Evaluating and specifying acceptable levels of indoor ambient noise is a key component of building design and performance assessment. Noise Criteria (NC), Room Criteria (RC) and Noise Rating (NR) curves provide standardized methods for describing both the level and spectral characteristics of background sound. Indoor ambient noise is typically dominated by building services, particularly heating, ventilation and air-conditioning (HVAC) systems, but it may also include contributions from external sources, such as road traffic, transmitted through the building envelope.
Excessive or spectrally unbalanced background noise can have significant adverse effects in indoor environments. Beyond acting as a distraction, it degrades speech intelligibility and makes verbal communication more difficult in offices, classrooms and conference rooms. Prolonged exposure to elevated noise levels can also cause listening fatigue, with negative implications for comfort, safety and productivity.
To manage these effects, acousticians and building designers rely on standardized single-number ratings derived from NC, RC and NR systems. These metrics provide a consistent framework for setting clear acoustic design targets, verifying performance in completed buildings and ensuring that indoor spaces are functional, comfortable and appropriate for their intended use.
Managing indoor ambient noise is essential—poorly balanced background sound harms speech intelligibility, causes fatigue, and reduces productivity (HVAC and external ingress are typical sources).
Noise Criteria (NC) is the foundational single-number metric tied to speech interference; it uses octave-band curves and is rated by tangency or NC-(SIL).
Room Criteria (RC) extends NC with diagnostics of sound quality; it adds descriptors—N (neutral), R (rumble), H (hiss), RV (rattle/vibration)—to flag tonal imbalance and low-frequency issues.
Noise Rating (NR) is the ISO-based international standard used widely outside North America; NC and NR are not interchangeable and can yield different values for the same spectrum.
Measurement: use calibrated Class 1 or 2 meters with octave analysis, follow ANSI/ASA placement/duration guidance, and design to ANSI/ISO recommended levels for the room type.
Noise Criteria (NC) is a widely used system for rating the acceptability of indoor background noise. Its strategic importance lies in its ability to translate a complex noise spectrum into a single, understandable number that directly relates to the potential for speech interference, making it a foundational tool for acoustical design and assessment.
The Noise Criteria method is a system that employs a set of standardized curves to evaluate noise levels across a range of frequencies, specifically from the 63 Hz to the 8000 Hz octave bands. These curves, which are similar to equal loudness contours, provide a benchmark against which measured background noise is compared. The primary goal is to ensure that the noise spectrum does not mask or interfere with critical activities like speech communication or create unwanted distractions.
Developed by acoustics pioneer Leo Beranek in the late 1950s, the NC system was created primarily in response to growing noise problems from HVAC systems in office buildings. The original intent was to relate the spectrum of a noise to the disturbance it causes to verbal communication. The NC curves were specifically designed so that their designating number corresponds approximately to the Speech Interference Level (SIL), a measure of how effectively noise masks speech.
This is the simplest and most common approach for determining an NC rating. The measured octave-band noise levels are plotted on a graph containing the standard NC curves. The NC rating is then defined by the value of the highest NC curve that is “touched” by any point in the measured spectrum. The frequency band where this touching occurs is always noted along with the rating, e.g., NC-45 (125 Hz).
The ANSI/ASA S12.2-2019 standard also describes an alternative method, designated NC-(SIL). This approach uses the Speech Interference Level—calculated as the arithmetic average of the sound pressure levels in the 500, 1000, 2000, and 4000 Hz octave bands—to approximate the NC rating.
While the NC system remains a powerful and widely used tool, its limitations in evaluating the subjective quality of noise, particularly low-frequency rumble, led to the development of a more diagnostic metric: the Room Criteria (RC).
Room Criteria (RC) represents a significant refinement of the NC method, developed to provide a more nuanced evaluation of indoor background noise. Its strategic importance lies in its ability to assess not only the overall loudness but also the subjective sound quality, diagnosing annoying characteristics like rumble or hiss that might otherwise go unflagged.
Developed in the 1980s by ASHRAE, the RC methodology was a direct response to the failings of the NC method in characterizing low-frequency rumble produced by newly energy-efficient HVAC systems. While NC focuses primarily on speech intelligibility, RC was designed to account for both the human ear’s sensitivity and the perceived character of the sound. This makes the RC method more diagnostic, as it provides not just an overall rating but also characterizes the tonal balance of the noise.
An RC rating consists of two parts: a numerical value and a set of letter designations that describe the sound’s character. These descriptors provide valuable diagnostic information about the noise spectrum.
RC Sound Quality Descriptors
While NC and RC are the predominant systems in the United States, a different standard is widely used internationally.
Noise Rating (NR) curves serve as the internationally recognized standard for evaluating indoor noise, particularly in Europe and other regions outside of North America. Like NC, it provides a single-number rating for a broad-spectrum noise, but it is based on a distinct set of criteria.
The NR system was developed by the International Organization for Standardization (ISO) and is widely used for specifying and assessing noise in indoor environments. It is frequently applied to evaluate noise from mechanical ventilation systems in buildings, serving as a key compliance tool in international design and construction projects.
The calculation of an NR value is conceptually similar to the NC tangency method. Sound pressure levels measured in octave bands are plotted against or compared to a standard set of NR curves. The final single-number NR value is determined by the highest NR curve that is crossed or “touched” by the measured noise spectrum. Modern sound level meters can perform this calculation automatically.
It is crucial for professionals to understand that the NC and NR methods are not interchangeable. They use different sets of curves and will produce different numerical results for the same noise spectrum. Therefore, a rating of NC 30 is not equivalent to NR 30. Specifying the correct metric based on project location and applicable standards is essential.
Beyond these three primary systems, other specialized criteria have been developed to address more acoustically sensitive environments.
Beyond the primary NC, RC, and NR systems, specialized criteria have been developed to evaluate noise in more demanding or problematic acoustic environments. These tools offer greater stringency and diagnostic capability for specific applications.
Preferred Noise Criterion (PNC) curves are an extension of the basic NC system, designed to be more stringent. They are considered superior to NC curves for critical listening environments such as recording studios, concert halls, and lecture theaters. This superiority stems from the fact that PNC curves are less tolerant of low-frequency noise than their NC counterparts, addressing a key area where the original NC system was found to be lacking.
Described in the ANSI/ASA S12.2-2019 standard, the Room Noise Criterion (RNC) method is a specialized tool developed specifically to evaluate environments where low-frequency noise from HVAC systems is high and may contain fluctuations or surging. It essentially functions as a “rumble criterion,” providing a procedure to identify and rate acoustically ill-behaved systems. When systems are well-designed, the RNC result is reduced back to a value consistent with the NC curves.
A direct comparison of the main methodologies can help professionals select the most appropriate tool for their specific needs.
Selecting the correct noise evaluation metric is a critical decision that depends on the project’s goals, geographic location, and the specific nature of the acoustic challenge. The following table directly compares the key features of Noise Criteria (NC), Room Criteria (RC), and Noise Rating (NR) to guide this selection process.
The comparison highlights the distinct applications of each metric. With a clear understanding of the tools, the next step is conducting accurate measurements.
| Attribute | Noise Criteria (NC) | Room Criteria (RC) | Noise Rating (NR) |
|---|---|---|---|
| Primary Purpose | To rate noise based on its potential to interfere with speech intelligibility. | To evaluate both the level and subjective sound quality of background noise. | To provide a single-number rating for assessing indoor ambient noise levels. |
| Key Differentiator | Focuses on loudness and Speech Interference Level (SIL). | Highly diagnostic, with descriptors for Rumble (R), Hiss (H), and Rattle/Vibration (RV). | An international compliance tool that does not provide detailed diagnostics on spectral balance. |
| Geographical Usage | Primarily used in the USA. | Primarily used in the USA. | Primarily used in Europe and internationally. |
| Governing Body/Origin | Leo Beranek / ANSI | ASHRAE / ANSI | International Organization for Standardization (ISO) |
Steps for conducting accurate and standardized indoor noise measurements for NC and RC evaluations:
The ultimate purpose of measuring indoor ambient noise is to compare the results against established criteria for specific room types. These benchmarks, provided by industry standards, help ensure that the acoustic environment is suitable for its intended function.
The following table provides a selection of recommended noise criteria for various unoccupied rooms as specified in the ANSI/ASA S12.2-2019 standard.
| Type of Room/Space | Recommended NC and RNC Curve | Recommended RC Mark Criteria |
|---|---|---|
| Bedrooms | 25–30 | 25–30(N) |
| Executive Offices | 25–35 | 25–35(N) |
| Open-Plan Offices | 35–40 | 30–40(N) |
| Conference Rooms (Large) | 25–30 | 25–35(N) |
| Hospitals (Private Rooms) | 25–30 | 25–35(N) |
| Hospitals (Wards) | 30–35 | 30–40(N) |
| Classrooms (< 566 m³) | 25–30 | 25–30(N) |
| Live Performance Theaters | 20–25 | — |
The following table outlines maximum recommended Noise Rating (NR) levels for various applications, commonly used in Europe and internationally.
| Noise Rating (NR) Level | Application |
|---|---|
| NR 25 | Concert halls, broadcasting and recording studios, churches |
| NR 30 | Private dwellings, hospitals, theatres, cinemas, conference rooms |
| NR 35 | Libraries, museums, court rooms, schools, hotels, executive offices |
| NR 40 | General offices, restaurants, halls, corridors, shops |
| NR 60 | Light engineering works |
| NR 70 | Foundries and heavy engineering works |
The most critical principle is to recognize that these noise curves move beyond simple loudness measurement to a more nuanced evaluation that considers frequency content and its impact on human comfort, communication, and productivity. Effective noise management requires selecting the appropriate metric for the task. Whether using the foundational NC for speech intelligibility, the diagnostic RC to assess sound quality, or the international NR standard for global projects, the choice must be informed by the specific goals of the project, its location, and the nature of the noise being evaluated.
An authorized SVANTEK consultant will help You with the details such as the required accessories for your noise monitoring task.
