I've helped to build many pairs of JBL short-throw bass horns, a.k.a. "double scoops" (the model number escapes me), following JBL's plans, yet I have never asked about or understood the acoustic principle(s) that are at work which allow the twin 15 inch woofers to act as direct radiators above 150hz and direct the lower frequencies down below out the "scoop". I believe that this model is referred to as a folded horn, but not sure. If someone in-the-know could enlighten me on the basic idea, I'd be grateful.

Scott