Recently we were asked “What is the difference between STC, NRC, and NC and are they interrelated?”
STC or Sound Transmission Class, describes how much sound a wall or a floor/ceiling construction will block from one room to the next. A good analogy to an STC rating is the fire rating of a partition. A two hour fire-rated assembly will keep the fire on the opposite side of the partition longer than a 20 minute assembly. Just like fire ratings, the higher the STC rating, the better the isolation. Therefore, an STC 50 partition will block more sound than an STC 30 partition.
NRC or Noise Reduction Coefficient, defines how much sound specific materials absorb. This is analogous to a room’s finishes. Just as various colors of paint, or textures, visually alter a room, various materials with different NRC ratings, such as carpet or tile, audibly alter a room. A material with a low NRC rating (tile) absorbs little sound and a material with a higher NRC rating (carpet) absorbs more sound.
Finally, NC or Noise Criteria measures how much mechanical noise can be heard in a room. This type of measurement can be compared to the amount of light in a room. A foot candle describes how much illumination a lighting fixture provides on a surface, NC describes how much mechanical system noise is heard in a room. The brighter the light on a surface the higher the foot candles, and the higher the NC rating, the more noise that is heard.
Are STC, NRC and NC related? Not really. They do interact in the total design of a room, just as the building structure, room finishes and lighting interacts on the visual design of a space. However, individually they all play separate roles. Room finishes are independent of the fire rating of the partition, just as the amount of sound a material absorbs is independent of its ability to block sound from one room to the next.
Hopefully these analogies of Sound Transmission Class to fire ratings; Noise Reduction Criteria to a room finish; and Noise Criteria to foot candles helps define and explain these acoustical terms.