Бас в бесконечном экране
In-wall speakers fall into two design camps: sealed and infinite baffle. Speakers intended to simply fit between stud (small) spaces for instance would be designed as sealed cabinets. Speakers that are provided with a large space behind them (4 - 10 times the driver's specified Vas) behave according to infinite baffle theory. In either case there should be no baffle step compensation needed, OTOH soundstage depth is severely compromised. Because of this diminished sound staging the best application for I.B. is for deep bass.
Just like O.B. bass use of multiple drivers and EQ is to be expected. I.B. bass has no front/rear wave cancellation, but finding a suitable domestic installation can be a challenge. Adjoining space is required. Garages or attics may get too hot or cold. Common ceiling/wall/floor construction to attics, spare bedrooms, or basements may be insufficient to prevent bleed through. Swinging of doors into those spaces (if small and/or tightly sealed) can damage the drivers.
But the performance can be truly great.
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.