Acoustic Requirements for Residential Buildings under the NCC

What it is

Acoustic separation is a Performance Requirement under the National Construction Code. The aim is to stop sound moving from one dwelling into the next at a level that affects health or amenity. The Code does not regulate noise inside a single house. It regulates noise between dwellings, between dwellings and common areas and around plant rooms or service spaces that can run all hours.

The provisions sit in two different volumes:

• Volume One Part F7 (formerly F5 in NCC 2019) covers Class 2, 3 and 9c buildings.
• Volume Two Part H4 covers Class 1 and 10 buildings and points to the Housing Provisions Part 10.7.

Single freestanding Class 1a houses with no shared wall do not trigger acoustic Deemed-to-Satisfy provisions. The moment a wall is shared with another dwelling (a duplex, a townhouse or a granny flat attached to the main dwelling) the Code wakes up.

Class 1a attached dwellings

Volume Two Performance Requirement H4P6 sets the floor. A wall separating dwellings or separating a dwelling from a plant room, lift shaft, stairway or public corridor must resist the transmission of airborne sound. A wall separating dwellings from a bathroom, sanitary compartment, laundry or kitchen of an adjoining dwelling must also resist impact sound.

The Deemed-to-Satisfy way to comply is through Part 10.7 of the Housing Provisions and Specification 28. The headline numbers for a wall separating Class 1a dwellings are:

• Weighted sound reduction index (Rw) of not less than 50, or
• A field-tested DnT,w + Ctr of not less than 45 measured under AS/NZS ISO 717.1.

If the adjoining room on the other side of the wall is a wet area (kitchen, bathroom or laundry) the wall also needs to be discontinuous construction or meet the impact insulation requirement. A timber stud wall with a single line of studs and plasterboard either side will not get there. A double stud wall with separate plates and an air gap will.

What discontinuous construction means

Discontinuous construction means the two leaves of the wall are not rigidly connected. No common studs. No common plates. A 20 mm minimum cavity. Wall ties are allowed if they are flexible. This stops impact sound (footsteps, taps closing, doors slamming) coupling from one leaf to the other through the structure.

Class 2 apartments

Class 2 buildings cop a tougher and more detailed set of provisions under Volume One Part F7 and Specification 28. Floors separating sole occupancy units need an Rw + Ctr of 50 and an Ln,w of not more than 62 for impact. Walls between sole occupancy units need Rw + Ctr of 50 (or 45 measured in-situ). Walls between a sole occupancy unit and a plant room or lift shaft need Rw + Ctr of 50 and discontinuous construction.

The Code also catches services. Sound-sensitive rooms (bedrooms or habitable rooms) cannot be located adjacent to a plant room unless the wall meets the higher acoustic floor. Pipes carrying soil, waste or water from another dwelling cannot run through a habitable room of a different dwelling without acoustic treatment of the duct.

Common compliance traps

Even a well specified wall can fail in the field if the build sequence is sloppy. The usual suspects:

• Penetrations for power points and switches on both sides of the wall back to back
• Gaps where the wall meets the floor or ceiling that get sheeted over without sealing
• Door undercuts on apartment entry doors that exceed the spec
• Plumbing in the acoustic cavity that bridges the two leaves
• Top of wall not packed and sealed to the underside of the slab or roof structure

Acoustic certificates from product suppliers are based on laboratory tests. The in-situ result is almost always worse than the lab number. If the spec says Rw 50 and the lab data sheet says Rw 50, the build is sitting on the edge. Most consultants ask builders to specify 5 above the minimum to leave headroom for site workmanship.

Verification methods

Performance Requirement H4P6 (Volume Two) and FP4.1 to FP4.3 (Volume One) can be met two ways:

1. Follow the Deemed-to-Satisfy Provisions (the prescriptive tables and construction details in Specification 28 and Part 10.7).
2. Use a Performance Solution backed by either laboratory test data on the same construction, in-situ field tests, expert judgment from a registered acoustic engineer or a Verification Method like V4.1.

Most townhouse and duplex projects use the Deemed-to-Satisfy path with a manufacturer system. Apartment builders almost always commission an acoustic consultant.

Practical builder steps

Before locking the wall system at tender, pull the manufacturer's full acoustic report (not the marketing brochure), check it has been tested in a configuration that matches what you are building (single or double stud, cavity width, insulation type, plasterboard thickness) and confirm with the consultant or the certifier that the spec meets the Code clause that applies to the project class. Pricing a single stud wall on a Class 1a duplex with wet areas on each side is the most common acoustic costing miss.