Way back in the later 60's I remember a cab maker (REH?) that had a rear port thatwas a door and hinge assembly. Supposedly the design was somewhat innovative , but developed from some old sound book. The theory was something like, ports are for specific freq. ranges to optimize speaker performance by limiting cone excursion or some such. The billow port automatically (from air pressure I guess) opened and closed thereby changing port size to the optimum size for the freq. being played. Now that I think about it, the door couldn't possibly react quick enough could it? I do know those were the loudest, deepest, smoothest sounding 2x15 JBL cabs I ever heard. Best of Luck, Wesley R.