What Is STC Rating? Sound Isolation vs Sound Absorption

If a meeting room sounds calm inside but conversations are audible next door, start by tracing where the sound crosses between rooms. A Sound Transmission Class (STC) rating describes how well a tested building assembly limits airborne sound transmission. Sound absorption addresses reflections within a room. A meeting room can need both, and each calls for different evidence.

What does an STC rating measure?

STC is a single-number rating calculated under ASTM E413 from laboratory sound transmission loss data typically measured under ASTM E90. A higher STC generally indicates better airborne sound isolation for the tested assembly across the frequency range used in the rating. An STC of 50 does not translate into a 50 dB reduction at every frequency or a guaranteed difference in sound levels between finished rooms. For sources with substantial low-frequency energy, review the transmission loss by frequency as well.

The rating belongs to the tested construction. For a wall, that includes details such as the board layers, framing, cavity insulation and connections represented in the test. A door or window can have its own test result. Once installed, sound can also travel through gaps or around the separating construction, so a laboratory wall STC and measured room-to-room isolation can differ. If field verification is required, agree on the metric and procedure before testing. Apparent Sound Transmission Class (ASTC) is one field metric associated with ASTM E336 measurements.

Follow the sound across the meeting room boundary

If the partition stops at a suspended ceiling, sound can travel up through one ceiling, across the shared plenum and down into the next room. Ceiling Attenuation Class (CAC) addresses this path for a tested ceiling system. A drawing that shows the wall stopping at the ceiling should prompt a review of the plenum path even if the wall schedule lists a high STC.

When NRC belongs in the same specification

Noise Reduction Coefficient (NRC) summarizes sound absorption under defined test conditions. It helps when speech sounds reverberant inside the meeting room. Perforated ceiling panels with acoustic backing or acoustic baffles with a suitable absorptive construction can contribute to that goal. An open baffle layout leaves gaps between blades, so room-to-room isolation still depends on how the room boundary and any shared plenum are detailed. Explore SEASON’s perforated aluminum lay-in panel.

Edge detail of a triangular aluminum lay-in ceiling panel

For two adjacent rooms, it can make sense to specify a partition and door assembly for airborne sound isolation and an absorptive ceiling treatment for reverberation. If a continuous ceiling is also intended to limit transmission through a shared plenum, request the CAC report for the proposed build-up. If the ceiling uses spaced blades, request absorption data for the proposed arrangement and review the isolation details separately. View SEASON’s aluminum wave baffle system.

Curved aluminum wave baffle profiles before ceiling installation

What to request before approving a substitution

Ask for the full assembly test report behind any scheduled STC or CAC value. Match the tested construction to the proposed product and details, then compare the reflected ceiling plan with a section through the dividing wall. Check where the wall ends and how doors, ducts and penetrations cross the boundary. For absorption, request the NRC or other appropriate absorption data for the proposed ceiling build-up or baffle layout. Read SEASON’s acoustic baffle ceiling guide.

Share your reflected ceiling plan, partition section and acoustic schedule with SEASON. Include where the partition terminates and whether the ceiling package is intended to provide room absorption or contribute to plenum attenuation. Contact SEASON to discuss the metal ceiling package.