You walk into a conference room, take your seat, and notice something immediately — but you cannot name it. The room feels slightly oppressive. There is a low, constant drone from the air conditioning. Not loud enough to be annoying, exactly, but present enough that you feel mildly uncomfortable. When the meeting ends and the HVAC fan cycles down, the room feels different. Quieter, but not in a pleasant way — in a way that makes you aware of every rustle of paper and throat-clearing at the table.
Both sensations — the intrusive drone and the sudden awareness of every small sound — are the result of getting background noise wrong. The first room had a background noise level that was too high and spectrally imbalanced. The hypothetical quiet room may have been so close to silence that ambient sounds become socially awkward. The metric that quantifies these judgements is the NC curve, and it has been the primary tool for rating building background noise since 1957.
The Definition: What NC Measures and Why It Exists
Noise Criteria (NC) is a family of reference curves used to rate the acceptability of steady-state background noise — primarily from HVAC systems and mechanical plant — in occupied building spaces. Each NC curve specifies an upper limit for sound pressure level at each of eight standard octave band frequencies. A higher NC number means a higher acceptable noise level.
The NC system was developed by Leo Beranek at Bolt Beranek and Newman in 1957, responding to a specific problem: the A-weighted decibel (dBA) scale, useful for rating many types of noise, was inadequate for rating HVAC noise because it failed to capture how annoying certain spectral profiles were.
A duct system that produces a loud, low-frequency rumble might measure the same dBA as a system that produces a high-pitched hiss — but the two sounds create completely different sensations in occupants. The rumble may cause physical discomfort and difficulty concentrating; the hiss may feel sharp and fatiguing. The dBA measurement tells you the total noise energy is similar; the NC curves tell you which one is worse and at what frequencies.
NC curves are published in ASHRAE Handbook — HVAC Applications (Chapter 48: Noise and Vibration Control) and are referenced in a wide range of building standards and guidance documents.
What the NC Curves Look Like: Octave Bands and Reference Levels
The NC curve family consists of a set of smoothly descending curves, each labelled with a number from NC-15 to NC-65. Each curve specifies maximum permissible octave-band sound pressure levels at eight standard octave band centre frequencies:
| Octave Band Centre Frequency | NC-25 | NC-30 | NC-35 | NC-40 | NC-45 | NC-50 |
|---|---|---|---|---|---|---|
| 63 Hz | 47 dB | 51 dB | 54 dB | 57 dB | 60 dB | 64 dB |
| 125 Hz | 36 dB | 41 dB | 45 dB | 49 dB | 53 dB | 56 dB |
| 250 Hz | 29 dB | 34 dB | 38 dB | 43 dB | 46 dB | 50 dB |
| 500 Hz | 22 dB | 28 dB | 32 dB | 37 dB | 40 dB | 44 dB |
| 1000 Hz | 17 dB | 24 dB | 28 dB | 32 dB | 36 dB | 40 dB |
| 2000 Hz | 14 dB | 21 dB | 24 dB | 28 dB | 32 dB | 36 dB |
| 4000 Hz | 12 dB | 19 dB | 21 dB | 26 dB | 30 dB | 34 dB |
| 8000 Hz | 11 dB | 17 dB | 19 dB | 25 dB | 28 dB | 33 dB |
Each curve slopes downward from low frequencies to high frequencies because human hearing is more sensitive at mid and high frequencies than at low frequencies for sounds in the background noise range. The NC curves are calibrated so that all sounds that fall on a given NC curve produce approximately the same degree of loudness and annoyance for typical occupants.
To determine the NC rating of a room, you take octave-band measurements of the background noise and plot them against the curve family. The NC rating is the value of the lowest NC curve that the measured spectrum falls at or below at every frequency — in other words, the curve that the measured spectrum just touches at its highest point above the curve family.
If your measured background noise level at 1000 Hz is 35 dB and at 250 Hz is 44 dB, the 1000 Hz value plots below NC-40 but the 250 Hz value sits between NC-40 and NC-45. The NC rating is NC-45 — determined by the worst frequency, not an average.
How to Read NC: A Worked Example
A mechanical engineer is commissioning a new headquarters building. She measures the background noise in the main open-plan work area with all HVAC running at design airflow. Her octave-band meter gives the following readings:
| Frequency | Measured SPL |
|---|---|
| 63 Hz | 52 dB |
| 125 Hz | 44 dB |
| 250 Hz | 38 dB |
| 500 Hz | 31 dB |
| 1000 Hz | 27 dB |
| 2000 Hz | 22 dB |
| 4000 Hz | 18 dB |
| 8000 Hz | 15 dB |
She plots these against the NC curve family:
- At 63 Hz: 52 dB falls between NC-35 (54 dB limit) and NC-30 (51 dB limit). Between NC-30 and NC-35, closer to NC-35.
- At 125 Hz: 44 dB falls between NC-40 (49 dB limit) and NC-35 (45 dB limit). Closest to NC-35.
- At 250 Hz: 38 dB falls between NC-35 (38 dB limit) and NC-30 (34 dB limit). Just on the NC-35 line.
- At 500 Hz: 31 dB falls between NC-35 (32 dB limit) and NC-30 (28 dB limit). Close to NC-35.
- At 1000 Hz: 27 dB falls between NC-30 (24 dB limit) and NC-35 (28 dB limit). Just below NC-35.
- At 2000 Hz and above: all values fall below NC-30.
An open-plan office target is typically NC 35-40, so this room is within specification. If the 63 Hz level had been 56 dB instead of 52 dB, the rating would have been NC-40 — still acceptable, but with more noticeable low-frequency rumble that could distract workers on quiet tasks.
NC Targets by Room Type: What the Standards Recommend
NC targets have been established through decades of occupant satisfaction surveys, productivity research, and speech intelligibility studies. The values below reflect the consensus of ASHRAE guidance, WELL v2 Feature 74, and acoustic consultancy practice:
| Room Type | Recommended NC Range | Notes |
|---|---|---|
| Recording studio, broadcast booth | NC 15-20 | Extremely quiet — any background noise is audible on microphone |
| Concert hall, recital hall | NC 15-20 | Audience hears room noise; background must not mask pianissimo passages |
| Opera house, theatre | NC 20-25 | Orchestral and vocal music requires very low background |
| Private residence (bedroom) | NC 25-30 | Sleep research supports NC 25 for restorative sleep |
| Hotel guest room (quiet) | NC 25-30 | Premium brand standards typically specify NC 25-30 |
| Executive office, private office | NC 30-35 | Confidential conversations; low background for concentration |
| Hospital patient room | NC 30-35 | Sleep and healing recovery; CDC guidelines align here |
| Conference room, meeting room | NC 30-35 | Clear speech required; too quiet and every whisper is audible |
| Classroom | NC 30-35 | ANSI S12.60 requires maximum 35 dBA background, roughly NC 30-35 |
| Open-plan office | NC 35-40 | Some background noise helps mask adjacent conversations |
| Restaurant (fine dining) | NC 35-40 | Conversation across the table should be easy |
| Library | NC 30-35 | Quiet study; lower than offices |
| Retail (quiet) | NC 40-45 | Browsing; some background acceptable |
| Restaurant (casual/lively) | NC 40-50 | Energetic atmosphere; near-shout conversation is acceptable |
| Gymnasium, sports facility | NC 45-50 | High activity, mechanical systems, high occupancy |
Two patterns are worth noting. First, the targets for performance spaces are dramatically lower (NC 15-20) than for offices (NC 35-40) because even faint background noise is audible against a soft musical passage or a pause in dialogue. Second, the open-plan office target of NC 35-40 is intentionally higher than private offices — a moderate level of background noise actually helps with speech privacy by masking the conversations at adjacent workstations.
NC Versus Other Background Noise Metrics
NC is not the only background noise rating system in use, and understanding how they differ prevents confusion in specifications.
NR (Noise Rating): The European precursor to NC, developed by Kosten and van Os in the 1960s. The NR and NC curve shapes are similar but not identical — NR curves are slightly more permissive at low frequencies. An NR 35 specification and an NC 35 specification will yield similar acoustic environments, but they are not mathematically equivalent. NR is the standard reference in the UK, continental Europe, and many Commonwealth countries. NC is dominant in North America.
RC (Room Criteria): Developed by Blazier at ASHRAE in the 1980s to address two significant weaknesses in NC: the inability to rate tonal noise, and the omission of very low frequencies below 63 Hz. RC extends the frequency range down to 16 Hz and adds a quality assessment that flags whether the noise spectrum is "rumbly" (excessive low-frequency energy), "hissy" (excessive high-frequency energy), or "vibration-prone" (energy below 32 Hz sufficient to cause perceptible mechanical vibration). RC is more diagnostic than NC — it tells you not just the level but the character of the noise problem. However, it requires measurements at frequencies below the capability of standard octave-band meters, making field measurement more demanding.
NCB (Balanced Noise Criteria): An update to NC introduced by Beranek in 1989 that extends coverage down to 16 Hz and adds a hiss/rumble qualifier, partly addressing the same limitations that RC aimed to fix. NCB sees less practical use than RC but is referenced in some North American specifications.
For most commercial building applications, NC remains the dominant specification language because measurement equipment is universally available, the targets are well-established, and the concept is straightforward for mechanical engineers and building owners to understand. RC or NCB should be specified when the room contains sensitive recording equipment, when the occupant has expressed sensitivity to tonal noise, or when very low-frequency mechanical vibration is a concern.
How NC Relates to Speech Privacy
Background noise level and NC are not just about comfort — they directly affect speech privacy. This is one of the less intuitive aspects of noise criteria.
In an open-plan office, you want workers to concentrate without being distracted by neighbouring conversations. Too little background noise (NC 25 or below) creates what acousticians call the "pin drop" effect — every whispered conversation across the office is audible to everyone. Too much (NC 50 or above) makes it difficult to hold your own conversation without raising your voice, fatigues occupants over the course of a day, and may cause stress responses associated with excessive noise exposure.
The sweet spot for open-plan offices — NC 35-40 — provides enough masking of distant speech to preserve privacy while remaining comfortable for sustained office work. When the background noise is insufficient, sound masking systems (electronic systems that introduce a spectrally shaped broadband noise) are used to bring the effective background up to the NC 40-45 range, improving speech privacy without creating an uncomfortable acoustic environment.
This is why NC is not purely a "lower is better" metric for all room types. The appropriate NC target depends on the function: performance spaces want the lowest achievable level; private offices want low-moderate levels for concentration and confidentiality; open-plan offices and certain reception areas benefit from a moderate background that provides some natural masking.
Common Mistakes When Specifying NC
Specifying NC only in the contract without testing during commissioning: HVAC systems are often designed to achieve a target NC, but supply-air grille velocities, duct resonances, fan characteristics at part-load, and plenum design details mean that the actual NC can be substantially different from the predicted value. NC should be specified as a verifiable performance requirement, with post-commissioning octave-band measurement required before practical completion is accepted.
Using dBA as a substitute for NC: Many mechanical specifications state a maximum background noise level in dBA without specifying the spectral distribution. A reading of 40 dBA could correspond to NC-35 (acceptable for a conference room) or NC-45 (too loud for a conference room) depending on whether the noise is concentrated at low or high frequencies. The dBA measurement cannot make this distinction. For critical listening environments, specify NC with octave-band limits.
Setting NC too low for spaces that benefit from masking: Specifying NC 25 in an open-plan office in the belief that "quieter is always better" will result in a space where occupants cannot have private phone calls without being heard across the floor, and where every keyboard click and chair squeak becomes a distraction. Match the NC target to the room function.
Forgetting that NC is a maximum limit, not a target range: An NC rating specifies the worst octave band relative to the curve family. A room with a very low NC could still feel unpleasant if its spectrum is strongly tonal — a pure hum at a single frequency that falls just under the NC limit still sounds annoying. The NC rating tells you about overall level, not about tonal quality.
How AcousPlan Helps You Verify NC Compliance
AcousPlan's room acoustics simulator includes background noise assessment alongside RT60 and STI calculations, giving you a complete picture of the acoustic environment in a single modelling session.
Within the platform, you can:
- Plot measured or predicted octave-band noise levels against the NC curve family to determine the NC rating of a space and identify which frequency bands are driving the rating
- Compare NC values to room-type targets drawn from ASHRAE guidance, WELL v2 Feature 74, ANSI S12.60 (classrooms), and other applicable standards, with automatic pass/fail flagging
- Assess the combined effect of NC and RT60 on speech intelligibility — because a room that is quiet enough in NC terms but too reverberant in RT60 terms can still produce poor speech transmission index (STI) results
- Generate compliance documentation that includes octave-band noise data, the NC curve plot, and a written assessment of compliance status, suitable for inclusion in a WELL or LEED acoustic documentation package
Ready to assess background noise in your room? Run the acoustic simulator at /calc — model your room, input background noise levels, and get NC rating, RT60, and STI compliance results in one place.