Quick Answer: What You Need to Know
STC (Sound Transmission Class) measures how well walls, floors, and ceilings block sound. Higher numbers mean better soundproofing. Ratings range from 25 (minimal soundproofing) to 60+ (nearly soundproof).
No one wants to hear their neighbors arguing or have employees blame low productivity on the talkative person in the office next door. That’s why sound transmission ratings are a critical measure for building products.
By having the sound transmission levels of products and assemblies tested, designers can know long before a space is occupied whether the level of sound reduction will meet end-user expectations.
Many factors influence sound transmission class ratings, and it’s important to understand and plan for these nuances. This article will explain STC ratings, what the testing entails, and how to interpret the ratings.
What Is Sound Transmission Class?
STC ratings are a standard way to rate the effectiveness of an assembly to attenuate airborne sound. The result is a numerical rating somewhere between 25 and 60+.
While decibels (dB) are a measurement of how loud a noise is, sound transmission class measures the difference in decibels through a product — in other words, how much sound is getting through.
In simpler terms, the STC rating offers an indication of how soundproof a wall or floor/ceiling structure may be.
STC vs Other Acoustic Ratings
It’s important to distinguish STC from related measurements:
- NRC (Noise Reduction Coefficient): Measures how much sound a material absorbs rather than blocks
- IIC (Impact Insulation Class): Specifically measures impact noise transmission through floors, like footsteps or dropped objects
- OITC (Outdoor-Indoor Transmission Class): Better for measuring low-frequency sounds like traffic noise
How Is Sound Transmission Class Rating Determined?
STC ratings are determined with acoustic testing that adheres to the ASTM E90 standard. The test measures the airborne sound attenuating characteristics of partitions, floors, ceilings, windows, doors, or barrier materials.
The transmission loss data is used to establish the STC rating of the unit being tested. Sound insulation is calculated according to the ASTM E413 standard using sound transmission measurements obtained in one-third octave bands as per ASTM E90.
STC Testing Details
Acoustic testing laboratories use specialized chambers to conduct STC tests:
- Setup: The test assembly (wall, floor, or ceiling) is installed between two reverberant chambers, creating a source room and a receiving room.
- Sound Generation: In the source room, controlled sound is generated across 16 different frequencies, ranging from 80 Hertz to 5,000 Hertz. This range covers the primary frequencies of human speech and common environmental noises.
- Measurement: Sensitive microphones measure sound levels in both the source room and receiving room. The difference between these measurements indicates how much sound the assembly blocks at each frequency.
- Calculation: The transmission loss data is plotted on a sound pressure graph, like the one below. According to ASTM E413 standard, this data is then fitted to a standardized contour curve to arrive at a single-number STC rating.
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Interpreting Ratings With a Sound Transmission Class Chart
Through acoustical testing, you’ve determined that an assembly has an STC rating of 50. But what does that really mean?
A sound transmission class chart is a simple way to understand the decibel scale and its relation to STC ratings. A higher sound transmission class rating indicates more effective sound insulation than a lower number.
STC |
Common Sounds Still Heard |
|
25 |
Normal speech can be easily heard and understood |
|
30 |
Loud speech can be easily heard and understood |
|
35 |
Loud speech heard, but not understood |
|
40 |
Loud speech only a murmur |
|
45 |
Loud speech not heard, music system/heavy traffic noise potentially audible |
|
50 |
Very loud sounds can be heard (musical instruments) |
|
60 |
Soundproof |
For reference, the International Building Code (IBC) requires a minimum lab-tested STC rating of 50 per ASTM E90 for walls, floors, and ceilings in new multi-family construction. Using the sound transmission class chart above, a rating of 50 means only very loud sounds can be heard through the assembly.
Factors That Impact STC Ratings
A single-number rating may not always tell the whole story. There are nearly unlimited factors in the field that will impact sound transmission, and many cannot be anticipated in laboratory testing.
For example, buildings in proximity to an outdoor concert venue or busy road will generate a considerable amount of low-frequency noise that could not be known when the assembly was tested. STC ratings are not appropriate for low-frequency noises, which require a detailed analysis of frequency bands.
Beyond that, material properties, construction techniques, and room acoustics will have an impact.
Sound can become structure-borne and travel through assemblies, such as walls connected to ceiling joists and subfloors. Known as flanking paths, this can increase the sound level, but could not be anticipated unless the actual assembly is evaluated.
Frequently Asked Questions About STC Ratings
What’s the difference between STC and NRC?
STC (Sound Transmission Class) measures how well a barrier blocks sound from passing through it. NRC (Noise Reduction Coefficient) measures how much sound a material absorbs within a room. STC is for soundproofing between spaces, while NRC is for acoustic treatment within a space.
What STC rating do I need to block music?
To significantly reduce music transmission, aim for STC 55 or higher. Music contains a wide range of frequencies, and bass frequencies in particular require substantial mass and isolation to block effectively. Consider OITC ratings for bass-heavy music.
Does adding insulation improve STC rating?
Yes, adding cavity insulation (fiberglass, mineral wool, or cellulose) typically improves STC by 5 to 10 points. However, insulation alone won’t achieve high STC ratings. You also need adequate mass, air sealing, and in some cases, decoupling strategies.
Can you improve STC rating without removing drywall?
Yes, you can add mass-loaded vinyl and an additional layer of drywall over existing walls, which can improve STC by 5 to 8 points. However, the most effective improvements (resilient channels, proper insulation) require accessing the wall cavity.
How does STC relate to decibel reduction?
While STC is not a direct decibel measurement, an increase of 10 STC points generally correlates to a perceived halving of loudness. For example, upgrading from STC 40 to STC 50 makes sounds seem approximately half as loud.
Why Professional STC Testing Matters
Sound transmission class ratings provide the most complete and reliable picture of acoustic performance. When designing spaces that share walls, floors, or ceilings, STC testing helps you:
- Meet code requirements with documented proof of compliance
- Avoid costly mistakes by identifying performance issues before construction
- Satisfy client expectations with verified sound privacy levels
- Prevent disputes between neighbors, tenants, or building occupants
- Make informed decisions about material selection and construction methods
- Optimize budgets by knowing exactly what performance you're getting for your investment
Our acoustical testing experts have decades of experience helping architects, contractors, and building owners achieve superior sound isolation. When you need lab testing for code compliance or to verify construction quality, we provide accurate, reliable results you can trust.
Ready to test your assembly? Contact our team of experts today to discuss your project requirements and learn more about acoustical testing.
