Sound insulation basics

The three effects

Mass

The heavier a layer, the harder it is for sound to set it vibrating. The most obvious route, and the most expensive and space-hungry.

Decoupling

If the two sides of the wall are not rigidly connected, the vibration cannot cross. Two independent, lighter layers can give a better result than one heavy, continuous one.

This is the great advantage of light gauge steel: decoupling is a natural option here.

Damping

Fibrous material in the cavity absorbs the vibration that forms in the air gap. Without it the air cushion between the two leaves would act as a “spring” and amplify the sound at certain frequencies.

Why the wall can be thinner

In brick the party wall has to be doubled from the outset, which takes a lot of room. In light gauge steel the doubled wall, a well-chosen build-up and sufficiently heavy lining boards together give a comparable result at roughly half the thickness.

The exact figure always depends on the specific build-up — there are many variants in masonry and in steel alike.

What the lining decides

Boards of higher density in several layers build a wall that feels duller and more solid than a thin partition-block wall.

Put up the cheapest board in a single layer and the same quality cannot be expected — and that will not be the fault of the structure.

What is easy to get wrong

A rigid connection between the two leaves

A single continuous, rigid element — a screw, a profile, a cable — spoils the decoupling.

Gaps and penetrations

Sound travels through a gap just as air does. An unsealed socket-box penetration measurably worsens the result.

Flanking paths

Sound can bypass a good wall through the floor, the ceiling and the flanking wall. Handling flanking transmission is a separate task.

The link with the other layers

The service cavity is useful acoustically too, because it decouples. Service cavity.

Fibrous insulation in the cavity is thermal and acoustic insulation at once.

The point

Mass is not the only route. Light gauge steel is strong on decoupling and reaches the same result from less space — as long as the work does not bridge it with a rigid connection.