FR striving for Coax Performance: Trends of Last Decade

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Compared with what was commonly available 10-15 years ago in small (3"-4") wide-band drivers, there's been significant performance gains in that size range, particularly in terms of extending the highs without adding a dedicated tweeter. Nevertheless, the very upper range is often still best addressed with a separate HF unit. The need for them is not quite as strong with the improved top end response in many small drivers available, but it hasn't obviated the need for a tweeter altogether. Smaller tweeter designs which don't begin to take on duty at common crossover points, and instead are employed at even higher crossover points, commensurate with the rise mid-bass sized drivers' capabilities now, would seem to me to have a role.

This, however, seems to have been declined in favour of producing mid-bass drivers that attempt to cover a range up to 20khz (see for example the dozens of Tangband drivers of this size which suggest this, and countless othes). Nevertheless, if a more airy, precise and detailed top end be achieved with an uncommon tweeter design utilised solely at the top end (and perhaps its more limited duty also allows for a more compact profile as well), then why isn't this to be found, i.e. why do we not see more 1/2 and 3/4 inch HF units coupling with the ever more capable small mid-bass drivers that have pretensions at covering the frequency spectrum at the top?

Your input and comments are appreciated. Thank you.
As logical as a coax may seem, I am inclined to believe the problems of doppler distortion just makes it harder than we would hope. Basically, a tweeter does not want it's lens moving. I had a set of Kef Q1's for a while. They were worse than the sum of their parts.

Next is marketing. The better of these driver are not cheap. To add a tiny neo magnet tweeter of comparable quality in the middle would be a real trick. The crossover can cost more than the speakers, and in a coax, the customer would expect plug and play. Of curse, if you don't know the baffle, you can't design a crossover.

I have been watching the 3 to 4 inch "fullrange" trend hoping we could get a decent mid out of it, but no, they only give the perception of high frequencies. These things just accentuate the breakup modes to fill the upper octaves with something other than music so you think they have treble. When you push the ability to get down to 120 or so, you will completely loose the ability to get above 4K or so cleanly.It's that little thing called physics. It turns out the old "decade rule of thumb" still holds true. TB could make some killer mid-ranges if they would only try. The market right now does not ask for mids, as the compact 2-way with one inch tweeter does well enough for most.

That's the way I see it.
Would the lens issue and doppler distortion in a coax be less problematic in one that utilises a flatcone for the mid-bass around the HF unit?

The disadvantage points on cost and marketing are quite clear.

Concerning the x-over, with cost considerations left aside for a moment, wouldn't powered speaker designs with active crossovers be capable of fully tailoring the x-over to each specific application using such a coax driver? Such optimal customization with that approach can also include baffle step compensation.
Second one is the easy one. Yes. However, back to marketing. How many successful powered speakers are out there? In theory, it is a far better systems approach as you point out, but packaging and cost makes it impractical. Look how physically large a decent amp is. Can you picture building in a Parasound 1200 into a 20L monitor? People will spend $1000 on an amp, and $1000 on a pair of speakers, but run from a $1800 set of powered speakers as they are too expensive. Humans are not always too smart. Almost all speakers are plugged into AVR's that have the amps built in and are not expecting to do the crossover work and power independent drivers. ( Oh how I WISH I could access all 9 amps in my Denon).

First question. Well, flat-cones have so far mostly not worked very well. Those horribly expensive ceramic ones are getting darn good though. So, flat means no lens effect and you lose the efficiency gain with the attendant lower distortion. Instead it is the effect of the baffle modulating. At any frequency below when the driver starts beaming, I would think it would still cause the same basic problem. Another problem I can think of, qualifying this as not being a driver designer, is the change in the interface between tweeter and mid. This is something the good Dr. Earl comments on in his designs addressing the diaphragm to horn transition better. Having it dynamic can't be good. In short, interesting idea. It would change the problem a bit, but I am not sure it would be any more manageable.

Hasn't someone made some sort of pancake concentric ring radiator? Anyone know about it or how well it works? Thinking further, that is exactly what Peter Walker did. Can't argue with the midrange of a Quad. Think I'll sleep on that and see what other ideas come up across the world.
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