# Noise Criteria (NC) Curves

Introduction
Noise Criteria (NC) curves are primarily used to rate indoor noise levels. It is important that the background noise levels are within acceptable limits to ensure optimal speech communication for structures ranging from residences and offices to hospitals and schools.

Indoor spaces are subject to several noise sources, including internal sources such as noise from an air conditioning unit or external sources such as road noise. Isolating the noise from external sources and ensuring internal noises are low is essential for providing an acceptable acoustic environment within a building.

Noise Criteria (NC) Calculation
Human perception of noise at different frequencies is very different. To compensate for the differential frequency sensitivity of the human ear, the criteria for the noise threshold needs to be defined for each frequency band. The Noise Criteria (NC) specifies the maximum noise levels that can be present in each octave band of noise to meet a specific Noise Criterion.

Noise Criteria is calculated by measuring the background noise is a building using a 1/1 octave spectrum and plotting the spectrum along with a set of curves, called NC Curves, ranging from 63 Hz to 8000 Hz. Noise Criteria for a specific measurement is defined as the criteria of the lowest NC Curve which is not exceeded by the noise spectrum.

The following table defines Noise Criteria Curves for the Sound Pressure Level measured as dB for each Octave Band center frequency in Hz.

Noise Criteria 63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz
Sound Pressure Level (dB)
NC-15 47 36 29 22 17 14 12 11
NC-20 51 40 33 26 22 19 17 16
NC-25 54 44 37 31 27 24 22 21
NC-30 57 48 41 35 31 29 28 27
NC-35 60 52 45 40 36 34 33 32
NC-40 64 56 50 45 41 39 38 37
NC-45 67 60 54 49 46 44 43 42
NC-50 71 64 58 54 51 49 48 47
NC-55 74 67 62 58 56 54 53 52
NC-60 77 71 67 63 61 59 58 57
NC-65 80 75 71 68 66 64 63 62
NC-70 83 79 75 72 71 70 69 68

Measuring Noise Criteria Using Spider Devices
Spider devices along with EDM DSA (Dynamic Signal Analyzer) are equipped with powerful algorithms for the computations of FFT and Octave spectra. Noise measurements through a microphone can be easily plotted along with the defined NC curves to visually determine the minimum NC curve that is not exceeded.

To view the NC Curves, right click on any Octave Spectrum with Sound Pressure Level as the measurement quantity and click the NC Curves setup.

When the limits are already known, select specific NC Curves to determine if the noise levels are within the acceptable limits.

Once selected, the EDM Software displays the Octave Spectrum along with the NC Curves.