Most acoustical flooring specifications are built on assumptions that haven't been tested in years. Before you choose an acoustical floor for a multi-family or hospitality project, here are five facts worth getting right.
1. The Sound Mat Assumption Is Outdated
For two decades, the standard approach to acoustical compliance in multi-family construction was a separate sound mat installed below the gypsum underlayment. Certified third-party testing (Intertek R5773.02) documents a 3mm acoustical glue-down LVT with an attached backing meeting target performance without that separate mat. The full data — four methods, four assemblies, side by side — is in The Silent Advantage.
2. Three Ratings Matter, Not One
Most specifications cite only IIC. That's the minimum required by the International Building Code. But it's not the whole picture:
| Rating | What It Measures | Code Requirement |
|---|---|---|
| STC | Airborne sound — voices, television, music | IBC §1206: STC 50 (lab) |
| IIC | Impact sound — footsteps, dropped objects | IBC §1206: IIC 50 (lab) |
| HIIC | Impact sound weighted toward frequencies occupants actually hear | Not required by code |
HIIC isn't required by any current code, but it's the metric that best predicts what occupants will actually experience.
3. The Assembly Carries the Rating — Not the Floor Covering
This is the most consequential fact in acoustical specification, and the most commonly misunderstood. STC in particular is governed almost entirely by the assembly's mass and decoupling. Neither an attached backing nor a separate mat rescues a weak assembly on its own. The practical implication: if the assembly you're building differs from the assembly that was tested, the published number may not apply to your project.
4. HIIC Best Predicts Perceived Quiet
IIC is measured with a tapping machine that strikes the floor across a fixed frequency range. HIIC adds weighting toward the higher frequencies human ears are most sensitive to. In the head-to-head comparison documented in the guide, the attached-backing assembly scores well on HIIC precisely because it controls the frequencies people notice — which is why two floors with the same IIC can feel very different underfoot.
5. Acoustical Specification Is a Structural and Financial Decision
Choosing an acoustical method isn't just a product selection — it's a decision about how many materials and trades your assembly requires:
- Separate mat below gypsum (conventional): ~10 steps, 4 materials, 2–3 trades
- Topical mat under LVT: ~8 steps, 2 materials, 1–2 trades
- Attached-backing glue-down (AFC Soundstop): ~6 steps, 1 material, 1 trade
Deleting a material and a trade has schedule and cost implications that compound across a 200-unit building.