What STC actually measures
STC is a single number that summarises how much sound a partition attenuates across 16 test frequencies from 125 Hz to 4 kHz. Higher STC = better isolation. STC 25 is a hollow-core door; STC 60 is a well-built cinema wall.
It's specifically designed for speech and mid-frequency noise. STC doesn't predict how well a wall blocks bass frequencies below 125 Hz — for home theatres and cinemas, you need to look at additional low-frequency ratings.
What STC ratings feel like in real rooms
STC 25–30: You can hear normal conversations through the wall. Standard hollow interior doors and single-drywall partitions fall here.
STC 35–40: You can hear loud speech through the wall but not normal conversation. Standard drywall partitions with insulation.
STC 45–50: Loud speech barely audible; music at normal volume audible. Good residential partition.
STC 55–60: Loud music barely audible; excellent isolation. Studios, cinemas, and premium hotel-room partitions.
STC 65+: Very loud sound barely audible. Broadcast studios and machinery enclosures.
How to hit your STC target
The recipe: mass, decoupling, damping, and airtightness. Mass = MLV inside the wall or heavier drywall layers. Decoupling = resilient channels or staggered studs. Damping = viscoelastic adhesive between drywall layers. Airtightness = acoustic sealant around every penetration.
STC 45–50 needs standard drywall + MLV + insulation. STC 55–60 needs mass-air-mass construction with staggered studs. STC 65+ needs floating rooms — the whole assembly decoupled from the surrounding structure.