Originally Posted by fbov
Specs vs reality... both drivers are conservatively rated in this regard. PE's telling folks that Ultimax rating of 19mm is purely mechianical - gap and coil geometry. Klippel testing gives -15% BL points around 22mm and Xmech is 30mm, as I recall. I was trying to show a Hoffman's Iron Law trade-off, so I may have given the Umax extra leaway...
DIYSound's flatpacks are "for" the driver cutout size. There's no reason the 4 cu ft box can't have a 15" driver cut-out, but he may not have it pre-cut. Erich's (best I can tell) a landscaper with an interest in audio who's been willing to act as an intermediary between a CNC shop and us audio nuts (and doing the same for horn/waveguide parts). You can always ask...
The reason has to do with "alignment," a fancy term for what kind of response the driver has, and power-vs-excursion.
"Alignment" of a sealed box depends on "Qtc" of the box-driver system. Q values relate to frequency response:
- Q=0.5 has great LF extension at the price of poor xmax control at the low end
- Q=0.7 has a very flat response and good Xmax control - our goal.
- Q=1.0 has a peaked response and reduced LF extension
Qtc depends on 1) the driver (specifically Qts - s for speaker), 2) the box size, through driver compliance (Vas), and 3) the density of fill or stuffing inside the box.
The Umax 15 driver sees a Qtc of ~0.7 when mounted in a sealed box ...
at 7.5 cu ft and no fill
at 4.5 cu ft and heavy fill
As we go smaller than 4.5 cu ft,
... at 4 cu ft and heavy fill, Qtc is 0.715 and F10 is 18Hz., and excursion at 400W is 19mm.
... at 3 cu ft and heavy fill, Qtc is 0.767 and F10 is 20Hz, and excursion at 400W is only 15.5mm, and the "peak" in frequency response is 0.1dB.
... at 1.25 cu ft and heavy fill, Qtc is 0.99, F10 is 26Hz, and there's a 1.25dB peak at 70Hz.
As you can see, it's a continuum; no sudden changes as Qtc moves, just regions that have differing response. The difference between 3 and 4 cu ft is peanuts compared with this driver's overall performance in either box. One just fits a little better...