After reading numerous subwoofer reviews, I am intrigued by something that some of them talk about. It seems that at certain tuning frequencies for vented woofer enclosures, the woofer cone hardly moves at all, and the port is handling most of the output, and vice versa. How does this occur? I always thought that the air coming from the port was a direct reflection of the woofer back pressure. Can anyone explain or point me to a site for an explanation? Thanks.