NC stands for Noise Criterion and refers to the quiescent or ambient background noise present in an acoustic space such as an auditorium or room. Curve, or contour, refers to the way in which our ears are sensitive to noise, which essentially follows the guidelines outlined by the Fletcher-Munson Curves, or other similar studies. In a nutshell this means that the human auditory system is not equally sensitive to noise at all frequencies. Further, as the noise level changes these relative sensitivities change with respect to one another. NC curves were developed to take all this into consideration, thus providing a reasonably objective way in which to document and communicate ambient noise levels in rooms. This is important because quite often the majority of noise in most auditoriums is caused by the ventilation system, where most of the noise is at relatively low frequencies – frequencies at which human hearing is relatively insensitive. There are ratings given for various levels across the spectrum that take these curves into account. So a room with a certain amount of noise at 100 Hz will rate significantly better than a room with the same amount of noise at 1 kHz. Typical ratings range from NC-15 to NC-70. For example, a room said to meet the NC-15 requirement would be so quiet that the average listener would not perceive any background noise at all, yet there could be noise at 30 dB SPL below 80 Hz. An NC-20 room is noticeably noisier, but still considered very quiet, while a room rated above NC-25 or 30 is generally considered too noisy for critical listening.