What is a folded waveguide enclosure and how do you design one for a driver?

Hey
I came across this video where the guy builds a subwoofer with a 3" Tang Band W3-1876S driver.
This is what it looks like - cross section of two halves
An externally hosted image should be here but it was not working when we last tested it.

He demonstrates the woofer going down to 50Hz in a small enclosure with a meter long folded waveguide.


Now how do you model such an enclosure design for any type of driver?What kind of limitations does it pose? What driver type/specs is best suited for this enclosure? And, is there some reliable calculator for this enclosure type?
 
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