Lab STC tests a carefully built wall in a test chamber where sound can only pass through the wall itself. In a building, sound also travels through floors, ceilings, ducts, outlets and gaps. Field measurements capture all of that:
- FSTC (field STC): the partition's performance in place, corrected for flanking (now largely replaced by ASTC).
- ASTC (apparent STC, ASTM E336): everything that reaches the receiving room, including flanking. Many codes now use ASTC 47 for apartments.
- NIC/NNIC: the plain level difference between rooms, normalized for the receiving room's absorption.
Expect field results 5–10 points below lab STC unless flanking paths are controlled.