About Noise Criterion (NC) Calculator

What is Noise Criterion (NC)?

Noise Criterion (NC) is a single-number rating system used to evaluate background noise levels in indoor spaces. It considers the sound pressure levels across different octave bands and compares them to a set of predefined NC curves.

Formula and Variables

The NC rating is determined by the following formula:

\[NC = \max\{i \mid SPL_f \leq NC_i(f) \text{ for all frequencies } f\}\]

Where:

• \(NC\) is the Noise Criterion rating
• \(SPL_f\) is the Sound Pressure Level at frequency \(f\)
• \(NC_i(f)\) is the NC curve value at frequency \(f\) for NC level \(i\)

Calculation Steps

1. Measure or input the sound pressure levels for each octave band (63 Hz to 8 kHz).
2. Compare these levels to the NC curves, starting from the lowest curve (NC-15).
3. Find the highest NC curve that is not exceeded by any of the input levels.
4. The NC rating is the number associated with this curve.

Example

Let's consider the following input sound pressure levels:

• 63 Hz: 47 dB
• 125 Hz: 40 dB
• 250 Hz: 35 dB
• 500 Hz: 32 dB
• 1 kHz: 30 dB
• 2 kHz: 28 dB
• 4 kHz: 26 dB
• 8 kHz: 25 dB

Comparing these levels to the NC curves, we find that NC-35 is the highest curve not exceeded at any frequency. Therefore, the NC rating for this example is NC-35.

The NC rating provides valuable information for architects, acousticians, and HVAC engineers to ensure that background noise levels are appropriate for various indoor environments, from quiet recording studios to bustling industrial spaces.