An STC (Sound Transmission Class) rating is a single number that describes how well a wall, floor, ceiling, door, or window blocks airborne sound between rooms. The higher the number, the less sound gets through: at STC 25, normal speech is easily understood; at STC 50, only very loud sounds come 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 rating calculated under ASTM E413 from sound transmission loss testing performed according to ASTM E90. 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 compared to a standard reference contour, and the STC rating is the value of that reference contour at 500 Hz after it has been 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 results (Apparent STC, or ASTC) are generally 3 to 7 points lower than the laboratory rating. This gap is why sealing penetrations and detailing the full assembly matter as much as the lab number, and why a finish that adds to the assembly's performance can help recover some of what the field takes away.

At the lower end of the scale, an STC of 25 to 30 means that speech can be easily understood through the partition. At STC 35 to 45, loud speech is audible but largely unintelligible. At STC 50 and above, only very loud sounds can be heard through the assembly. These numbers have direct regulatory implications as well. Section 1206 of the 2021 International Building Code requires a minimum lab-tested STC of 50, or 45 if field-tested, for assemblies separating dwelling and sleeping units from each other and from public or service areas. For more demanding program types, acoustic designers commonly work to higher targets: STC 45 to 50 for private offices and meeting rooms, and STC 55 to 60 for healthcare and hospitality. Architects specifying for sensitive environments should treat the code minimum as a floor, not a target.

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How BASWA Systems Perform

BASWA Phon is applied directly over a rated wall or ceiling assembly and adds 5 to 7 STC points to that assembly's performance. For a partition sitting just under a project's target, that can close the gap without rebuilding the wall. At the same time, Phon absorbs sound within the room, with NRC ratings from 0.80 at 30 mm to 1.00 at 70 mm depending on finish and thickness. That means one seamless surface both reduces the sound passing between rooms and controls the echo and reverberation inside them, work that would otherwise take two separate products. Phon carries a Class A fire rating per ASTM E84 and installs on flat, curved, and vaulted surfaces. Visit our Technical Data section to review tested performance specifications for each BASWA system.

The consequences of under-designing for STC extend well beyond regulatory risk. In healthcare settings, HIPAA requires reasonable safeguards for patient conversations, which makes speech privacy a compliance question, not just a comfort one, and the Facility Guidelines Institute sets minimum STC ratings by room adjacency for hospitals and outpatient facilities. In offices, the evidence is just as clear. A peer-reviewed analysis of 42,764 occupant surveys from 303 office buildings found sound privacy was the most common source of dissatisfaction in open-plan offices, and nearly one in five occupants of enclosed private offices were still dissatisfied with it. The authors attributed that to lightweight partitions that underperform occupants' expectations. 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 specified for in-room sound absorption and work alongside high-STC walls and ceilings as part of a complete acoustic plan. Our Continuing Education resources include AIA-accredited coursework covering how acoustical plaster systems are specified to meet today's most demanding acoustic standards.

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