COMPARISON OF DAY-NIGHT SOUND LEVEL WITH OTHER MEASURES OF NOISE USED BY FEDERAL AGENCIES

The following subsections compare the day-night sound level with three measures utilized for airport noise, CNR, NEF, and CNEL, the HUD Guideline Interim Standards and the Federal Highway Administration standards.

Comparison of L_dn with Composite Noise Rating (CNR), Noise Exposure Forecast (NEF), and Community Noise Equivalent Level (CNEL)

CNR, NEF, and CNEL are all currently used expressions for weighted, accumulated noise exposure. Each is intended to sum a series of noise while weighting the sound pressure level for frequency and then adding appropriate nighttime weightings. The older ratings, CNR and NEF, are expressed in terms of maximum Perceived Noise Level and Effective Perceived Noise Level, respectively; each considers a day-night period identical to L_dn.

The measure CNEL itself is essentially the same as L_dn except for the method of treating nighttime noises. In CNEL, the 24-hour period is broken into three periods: day (0700-1900), evening (1900-2200), and night (2200-0700). Weightings of 5 dB are applied to the evening period and 10 dB to the night period. For most time distributions of aircraft noise around airports, the numerical difference between a two-period and three-period day are not significant, being of the order of several tenths of a decibel at most.

One additional difference between these four similar measures is the method of applying the nighttime weighting and the magnitude of the weighting. The original CNR concept, carried forward in the NEF, weighted the nighttime exposure by 10 dB. Because of the difference in total duration of the day and night periods, 15 and 9 hours respectively, a specific noise level at night receives a weighting of 10 + 10 log (15/9), or approximately 12 dB in a reckoning of total exposure. Given the choice of weighting either exposure or level, it is simpler to weight level directly, particularly when actual noise monitoring is eventually considered.

The following paragraphs describe the method utilized to calculate CNR, NEF, and CNEL, as applied principally to aircraft sounds, together with the analogous method for calculating L_dn.

Composite Noise Rating Method (CNR)

The original method for evaluating land use around civil airports is the composite noise rating (CNR). It is still in wide use by the Federal Aviation Administration and the Department of Defense for evaluating land use around airfields (Civil Engineering Planning and Programming, "Land Use Planning with Respect to Aircraft Noise," AFM 86-5, TM 5-365, NAVDOCKS P-98, October 1, 1964). This noise exposure scale may be expressed as follows:

(Eq. A-16)
The single event noise level is expressed (without a duration or tone correction) as simply the maximum perceived noise level (PNL_max) in PNdB.

The noise exposure in a community is specified in terms of the composite noise rating (CNR), which can be expressed approximately as follows:

= approximate energy mean maximum perceived noise level (PNL) at a given point N_f = (N_d + 16.7 N_n),
where N_d and N_n the numbers of daytime and nighttime events, respectively.

The constant (-12) is an arbitrary constant, and the factor 16.7 is used to weight the nighttime exposure in the 9-hour night period on a 10 to 1 basis with the daytime exposure in the 15-hour daytime period.

Noise Exposure Forecast (NEF)

This method, currently in wide use, for making noise exposure forecasts utilizes a perceived noise level scale with additional corrections for the presence of pure tones. Two time periods are used to weight the number of flights (Galloway, W.J. and Bishop, D.E., "Noise Exposure Forecasts: Evolution Evaluation, Extensions and Land Use Interpretations," FAA-NO-70-9, August 1970).

The single event noise level is defined in terms of effective perceived noise level (EPNL) which can be specified approximately by:

(Eq. A-17)
where

PNL_max = maximum perceived noise level during flyover, in PNdB,

= "10 dB down" duration of the perceived noise level time history, in seconds,
and

F = pure tone correction. Typically, F = 0 to + 3 dB.

Community noise exposure is then specified by the Noise Exposure Forecast (NEF). For a given runway and one or two dominant aircraft types, the total NEF for both daytime and nighttime operations can be expressed approximately as:

(Eq. A-18)

where

= energy mean value of EPNL for each single event at the point in question

N_f = same as defined for CNR.

Community Noise Equivalent Level (CNEL)

The following simplified expressions are derived from the exact definitions in the report, "Supporting Information for the Adopted Noise Regulations for California Airports." They can be used to estimate values of CNEL where one type of aircraft and one flight path dominate the noise exposure level.

Single event noise is specified by the single event noise exposure level (SENEL) in dB and can be closely approximated by:

SENEL = NL_max + 10 log₁₀ T/2 (dB) (Eq. A-19)

where

NL_max = maximum noise level as observed on the A scale of a standard sound level meter

and

T = duration measured between the points of (L_max - 10) in seconds. The effective duration is equal to the "energy" of the integrated noise level (NL), divided by the maximum noise level, NL_max, when both are expressed in terms of antilogs. It is approximately 1/2 of the 10 dB down duration.

A measure of the average integrated noise level over one hour is also utilized in the proposed standard. This is the hourly noise level (in dB), defined as:

= (dB) (Eq. A-20)
where

SENEL = energy mean value of SENEL for each single event,

and

n = number of flights per hour.

The total noise exposure for a day is specified by the community noise equivalent level (CNEL) in dB, and may be expressed as:

= (dB) (Eq. A-21)
where

N_c = (N_d + 3N_e + 10N_n)

= total number and average number per hour, respectively, of flights during the period 0700 to 1900
= total number and average number per hour, respectively, of flights during the period 1900 to 2200
and
= total number and average number per hour, respectively, of flights during the period 2200 to 0700

Day-Night Sound Level (L_dn)

The following simplified expressions are useful for estimating the value of L_dn for a series of single event noises which are of sufficient magnitude relative to the background noise that they control L_dn:

Single event noise is specified by the sound exposure level (L_ex) measured during a single event. It can be closely approximated by:

(dB) (Eq. A-22)

Where

L_max = maximum sound level as observed on the A scale of a standard sound level meter on the slow time characteristic

and

_T = duration measured between the points of (L_max - 10) in seconds

The day-night sound level may be estimated by:

(dB) (Eq. A-23)

where

= the energy mean value of the single event L_ex values

N = (N_d + 10N_n)

or

N_d = total number of events during the period 0700 to 2200

and

N_n = total number of events during the period 2200 to 0700

There is no fixed relationship between L_dn or CNEL and CNR or NEF because of the differences between the A-level and PNL frequency weightings and the allowance for duration, as well as the minor differences in approach to day-night considerations. Nevertheless, one may translate from one measure to another by the following approximate relationship:

(Eq. A-24)
For most circumstances involving aircraft flyover noise, these relationships are valid within about a +/-3 dB tolerance.

Comparison of Leq with HUD Guideline Interim Standards (1390.2 Chg. 1)

The interim HUD standards for outdoor noise are specified for all noise sources, other than aircraft, in terms of A-weighted sound level not to be exceeded more than a certain fraction of the day. Aircraft noise criteria are stated in terms of NEF or CNR.

The HUD exposure criteria for residences near airports are "normally acceptable" if NEF 30 or CNR 100 is not exceeded. A "discretionary acceptable" category permits exposures up to NEF 40 or CNR 115.

For all other noise sources, the HUD criteria specify a series of acceptable, discretionary, and unacceptable exposures. Since these specifications are similar to points on a cumulative statistical description of noise levels, it is of interest to compare the HUD criteria with L_eq for different situations. For discussion purposes, consider the boundary between the categories "discretionary-normally acceptable" and "unacceptable."

The first criterion defining this boundary allows A-weighted noise levels to exceed 65 dB up to 8 hours per 24 hours, while the second criterion states that noise levels exceeding 80 dB should not exceed 60 minutes per 24 hours. These two values may be used to specify two limit points on a cumulative distribution function, L33.3 = 65 dB and L4.2 = 80 dB. The relationship between L_eq and the HUD criteria may then be examined for different types of distribution functions, restricting the shape of the distribution only so that it does not exceed these two limit points.

First consider two cases of a normal distribution of noise levels, comparable to vehicle traffic noise. For the first case, assume a distribution with quite narrow variance so placed on the graph that the 65 dB point is not exceeded (see Figure A-8). For this curve, to the nearest decibel, L₅₀ = 64 dB, and the corresponding standard deviation (arbitrarily chosen small) is 2.3 dB. The resulting L_eq is equal to 64.6 dB.

Now consider a normal distribution with the widest permissible variance (the curve marked Maximum Variance in Figure A-8); if the variance were any greater, the distribution would violate HUD's requirement that the level not exceed 80 dB for more than 60 minutes per 24 hours. This distribution, to the nearest decibel, has L₅₀ = 60 dB, L₁₀ = 74 dB and a standard deviation of approximately 11 dB. The resultant L_eq = 74 dB, is almost 10 dB higher than for the previous case. Both curves meet HUD's interim standards.

Next, consider a series of intermittent high level noises, superposed on a typical urban/ suburban background noise level, such that 80 dB is not exceeded more than 60 minutes per 24 hours, say 4%. Choosing a series of repeated triangular-shaped time signals of 90 dB maximum sound level will produce an L_eq value of 72.4 dB without exceeding an L₄ value of 80 dB.

However, one can allow the maximum level to increase indefinitely provided L₄ remains at 80 dB or less. The limiting case is that of a square-shaped time pattern, switched on and off. In this instance, if the total "on-time" is 4% or less, the value of L_eq is equal to L_max - 14 dB, and both L_max and L_eq can increase without limit and still remain acceptable within the HUD interim standards. Maximum A-levels for an aircraft can be as high as 110 dB, which would permit L_eq values of 96 to be obtained without exceeding the L₄ limit of 80 dB.

It is clear that no unique relationship can be specified between the HUD non-airport standards and L_eq. Values of L_eq ranging up to 95 dB can be found in compliance with the HUD outdoor noise standard depending on the time distribution of noise levels considered. Even if the nighttime penalty were applied to L_eq to yield L_dn there would still be no unique relation with the HUD standards.

Comparison of L_eq with Federal Highway Administration Noise Standards, PPM 90-2, February 8, 1973

The primary criteria of PPM 90-2 are that L₁₀ for noise levels inside people-occupied spaces shall not exceed 55 dB, or for sensitive outdoor spaces " -- in which serenity and quiet are of extraordinary significance --," 60 dB.

Figure A-8. Permissible normal distribution of L_eq under HUD standards. [Ref. 25]

Highway noise often has a random distribution of noise level, the distribution function being approximately normal in many instances. In this case, the relationship between L_eq and L₁₀ is given by the expression:

Where s is the standard deviation of the noise level distribution. The difference between L₁₀ and L_eq for normal distribution of sound level is plotted in Figure A-6. It can be noted that Leq = L₁₀ -2 dB within +2 dB, for s ranging from 0 to 11 dB. Highway noise rarely has a standard deviation of 11 dB; 2 to 5 dB is more typical.

Thus, setting L₁₀ at 60 dB for highway noise impacting a sensitive outdoor space, we find that a L_eq value of 60 -2 = 58 +2 dB would meet the most sensitive FHWA criterion.

1	"Report to the President and Congress on Noise," Environmental Protection Agency, NRC 5 00. I , December 31 , 1971 .
2	Bishop, D.E., "Judgements of the Relative and Absolute Acceptability of Aircraft Noise," J. Acoust. Soc. Am. 40:103, December 1966.
3	Kryter, K.D., "The Effects of Noise on Man," Academic Press, New York, 1970.
4	"House Noise - Reduction Measurements for Use in Studies of Aircraft Flyover Noise," Society of Automotive Engineers, Inc., AIR 1081, October 1971.
5	Bishop, D.E., and Horonjeff, R.O., "Procedures for Developing Noise Exposure Forecast Areas for Aircraft Flight Operations," FAA Report DS-67-10, August 1967.
6	Stevens, K.N., and Pietrasanta, A.C., and the Staff of Bolt Beranek and Newman, Inc., "Procedures for Estimating Noise Exposure and Resulting Community Reactions from Air Base Operations," WADC Technical Note 57-10, Wright-Patterson Air Force Base, Ohio, Wright Air Development Center, 1957.
7	Eldred, K.M., Gannon, W.J., and von Gierke, H.E., "Criteria for Short Time Exposure of Personnel to High Intensity Job Aircraft Noise," WADC Technical Note 55-355, Wright-Patterson Air Force Base, Ohio, 1955.
8	Air Force Regulation 160-3, "Hazardous Noise Exposure," USAF, October 29, 1956.
9	Burck, W., Grutzmacher, M., Meister, F.J., Muller, E.A., and Matschat, K., "Fluglarm, Gutachten erstattet im Auftrag des Bundesministers fur Gesundheitswesen," (Aircraft Noise: Expert Recommendations Submitted under Commission from the German Federal Ministry for Public Health), Gottingen, 1965.
10	Bruckmayer, F. and Lang, J., "Storung der Bevolkerung durch Verkehrslarm" (Disturbance of the Population by Traffic Noise), Oesterreiche Ingenieur-Zeitschrift, Jg. 1967, H.8, 302-306; H.9, 338-344; and H.10, 376-385.
11	Bruckmayer, F., and Lang, J., "Storung durch Verkehrslarm in Unterrichtstraume" (Disturbance Due to Traffic Noise in Schoolrooms), Oesterreichische IngenieurZeitschrift, 11 (3): 73-77, 1968.
12	"Schallschutz: Begriffe" (Noise Control: Definitions), TGL 10 687, Blatt I (Draft), Deutsche Bauinformation, East Berlin, November 1970.
13	"Mittelung zeitlich schwankender Schallpegel (Aquivalenter Dauerschallpegal)" (Evaluation of Fluctuating Sound Levels (The Equivalent Continuous Sound Level)), DIN 54 641, (Draft), Deutsche Normen, Beuth-Vertrieb GmbH, Berlin 30, April 1971
14	"Schallschutz: Territoriale und Stadtebauliche Planung" (Noise Control: Land Use and City Planning), TGL 10 687, Blatt 6, (Draft), Deutsche Bauinformation, East Berlin, November 1970.
15	"Schallschutz in Stadtebau" (Noise Control in City Planning), DIN 18 005, (Draft), Deutsche Normen, Beuth-Vertrieb GmbH, Berlin 30, August 1968.
16	Benjegard, Sven-0laf, "Bullerdosimetem" (The Noise Dose Meter), Report 51/69, Statens institut fur byggnadsforskning, Stockholm, 1969.
17	Robinson, D.W., and Cook, J.P., NPL Aero Report No. Ac 31, National Physical Laboratory, England, June 1968.
18	Meister, F.J., "Den Einfluss den Einwirkdauer bei den Beschallung des Ohres" (The Influence of the Effective Duration in Acoustic Excitation of the Ear), Larmbekampfung 10 (3/4), June/August 1966.
19	Pearsons, K.S., "The Effects of Duration and Background Noise Level on Perceived Noisiness," FAA ADS-78, April 1966.
20	Galloway, W.J., and Bishop, D.E., "Noise Exposure Forecasts: Evolution, Evaluation, Extensions and Land Use Interpretations," Bolt Beranek and Newman, Inc., Report No, 1862, August 1970; also FAA-No-70-9.
21	"Procedure for Describing Noise Around an Airport," R-507, International Standards Organization, Geneva, 1970.
22	"Noise Assessment with Respect to Community Noise," R-1996, International Standards Organization, Geneva, 1970.
23	"Assessment of Noise-Exposure During Work for Hearing Conservation," R-1999, International Standards Organization, Geneva, 1970.
24	Galloway, W.J., "Review of Land Use Planning Procedures," Interim Technical Report, Aerospace Medical Research Laboratory, WPAFB, Ohio, March 1972.
25	"Impact Characterization of Noise Including Implications of Identifying and Achieving Levels of Cumulative Noise Exposure," Environmental Protection Agency, NTID 73.4, 1973.