The Sound Transmission Class (STC) rating is a single number which indicates the sound isolation value of a partition system. The higher the STC number, the better the sound isolation. This rating was developed to characterize the effectiveness of a partition with respect to speech. This rating should never be used where the noise source has significant low frequency content, such as is the case with music or mechanical noise. The IBC requires STC 50 (theoretical) and STC 45 (Tested) for party walls of multi-unit dwelling units.

An NC level is a single number rating of the noise spectrum within a space. A low number is quieter than a higher number. NC 15 is very quiet. NC 65 is very loud. NC levels are intended to be used for steady, continual background noise levels within a space, as opposed to specific noises or intermittent activity occurring there.

NRC is a single number metric that is intended to indicate the absorptivity of a material. NRC’s could theoretically range from 0 – 1.0. Practically speaking, they usually are in the range from 0.5 – 1.0. A higher number generally indicates a material with more absorptivity than a lower number. NRC is the arithmetic average of the measured sound absorption coefficients in the 250, 500, 1,000 and 2,000 octave bands.

IIC is a single-number rating of the sound isolation of impact noise, such as footfall noise or chair scraping on one floor, and the sound level on the floor below. A higher number is better than a lower number. The IBC requirement for multi-unit apartments, as well as the ANSI S12.60 recommendation is for IIC 50. A concrete floor/ceiling assembly is approximately IIC 26. Generally, for hard floor surfaces, adding carpet to the floor will almost always result in at least IIC 50.