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COMMUNITY NOISE EQUIVALENT LEVEL (CNEL)

A noise measurement system introduced in the early 1970's by the State ofCalifornia as a simplified alternative to the NEF system (see NOISE EXPOSURE

FORECAST) for community noise exposure, with particular emphasis on airportnoise. The major difference is that CNEL can be measured using ordinary dBAreadings (see SOUND LEVEL METER), as opposed to the computer calculation ofEFFECTIVE PERCEIVED NOISE LEVEL used in the NEF.

As well, the CNEL system gives a higher weighting to evening flights (1900 to2200) and includes a table of corrections (see below) based on seasonal, residentialtype, previous community noise experience and pure tone/impulse differences. Inpractice, CNEL values are comparable to NEF and exceed them by 35 ± 2 dB.The total noise exposure per day (CNEL) is calculated from the equation:

CNEL = SENEL + 10 log10 (ND + 3NE + 10NN) - 49.4 (dB)

where ND, NE and NN are the number of flights during the day (0700 to 1900),

evening (1900 to 2200) and night (2200 to 0700) respectively, and SENEL is theenergy mean value of the single event noise exposure level which may becalculated from the equation:

SENEL = NLmax+10 log10tea (dB)

where NLmax is the maximum noise level in dBA and tea, is the effective time

duration (in seconds) of the noise level (on the A scale) and is approximately equalto one-half of the duration during which the noise level is within 10 dB of themaximum.

The above expressions are simplifications of the actual procedure and only applyto a single type of aircraft and single flight path. An hourly average of the noiselevel (HNL) including number of flights per hour N is also used and defined as:

HNL = SENEL + 10 log10N - 35.6 (dB)

A variation in the calculation uses the first equation above but counts the flights asin the NEF system (without evening specification). The normalized CNEL uses thecorrections listed in the table below (see also SOUND INTRUSION). Correlation tocommunity reaction is shown in the following chart for the CNEL scale (withapproximate CNR and NEF values).

Compare: NOISE AND NUMBER INDEX, NOISE CRITERION, NOISE EXPOSURE

FORECAST, NOISE LEVEL, NOISE POLLUTION LEVEL, NOISE RATING, PERCEIVED

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NOISE LEVEL, SPEECH INTERFERENCE LEVEL.

Type of

CorrectionDescription

Amount of

Correctionto be Added

to

Measured

CNEL in

dB

Seasonal

Correction

Summer (or year-round operation)

Winter only (or windows always closed)

0

- 5

Correction for

Outdoor Residual

Noise Level

Quiet suburban or rural community (remote from large cities

and from industrial activity and trucking)+10

Normal suburban community (not located near industrial

activity)+5

Urban residential community (not immediately adjacent toheavily traveled roads and industrial areas)

0

Noisy urban residential community (near relatively busy roads

or industrial areas)-5

Very noisy urban residential community -10

Correction forPreviousExposure &

CommunityAttitudes

No prior experience with the intruding noise

+5

Community has had some previous exposure to intruding noisebut little effort is being made to control the noise. This

correction may also be applied in a situation where thecommunity has not been exposed to the noise previously, butthe people are aware that bona fide efforts are being made to

control the noise.

0

Community has had considerable previous exposure to the

intruding noise and the noisemaker's relations with thecommunity are good.

-5

Community aware that the operation causing noise is very

necessary and it will not continue indefinitely. This correctionmay be applied for an operation of limited duration and under

emergency circumstances.

-10

Pure Tone orImpulse

No pure tone or impulsive character 0

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Pure tone or impulsive character present +5

Table of correction factors used to normalize CNEL values (after U.S.Environmental Protection Agency document NTID300.3, CommunityNoise, 1971).

Community reaction to noise as a function of normalized CNEL valuesas calculated from case histories (after U.S. Environmental ProtectionAgency document NTID 300.3, Community Noise, 1971).

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