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Acoustic performance is measured with a confusing set of terms — Rw, R'w, DnT,w, Dn,w, and others. Each measures something slightly different, in a slightly different way, and they're not interchangeable. If you specify Rw when the Building Regulations require DnT,w, you may fail the acoustic test even if the wall performs well. This guide explains what each term means and how to specify correctly.

Acoustic basics — what we're measuring

Sound travels from one room to another in two ways:

• Airborne sound — voices, TVs, music, traffic. This is what partition performance measures. It's measured as a sound reduction index (R or D), which is the difference in sound level between the source room and the receiving room.
• Impact sound — footsteps, dropped items, furniture being moved. This is measured for floors, not walls, so it's not relevant to most SFS partitions.

For SFS walls, we're concerned with airborne sound. The performance is measured in decibels (dB), but there are several different ways to measure and express it. That's where Rw, DnT,w and R'w come in.

Rw — laboratory sound reduction

Rw is the "Weighted Sound Reduction Index" — the sound reduction of the wall measured in a laboratory, with no flanking transmission. It measures only the performance of the wall itself, in ideal conditions.

Rw is the number you see on manufacturer datasheets and test reports. It's a measure of the wall's inherent acoustic performance. Higher is better.

Rw is measured to BS EN ISO 10140 (or the older BS 2750) and rated to BS EN ISO 717-1. The laboratory test sends sound through the wall from one room to another, measures the reduction, and calculates the single-figure Rw rating.

Rw + Ctr — spectrum adaptation

Rw + Ctr is a variant of Rw that includes a spectrum adaptation term (Ctr) for low-frequency sound. This is important because low-frequency sound (traffic, bass, machinery) is harder to block, and a wall that performs well at mid-frequencies may perform poorly at low frequencies.

Rw + Ctr is typically 5–10 dB lower than Rw, depending on the wall build-up. Building Regulations increasingly require Rw + Ctr rather than just Rw, because it better reflects real-world sound (traffic noise, music) rather than the standard reference spectrum.

For the same wall, Rw + Ctr will always be a lower number than Rw, but it's a more honest measure of performance.

Dn,w — on-site normalised

Dn,w is the "Weighted Normalised Level Difference" — measured on site, with flanking transmission included. The "normalised" part means it's adjusted for the reverberation time of the receiving room (to a standard of 0.5 seconds).

Dn,w is closer to real-world performance than Rw, but it's less common than DnT,w (below) in UK specifications.

DnT,w — on-site standardised

DnT,w is the "Weighted Standardised Level Difference" — measured on site, with flanking transmission included, and standardised to a reference reverberation time (0.5 seconds for dwellings, 0.8 seconds for other building types).

DnT,w is the most common acoustic requirement in UK Building Regulations. When Approved Document E says "the separating wall must achieve DnT,w 45 dB", it's this measurement. It reflects what a person in the receiving room actually experiences.

DnT,w is typically 3–8 dB lower than Rw (because of flanking transmission), but can be higher if the wall's actual performance is better than the test suggests (rare).

R'w — on-site apparent

R'w is the "Weighted Apparent Sound Reduction Index" — measured on site, with flanking transmission included, but without normalising for reverberation time.

R'w is similar to DnT,w but is used in different contexts. For dwellings, DnT,w is usually the required metric; for schools and some commercial buildings, R'w is often used. Both are on-site measurements and both are typically 3–8 dB lower than Rw.

The differences in practice

For a single wall build-up, the numbers typically look like this:

So a wall that has Rw 55 on a manufacturer datasheet may only achieve DnT,w 49 on site — a shortfall of 6 dB. This is normal. It doesn't mean the wall failed — it means the lab test and the site test measure different things.

What the regulations require

In the UK, acoustic requirements for walls are set by:

• Approved Document E (England and Wales) — for dwellings and some other buildings
• Section 5 of the Scottish Building Standards
• Part G of the Northern Ireland Building Regulations

Approved Document E typically requires DnT,w for separating walls:

• New-build dwellings — DnT,w 45 dB minimum between habitable rooms and other dwellings
• Conversions — DnT,w 43 dB minimum
• Rooms for residential purposes — DnT,w 45 dB

Some specifications also require Rw + Ctr for external walls (to reflect traffic noise), especially near busy roads or airports.

For schools, hospitals and commercial buildings, the requirements come from BB93 (schools), HTM 08-01 (hospitals), and BREEAM or the client's own specification. These typically specify Rw + Ctr or R'w rather than DnT,w.

How to specify correctly

When writing an acoustic specification:

1. State the metric clearly — Rw, DnT,w, R'w, or Rw + Ctr. Don't just say "acoustic performance" — say which measurement.
2. State the required value — e.g. "DnT,w 45 dB" or "Rw + Ctr 43 dB".
3. Specify the wall build-up if it's critical — a tested build-up with the achieved Rw gives confidence that the target will be met on site.
4. Allow for lab-to-site difference — a wall with Rw 55 will typically achieve DnT,w 48–52 on site. If the target is DnT,w 45, an Rw of 50–52 is usually sufficient. Don't specify Rw 45 for a DnT,w 45 requirement — you'll fail.
5. Consider flanking — the wall may perform well, but flanking transmission (through floors, ceilings, adjacent walls) can dominate the on-site performance. Coordinate with other trades.

Summary

Acoustic performance of SFS walls is measured in several different ways, and the metrics are not interchangeable. Rw is the laboratory performance; DnT,w and R'w are on-site measurements that include flanking and are typically 3–8 dB lower than Rw. Building Regulations for dwellings require DnT,w; commercial specifications often use Rw + Ctr or R'w.

Always specify the metric and the value clearly, and allow for lab-to-site difference when selecting the wall build-up. A wall with Rw 55 will typically achieve DnT,w 48–52 on site.