Impact sound | Chalkline

Impact sound is structure-borne noise caused by physical impact on a floor or wall: footsteps, chair scraping, dropped items, a hammer strike. The energy enters the building fabric directly and travels through the structure into the unit below or beside, rather than through the air. It is the harder of the two noise categories to design out, because once the energy is in the slab or wall it travels long distances unless something stops it.

How it is measured

Impact sound is rated under AS ISO 717.2, the impact sound insulation standard:

Ln,w is the weighted normalised impact sound pressure level measured in a laboratory; lower numbers are better (less sound passes through).
LnT,w is the in-situ field equivalent, measured on the finished building.

A floor system is given a single-number Ln,w rating you can compare against the regulatory bar.

What the NCC requires

For floors between sole-occupancy units in Class 2 apartment buildings (and Class 3 hotels), NCC 2022 Volume One F7D5 requires Ln,w of not more than 62, with a paired airborne rating (see NCC sound insulation in residential for the full picture). Class 1 (houses, townhouses, duplexes) has no equivalent DTS floor impact provision.

Why it is the hard one

Airborne sound (speech, TV) is stopped with mass and air gaps. Impact sound is different: the impact energy bypasses an air gap entirely and travels through the structure. Stopping it usually means discontinuous construction: a resilient layer (rubber pad, acoustic underlay, isolated floating floor) that breaks the structural path between the impact surface and the structure below. Retro-fitting a discontinuity to a poured slab is awkward and expensive, which is why impact sound is mostly a design-stage decision, not a build-stage one.

Also known as: impact-generated sound, structure-borne impact noise, footfall noise.

Category: Acoustics

How it is measured

What the NCC requires

Why it is the hard one

Related

See also

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