Noise Criteria (NC) and Room Criteria (RC) Curves

Noise Criteria (NC) and Room Criteria (RC) curves are two related tools to evaluate and specify acceptable levels of indoor ambient broad-spectrum noise (often called background noise. Typically, this noise comes from internal sources, including building mechanical systems like HVAC, but external sources, like road noise are also considered. 

What are the Negative Impacts of Background Noise?

Background noise that is too loud or annoying can not only interfere with speech intelligibility, it can also be a source of fatigue that can have a negative effect on safety and productivity.

Indoor Background Noise Standards

It is common in new construction for businesses, education, and government to specify the maximum allowed level of background noise in a room using an NC or RC level. Both Noise Criteria and Room Criteria are specified by the American National Standards Institute (ANSI) in the standard ANSI/ASA S12.2-2019: Criteria For Evaluating Room Noise.

An NC or RC level is a single number metric that characterizes the background noise in a room. Room Criteria measurements include additional parameters described later.

How were the curves developed?

NC Curves were plots of SPL vs frequency first developed in the 1950s based on surveys of acceptable noise level. They can be thought of as equal perceived loudness contours. Room Criteria curves, first proposed in the 1980s, aimed to improve on NC curves by taking into account a subjective understanding of the character of the sound or sound quality. While NC curves were most focused on speech intelligibility over background noise, the developers of RC curves also wanted to ensure that background noise did not have annoying characteristic like high frequency hiss or low frequency rumble that would not be flagged by the NC rating.

How Does a Sound Level Meter Compute an NC or RC Level?

In both cases, a sound level meter plots background noise levels over a range of frequencies using 1/1 octave band L_eq, compare that plot to the established NC or RC curves, and provide an NC or RC level or rating. So, how is this level determined? In short, the NC or RC level is defined by the highest curve that the measured sound in any octave band reaches. Even if the L_eq measurements for every other octave band are lower, this single touch point determines the NC rating In the example below, the measured result does not go above the NC 24 curve, so the NC level is 24.

This method of determining NC Rating is know as the tangency method.

Alternative Computation Method for NC Rating

An alternative method for approximating the NC rating uses Speech Interference Level (SIL) rounded to the nearest dB noted as NC-(SIL). The SoundExpert meter computes and reports the SIL level using the following formula.

What are the Additional Metrics in Room Criteria Measurements?

The RC rating has two parts: a numerical rating known as LMF and additional designations.

• “R” for Rumble, when there is excessive low frequency noise in bands at or below 500 Hz
• “H” for Hiss, when there is excessive high frequency noise in bands at or above 1000 Hz
• “RV” for Rattle and Vibration
  • Rattle: The audible sound produced when building components, such as light fixtures, windows, and ceiling tiles, are agitated by low-frequency, structure-borne vibrations. This often occurs when noise levels in the 16 Hz to 32 Hz octave bands exceed a certain threshold, causing lightweight building elements to rattle.
  • Vibration (perceptible): A physical sensation felt by building occupants, transmitted through the floor, walls, or furniture, usually in the frequency range below 20 Hz. While low-frequency sound is often the cause, the vibration itself is a physical motion of the building structure.
• “N” for Neutral, when the noise has no particular identity with frequency.

Example of a room with hiss.

Tips for Making Noise Criteria and Room Criteria Measurements

• Use a Class 1 or Class 2 Sound Level Meter with Octave Band Analysis (for example SoundAdvisor Sound Level Meter Model 831C or SoundExpert Sound Level Meter Model 821ENV-D
• The Sound Level Meter can be equipped with a random incidence microphone or a freefield microphone (included as standard with both SoundAdvisor and SoundExpert meters) with microphone corrections set to random incidence in the meter settings
• Choose a time to measure when the environment is typical for when the room is being used. (For example, the HVAC is running normally and any external noise sources are typical)
• Ensure the room is unoccupied
• Calibrate the Sound Level Meter with a handheld acoustic calibrator in the measurement environment before and after the measurement.
• Make several measurements at occupant ear height in the loudest areas of the room, or for a rating of the entire room, slowly scan the room
• Measure for 15 seconds to several minutes at each location
  • For steady-state noise: A measurement period of 15–30 seconds per location is generally sufficient. This is typically the case for the continuous and predictable noise from HVAC systems.
  • For fluctuating or intermittent noise: A longer measurement period of 5–15 minutes or more may be necessary. This allows the measurement to capture the different noise levels that occur over time.
• Advanced instruments like the SoundAdvisor or SoundExpert meters with the appropriate firmware have built-in functionality to display the octave band data and determine and report the NC or RC rating

What Are Examples of Common Acceptable NC Levels?

Noise Rating Curves

Note that a similar set of curves known as Noise Rating Curves were developed by the International Organization for Standardization (ISO). NR Curves are most commonly used outside of North America. It is important to understand whether you are measuring NC or NR, as each method will produce different results (for example, NC 30 is not equivalent to NR 30). Larson Davis Sound Level Meters that offer NC/RC include the ability to measure according to Noise Rating Curves as well.