Kosst, thank you for your reply. You are absolutely correct that I was not clear about what I meant by "good radiation pattern control".
First, I like for the pattern to be uniform over as much of the spectrum as is reasonably feasible, at least in the horizontal plane. Second, I would like for the pattern to be fairly narrow - say, 90 degrees wide (45 degrees on either side of the centerline) over as much of the spectrum as is reasonably feasible, at least in the horizontal plane. These characteristics tend to do two things: Give us a spectrally-correct reverberant field, and minimize detrimental early reflections. They also give us a very wide sweet spot with proper set up.
The midwoofer diameter usually sets the lower limit on radiation pattern control in the horizontal plane. Ideally we’d like to have the above-described "good pattern control" down to about 700 Hz, but that’s usually not practical. With a 12" midwoofer we can get down to about 1.4 kHz, which is definitely low enough to be a worthwhile improvement in my experience.
The idea behind all of this is to minimize detrimental room interactions by not causing them in the first place, and then to encourage beneficial room interactions. I happen to think this is something that matters a lot, and obviously most designers give higher priority to other considerations.
There are plenty of prosound drivers in the 10-15 inch range that perform well at these higher crossover points, but in turn they usually do not go down as low as most home audio woofers. So there is some compromise involved (in addition to the large enclosures required).
Some of these prosound woofers have well-behaved cone breakup, and then their accordion surrounds are generally superior to half-roll rubber surrounds from a damping standpoint. The downside is the stiffer accordion surrounds impose a higher resonant frequency.
The detriment from cone breakup is twofold: Peaks in the frequency response, and a slight smearing of arrival times from the surface of the cone. In many cases the frequency response peaks can be tamed via the crossover, leaving only a small arrival-time smear. A single small ultra-rigid cone is theoretically superior in this regard, of course. But TWO or more small ultra-rigid cones will often have WORSE arrival-time smear than a single large cone! So in this area an array of very expensive small cones is not necessarily superior to one big cone.
Instinctively people expect big woofers to be "slow". The 12" woofer I mentioned a couple of posts up has a motor-strength-to-moving-mass ratio almost TWICE that of the famously "fast" 5" Scan-Speak Revelator. So these prosound woofers give up nothing in that regard.
As a ballpark rule of thumb, a driver often has around 1 dB of thermal compression (or more precisely thermal modulation) at about 1/10th its RMS rated power. I like to have enough thermal headroom that the peaks are never going to be rounded off. The 12" woofer in my example above has about 1 dB of thermal compression at 112 dB (at 1 meter), which translates to somewhere around 100 dB peaks at the listening position once we factor in the other speaker + room reflections. This is 10 dB more thermal headroom than three 7" Scan-Speak Revelators. But the three 7" Revelators will go over half an octave deeper, so that’s something we trade off with the prosound woofers.
I guess it boils down to what problems one thinks are most in need of solving. I readily admit to being in the minority among speaker designers in that area.
Duke
