STC, or Sound Transmission Class, is a single-number rating that measures how well a wall, floor, ceiling, door, or window blocks airborne sound. The higher the number, the more sound the partition stops. An STC 50 wall, for example, reduces noise by roughly 50 decibels, making loud speech on the other side barely audible. It’s the most widely used acoustic rating in the United States for interior building assemblies.
How STC Is Measured
STC ratings come from laboratory testing that follows ASTM E90, a standardized method where sound is generated on one side of a partition and measured on the other. The test captures how much sound the partition blocks across sixteen different frequencies, ranging from 125 Hz to 4,000 Hz. That range covers most of the human speech spectrum, from deep male voices up through higher-pitched consonants.
The sound reduction at each frequency is plotted on a graph, and the resulting curve is compared against a standard reference contour defined by ASTM E413. Where the curve fits against that reference determines the final STC number. This curve-fitting approach means the STC rating is a weighted average, not a simple measurement at one frequency. It’s designed to reflect how we actually perceive sound through a barrier in everyday situations like overhearing a conversation through a shared wall.
What Different STC Ratings Sound Like
The numbers are more meaningful when you connect them to real experience:
- STC 30 to 35: Basic separation. Normal conversation on the other side is clearly audible and easy to follow.
- STC 40 to 45: Moderate privacy. You can faintly hear raised voices but can’t make out normal speech.
- STC 50 to 55: The typical code minimum for walls between apartments and condos. Loud speech is barely audible.
- STC 60 and above: High privacy. This is the range recommended for recording studios, theaters, and hospital rooms where confidentiality matters.
Each jump of about 10 STC points roughly corresponds to a perceived halving of loudness. Going from STC 35 to STC 45 is a noticeable, meaningful improvement in privacy.
Common Walls, Floors, and Windows
A standard residential wall built with 2×4 wood studs, a single layer of 5/8-inch drywall on each side, and fiberglass batt insulation in the cavity typically rates STC 34 to 39. That’s enough to muffle casual sound but not enough to keep a neighbor’s conversation from drifting through. Metal stud walls with the same drywall but no insulation land in a similar range, around STC 38 to 40, because the thinner metal studs transmit slightly less vibration than rigid wood framing.
Windows vary widely. A basic double-pane window with thin glass and a narrow air gap rates around STC 28 to 31. Upgrading to thicker glass, wider air spaces, and laminated panes pushes the rating into the low-to-mid 40s. A double-pane unit with laminated glass on both sides and a wide air gap can reach STC 44 to 46 for the glass alone, though the finished window assembly (frame, seals, and all) typically scores a few points lower.
Improving an Existing Wall’s STC
You don’t always need to tear out a wall to improve its rating. Two of the most common retrofit approaches are mass-loaded vinyl (MLV) and damping compound applied between layers of drywall.
Mass-loaded vinyl is a thin, dense sheet material, usually about 1/8 inch thick. Adding it to a wall that starts around STC 30 can bring the rating up to somewhere between STC 45 and 55, depending on the product and the wall’s construction. A damping compound like Green Glue works differently: you apply it between a new layer of 5/8-inch drywall and the existing wall surface. The compound converts sound vibrations into small amounts of heat, and this approach can push a wall into the low 50s without gutting anything. Both methods add mass, reduce vibration, or both, which are the two main levers for blocking airborne sound.
Lab Ratings vs. Real-World Performance
STC ratings are measured in controlled laboratory conditions with no flanking paths, meaning sound can only travel directly through the test specimen. In a real building, sound also leaks around the edges of walls, through electrical outlets, along ductwork, and through gaps at the floor and ceiling. Because of this, the Field Sound Transmission Class (FSTC) of an installed partition is typically about five points lower than its lab rating. Building codes account for this gap.
Here’s what that looks like in practice:
- A wall rated STC 50 in the lab will likely perform around FSTC 45 once installed. At that level, you can understand a loud voice through the wall but won’t hear a raised voice.
- A wall rated STC 55 in the lab drops to about FSTC 50, where a shouting voice is understandable but a loud voice is merely audible without being clear.
- At STC 65 (FSTC 60 in the field), shouting is audible but a loud conversational voice is not.
This five-point gap is one of the most important things to understand when shopping for acoustic assemblies. If your goal is to meet an FSTC 45 requirement after construction, you need a wall assembly rated at least STC 50 in the lab.
Where STC Falls Short
STC’s biggest limitation is its frequency range. Because it only measures from 125 Hz to 4,000 Hz, it captures speech and most indoor sounds well but misses deep bass. Low-frequency noise from traffic, aircraft, subwoofers, and mechanical equipment falls partly or entirely below 125 Hz, where STC has nothing to say.
For exterior noise, a different rating called OITC (Outdoor-Indoor Transmission Class) is more useful. OITC measures from 80 Hz to 4,000 Hz and weights the lower frequencies more heavily, reflecting the rumble of trucks and planes that dominates outdoor sound. A window or wall can have a strong STC rating and still let bass frequencies through because STC simply wasn’t designed to evaluate that part of the spectrum. If you’re trying to block highway noise or live near an airport, OITC is the number to look for. If you’re trying to keep conversation from passing between rooms or apartments, STC is the right metric.
STC also doesn’t account for impact noise, like footsteps on a floor above you. That’s measured separately using Impact Insulation Class (IIC), which is why multi-family building codes typically require both an STC and an IIC rating for floor-ceiling assemblies.