IIC, STC & ΔIIC — Flooring Acoustic Ratings Explained
Three numbers decide whether a floor passes acoustic code in apartments, condos and hotels: IIC (impact noise), STC (airborne noise) and ΔIIC (the improvement a covering adds). This guide explains what each one means, how it is tested, the IBC 50 minimums, and the single most misunderstood point — why the same floor gives different numbers in different buildings.
IIC (Impact Insulation Class) measures impact noise such as footsteps, STC (Sound Transmission Class) measures airborne noise such as voices, and ΔIIC (Delta IIC) measures the impact-noise improvement a floor covering adds over a bare concrete slab. IIC is tested by ASTM E492 (lab) and E1007 (field), STC by ASTM E90 (lab) and E336 (field), and ΔIIC by ASTM E2179. For multi-family dwellings the International Building Code (Section 1207) sets minimums of IIC 50 and STC 50 between units. Critically, IIC and STC describe a whole floor-ceiling assembly, not the flooring alone — the same covering gives different ratings on different structures. Reference compiled by Ecoflors, a factory-direct SPC and LVT flooring manufacturer.
IIC, STC and ΔIIC — what each one measures
They sound similar but answer different questions. Specifying the wrong one is a common cause of noise complaints that pass on paper.
Impact Insulation Class
How well the floor stops impact noise travelling down — footsteps, dropped objects, dragged chairs. A standard tapping machine runs on the floor; a microphone measures the noise in the room below.
Sound Transmission Class
How well the assembly stops airborne noise — voices, TV, music passing through the structure. A broadband sound source plays on one side; the transmitted level is measured on the other.
Delta IIC
How much impact isolation a covering or underlayment adds over a bare slab. The slab is tested alone, then again with the covering; the difference is ΔIIC. It lets you compare products directly.
Two different kinds of noise
IIC and STC exist because a building has to stop two physically different problems. A floor can be excellent at one and poor at the other.
IIC tracks impact energy travelling down through the structure; STC tracks airborne sound passing through it. Multi-family code requires both.
IIC vs STC vs ΔIIC
The quick comparison specifiers come back to.
| IIC | STC | ΔIIC | |
|---|---|---|---|
| Measures | Impact noise (footfall) | Airborne noise (voices) | Improvement a covering adds |
| Rates | Whole assembly | Whole assembly | A product, vs bare slab |
| Lab standard | ASTM E492 | ASTM E90 | ASTM E2179 |
| Field standard | ASTM E1007 (FIIC) | ASTM E336 (FSTC) | — |
| IBC minimum | 50 (multi-family) | 50 (multi-family) | Not a code value |
| Use it to | Check code compliance | Check code compliance | Compare two products |
Walls are rated by STC only — impact transmission through a wall is rare and not regulated. Floor-ceiling assemblies in multi-family housing need both IIC and STC.
An IIC number describes a building, not a floor
This is where most specification mistakes happen. IIC and STC rate the entire floor-ceiling assembly — the structural slab or joists, the underlayment, the finish floor, and the ceiling below — combined. Change any one layer and the rating changes. The same flooring product gives a different IIC on a 6″ concrete slab than on a wood-joist floor.
That is exactly why a single “IIC 60” on a product sheet, with no assembly stated, tells you very little. A meaningful number always comes with the tested assembly and a test report.
Why ΔIIC exists
Because absolute IIC changes with the building, ΔIIC was created to compare products fairly — every product is tested on the same bare reference slab, so the improvement figures are comparable.
Lab vs field
Field results (ASTM E1007 / E336) typically run 5–10 points below lab results, because real installations add flanking paths and workmanship variables the lab does not have.
What the code requires
The International Building Code (IBC) Section 1207 sets minimum ratings of IIC 50 and STC 50 between dwelling units in multi-family residential occupancies — apartments, condos, hotels, dormitories and stacked mixed-use.
Many specifiers target higher (often IIC 55–60) to keep a margin for the field penalty, and some jurisdictions require post-construction field testing as final proof.
IBC §1207 · IIC 50 · STC 50How a floor reaches the target
Impact performance comes from the whole assembly: slab mass, decoupling, an acoustic underlayment, and the finish. A hard finish over a bare slab with no underlayment isolates impact poorly; adding a resilient underlayment is what lifts the IIC.
This is the role an attached acoustic pad plays — it builds the resilient layer into the floor product itself, which is the basis of ABA / RSVP flooring.
Assembly = slab + underlayment + finish + ceilingCommon questions on IIC, STC & ΔIIC
What is the difference between IIC and STC?
What is Delta IIC (ΔIIC), and how is it different from IIC?
What IIC and STC ratings does building code require?
Why do the same floors get different IIC ratings?
Why are field ratings lower than lab ratings?
How does an attached acoustic pad help meet IIC?
Specifying for an acoustic project?
If your project has an IIC / STC requirement, tell us the floor-ceiling assembly and target. We supply the acoustic-backed (ABA / RSVP) product options and the third-party test report for the matching assembly — with an FOB price within one business day.
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