STC Rating Explained: How to Measure Soundproofing Effectiveness
When designing a building, controlling sound is essential for creating functional and comfortable spaces. Whether the goal is to ensure speech privacy in an office, protect a recording studio from external noise, or prevent sound from traveling between apartments, engineers need a reliable way to measure how well a wall or floor assembly blocks sound. The primary metric used for this in the United States is the Sound Transmission Class (STC) rating.
A Sound Transmission Class (STC) rating is a single-number rating that describes how well a building partition, such as a wall, floor, ceiling, door, or window, reduces the transmission of airborne sound. A higher STC rating indicates better sound insulation performance. This rating system provides a standardized method for architects, engineers, and builders to compare the acoustical performance of different materials and assemblies. Understanding this rating is the first step in effective acoustic design.
How Is an STC Rating Measured?
The process for determining an STC rating is highly standardized to ensure that results are consistent and comparable. The measurement is performed in a controlled laboratory environment according to the ASTM E90 standard.
The test involves the following steps:
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1. Test Assembly: A sample of the partition to be tested (e.g., a wall assembly) is constructed between two isolated rooms, known as the source room and the receiving room. The setup is designed so that the only significant path for sound to travel between the rooms is through the test partition itself.
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2. Sound Generation: A sound source in the source room generates noise across a wide range of frequencies, specifically covering 16 standard one-third-octave bands from 125 Hz to 4000 Hz. This frequency range was chosen because it effectively covers the range of normal human speech.
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3. Measurement: Microphones in both rooms measure the sound pressure levels. The difference between the sound level in the source room and the receiving room is calculated at each of the 16 frequencies. This difference is known as the Transmission Loss (TL).
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4. Calculation: The 16 Transmission Loss values are plotted on a graph. This data curve is then compared to a standard reference contour defined by the ASTM E413 standard. The reference contour is shifted vertically until it fits the test data according to a specific set of rules. The STC rating is the value on the fitted reference contour at the 500 Hz frequency.
This rigorous process produces a single, reliable number that allows for an apples-to-apples comparison of different assemblies.
What Do Different STC Ratings Mean?
The STC rating gives a good indication of what you can expect to hear through a partition in practice. A higher number means more sound is blocked.
| STC Rating | Audibility of Speech | Example Environment |
|---|---|---|
| STC 25-30 | Normal speech can be easily understood. | Basic interior wall, no insulation |
| STC 35-40 | Loud speech is audible but not intelligible. | Standard office or residential wall |
| STC 45-50 | Loud speech is not audible. This is where privacy begins. | IBC-compliant apartment walls, conference rooms |
| STC 55-60+ | Very loud sounds like music or a shouting voice are faintly heard. | Luxury condos, recording studios, sensitive labs |
For many applications, certain STC levels are considered standard. For example, the International Building Code (IBC) requires a minimum STC rating of 50 for wall and floor assemblies separating different dwelling units (e.g., in apartment buildings or hotels). This is considered the threshold for providing good sound isolation and minimizing noise complaints between neighbors. For more sensitive spaces, like conference rooms or executive offices, an STC of 55 or higher is often specified.
STC Ratings of Common Building Assemblies
It is important to remember that an STC rating applies to the entire assembly, not just one component. A wall’s performance depends on its studs, drywall, insulation, and how it is constructed.
– Standard Interior Wall: A basic wall made with 2×4 wood studs and a single layer of 1/2″ drywall on each side has an STC rating of about 33-35. Normal speech can often be heard through this type of wall.
– Wall with Insulation: Adding fiberglass or mineral wool insulation into the stud cavity of the same wall can increase its rating to STC 36-39. The insulation absorbs sound energy, improving performance.
– Staggered or Double-Stud Walls: A very effective way to boost an STC rating is to decouple the structure. Building a wall with staggered studs or two separate rows of studs creates a break in the vibration path. A double-stud wall with insulation and two layers of drywall on each side can achieve an STC rating of 60 or higher.
– Concrete Block Wall: Mass is a key factor in blocking sound. A standard 8-inch hollow concrete block wall has an STC rating of about 45.
Doors and windows are often the weakest acoustic links in a wall. A standard hollow-core interior door may have an STC rating of only 20-25, while a solid-core door can be in the STC 30-35 range. Similarly, a standard single-pane window has an STC of about 26-28, while specialized acoustic windows can achieve ratings of 45 or more.
Limitations of the STC Rating System
While the STC rating is a very useful tool, it has important limitations that professionals must understand.
The most significant limitation is its frequency range. The STC test only measures sound down to 125 Hz. It does not account for low-frequency noise, such as: the bass from a home theater system or music venue, the rumble of heavy machinery, or noise from traffic, trains, and aircrafts.
Because STC is designed around speech frequencies, it is not the best indicator for projects where low-frequency noise is the primary concern. For these situations, the Outdoor-Indoor Transmission Class (OITC) rating is a more appropriate metric. OITC was developed specifically to measure the transmission of exterior noise (like traffic) and its frequency range extends down to 80 Hz, providing a better assessment of low-frequency sound insulation.
Another limitation is that STC is a laboratory rating. Real-world installations are subject to “flanking paths,” which are indirect routes for sound to travel, such as through ductwork, ceiling plenums, or gaps around electrical outlets. These flanking paths can significantly reduce the actual sound isolation of a wall, a factor not captured in a lab test.
How to Improve an Assembly's STC Rating
Improving the sound-blocking performance of a partition involves four key strategies. These principles are fundamental to our work in Architectural Acoustics.
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1. Add Mass: Heavier and denser materials are harder for sound waves to vibrate. Using thicker drywall (5/8″ instead of 1/2″), adding extra layers of drywall, or using materials like mass-loaded vinyl (MLV) increases the mass of a partition and improves its STC rating.
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2. Improve Absorption: Filling the empty cavity in a stud wall with absorptive material like fiberglass or mineral wool insulation is a cost-effective way to increase its STC rating. The insulation dissipates sound energy as it tries to pass through the wall.
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3. Decouple the Structure: Decoupling, or creating a break in the structure, is one of the most effective soundproofing techniques. This prevents vibrations from traveling from one side of the partition to the other. Methods include building a double-stud wall, using staggered studs, or attaching drywall to resilient channels or acoustic isolation clips.
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4. Seal All Gaps: Sound acts like water and will find any opening. Even a small gap around the perimeter of a wall, an unsealed electrical box, or a gap under a door can drastically lower the effective STC rating of an otherwise well-built assembly. Using acoustic sealant to seal all joints and penetrations is a critical final step.
Using STC Ratings Effectively
The Sound Transmission Class (STC) rating is an essential tool in acoustic design. It provides a standardized way to evaluate the ability of walls, floors, and other building elements to block the sound of speech. However, for a truly effective design, an engineer must look beyond the single number.
Understanding the limitations of STC—particularly with low-frequency noise—and considering other ratings like OITC when appropriate is crucial. Furthermore, success depends on a holistic approach that considers the entire assembly, from the materials used to the quality of construction and the sealing of all potential flanking paths. This is the foundation of creating spaces that are not only functional but also acoustically comfortable.
For expert guidance on your next project, from material specification to a complete acoustic enclosure design, please contact our team for a consultation.