Airborne sound | Chalkline

Airborne sound is sound transmitted through the air from a source into an adjoining space: speech, music, TV, the dog next door. Distinct from impact sound, which is structure-borne noise from physical contact (footsteps, dropped items, furniture moves). The NCC uses Rw and Rw+Ctr ratings under AS/NZS ISO 717.1 to set minimum airborne-sound insulation across separating walls and floors.

The two main ratings

Rw (Weighted Sound Reduction Index): the lab-tested airborne-sound insulation across a building element. A higher Rw means a quieter neighbour.
Rw+Ctr (Rw plus a low-frequency spectrum adaptation term): Rw with a correction for low-frequency content (bass-heavy music, traffic noise). NCC residential separating walls use Rw+Ctr where bass-frequency transmission matters; commercial / general partitions often use bare Rw.

A typical NCC minimum for an inter-tenancy wall in Class 2 is Rw+Ctr ≥ 50; bedroom walls within a single dwelling have lower targets.

Lab vs field

Rw = lab rating (controlled, single element).
DnT,w = field rating (real installed assembly with flanking).

Field DnT,w typically runs 2 to 5 dB lower than lab Rw because of flanking through floors, ceilings, and junctions.

NCC airborne rules

Class	Element	Minimum
Class 2/3	Inter-tenancy wall	Rw+Ctr ≥ 50
Class 2/3	Inter-tenancy floor	Rw+Ctr ≥ 50 + impact
Class 2/3	Wall to corridor/stairwell	Rw ≥ 50
Class 1	No statutory minimum (best practice)	n/a

Full detail: NCC sound insulation in residential.

For a builder

Spec from a tested system, not a bare Rw number. Plasterboard makers publish system ratings; the system is the proof.
Control flanking. A Rw 55 wall with a sound-transmitting floor junction tests at Rw 48.
Soundshield / Soundchek are airborne products. They won’t fix an impact-sound complaint from above.

Category: Acoustics / NCC.

The two main ratings

Lab vs field

NCC airborne rules

For a builder

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