I give some guidance (spacing, choice of driver type, the freqeuncy range coverage,etc.) in the white paper but the best answer is to build and measure. The white paper stuff is intented to get you in the ball park but ultimately the correct answer is to measure. From a design viewpoint you would be safest if you use a conservative critieria on spacing--like c-t-c spacing of less than a half wavelength vs. one wavelength.

Frankly, the Axon array isn't a high quality design based on the drivers used and the limited length and spacing of the tweeter line. Using either planar or ribbon tweeters would be much preferred to the large tweeters used in the Axon design. The 98 dB SPL value is clearly bogus or based on very optimistic assumptions. Maybe they have reinvented acoustical physics.

Jim

That's also what I thought on the designs like the Axon array. There have been several that I've seen lately - a few too many people jumping on the LA bandwagon, trying to make a profit. This part was more of an (in)sanity check on my part.