Size of Midrange Drivers

Johnnyb53, are you saying that all 6.5 inch drivers start beaming at 2086hz....???

It's not hard to look up Tim.

Hi Ptss,
I've been down this road before, I won't push it, but I've been in the discussion of calling a frequency peak vs reduced dispersion before. I've experimented with curved cones, phase plugs and even flat cones.... I have found some variance in dispersion characteristics. overall the formula is accurate, but I can't say it is cast in stone.
I'll leave it at that and hope that makes sense.

08-22-14: Timlub
Johnnyb53, are you saying that all 6.5 inch drivers start beaming at 2086hz....???

Actually no; it's not that simple. The significant measurement is the actual moving cone's diameter. A 6.5" driver with a large surround might have a 5" cone, but that would still beam at about 2700 Hz, somewhat shy of 3K. However, this can be offset by a 1st order slope for the tweeter crossover. If a tweeter crosses over at 3K at a 6dB/octave slope, then it's still putting out significant output at 1.5K, and would be down only 3dB at 2250Hz, which would be enough for good dispersion at the crossover point.

There are other ways to cheat the dispersion formula somewhat with phase plugs, waveguides, and whizzer cones. Still, I mentioned the formula with a link to the wavelength calculator to answer the OP's question--why have a small diameter midrange when a larger one would provide larger radiating surface? One answer is that the larger driver has the potential to create an in-room 1/2-octave wide suckout around the crossover point.

A better solution might be dual mids or dual or triple tweeters. Dual small mids would provide more radiating surface while maintaining small diameter diaphrams for better dispersion. Multiple tweeters would allow using a lower crossover point without overdriving the tweeter. This is what Tekton does in mating a 10" woofer with a 1.5" ring radiator. Some of the 10" woofers have whizzer cones with 3 tweeters instead of one on his top line models. This enables lowering the crossover point to let the tweeters cover for the woofer's beaming.

The worst case is a 2-way with a large woofer, small tweeter, and 4th order (24 dB/octave) crossover to increase power handling. It means the tweeter wouldn't be able to improve dispersion just below the crossover point because output would be too far down to help out.

Why do you think there are so many speaker manufacturers and DIYers? Dynamic speakers in boxes present balancing acts with an infinite number of possibilities in driver sizes, driver numbers, crossover points, crossover slopes, as well as cabinet size and bass alignment. Not to mention the actual individual driver characteristics.

Some of the more important reasons why we rarely see midrange drivers above, or even approach 7-8" in the typical regime of "hifi" speakers seem dictated by the use of direct radiating dome tweeters and their lack of <2kHz handling, as well as a common demand for midrange drivers to co-act as low fs bass units in 2-way systems, where 6.5" driver would see limitations in speed and agility starting already in the upper bass and lower mids. Even "dedicated," smaller midrange drivers (up to ~5") in 3-ways systems seem to emulate almost the design of the more typical hifi bass drivers, certainly going by their efficiency and overall specs, making you wonder what is gained by the smaller diaphragm diameter when this again limits the preservation of energy (and not least its nature) that comes through added, sheer displacement area. My own speakers sport a 12" bass/mid driver (crossed at ~1.3kHz to a 12" compression driver-driven OSWG waveguide) with a relatively lightweight paper cone and moderate sensitivity of about 94dB, and though this sits close to the beaming wavelength of the effective cone diameter the audible outcome is nonetheless more dynamic, natural, relaxed and uncolored than most mids I encounter from other, even much more expensive speakers with smaller, more "exotic" midrange drivers. Perhaps the best mids I've ever heard comes from 3' diameter fiberglass JMLC-horns (300 - 3.5kHz) driven by compression drivers as well - go figure.