All I Can Say is... Wow!
The concept: use several midrange drivers ringed around a very efficient tweeter to create a high sensitivity point source. The mids / tweeter act like a dual concentric driver (coax) without the main problem that come from loading a tweeter into a moving horn - doppler distortion.
I tried 4 mids to start (the first set of measurements) and found that the tweeter was still a good 8db more sensitive. I could pad it down, but decided to try adding two more mids and that raised the sensitivity of the mids by another 3db. The second set of measurements show the on and off axis response.
The results are quite good and certainly better than I expected. With the crossover set to around 1400Hz, there's no significant amount of comb filtering. and the off-axis response is excellent.
Of course this is just a very roughly done test to determine whether it's worth pursuing, and what I've seen so far pretty much guarantees that.
The drivers I used are the Hi-Vi B3N 90mm fullrange and the Peerless BC25SC06-04 horn loaded tweeter. Both drivers are very low cost but my initial impressions are that they sound very good.
Is it worth doing?
6 of the B3N drivers cost less than a good quality midrange driver, but they require a much wider baffle to fit into. The most common thinking these days is that a wide baffle destroys imaging and soundstage, but I haven't found that to be a written in stone rule. I've made many different speakers of many different sizes and I can't positively say with certainty whether narrow speakers produce better a better defined soundstage. It's been my experience that speaker position plays a MUCH greater roll in how well a pair of speakers image. Also the room is a big factor, especially an excess of early reflections. And this is also at odds with the standard thinking on this today, with most saying you NEED the early reflections to produce a good soundstage.
Since there are 6 midranges to install, the build is more complex. But I think that's offset at least a little bit by how interesting it looks. Looks count!
I'll admit that I'm disappointed that I'm not getting a higher db output from 6 of these drivers (at the same power, each additional driver should add 3db). I thought that 4 would have come closer to the 96db output of the tweeter than they did. Some of that is from the mids operating on an open baffle and the efficiency should increase when they are enclosed in the speaker cabinet.
Also, you can think of it this way: instead of one motor (magnet / voice coil) for the midrange frequencies, now there are several, which greatly increases power handling capacity and reduced the distortion associated with power compression. Also a much greater cone area translates to less cone movement to produce the same output. In other words, you get the advantages of a large speaker without the expense and other drawbacks.
The mids are wired 3 in parallel x 2, and then the triplets wired in series for a total nominal impedance of around 5.3 ohms.
The woofer I'll use for these speakers is an 8" Seas that has a 93db efficiency.
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