03-18-25
Whether you're managing an oil and gas operation, running a data center, or hosting a concert, understanding Sound Transmission Class (STC) is crucial for effective noise control.
This guide cuts through the technical jargon to explain what STC ratings really mean for your project and why the right noise mitigation strategy requires more than just looking at a single number.
What Does STC Mean?
STC stands for Sound Transmission Class (STC) and describes how effectively a material blocks sound. For example, many acoustic barrier blankets are rated STC-25. The higher the number, the more sound it blocks. However, there are several exceptions.
Performance Depends on Physical Gaps
Sound is a lot like water; it follows the path of least resistance. If there are gaps between your acoustic barriers, sound will leak through. Sound will also go around your barriers, so placement is just as important as STC rating.
Performance Depends On Accurate Testing
Determining a material's STC value is complicated. It requires advanced monitoring equipment and a controlled environment like a laboratory. If done improperly, the STC rating won’t be accurate, and the material will not perform as you need it to. When evaluating vendors, check to see if their products are tested by a reputable third party.
Performance Depends On Sound Frequency
First, some key definitions:- Volume of sound is measured in decibels (dB)
- Frequency of sound is measured in hertz (Hz)
STC-rated materials are more effective at different frequencies. For example, an STC-25 acoustic barrier blanket from Environmental Noise Control will reduce sound by 11 decibels (dB) when the frequency is around 100 Hz. However, it will reduce sound by 28 dB when the frequency is around 500 Hz.
| Sound Frequency | Reduced Volume With STC-25 Barrier |
| 100 hertz | 11 decibels |
| 500 hertz | 28 decibels |
| 1000 hertz | 39 decibels |
| 5000 hertz | 50 decibels |
To fully understand how effective an STC-rated material will be, you need to know what types of frequencies your project is creating.
Low-Frequency Sounds (20-250 Hz)
- 31.5-63 Hz: Diesel generators, bass notes, HVAC units
- 63-125 Hz: Heavy machinery vibrations, compressors, server cabinet fans
- 125-250 Hz: Transformers, idling trucks, distant traffic
Mid-Frequency Sounds (250-2000 Hz)
- 250-500 Hz: Machinery noise, most musical instruments
- 500-1000 Hz: High-density server operations, light equipment, power tools
- 1,000-2,000 Hz: PA systems, telephones, high-pitched alarms
High-Frequency Sounds (2000-20,000 Hz)
- 2,000-4,000 Hz: String instruments, sirens, electronic whine
- 4,000-8,000 Hz: High-pitched machinery noises, metal-on-metal contact
- Above 8,000 Hz: Air hisses, pneumatic tools, steam releases
STC vs. NRC: What’s the Difference?
People often mistake STC (Sound Transmission Class) and NRC (Noise Reduction Coefficient):- STC measures how well a material blocks sound.
- NRC measures how well a material absorbs sound.
Many projects need a combination of blocking and absorbing solutions. For example, a construction site might produce high-frequency sounds from pneumatic tools and low-frequency sounds from diesel generators. An STC-32 Temporary Sound Wall with NRC overlays will block high frequencies and absorb low frequencies while minimizing reverberation within the site.
Environmental Noise Control also offers solutions specifically for low-frequency sound, including SK8 and SK6 sound barriers. Fabricated with a galvanized exterior sheet metal and an acoustical absorber with a septum barrier, these panels are used for low-frequency and heavy-duty applications.
Industrial Vs. Residential STC Ratings
It's important to note that STC ratings for industrial sound barriers are often lower than those for residential sound barriers, which can cause confusion. In your research on STC, you’ll see articles that list STC-25 at the lowest end of the spectrum. That doesn't mean STC-25 products are ineffective. Rather, it’s important to consider the differences between residential and commercial applications.
First, residential settings aim to create quiet environments. Industrial settings simply aim to make loud noises less loud. Trying to completely soundproof a construction site is just not realistic or necessary.
Second, high-quality industrial sound barriers both block and absorb sound. They combine STC-rated and NRC-rated materials, so their STC ratings do not need to be as high.
Choose the Right Sound Solution for Your Project
When planning your next project, remember that:
- STC ratings are based on mid-to-high frequencies and don’t indicate how well a material will block low-frequency noise.
- Physical gaps can compromise even high-rated barriers.
- Most projects benefit from combining STC and NRC-rated solutions.
With four decades of nationwide experience across many industries, Environmental Noise Control has the expertise to assess your job site and recommend the most effective solutions. Ready to get started?
