What kind of design

[kuifje19]

Does someone know what this design is and how to calculate for it?
I know about bandpass with 1 speaker or 2 tied together, but never seen with a chamber between.

This one for example :

I do not need the complete formulas, but just a link to an explanation would be fine.
I just found Bassbox, which is a free and complete program, but also no band pass with some room between 2 speakers.

 

[kuifje19]

Update: Instead of bassbox, I meant Basscad ->-> </url>http://www.selfmadehifi.de/basscade_en.html</url>

B.t.w. : It looks to me as some variation on a double vented bandpass...
But I can't read japanese or chinese ...