STC tests start by setting up speakers in a room, called the source room. The speakers play pink noise, a noise that sounds like white noise TV static but contains equal energy across frequency bands. Microphones are used to measure the sound pressure level in the source room as well as the sound pressure level in the room on the other side of a wall or similar partition. This room is called the receiving room.
By comparing the sound pressure levels in the source and receiving rooms the noise reduction (NR) can be determined. Because some frequencies might be reflected or absorbed more than others, the NR is determined for 16 frequency divisions between 125 Hz and 4000 Hz, although the standard recommends extending this range down to 100 Hz and up to 5000 Hz. The frequency bands are broken up into one-third-octave groups. If that doesn’t mean anything to you, that’s okay. Just know that the measurement is chunked into 16 groups from low to high based on the pitch of the noise.
Since rooms can differ drastically in their acoustic responses due to shape, furniture, flooring type, and other factors, acoustics professionals needed a way to account for room variation. The solution was to use the reverberation time to normalize the noise reduction. By measuring how long it takes for sound to completely die away (or decay) in the receiving room, the noise reduction could be normalized to the amount of noise reduction you would get if the reverberation time was half a second. Normalized noise reduction (NNR) measurements are useful for characterizing how much sound can get from one room into an adjacent room.
While NR and NNR measurements can be useful for understanding the strengths and weaknesses of a room for various frequencies, it can be difficult to compare rooms that have strengths and weaknesses in different frequency ranges. To solve this, acoustics professionals created several single-number ratings to quickly compare and reference different partitions. These single-number ratings are the Noise Isolation Class (NIC) and the Normalized Noise Isolation Class (NNIC), which are respectively derived from the noise reduction and normalized noise reduction data.
The exact process for going from NR data to an NIC value or from NNR data to an NNIC value is set out in the ASTM E 413 standard. Essentially, the method involves plotting the one-third-octave data and matching it to a predetermined curve. The matching curve gives the single-number value. The value loosely corresponds with the average decibel drop in sound you get between two rooms.