I did some messing around last night with the 24" cube (6ft) XXX and ran into some interesting things. First if I push it hard it will start to rock the whole cabinet, just like the LMS did at the GTG. Second these drivers are a real PITA to remove from the cabinet once they are in there. I smashed another finger and lost the first couple of layers of skin due to the driver getting wedged.
. I took the driver out so that I could stuff some pillows in there and smooth things out a bit. I'll post up the data from the WT3 impedence traces and close mic FR measurements for before and after stuffing the enclosure. One thing I am concerned about in an enclosure this small with stuffing the 5 pillows in there is heat build up when pushing it hard for extended periods. In this size of a box the Q is getting up close to 1.0 with this driver, so it's a bit peaky at 30hz. The pillows seemed to help flatten it out a bit. Still a larger enclosure volume would help out a lot there.
It seems like the driver in the smaller box distorts earlier than it should. Once the excursion gets up around 2.5" p2p things start to sound different. In the larger 10ft box, or the 25 footer it does not do this. I free air all of my drivers and note the point at which they start to "lose it" and this one is the same as the rest of the XXX's which is utterly clean mechanically until you start approaching the outer limits. I don't think it's related to the power either. The CE4000 is not clipping at all and it is not happening at the higher bass freq's where the power related issues should be worse. Neither the amplifier or the driver are out of their operational envelope at the point it starts happening. It's only once the excursion starts to get high. I've not noticed it with actual material either. Just sine waves or sweeps. Could this be the dreaded ASD kicking in, or perhaps it's related to an air leak? Something else? I'll be dragging my 10ft test box out there for a heads up test between the 2.
One other thing that I figured out last night is that somewhere around 120db the ECM8000 is overloaded and it probably starts distorting well before then.
At some point it quits increasing the signal it puts out and compresses like all get out. I figured this out doing close mic stuff. If you put the mic 3" from the center of your 18" subwoofer cone it doesn't take much at all to hit 120db there. I was trying to set all of my gains ( around 7 total) and was running into huge compression somewhere in the chain. After checking nearly everything else I figured out it was the mic. It also seems to vary somewhat with the frequency range. I'm going to buy another ECM8000 that's professionally calibrated and see what happens. Maybe mine is defective. This also makes me wonder about how much of the compression I encountered during my sweep tests was actually the subwoofer and what was the microphone's contribution. I'm starting to think that the mic had a significant impact on the highest sweep levels, since I should've easily heard that much, but didn't. I don't know how much it may have affected the rough THD measurements too. Someone had brought this up as probable before. I think it was Noah Katz. I just assumed that it wouldn't be more than a very tiny effect compared to the rest of the system. Maybe it's time to look into something a little better in quality.