A practical comparison of the common ways to add transmission loss to a partition, and which one to reach for in each situation.
An extra layer of dense board is cheap per square metre and easy to specify. It adds mass and, on a decoupled frame, works well. The cost is depth and weight, and on a refurbishment it may mean moving services, skirtings and door frames.
Cavity absorption is not a barrier — it damps resonance inside the cavity and improves the performance of a double-leaf partition. It is nearly always worth including, and nearly never sufficient on its own. Filling a cavity does not compensate for an under-massed leaf.
Constrained-layer damping between two board layers targets coincidence dip and resonance. It performs well in the frequency range where board assemblies are weakest, but it requires two layers of board to work at all, so it does not solve a depth problem.
MLV adds surface density in a few millimetres. That makes it the tool of choice when depth is fixed, when an existing partition has to be upgraded in place, or when the element to be treated is not a wall at all — ductwork, pipework, an enclosure, a floor build-up.
Per decibel it is not the cheapest option in new build, where you can simply design a deeper partition. It becomes the economical option the moment depth, weight or demolition is constrained.
New build with space: design the assembly properly. Refurbishment, tight build-up, or a non-wall element: MLV. Reverberation complaint: neither — you need absorption.
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