IB refers to infinite baffle.
basically all it means is placing a barrier between the back wave and the front wave without an enclosure on either side. or if you are trying to install one in a room, both areas (the front and back of the cone) should be 10x the Vas of a speakers paramaters.
for car installations, its usually refered to as IB, but technically a trunk is not going to be 10x the Vas, it will be just a giant sealed box. but it is commonly accepted because it mimics the response of a true IB fairly close. to be honest, if you did 10x the Vas, versus 8x the Vas, you would not be able to hear the difference by ear.
the benefits of IB are an extremely smooth frequency response that is natural to the speakers abilities. here is an example using an IDmax12v3
as you can see, the frequency response is very smooth and has way less peaks than a ported enclosure and is better suited for sound quality. the trade off you get with IB is usually a lower overall output and less mechanical power handeling. but you gain in the fact that there is usable area because there is no box, and you can make up for the lack of output with more cone area (bigger\more speakers).
here is a few pictures of my infinite baffle installation.
hope that helped //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif