Аудио-википедия

Бас в бесконечном экране
https://www.subwoofer-builder.com/ib.htm

https://www.audiocircle.com/index.php?topic=133797.0
IB is usually used for bass. It allows for the air space «behind» the woofer to act as the enclosure. The rule of thumb for the amount of space is 10X VAS of the woofer.

OB are often/usually run full range, and therefor the rear wave off the driver is used, after it reflects off the room and furnishings etc.

Sound staging is affected which is why it's typically used only for lower bass (subwoofer) frequencies. In open baffle (dipole) designs at those frequencies front and rear sound waves (versus directly sound rays, search Audio Circle for «constant directivity» and «Swarm») are large enough to cancel each other out along the plane of the baffle. Mark is right about the 10 times Vas rule of thumb, but the space behind at that arbitrary point is large enough to not act as any kind of enclosure. Some use attics or even exterior walls (preferable in temperate climates due to thermal transfer) as outdoors the bass energy quickly dissipates. If using an interior back space care must be taken not to use rooms with hinged doors (swinging of large surfaces could pressurize against the driver and blow it out) or loose contents (sympathetic vibrations), so attached garages can be a viable option.