65 DNL (Day-Night Average Sound Level)

What is the significance of 65 DNL?

DNL is the acronym for Day-Night Average Sound Level.

The Federal Aviation Administration’s Office of Environment and Energy (AEE) issued a document titled Aircraft Noise.  The document states, in part, that federal agencies have certain guidelines for compatible land uses and environmental sound levels.  Land use is normally determined by property meaning, such as residential, industrial, or commercial.  Noise levels that are unacceptable for homes may be quite acceptable for stores or factories.  The Federal Aviation Administration (FAA) has issued these guidelines as part of its Airport Noise Compatibility Program, found in Part 150 of the Federal Aviation Regulations.

Most land uses are considered to be compatible with airport noise that does not exceed 65 decibels (dB), although Part 150 declares that “acceptable” sound levels should be subject to local conditions and community decisions.  Nevertheless, 65 dB is generally identified as the threshold level of aviation noise which is “significant.”

The FAA has established 65 DNL as the threshold above which aircraft noise is considered to be incompatible with residential areas.  In addition, the FAA has determined that a significant impact occurs if a proposed action would result in an increase of 1.5 DNL or more on any noise-sensitive area within the 65 DNL exposure justify.

Three categories of impacts are examined in this analysis, based on FAA Order 1050.1E:

• Significant Impacts: 1.5 DNL minimum increase resulting in 65+ DNL noise exposure, or 1.5 DNL minimum increase where noise exposure already exceeds 65 DNL
• Slight to Moderate: 3 DNL minimum increase resulting in noise exposure between 60 and 65 DNL, or 3.0 DNL minimum increase where noise exposure is already between 60 and 65 DNL
• Slight to Moderate: 5 DNL minimum increase resulting in noise exposure between 45 and 60 DNL, or 5 DNL minimum increase where noise exposure is already between 45 and 60 DNL