Two Numbers. Two Kinds of Noise. One Assembly.
Every multifamily floor/ceiling assembly is fighting two different enemies, and the code gives each one a number.
STC — Sound Transmission Class — is airborne sound. Voices, televisions, music. Sound waves traveling through the air hit the floor assembly and try to push through it. The defense is mass: heavy materials resist being vibrated by air. A bare wood subfloor is light, so it transmits conversation freely. Pour 1.5 inches of gypsum concrete at 110–120 lb/ft³ over it and you have added the mass the assembly needs. Higher STC = less of the neighbor’s television.
IIC — Impact Insulation Class — is structure-borne sound. Footfalls, dropped objects, dragged chairs. Here the noise does not travel through air first — it is hammered directly into the structure, and mass alone will not stop it. The defense is decoupling: a resilient layer that breaks the rigid path between the finish floor and the framing. That is the acoustic mat’s job. The gypsum topping floats on the mat, so impact energy is absorbed instead of transmitted to the ceiling below. Higher IIC = fewer footsteps overhead.
What the code requires: the IBC sets a minimum STC 50 and IIC 50 between dwelling units, or 45 for each when field-tested. Wood-frame construction cannot reach those numbers on framing and subfloor alone — which is why an acoustic mat with a gypsum concrete topping is the standard path to compliance in wood-frame multifamily, hotels, and senior living. It adds the mass for STC and the decoupling for IIC in a single low-profile assembly.
One more distinction worth knowing: STC and IIC lab ratings describe a whole tested assembly — framing, subfloor, mat, topping thickness, ceiling. Change any layer and the rating no longer applies. That is why the mat product, the topping thickness, and the topping density in your spec are not suggestions.
Tested in a Lab. Built in the Rain. Argued in the Field.
The most common way a sound assembly fails is not exotic — it is a topping that went down lighter or thinner than the tested assembly, or a mat installed under a topping batched so wet it lost density. When a field test comes back at IIC 43 and the tenants below are complaining, the question becomes: was the assembly built as tested?
We install the mat and pour the topping as one scope, and our Smart Batch® GC-1 weighs every batch against the mix design the assembly was engineered around — then archives the record to the cloud. If the acoustic performance of a floor is ever questioned, you hold a per-batch document showing the topping went down at the specified mix. That is a conversation no hand-batched pour can have.
Mat. Topping. Rating.
- Acoustic mat + gypsum concrete topping — the standard wood-frame multifamily assembly; sound attenuation mats (including Formulated Materials M-series and comparable systems) under FIRM-FILL® or Treadstone® gypsum toppings
- Sound mat under resilient finish floors — attenuation mats installed for sound-rating compliance where LVP, sheet vinyl, or engineered flooring is going down
- Retrofit sound upgrades — adding mass and decoupling over existing subfloors in renovation and change-of-use projects
- Assembly consultation at bid time — we will read your architect’s tested-assembly callouts and flag mismatches between the drawings and the spec before they become change orders
