Federal Government's Method of Assessing Noise Impacts

The following text is an excerpt from the FAA's proposed noise abatement policy, published in the Federal Register, July 14, 2000.

Section 4: Assessing Aviation Noise

4.1 Foundations

The Federal government's methods and standards for measuring and assessing noise impacts derive from scientific research and a series of interagency committee reviews.

Federal Interagency Committee on Urban Noise

In 1979 the Federal Interagency Committee on Urban Noise (FICUN) was formed to develop Federal policy and guidance on noise. The committee's membership included the Environmental Protection Agency (EPA), the FAA, the Federal Highway Administration, and the Departments of Defense (DOD), Housing and Urban Development (HUD), and Veterans Affairs (VA). Among other things, it developed consolidated Federal agency land use compatibility guidelines using Yearly Day-Night Average Sound Levels (DNL) as the common descriptor of noise levels. In order to develop the guidelines, it was also necessary to establish a correlation between land use and noise exposure classifications.

The FICUN issued its report entitled Guidelines for Considering Noise in Land Use Planning and Control in June 1980. This report established the Federal government's DNL 65 dB standard and related guidelines. The FICUN generally agreed that standard residential construction was compatible for noise exposure from all sources up to DNL 65 dB. Their land use compatibility guidelines for noise exposure between DNL 65-70 dB called for building codes to require at least 25 dB outdoor to indoor noise level reduction (NLR); between DNL 70-75 dB, at least 30 dB NLR.

The FICUN considered noise exposure above DNL 75 dB to be "incompatible" with all residential uses except transient lodging with NLR of at least 35 dB. The report contained a comprehensive guidelines table. This table contains the following footnote regarding residential and certain other noise-sensitive uses in the moderate exposure zone from DNL 55-65 dB:

The designation of these uses as "compatible" in this [moderate impact] zone reflects individual Federal agencies' consideration of general cost and feasibility factors as well as past community experiences and program objectives. Localities, when evaluating the application of these guidelines to specific situations, may have different concerns or goals to consider.

The designations contained in the FICUN's land use compatibility table do not constitute a Federal determination that any use of land covered by the program is acceptable or unacceptable under Federal, State, or local law. The responsibility for determining the acceptable and permissible land uses and the relationship between specific properties and specific noise contours rests with the local authorities.

Aviation Safety and Noise Abatement Act of 1979

The ASNA was the first Federal legislation specifically addressing airport noise compatibility. The FAA implemented the ASNA's provisions in Part 150. This regulation adopted the DNL metric and the 65 dB land use compatibility guideline. This Federal guideline has been widely accepted by airport proprietors as a threshold for limiting new residential development and for sound insulation where new development is permitted above this guideline. The subsection on Airport Noise Compatibility Planning in Section 2 addresses Part 150 provisions in greater detail.

Federal Interagency Committee on Noise

In 1991, the FAA and EPA initiated the Federal Interagency Committee on Noise (FICON) to review technical and policy issues related to assessment of noise impacts around airports. Membership included representatives from DOD, DOT, HUD, the Department of Justice, VA, and the Council on Environmental Quality. The FICON review focused, among other things, on the manner in which noise impacts are determined and described and the extent of impacts outside of DNL 65 dB that should be reviewed in a NEPA document. The FICON's findings and recommendations were published in an August 1992 report, Federal Agency Review of Selected Airport Noise Analysis Issues.

With respect to DNL, the FICON found that there are no new descriptors or metrics of sufficient scientific standing to substitute for the present DNL cumulative noise exposure metric. It further recommended continuing the use of the DN metric as the principal means for describing long-term noise exposure of civil and military aircraft operations. The FICON reaffirmed the methodology employing DNL as the noise exposure metric and appropriate dose-response relationships (primarily the Schultz curve for Percent Highly Annoyed) to determine community noise impacts.

Based on these findings, the FICON supported agency discretion in the use of supplemental noise analysis. It also recommended that further analysis should be conducted of noise-sensitive areas between DNL 60-65 db having an increase of 3 dB or more if screening analysis shows that noise-sensitive areas at or above DNL 65 dB will have an increase of DNL 1.5 dB or more. The FICON decided not to recommend evaluation of aviation noise impact below DNL 60 dB because public heath and welfare effects below that level have not been established.

The FICON strongly supported increasing research efforts on methodology development and on the impact of aircraft noise. It recommended a standing Federal interagency committee be established to assist agencies in providing adequate forums for discussion of public and private sector proposals identifying needed research and in encouraging research and development in these areas.

Federal Interagency Committee on Aviation Noise

The Federal Interagency Committee on Aviation Noise (FICAN) was formed in 1993 based on the FICON report's policy recommendation to form a standard interagency committee for facilitating research on methodology development and on the impact of aircraft noise. Membership includes representatives from DOD, HUD, DOT and the Department of the Interior, as well as NASA and the EPA. Each of the Federal agencies conducting significant research on aviation-related noise is represented on FICAN. Some member agencies, such as HUD and EPA, are not currently conducting research but have broad policy roles with respect to aviation noise issues.

The FICAN does not conduct or directly fund any research. Rather, it serves as a clearinghouse for Federal aircraft noise research and development (R&D) efforts and as a focal point for questions and recommendations on aviation noise R&D. Products include various reports, studies, analyses, findings, and conclusions. The FICAN holds periodic meetings, including a public forum, and issues a report on its activities annually. Since its inception in 1993, it has reached the following conclusions:

Current and future FICAN activities include:

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:

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 65 dB. 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.

4.3 Aircraft Research in National Parks

In 1987, the U.S. Congress enacted Public Law (PL) 100-91, the National Parks Overflight Act, which called for the NPS to recommend to the FAA actions for the substantial restoration ``natural quiet'' to the Grand Canyon National Park (GCNP). One year later, the FAA issued the Special Federal Aviation Regulation (SFAR) 50-2, creating a Special Flight Rules Area, flight-free zones, and defined routes for commercial air tours and sightseeing within the GCNP. Another milestone occurred in 1995 when the NPS presented a report to Congress on aircraft noise in national parks.

The FAA and the NPS initiated a model validation process. In August 1999, the agencies hosted a two-day meeting at Grand Canyon National Park of eight internationally recognized acoustics experts (the Technical Review Committee (TRC)). Representatives from Harris, Miller, Miller and Hanson; Volpe National Transportation System Center; and Wylie Laboratories worked with the TRC to develop a protocol that would measure the output of various acoustic models against the actual acoustic environment in the Grand Canyon National Park. The desired outcome of the process is a level of confidence in the ability of the tested models to replicate the conditions found in the park. The on- site data was collected during the month of September 1999 and a Spring 2000 report is planned. The TRC will be asked to review and comment on the results.

4.4 Research on Low Frequency Noise

The issue of low frequency aircraft noise and its impact on structural integrity and human health was explored in detail as part of the environmental assessment of the introduction of Concorde supersonic transport operations into the United States. Potential impacts were found to be negligible. Field studies found that the noise-induced vibrations as a result of Concorde operations cause no structural damage. In addition, the Concorde sound pressure levels at low frequencies were found to be well below the EPA threshold for potential health impact. As a result of these findings, the FAA concluded that low frequency noise of subsonic aircraft in a typical airport environment had no significant impact on structures or human health. This does not mean that there may not be some noticeable vibration in certain cases.

Human annoyance resulting from the effects of aircraft noise induced structural vibration is a recently raised concern. Low frequency noise and perceptible vibration may be experienced when aircraft noise levels are high (near the start of takeoff roll) and there are many aircraft events. This same combination of factors also tends to lead to high DNL levels (generally within the 65 DNL contour or higher). However, unlike the widely accepted relationship between aircraft noise exposure in DNL and community annoyance, there does not currently exist a scientific consensus or Federal guidelines on the human annoyance effects of noise-induced structural vibration.

Overall evidence recently evaluated by the FAA suggests low frequency noise is not a separate impact phenomenon, but rather is connected to high cumulative aircraft noise exposure levels. It may be of concern under certain conditions in areas already within the 65 DNL contour due to higher frequency noise. Perceptible vibration due to low frequency noise may be a secondary effect under certain conditions (e.g., home location relative to takeoff roll and aircraft fleet composition) in homes that are exposed to high levels of aircraft noise as calculated with the DNL metric. The FAA supports and promotes further research on this issue through FICAN.