This is an excerpt from a proposed FAA Aviation Noise Abatement Policy 2000
printed in the Federal Register Vol 65, No 136, July 14, 2000.
"4.2 Assessment Methodologies
Yearly Day-Night Average Sound Levels (DNL)
The FAA and other Federal agencies use DNL as the primary measure
of noise impacts on people and land uses. This cumulative metric is the
Federal standard because it:
Correlates well with the results of attitudinal surveys of
residential noise impact;
Increases with the duration of noise events, which is
important to people's reaction;
Takes into account the number of noise events of the full
24 hours in a day, which also is important to people's reaction;
Takes into account the increased sensitivity to noise at
night by including a 10-dB nighttime penalty between 10:00 p.m. and
7:00 a.m. to compensate for sleep disturbance and other effects;
Allows composite measurements of all sources of community
noise; and
Allows quantitative comparison of noise from various
sources with a community.
DNL is the only metric backed with a substantial body of scientific
survey data on the reactions of people to noise. It provides a simple
method to compare the effectiveness of alternative airport scenarios.
Land use planners have acquired over 20 years of working experience
applying this metric to make zoning and planning decisions. DNL is a
sound and workable tool for land use planning and in relating aircraft
noise to community reaction. Experience indicates that DNL provides a
very good measure of impacts on the quality of the human environment,
forming an adequate basis for decisions that influence major
transportation infrastructure projects. In an August 1992 report, the
FICON reaffirmed both DNL as the appropriate metric for measuring
aviation noise exposure and DNL 65 dB as the Federal Government's level
of significance for assessing noise impacts.
Some people challenge the use of DNL to assess aviation noise
because it is a measure of exposure from cumulative events over time
rather than a measure of exposure from a single noise event. Commonly
cited as potential alternative metrics are the Sound Exposure Level
(SEL), which describes cumulative noise exposure from a single event,
and Maximum Level (Lmax ), the highest level during a single
event. Although sometimes useful as supplemental measures of noise
exposure, single event metrics pose problems. They present neither an
accurate picture of noise exposure nor the overall impact of noise on a
community. Because single event metrics by definition are not
composites of cumulative events, 100 aircraft operations a day would be
no worse than one operation. Similarly, one event at 90 dB would be
assessed as worse than 100 events at 89 dB. These effects clearly do
not reflect noise impacts or annoyance reactions accurately.
Alternatively, DNL increases with the number of operations, while
single event measures do not. DNL combines the number of operations
with the loudness of each operation into a cumulative noise dose. The
resulting values correlate well with independent tests of annoyance
from all sources of noise.
Human response to noise involves both the maximum level and its
duration, so the maximum sound level alone is not sufficient to
evaluate the effect of noise on people. Clearly, people are bothered by
individual noise events, but their sense of annoyance increases with
the number of those noise events, and with those that occur late at
night. The DNL metric provides a combined measure of these factors that
can be used to evaluate existing and predicted future conditions on an
unambiguous, single-number basis. Although DNL is an average of
cumulative noise levels, sound levels of the loudest events control the
DNL calculation. Both Lmax and SEL measure individual sound
events that may occur only once, or may occur several times during the
day. The number of times these events occur and when they occur are
important in measuring the noise environment. DNL is a time-average of
the total sound energy over a 24-hour period, adjusted by providing a
10 dB penalty to sound levels occurring between 10PM and 7AM. This 10
dB penalty means that one nighttime sound event is equivalent to 10
daytime events of the same level. Accordingly, DNL combines both the
intensity and number of single noise events with a nighttime weighting
factor in a manner that is strongly influenced by maximum sound levels.
Recognizing that DNL often is criticized based on perceptions of
community annoyance, the FICON reaffirmed that complaints are an
inadequate indicator of the full extent of noise effects on a
population. The DNL 65 dB level of significance does not mean that no
one is annoyed below that level. Extensive research has been conducted
to evaluate annoyance. In an attempt to meet demand for a usable and
uniform relationship between noise and annoyance, T.J. Schultz reviewed
the results of 161 social surveys where data were available to make a
consistent judgment concerning what percent of the population was
``highly annoyed'' (%HA). The surveys were of community reactions to
several types of transportation noises such as road traffic, railroad,
and aircraft noises. The results agreed fairly well with one another,
and Schultz developed an equation for describing the relationship
between the level of exposure (in DNL) and percent of population highly
annoyed. Schultz published the results of the surveys in 1978 in
``Synthesis of Social Surveys on Noise Annoyance.'' In 1992, the US Air
Force updated Schultz's research with a total of 400 surveys.
Comparison of the original and updated results indicate that they
differ by less than two percent in the DNL range from 45 to 75 dB. The
following chart presents the relationship between %HA and DNL:
The Schultz curve indicates that about 12 percent of people living
at DNL 65 dB report themselves to be ``highly annoyed'' by
transportation noise. About 3 percent are highly annoyed at a DNL of 55
dB.
Noise Analysis Criteria for Changes in DNL
The DNL 65 dB contour remains the FAA's lower limit for defining
significant noise impact on people. For a variety of reasons, noise
predictions and interpretations are frequently less reliable below DNL
65 dB. DNL prediction models tend to degrade in accuracy at large
distances from the airport. Smaller proportions of the population are
highly annoyed with successive decreases in noise levels below DNL 65
dB. The FICON studied criteria for predicting changes in community
annoyance below DNL 65dB. It found that a DNL 3 dB increase at the DNL 60 dB level is
generally consistent with the existing DNL 1.5 dB screening criterion
at the DNL 65 dB level. This finding was based on using the Schultz
curve to relate changes in impact level with changes in DNL. Increases
of 5 dB at DNL 55 dB, 3 dB at DNL 60 dB, and 1.5 dB at DNL 65 dB all
resulted in a three percent increase in %HA.
For airport development and other actions in the vicinity of an
airport, the FAA guidelines for screening based on changes in aviation
noise impacts above and below DNL 65 dB follow:
DLN 65 dB and above--An increase in noise exposure of 1.5 dB or
more at these levels is considered a significant addition of noise. A
Federal action resulting in such an increase would require an
environmental impact statement (EIS).
DLN 60-65 dB--Increases in noise of 3 dB or more that remain
between DNL 60-65 dB do not result in significant exposure but can be
noticeable and may be highly annoying to some people. The FAA will
consider mitigation options but would not require an EIS in noise-
sensitive areas between DNL 60-65 dB that are projected to have an
increase of 3 dB or more as a result of the proposed changes.
For air traffic changes farther away from an airport, FAA
recognizes that some actions in areas below DNL 60 dB may produce
noticeable noise increases and generate adverse community reaction.
Although increases in noise in these areas are well below the standard
criteria for significant impact, the FAA's air traffic screening
procedures provide mechanisms to identify whether there are
extraordinary circumstances warranting an EA.
Supplemental Metrics
The FICON recognized that DNL can be supplemented by other metrics
on a case-by-case basis, but advised continued agency discretion in the
use of supplemental noise analysis. It found that the use of
supplemental metrics is limited because threshold levels of significant
impact have not been established and there is no accepted methodology
for aggregating these values into a cumulative impact description.
Supplemental metrics can be useful in characterizing specific events
and enhancing the public's understanding of potential effects resulting
from proposed changes in aircraft operations. Supplemental single event
analysis sometimes is conducted to evaluate sleep disturbance and, less
frequently, specific speech interference issues. For proposed FAA
actions in the vicinity of national parks in pristine areas and land
uses such a wildlife refuges where the Part 150 land use compatibility
guidelines bear little relevance, the FAA supplements DNL noise
analysis with other metrics on a case-by-case basis. The following
metrics are useful for site-specific applications on a case-by-case
basis:
Equivalent Sound Level (Leq ) is a cumulative metric that
can be appropriate where aircraft noise can affect activity periods of
less than 24-hour duration.
Maximum Sound Level (Lmax ) is a single event metric that
can be used to describe the greatest sound level in decibels during a
given time period at a noise-sensitive location.
Sound Exposure Level (SEL) is a single event metric that can be
used to describe noise exposure at noise-sensitive locations. This
metric can be expressed both in terms of maximum levels and number of
occurrences at varying levels.
Time Above dBA Threshold (TA) is a metric that can be used in the
same situations as Leq , such as measuring noise exposure
within specific time periods. The designation of threshold to be used
in supplemental TA measurements may be defined with respect to speech
interference or the ambient (background) noise level."