Section 1207.2/Airborne Sound Transmission (STC Ratings)
IBC 1207.2 requires walls and floor-ceiling assemblies between dwelling units to achieve a minimum Sound Transmission Class (STC) of 50 laboratory or ASTC 45 field tested.
Walls, partitions, and floor-ceiling assemblies separating dwelling units from each other or from public and service areas must have a Sound Transmission Class (STC) of not less than 50 when tested in a laboratory per ASTM E90, or an Apparent Sound Transmission Class (ASTC) of not less than 45 when field tested per ASTM E336. This applies to all multifamily housing including apartments, condominiums, townhouses, hotels, and dormitories. Penetrations through sound-rated assemblies for electrical outlets, plumbing, HVAC ducts, and other services must be sealed, lined, insulated, or otherwise treated to maintain the required STC rating. The requirement applies to the entire assembly, not just the wall or floor material. Section 1207.3 separately requires Impact Insulation Class (IIC) 50 (lab) or AIIC 45 (field) for floor- ceiling assemblies between dwelling units.
Why this section exists
Sound transmission between dwelling units is the most common complaint in multifamily housing and a leading cause of tenant dissatisfaction. Airborne sound includes speech, music, television, and general activity noise. The STC 50 laboratory rating provides adequate speech privacy between units: loud speech can be faintly heard but is not intelligible. The 5-point reduction allowed for field testing (ASTC 45 vs STC 50) accounts for flanking paths, construction tolerances, and real-world conditions that reduce performance compared to laboratory testing. The field test option provides a compliance path for verifying as-built performance.
What plan reviewers look for
Plan reviewers check wall and floor assembly specifications for documented STC ratings. They verify the specified assembly matches a tested configuration from a recognized source (such as the Gypsum Association's Fire Resistance and Sound Control Design Manual or UL's Fire Resistance Directory). They check for penetrations through rated assemblies and verify that sealing details are provided. Common flanking paths they check include back-to-back electrical outlet boxes (which must be staggered and sealed), medicine cabinets, HVAC duct penetrations, and plumbing penetrations through fire-rated assemblies that also carry sound.