Noise FAQ's

Following are frequently asked questions about the effects of vegetative removal on noise exposure.   

1. How does FAA assess overall aircraft noise exposure?    
The 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, whether 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 seasonal fluctuations in aircraft operations, runway use and weather conditions. 

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?
In 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 run-up.

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 Figure 1.

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?
Aircraft 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 community locations.

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 levels from

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?
Even 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 operations noise.

5. Do areas of trees and other vegetation near airports reduce aircraft noise?
Trees 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 level.

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?
Even 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?
Because 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 when outside.

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.