Why Acoustics Matter
Building occupants notice acoustic problems even when they don’t consciously think about acoustics. Neighbours in a duplex who can hear every footstep through the floor. Office workers who can hear conversations from the next room. Residents kept awake by traffic noise because the bedroom windows don’t provide adequate attenuation. These problems are the result of inadequate acoustic design — and they generate client complaints, disputes, and in some cases legal claims against builders and developers.
New Zealand’s Building Code clause G6 (Airborne and Impact Sound) establishes minimum performance requirements for inter-tenancy elements in multi-unit residential buildings. Understanding what the code requires and how to achieve it is important for builders and developers working on multi-unit projects.
Sound Transmission Concepts
Sound travels between building elements in two ways: airborne sound transmission (through the air, finding gaps and penetrations in the building envelope) and structure-borne (impact) sound (vibrations transmitted through the structure — footfall, plumbing, mechanical equipment). Effective acoustic design addresses both transmission paths, which often require different design approaches.
The performance of a wall or floor assembly in resisting airborne sound is described by its STC (Sound Transmission Class) or Rw (weighted sound reduction index) rating. For impact sound — the noise of footfall through a floor — the rating is IIC (Impact Insulation Class) or Ln,w. The Building Code minimum requirements specify Rw and Ln,w values that party wall and floor assemblies must achieve.
Construction Details That Matter
Even acoustically rated assemblies fail when construction details create flanking paths — routes by which sound bypasses the rated element. Common flanking issues include: continuous floor structures that carry impact sound around rated wall assemblies; gaps in linings at penetrations (pipes, electrical, ducts); and direct connections between linings on opposite sides of a party wall that short-circuit the decoupling in the wall design. Acoustic performance is only as good as the weakest detail — careful construction and on-site quality assurance are essential.