STC and IIC ratings describe two different ways sound moves through floor assemblies. STC measures airborne sound, such as voices, televisions, and music. IIC measures impact sound created by footsteps, dropped objects, and other contact with the floor above.
For Wisconsin multifamily projects, these ratings matter because residents experience both types of noise. A floor can control airborne sound well and still allow noticeable impact noise to travel between units. The complete floor-ceiling system has to be considered, including the structural deck, acoustic layer, gypsum underlayment, flooring, insulation, ceiling construction, and other assembly details.
That distinction becomes especially important when materials are changed during construction. STC and IIC ratings apply to tested assemblies, not to one product by itself.
What Is the Difference Between STC and IIC?
STC stands for Sound Transmission Class. It describes how well a wall or floor-ceiling assembly reduces airborne sound transmission.
Conversations, televisions, and music are common examples of airborne noise in apartments, senior living communities, dormitories, and hotels. In general, a higher STC rating indicates that less airborne sound passes through the tested assembly.
IIC stands for Impact Insulation Class. It measures how well a floor-ceiling assembly reduces impact sound.
Footsteps are the easiest example. When someone walks across a floor, the impact creates vibration that can move through the flooring and building structure. Moving furniture or dropping an object can create the same type of structure-borne sound.
The two ratings answer different questions:
- STC measures airborne sound.
- IIC measures impact sound.
A multifamily building usually needs to account for both.
Why Are STC and IIC Ratings Based on the Entire Assembly?
This is where sound-control specifications can become confusing.
A gypsum underlayment does not have one universal STC or IIC rating that applies to every project. Neither does an acoustic mat, flooring material, or ceiling component when it is considered by itself.
The rating comes from the complete tested construction.
A floor-ceiling assembly might include a structural deck, acoustic mat or membrane, gypsum underlayment, finished flooring, insulation, resilient components, and the ceiling below. Change one part of that system, and the acoustic performance may change with it.
This matters during product substitutions and value engineering. Changing from carpet to hard-surface flooring, switching acoustic membranes, modifying the ceiling, or changing the specified underlayment depth may create a different assembly from the one that was originally tested.
The better question is not, “What is the STC rating of this product?”
It is, “What complete assembly was tested to achieve this rating?”
Elastizell of Wisconsin installs sound control systems using gypsum underlayments with acoustic mats or sound-control membranes where specified. These systems are used in multifamily, mixed-use, senior living, dormitory, hotel, and commercial construction.
What STC and IIC Ratings Does Code Require?
The 2021 International Building Code addresses sound transmission between dwelling units and sleeping units, as well as between those spaces and certain public or service areas.
Under Section 1206, applicable floor-ceiling assemblies must generally have an STC rating of at least 50 when laboratory tested under ASTM E90. Floor-ceiling assemblies subject to the impact-sound provisions must generally have an IIC rating of at least 50 when tested under ASTM E492. Field-tested assemblies use separate measurements and thresholds.
Wisconsin currently uses the 2021 International Building Code as part of its Commercial Building Code. Plans submitted on or after November 1, 2025, must comply with the updated code and Wisconsin amendments.
Project specifications can also call for performance above the code minimum. Developers, architects, and acoustical consultants may set higher targets based on the building type, flooring choices, ownership goals, or expectations for the finished space.
The current building code provides the baseline, but the plans and tested assembly for the specific project should drive material and installation decisions.
Why Hard-Surface Flooring Can Change the Acoustic Picture
One practical reason IIC deserves close attention is the continued use of hard-surface flooring in multifamily buildings.
LVT, hardwood, and similar finishes do not interact with impact sound the same way carpet and pad do. The finished flooring is one part of the tested system, so changing the finish can affect how the complete assembly performs.
That does not mean hard-surface flooring cannot meet acoustic requirements. It means the sound-control system underneath it has to be selected as part of the entire floor design.
Elastizell’s gypsum underlayments can be installed as lightweight pour-over toppings that create a flat substrate for flooring such as LVT, carpet, and hardwood. When paired with the specified acoustic mat or membrane, the system can become part of an assembly designed around both airborne and impact sound control.
The key is matching the construction in the field to the assembly shown in the plans or supporting test documentation.
What Can Change an IIC or STC Result in the Field?
Two buildings can use similar materials and still perform differently acoustically.
That is because sound does not travel through just one layer. The framing, ceiling system, insulation, penetrations, transitions, flooring, acoustic membrane, and installation details can all influence the finished result.
Common project changes worth reviewing include:
- Switching from carpet to LVT or another hard flooring
- Changing the specified acoustic mat
- Changing gypsum underlayment thickness
- Altering ceiling or framing details
- Adding penetrations through the assembly
- Substituting materials that were not part of the referenced test
- Changing floor transitions or elevation requirements
This is one reason acoustic decisions should not be left until the flooring phase.
Once several trades are already working from the established floor elevations, correcting a conflict can become much more difficult than identifying it during preconstruction.
Where Does Gypsum Underlayment Fit Into the System?
Gypsum underlayment serves as more than a surface for finished flooring.
On multifamily and multi-story projects, it can create a smooth, flat substrate while contributing to assemblies that also address acoustics and fire-rating requirements. Elastizell uses gypsum underlayments in applications where sound attenuation and flooring preparation are both important.
The pour can be installed over an acoustic mat or membrane when the specified system calls for one. Finished flooring is then installed above the underlayment based on the project requirements.
The underlayment itself should not be treated as a shortcut to a target STC or IIC value. Its role has to be viewed in relation to everything above and below it.
Projects dealing with uneven elevations, transitions, or substrate conditions may also require separate floor leveling work before the final flooring system is complete. Elastizell provides gypsum and cement-based underlayments for those applications as well.
What Should Be Checked Before the Gypsum Pour?
By the time a gypsum pour is scheduled, many of the decisions that affect acoustic performance have already been made.
A review before installation can help confirm that the construction being installed still matches the intended assembly.
Items worth checking include the specified gypsum depth, acoustic mat or membrane, finished flooring, floor elevations, transitions, penetrations, tested assembly reference, and any STC or IIC requirement listed in the plans.
This is particularly important when substitutions have occurred earlier in the project.
A product may appear similar to the material originally specified, but that does not automatically mean the revised floor-ceiling system has the same tested acoustic performance.
Elastizell of Wisconsin has worked in specialty concrete since 1963 and operates gypsum underlayment, lightweight cellular concrete, and structural concrete divisions. Our company history reflects more than six decades of work throughout Wisconsin and the surrounding region.
Build Floor Assemblies Around the Whole System
STC and IIC ratings describe different parts of acoustic performance. STC addresses airborne sound, while IIC addresses impact sound. For Wisconsin multifamily floor assemblies, both need to be understood in the context of the full floor-ceiling construction.
The most important takeaway is that an acoustic rating does not belong to one layer. Gypsum underlayment, acoustic materials, structural components, finished flooring, insulation, and ceiling details work together as one tested system.
If your multifamily, mixed-use, senior living, dormitory, hotel, or commercial project calls for gypsum underlayment with integrated sound control, Elastizell of Wisconsin can help review the scope and coordinate the underlayment installation with the requirements of the project.
