4.1 Subjective Evaluation and Conversion between Rw and Dw

Sound Insulation
Sound insulation describes the reduction in sound across a partition. The sound insulation across a good conventional, lightweight, office to office construction is typically in the order of 45 dB Dw. This means that if the sound level in the source room is in the order of 65 dB, (a typical level for speech) the sound level in the adjacent room, the receiver room, will be in the order of 20 dB (barely audible). If sound levels are increased in the source room to 75 dB (raised voice), sound levels within the adjacent room will also increase to around 30 dB (audible). Sound insulation therefore describes the level of sound lost across a partition and not the level of sound within a room.

Privacy
Privacy describes the perceived sound reduction across a wall. Privacy is a function of both sound insulation and background noise. Background noise is a function of services noise and noise break-in through facades or open windows, vents etc.
If the background noise within a room is increased by 5 - 10 dB, the perceived level of privacy across a partition is also increased by 5 to 10 dB. Therefore, when looking at required sound insulation levels on-site, it is important to consider both the background noise in the receiver room and the sound insulation reduction across the partition.

Subjective Description of Sound Insulation
The table to the right provides an illustrative representation of privacy. This table specifies the two Dw levels of the partition, Column 1. Two levels are provided in this column, one for background noise levels of 35 dBA, 1, and the other for background noise levels of 40 dBA, 2. Please see the text above for an explanation.

Rw (Lab Tested Sound Reduction Index) and Dw (On SIte Sound Reduction Index)
Two parameters are used to describe the sound insulation of a partition; Dw and Rw. Dw represents the sound insulation of a partition on-site. Since these figures describe the final site requirements, Dw levels are specified by clients and Building Regulations. Rw represents the lab tested sound insulation of a product wall type/floor type. Due to flanking and other factors, lab rated sound reduction levels will not be achieved on-site. Conventionally, there is a 5 to 10 dB reduction between a Rw lab tested figure and an on-site Dw figure. The conversion between Dw and Rw is relatively complex and takes into consideration receiver room volume, receiver room reverberation times and the area of the separating partition. The conversion between Rw and Dw should always be calculated.