Differences between small vs. large mid driver

El, 600Hz? That's over an octave above "middle" C on the piano! Most mids that cross that high have coherence problems, especially obvious with the human voice. And asking a "big" woofer (8+) to work cleanly at least an octave past there is pretty risky too....
I'm a fan of running a mid "naked", crossing acoustically at
150-200 Hz. Does rule out very small ones, of course. 5.25-6.5 is the generalized best starting point. Cone breakup issues at the top of the passband are minimiwed by use of outstanding cone design/material and a proper motor. Great midrange drivers are usually not cheap, although it's clear that many of them can be made to work pretty well at least for a portion of the 200-4k band (what I call the mids: an octave above the top of the piano).

Eldartford, you are correct in the classic definition of a midrange driver - your frequency range excepted. However, you must allow that a lot of great two way speakers, such as the Merlins, have fabulous midrange, despite "missing a driver" as you say. A midrange should not go down to 600 Hz, but much below that, I would say it should do 100 Hz, as the crossover between midrange and woofer is ideally around 200 - 400 Hz, and there will be a lot of output below the crossover frequency as a matter of course.

Your assertion that the best 7" drivers develop ripples starting at 1500 puts up a red flag in this thread that many a 4" driver also shows. In my experience, the 18 cm (7"+) ScanSpeak drivers are fine out to 3000 Hz. I personally would set the crossover point a bit under that, closer to the 2500 Hz area, but being a big believer in first order crossovers, there is going to still be a heck of a lot of output at 3000 Hz and more.

My favorite midrange drivers are 6.5", which we're using in the new Frieds. Again, my Coincident Digital Masters are some of the finest speakers I have come across (which is why I own them - they are a lifetime purchase), and they use an 8" midwoofer, crossed over in the 2250 Hz neighborhood. As they were also intended to be used on their own, without the matching Troubass subwoofer which they use as a stand, they go just about all the way down. After I rebuilt the Seas P21 Excels, they have some of the finest midrange I have come across.

What the above posters have said is correct, and there is some basic physics that can be used to predict a driver's characteristics. If you're not technically inclined, I apologize in advance. This is as simple as I know how to make it while still presenting the basic math involved.

When the wavelength of sound being reproduced is longer (lower in frequency) than the circumference of the moving part of the driver (the piston), the radiation pattern of a driver in free space is approximately spherical. When mounted to a baffle, the radiation pattern is about 180 degrees. As the frequency rises (wavelength gets smaller), the radiation pattern gets narrower. When the wavelength of the sound is about half of the circumference of the driver, the driver starts to "break up", indicated by ripples in the frequency response. At one third of the circumference of the driver, which just happens to be about the diameter of the driver, the response is down about 7dB at 60 degrees off axis. This represents a practical limit on the upper frequency of the driver. There are techniques that can be used to extend this range, but for the sake of discussion assume this is a conventional driver.

As a practical example, take a Scanspeak 18W/8545, a popular 7 inch midwoofer. You can view its data at http://www.d-s-t.com/scs/index.htm. From its datasheet, the pistonic area is 145 square cm. This translates into a diameter of 5.35 inches, and a circumference of 16.8 inches, or 1.4 feet. The speed of sound is approximately 1130 ft/sec, so 1.4 feet corresponds to a wavelength of 1130/1.4=807Hz. If you look at the 8545's frequency response graph you'll see a bump at around 800Hz, and a downward slope from that point upward. At 1600Hz (half the wavelength of 800Hz), the onset of cone breakup is evident. At 2400Hz, the response at 60 degrees off axis is down about 6 dB, which is very close to the predicted value.

Depending on the slope of the crossover, this driver should probably be limited to about a 2KHz maximum crossover frequency. Higher order crossovers can push the limit, while lower order crossovers should be around 1KHz or so. Since it is a rare tweeter that can be crossed over at 1Khz, a higher order crossover will have to be used if you want to make a decent sounding 2-way. Also, while you'd like to have the crossover above 3KHz to get it out of the most audible range, the breakup modes of the driver prevent you from doing that. It would sound too distorted.

If you had a driver with a 4" piston diameter (a roughly 5.25" driver), the 3x wavelength would be about 3200Hz, which would allow the crossover frequency to be moved upward. However, the bass response would suffer, and would likely require a 3-way system instead of a 2-way.

What does all of this have to do with the sound, you ask? If you only use drivers in their linear range, it is pretty much impossible to cover the 20-20KHz range with only 2 drivers. However, you can certainly do 40-20KHz, which is good enough for most people. Full range sound requires at least 3-way operation. But the more crossovers and drivers you have, the more expensive the speaker, and the harder it is to get the drivers to blend well.

If you'd like to read more about this subject, check out "High Performance Loudspeakers" by Martin Colloms.

I can smell the can of worms that just opened in this thread : )

My personal thoughts are that 100 Hz is too low for a driver expected to cover the entire upper midrange region. I also agree with Trelja's comments that optimal woofer crossovers are quite low. What to do?

Well, if you use a "filler" driver aka a "upper bass /lower midrange" driver to fill in the gap, you've now got more dispersion, phase, impedance, etc.. characteristics to blend together. On top of that, you've just made your three way a four way. More cost and far more complex. As such, it is a step forward or backwards?

As far as larger drivers covering the range above 1.5 KHz, there are a LOT of variables here. The size of the voice coil, the contour and slope of the cone used, the type, size and shape of the dust cap, the type and size of the surround, the design of the motor structure ( magnet & pole piece ), etc... All of these things will alter frequency response, dispersion characteristics, transient response, etc...

While not the "perfect" driver, take a look at this Eminence Beta 12LT. The bandwidth that they get out of this driver is VERY impressive for its' size to say the least. I'm using these as midranges in PA cabinets as they easily cover that entire pass-band with admirable results. The cost is phenomenally reasonable for what you get too. Maybe not "hi-fi" quality, but it just goes to show what proper engineering can do. Sean
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The midrange driver range that I suggested (600 to 3000) is based on what I have seen done in three way speakers. I think that Nighthawk agreed with me, and provided all the details.

If you suggest that a woofer is good to 2500Hz, what's wrong with using it to 600 Hz?