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Acoustic

Understanding STC and IIC Ratings for Impact Sound Insulation

STC and IIC are the two numbers that describe how well a floor stops sound between spaces. Here is what each one measures, why field results run lower than the lab, and where a resilient underlay actually helps.

By Rukn Al Zain 7 min read

If you are specifying or buying acoustic flooring, two acronyms come up in every consultant’s report and every product datasheet: STC and IIC. They are not interchangeable, they measure different things, and confusing them leads to floors that pass on paper and still generate noise complaints. Here is what they actually mean.

Airborne sound versus impact sound Two floor cross-sections. On the left, airborne sound from a source in the upper unit spreads through the air, passes through the concrete slab and is mostly blocked by the slab mass; this is what STC rates. On the right, a footstep on the floor drives impact energy straight into the structure and radiates into the lower unit; this is what IIC rates, and a resilient layer under the finish reduces it. Airborne sound Rated by STC Upper unit Lower unit Slab mass is the barrier STC · airborne sound Impact sound Rated by IIC Upper unit Lower unit Resilient underlay reduces it here IIC · impact sound
Airborne sound (rated by STC) is stopped mainly by the mass of the slab. Impact sound (rated by IIC) travels straight into the structure from a footstep, and is reduced by a resilient layer under the finish.

STC — airborne sound

Sound Transmission Class rates how well a building element — a wall, a floor, a door — reduces airborne sound passing through it. Airborne sound is anything that travels as pressure waves in the air before it hits the structure: conversation, television, music, a barking dog.

  • It is derived from the reduction the assembly provides across a range of frequencies (in the standard method, 125 Hz to 4000 Hz), collapsed into one number.
  • Higher is better. Each point is roughly a 1 dB improvement in the fitted curve.
  • For floors, STC is driven mostly by mass — the thickness and density of the concrete slab — plus any isolated ceiling below.

A resilient flooring underlay adds very little to STC, because it adds almost no mass. If your problem is hearing the neighbour’s television, the answer is slab mass, a separating ceiling or wall treatment — not underlay.

IIC — impact sound

Impact Insulation Class rates how well a floor-ceiling assembly reduces structure-borne impact sound: footsteps, high heels, a dropped remote, a dragged chair, children running. A standard tapping machine strikes the floor and the sound radiated into the room below is measured and converted to a single number.

  • Higher is better, same as STC.
  • IIC is driven by the floor finish and any resilient layer as much as by the slab. A bare concrete slab might rate around IIC 25–30; carpet on underlay can reach the high 60s or 70s; hard finishes like tile, stone and timber sit low unless a resilient underlay is introduced.

This is where an acoustic underlay earns its place. Putting a resilient recycled rubber layer such as ZainAcoustic Underlay between a hard finish and the slab is one of the most effective ways to lift the impact performance of a floor that would otherwise transmit every footstep.

Where the resilient layer works in a floor build-up A section through a floor build-up, from the bottom up: a thick structural concrete slab, a thin resilient rubber underlay, and the floor finish. A footstep above the finish sends impact energy downward; the energy is absorbed at the resilient layer before it reaches the slab. A note explains that the resilient layer raises IIC but adds almost no mass, so STC is largely unchanged. Where the resilient layer works Floor build-up, section view Structural concrete slab Resilient rubber underlay Floor finish — tile, stone or timber Impact energy absorbed here Why it matters The resilient layer mainly raises IIC (impact sound). It adds almost no mass, so STC (airborne) barely changes.
A resilient rubber underlay such as ZainAcoustic Underlay sits between the floor finish and the structural slab. It absorbs impact energy before it enters the structure — which lifts IIC — but adds almost no mass, so STC is largely unchanged.

Lab versus field: why the number drops

Both ratings can be quoted as a laboratory value or a field value, and the difference matters:

LaboratoryField
AirborneSTC (ASTM E90 / E413)FSTC or DnT,w (ASTM E336 / ISO)
ImpactIIC (ASTM E492 / E989)FIIC or L’nT,w (ASTM E1007 / ISO)

Laboratory tests deliberately isolate the assembly and suppress flanking — sound going around the element through walls, the slab perimeter, columns and poorly sealed penetrations. A real building has all of those paths, so the same floor measured on site typically returns a figure around five points lower than its laboratory rating.

Why field ratings run lower than the laboratory A section through two stacked units separated by a floor assembly, with walls on each side. A bold arrow shows the direct path straight through the floor, which is what the laboratory test isolates and measures. Curved arrows on each side show flanking paths that carry sound around the floor through the walls and slab edge, which are present only on site. A small bar chart shows a laboratory rating of about IIC 55 against a field rating of about FIIC 50, a gap of roughly five points. Why field ratings run lower Direct path versus flanking paths Upper unit Lower unit Direct path measured in the lab Flanking paths walls, slab edge and penetrations — present only on site Typical gap IIC 55 FIIC ~50 Laboratory Field ≈ −5
Laboratory tests measure only the direct path through the floor. On site, sound also flanks around the assembly through the walls, slab edge and service penetrations, so field ratings typically come in about five points lower.

Because of that gap, a specification must say which it means. “IIC 55” as a lab target and “FIIC 55” as a field target are very different requirements.

What to specify

Many codes and rating systems use STC 50 and IIC 50 (laboratory) for separating floors between dwellings as a baseline, with field values a few points lower considered acceptable. Premium residential, hotels and mixed-use developments in the UAE are often specified higher — into the high 50s or 60s — because occupant expectations are higher and remediation after handover is expensive.

A workable approach:

  1. Get a target from an acoustic consultant, stated as lab or field, for both airborne and impact.
  2. Ask the flooring manufacturer for a tested assembly — slab, screed, finish and underlay — that meets it. Not a product “IIC value” in isolation, which does not exist independently of an assembly.
  3. Detail the flanking control: perimeter isolation strips, sleeved penetrations, and separation from corridor screeds. Our guide on choosing acoustic underlay thickness covers those details.
  4. Consider field testing a completed sample area before rolling out, so any shortfall is caught while it is cheap to fix.

The short version

  • STC = airborne sound (voices, TV). Driven by mass. Underlay barely affects it.
  • IIC = impact sound (footsteps). Driven by finish and resilient layer. Underlay is central to it.
  • Field beats lab claims: expect roughly five points lower on site, and make sure the spec says which one it wants.
  • There is no product IIC without an assembly. Ask for tested build-ups.

If you have a consultant’s STC and IIC targets, send them to Rukn Al Zain with your floor make-up and we will identify the acoustic & soundproofing flooring build-up — ZainAcoustic Underlay, ZainSilence screed isolation or a Zain Acoustic Strip perimeter detail — with data to match.

Frequently asked questions

What is the difference between STC and IIC?

STC (Sound Transmission Class) rates how well an assembly blocks airborne sound — voices, TV, music. IIC (Impact Insulation Class) rates how well a floor-ceiling assembly blocks structure-borne impact sound — footsteps, dropped objects, dragged furniture. A floor needs both: mass for STC, and a resilient layer or soft finish for IIC.

Is a higher STC or IIC number better?

Yes, higher is better for both. Each is a single-number rating derived from performance across a range of frequencies, so an increase of a few points is a meaningful improvement in what occupants hear.

Why is the field IIC lower than the lab IIC?

Laboratory tests isolate the floor assembly and remove flanking paths. In a real building, sound also travels through walls, the slab edge and service penetrations, so measured field values — reported as FIIC or as L'nT,w — are typically around five points below the laboratory rating for the same assembly. Specifications should state whether the target is a lab or a field figure.

How much does an acoustic underlay improve IIC?

A resilient underlay mainly improves impact (IIC) performance, and the gain depends on the whole assembly — slab thickness, any screed, the floor finish and the underlay's properties — not the underlay alone. The only reliable figure is a test result for an assembly that matches yours, which is why manufacturers publish tested build-ups rather than a single "IIC value" for the product.

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