Aircraft Noise 101
There are many terms used in the discussion of aircraft noise. This section is designed to give you a broad overview of the most commonly used terms.
All sound comes from a sound source. It takes energy to produce sound, and this energy is transmitted through the air in form of sound waves. These waves, are tiny, quick oscillations of pressure just above and just below atmospheric pressure. These oscillations, or sound pressures, interact with the ear and its auditory system, creating the sounds we hear, and noises we experience.
The Decibel (dB)
A measure of the sound pressure of a given noise source relative to a standard reference value is the sound pressure level (SPL). This reference pressure is typical of the quietest sound that a young person with good hearing is able to detect and is measured in decibels (dB). The logarithmic conversion of sound pressure to sound pressure level means that the quietest sound we can hear (the reference pressure) has a sound pressure level of 0 dB, while the loudest sounds we can hear without pain have sound pressure levels on the order of 120 dB. Most sounds in our day-to-day environment have sound pressure levels on the order of 30 to 100 dB.
The A-weighted Sound Level dB(A)
Another very important characteristic of sound is its frequency, or "pitch". This is the rate of repetition of the sound pressure oscillations as they reach our ear. Thus, the closer the vibrations/oscillations, the higher the pitch and vice versa. Our ears are better equipped to hear mid and high frequencies than low frequencies, thus we find mid and high frequency noise to be more annoying. In general, people respond to sound most readily when the predominant frequency is in the range of normal conversation. Several filters have been defined to approximate this sensitivity level. The A-weighted sound level/filter is used for most environmental reviews. A-weighted sound levels best match the human ears' sensitivity.
The Day-Night Average Sound Level (DNL) and Community Noise Exposure Level (CNEL)
In simple terms, Ldn or DNL is the average noise level over a 24 hour period except that noise occurring at night (between the hours of 10PM and 7AM) are artificially increased by 10 dB. This weighting reflects the added intrusiveness of night noise events attributable to the fact that community background noise typically decreases by 10 dB at night, and communities are more sensitive to noise during these hours.
The Community Noise Exposure Level (CNEL), the standard used for environmental noise studies in California (and at the non-federal level), represents DNL plus with an additional artificial increase of 5 dB between the hours of 7 PM to 10 PM.
Under Federal Aviation Regulation (FAR) Part 150, the FAA has established Ldn/DNL as sole cumulative noise exposure metric for use in airport noise analyses and has developed recommended guidelines for noise/land use compatibility evaluation using that standard.