Rw and DnT,w - What do they mean?​

A common confusion in understanding acoustics and specifying wall types for a building lies in the difference between Rw and Dw. Misunderstanding this can lead to under or over performance, whilst grasping this simple difference ensures a better result on-site, as well as presenting the opportunity to value engineer over specifications.

Dw is a term that relates to onsite sound insulation. It is as simple as the noise level in my source room, minus the noise level in my receiver room, the level difference. This is a performance standard, a value you can physically measure on site after completion, and often have to to demonstrate compliance with building regulations for schools and residential developments, and to achieve BREEAM credits.

The sound transmission between two rooms is the sum of many paths. The dominant and obvious path, is directly through the separating partition. However sound also transmits through junctions with the floor, soffit, facade and corridor wall. For example if a wall is only built to the underside of a ceiling, and not to the slab, a significant flanking path around the wall exists. Flanking through mechanical ducts, services routes and penetrations and pipes and steels running between rooms also need to be considered. Flanking is particularly important to control on high performance walls, the margin for error rapidly decreases around rooms such as music and performing arts spaces.

You may hear variations of Dw, for example BB93 and HTM specifies DnT,w for schools and healthcare buildings. The nT in DnT,w is the normalisation of reverberation, which allows us to compare sound insulation results objectively on a level field, irrespective of differences in reverberation. Part E for residential uses DnT,w + Ctr, the Ctr being a low frequency correction, making this target more onerous than a DnT,w. A Dw is also referred to in BREEAM documents for the performance targets of other building types, and is the term that most effectively follows the subjective level of sound insulation heard on site, though most standards will use the normalised version. The important thing to remember, is that these are all onsite performance targets. 

Rw relates to the laboratory rated sound reduction index of a single element, i.e. a wall. A laboratory test measures the wall performance in isolation from any other sound flanking paths. So if you were to build a 50 dB Rw wall, in a perfect building with infinitely high mass surrounding constructions with no flanking whatsoever, it could theoretically achieve 50 dB Dw on site. Of course, we cannot build perfect buildings, and therefore we have to account for flanking. We also cannot guarantee that the Rw was determined correctly, or that the element tested in a lab was of a much different surface area to our actual element, but this is a topic for another day.

The important thing is, we need to choose walls, floors, glazing and doors with a sufficient Rw rating, and then build these well through good detailing and adequate workmanship, to achieve our onsite Dw, DnT,w, or DnT,w + Ctr targets.

How do I get from Dw to Rw?

The same construction measured in a lab will get the same Rw result every time. But when measured on-site, the result will vary from room to room, project to project.

The calculation to convert from Rw to Dw has to account for:

• The area/size of the separating partition, a bigger wall means more area for sound energy to transmit through. Bigger area = Higher Rw.
• The volume of the ‘receiving’ room. The smaller the space, the greater the concentration of sound energy, the higher the sound pressure level. Smaller volume = Higher Rw.
• The reverberation time of the ‘receiving’ room, which is the time taken for sound to decay by 60 dB. A sports hall or church, with a large volume and hard reflective surfaces such as concrete or plasterboard, has a long reverberation time. A small space with lots of soft absorbent materials such as ceiling tiles, acoustic wall panels or soft furnishings, will have a low reverberation time. A high reverberation time means a build up in noise levels from sound reflecting around the room, with energy being dissapated slowly from a lack of absorbent materials. Higher reverberation time = Higher Rw.
• The possibility of flanking sound transmission on-site due to a potential downfall in construction quality or inattention in design to junction and penetrating detailing.

Therefore it is not a simple case of Rw = Dw + X dB. The Rw can vary significantly between partitions, even if they require the same Dw. The image below shows this. If we look at DnT,w, or DnT,w + Ctr, theoretically the DnT,w on site should not change with reverberation time (RT), as the nT refers to the normalisation of reverberation. The DnT,w allows us to compare sound insulation results objectively on a level field, irrespective of reverberation. However calculation methods in standards like BB93 still use RT within the formula to calculate Rw from DnT,w. The RT still effects the Dw, which is the true level difference, the one we subjectively hear on site!

How does this help my design?

The temptation by some consultants is to simply say ‘Rw = Dw + 8 dB’. Why? This is a comfortable safety margin, and avoids the time consuming exercise of measuring each wall, volumes of each room and calculating the Rw.

So what’s wrong with this? Clearly we see from the illustration above, if the same wall type Rw (say 48 dB) is used everywhere where the Dw is 40 dB, there will be rooms that fail, as well as rooms that exceed the required performance standard by some distance. So not only will I have walls in my building that fail and may require expensive remedial work, I may have overspent unnecessarily on the walls that pass. If we just take the worst case at 53 dB Rw and minimise the risk of failure, then most of the walls will be over designed.

Therefore doing these calculations correctly, wall by wall, and paying close attention to the construction details, is important in achieving a successful, cost effective design.

Of course we don’t want to end up with 99 different wall types. But if we carefully design the construction details, limit the number of plasterboard types and convey this clearly to the site team, we can cut down on safety margins and use a handful of wall types.

Reducing every dB of over-design quickly sums up when applying over projects, schemes and larger frameworks, particularly those with common shared constructions. And of course, simply getting things right in the first place cuts down on costly post construction remedial work!