iNuke DSP below 20Hz
I've been running a sealed Ultimax with an inuke DSP amplifier for a couple years now. Only recently did I learn that there are ways around the limitations on the DSP filters going only down to 20 Hz. Instead of using a low shelf filter, a negative high shelf will give lower extension to the filter. You probably have a couple questions.
Q: Won't a negative filter reduce my output?
A: Yes, but you can compensate with increased gain
Q: How does a negative high shelf go lower in frequency than a low shelf filter?
A: It all comes down to which side of the filter the 20Hz knee is at. A 20Hz +6db low shelf and 20Hz -6db high shelf filter will have the same curve shape but at different frequencies. The peak of the low shelf will be at 20Hz, meaning the rise begins at a higher frequency than 20Hz until it levels out at 20Hz. The bottom of the negative high shelf filter will be at 20Hz, meaning that the overall response rises at 20Hz through lower frequencies. The strength of the filter determines how low in frequency the filter peaks.
If the DSP interface isn't frequency limited, this doesn't make a difference because you can do a low shelf at the frequency you want. Anything that is frequency limited and dealing with subwoofers, it can make a big difference. It definitely transformed the low bass from my sealed Ultimax by feeding it the boost it needs.
My one caution when using this with subwoofers is to slowly make changes, measure, and then test with some low frequency content. It's amazing how fast excursion can increase even on a sealed subwoofer once you get below 20 Hz. 16-18 Hz may not seem like much, but it's similar to the changes in risking blowing out a tweeter changing the crossover from 1.8k to 1.6k. I was playing around with settings and testing it with content and decided to back it off when I suddenly saw huge excursion that scared me. It happened too fast to measure exactly where it was in the large limits of an Ultimax, so I don't know if I was close to the limits. It certainly scared me enough to back off by about 1db on the filter. That was enough to significantly reduce the excursion.