Enter your acoustic data
Formula used
NC determination: select the lowest curve containing every measured band.
Combined sound level: L = 10 log10(Σ10^(Li/10)).
Required reduction: max(0, measured dB − target limit).
NC uses eight octave-band sound-pressure levels. One broadband value cannot define an NC rating. Always retain the measured frequency spectrum for review.
How to use this calculator
- Select the required calculation mode.
- Enter an NC rating or measured levels.
- Choose a target and curve method.
- Press Calculate and review every frequency.
- Export the table for documentation.
Use calibrated octave-band measurements whenever possible. Check the controlling band before choosing treatments. Low-frequency problems often need specialized acoustic control measures.
Worked example
| Frequency | Example level | Target NC-30 | Expected status |
|---|---|---|---|
| 63 Hz | 44 dB | 57 dB | Pass |
| 125 Hz | 35 dB | 48 dB | Pass |
| 250 Hz | 29 dB | 41 dB | Pass |
| 500 Hz | 25 dB | 35 dB | Pass |
| 1000 Hz | 22 dB | 31 dB | Pass |
| 2000 Hz | 21 dB | 29 dB | Pass |
| 4000 Hz | 20 dB | 28 dB | Pass |
| 8000 Hz | 18 dB | 27 dB | Pass |
The example spectrum remains below NC-30 limits. Its closest band controls the reported curve. Review margins before approving sensitive indoor acoustic spaces.
Interpretation guide
Calculated NC rating
The result names the lowest containing curve. Lower numbers indicate quieter background sound conditions. Occupancy needs determine whether that result works.
Controlling frequency
This band has the smallest governing margin. It usually drives the final NC rating. Treating another band may not improve compliance.
Estimated dBA
The estimate applies octave-band A-weighting corrections. It does not replace direct meter readings. Use it only for preliminary comparisons.
Limitations and measurement guidance
This tool evaluates octave-band data mathematically. It cannot verify microphone placement or calibration. Field conditions still require professional acoustic judgment.
- Measure during representative equipment operation.
- Avoid reflections near walls and corners.
- Document instrument class and calibration status.
- Repeat readings when conditions vary significantly.
- Use project standards for formal acceptance.
Frequently asked questions
Is NC the same as dB?
No. NC represents an octave-band criterion curve. Decibels describe sound-pressure levels within each band.
Can one dBA reading determine NC?
No. A broadband dBA value hides spectral information. NC needs octave-band measurements for reliable classification.
What frequency bands are used?
The calculator uses 63 through 8,000 hertz. These are standard octave-band center frequencies.
What controls the final NC rating?
The highest required curve across all bands controls. One dominant band can set the result.
Why does the calculator round upward?
Published curves commonly use five-point NC increments. Rounding upward avoids understating the measured noise.
What does interpolation change?
Interpolation estimates one-point curves between published values. It should be disclosed in formal reports.
What does required reduction mean?
It shows attenuation needed at each failing band. Actual treatment performance depends on installation details.
Can NC values exceed NC-70?
Yes. This tool flags spectra above NC-70. Higher environments may require another evaluation method.
Are room presets mandatory limits?
No. Presets are editable planning examples only. Project criteria should control final acceptance decisions.
Why is estimated dBA approximate?
It uses center-frequency corrections for broad bands. Direct instruments capture more detailed spectral behavior.
Can results be exported?
Yes. Copy, CSV, PDF, sharing, and printing are included. Exports retain the visible result table.