Masking does not change the wall. It changes what the listener hears on the other side of it.
Compact maskers fitted in the ceiling of the receiving space (the corridor or adjacent area) produce a calibrated broadband signal tuned to speech frequencies. At roughly 45–48 dB(A) the masking pushes overheard speech below the threshold of intelligibility.
People inside the room are unaffected; people working outside it stop registering the masking within a few hours, and most visitors never notice it at all.