When architects and building owners begin researching how to control noise between rooms, one of the first metrics they encounter is the Sound Transmission Class (STC) rating. STC is a single number that describes the ability of a building material or partition to block airborne sound, and a higher STC rating indicates better performance at preventing sound from passing through. In practical terms, STC is the foundational language of acoustic privacy in the built environment, used to evaluate and compare walls, floors, ceilings, doors, and windows. If you are designing a home theater, a private office suite, a healthcare exam room, or a multifamily residence, the STC rating of your partition assemblies will largely determine whether conversations stay private or bleed into adjacent spaces. Understanding STC is therefore an essential first step in creating any serious acoustic plan, and it works alongside other metrics like NRC (Noise Reduction Coefficient) and SAA (Sound Absorption Average) to define the full acoustic character of a space.

How STC Is Measured

STC is a laboratory result defined by ASTM E413 for sound transmission loss testing performed according to ASTM E90, and the rating is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4,000 Hz, which closely corresponds to the primary frequency range of human speech. During the test, sound is generated in one room and measured in an adjacent receiving room. The resulting Transmission Loss values are plotted on a sound pressure level graph and compared to a standard reference contour provided by ASTM, and the STC rating is the value of that reference contour at 500 Hz after it has been optimally fit to the measured data. One important nuance for architects is that lab STC ratings represent ideal conditions. Real-world field conditions such as flanking paths, installation errors, or unsealed penetrations typically reduce actual performance, and field STC results (also called Apparent STC, or ASTC) are generally 3 to 7 points lower than the laboratory rating. This gap underscores why specifying a high-quality, continuous ceiling finish like acoustical plaster, rather than a system dependent on perfect tile edge seals, is critical to maintaining performance over the life of a building.

‍

At the lower end of the scale, an STC of 25 to 30 means that speech can be easily understood through the partition, a condition commonly associated with minimal soundproofing. An STC of 35 to 45 means loud speech is audible but less intelligible. At STC 50 and above, only very loud sounds can be heard through the assembly. (source) These numbers have direct regulatory implications as well. Section 1206 of the International Building Code (IBC) 2021 requires a minimum lab-tested STC rating of 50 for separation between dwelling units and public or service areas in multifamily construction. For more demanding program types, recommended thresholds are considerably higher. Private offices and meeting rooms generally benefit from an STC of 45 to 50 to ensure speech privacy, while healthcare spaces and hotels are better served by ratings of STC 55 to 60. (source) Architects specifying for sensitive environments should treat the IBC minimum as a floor, not a target. Visit our Technical Data section to review tested performance specifications for BASWA acoustical plaster assemblies.

The consequences of under-designing for STC extend well beyond regulatory risk. In healthcare settings, physician-patient privilege under HIPAA requires that private information be protected, making acoustic performance a legal consideration, not merely a comfort preference. The Facility Guidelines Institute (FGI) specifies minimum STC ratings for numerous room adjacencies, ranging from STC 45 between patient rooms on the same floor to STC 60 between an operating room and an MRI scanner room. In offices, the stakes are similarly concrete. Research consistently links uncontrolled noise intrusion to elevated stress and measurable reductions in cognitive performance, particularly for complex, knowledge-intensive work. Designing to an appropriate STC from the start is far less costly than retrofitting a completed building. Explore how BASWA has addressed these challenges across a range of project types in our Portfolio.

‍

STC is one of several interlocking metrics that together define how a space performs acoustically. A wall or ceiling assembly that excels at blocking sound between rooms (high STC) performs a fundamentally different function than materials that absorb sound within a single room, which is measured by NRC and SAA ratings. Sophisticated acoustic design balances both: blocking sound where separation is needed and absorbing it where reverberation and echo must be controlled. It is also important to distinguish STC from CAC (Ceiling Attenuation Class), which specifically rates a suspended ceiling's ability to block sound traveling through a shared plenum between adjacent rooms. BASWA acoustical plaster systems are engineered to contribute to both in-room absorption and partition assembly performance within a seamless, monolithic ceiling finish, eliminating the performance compromises associated with modular tile systems. Our Continuing Education resources include AIA-accredited coursework covering how acoustical plaster systems are specified to meet today's most demanding acoustic standards.