Rw+Ctr

Rw+Ctr is the single-number rating used under NCC 2022 to measure how well a separating wall blocks airborne sound between adjoining dwellings. It is made up of two components: Rw (the weighted sound reduction index, rating the wall’s overall sound isolation across a broad frequency band) and Ctr (the spectrum adaptation term, a negative adjustment for low-frequency sound such as traffic noise and home theatre bass). The sum of the two is the number that matters for NCC compliance.

Under ABCB Housing Provisions Part 10.7.1, a separating wall between Class 1 buildings must achieve Rw+Ctr of not less than 50 (verified 2026-05-07). The rating is determined per AS/NZS ISO 717.1 from laboratory test results, or by building one of the Deemed-to-Satisfy construction forms listed in Part 10.7.5 to 10.7.8, typically a discontinuous construction wall to control flanking paths.

The practical trap: a lightweight wall may show a raw Rw of 55 but carry a Ctr of -6, giving Rw+Ctr = 49, which fails. Always confirm both values from the product data sheet before selecting a wall system. The deeper how-to lives in NCC sound insulation in residential, which also sets out the FRL requirements that often apply to the same wall.

Also known as: Rw, weighted sound reduction index, sound reduction index.

Category: Compliance & approvals.

Related

• NCC sound insulation, full Part H4P6 and Part 10.7 requirements

See also

• NCC, the National Construction Code
• Deemed-to-Satisfy (DTS), the compliance pathway these numbers feed into