Definition
STC (Sound Transmission Class) is a single-number rating of the airborne sound insulation of a building partition or element — a wall, floor, ceiling, door or window. A higher STC means the element blocks more sound passing through it.
STC is a rating, not a direct measurement. It is derived from frequency-dependent sound transmission loss measurements using the classification procedure in ASTM E413.
What Does STC Measure?
STC measures airborne sound insulation: how much sound energy is reduced when sound passes through an element from one room to another. It is the quantity you use when deciding whether a wall will keep a conversation, a television or street noise from being heard on the other side.
STC does not measure sound absorption (echo inside a room) and does not measure impact noise (footsteps on a floor), which uses a separate rating (IIC).
Unit / Rating
STC is expressed as a dimensionless rating (it is written without units, e.g. "STC 50"). Although it is derived from transmission loss measured in decibels, STC itself is a classification number, not a literal decibel reduction at every frequency.
Airborne Sound Insulation
Airborne sound insulation concerns sound that travels through the air and then through a building element — speech, music, traffic and similar sources. It is distinct from impact sound (footsteps, dropped objects), which is generated by direct mechanical contact and rated separately (IIC in the ASTM system).
Sound Transmission Loss
The foundation of STC is Sound Transmission Loss (TL), the frequency-dependent measure of how much sound an element blocks. It is related to the transmission coefficient (τ), the fraction of incident sound power transmitted through the element:
TL = 10 × log10(1 / τ)
TL is measured in decibels across many frequency bands. STC is the single number produced by rating that frequency-dependent TL curve.
How STC Is Determined
In outline:
- The element is tested in a laboratory to obtain the sound transmission loss (TL) at sixteen one-third-octave bands, from 125 Hz to 4000 Hz (per ASTM E90).
- The TL curve is compared to a reference contour defined in ASTM E413. The contour is shifted vertically until the sum of the deficiencies (where the measured TL falls below the contour) is as large as possible but not more than 32 dB, and no single band is deficient by more than 8 dB.
- The STC value is the value of the shifted reference contour at 500 Hz.
This multi-step procedure is why STC is a rating rather than a simple "STC = X dB" measurement.
Frequency Dependence
Sound transmission loss varies with frequency. Most partitions insulate better at mid and high frequencies and worse at low frequencies. STC condenses the whole frequency curve into one number, so it does not mean the element performs equally at every frequency.
Low-Frequency Limitation
STC is based on the 125 Hz to 4000 Hz range, so it does not fully capture performance below 125 Hz. Some noise sources — music with strong bass, heavy traffic, industrial equipment — contain significant low-frequency energy that a single STC number may not represent. When low-frequency isolation matters, the full TL curve (or additional low-frequency descriptors) should be reviewed, not just the STC rating.
Laboratory Testing
Laboratory measurement is defined in ASTM E90. A sample assembly is built in an opening between two rooms (a source room and a receiving room). Sound is generated in the source room and measured on both sides across frequency bands to obtain the transmission loss. Laboratory conditions suppress flanking paths so the test isolates the assembly itself.
Field Testing
Field performance is measured differently and reported as Field STC (FSTC) or an apparent rating, defined in ASTM E336. The field result includes all real-world transmission paths — flanking, workmanship, penetrations and junctions — and is therefore usually lower than the laboratory STC. A laboratory STC for a given assembly does not guarantee that the same number will appear on site.
STC vs Rw
| STC | Rw |
|---|---|
| ASTM-based rating system | ISO-based rating system |
| Commonly used in North America | Commonly used internationally |
| Airborne sound insulation | Airborne sound insulation |
| Based on its own rating procedure (ASTM E413) | Based on ISO 717-1 rating procedure |
| Not automatically equal to Rw | Not automatically equal to STC |
STC 50 is not automatically Rw 50, and Rw 50 is not automatically STC 50. The two systems use different reference contours, frequency ranges and procedures, so there is no fixed conversion between them. The numbers are often close in practice (frequently within a decibel or two for many common constructions), but they should be treated as related, not identical.
STC vs NRC
STC and NRC measure different things:
- STC — sound transmission / insulation (blocking sound passing through an element).
- NRC — sound absorption (reducing echo inside a room).
A wall can have a high STC and still provide poor absorption treatment inside the room. Conversely, an acoustic PET panel may be excellent at absorption but does not by itself provide high sound isolation. The two ratings answer different questions.
STC vs RT60
STC and RT60 are also unrelated:
- STC — transmission of sound between two spaces.
- RT60 — reverberation (decay of sound) inside a single space.
They are not substitutes for each other: a room can be well isolated (high STC) yet reverberant inside (long RT60), or the reverse.
STC of Complete Building Systems
STC is a property of a complete tested assembly — a specific wall, floor, ceiling, door or window construction — not a fixed property of a material in isolation. A statement such as "material X has STC 50" is meaningless unless it refers to a tested assembly of specified construction. When discussing STC, always refer to the complete tested assembly/system.
Walls
Wall STC depends on mass, the number of layers, cavity, decoupling and sealing. A single-leaf gypsum wall and a double-leaf wall with a cavity can have very different STC ratings even when they contain similar materials.
Doors
Doors are often the weak point in a partition. A heavy solid-core door with perimeter and threshold seals performs far better than a hollow-core door with gaps, because even small air leaks sharply reduce the effective insulation.
Windows
Windows typically have lower STC than the surrounding wall. Insulating glazing (laminated glass, wider air gaps, secondary glazing) raises the window's contribution, but the overall partition performance is limited by the weakest element.
Ceilings
For rooms sharing a common ceiling void, sound can travel over the partition through the ceiling cavity. A ceiling system's STC only applies to the specific suspended-ceiling and plenum construction, and flanking through the void must be addressed separately.
Flanking Transmission
In a real building, sound also travels around a partition through flanking paths — ceiling voids, ducts, electrical penetrations, junctions and structural connections. Flanking is a major reason field performance is lower than laboratory STC, and it must be designed out of the whole building, not just the partition.
Factors Affecting STC
- Mass — heavier constructions generally insulate better.
- Layering and cavity — decoupled multi-leaf systems outperform single-leaf systems of similar weight.
- Cavity absorption — insulation inside a cavity improves performance.
- Decoupling — resilient channels and staggered framing reduce structural coupling.
- Sealing — gaps and leaks sharply reduce effective STC.
- Frequency — performance varies across the spectrum.
Common Misunderstandings
- "STC is sound absorption" — incorrect; STC is insulation, not absorption.
- "STC is RT60" — incorrect; STC is transmission, RT60 is reverberation.
- "STC guarantees field performance" — incorrect; on-site performance is usually lower due to flanking and workmanship.
- "STC automatically equals Rw" — incorrect; different rating systems.
- "STC is a property of one material" — incorrect; it belongs to a tested assembly.
- "Higher STC automatically means better room acoustics" — incorrect; room acoustics (absorption, reverberation) are separate from insulation.
Limitations
- STC does not describe impact noise (footsteps) — that requires IIC.
- STC does not capture performance below 125 Hz, so it may under-represent low-frequency noise.
- STC does not describe absorption, reverberation or speech intelligibility.
- A high STC does not mean the element "soundproofs" the room in an absolute sense; real performance depends on the complete building and installation.
Relevant Standards
- ASTM E90 — Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.
- ASTM E413 — Classification for Rating Sound Insulation (the procedure that converts transmission loss into the STC single-number rating).
Related Yamamah Solutions
Yamamah supplies components used within sound-insulating assemblies. Any stated STC/Rw performance applies only to a complete, tested assembly — not to an individual material:
To verify the performance of a tested assembly, contact us about acoustic testing and consulting.
See also the Sound Absorption vs Sound Insulation hub.
Read more in the four-part acoustic fundamentals series: What Is NRC? · What Is RT60? · What Is Rw?
Frequently Asked Questions
What is a good STC rating? It depends on the application. Basic interior partitions may be around STC 35–45, while high-performance separating walls can exceed STC 55. The required value should come from the relevant building code or project specification.
Is STC the same as dB? No. STC is a rating derived from transmission loss measured in dB, but it is not a literal decibel reduction at every frequency.
Does STC include low-frequency noise? Only partially. STC is based on 125–4000 Hz and does not fully represent performance below 125 Hz.
Can I convert STC to Rw? Not with a fixed formula. They use different procedures; treat them as related but not automatically equal.
Verified Sources
- ASTM E90 — Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.
- ASTM E413 — Classification for Rating Sound Insulation.