Noise has a powerful impact on how we experience a space. A well-designed building should not only look good but also sound right — allowing conversations to stay private and distractions to stay out.
Achieving that balance requires an understanding of how sound moves through walls and structures, and how effectively those materials can block it. That’s where Sound Transmission Class (STC) comes in.
What Is STC?
Sound Transmission Class is a numerical rating that indicates how well a building element or specific material can block airborne sound from traveling through it. In simpler terms, it measures how much noise gets through from one side to the other.
The higher the STC number, the better the sound isolation. For example:
- A wall with an STC of 30 might allow you to clearly hear a normal conversation in the next room.
- A wall with an STC of 55 would make that same conversation barely noticeable. You might sense muffled speech but not distinguish words.
- A wall rated STC 65+ provides excellent isolation, suitable for recording studios or home theaters.
STC is expressed in decibels (dB) and defined by standardized tests — ASTM E90 (for laboratory sound transmission loss measurements) and ASTM E413 (for determining the STC rating from test data). These standards ensure that sound performance can be compared consistently across materials and manufacturers.
The measurement primarily focuses on airborne sounds in the 125 Hz to 4000 Hz frequency range — which covers the bulk of human speech and most everyday noise sources, such as conversation, radio, and television.
How Is STC Measured?
To determine the STC of a partition, a controlled laboratory test is conducted using two adjacent, acoustically isolated rooms separated by the material being tested. Sound is played in one room, and microphones measure how much of it passes into the other.
By comparing sound levels across a range of frequencies (125–4000 Hz), engineers determine the Transmission Loss (TL) — essentially, the difference (in decibels) between the sound level in the source room and the receiving room.
For example, if a wall allows 60 dB of sound to drop to 30 dB on the other side, the TL is 30 dB at that frequency.
These TL values are then matched to a standard STC reference curve. The position of this curve determines the final STC number for that material or assembly.
In simple terms: STC condenses a detailed sound performance graph into one easy-to-understand number.
What Do STC Ratings Mean? (Table)
The chart below shows how STC ratings translate into real-world sound performance.
| STC Rating | Performance | Example |
| 25–30 | Poor – you can clearly hear speech | Thin wall, hollow-core door |
| 35–40 | Fair – speech audible but muffled | Typical interior wall |
| 45–50 | Good – loud speech faintly heard | Apartment or hotel wall |
| 55–60 | Very good – most sounds blocked | High-end condo, private office |
| 65+ | Excellent – nearly soundproof | Recording studio, cinema |
For most residential and commercial applications, an STC rating of 50 or higher is considered a good benchmark for acoustic privacy.
What STC Tells You (and What It Doesn’t)
STC focuses on mid-range frequencies — roughly the range of human speech. That means it’s great for predicting how well a wall blocks conversation or television noise, but it’s less accurate for low-frequency sounds like bass music, machinery, or subwoofers.
It also only measures airborne sound. For impact noise — footsteps, dropped objects, or furniture movement — another rating called the Impact Insulation Class (IIC) is used.
Note: STC is one of several acoustic ratings used in building design. While STC measures sound blocking through walls and floors, NRC describes how much sound a surface absorbs within a space, and CAC measures how well a ceiling system prevents noise from traveling between rooms.
How to Improve STC in a Building
Improving STC is all about controlling how sound energy moves through a structure. Sound can pass through solid materials, small gaps, and even indirectly through connected elements like studs, ceilings, and floors. Effective soundproofing requires a combination of strategies that address each of these paths.
Below are the most common and effective methods used in architectural acoustics to improve STC ratings.
Add Mass
According to the mass law of acoustics, heavier materials block sound better. Every time you double the mass per unit area of a wall, you gain roughly 6 dB of sound transmission loss, which usually increases the STC by about 5–6 points.
Some practical ways to add mass include:
- Choosing dense materials like concrete, brick, or plaster over lightweight ones.
- Using multiple layers of drywall (for example, two layers of 5/8” gypsum board).
- Applying mass-loaded vinyl (MLV), a flexible soundproofing barrier that adds density without taking up much space.
Seal Gaps and Cracks
Sound behaves much like air — if air can pass through, so can noise. Even small gaps can significantly undermine an otherwise high-STC wall.
Common leak points include electrical outlets and switch boxes, HVAC ducts and ceiling penetrations, door and window frames, baseboards and floor-wall joints, etc.
Acoustic caulking, backer rods, and gaskets should be used around all joints and penetrations to ensure airtight construction. In some real-world tests, sealing leaks raised a wall’s effective STC by 5–10 points.
Use Specialized Soundproofing Components
In spaces where superior sound control is essential, such as hotels, theaters, or recording studios, specialized materials can greatly improve isolation performance. Some of the most effective options include:
- Resilient channels for drywall mounting.
- Damped drywall panels, which have viscoelastic layers that absorb vibration energy.
- Acoustic doors and windows rated for specific STC levels.
- Floating floors or acoustic underlayments to reduce both airborne and impact transmission.
These materials can be more expensive but offer measurable and consistent performance improvements.
Address Flanking Paths
Even with a high-STC wall, sound can still bypass the partition by traveling through ceilings, floors, ductwork, or structural framing — a phenomenon known as flanking transmission.
To control it, extend soundproofing measures to adjoining elements such as ceilings and floors. For example, use acoustic breaks or isolation mounts at junctions, and line ducts or install sound baffles to prevent noise leakage through ventilation systems.
Flanking transmission often explains why a design that performs well in the lab doesn’t achieve the same STC rating in the field.
STC in Real-World Conditions
While laboratory STC ratings are essential for comparing materials under controlled conditions, they don’t always tell the full story of how a wall or partition will perform once it’s actually built.
In the lab, testing is done under ideal circumstances. The test wall is perfectly sealed, adjacent surfaces are acoustically isolated, there are no ducts, outlets, or gaps, etc.
In the real world, small imperfections, flanking paths (sound traveling through adjacent structures), and construction differences often reduce performance by 3–5 points.
That’s why on-site testing and careful detailing are just as important as choosing high-STC materials.
STC vs. Rw — International Equivalents
Although STC is widely used in North America, many other parts of the world use a similar metric called the Rw rating (short for Weighted Sound Reduction Index).
Rw is defined under ISO 717-1, an international standard that measures and classifies the sound insulation performance of building elements using a similar methodology to STC. Like STC, it condenses detailed frequency data into a single-number rating that represents airborne sound isolation.
While the test methods differ slightly, STC and Rw values are usually within 1–2 dB of each other, making them roughly interchangeable for comparison purposes.
To Sum Up
Understanding what Sound Transmission Class means — and how to improve it — can help you make smarter design and construction choices that reduce unwanted noise and enhance overall comfort and quality of life.
Hopefully, you found what you were looking for! Here’s a quick recap of the main points:
- STC measures how effectively a surface or material blocks airborne sound.
- Higher STC = better sound isolation — higher ratings mean less noise passes through.
- STC focuses on speech frequencies, so it doesn’t account for impact noise (like footsteps) or low-frequency sounds (like bass).
- Real-world performance can be slightly lower than lab results due to flanking paths, gaps, and construction details.
- The best sound control comes from a balanced combination of mass, insulation, and airtight sealing — not from any single technique alone.
RELATED POSTS: