What Is Noise Criterion (NC)? NC Ratings Explained

Noise Criterion (NC) is a single-number method for evaluating relatively steady background noise in an indoor space from its frequency spectrum. Engineers compare measured or predicted sound pressure levels with standardized NC curves. The result is written as NC followed by a number, such as NC 25, NC 30, or NC 40; a lower number generally represents a quieter background-noise target.

NC is widely used when designing and commissioning HVAC systems, but it is not a rating for acoustic panels, wall sound insulation, or equipment sound power. Correct use starts by measuring the room under defined operating conditions and identifying which octave band controls the result.

Key takeaways

• Noise Criterion evaluates indoor background sound as a frequency spectrum, not just one broadband decibel value.
• The NC rating is governed by how the measured spectrum relates to a family of criterion curves.
• Lower NC numbers generally indicate quieter design targets, but the correct value depends on room use.
• ASHRAE’s 2023 guidance lists NC/RC 30 for hotel guest rooms, private offices, conference rooms, classrooms, and libraries, but project-specific variation may be appropriate.
• NC and dBA are not directly convertible because NC retains octave-band information while dBA combines weighted sound energy into one total.
• NC is different from NRC, STC, RC, NCB, NR, and reverberation time.
• Acoustic panels may reduce reflected contributions to room noise, but they do not automatically correct noisy equipment, duct breakout, air turbulence, tones, or structure-borne vibration.

Table of Contents

1. What is Noise Criterion?
2. Why NC ratings matter in buildings
3. How NC curves work
4. How is an NC rating determined?
5. Typical Noise Criterion targets by room type
6. Noise Criterion vs dBA
7. NC vs NRC, STC, RC, NCB, and NR
8. What causes a high or unacceptable NC rating?
9. How to reduce background noise and improve NC performance
10. When do acoustic panels help?
11. NC measurement and commissioning checklist
12. Common Noise Criterion mistakes
13. Frequently asked questions

What Is Noise Criterion?

Noise Criterion is a method for judging whether continuous or relatively stationary indoor background sound is suitable for the activity taking place in a room. It is especially associated with noise from heating, ventilation, and air-conditioning systems: fans, air terminals, grilles, diffusers, dampers, ductwork, pumps, chillers, fan-coil units, and similar building services.

The current US consensus standard is ASA/ANSI S12.2-2026, Criteria for Evaluating Room Noise. Published on March 23, 2026, it describes A-weighted and NC methods for evaluating room noise and introduces an NC3 method based on one-third-octave-band levels. The standard recognizes that acceptability depends on amplitude, frequency spectrum, and temporal behavior—not on a single number alone.

NC is a receiver-room result. It describes the sound that reaches a measurement position in a particular room under stated conditions. It does not describe the sound power emitted by a fan, the absorption coefficient of a panel, or the transmission loss of a wall.

What does an NC number mean?

An NC designation combines the letters NC with a numerical rating. In general:

• A lower value indicates a quieter background-noise condition;
• A higher value indicates a more noticeable background-noise condition;
• The acceptable target depends on the room’s function and occupant expectations;
• The band controlling the rating matters because it may reveal rumble, roar, or hiss;
• Meeting the number does not guarantee that tones or fluctuations will be unobjectionable.

NC 30 is therefore not a universal definition of “good acoustics.” It may be a reasonable HVAC background-noise design target for one room, too noisy for a critical recording environment, and unnecessarily strict for a corridor or service area.

Why NC Ratings Matter in Buildings

Background noise can either support or interfere with a room’s purpose. If it is too high, speech may become harder to understand, concentration may suffer, and guests or occupants may complain about hiss, rumble, or equipment noise. If it is extremely low in a space where privacy matters, conversations and small intermittent sounds may become more noticeable.

NC gives designers a frequency-sensitive target. This is important because an overall sound level can hide the difference between a low-frequency rumble and a high-frequency hiss. Two rooms may produce similar broadband readings but sound very different to occupants.

The 2023 ASHRAE Handbook—HVAC Applications, Chapter 49 recommends evaluating HVAC-related room background sound with octave bands and balanced target contours. It notes that loudness and sound quality both influence acceptance, and that unwanted rumble, throb, roar, hiss, or tones can trigger complaints even when the overall level appears reasonable.

How NC Curves Work

An NC chart is a family of curves plotted against frequency. The horizontal axis represents octave-band center frequency in hertz; the vertical axis represents sound pressure level in decibels. Each curve represents one Noise Criterion rating.

The method retains spectral information. Instead of asking only “How many dBA is the room?”, it asks “How much sound is present in each relevant frequency region, and which region is closest to or exceeds the selected limit?”

ASHRAE’s description of the established NC method shows curves extending from 16 Hz to 8,000 Hz. The curves are conventionally displayed in 5 dB steps, while measured results can be interpolated and reported more precisely under the applicable procedure. The most important principle for non-specialists is that every relevant band must be reviewed; a quiet-looking average cannot cancel an excessive band.

The controlling octave band

Imagine that most of a room’s spectrum lies comfortably below an NC 30 curve, but the 125 Hz band rises close to a higher curve. That low-frequency band may control the rating. The same numeric result caused by an excessive 2,000 Hz band would suggest a different audible character and probably a different physical cause.

This is why a good test report should provide more than “NC 35.” It should include the measured spectrum, the criterion curves or calculation method, the controlling band where applicable, measurement conditions, and the operating state of the building systems.

How Is an NC Rating Determined?

The exact method must follow the project’s specified standard. A simplified field workflow is shown below for understanding, not as a substitute for the licensed standard or a qualified acoustic test procedure.

Step 1: Define the purpose and operating condition

Decide whether the measurement is for design verification, commissioning, troubleshooting, or comparison. Record which HVAC equipment should operate, its load or control mode, room occupancy, furnishing, doors and windows, and other relevant conditions.

ASHRAE’s room-design table applies to unoccupied rooms with HVAC systems operating at normal or full-load design conditions. A reading taken during conversation, housekeeping, traffic intrusion, or an atypical low-load period may not answer the same question.

Step 2: Measure the room spectrum

Use suitable calibrated instrumentation and measure sound pressure level by frequency band at representative receiver locations. Measurement position, duration, temporal variation, microphone placement, and instrument performance should follow the specified method and project plan.

A phone application may help identify an obvious trend, but it should not be treated as contractual evidence without verified microphone response, calibration, band analysis, and a documented procedure.

Step 3: Compare the spectrum with the NC curves

Plot or calculate the measured band levels against the standardized curve family. Under the traditional tangency method, select the lowest curve that lies entirely above the measured spectrum; equivalently, the controlling band reaches or “touches” that curve after any permitted interpolation.

The 2026 edition of ASA/ANSI S12.2 should be consulted for its current calculation and reporting rules, including the introduced NC3 option. Do not mix a result calculated under one method with a target written for another method unless the project’s acoustic professional confirms that approach.

Step 4: Identify the controlling band and sound character

Record the band or bands responsible for the result. Listen and check for:

• Low-frequency rumble or pulsing;
• Mid-frequency fan, motor, or duct noise;
• High-frequency air hiss;
• Audible hum, whine, or other tones;
• Intermittent cycling or control noise;
• Structure-borne vibration and rattling.

This diagnostic information is more useful for remediation than the rating alone.

Step 5: Compare with the project criterion

Use the criterion written into the design documents, applicable standard, client brief, or commissioning plan. Generic room tables are starting points, not automatic legal requirements. Local codes, owner standards, brand requirements, and acoustically sensitive activities may justify different values.

Typical Noise Criterion Targets by Room Type

The table below summarizes selected values from ASHRAE’s 2023 HVAC Applications guidance. These are design guidelines for HVAC-related background sound in fully built-out, furnished, unoccupied rooms under normal or full-load design operation—not universal limits for every project.

Building or room type	Suggested NC/RC design value	Approximate dBA reference in ASHRAE table
Hotel individual room or suite	30	35 dBA
Hotel meeting or banquet room	30	35 dBA
Hotel corridor or lobby	40	45 dBA
Executive or private office	30	35 dBA
Conference room	30	35 dBA
Teleconference room	25	30 dBA
Open-plan office	40	45 dBA
Classroom	30	35 dBA
Large lecture room without amplification	25	30 dBA
Library	30	35 dBA
Hospital patient room	30	35 dBA
Drama theater, concert hall, or recital hall	20	25 dBA

ASHRAE states that room use and project judgement matter and that an individual target may vary by approximately ±5 dB from its table. It also recommends experienced acoustic guidance for acoustically critical spaces below NC/RC 30 and for performing arts spaces.

How to use the room table correctly

Do not copy a value into a specification without answering these questions:

1. Is the criterion intended for HVAC noise only or all building-services noise?
2. Is it a design target, a maximum acceptance limit, or a diagnostic reference?
3. Under what load, occupancy, and room condition will it be checked?
4. Which version of the rating method applies?
5. Are tones, fluctuation, low-frequency noise, and vibration addressed separately?
6. Does a local code, school standard, healthcare guideline, hotel brand, or owner requirement take precedence?

Noise Criterion vs dBA

NC and dBA both express sound with a single number, but they are not the same measurement.

Comparison	NC rating	A-weighted sound level, dBA
Input	Frequency-band spectrum	Broadband sound processed through A-weighting
Main strength	Shows whether a frequency band controls a room-noise criterion	Convenient overall level associated with perceived loudness
Main limitation	Different spectral shapes can receive the same NC	Spectral imbalance and low-frequency problems can be hidden
Typical use	Indoor HVAC/background-noise design and evaluation	Environmental, occupational, product, and general sound-level reporting, depending on the standard
Direct conversion	Not applicable	Not applicable

Can NC be converted to dBA?

There is no universal NC-to-dBA conversion formula. ASHRAE publishes approximate dBA references beside recommended NC/RC values—for example, NC/RC 30 is paired with approximately 35 dBA in several room categories—but it explicitly treats dBA as an approximate reference rather than an exact substitute for octave-band analysis.

Two spectra with the same NC can produce different A-weighted totals, and two sounds with the same dBA can fall on different NC curves. A calculator needs the actual band levels and a defined procedure; one NC number alone does not contain enough information to reconstruct the spectrum.

For the fundamentals behind weighted sound levels, read What Is a Decibel (dB)? Understanding Sound Level in Acoustics and What Is Hertz (Hz)? Understanding Sound Frequency in Acoustics.

NC vs NRC, STC, RC, NCB, and NR

Confusing these abbreviations can send a project toward the wrong product or test.

Metric	What it describes	Typical question it answers
NC	Frequency-sensitive indoor background-noise criterion	Is the steady HVAC-related background sound suitable for this room?
NRC	Averaged sound absorption of a tested material or assembly	How much mid-frequency reflected sound does this treatment absorb?
STC	Laboratory airborne sound insulation of a tested building assembly	How well does this wall, door, or floor-ceiling assembly resist airborne sound transmission?
RC / RC Mark II	Room background-noise rating with added sound-quality diagnosis	Does the HVAC noise have a neutral spectrum or signs of rumble/hiss?
NCB	Balanced Noise Criterion method	How does room noise compare with a balanced criterion framework?
NR	A different family of Noise Rating curves used in some international practice	Does the spectrum meet the separately specified NR curve?
RT / RT60	Time required for sound energy in a room to decay by 60 dB, usually estimated from a measured decay range	How reverberant is the room?

NC is not NRC

This is the most important distinction for acoustic-panel buyers. NC evaluates background sound in a room. NRC, the Noise Reduction Coefficient, summarizes sound absorption at 250, 500, 1,000, and 2,000 Hz under ASTM C423.

A panel with NRC 0.90 does not guarantee NC 30. The final NC depends on source strength, HVAC operation, transmission paths, room absorption, location, spectrum, and measurement conditions. Review Understanding Noise Reduction Coefficient (NRC) Rating before comparing absorber test reports.

NC is not STC

STC concerns airborne sound passing through a separating element. NC concerns background sound present at the receiver position. A mechanical-room wall might need sufficient sound isolation, while the occupied room on the other side has an NC target; both requirements may be relevant, but they are not interchangeable.

For a detailed comparison of absorption and isolation, see NRC vs STC: What’s the Difference?.

NC is not reverberation time

Reverberation affects how sound accumulates and decays in a room. NC evaluates the background-noise spectrum under operating conditions. Adding absorption may change the measured room levels from a continuous source, but an NC rating does not state the room’s reverberation time.

What Causes a High or Unacceptable NC Rating?

Use the source–path–receiver model. A high band level can originate at the equipment, increase along a transmission path, or be reinforced by the receiving room.

Symptom or controlling band	Possible sources and paths	Investigation direction
Low-frequency rumble	Fan operation, large duct modes, compressors, pumps, rooftop equipment, structural vibration	Check equipment selection, speed, isolation, duct path, structural connections, and low-frequency spectrum
Mid-frequency roar	Fan noise, duct breakout, insufficient attenuation, mechanical-room transmission	Review sound power data, duct construction, silencers, barriers, walls, doors, and penetrations
High-frequency hiss	Excessive air velocity, restrictive grilles, diffusers, dampers, or terminal units	Check pressure drop, neck velocity, diffuser selection, balancing, and local turbulence
Hum or whine	Motor, electrical equipment, variable-frequency drive, bearing, or tonal fan noise	Identify tonal frequency and source; do not rely on NC alone to describe annoyance
Pulsing or cycling	Control instability, variable-air-volume operation, compressor or fan cycling	Measure over representative time; inspect controls and operating sequence
Rattle or buzzing surface	Loose grille, ceiling element, duct, pipe, panel, or light fitting excited by vibration	Locate the secondary radiator and correct the mechanical connection
Acceptable equipment data but poor room result	Transmission through ducts, structure, shafts, doors, ceilings, or façades; reflective room	Trace each path and verify installed construction and room absorption

ASHRAE emphasizes that HVAC noise can travel through both airborne and structure-borne paths. Fans, dampers, compressors, ductwork, piping, mounts, walls, floors, and ceilings can all be part of the chain. Treating only the receiving-room surface may leave the dominant path unchanged.

How to Reduce Background Noise and Improve NC Performance

The most reliable sequence is to control the source first, then the path, and finally the receiver-room response.

1. Select quieter equipment

Compare manufacturer sound power data by octave band at the intended speed, pressure, flow, and control position. Do not select a fan, fan-coil unit, diffuser, or terminal device only from one overall dBA value or a catalogue NC claim detached from room assumptions.

2. Reduce aerodynamic noise

Avoid unnecessarily high air velocities, abrupt transitions, poorly placed dampers, and fittings that create turbulence near occupied rooms. Select duct sizes, grilles, diffusers, and terminals for both airflow and acoustic performance.

High-frequency noise near a supply outlet may indicate local air noise rather than a need for more wall panels.

3. Attenuate duct transmission

Where calculations show a need, use correctly designed silencers, lined duct sections, plenums, or other attenuation measures. Also check duct breakout and cross-talk through common air paths. Pressure loss, regenerated noise, hygiene, fire, moisture, maintenance, routing, and ceiling/plenum details must remain coordinated.

5. Isolate vibration

Use suitable equipment mounts, inertia bases where required, flexible connectors, pipe isolation, and structural detailing. The isolation system must match equipment mass, operating speed, forcing frequency, support stiffness, and installation conditions.

5. Improve enclosing constructions

Mechanical-room walls, floors, ceilings, doors, louvers, shafts, and penetrations may need coordinated airborne sound isolation. Small gaps and weak components can dominate. The appropriate laboratory and field sound-isolation ratings should be specified separately from NC.

6. Adjust room absorption where reflections contribute

Acoustic ceilings, wall panels, baffles, or clouds can reduce reverberant build-up from a source inside or radiating into the room. Their value depends on tested frequency response, installed area, mounting, placement, and the room’s existing absorption.

7. Rebalance, recommission, and verify

Incorrect airflow, control sequences, damper positions, fan speeds, or equipment schedules can make an otherwise sound design fail. After corrections, repeat measurements under the same defined condition and document the spectrum—not only the final NC number.

When Do Acoustic Panels Help?

Acoustic panels can help when reflected sound in the receiving room materially contributes to the measured background level or to occupant discomfort. They are especially useful for controlling reverberation in hard-finished offices, lobbies, restaurants, meeting rooms, classrooms, and multipurpose spaces.

They are unlikely to be the primary correction when the dominant problem is:

• A noisy fan or terminal unit;
• High air velocity at a grille or diffuser;
• A tonal motor or variable-frequency drive;
• Duct breakout or cross-talk;
• An unsealed mechanical-room boundary;
• Structure-borne vibration;
• A rooftop unit transmitting through the structure;
• Low-frequency fluctuation outside the practical strength of a thin porous absorber.

A useful buyer question

Before requesting “panels to achieve NC 30,” ask:

Is the measured NC controlled by reverberant sound that added absorption can reduce, or by a source and transmission path that must be corrected elsewhere?

The answer requires spectrum and path information. If panels are appropriate, select them from a verified absorption report and calculate the necessary area. Leeyin’s guide to calculating acoustic panel quantity connects room volume, reverberation targets, and absorptive coverage.

NC Measurement and Commissioning Checklist

Use this checklist when requesting or reviewing a Noise Criterion test.

Before measurement

• Identify the governing standard and edition, such as ASA/ANSI S12.2-2026;
• State the project NC target and whether other criteria also apply;
• Define room condition: finished, furnished, unoccupied, doors/windows positioned as specified;
• Define equipment condition: normal/full load, fan speed, VAV state, diffuser flow, adjacent equipment operation;
• Decide how intrusive external noise and unrelated intermittent events will be handled;
• Confirm suitable instrumentation, calibration, frequency-band capability, measurement positions, and durations.

In the report

• Room name, dimensions, finishes, and furnishing condition;
• Date, time, measurement positions, and microphone height;
• Instrument make/model, calibration information, and analysis settings;
• Operating state of all relevant HVAC and building-services equipment;
• Octave- or one-third-octave-band sound pressure levels as required;
• Calculated NC result, method, and controlling band where applicable;
• Notes about tones, fluctuation, vibration, or non-HVAC contamination;
• Comparison against the contractual criterion;
• Diagnostic recommendations tied to source and transmission path.

The National Institute for Occupational Safety and Health has used comparisons between frequency-dependent measurements and NC curves to evaluate whether workspace background noise exceeds selected criteria. That example reinforces why the underlying spectrum is more diagnostic than a single broadband reading.

Common Noise Criterion Mistakes

Mistake 1: Treating NC as an acoustic-panel rating

NC describes room background noise. NRC and αw describe absorption. A product data sheet cannot promise a final room NC without source, path, room, coverage, and operating information.

Mistake 2: Converting NC to dBA with a fixed offset

Approximate room-target tables are not conversion formulas. Always retain the measured spectrum or calculate both metrics from the same valid measurement data.

Mistake 3: Reporting the number without the controlling band

The band that controls the rating can point toward rumble, duct roar, or air hiss. Without the spectrum, the contractor may apply the wrong treatment.

Mistake 4: Ignoring tones and fluctuation

A steady curve comparison may not fully describe hum, whine, pulsing, cycling, or other temporal and tonal characteristics. Apply the appropriate additional assessment.

Mistake 5: Using terminal-unit NC data as the finished-room result

Manufacturer ratings depend on defined assumptions and test conditions. The final room includes multiple sources, paths, room absorption, installation effects, and operating points.

Mistake 6: Choosing one generic target for the whole building

A teleconference room, hotel lobby, guest room, classroom, theater, and plant area do not have the same purpose or occupant expectation. Set criteria by room type and risk.

Frequently Asked Questions

Use Noise Criterion as a Design and Diagnostic Tool

Noise Criterion turns a complex background-noise spectrum into a practical room rating. Used correctly, it helps architects, HVAC engineers, contractors, and facility teams set targets, select systems, commission buildings, and identify the frequency region driving a complaint.

Used incorrectly, NC becomes just another number. It cannot replace octave-band data, diagnose tones or vibration by itself, predict a finished room from one equipment catalogue value, or tell you how many acoustic panels to install.

Start with the room function and the project criterion. Then trace the sound from source through path to receiver, control the dominant mechanism, and verify the completed condition. To review suitable wall and ceiling acoustic products where room absorption is part of the solution, send Leeyin Acoustic your room drawings, noise spectrum, operating conditions, reverberation target, and required documentation through the project contact page.

Authoritative Sources

• Acoustical Society of America. ASA/ANSI S12.2-2026: Criteria for Evaluating Room Noise. Published March 23, 2026; accessed September 2, 2026.
• Acoustical Society of America Standards. Newly Published, Recently Revised, and Reaffirmed Standards. Lists ASA/ANSI S12.2-2026 as a revision of the 2019 edition; accessed September 2, 2026.
• ASHRAE. 2023 ASHRAE Handbook—HVAC Applications, Chapter 49: Noise and Vibration Control. Room criteria, design conditions, limitations, and source-path-receiver guidance; accessed September 2, 2026.
• ASHRAE. 2021 ASHRAE Handbook—Fundamentals, Chapter 8: Sound and Vibration. NC curve method and limitations; accessed September 2, 2026.
• Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. Health Hazard Evaluation Report 2023-0087-3401. Example of octave-band measurements compared with NC curves; accessed September 2, 2026.