NTi Audio
XL2 Audio and Acoustic Analyzer

Noise Curves

Noise curves specify the acoustic ambient noise in an indoor environment. The unoccupied room is measured to produce a single maximum value across the complete sound spectrum. This value is used to determine if the ambient noise will be annoying to people occupying the room. The value also influences of the intelligibility of speech.

Different rooms, locations, regulations and applications may allow different acceptable noise ratings. In most cases, the goal is that background noise should not interfere with the purpose of the room, e.g. the noise of an office air-conditioning system and consistent noise of traffic outside the building should not interfere with telephone calls or conversations.

Background noise that is annoying creates fatigue and can negatively affect productivity and safety. Too much noise also affects the ability to communicate. Therefore standard methodologies for quantifying such noise have been developed.

Methods and Standards

For all ambient noise measurement methods, a measured noise spectrum is superimposed on a family of contours (or curves).

NC Result
Result: NC45

Noise Rating NR

(in accordance with ISO 1996)

In Europe, the International Organization for Standardization (ISO) defines Noise Rating (NR) as a graphical method for assigning a single number rating to a noise spectrum. It can be used to specify the maximum acceptable level in each octave band of a frequency spectrum, or to assess the acceptability of a noise spectrum for a particular application. The method was originally proposed for use in assessing environmental noise, but it is now used frequently for describing noise from mechanical ventilation systems in buildings. The NR of the spectrum corresponds to the value of the first NR contour that is entirely above the spectrum.

Noise Rating
Noise Rating NR32

Noise Criteria NC

(in accordance with ANSI S12.2-2008 and -1995)

The American National Standards Institute (ANSI) defines the NC rating to describe the noise in a space by examining a range of frequencies. The NC rating of a spectrum is designated as the value of the lowest NC curve above the measured octave-band spectrum. The measured noise criteria, e.g. NC30, informs that the room performs better than that. The designating number for any NC curve is, approximately, its Speech Interference Level (SIL): the average of the levels in the 500, 1000, 2000 and 4000 Hz octave bands. SIL is a simple metric, which measures the effects of noise on speech intelligibility. The XL2 Analyzer includes the tangency method adaptation in accordance with the standard.

Noise Criteria
Noise Criteria NC30

Room Noise Criteria RNC

(in accordance with ANSI S12.2-2008)

The RNC method is used to determine noise ratings when the noise from heating, ventilating and air-conditioning (HVAC) systems at low frequencies is high, and which is also suspected of containing sizeable fluctuations or surging. It essentially represents a rumble criterion. The RNC curves also provide a procedure that reduces the result essentially back to the NC curves when systems are well designed and acoustically well-behaved.

Room Noise Criteria
Room Noise Criteria RNC48

Room Criteria RC

(in accordance with ANSI S12.2-1995)

The RC criterion curves are a system for use in the design of heating, ventilating and air-conditioning (HVAC) systems in office buildings, dwelling units, etc., where the desired mid-frequency levels are in the range of 25 to 50 dB. Each RC criterion curve bears a rating number equal to the level at 1000 Hz.

Spectrum classification

  • Neutral spectrum (N): The levels at 500 Hz and below do not exceed the RC curve corresponding to a sound level spectrum by more than 5 dB; and the spectrum levels in Band 1000 Hz and higher do not exceed the corresponding RC curve by more than 3 dB.

  • Rumble (R): Excessive noise in low-frequency band. The level in one or more of the octave bands at and below 500 Hz exceeds the RC curve corresponding to a spectrum by more than 5 dB.

  • Hiss (H): Excessive noise in high-frequency bands. The level in one or more of the octave bands at and above 1000 Hz exceeds the RC curve corresponding to a spectrum by more than 3 dB.

  • Vibration and rattle (RV): The level in one or more of the octave bands from 16 Hz through 63 Hz exceeds the criterion for moderately noticeable rattle.

Room Criteria
Room Criteria RC32

Preferred Noise Criteria PNC

(in accordance with ASA 1971)

The American Statistical Association (ASA) defines the PNC curves as an extension of the basic Noise Criteria system. They have been used in the past to judge the acceptability of ventilation and other background broadband noise.

PNC curves are less often used than Noise Criteria curves because they are more stringent at lower frequencies than the Noise Criteria curves, and also because the latest (2008) version of Noise Criteria curves includes an extended frequency range somewhat mitigating the original motivation for PNC.

Preferred Noise Criteria
Preferred Noise Criteria PNC34


The standard ANSI/ASA S12.2-2008 recommends the following criterias:

Type of Room NC and RNC Curve RC Curve
Concert halls, opera houses 15-18 -
Small auditoriums (≤500 seats) 25-30 -
Large auditoriums, churches (>500 seats) 20-25 -
TV and broadcast studios 15-25 -
Legitimate theaters 20-25 -
Private residences: Bedrooms 25-30 25-30 (N)
Private residences: Living rooms 30-40 25-30 (N)
Schools 25-35 25-40 (N)
Hotels: Suites 30-35 25-35 (N)
Hotels: Meeting/banquet rooms 25-35 25-35 (N)
Hotels: Service support areas 40-50 35-45 (N)
Offices 25-40 25-35 (N)
Conference rooms 25-35 25-35 (N)
Hospitals: Private rooms 25-30 25-35 (N)
Hospitals: Public areas 40-45 40-50 (N)
Movie theatres 30-40 -
Small Churches 30-35 25-35 (N)
Courtrooms 30-35 25-35 (N)
Libraries 30-35 30-40 (N)
Restaurants 40-45 -
Light maintenance shops 45-55 -
Shops and garages 50-60 -

Measuring Noise Curves with XL2 Analyzer

The XL2 Acoustic and Audio Analyzer, with the Spectral Limit Option installed, supports these Noise Curve types:

NC Result

The XL2 measures all noise curve types at the same time, thus you can switch to your required report after the measurement. The result also indicates the frequency band with the highest noise rating  between 16 Hz and 8 kHz.

For RNC curve types the XL2 reports any large fluctuations or surging at low frequencies, e.g. caused by fans.

Further following the RNC specification, the XL2 measures the octave-band sound pressure level every 100 ms. The minimum measurement time is 20 seconds.

For curve types NC-2008 and NC-1995, the speech interference level (SIL) result is calculated by averaging the octave-band sound pressure levels at 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The XL2 Analyzer includes any tangency method adaptation as applicable.

Order Information

Exel Set for Noise Curves Testing

Exel Kit for Noise Curves Measurements

  • XL2 Audio and Acoustic Analyzer
  • Spectral Limits Option for XL2
  • M2211 Measurement Microphone
  • ASD Cable for XL2
  • Mains Power Adapter
  • 2x Calibration Certificates
  • Exel System Case

Get more Information

Buy at the web shop or contact NTI Americas .

Contact NTi Audio

NTI Americas Inc.
Tigard, Oregon
Thomas E. Mintner
+1 503 684 7050

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