NRC
Noise Reduction Coefficient
Average absorption coefficient across 250, 500, 1000 and 2000 Hz. Sets reverberation time and speech clarity within a space. Has no bearing on sound transfer between spaces. Results depend on mounting, so compare products at the same tested condition.
Tested to ASTM C423 or ISO 354, mounted per ASTM E795 or ISO 354. ISO single number rating: αw to ISO 11654.
Sabins
Equivalent absorption area
Absorption of a discrete object such as a baffle, cloud, raft or screen, stated per unit rather than per square metre because the object absorbs on more than one face. Multiply by quantity for the total absorption added to the room, then check reverberation time with Sabine's equation, RT = 0.161 V / A.
Tested to ASTM C423 (sabins per unit) or ISO 354 (A obj, m² per object). One metric sabin is 1 m² of perfect absorption; one imperial sabin is 1 ft².
STC
Sound Transmission Class
Airborne sound insulation of a complete wall, floor or door assembly. The rating belongs to the tested build up, so framing, linings, cavity insulation, seals and penetrations all govern the result. Field performance (FSTC) sits below the lab figure.
Tested to ASTM E90, rated to ASTM E413. ISO single number rating: Rw to ISO 717-1. NZBC G6: STC 55 for walls, floors and ceilings between household units and other occupancies, FSTC 50 on site.
CAC
Ceiling Attenuation Class
Room to room attenuation through a shared ceiling plenum where the partition stops at the ceiling line. On those builds it caps the achievable privacy regardless of the wall's STC. High NRC tiles usually rate low, so the options are a dual layer tile, a plenum barrier above the partition, or taking the wall to slab.
Tested to ASTM E1414, rated to ASTM E413.
ΔLw
Weighted impact sound reduction
The improvement in impact sound insulation a floor covering or underlay adds to a heavyweight reference floor, in dB. It rates the product, not the floor: the site result depends on the slab or joist system and the ceiling below, so compare products tested on the same base floor and assess the full build up for compliance.
Tested to ISO 10140-3 on the ISO reference floor, rated to ISO 717-2. Full floor ratings: Ln,w, or IIC to ASTM E492 and E989. NZBC G6: IIC 55 for floors between household units and other occupancies, FIIC 50 on site.