This would be the so-called EBS alignment, where the tuning frequency is set very low and peaked so that a shelf is formed. It provides flat response down to fh, and then response is reduced but relatively flat below fh down to fl. That's why it requires increased power and is why guys today call it an "Extended Bass Response" or EBS alignment. Apparently Electrovoice called it "Step Down" and I called it "EBS Stepped Response from ported cabinet with specific peaking." This and other characteristic response curves are shown in a post called "Response curves of closed vs. vented systems."