The aim of this article is to discuss the importance of Rw and Dw in building acoustics, without delving too deeply into its mathematics and technical aspects.
What is Dw?
There are two parameters that are used to describe the sound insulation of a partition – Dw and Rw. Dw is a term that relates to the sound insulation between rooms on-site. Put simply, it is the noise level in the source room minus the noise level in the receiving room, the level difference as it’s termed. This is a performance standard used to describe final site requirements, which commonly is used to demonstrate compliance with building regulations for schools and residentials developments, and to achieve BREEAM credits.
Variations of Dw are specified more in-depth in documentation such as BB93 and HTM. For example, DnTw is the performance parameter required for schools and healthcare buildings. The nT represents the normalisation (n) of reverberation (T) which allows us to compare sound insulation results objectively on a level field, irrespective of the differences in reverberation. Effectively, normalization allows the measured reverberation to be weighted against the reference reverberation standard of ≤0.5 seconds, a value which represents the time taken for all the emitted sound in the source room to completely decay. Part E for residential purposes uses DnTw + Ctr, the Ctr accounting for a low frequency correction. This is the performance standard we use for acoustic testing at Air Tightness Solutions, as it is in compliance with the Building Regulations Document relevant to all new builds and refurbishments. The important thing to remember is that these acoustic parameters are all onsite performance targets.
What is Rw?
Now, before moving onto understanding Rw, it is useful to understand the process of sound transmission between two rooms subject to testing. Sound transmission between two rooms is the sum of many paths, with the dominant and obvious path transmitting directly through the separating partition. However, sound transmission paths are very much random and unpredictable, passing through undesirable barriers known as flanking. For example, if there is acoustic panelling hanging from the ceiling, then a flanking path exists around the panelling. This process is the same for mechanical ducts, penetrations and pipes running between rooms so also need to be considered.
Rw relates to the laboratory rated sound reduction index of a single element making up a partition wall/floor type. A laboratory test measures the wall performance in isolation of potential flanking paths, it is achieved off-site. This method suggests that if you were to build a 40 dB Rw wall of sound insulation then theoretically it could achieve 40 dB Dw on-site, given that the surrounding constructions provide no flanking whatsoever. In practise, a perfect value is of course unattainable, and therefore we must account for flanking. We also cannot be certain that Rw was determined correctly, the element tested in a lab could well have been of a different surface area to the actual element.
How do Dw and Rw affect each other?
To achieve the onsite Dw target, there are some factors to consider when using partitions with a sufficient Rw rating. It is useful to note that utilizing figures of both Dw and Rw give a representative sample of the wall type within a construction build for the benefit of minimizing the amount of testing required for multiple partitions.
If we consider DnTw, or DnTw + Ctr, theoretically, the DnTw on site should not change with reverberation time as nT refers to the normalisation of reverberation, allowing us to compare sound insulation results objectively on a level field irrespective of reverberation. However, calculation methods in standards such as BB93 still use RT within the formula to calculate Rw from DnTw. The problem therefore arises in that the Rw can vary significantly between partitions, even if they require the same Dw.
The RT affects the Dw (which is the true level difference, this is what we hear subjectively when testing on site), due to this, values in Rw can vary significantly. For example, if one party wall passes by a value of 53 dB but its target is to achieve 40 dB of sound insulation, then ascertaining each wall type on-site would be difficult, we do not want to be left with a vast multifarious blunder! The reduction of dB for walls that have passed in this manner may well be considered as over-designed, getting things right in the first place would greatly cut down on costs on post construction remedial work. At Air Tightness Solutions, we test partitions using DnTw + Ctr. Not only does this method account for the acoustic characteristics of sound decay in a particular room but also for the low frequency correction of white noise emission from our multi-directional speaker, giving very accurate and more valid measurements overall.
Author: Jashan Goodary, Acoustic Engineer