A single-cavity partition is one volume of air between two board faces. Sound crosses it in one step: the first face vibrates, the air carries the energy, the second face radiates it into the next room. Adding mass or thickening the cavity helps, but the curve flattens quickly — doubling the mass buys you only about 5 dB.
A double-cavity wall inserts a third element into that path. The stud carries two separate cavities, and an acoustic board sits between them. Sound now has to excite the first face, cross cavity one, excite the middle board, cross cavity two, and excite the far face. Each crossing loses energy, and the failure frequencies of the two cavities do not line up, so the coincidence dip that limits a single-cavity wall is filled in.
In the 122 mm system the stud section is 120 × 45 mm with a wall thickness of 0.6 or 0.7 mm, and the two cavities are filled with rock wool or glass wool. The middle layer is a non-combustible acoustic board of 10 or 12 mm, clipped into the stud rather than screwed. The clip detail matters: screws create hard points that transmit vibration, while a clip holds the board without clamping it rigidly.
The practical result is that a 122 mm wall reaches the acoustic class of a masonry wall several times its thickness, while staying light enough to hang from a structural slab without additional framing. Where the project needs a higher rating again, the same logic can be extended to a 148 or 172 mm stud.