When acoustic performance is quoted, the letters after the dB value show how and where the sound insulation is measured—this is why the difference matters.

dB (Rw)
This is a laboratory test result. It shows how well a wall or system blocks sound under controlled conditions, with no influence from the rest of the building.
Approved Document E (AD‑E) uses Rw for internal (non‑separating) walls, because these walls are assessed based on their product/system performance.

dB (DnT,w)
This is a real‑world, on‑site test result. It measures sound insulation between rooms or dwellings once the building is complete, and includes sound travelling around the wall via floors, ceilings and other connections (known as flanking sound).
AD‑E requires DnT,w for separating walls because it reflects what occupants actually experience.

What does Ctr mean?

You may also see:

• Rw + Ctr or
• DnT,w + Ctr

Ctr adjusts the result to account for low‑frequency noise such as music and TV, which is common in homes.
For this reason, AD‑E specifies DnT,w + Ctr for separating walls, not just DnT,w.

Why this matters in practice

• Rw and DnT,w values cannot be directly compared—one is measured in a lab, the other on site.
• In typical buildings, the on‑site DnT,w result is usually 5–10 dB lower than the laboratory Rw value, depending on room size, junction detailing and how well flanking sound paths are controlled.
• A wall with a good Rw rating can still fail AD‑E if installation quality or flanking conditions are poor.

This is why AD‑E asks for Rw on internal walls, but DnT,w + Ctr on separating walls.

Siniat systems are normally quoted with Rw or Rw + Ctr values, which designers use to select constructions capable of achieving the required on‑site DnT,w + Ctr performance when properly detailed and built.