A common way of indicating the level of noise in a room, be it control room, studio, or office space, residence, etc., is the NC value.
NC stands for “noise criteria,” and each of the single number NC values actually refer to a curve of specific sound pressure levels plotted against octave band frequency for eight different octave bands. These are 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz.
The larger the NC value, the greater the noise it represents.
For each NC-curve, higher levels of low frequency noise are “allowed” compared to mid and high frequencies, because the ear isn’t as sensitive to the lows, especially at lower levels. But when mixing very low frequency material, like for an LFE channel, make sure the noise level at these frequencies is lower than the lowest level of sound you need to monitor (not just for program, but for annoying background noise as well). Otherwise, sound at these frequencies will be masked by the room noise.
The table below lists the sound pressure levels for each octave band for four NC values. Studios should aim for NC-25 or lower. As a comparison, good noise levels for offices range from NC-30 to NC-40, with open plan offices typically noisier. In general, the quieter you make a space the more expensive it will be.
