IB = Infinite Baffle. It refers to speakers mounted in a way that leaves their cones unrestricted by a typical enclosure and basically operating in "free air." Basically, it's a boxless speaker. The advantage of infinite baffle, especially when you're talking about subwoofers, is flat response. The disadvantage is that you trade off power and depth for that flatness. To regain that power and depth, you need to use more drivers. Here's how one guy did it.