How does the ipal system work?

I've already shared some information and experience about this topic.

IPAL drivers have a lot of linear xmax and motor force and will outperform almost all other drivers in therms of spl/size.

That said, the combination of a very strong driver with low qts and a class d amp as the ipal mod with pressure sensor sounds very powerful, controlled and tight, but also has a steeper rolloff below tuning frequency and is missing the typical bassreflex "resonating" sound because of class d and especially because of the pressure sensor. 

It's like you can't have all from just one enclosure (controlled clean sound, strong punch, high spl, low box size, strong lowend with warm full subbass) 

Practical experience: Compared 18NTLW5000 in 165L net with 35Hz tuning against 18IPAL in 105L net 35Hz tuning: sine wave they are almost the same at tuning but the 18sound sounds lower and cleaner in subbass frequencies but the ipal sounds tighter with more punch, but still quite similar by the way.

If your goal is to archive more subbass you could turn the sensor off or set lower sensor sensivity, or as you said use another amplifier. I tested Lab Gruppen fp/plm14000 with ipal, it is in general nicer sounding with  "deeper or fuller" sounding bass but as i mentioned not as tight as the ipal mod.

some things you don't see with pn or sinewave measurements, music means impulses and some small changes in the waveform for example a higher group delay or "reverberation time" can cause huge difference in the subjective sound impression of a loudspeaker/subwoofer and make it sound deeper or louder even if you measure sine wave or pink noise and they seem to have the same response.

Don't forget ipal mod is made for professional subwoofers for touring. it has not as much continuous output power as a 5000w car hifi amplifier, but fits the ipal drivers and is very efficient and you can use several of them on a C16 circuit breaker with low average current draw.

Btw. 290L is way too much for the 21IPAL except you mean brutto/the total enclosure volume.

What is your tuning frequency, port area and geometry?

Last but not least, don't forget that the drivers could actually be used in a tapped, hybrid or front loaded horn enclosure, they have perfect parameters for it.

But it's not made for a 1 bass solution imo, you need kick bins from 50Hz upwards then.

Btw. the topic headline "how does it work" is a different actually very interesting topic, i think the math behind it is quite complicated, so far I know its a "current driven amplifier". Sounds similar to a thing that is used to control electric motors, its called FOC (field oriented control) that keeps the current in a spinning motor constant or better "sine wave". with constant voltage you will get a torque rippen because when the windings are at the nearest position close to the magnet the magnetic force is the highest. This can cause the typical annoying hf noise of an rc car or cheap ebike.

I think the princip about the ipal system is not the same but kind of similar, it controls the transducers calculated position according to the input signal.

Take a look at this at page 2

without the dpc sensor activated the loudspeaker still produces some subbass even after the input signal disappeared. I think this is normal for most loudspeakers, some more some less, and makes the typical "ported subbass sound".with the dpc activated the whole bass sounds more tight but also less "full and warm"...

Tried to play around with the parameters, going to work more on that. 

What kind of cabinet did you make for 24" PD? I like the sound from it. Not sure if I got the port and box perfect but it gets pretty loud probably not far from a double 18" box from 30-80hz on my bridged K3 amp. More testing during the weekend and next week.