Enter octave-band SPL values measured in a room and the tool compares them to Beranek's NC family, returning the NC rating, governing frequency and A-weighted level. Practical for HVAC noise design and indoor acoustics in bedrooms, offices and meeting rooms.
Octave-band SPL
63 Hz band SPL
dB
Low-frequency rumble. Dominated by fans and structure-borne paths
125 Hz band SPL
dB
250 Hz band SPL
dB
500 Hz band SPL
dB
Most important mid band for speech intelligibility
Light grey lines are NC15-NC65 ceilings, the blue dots are the entered SPL, and the red dashed line is the resulting NC curve. The green dot marks the governing band (smallest margin).
Definition of the NC rating. NC curves are SPL ceilings vs frequency; the smallest NC number whose ceiling envelops every measured band (63 Hz to 8 kHz) is the rating.
Governing frequency. The band where the margin against the resulting NC ceiling is smallest is the limiting band; reducing its SPL is the cheapest way down to the next NC tier.
Rating room noise with the NC curve family
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I've only ever used dB(A) for room noise targets. What does an "NC curve" buy me on top of that?
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dB(A) collapses the whole spectrum into one number, so it only tells you "overall loudness". But HVAC noise is rarely uniform: low-frequency rumble that shakes the floor and high-frequency hiss that pierces the ear can produce the same dB(A) yet feel completely different. Beranek's NC family (1957) defines an SPL ceiling for every octave band from 63 Hz to 8 kHz. The lowest NC number whose ceiling is never exceeded becomes the room's NC rating. That way the spectral balance is built into the score: a bass-heavy spectrum lands on a higher (worse) NC number than a flat one of the same dB(A).
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So "lowest curve where all bands stay below" — does a single band crossing the ceiling really disqualify the whole curve?
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Exactly, one band over is a fail. Try the default input: 60, 55, 50, 45, 42, 38, 35 dB. NC35 has a 125 Hz ceiling of 52 dB but the measurement is 55, so NC35 fails. NC40 fails at 1 kHz (41 vs 42). NC45 (67/60/54/49/46/44/43) finally envelops everything, so the rating is NC45. The tool also shows you the "governing band" — the bands tied at the smallest margin (4 dB at 250, 500, 1k Hz). Those are the bands you must lower to drop a tier.
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That's actually a great design hint — it tells you where to spend your money. People say a bedroom should hit NC25. How quiet is that really?
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NC25 sounds like a faint refrigerator compressor — roughly 30 dB(A). A general open office is NC35-40, where multiple conversations still let you concentrate. Concert halls and recording studios push for NC15-20. Above NC50 conversation gets effortful, and NC55+ is the world of factories and gymnasiums. Seeing NC45 in a residence almost always points at low-frequency HVAC content and an under-silenced duct run.
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Right — and low frequencies are notoriously hard to absorb. If the bass band is the governing one, what actually works?
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Good catch — porous absorbers fail at long wavelengths. The cure ladder for low-frequency dominance is (1) isolate the source (fan, compressor) from the structure, (2) add reactive duct silencers and large plenum mufflers, and (3) use double walls or floating floors to break flanking paths. If 4 kHz dominates, by contrast, just upsizing diffusers so outlet velocity drops below 3 m/s often clears a whole NC tier. The point of NC analysis is to pick the right intervention for the right band, and that's what this tool visualises.
Frequently asked questions
For each octave band from 63 Hz to 8 kHz, the NC family defines an allowable SPL ceiling. The NC rating of a measured spectrum is the lowest NC curve number whose ceiling is not exceeded by any band. A single band crossing the curve disqualifies it, and the next larger curve is tried. This tool stores NC15 to NC65 in 5-step increments and returns the rating from the seven bands you enter.
dB(A) compresses the spectrum into a single "overall loudness" number, while the NC rating captures the spectral balance as well. Two rooms with identical dB(A) can feel very different if one has strong low-frequency rumble: its NC rating will be worse. HVAC equipment that meets a dB(A) target often fails an NC target because the low-frequency content is too rich. Always check both.
Typical values from ANSI S12.2 and the ASHRAE Handbook are: concert halls and recording studios NC15-25, bedrooms/dwellings/libraries NC25-30, classrooms/meeting rooms/medical exam rooms NC30-35, general open offices NC35-40, retail/lobbies NC40-45, factories/gymnasiums NC50-55. Values above these ranges are considered to interfere with speech intelligibility and concentration.
Start by identifying the governing band (smallest margin). Low frequencies (63-250 Hz) are usually fan and duct regenerated noise: add inline silencers, low-frequency absorbers and structure-borne isolation. Mid frequencies (500-1000 Hz) point to diffuser jet noise and reflections, fixed by limiting outlet velocity to 3 m/s and adding absorptive ceilings. High frequencies (2-4 kHz) often come from throttled diffusers, cured by better static-pressure balancing.
Real-world applications
Office and workplace HVAC design: General offices target NC35-40 while executive offices and meeting rooms aim for NC30-35. At design stage, octave-band sound-power data from the air-handler manufacturer is propagated through the duct losses and diffuser regeneration, then the room SPL is estimated and rated with this tool. Post-commissioning, the most common surprise is NC42 against an NC40 spec, and 80% of the deviation comes from low-frequency (63-250 Hz) prediction error. The measure-then-rerate-then-fix loop (silencer addition, isolator tuning) is exactly what this calculator is built for.
Theatres, concert halls and recording studios: Acoustically critical spaces demand NC15-20, and benchmark venues like Boston Symphony Hall achieve NC10-15. Reaching these levels requires diffuser velocities under 1 m/s, double structural skins, floating floors and outdoor sound insulation of 70 dB or more, all of which dominate the construction budget. NC analysis tells you exactly which band needs the next few dB, so cost-benefit decisions on each treatment become explicit.
Residential and hotel guest rooms: Sleeping environments aim at NC25-30, with premium hotel rooms specified at NC25. The two NC-dominating sources are neighbour activity (impact noise) and outdoor traffic, so window grading (T-2 to T-4) and floor impact ratings (LL-45 to LL-50) decide whether the NC target is met. Feeding a measured spectrum into this tool tells you immediately whether the window or the wall is the limiting element — the answer lives in which band governs.
Hospitals, clinics and intensive-care units: WHO guidelines and most national healthcare codes recommend NC30 for patient rooms, NC35 for ICUs and exam rooms. Medical-device alarms and central piping noise often produce narrow peaks that dominate the NC rating, and visualising "which band is sticking up" lets you target a single shroud or pipe insulation upgrade rather than reworking the whole HVAC system.
Common misconceptions and pitfalls
The first misconception is that meeting a dB(A) target automatically meets the NC target. The reverse is closer to the truth: NC is usually stricter, especially for low-frequency-rich spectra like HVAC noise where a unit can satisfy its dB(A) spec yet land one or two NC tiers worse. Many equipment data sheets list only dB(A), so insist on octave-band tables from the manufacturer and convert with this calculator before signing off. Designing on dB(A) alone is the most common reason for "noise complaint after handover" disputes.
The second trap is to assume the NC curve is obsolete because it dates to 1957. RC (Room Criterion, ASHRAE 1989) and NCB (Balanced NC, 1989) add finer descriptors for rumble and hiss, but ANSI S12.2 and the ASHRAE Handbook still treat NC as the headline rating. Contracts and tenant fit-out specs are almost always written as "NC30" or "NC35", not RC numbers, so NC literacy is non-negotiable.
The third pitfall is microphone position and background noise correction. NC must be measured at the listener's ear position (1.2 m above the floor seated, 1.5 m standing) and at least 1 m away from any wall surface to escape standing-wave peaks that can boost the 63 Hz band by 10 dB or more. Background noise from traffic or other floors must be at least 10 dB below the source under test. If only the 63 Hz band looks oddly high, suspect the microphone location and the background level before redesigning the silencer.
How to Use
Measure or input octave-band sound pressure levels (SPL) in dB for frequencies 63 Hz, 125 Hz, 250 Hz, and 500 Hz using a calibrated sound level meter in octave-band analysis mode.
Enter each measured SPL value in the corresponding numeric field; optionally specify the measurement uncertainty range (±dB) for each band.
The simulator compares your measured spectrum against Beranek NC curve criteria, outputs the limiting NC rating, identifies the governing frequency band, calculates margin above the NC threshold, and provides A-weighted equivalent and room use classification.
Worked Example
An HVAC-dominated office space measures: 63 Hz = 58 dB, 125 Hz = 52 dB, 250 Hz = 48 dB, 500 Hz = 45 dB. The 63 Hz band exceeds NC-55 criterion (57 dB) by 1 dB, making this room NC-60 rated. Governing frequency is 63 Hz with a -2 dB margin to NC-60. A-weighted sum approximates 54 dB(A). Classification: NC-60 suits mechanical equipment rooms but exceeds open office targets (NC-50 preferred). Verdict: Install low-frequency absorption or reduce compressor vibration transmission.
Practical Notes
NC-45 to NC-50 is standard for general office spaces; NC-35 to NC-40 for conference rooms; NC-55+ indicates HVAC/mechanical dominance requiring duct silencers.
Low-frequency exceedance (63–125 Hz) typically signals structure-borne vibration or undersized ductwork; high-frequency excess suggests fan noise or air turbulence.
Always measure with sound level meter in slow response, A-weighting off for octave-band data; uncertainty ranges help identify measurement repeatability and guide remediation priorities.