When builders start paying attention to acoustics, STC is usually the first number they encounter. It shows up on spec sheets, product data, and assembly guides, and it gives the impression of being a complete picture of how well a wall or floor controls sound. For airborne noise, it mostly is. For everything else, including the footfall complaint that accounts for a significant share of post-occupancy callbacks in multifamily construction, STC tells you almost nothing. That’s where IIC comes in, and understanding the difference between the two is what separates a spec that looks good on paper from one that actually performs.
This piece covers what each rating measures, what it doesn’t, and how to think about both when you’re specifying for a real project. For the broader foundation on how sound moves through buildings and the four methods used to control it, that’s covered in How Sound Works in Buildings: A Complete Guide for Builders and Architects.
What STC measures
STC stands for Sound Transmission Class. It measures how well a building assembly reduces airborne sound transmission, meaning sound that travels through the air before it reaches a surface. Voices, music, television, a conversation carrying through a wall from one room to the next, these are all airborne noise sources and STC is the metric that describes how well an assembly handles them.
The test runs across 16 frequency bands, from roughly 80 Hz up to 4,500 Hz, covering the range where most speech and common indoor noise sources fall. The results across those frequencies are combined into a single number using a standardized curve, and that number is the STC rating. Higher is better. A standard wood-framed wall with half-inch drywall on each side and no insulation in the cavity lands around STC 33 to 35. Add cavity insulation and you get into the low 40s. A well-built assembly that combines multiple control methods, mass, absorption, decoupling, and damping, can reach STC 50 or above.
To put those numbers in practical terms, Quieture’s tested wall assemblies show what layering methods actually achieves:
Each step adds a layer of control and the performance compounds accordingly. What STC does not measure is equally important to understand. The rating is based on airborne sound transmission across a specific frequency range. It says nothing about how an assembly performs against impact noise, and it says nothing about very low frequencies, the deep bass from a subwoofer or heavy mechanical equipment, which fall below the test range. A wall or floor can carry an impressive STC rating and still transmit both of those things clearly.
What IIC measures
IIC stands for Impact Insulation Class. It measures how well a floor assembly reduces impact noise transmission, meaning noise generated by a physical strike on the surface rather than by sound traveling through the air. Footsteps are the most common source, which is why IIC is the metric that matters most in multifamily construction and why it tends to be the rating behind the most common post-occupancy complaint builders receive.
The test works differently from STC. A standardized tapping machine is placed on the floor above and operated at a set impact level. Measurements are taken in the space below across the same frequency range, and the results are combined into a single IIC number. Higher is better here too.
The key distinction is that impact noise is structurally transmitted. When someone walks across a floor, that footfall sends vibration directly into the floor structure, which travels through the building materials and radiates sound into the space below. This is a fundamentally different physics problem from airborne sound transmission, and it requires a different approach to address it. Mass helps with both, but impact noise requires isolation, a way to interrupt the structural path that the vibration is traveling through, in addition to mass. A gypcrete layer adds mass to a floor system and improves both STC and IIC, but without a decoupling underlayment working alongside it, the impact transmission path through the structure remains largely intact. The underlayment is what provides the isolation that gypcrete alone can’t deliver.
Why both numbers matter for floor assemblies
The practical reason to understand the difference between STC and IIC is that floor assemblies in any multi-story building need to be evaluated against both. A floor spec that addresses airborne sound transmission but ignores impact noise will produce a space where residents can’t hear the television from the unit above but can hear every footstep clearly, and that’s a complaint that’s entirely preventable with the right spec from the start.
The other reason is that the two ratings don’t always move together. An assembly change that improves STC might have a neutral or even negative effect on IIC depending on how it’s constructed. Adding mass improves airborne performance. Adding isolation improves impact performance. Adding both, along with absorption and damping, is what produces an assembly that performs well against the full range of noise sources a resident is likely to encounter.
For walls in single-story residential construction, STC is typically the relevant metric and IIC doesn’t apply. For floors in multifamily buildings, townhomes, or any project where one occupant’s floor is another’s ceiling, both ratings need to be part of the conversation from the time the assembly is being designed, not after the first callback arrives.
Reading the numbers in context
A few practical things worth knowing when you’re looking at STC and IIC ratings on a spec sheet or product data sheet.
Lab ratings versus field performance. Tested ratings are produced in controlled laboratory conditions with no flanking paths, sealed perimeters, and no competing variables. Field performance is almost always lower because real buildings have penetrations, rigid connections at plates, mechanical systems, and other paths that the test environment eliminates. Treat lab ratings as a ceiling, not a guarantee.
The frequency limitation. Both STC and IIC testing covers a specific frequency range. Very low-frequency noise, below roughly 80 Hz, is not captured by either rating. This matters in multifamily buildings where bass from sound systems or mechanical equipment is a complaint source, because an assembly that rates well on both metrics can still transmit low-frequency energy that the ratings don’t account for.
Assembly matters as much as product. A product’s contribution to STC or IIC is always a function of the full assembly it’s part of. The ratings on a product data sheet reflect tested assemblies, not standalone material performance. Specifying a high-performing product into a poorly detailed assembly will not produce the tested result.
Want to go deeper?
Michael Crane, acoustic specialist at MP Global Products, walks through how STC and IIC work in practice, including how he has field-tested assemblies on real projects, in a conversation with builder Ben Bogie recorded for an episode of Ben’s Basement Tapes.
For the full picture on how sound moves through buildings and the four methods used to control it, start with How Sound Works in Buildings: A Complete Guide for Builders and Architects.
FAQ
What is the difference between STC and IIC ratings?
STC measures how well an assembly reduces airborne sound transmission, the kind generated by voices, music, and television. IIC measures how well a floor assembly reduces impact noise transmission, the kind generated by footsteps and physical strikes on the surface. They address different types of noise and require different design strategies to improve.
What is a good STC rating for a residential wall?
A standard wood-framed wall with drywall on each side lands around STC 33 to 35. Adding cavity insulation brings that into the low 40s. A well-built assembly that combines mass, absorption, decoupling, and damping can reach STC 50 or above. The right target depends on the project type and what the adjacent spaces are being used for.
Why does my floor have a good STC rating but residents still complain about footsteps?
STC measures airborne sound transmission, not impact noise. Footsteps generate impact noise, which is structurally transmitted through the floor assembly rather than through the air. A floor can perform well on STC and still transmit footfall clearly if the assembly doesn’t include adequate isolation to interrupt the structural vibration path. IIC is the relevant metric for footfall performance.
Do I need to consider both STC and IIC for every project?
For walls in single-story residential construction, STC is typically the relevant metric. For any floor assembly where one occupant’s floor is another occupant’s ceiling, both ratings need to be part of the spec. Multifamily buildings, townhomes, and any multi-story project where impact noise is a likely complaint source require attention to both.
Why do field STC ratings differ from lab ratings?
Lab testing is conducted in controlled conditions with sealed perimeters, no flanking paths, and no competing variables. Real buildings have penetrations, mechanical systems, rigid connections at plates, and other paths that reduce field performance relative to the lab result. Field performance is almost always lower than the tested rating, which is why assembly detailing and air sealing matter as much as the product spec itself.
