Sound Ratings Explained IIC · STC · Test Assemblies · Multi-Family Requirements
Sound ratings for rigid core LVT are published as IIC (Impact Insulation Class) and STC (Sound Transmission Class) numbers. Both describe how a specific floor assembly performed in a laboratory test — not how the floor will necessarily perform in a given building. Understanding what is and is not captured by these numbers is essential for projects where acoustics matter, particularly multi-family residential, hospitality, and healthcare settings. Reference-only: no product recommendations.
IIC measures impact noise reduction (footsteps, drops); STC measures airborne noise reduction (voices, music). Both are assembly ratings — the concrete slab or wood frame and ceiling below contribute to the number, not just the flooring product. Published ratings are tested in specific lab assemblies and may differ from real-world performance. US multi-family codes commonly require a minimum IIC and STC of 50 for floor/ceiling assemblies.
What IIC and STC Measure
IIC — Impact Insulation Class
IIC measures how effectively a floor/ceiling assembly reduces impact-generated noise from reaching the space below. The test uses a standardized tapping machine that strikes the floor surface repeatedly, and the sound level in the room below is measured across multiple frequencies. A higher IIC number means the assembly reduces more impact sound. Hard-surface flooring like LVT generates more impact noise than carpet because it does not absorb the impact — the sound passes directly through the rigid material into the structure. Attached underlayment or a separate underlayment layer is the primary tool for improving IIC in LVT installations. Typical values for rigid core LVT with attached underlayment range from IIC 50 to IIC 72 depending on the assembly.
STC — Sound Transmission Class
STC measures how effectively a floor/ceiling assembly blocks airborne sound — voices, music, TV audio — from transmitting from one space to another. The test generates broadband sound in one room and measures the reduction in the room on the other side of the assembly. For flooring products, STC is typically less affected by the flooring material choice and more affected by the structural assembly (concrete slab vs. wood joists, ceiling insulation, ceiling treatment). Hard-surface flooring like LVT has less impact on STC than on IIC, because STC is dominated by the mass and construction of the structural elements rather than the surface treatment.
Why Assembly Context Matters
Published IIC and STC ratings are test results for a specific assembly — a combination of flooring product, subfloor structure, and ceiling below. The most common test assemblies for LVT are: (1) 6-inch bare concrete slab with no ceiling treatment, (2) 6-inch concrete slab with drop ceiling below, and (3) wood-frame floor joist assembly. The same flooring product can yield IIC ratings that vary by 10–20 points or more depending on which assembly is used. When comparing products, check whether the published ratings were obtained from the same test assembly. When the assembly type is not published, the rating cannot be reliably compared to another product's differently-assembled rating.
Common Benchmarks
| Rating level | What it means |
|---|---|
| IIC below 50 | Does not meet typical US multi-family code minimum; impact noise clearly audible below |
| IIC 50–54 | Meets base code minimum; impact noise reduced but still present |
| IIC 55–64 | Above code minimum; good performance in most multi-family applications |
| IIC 65+ | High-performance assembly; common in premium or acoustically demanding applications |
Spec Sheet Checklist
- Confirm which ratings are published (IIC, STC, or both) and which test standard was used (ASTM E492/E413 for US, ISO 717 for international).
- Identify the test assembly used — concrete slab vs. wood frame vs. other — as it significantly affects the rating.
- Check whether the rating includes a ceiling treatment below the slab — ceiling assemblies dramatically improve IIC and comparing with-ceiling to without-ceiling ratings is not valid.
- For products with attached underlayment, confirm that the published rating reflects the product with pad installed (it typically does).
- Verify whether additional underlayment is permitted and whether tested assemblies with supplemental underlayment are published.
- Compare published ratings against project code requirements or HOA minimums.
FAQ
Do higher IIC and STC numbers always mean better acoustic performance? ⌄
Higher numbers indicate better lab performance in the specific test assembly — but real-world performance depends on whether your assembly matches the tested one. A product rated IIC 72 was tested in a specific lab configuration. If your building has different framing or ceiling construction, actual IIC performance will likely differ. Published ratings are best used for relative comparison between products tested to the same standard, not as absolute guarantees.
What is the difference between IIC and STC? ⌄
IIC measures reduction of impact noise — footsteps and dropped objects — traveling through the floor structure. STC measures reduction of airborne sound — voices and music — through the same assembly. For flooring, IIC is usually the primary concern because hard-surface flooring transmits more impact noise than carpet. Building codes commonly specify minimum IIC requirements of 50 or higher for multi-family construction.
Can I increase sound ratings by adding extra underlayment? ⌄
For products with factory-attached underlayment, most manufacturers prohibit adding additional underlayment — the published rating applies to the product as manufactured. For products without attached pads, adding a heavier underlayment generally improves IIC, but the product's published rating no longer applies and you would need the underlayment manufacturer's test data for the modified assembly.
What IIC rating is required by building codes for multi-family housing? ⌄
US building codes commonly require a minimum IIC of 50 and STC of 50 for floor/ceiling assemblies in multi-family construction under IBC. Some jurisdictions require 55 or 60. HOA rules may set higher standards than base code. These minimums apply to the full floor/ceiling assembly, not just the flooring product — the structural system and ceiling treatment below both contribute.
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Reference-Only Information
This page provides general informational reference about sound ratings for rigid core LVT. It does not provide installation guidance, professional advice, or product recommendations.