Rebuilding the Enigma...

The only thing that sticks out at me is perhaps the sweep length (unfortunately I didn't record it, but I believe that I used 512K long test signals - REW suggests using 1M long test signals as shorter ones apparently result in less accurate harmonic information in the resulting distortion graph).

I don't have the Enigma on-hand to repeat the distortion tests on it. However, I do have on-hand a 4th order BP subwoofer that I built many years ago (see The Subwoofer DIY Page v1.1 - El Uglito for more details. Note however that it now uses different drivers, so FR and distortion measurements done back then don't apply), so I did some distortion testing on that subwoofer to see if the signal length made a difference. The results suggests that the signal length makes little difference at bass frequencies, at least when set to 512K or 1M. If anything, distortion measurements using a 512K signal seem to be a little higher than distortion measurements using a 1M signal.

Note: cursor set to 34.9 Hz, mic about 3" from the subwoofer's vent (to minimize impact of the room on the measured response).

Test signals:
1. 10-20K, 512K
2. 10-20K, 1M
3. 10-2K, 512K
4. 10-2K, 1M

Attached:
1. Measured FR using test signals indicated above. Basically identical at bass frequencies.
2. Distortion measurement using 1st test signal
3. Distortion measurement using 2nd test signal
4. Distortion measurement using 3rd test signal
5. Distortion measurement using 4th test signal
 

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Just a quick update to this ongoing saga:

I installed "Enigma v2" in my brother's bar just over a week ago, where it sees somewhere between 8~12 hours of continuous play every day.

...and 6 months later, the Enigma v2 is still going strong :).

I might pull it later this year, to perform more detailed checks on it, but the only thing that it seems to be suffering from now is a bit of wear and tear on the front of the box (due to its location in the bar).

I wish there was a way though to find out if my dual-stage protection circuit was actually triggered in use. I'm hardly at the bar, so I don't know if they've managed to crank the volume up higher than recommended at any time. Maybe the impedance of the PTCs would measure a little differently if they were triggered.

I'll find out in five months :).
 
Silly question; Do you position this subwoofer so that the ports face the listening area? I ask because I have a Paradigm PS1200a (4th order bandpass design) and, at least cosmetically, it seems that the port openings were intended to face away from the listening area. I always wondered if it acoustically made more sense to have the ports face into the listening area.