Noise Criteria (NC) Curve Calculator

Concepts behind this calculation

Compare eight unweighted octave-band sound-pressure levels with traditional NC 15–70 curves. See the lowest containing curve, controlling bands, interpolation estimate and optional target margins.

Traditional NC compares a receiver’s unweighted octave-band sound-pressure spectrum with a family of shaped limits. The tabulated result is the lowest curve that contains every band, so a single frequency region can govern even when the other bands are well below that curve. This is an envelope comparison rather than an energy sum, and the controlling frequencies help identify where a noise-control effort could change the rating.

The optional interpolated estimate describes a position between adjacent source curves; it does not replace the tabulated result or relax a selected target. All eight receiver bands are needed, with relevant background and source contributions already represented. Product sound power, catalog NC and overall dBA cannot supply that spectrum. The method also leaves very-low-frequency sound, tones and temporal character incompletely described.

How to use this calculator

1. Complete Receiver and spectrum reference with the location, operating condition and receiver sound-pressure basis, then establish Spectrum and method confirmation.
2. Enter all eight sound-pressure levels from 63 Hz through 8000 Hz. Use unweighted receiver bands, not product sound power or already weighted values.
3. Choose Separately selected project target when a documented criterion exists. Review the containing curve and each band’s margin; keep the interpolated estimate separately identified.

Calculation method and applicability

Choose the lowest tabulated N for which Lp(f) ≤ NC_N(f) in all eight bands. Optional estimate: interpolate between adjacent tabulated limits per band, then take the largest band estimate. Target margin = target-band limit − supplied Lp.

Lp(f) = unweighted receiver band sound pressure level (dB re 20 µPa); NC_N(f) = limit of named NC curve N at band center frequency f (Hz). Band margin is limit minus input level (dB).

Input definitions and units

Receiver and spectrum reference

Identify receiver location, operating/time condition, measurement or model source, unweighted octave-band sound pressure and the separately established project criterion. Include all relevant room/background contributions before using this tool.

Spectrum and method confirmation

Confirm all eight bands are unweighted sound-pressure levels at the receiver, re 20 µPa, and the traditional eight-band NC method applies. Product sound power, overall dBA and partial spectra are not interchangeable inputs. Choices: Not established; Complete receiver sound-pressure spectrum and traditional method established.

63 Hz sound-pressure level (dB re 20 µPa)

Unweighted octave-band level, not sound power or an overall weighted level. Supported range −100 to 200 dB; negative levels are valid. Enter every band.

125 Hz sound-pressure level (dB re 20 µPa)

Complete receiver octave-band sound pressure. Missing or unsupported bands must not be entered as invented zeros.

250 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 250 Hz octave-band sound-pressure level at the stated receiver.

500 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 500 Hz octave-band sound-pressure level at the stated receiver.

1000 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 1 kHz octave-band sound-pressure level, not a dBA value.

2000 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 2 kHz octave-band sound-pressure level at the stated receiver.

4000 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 4 kHz octave-band sound-pressure level at the stated receiver.

8000 Hz sound-pressure level (dB re 20 µPa)

Enter the unweighted 8 kHz octave-band sound-pressure level. Complete eight-band input is required for a result.

Separately selected project target (NC curve)

Optional. No room-use limit is chosen automatically. A target must be an explicitly applicable tabulated curve; the worked example's NC 30 is illustrative. Choices: No target comparison; NC 15; NC 20; NC 25; NC 30; NC 35; NC 40; NC 45; NC 50; NC 55; NC 60; NC 65; NC 70.

How to interpret the result

Retain the tabulated rating or explicit outside-family status, controlling bands and target margins. Below-family and above-family results are range statements rather than extrapolated ratings. Equal NC values can still describe different spectral character.

Worked examples

Assumptions and limits

• Pinned traditional eight-band NC curve family: Crystal Instruments public 2018 table, NC 15–70, 63–8000 Hz. This is not a full implementation or certification of the latest ANSI/ASA S12.2 or any adopted local requirement.
• The table result is the lowest source curve containing every band. Its controlling bands need not be the sole controlling band of a discretionary interpolated estimate. All tied bands requiring the tabulated bound are retained.
• The separate interpolation estimate is piecewise linear between each pair of adjacent source curves in each band. Take the largest resulting band estimate; do not round down the tabulated result or use a rounded interpolation value for target acceptance.
• All bands below NC 15 produce an explicit below-family result. Any band above NC 70 produces an above-family result. No numeric rating is extrapolated outside the source family; boundary comparison margins remain available.
• Input must be unweighted octave-band sound-pressure level at the receiver, re 20 µPa. This tool does not sum source/background contributions, convert sound power to pressure, select measurement positions or verify calibration, averaging time or operating conditions.
• Traditional NC does not address sound below the 63 Hz octave band, strong tones or temporal variation adequately. Equal NC numbers do not imply equal sound quality. NR, RC/RC Mark II, dBA/dBC and hearing exposure are separate methods.
• Targets are user-selected curve limits, not automatic room recommendations. The example uses a source curve as a constructed receiver spectrum, not an actual measured room. Product catalog NC or an individual duct path alone is not proof of the complete room spectrum.

Sources

• Crystal Instruments: Noise Criteria (NC) Curves (2018)Direct public manufacturer table supplies all 96 limits for twelve traditional NC curves, 15–70, at eight octave bands. The source's room guidance is not used as an automatic target.
• ASHRAE 2019 Applications, Chapter 49: Noise and Vibration ControlThe NC method section describes the 63–8000 Hz family, tangency and controlling frequency, formal 5-point increments and discretionary interpolation. It explicitly identifies low-frequency and sound-quality limitations; current ANSI/ASA conformity is not inferred.
• CalcREX: Acoustics — Noise Criteria CalculatorDirect English peer controls and one live NC 25 curve result observed. It is task/numeric cross-check evidence, not the authority for curve values or room limits. A second live attempt was blocked by the peer's cookie overlay and is not claimed as reproduced.