Understanding Sound Transmission Class (STC) Ratings and Testing: What Is It and What Do The Results Mean?

When creating acoustically optimized spaces, understanding how sound travels through walls, floors, and ceilings is crucial. One of the key measures in this domain is Sound Transmission Class, a rating system that evaluates how effectively a building partition attenuates airborne sound.

What is Sound Transmission Class?

Sound Transmission Class (STC) is a single number that describes the ability of a building material or partition to block airborne sound. STC is a laboratory test result defined by ASTM E413 for sound transmission loss testing performed according to ASTM E90.  

STC ratings are intended for the fair comparison of sound isolation performance provided by architectural building products and systems. The higher the STC rating, the better the material prevents sound from passing through. STC ratings are particularly important in spaces where noise control is critical, such as residential buildings, offices, schools, and hospitals.

How is STC Testing Performed?

STC testing is a methodical process performed explicitly according to ASTM E90 and E413 to ensure accuracy and repeatability.

Here’s a step-by-step overview of how STC testing is typically conducted:

1. Preparation of the Test Specimen: The building partition or material to be tested is installed in a test opening between two rooms in a lab environment. This setup simulates real-world conditions as closely as possible, but the perimeter is typically sealed with laboratory mastic to reduce sound flanking around the sample.
2. Generating Sound in the Source Room: In one of the rooms (the source room), a powerful loudspeaker system generates broadband “pink” noise across a wide range of frequencies (32 Hz and 10,000 Hz). The average sound pressure level is measured to establish the sound levels in the source room.
3. Measuring Sound Levels in the Receiving Room: The sound that passes through the test specimen is then measured in the adjacent room (the receiving room). The difference in sound levels between the source room and the receiving room at each frequency is calculated. To determine Sound Transmission Loss (TL), these values are corrected for the sample size and sound absorption in the receiving room.
4. Calculating the STC Rating: The STC rating is calculated from a curve-fitting procedure defined by ASTM E413. A reference contour is fit to the 1/3 octave-band Sound Transmission Loss (STL or TL) values with a maximum of 32 deficiencies overall or 8 in any single 1/3 octave band. Deficiencies are points where the actual TL values are lower than the reference contour. The STC Rating is the value of the reference contour at 500Hz after it has been fit to the TL data.

What Do STC Results Mean?

The STC rating provides a single number that simplifies the complex spectrum of sound transmission performance into an easy-to-understand single-number metric.

Here’s a quick guide to interpreting STC ratings:

• STC 25-30: Speech can be easily understood through the partition. This rating is common in residential buildings with minimal soundproofing.
• STC 35-45: Loud speech is audible but less intelligible. This range is suitable for offices or moderately noisy environments.
• STC 50-55: Speech privacy is achieved at normal voice levels, but loud sounds are still audible through the wall. This rating passes the minimum building code requirements for shared walls in multi-family dwellings. This is also a minimum range for sound isolation between hotel rooms.
• STC 60-65: Loud sounds are barely audible. This range is ideal for spaces requiring high levels of privacy, such as luxury condominiums, conference rooms, or executive offices.
• STC 65+: Very high soundproofing, with most sounds virtually inaudible. This rating is used in specialized environments like recording studios or luxury apartments.

What to Consider When Hiring a Lab for STC Testing?

When selecting a lab to conduct STC testing, it’s important to consider a few key factors to ensure you receive accurate and reliable results:

1. Accreditation and Standards Compliance: Ensure the lab is accredited to ISO 17025 by an International Laboratory Accreditation Corporation (ILAC) body such as NIST’s National Voluntary Laboratory Accreditation Program (NVLAP). ISO 17025 is the international standard for quality management of a laboratory system. The laboratory shall also comply fully with all requirements of the ASTM test procedure, laboratory qualification testing, and all referenced standards. This guarantees the testing procedures and results meet industry-recognized benchmarks.
2. Experience and Expertise: Look for a lab with extensive experience in acoustics testing, particularly STC testing. Experienced labs are more likely to provide precise measurements and understand the nuances of different materials, construction methods, mounting types, and their implications for field performance.
3. Facilities and Equipment: The lab should have state-of-the-art testing facilities and equipment that are regularly calibrated and maintained. This ensures that the test conditions accurately simulate real-world scenarios.
4. Reporting and Analysis: A good lab will provide detailed reports that not only give you the STC rating but also explain the results in the context of your specific project. Modern test reports include photos of sample details and the installation conditions.
5. Customer Support: Consider the level of customer support provided by the lab. Open communication, timely responses, and the ability to discuss results and potential implications are all indicators of a lab that values its clients.

Understanding STC ratings and testing can significantly impact the success of your building projects, especially when sound control is a priority.