Tls do u know of any subs that have a f3 of 20hz in sealed box?
Personally, I haven’t seen any I can think of offhand.
From a design perspective, I’m not certain what it would take to design a Driver with such a low resonant frequency that, once put in a cabinet, would maintain higher efficiency at infrasonic frequencies. Simple physics mandates that the tuning of the system will always be higher than Driver Fs.
Where you get away from this is with Power Handling, Excursion and DSP. When you apply a Linkwitz Transform as Nick would recommend, you also need a Driver that can handle the additional Power required to force excursion to its limits, thus “allowing” the Driver to perform beyond its theoretical capabilities.
In a Sealed cabinet, this is a matter if brute force. In a ported cabinet it changes somewhat. Either requires a Motor and Suspension capable of handling the demands placed upon them.
This then gets back to what I said earlier about how the actual Driver(s) you own or buy behave against advertised spec.
How you are able to push s Driver in an extreme Subwoofer build is a dice roll against its theoretical performance capabilities. If you push too hard, it will not sound good. If you push s little further it will burn out. Possibly, you will take the Amp with it.
Like it or not, I’m just advising based on my experience with these drivers. Specifically the two I own and have tested.
There are many that seem to be quite happy with the product. Others I’ve seen are satisfied yet experienced slightly similar changes to spec as I have reported. I expect when I build and test, I will find s mixed bag.
In the end, your best option is to make certain you have a good amp, good DSP, understand how to set them up, then build and test a modeled Sub.
Then you can make changes and have some parameters to guide you.