Imagine you are in a hotel room and the couple next door is having a heated argument. You can hear every word through the wall — tone, volume, even specific phrases. Now imagine a different hotel, a better one. You know there are guests next door because you hear a faint murmur when you press your ear to the wall, but from your bed you cannot make out a single syllable. The wall in the second hotel has a higher STC rating than the wall in the first.
STC — Sound Transmission Class — is the single number that separates those two experiences. It is stamped on every partition system, wall assembly, door, and window that claims any acoustic performance at all. Understanding what it measures, what it does not measure, and what the numbers actually mean in practice is essential knowledge for anyone involved in building design, construction, or renovation.
The Definition: What STC Actually Measures
Sound Transmission Class (STC) is a single-number rating that describes how much a building assembly reduces airborne sound transmission across a standardised frequency range. A higher number means more reduction — more blocking — and less sound transmitted from one side to the other.
STC is defined by ASTM E413: Classification for Rating Sound Insulation, the governing North American standard. The test procedure that generates the raw data is ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.
In the laboratory, the assembly to be tested — a wall, floor, door, or window — is built into an opening between two rooms that are acoustically isolated from each other. A loudspeaker in the source room generates a standardised noise signal covering the test frequency range. A microphone in the receiving room measures how much sound passes through. The difference between the levels in the two rooms, after correcting for the receiving room's absorption, gives the Transmission Loss (TL) in decibels at each test frequency.
The test is performed at 16 one-third octave band frequencies from 125 Hz to 4000 Hz. The result is 16 TL values — for example: 25 dB at 125 Hz, 31 dB at 160 Hz, 38 dB at 200 Hz, and so on up to 4000 Hz.
These 16 numbers are then compared to a standardised STC contour — a reference curve defined in ASTM E413. The contour is shifted vertically until the total unfavourable deviation (the sum of all frequencies where the measured TL falls below the contour) is as large as possible without exceeding 32 dB. The STC rating is the value of the contour at 500 Hz at its final position.
In plain language: the STC is the number that best summarises the assembly's sound blocking performance across the speech frequency range, penalising assemblies that have weak spots at specific frequencies.
What the Numbers Mean: An STC Scale for Real Life
The abstract definition becomes useful when you anchor the numbers to real-world listening experiences.
| STC Rating | What You Hear Through the Assembly |
|---|---|
| 25 | Normal speech is clearly intelligible. Loud speech sounds like normal speech. |
| 30 | Loud speech is intelligible. Normal speech is audible but not clear. |
| 35 | Loud speech is audible but not fully intelligible. Normal speech is heard as a murmur. |
| 40 | Loud speech is heard as a murmur. Normal speech is barely audible. |
| 45 | Loud speech is barely audible. Normal speech is inaudible. |
| 50 | Loud speech is inaudible. This is the minimum typically required between dwelling units. |
| 55 | Very loud sound systems at low volume are audible. Normal conversation is completely private. |
| 60 | Only very loud sounds — a rock band, a home theatre at high volume — are faintly audible. |
| 65+ | Near-complete acoustic isolation for speech and most music. |
The critical threshold for most residential and commercial applications is STC 50. Below this level, occupants in adjacent spaces can hear speech. At STC 50, normal conversation is inaudible; loud conversation and music remain audible but not intelligible. Most building codes in the United States — including the International Building Code Section 1207 — require STC 50 as a minimum between dwelling units.
How Walls Achieve Different STC Ratings
The STC of an assembly depends on four physical mechanisms, each contributing differently to the total performance.
Mass: Heavy assemblies transmit less sound than light ones. A wall built from 200 mm solid concrete blocks has much higher STC than a wall built from 90 mm steel studs with a single layer of drywall. The relationship between mass and transmission loss follows the Mass Law: every doubling of mass per unit area adds approximately 6 dB of transmission loss.
Decoupling: When two surfaces are structurally connected, vibration passes directly through the connection points. Breaking that connection — by using resilient channels, double-stud framing, or floating floor systems — dramatically reduces the transmitted vibration. This is why a wall with two completely separate stud frames (a double-stud wall with a gap between them) consistently outperforms a single-stud wall even if both use the same amount of drywall.
Damping: Viscoelastic materials (such as constrained layer damping compounds applied between drywall layers) convert acoustic vibration into heat within the assembly, reducing the amplitude of transmitted sound. Purpose-built damping compounds can add 8-10 STC points to a standard drywall assembly without changing its thickness.
Absorption in the air cavity: Filling the cavity between wall faces with absorptive insulation (mineral wool or fiberglass batt insulation) prevents the air inside from acting as an additional transmission path. An unfilled cavity allows sound to bounce around inside the wall and re-radiate on the other side. A cavity filled with 90 mm of mineral wool insulation typically adds 4-8 STC points over the same assembly without insulation.
A Worked Example: Upgrading a Standard Partition
Consider a standard interior partition: 90 mm metal studs with a single layer of 13 mm drywall on each face, no insulation.
Based on standardised test data (ASTM E90 laboratory tests), this assembly achieves approximately:
- Transmission loss at 125 Hz: 20 dB
- Transmission loss at 500 Hz: 35 dB
- Transmission loss at 2000 Hz: 44 dB
- STC: approximately 33
Now let us add three upgrades, each building on the last:
Upgrade 1 — Add mineral wool insulation to cavity: TL improves by 4-6 dB at mid and high frequencies. New STC: approximately 39
Upgrade 2 — Add a second layer of 13 mm drywall to each face (now two layers per side, staggered joints): The additional mass adds 4-5 STC points. New STC: approximately 44
Upgrade 3 — Mount the drywall on resilient channels (decoupling the drywall from the studs): The decoupling adds 6-8 STC points by breaking the structural vibration path. New STC: approximately 52
This STC 52 assembly — metal studs, mineral wool insulation, two layers of drywall on resilient channels — is a common specification for offices and residential party walls. The total cost increase over the STC 33 base assembly is modest, but the acoustic performance improvement is enormous.
Where STC Falls Short: The Important Limitations
STC is a useful and widely used rating, but it has well-documented limitations that every designer should understand.
STC Does Not Measure Low-Frequency Performance
The ASTM E413 STC contour is intentionally weighted for speech. It covers frequencies from 125 Hz to 4000 Hz, and its weighting penalises deficiencies in the speech frequency range most heavily. This means an assembly can score well on STC while performing poorly at frequencies below 125 Hz.
Bass-heavy sounds — home theatre subwoofers, nightclub music, mechanical plant vibration, music in the 60-100 Hz range — bypass STC-rated assemblies far more readily than speech. A wall rated STC 55 that blocks conversation effectively may still transmit significant bass energy from a neighbour's music system.
This is not a flaw in the STC standard — it was designed for speech. But it means STC alone is insufficient for spaces with known low-frequency sources. In those situations, transmission loss data should be examined per octave band from 63 Hz or lower, not compressed into a single STC number.
STC Is a Laboratory Rating — Real-World Performance Is Lower
The "STC" stamped on a product datasheet is a laboratory measurement, made under controlled conditions with the assembly sealed at all edges and tested through a calibrated opening. Real-world installations suffer from flanking transmission — sound that travels around or through the assembly via indirect paths: through the shared floor slab, through HVAC ductwork, through the ceiling plenum above a partition that does not reach the structural deck.
The difference between laboratory STC and field STC (measured after construction, termed FIIC or FSTC — Field Sound Transmission Class per ASTM E336) is typically 3-8 STC points. A wall specified at STC 55 may deliver only STC 48 in the finished building if flanking paths are not addressed. Codes and standards account for this by specifying minimum FSTC values, which are typically 3-5 points below the required STC.
STC Is Airborne Sound Only
STC measures how well an assembly blocks sound that travels through the air — speech, music, television noise, HVAC noise. It says nothing about impact sound — the noise generated by footsteps, dropped objects, or furniture being dragged across a floor above you. Impact sound performance is rated by a separate metric called IIC (Impact Insulation Class), which uses a completely different test method. A floor assembly can have an excellent STC rating and a terrible IIC rating simultaneously.
Building Code Requirements for STC
Most building codes specify minimum STC requirements for specific assembly types. Key references:
International Building Code (IBC) 2021, Section 1207: Requires STC 50 (or FSTC 45) between dwelling units in residential occupancies, between dwelling units and public areas, and between bedrooms and other spaces within a dwelling unit.
International Residential Code (IRC): Similar STC 50 requirements for walls and STC 50 / IIC 50 requirements for floor-ceiling assemblies between dwelling units.
LEED v4 Acoustic Performance: Requires documentation of assembly STC ratings and verification of FSTC in field testing as part of the acoustic performance credit.
WELL v2 Feature 75 (Acoustics): Specifies minimum STC values for different wall types based on adjacency — for example, STC 50 between an office and a conference room, STC 55 between a conference room and a corridor, STC 60 between an executive office and a conference room.
These codes represent minimums — the floor below which a building cannot legally fall. They do not represent optimal acoustic comfort. An apartment built exactly to IBC minimums (STC 50 party walls) will technically comply with the code but will not provide the level of acoustic privacy that most occupants expect in a well-built building. Premium residential and commercial projects routinely target STC 55-65.
Common Mistakes When Specifying STC
Specifying only the STC rating without the full assembly detail: An STC 50 rating can be achieved by many different assemblies. A thin wall with damping compound may hit STC 50 at minimal thickness. A thick, massive wall hits STC 50 with a completely different profile across the octave bands. The designer should specify the full wall assembly (stud size, stud spacing, drywall thickness and layers, cavity fill, mounting method), not merely the target STC.
Ignoring flanking paths during design: Many projects specify high STC walls and then install them in buildings with open ceiling plenums, shared HVAC systems, or concrete slabs that bypass the partitions entirely. The acoustic consultant's job is not just to specify the partition STC but to identify and close all flanking paths.
Confusing STC with NRC: STC measures sound transmission through an assembly — how much sound is blocked. NRC (Noise Reduction Coefficient) measures sound absorption — how much sound energy a material absorbs rather than reflecting it back into the room. They address completely different acoustic problems. Adding acoustic foam panels to a wall does not increase its STC, but it does increase its NRC. Both parameters matter, but for different reasons.
Overlooking weak links: Every assembly is only as good as its weakest element. A STC 55 wall with a hollow-core door (STC 20-25) in it effectively becomes an STC 25 partition. Small gaps around electrical outlets, plumbing penetrations, and HVAC grilles can reduce field performance dramatically. A 1% open area in an otherwise STC 50 wall limits the effective field performance to approximately STC 30 — because sound takes the path of least resistance, and a gap is essentially no barrier at all.
How AcousPlan Helps You Get STC Right
AcousPlan's Sound Insulation Calculator gives you the tools to model, specify, and verify STC performance from the early stages of design through to construction documentation.
The platform draws on a library of tested wall, floor, and facade assemblies. For each assembly, you can:
- See the full octave-band transmission loss profile, not just the STC summary number, so you can identify low-frequency weaknesses before they become construction defects
- Compare multiple assemblies side by side to understand the performance-cost tradeoffs between mass, decoupling, and damping strategies
- Model flanking transmission paths and estimate the likely field FSTC, so your specification is realistic about what will be achieved after construction
- Check compliance automatically against IBC 2021 Section 1207, WELL v2 Feature 75, and other applicable standards for your project type
- Generate specification-ready documentation with full assembly details, STC ratings, and compliance notes for inclusion in project specifications
Ready to model your wall assemblies? Try the Sound Insulation Calculator at /insulation — input your room adjacencies, select assemblies from the library, and get STC ratings with compliance checking instantly.