APPLICATION NOTE

Noise Curves
CRITERIA FOR EVALUATING ROOM NOISE

Noise curves specify the acoustic ambient noise in an indoor
environment. Typical disturbing noises are the backgroundnoise caused by an air conditioning system as well as noises
from heating or ventilation systems (HVAC).

This application note describes how to interpret noise curves,and how to measure noise curves with the XL2 Acoustic Analyzer. We also detail why it is necessary to measure, givea history of the development and finally describe the major
types of noise curves.

XL 2IndexRequirements

3— Room Criteria Curves (RC)9

How to Measure the Noise Curves

3— Noise Rating Curves (NR)10

How to interpret Noise Curves

4— Preferred Noise Criterion Curves (PNC)

11History

5Tolerances

12

Measurement Positions

6

Recommendations

13

Supported Noise Curves Types

6

Privacy Masking

15— Noise Criterion Curves (NC)

7References

15— Room Noise Criteria Curves (RNC)

8

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NOISE CURVESAPPLICATION NOTE

Background noise that is annoying creates fatigue and can negatively affectproductivity and safety. Too much noise also affects the ability to communicate.
Therefore, standard methodologies for quantifying such noise have been developed. Different rooms, locations, regulations and applications may allow individual 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 airconditioning system should not interfere with telephone calls or conversations.
Such residual noise may be caused from outside road traffic noise, HVAC systems (heating, ventilating, air-conditioning) or other sources. Background noise
may also be deliberately introduced to mask private conversations.

NC Noise Curves - Screenshot Room Acoustics Reporter

Noise curves provide a uniform measuring standard and are referred to by several noise regulations covering a variety of common locations including manufacturing environments, concert halls, schools and lecture theatres, hospitals
and offices.

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NOISE CURVESAPPLICATION NOTEREQUIREMENTS• XL2 Acoustic Analyzer• M4261 Measurement Microphone (Class 2) or better
Typical measurement range of measurement microphones— M4261: NC27
— M2211: NC20
— M2230: NC15
— M2340: NC15
• Spectral Limits Option installed on XL2 for onboard noise curve reading• Room Acoustics Reporter PC Software available as— Room Acoustics Reporter 365 (annual subscription service)— XL2 Room Acoustics Option(permanently installed option in XL2 Sound Level Meter)

HOW TO MEASURE THE NOISE CURVESThe XL2 Acoustic Analyzer measures the background noise spectrum of the unoccupied room in accordance with the standard. The resulting frequency spectrum gets classified by the Noise Curves.
• Power up the XL2 Acoustic Analyzer.
• Choose the measurement function Noise Curves. This requires the Spectral
Limits Option be installed on the XL2. Alternatively, select the LZeq spectrum
within the function SLMeter.
• Ensure that the level of background noise in the room is typical for the room
when it is unoccupied i.e., nobody should be talking and there should be no
extraordinary noise during the measurements.
• Start the measurement.
• Move the microphone slowly (not more than 0.5 m/s) around the entire room
for at least 20 seconds. Alternatively, use multiple fixed microphone positions
randomly distributed in the room.
• Save the reading(s).
• Load the data into the software Room Acoustics Reporter and generate themeasurement report.

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Page 315NOISE CURVESAPPLICATION NOTEHOW TO INTERPRET NOISE CURVES
A noise curve may be used to characterize room noise or other environments.
In the design documentation of a building the noise curve may be specified foreach room. The noise rating is designated as the value of the highest noise
curve “touched” by the measured noise spectrum. When all the noise levels fallon or below a noise rating curve, that noise rating is attributed to the room. Forexample, the Noise Criterion (NC) of the sample noise spectrum below is NC40.
This means, that the ambient noise in the room must be on or lower than theNoise Criterion curve NC40 at any position in the frequency spectrum.

NC Noise Curves - Screenshot Room Acoustics Reporter

The standards list the noise criteria in 5 dB steps. The XL2 measures the noise
criteria’s more precisely in 1 dB steps, which are calculated by linear interpolation between the standardized 5 dB levels.

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Page 415NOISE CURVESAPPLICATION NOTEHISTORY
Fully describing the tonal and temporal characteristics of acoustic sound inbuildings is a complex undertaking. Throughout the history of acoustic measurement, therefore, there have been many attempts to simplify by create singlenumber rating methods.
SIL: To evaluate the interference of noise upon speech communication in passenger aircraft, Leo Beranek (1947) introduced the Speech Interference Level(SIL). The SIL was defined as the arithmetic average of the sound pressure levels
measured in the octave bands 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. It also
served as a convenient single-number rating for evaluating the interference ofnoise on speech communication in enclosed spaces and outdoors.
SC: The Sound Communication (SC) curves were first introduced in 1953 (Beranek et al., 1953 and 1954). The SC curves were defined in 10 dB increments,
but later interpolated to 5 dB and 1 dB increments.
NC: The Noise Criteria curves were first published in 1957 by Beranek, and, likethe SC curves that preceded them, are curves of approximate equal loudness.
They were developed from a table of Speech Interference Level (SIL) values
found to be acceptable in a survey of a person’s working in a wide variety ofoffice environments. The curve shapes were set to be monotonic in shape andto have loudness levels in phons that are 23 units above the corresponding SIL
values. It is to be noted that the NC curves are not intended to be the mostdesirable noise spectrum shapes, but rather they are intended to be octave
band noise levels that just permit satisfactory speech communication without
being annoying (Beranek, 2000). The NC curves have been revised in 1960 andextended in 2004.
It was originally presumed that an octave band spectrum that generally follows
an NC curve shape would be perceived as equally balanced in low, mid, andhigh frequency energy. Although this was shown not quite to be the case, thisled to the development of other curve sets.
SI: Leo Beranek developed the originals for Speech Interference (SI) rating inaircraft for face-to-face communication. These curves were found reasonable
for noise annoyance as well.
RC: Warren Blazier sought “a simpler and more apt” straight-line version for heating, ventilating and air-conditioning (HVAC) mechanical noise equipment design
purposes, sloping at the rate of 5 dB/octave and down to 31 Hz.
NR: The common understanding stemming from the 1970s was that NR curves
were intended for external environmental application, as distinct to NC curves
etc. that were derived for assessing/rating/design in the context of internal
spaces.

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NOISE CURVESAPPLICATION NOTEMEASUREMENT POSITIONS
The microphone locations shall be near the average normal standing or seatedheight of human ears in the space in accordance with ANSI/ASA S12.2-2019• Height of 1.6 m for standing and 1.2 m for seated adults• Height of 1.1 m standing and 0.75 m for seated children• Minimum microphone distances— to any sound reflecting surface is 0.6 m— to any room edge is 1.2 m— to any room corner is 2.4 m

SUPPORTED NOISE CURVES TYPESThe XL2 Acoustic Analyzer support the five the most widely used standardized
noise curve types for evaluating background noise in rooms.
• ANSI/ASA S12.2-2019 - Noise criteria curves NC• ANSI/ASA S12.2-2019 - Room noise criterion RNC• ISO R 1996-1971 - Noise rating curves NR• Preferred Noise Criterion curves (PNC)• Room Criteria curves (RC)

Room Acoustics Reporter offers a dedicated solution for generating standardized noise curve reports. The software calculates the average background noisespectrum in the room. This may be compared with the target range - based on
the usage of the room. The measurement data of the following XL2 functions
can be imported: SLMeter, Noise Curves, 1/12 Octave and Cinema Meter. Thesoftware supports the following Noise Curve standards.
• ANSI/ASA S12.2-2019 - Noise criteria curves NC• ANSI/ASA S12.2-2019 - Room noise criterion RNC• DIN 15996:2020 - Grenzkurven GK• ISO/R 1996-1971 - Noise rating curves NR

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NOISE CURVESAPPLICATION NOTE

Noise Criterion Curves (NC)The American National Standards Institute (ANSI) specifies the NC rating inANSI/ASA S12.2-2019. The XL2 Analyzer reports the NC curves using thespecified tangency method adaptation in accordance with the standard. Additionally, the Speech Interference Level (SIL) is presented on the screen.
This is calculated by the linear average of the measured sound pressure level in the octave bands 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The designating number for any NC curve is, approximately, its speech interference level, which measures the effects of noise on speech intelligibility.

NC Noise Curves - Screenshot Room Acoustics Reporter

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NOISE CURVESAPPLICATION NOTE

Room Noise Criteria Curves (RNC)
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. Itessentially 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. The Room Noise Criteria RNC
are defined in ANSI/ASA S12.2-2019. The XL2 Analyzer measures the RNC andreports any large fluctuations or surging at low frequencies, e.g. caused by fans.

RNC Rating Curves - Screenshot Room Acoustics Reporter

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NOISE CURVESAPPLICATION NOTERoom Criteria Curves (RC)The Room Criteria RC Mark II are defined in the informative Annex of the standard
ANSI/ASA S12.2-2019. The RC Mark II criterion curves are used for the designof 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 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 thespectrum 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 moreof 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.
• Rattle and Vibration (RV): The level in one or more of the octave bands from
16 Hz through 63 Hz exceeds the criterion for moderately noticeable rattle.

RC Noise Curves

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NOISE CURVESAPPLICATION NOTE

Noise Rating Curves (NR)The Noise Rating Curves (NR) are defined in the old standard ISO/R 1996-1971.
This standard has been revised; today’s edition does no longer contain the NR
curves - still some local standards refer to the NR curves. The NR curves offer
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 ofa frequency spectrum, or to assess the acceptability of a noise spectrum for aparticular 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.

NR Noise Rating Curves - Screenshot Room Acoustics Reporter

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NOISE CURVESAPPLICATION NOTE

Preferred Noise Criterion Curves (PNC)
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 considered superior to NC curves for critical uses such as instudios, concert & lecture theatres.
These curves are less steep in the low frequencies and steeper in the high frequencies than the NC curves. 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 (2019) version of Noise Criteria
curves includes an extended frequency range somewhat mitigating the original
motivation for PNC.

PNC Noise Rating Curves

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NOISE CURVESAPPLICATION NOTETOLERANCES
A tolerance may be added in the report visualizing the specified maximum noisecurve. This allows a quick verification if the background noise in the room under
test meets the requirements and shall not annoy people occupying the room.

NC Noise Curves with NC40 Tolerance - Screenshot Room Acoustics Reporter

The set of noise curve tolerances is available for download at theXL2 Support Page upon registration of the XL2 Analyzer.

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NOISE CURVESAPPLICATION NOTERECOMMENDATIONSThe standard ANSI/ASA S12.2-2019 provides three primary methods for evaluating room noise:• a survey method that employs the A-weighted sound level with Slow timeweighting measured at a point (or average of points) in the space under test
• an engineering method that employs expanded noise criteria (NC) curves• a method for evaluating low-frequency fluctuating noise using room noise
criterion (RNC) curves.

These are the recommended criteria for various unoccupied rooms:
Type of RoomSound Level
LAeq

NC and RNC
Curve

RC Mark Criteria

-

15-18

-

Small auditoriums (≤500 seats)

35-39

25-30

-

Large auditoriums (>500 seats)

30-35

20-25

-

TV and broadcast studios

16-35

15-25

-

Live performance theaters

25-30

20-25

-

Premier movie theaters

25-30

20-25

-30

25

-

• Bedrooms

35-39

25-30

25-30(N)• Apartments

39-48

30-40

30-35 (N)• Family rooms and living rooms

39-48

30-40

30-35 (N)• Individual rooms or suites

39-44

30-35

25-35 (N)• Meeting/banquet rooms

35-4425-35

25-35 (N)• Service support areas

48-57

40-50

35-45 (N)

35-44

25-35

25-35 (N)Concert and recital halls

Normal theater

Private residences

Hotels/Motels

Office buildings• Offices executive

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NOISE CURVESAPPLICATION NOTESound Level
LAeq

NC and RNC
Curve

RC Mark Criteria• Offices small

44-48

35-40

25-35 (N)• Offices large

39-44

30-35-• Conference rooms large

35-39

25-30

25-35 (N)• Conference rooms small

39-44

30-35

25-35 (N)• Open-plan areas

44-48

35-40

30-40 (N)• Business machines/computers

48-5340-45-• Public circulation

48-57

40-50

40-45 (N)• Private rooms

35-39

25-30

25-35 (N)• Wards

39-44

30-35

30-40 (N)• Operating rooms

40-50

40-50

40-45 (N)• Laboratories

44-53

35-45

35-45 (N)• Corridors

44-53

35-45

35-45 (N)• Public areas

48-52

40-45

40-50 (N)• Lecture and classrooms < 566 m3(20,000 ft3)

35

25-30

25-30 (N)• Lecture and classrooms > 566 m3(20,000 ft3)

4030-35

35-40 (N)• Open-plan classrooms

35

25-30

-Churches small

39-44

30-35

25-35 (N)

Courtrooms

39-4430-35

25-35 (N)Libraries

44-48

35-40

30-40 (N)Restaurants

48-52

40-45

-

Control rooms, kitchens, and laundries

52-62

45-55

-Shops and garages

57-67

50-60

-

Type of Room

Hospitals and clinics

Schools

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NOISE CURVESAPPLICATION NOTE

The Noise Rating level should not exceed the levels indicated in the table below:
NoiseRating

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, hospitals operating
theaters and wards, apartments, hotels, offices

NR 40Halls, corridors, cloakrooms, restaurants, night clubs, offices,shops

NR 45

Department stores, supermarkets, canteens, general offices

NR 50

Typing pools, offices with business machines

NR 60

Light engineering works

NR 70Foundries, heavy engineering works

PRIVACY MASKING
Sound masking for privacy is the addition of sound, created by a sound generator, to an indoor environment in order to mask conversations. In a simple form,a broadband spectrum sound similar to a pink noise may be introduced to acorridor outside a meeting room to mask the conversations within the room fromanybody standing in the corridor. Sound masking is used in homes, commercialoffices, medical facilities, court rooms, and in secure facilities to provide secrecy.

REFERENCES• ANSI/ASA S12.2-2019, Criteria for Evaluating Room Noise• ISO-R-1996-1971, Assessment of Noise with respect to community response• Beranek, L.L., “Revised criteria for noise in buildings.” Noise Control 3, 19-27(1957)• Beranek, L.L., Noise Reduction, McGraw-Hill Book Co., p. 519 (1960)

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