The 84 mm system uses a stud of 82 × 45 mm with 0.7 or 0.8 mm wall thickness, or a 84 × 35 mm section, with matching top and bottom tracks in 3000 mm standard lengths. It keeps the same idea as the 122 system — two cavities, rock wool or glass wool fill, an acoustic board in the middle — but in a thinner overall build-up.
The trade-off is straightforward. A thinner cavity holds less absorbing material and has less distance between faces, so the low-frequency performance drops before the mid-range does. For a meeting room, a consulting room or a hotel corridor wall, that is usually an acceptable trade; for a plant room or a cinema, it is not.
Where the 84 mm system earns its place is in refurbishment and in buildings with a tight floor-to-ceiling zone. Losing 40 mm per wall across a corridor can release enough width for a compliant escape route. It also suits internal subdivisions where the wall line may move later: the sections are light enough for two people to reposition without plant.
The rule we use on projects is simple. Start from the acoustic requirement, not from the wall thickness: if the required rating is met by the 84 mm build-up, use it; if not, go to 122 mm before adding layers to a thinner wall, because extra layers on a thin cavity cost more and perform worse than a properly sized cavity.