Noise curve
![The lowest curve which is not touched by the measured spectrum at any position in the audio frequency range is NC-45. This room has an NC45 rating.](/Images/godic/202501/30/Noise_Curves_Graph_Spectrum_with_NC0058.jpg")
Noise curves are a common way to measure and specify background noise in unoccupied buildings and spaces. Their purpose is to produce a single value representation of a complete sound spectrum. International standards organizations (ISO, ANSI and ASA) recognize the need to objectify judgements on the amount of ambient noise in enclosed spaces, and provide us with definitions for various Noise Curves. The ANSI/ASA S12.2-2008 standard, for example, recommends an NC value of 25-35 for Schools and 15-18 for Concert halls.