The tenant on the third floor calls the property manager: "We can hear every footstep from the office above us. It sounds like they're wearing boots on a dance floor." The property manager calls us. It's a conversation we have monthly.
Noise between office floors comes in two forms: airborne sound (voices, music, phone conversations) and impact sound (footsteps, dropped objects, rolling chairs, moving furniture). Each requires a different approach, and fixing one doesn't necessarily fix the other.
Understanding the Ratings
STC — Sound Transmission Class
STC measures how well a floor-ceiling assembly blocks airborne sound. Higher is better. A bare concrete slab (6" normal weight) tests around STC 53-55. That's decent for airborne sound — voices and music are mostly blocked. Add a suspended ceiling below and the STC climbs to 58-65 depending on the ceiling system and plenum depth.
For reference: STC 50 means loud speech is barely audible through the assembly. STC 55 means most sounds are inaudible. STC 60+ means even loud music is barely perceptible. Most office building specs target STC 50-55 between floors.
IIC — Impact Insulation Class
IIC measures how well the assembly isolates impact sound. This is the rating that matters for footsteps and dropped objects. Bare concrete scores poorly on IIC — around 28-34 for a typical 6" slab. That's why footsteps transmit so clearly through concrete floors. The concrete is rigid and dense, which is great for blocking airborne sound but terrible for impact isolation.
Office buildings should target IIC 50+ between floors. Residential buildings and hotels typically require IIC 50-55 by code.
Solutions That Work
1. Floating Floor Systems
The most effective solution for impact noise. A floating floor is a secondary floor surface that sits on resilient pads or mats above the structural slab. The resilient layer decouples the walking surface from the structure, so impact energy doesn't transmit directly into the slab.
Options range from simple resilient underlayment (rubber or cork sheeting under carpet or LVP) to full floating floor systems with plywood over isolation pads. A quality resilient underlayment under carpet can improve IIC by 15-25 points. A full floating floor system can push IIC above 60.
2. Ceiling Treatment Below
Adding or improving the suspended ceiling on the floor below helps both STC and IIC. A standard 2×4 acoustical tile ceiling on 15/16" grid improves STC by 5-10 points over the bare slab. Adding a layer of sound isolation batt (unfaced fiberglass or mineral wool) in the plenum improves both ratings further.
For maximum performance, use resilient channel or sound isolation clips on the ceiling framing. This decouples the ceiling plane from the structure — similar concept to a floating floor, but on the bottom side. The combination of a floated ceiling below and resilient flooring above can achieve STC 65+ and IIC 55+.
3. Concrete Deck Treatments
For existing buildings where adding a floating floor or new ceiling isn't practical, treatments applied directly to the concrete deck can help. Spray-applied damping compounds (viscoelastic coatings) reduce the slab's ability to vibrate and radiate sound. They're applied to the underside of the slab in the ceiling plenum. Improvement is modest (3-5 STC/IIC points) but sometimes that's enough to resolve a specific complaint.
4. Mass-Loaded Vinyl (MLV)
MLV is a dense, flexible sheet material that adds mass to the floor-ceiling assembly. Draped over the ceiling grid or laid over the existing ceiling tiles, it increases STC by 3-7 points. It doesn't help much with impact noise, but for airborne sound (voices from the floor above), it's a cost-effective addition.
What Doesn't Work
- Adding carpet alone: Carpet with pad improves IIC on the upper floor, but if the complaint is on the lower floor, you need to treat either the floor above or the ceiling below. Carpet is a good part of the solution, not the whole solution.
- Thicker ceiling tiles: Swapping from 5/8" to 1" ceiling tiles provides minimal improvement in sound isolation. The mass difference is too small to matter.
- Acoustic foam on walls: Wall treatment absorbs sound within a room. It does nothing for floor-to-floor transmission. Different problem, different solution.
Practical Steps for Building Owners
- Identify the noise type. Is it footsteps and impact (IIC problem) or voices and music (STC problem)? The solution differs.
- Test before you treat. An acoustic consultant can measure existing STC and IIC performance and identify the weak points in the assembly.
- Address the source first. Treating the floor above (resilient underlayment, carpet) is usually more effective and less disruptive than treating the ceiling below.
- Combine strategies. The best results come from treating both sides — floor above and ceiling below. Each layer adds to the overall performance.