What Is Sound Transmission Class (STC)?

Sound Transmission Class (STC) is a laboratory rating of how well a complete building assembly reduces airborne sound passing through it. The rating applies to tested walls, floors, or ceilings, not to a flooring product by itself. It does not measure footfall noise, and real-world results can differ when sound travels around the tested assembly.

If you are choosing flooring or planning a renovation, an STC number can look like a simple promise of quiet. It is not. To use it well, you need to know what was tested, what kind of noise matters, and whether the installed floor matches the tested design. I start by separating airborne sound from impact sound, then look at the whole floor-and-ceiling system.

That approach helps avoid a common and costly mistake: choosing an underlayment because it has a high-sounding rating, without checking whether that number describes the finished assembly. The sections below explain the rating, how to read it, and how to reduce the chance that installation details undermine it.

What the STC rating measures

Sound Transmission Class is a single-number rating that summarizes how a tested building assembly reduces airborne sound across a defined range of frequencies. A higher STC generally means less airborne sound passes through that assembly under laboratory test conditions. It is not a guarantee of quiet in every room or a rating for a loose flooring layer.

Airborne sound includes speech, television, and music. A laboratory sends sound toward one side of a test specimen and measures how much passes through to the other side. The resulting sound transmission loss data are then classified as an STC rating.

STC applies to the tested assembly: its layers, framing, cavity, and installation details. For a floor, that may include the slab or joists, subfloor, insulation, ceiling, resilient components, and specified finish. If a product listing gives a number but does not identify the tested assembly and method, that number alone may not help you predict the finished floor’s performance.

How the rating is determined

ASTM E90 is the laboratory method used to measure airborne sound transmission loss through building elements. ASTM E413 provides the method for classifying the measured data as an STC rating. The classification uses one-third-octave-band data from 125 to 4,000 Hz.

That range matters. The STC number does not describe performance at every frequency. In particular, it does not fully represent very low-frequency sound, such as some bass sounds, or the distinct thuds and taps caused by impacts. When those sounds are the concern, ask for the relevant test data or another appropriate rating instead of relying on STC alone.

A higher rating is useful for comparing assemblies tested on a comparable basis. But a difference in rating is not a direct promise that one room will seem a fixed amount quieter. Room layout, sound source, background noise, and paths around the assembly all affect what people hear.

STC, ASTC, and IIC: choosing the right measure

STC and apparent STC (ASTC) concern airborne sound, while Impact Insulation Class (IIC) concerns standardized impact noise. The right measure depends on the noise you want to control. A floor can perform differently for voices than for footsteps, so one number cannot answer both questions.

ASTM E336 covers field measurement of airborne sound attenuation between rooms. Field results may be reported or classified as apparent STC, or ASTC, under applicable procedures. Unlike a laboratory test of a specimen, a field measurement reflects the rooms and building paths present at the time of testing.

IIC is a separate classification for standardized impact-noise performance. ASTM E492 is used for laboratory impact-sound measurements, and ASTM E989 provides the classification method. IIC is often relevant when footsteps or dropped objects are the main concern. Neither IIC nor STC predicts every sound in every building.

Before selecting a system, identify whether you need airborne control, impact control, or both. Then check local code and project specifications, since required ratings and test conditions can vary by project.

Read test results as assembly specifications

An STC result belongs to a specific tested build-up, not to a general category such as “wood floor” or “soundproof underlayment.” When comparing products, check the full test description, including the support structure, finish layers, ceiling, insulation, and resilient parts. A mismatch between tested and installed details makes the rating less useful as a prediction.

Ask for the test report or assembly details. Confirm whether the documented result is for the exact combination you plan to install, or whether a qualified design professional has approved a change. An underlayment alone does not establish the STC of a finished floor; its effect depends on the other layers and how they connect.

There is no universal underlayment thickness, moisture limit, or material choice that guarantees a certain floor STC. Follow product-specific installation limits and the test or design basis for the whole system.

Substrate inspection, floor/ceiling assembly, and subfloor preparation

A reliable plan starts with documenting what is already in the building. Record whether the floor is a concrete slab or a framed system, and note the subfloor, ceiling, cavity, penetrations, and connections to nearby walls. These details matter because the tested rating belongs to an assembly, not just its visible surface.

Before covering the floor, inspect the substrate and structure. Check floor flatness against the selected flooring manufacturer’s requirements, and assess moisture using methods and limits suited to the materials involved. Moisture limits are product-specific, so do not substitute a general rule for the flooring and underlayment instructions.

Acoustic planning does not replace structural checks. Common deflection criteria are L/360 maximum for ceramic tile and L/720 for natural stone, subject to applicable standards and project requirements. These figures describe structural deflection criteria, not sound ratings. Confirm that the floor meets the requirements for the chosen finish independently of its acoustic design.

Acoustic layer installation and flooring technique

Once the assembly is defined, install its layers as specified in the documented test or approved design. That may include insulation, resilient channels, a membrane, underlayment, or a ceiling system. Changing a layer, omitting a component, or using different attachment details can change performance, so avoid treating two similar-looking systems as interchangeable.

Resilient components work by limiting direct, rigid connections between parts of an assembly. Fasteners that pass through a resilient layer, or other unintended rigid contacts, can bridge that separation. Follow the assembly instructions for fastener type, placement, and length. Do not add fasteners or adhesives simply because they seem likely to make a floor more secure.

For a floor installation, coordinate the acoustic layer with the flooring manufacturer’s instructions. Confirm that the materials are compatible and that the finished surface will meet required flatness and structural limits. There is no single underlayment thickness that works for every floor or guarantees an STC result; use the documented system and product-specific installation rules.

Transitions, perimeter isolation, and flanking paths

Sound can travel around a floor or wall instead of passing straight through it. This is called flanking transmission. Continuous walls, rigid perimeter connections, ducts, and unsealed penetrations can carry sound around a high-performing assembly, so a laboratory rating does not by itself predict the result between finished rooms.

At transitions, preserve the designed separation between resilient components and adjoining construction. Avoid rigid perimeter contacts that short-circuit the isolation layer, and seal gaps and penetrations with the specified acoustic sealant. Keep required movement joints; filling or bridging them in the wrong way can conflict with the flooring system’s needs.

A high laboratory STC can therefore coexist with disappointing field performance. The tested assembly may be sound, while an untreated path at a wall, pipe, or duct lets sound bypass it. When project compliance matters, arrange field testing using the specified ASTM method and have the results interpreted against the project requirements.

Illustrative floor scenarios

These examples show how the rating should guide decisions; they are not measured case studies or promises of specific performance. In each scenario, the key question is whether the proposed work matches a tested or approved assembly and addresses the noise that people actually hear.

Concrete floor with footfall complaints: If residents mainly hear footsteps or dropped objects, ask for IIC information on the relevant floor-ceiling assembly. A high STC alone does not answer that concern. Check the test details and installation instructions for the chosen floor system.

Wood-joist floor with voices from below: For airborne speech, compare STC-tested assemblies that include the relevant joists, subfloor, cavity treatment, ceiling, and resilient components. A rating for an underlayment alone cannot establish the performance of this floor.

Renovation with a strong laboratory rating but poor room-to-room results: Inspect likely flanking paths, such as rigid wall connections or penetrations. If required, use field measurements under ASTM E336 to assess airborne sound between rooms rather than assuming the laboratory rating carried over unchanged.

The practical takeaway is to match the metric to the noise, then match the installation to the tested build-up.

Maintenance, sealant integrity, and long-term acoustic protection

Acoustic performance can be affected when gaps open, seals fail, or later work creates rigid connections through resilient parts. Keep installation and product records so future repairs can follow the same design. If flooring, walls, ducts, or penetrations are changed, check whether the work alters the assembly or its sound paths.

During repairs, use the specified sealant and preserve movement joints. Do not fill every visible gap with an arbitrary material; the correct treatment depends on the assembly and the flooring system. If sound transmission changes after building work, inspect the altered area and consider professional testing when the project requires verified performance.

Key steps before you choose a system

Use this sequence to turn a rating into a useful project decision:

• Decide whether the main concern is airborne sound, impact noise, or both.
• Check local code and project specifications for required performance and test methods.
• Document the slab or framing, subfloor, ceiling, cavity, penetrations, and possible flanking paths.
• Verify structural adequacy and floor flatness separately from acoustic performance.
• Select a complete tested or approved assembly, and follow its installation details.
• Seal specified gaps, preserve movement joints, and avoid rigid bridges through resilient layers.
• Arrange field testing when compliance or actual room-to-room performance must be verified.

Frequently asked questions

Is STC a rating for flooring underlayment?
No. STC describes the airborne-sound performance of a tested building assembly. Underlayment alone does not establish the STC of the completed floor.

Does a higher STC always mean a quieter room?
A higher STC generally indicates less airborne sound transmission in the tested assembly. It does not guarantee the same result in a finished room, where flanking paths and other site conditions matter.

Does STC measure footsteps?
No. STC addresses airborne sound. IIC is the separate classification commonly used for standardized impact-noise performance.

What does ASTM E90 test?
ASTM E90 is a laboratory method for measuring airborne sound transmission loss through building elements and partitions.

What does ASTM E413 do?
ASTM E413 classifies measured transmission-loss data to produce an STC rating.

What is ASTC?
Apparent STC is a field classification of airborne sound attenuation between rooms, based on applicable field measurement procedures such as ASTM E336.

What frequency range does STC cover?
The STC classification uses one-third-octave-band data from 125 to 4,000 Hz. Performance outside that range is not represented by the STC number.

Can I use an STC result from one floor on another?
Only if the tested or approved assembly matches the new floor closely enough for the design basis to apply. Differences in framing, layers, ceiling, or connections can affect results.

Can resilient underlayment be fastened through?
Only as the documented assembly directs. Unintended rigid fasteners or perimeter contacts can bridge resilient components and undermine the intended separation.

How can I confirm performance after construction?
If verification is required, arrange field testing using the method specified for the project. ASTM E336 is used for field measurement of airborne sound between rooms; the required criteria should come from the applicable code or project documents.

The most dependable way to use an STC rating is to treat it as evidence about a particular tested assembly, not as a promise attached to one product. Identify the kind of noise, match the complete floor-and-ceiling system, and pay attention to connections and flanking paths. When required, verify the finished building with the specified field test.

(This article was written by one of our staff writers, Andrew P. Morrison. Visit our Meet the Team page.)