Following are frequently asked questions about the effects of vegetative removal on noise exposure.
1. How does FAA assess overall aircraft noise exposure?
Federal Aviation Administration (FAA) requires the use of the Day-Night
Average Sound Level (DNL) as the primary metric for aircraft noise
exposure. Despite its name, DNL is not a typical average, but instead
is a cumulative measure of all noise exposure during a 24-hour period,
wherther it is a loud event or a quieter event, increases the DNL
value. To reflect the added intrusiveness of noise between the
nighttime hours of 10p.m. and 7 a.m., DNL counts each nighttime noise
event as if it occurred 10 times. FAA noise evaluations typically
average daily DNL values over a one-year period to account for daily or
searonal fluctuations in aircraft operations, runway use and weather
Figure 1 shows a range of typical DNL values for various outdoor
environments along with federal goals and criteria. The ambient sound
levels indicated on the figure’s left side represent approximate
background sound levels in different settings. These ambient levels may
include natural sounds such as wind blowing in trees and also
human-caused sounds such as distant or local traffic, lawn mowers, air
conditioners, etc. depending upon the location. FAA considers all land
uses to be compatible with aircraft noise at annual-average exposure
levels below DNL 65 dB.
Click here to view Figure 1
2. How does FAA assess noise caused by a single aircraft event?
some cases, noise metrics other than DNL are helpful "to describe
aircraft noise impacts for specific noise-sensitive locations and to
assist in the public’s understanding of the noise impact." These
supplemental metrics include descriptors that provide information on
single events such as an individual aircraft departure or a pre-flight
The maximum sound level (Lmax) of a particular noise event is one
example of a supplemental metric. Although Lmax is easy to understand
and helps to predict certain noise effects such as speech interference,
it does not account for a noise event’s duration. Figure 2 provides
examples of common Lmax values. It is important to note that these Lmax
values should not be compared directly with the DNL values shown in
Click here to view Figure 2.
Another common supplemental metric, the Sound Exposure Level (SEL),
accounts for both the loudness and the duration of a single event.
Because it is a cumulative measure, a higher SEL can result from a
louder event, a longer event, or from some combination of the two. The
shaded area in Figure 3 represents the noise "dose" associated with a
single event such as an aircraft departure. The SEL value for the single
event (represented by the vertical bar) is proportional to the area of
the noise dose.
Click here to view Figure 3.
3. How do aircraft ground operations contribute to overall noise exposure near airports?
ground operations may include aircraft idling, taxiing, pre-flight
run-ups of propeller aircraft, and start-of-takeoff roll. Typically,
however, noise from airborne flight operations (i.e. aircraft departures
and arrivals) dominates overall noise exposure near airports. Although
aircraft ground operations sometimes are audible near airports,
generally they are quieter than airborne aircraft when heard in
Ground operations noise often is reduced by interaction with the
ground ("ground effects") and shielding provided by terrain and other
obstructions. Because these factors are less likely to reduce noise
airborne departures or arrivals, the louder flight operations
dominate noise exposure and ground operations noise seldom makes a
significant contribution to DNL.
4. What are the potential effects of aircraft ground operations noise?
when making only a minor contribution to overall noise exposure
(measured in DNL), aircraft ground operations noise still has the
potential to cause speech interference, sleep disturbance, and community
annoyance in nearby residential areas. Sound levels sufficient to cause
speech interference may make conversation difficult or interfere with
use of the telephone or with listening to television or radio.
Sufficiently high sound levels also may cause sleep disturbance,
especially during warmer months when windows are more likely to be open.
Sound levels that are not loud enough to cause speech interference or
sleep disturbance still may cause community annoyance, especially during
events of unpredictable or indefinite duration such as aircraft idling
or pre-flight run-ups. For these reasons, supplemental metrics such as
Lmax and SEL can be useful in describing and understanding ground
5. Do areas of trees and other vegetation near airports reduce aircraft noise?
and vegetation around airports are more likely to affect sound levels
caused by aircraft when they are on the ground than when they are in the
air. When airborne aircraft are sufficiently high above the ground that
trees do not break the line of sight from the listener, the trees
provide no noise reduction. When trees do break the line of sight from
the listener to an aircraft on the ground, a relatively broad area of
dense vegetation is required to provide a noticeable reduction in sound
Although the FAA does not provide specific guidance on noise
reduction provided by trees and other vegetation, the Federal Highway
Administration (FHWA) attributes approximately one to three decibels of
noise reduction for every 100 feet of vegetation that is "sufficiently
dense to completely block the view along the sound propagation path." To
provide this level of noise reduction, such vegetation zones must
consist of "long, wide regions of heavy . . . woods and undergrowth, not
just individual trees or several rows of trees." When considering
changes in sound levels two useful rules of thumb are: (1) most
individuals perceive a six to 10 decibel change to be either about a
doubling or a halving of loudness, and (2) changes of less than about
three decibels are not easily detected outside of a laboratory.
6. What are other possible effects of trees and other
vegetation near airports?
when not providing measurable noise reduction, vegetation can influence
a listener’s perception of the noise environment in other ways. Trees
can provide a visual buffer and thereby eliminate a visual reminder of
one’s proximity to an airport or other noise source. Trees scatter the
very high frequency sounds that can convey "mechanical harshness," and
also may provide a type of forest reverberation further reducing
harshness and the impulsive nature of some noise sources. "In addition,
wind motion through leaves produces a pleasant sound, which can
partially mask more annoying sounds." Although these effects do not
reduce the overall noise level, they may affect the listener’s
perception of the noise environment and thereby decrease annoyance.
Oftentimes "even when measurements show no significant [noise reduction]
from intervening trees, many people believe strongly that such trees do
quiet their environment."
7. What changes may residents near an airport notice due to vegetation removal?
cumulative noise exposure (measured in DNL) near airports typically is
dominated by airborne departures and arrivals, vegetation removal at an
airport is unlikely to have a significant effect on cumulative noise
exposure in nearby communities.
In some areas, however, such as near the approach ends to runways,
sufficient areas of dense vegetation will be cleared that residents may
notice increased sound levels during particular types of aircraft ground
operations. For example, clearing 100 feet or more of trees and dense
undergrowth may allow community sound levels during certain events to
increase by three decibels or more. If resulting single-event sound
levels are sufficiently high, residents may notice increased occurrences
of speech interference for brief periods during some events, especially
Even without a measurable increase in sound levels, residents may
notice a change in the character of the sound environment due to
reduction in scattering of high-frequency sound, reduction in "forest
reverberation," and decrease in masking noise caused by rustling leaves.
In addition, residents are more likely to be generally aware of airport
operations in locations where the airport formerly was hidden from view
and has become visible due to vegetation removal.