As we learned from the wall, the solution to cutting airborne noise is the "Mass – Spring – Mass" system. In the case of the ceiling, Mass 1 is the concrete slab, the Spring is the trapped air with rock wool, and Mass 2 is our new suspended plasterboard ceiling.
Sounds simple, right? However, because the ceiling… hangs, we are forced to support it with screws and metalwork on the very same concrete we are trying to insulate! If we don't pay attention to how this connection is made, all the noise will travel down through the screws.
Forget the simple steel brackets used for decorative plasterboard ceilings. The frame of the new ceiling must be hung from the concrete using specialist resilient mounts (hangers). These fittings have an integrated elastic material (neoprene or specialist rubber). This way, the vibration of the slab "dies" on the rubber and never reaches the metal frame of our plasterboard.
Above the metal frame (and before we close with plasterboard), we lay rolls or slabs of rock wool. As we said, its role is to "drown" the echo that would otherwise build up inside the air gap, preventing our ceiling from sounding like a hollow drum.
We screw onto the frame two layers of acoustic (heavy) plasterboard, once again taking care to stagger the joints.
Our ceiling MUST NOT touch the side walls! We leave a gap of 3-5 millimetres all the way round and fill it with elastic acoustic sealant. If the plasterboard is wedged into the walls, the walls will transfer building vibrations to it (flanking transmission) and we will lose a large part of our sound insulation.
This is the most common question. For the "spring" (air gap) to work properly, an acoustic suspended ceiling typically requires 10 to 15 centimetres of your room height. If you have a low-ceiling home (below 2.60 m), you may feel the space shrinking, but it is the necessary price for silence.
The upstairs neighbour (on the 1st floor) talks loudly and watches TV, resulting in 35 dB measured at our bed.
The tradesman screws the metal frame directly to the concrete with plain steel rods ("threaded rods"). He adds a single thin plasterboard with a little rock wool. The upstairs TV makes the slab vibrate, the vibration travels down the rods, shakes the lightweight plasterboard, and we hear the TV almost as loudly as before. We have thrown away €600.
We buy resilient mounts. We hang the frame, lay 5 cm of rock wool, and screw on two layers of heavy acoustic plasterboard, leaving a perimeter gap. We have lost 12 centimetres of height. At night, the neighbour's voices vanish completely (−15 dB). Our ceiling is a disconnected, heavy "lid" that does not participate in the building's vibrations!
The Final Conclusion: The ceiling is essentially a wall turned horizontal. The only way soundproofing can succeed is to "break" the mechanical connection with the concrete slab using elastic, resilient mounts. Without them, you are simply building a beautiful, decorative ceiling.
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