Window acoustic performance is measured by the reduction in sound level between the outside and inside of a building achieved by the window assembly. Two primary rating systems are used in Australia:
- Rw (Weighted Sound Reduction Index) — the primary international metric, measured in decibels (dB); higher values indicate better sound reduction; used in Australian Standards and NCC compliance assessments
- STC (Sound Transmission Class) — an alternative rating system more commonly referenced in older documentation; broadly comparable to Rw for typical window applications
A window rated Rw 35 reduces sound by approximately 35 decibels under test conditions. In practice, the installed performance of a window system depends on both the window rating and the quality of installation — gaps, seals and the surrounding construction all affect the overall acoustic outcome.
Acoustic performance of a window system is determined by a combination of factors:
- Glazing thickness — heavier glass reduces sound transmission; thicker glass generally performs better acoustically
- Double glazing — two panes of glass with an air cavity significantly improve acoustic performance compared to single glazing; cavity width and gas fill also affect outcomes
- Asymmetric glazing — using different thicknesses of glass in a double glazed unit avoids the coincidence effect where both panes resonate at the same frequency, improving performance across a broader frequency range
- Laminated glass — the interlayer in laminated glass damps sound transmission, particularly at mid-range frequencies; laminated glass is often specified for its acoustic properties independently of safety requirements
- Frame depth and sealing — frame depth and the quality of weather sealing around the sash contribute to overall acoustic performance; a well-sealed frame prevents sound flanking around the glazing
- Operable vs fixed — fixed glazing consistently achieves higher acoustic ratings than operable systems of the same glass specification; operable systems introduce seal and hardware variables that affect performance
The National Construction Code specifies minimum acoustic performance requirements for habitable rooms in residential buildings adjacent to noise sources. Requirements are expressed in terms of weighted standardised level difference (DnTw) for the building element, which reflects both the window performance and the overall construction.
NCC acoustic requirements relevant to window specification include:
- Minimum performance requirements for habitable rooms in Class 2 buildings (apartments) adjacent to road, rail and mixed transport noise corridors
- Requirements vary by noise level at the facade and NCC climate zone
- Development consent conditions for noise-affected sites often specify more stringent requirements than the NCC minimum — always confirm against the project’s development consent and acoustic consultant’s specification
For NSW projects, acoustic requirements should be confirmed against both the NCC and any development application conditions imposed by the consent authority or acoustic report.
Different project types have distinct acoustic performance requirements:
- Multi-residential apartments — NCC minimum requirements apply for noise-affected facades; development consent conditions and acoustic consultant specifications often require higher performance; glazing selection is the primary means of achieving required ratings
- Education buildings — acoustic separation between classrooms and from external noise is addressed through building fabric including windows; NCC and relevant education facility guidelines specify performance requirements
- Aged care facilities — acoustic comfort is a quality-of-life consideration as well as a compliance requirement; quieter internal environments support sleep, communication and wellbeing for residents
- Commercial offices — acoustic performance of perimeter glazing affects both occupant comfort and NCC Section F compliance for noise-sensitive environments
- Hospitality — acoustic performance requirements vary by location and application; venues in noise-affected areas may have development consent conditions specifying facade performance
