Identify How Aircraft Noise Is Described and What It Means

There are many ways to describe noise metrics. The most commonly used metric is the decibel (dB), which includes a weighting system that closely reflects the human ear (the A-weighted decibel – dBA). Noise metrics can be categorized as either single-event metrics or cumulative metrics. Single-event metrics describe the noise from an individual event such as an aircraft flyover or a car horn. Cumulative metrics describe noise in terms of the total noise exposure throughout the day.

Federal law requires the use of the DNL – a cumulative, 24-hour average sound level that includes a weighting for noise during the nighttime hours – to quantify aircraft noise with regard to land use compatibility.

One of the frequent criticisms of the DNL metric is that it does not truly represent what people hear because the DNL is a cumulative measurement while people hear single-event noise. Although single-event noise often seems to the public to be a better description of what they are experiencing, the FAA uses cumulative noise metrics to assess community response to noise because these scales attempt to include the loudness and duration of all noise, the total number of noise events, and the time of day and frequency into one rating scale.

• DNL – A cumulative noise metric with a 10 dB penalty for nighttime operations.
• LMax – the maximum noise level an individual aircraft makes, what people hear
• SEL – the sound exposure level of an aircraft flyover, takes into consideration loudness and duration.