Understanding Sound Insulation Quantities — Rw, DnT,w and Ctr Explained

Sound insulation performance is often described using a range of different quantities, and these are frequently misunderstood or incorrectly used interchangeably. In practice, each quantity represents something slightly different, and confusion between them can lead to inappropriate specifications or unrealistic expectations of on-site performance.

This note explains the main sound insulation quantities typically encountered in building acoustics and how they relate to one another in practice.

Why there are different sound insulation quantities

Sound insulation can be measured in controlled laboratory conditions or in real buildings. Because the acoustic environment differs significantly between these scenarios, different metrics are used to describe performance.

Laboratory values tend to represent the intrinsic performance of a construction element, while on-site measurements reflect how that element performs within the completed building, including the influence of flanking transmission and workmanship.

Rw — laboratory sound insulation

R_w is a laboratory measurement of airborne sound insulation. It represents the performance of a construction element tested under controlled conditions, where flanking paths and site variables are minimised.

For example, a British Gypsum double stud system comprising two sets of 90 mm studs, two layers of 25 mm mineral wool and two layers of 15 mm SoundBloc board on each frame may have a stated laboratory performance of:

R_w = 61 dB

Laboratory values are useful for comparing constructions but do not represent the performance that will necessarily be achieved on site.

DnT,w — on-site sound insulation

D_nT,wis an on-site measurement of airborne sound insulation. It is effectively the real-world equivalent of the R_wlaboratory measurement and is often used for demonstrating building acoustics compliance in schools, healthcare, commercial and office developments.

Because site conditions introduce flanking transmission, junction effects and construction tolerances, on-site results are typically lower than laboratory performance. A difference of around 3–5 dB is common, although it can be greater.

For this reason, when specifying constructions, the laboratory Rw value should usually exceed the required on-site DnT,w by an appropriate margin. We normally recommend a difference of 7 dB when specifying construction systems, to provide some tolerance margin.

Rw + Ctr — laboratory value with low-frequency adaptation

R_w+ C_tris the same laboratory measurement with an additional adaptation term, Ctr, applied.

The Ctr term adjusts the result to better represent performance against noise with a greater low-frequency content, such as traffic noise – the ‘tr’ is an abbreviation for ‘traffic’. Because the adaptation is typically negative, the R_w+ C_tr value is usually lower than the R_w.

In the example above, if the C_tr value is −8 dB, the resulting figure becomes:

R_w+ C_tr = 53 dB

DnT,w + Ctr — the Building Regulations ADE compliance metric

D_nT,w+ C_tr is the on-site measurement used in sound insulation tests for compliance with Building Regulations Approved Document E for separating walls and floors in dwellings, flats and rooms for residential purposes such as HMO’s.

This is effectively the on-site equivalent of the R_w+ C_tr laboratory value. As before, the measured result is typically lower than laboratory performance due to real-world conditions and flanking transmission.

It is therefore prudent for the specified laboratory R_w+ C_tr value of a partition to exceed the regulatory on site D_nT,w+ C_tr requirement by a suitable margin to ensure compliance can realistically be achieved on site.

How the quantities relate in practice

Using the example above, the difference between laboratory and on-site performance can be significant.

A wall with a laboratory performance of:

R_w = 61 dB
R_w+ C_tr = 53 dB

might reasonably be expected to achieve an on-site result of approximately:

D_nT,w ≈ mid-50s dB
D_nT,w+ C_tr ≈ mid- to high-40s dB

This illustrates how the quantity used for Building Regulations ADE compliance in residential development can be around 15 dB lower than the headline laboratory R_w value.

Why this matters for developments

Misunderstanding these quantities is one of the most common causes of sound insulation problems at design stage.

Typical issues include:

Assuming laboratory values represent on-site performance
Failing to allow sufficient performance margin
Selecting constructions based on R_walone without considering C_trin applicable residential developments.
Underestimating the influence of flanking transmission

Understanding how these quantities relate helps ensure that specifications are realistic and that the risk of failing on-site testing is minimised.

Summary

Sound insulation performance can be expressed using a number of different quantities, each representing a slightly different aspect of acoustic behaviour.

Laboratory metrics such as R_wand R_w+ C_trdescribe intrinsic construction performance, while on-site measurements such as D_nT,wand D_nT,w+ C_tr reflect how buildings perform in reality. Recognising the differences between these values is essential to ensuring that constructions are specified appropriately and that compliance targets are achievable in practice.

If you are unsure how these quantities apply to a particular project, early acoustic advice can help ensure that specifications are appropriate and that compliance targets are realistic.

Last reviewed: 2026