Specifier Briefing · Acoustics

Cork Acoustic NIC vs IIC, Which Number Your Project Actually Needs

Cork's headline acoustic number is IIC 71. But specifier briefs often ask for NIC, STC or "ΔL_w". They are not the same test, and they don't all mean the same thing. Here is the short version of which number you ask for, and why.

The three numbers a floor can be tested against

Which test matters for which scenario

Scenario	Critical metric	Why
Apartments stacked above each other	IIC	Impact noise (footfall, chairs) is what the neighbour below complains about
Open-plan office over a meeting room	IIC	Chair-roll and footfall transmitting to the room below
Adjacent offices separated by a partition	NIC / STC	Airborne voice transmission across the partition, not floor
Hotel room above a corridor	IIC	Footfall + luggage wheel impact transmission
Recording studio / control room	NIC / STC and IIC	Both airborne and impact isolation matter
Hospital ward over a ward below	IIC	Trolley wheels, dropped equipment, footfall

For the majority of UK specifier projects, residential, offices, hotels, hospitality, healthcare, IIC is the metric the floor specification is actually scored on. NIC and STC are tested at the wall, not the floor.

How cork delivers on IIC specifically

Cork's IIC performance is a function of the material's closed-cell structure: roughly 50 % of cork by volume is trapped air, in millions of microscopic sealed cells. Impact energy dissipates through cell-wall deformation rather than transmitting through the floor as a vibration wave. The result, on a typical concrete-deck building envelope:

Floor finish	IIC (typical, concrete deck)
Bare concrete	~28
Engineered oak on plywood underlay	49–55
LVT on standard underlay	50–58
Cork plank (7 mm, Classic Cork)	71–73
Cork plank (11 mm, Cork Styles)	72–74

The 16–22-point gap between cork and the engineered/LVT bracket is roughly one perceptual halving of impact noise, the floor-below occupant hearing "muted thud" instead of "clear footstep".

Where NIC matters for a floor (it does, occasionally)

NIC at the floor is relevant where the floor assembly itself forms part of an airborne-noise barrier, for example, a recording-studio floating-floor build-up over a control room, or a club space over a residential unit. In those cases the specifier wants both IIC and NIC documented. Cork's closed-cell structure contributes to both: the impact attenuation is direct, and the airborne attenuation comes from cork acting as a damping mass in the assembly.Where NIC is the critical metric, we recommend pairing cork plank with a project-specific resilient underlay and consulting the acoustician on the full ceiling-floor-ceiling build-up. Cork plank alone is not an airborne-noise solution; cork plank in the right build-up materially improves both numbers.

What we issue for acoustic-sensitive projects

• ASTM E492 / ISO 717-2 IIC test certificate for the cork plank
• ASTM E336 NIC report where the build-up has been tested as a system
• ΔL_w (impact sound improvement) figure for cork plank over standard concrete deck
• Underlay recommendation for the project's specific brief (residential / office / hospitality / studio)
• BREEAM Hea 05 evidence pack where the project targets the credit

The honest framing

"Cork is IIC 71" is true and it is the right marketing line. The specifier line is: cork plank delivers exceptional IIC performance with no additional underlay required for the vast majority of UK schemes, and where NIC also matters, cork contributes positively but the full assembly is what gets tested. Ask for the metric your spec is actually scored on, in 90 % of UK projects, that's IIC.