As mold, mildew, and moisture resistance have become standard in building projects, effective sound control is now equally essential in today's market.
The demand for quieter environments comes from various sources: apartment residents seeking peace from their neighbors' noise, office workers needing quiet despite shared walls, and medical facilities requiring patient privacy. Regardless of the building type, the approach to sound control remains consistent.
Since sound travels through walls, floors, ceilings, and even hidden air gaps, the most effective way to reduce noise involves a combination of strategies: acoustically separating spaces, sealing air leaks, adding mass, and using sound-damping materials. A mix of these methods often yields the most favorable sound ratings.
When determining sound ratings for your next project, it’s important to understand the differences between sound transmission class (STC) and outdoor-indoor transmission class (OITC) and know when to use one over the other.
What Are STC and OITC Ratings?
STC and OITC ratings are measures of how effectively a building material or structure blocks sound. STC ratings focus on blocking indoor noise, such as voices and music, while OITC ratings are specifically designed to measure exterior noise from sources like traffic, airplanes, and construction.
Both ratings are determined through rigorous acoustical testing to assess a material's ability to reduce sound transmission. STC and OITC can be derived from a single test—separate tests are not required.
Sound Transmission Class
The STC rating is a single-number measurement that indicates how well a building material or assembly blocks airborne sound between interior walls. It reflects the effectiveness of a wall or floor-ceiling assembly in reducing sound transmission.
When designing interior spaces, it’s essential to consider the STC rating of materials to ensure effective sound control. The higher the rating, the better the material is at reducing sound within the test's frequency range.
Sound transmission class ratings are determined by testing assemblies per ASTM E90, the Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, and ASTM E413, which classifies sound insulation ratings.
How Is STC Measured?
Sound transmission loss between the source and receiving rooms is measured and plotted on a graph, showing both frequency and sound level decibels (dB).
The STC curve is an adjustable contour that fits the performance data, ensuring deficiencies do not exceed 32 dB below the contour. At any given frequency, the maximum allowable deficiency is 8 dB.
STC is measured across one-third octave frequencies ranging from 125 to 4,000 hertz (Hz), which encompasses everyday sounds like normal conversation and typical background noise.
During STC testing, a wall or floor-ceiling assembly is mounted between a source room and a receiving room. Sounds are played in the source room at 16 different frequencies between 125 Hz and 4,000 Hz, replicating common sounds such as human speech.
The test then measures the reduction in decibel levels for each frequency within the receiving room and plots the transmission loss values on a graph. This data forms a curve that fits the performance, allowing up to 32 dB of deficiencies below the contour, with no frequency exceeding an 8 dB deficiency.
The final STC rating is determined by the intersection of the STC contour and 500 Hz frequency, yielding a decibel value that is expressed as a single number (e.g., STC 38). For projects requiring enhanced soundproofing, investing in higher STC ratings can significantly improve noise control and overall acoustic performance.
Outdoor-Indoor Transmission Class
OITC is a newer rating system that measures how effectively a roof, wall, window, or door blocks outside noise from entering a building. This system specifically targets low- and mid-frequency sounds commonly associated with transportation noise through exterior assemblies.
Like STC ratings, outdoor-indoor transmission class ratings are calculated using ASTM E90, but the OITC test covers a broader frequency range, from 80 Hz to 4,000 Hz. This range better represents outdoor noises, such as traffic, aircraft, and industrial sounds.
How Is OITC Measured?
To measure the OITC rating of an exterior assembly, a source will generate sounds at predetermined frequencies within the given range, and the receiver will measure the sound levels within the building structure as the sound passes through the tested element.
The test then uses the data collected over the 80 to 4,000 Hz frequency range to determine the OITC rating. The higher the rating, the better the building assembly’s ability to resist sound transmission from exterior elements.
Key Differences Between STC and OITC
There are a few key differences between the two sound ratings to consider.
Types of Noise Measured
While STC focuses on mid-to-high frequencies, OITC is used to measure lower-frequency sounds.
Testing Standards Used
STC ratings are determined by using frequencies ranging from 125 Hz to 4,000 Hz to replicate common sounds found indoors, while OITC ratings are determined by using a frequency range of 80 Hz to 4,000 Hz.
Application-Specific Considerations
STC is used for interior spaces and minimizing airborne sound transmission, while OITC is used for exterior barriers where environmental noise poses an issue.
Factors That Influence STC and OITC Ratings
Six variables can influence STC and OITC ratings:
- Mass: The thicker the material, the less sound that will be transmitted. For example, a heavy, solid door will allow less sound to pass through as opposed to a lighter, hollow door.
- Cavity Absorption: Adding insulation to a space can have a significant effect on the wall assembly’s overall STC rating because it adds sound-absorbing materials that fill in the cavity and reduce noise.
- Cavity Depth: Deeper studs offer more space for insulation to be added, which can increase the material assembly’s STC rating.
- Stiffness: Controlling an assembly’s stiffness level can also affect STC testing considerably. Reducing the stiffness will increase STC, improving STC performance when utilizing more flexible materials such as metal studs rather than wood studs.
- Decoupling: Since sound moves more easily through solids as opposed to air, separating different pieces of the assembly allows air gaps to help slow down sound waves.
- Damping: Sound-dampening materials can dissipate vibrational energy produced by sound waves, which helps decrease noise levels.
How to Choose the Right Sound Rating for Your Project
Selecting the right materials and sound rating for your project involves a few key considerations. To achieve optimal STC levels, it's essential to assess your budget to determine the types of wall assemblies that fit your needs. Here are some examples to guide your decisions:
- For interior partitions, a wood stud wall with fiberglass batt insulation and ½" (12.7 mm) gypsum board on both sides typically achieves an STC of about 38.
- Apartment party walls generally require an STC of at least 50.
- Area separation walls with an STC of 60 or higher are rarely the source of noise complaints.
However, it's important to remember that a higher STC value doesn't always guarantee superior performance in every situation.
When considering OITC levels, similar strategies can be applied, but it's also important to factor in how mass and multi-layer construction can help reduce low-frequency noise.
By combining both STC and OITC ratings, you can create a quieter, more comfortable environment for your building’s occupants.
Ultimately, addressing acoustics early in the design process is crucial. Without proper sound control, privacy, sleep quality, and overall client satisfaction can be compromised. However, finding the right acoustic solution that meets building codes, material constraints, and client expectations can be challenging.
Working with independent testing facilities like NGC Testing Services ensures your construction is thoroughly vetted through full-scale acoustical testing, compliant with ASTM E90 standards.
Learn more about how NGC Testing Services can help you get started.
