Sound at its most basic definition is a mechanical wave that results from the vibration of particles that travel through the air or another medium and can be heard by the human ear. Not all sound frequencies are detectable by the human ear. The normal range of human hearing is considered to be between 20 Hz (lowest pitch) and 20 kHz (highest pitch) but is most sensitive to sounds between 1 kHZ and 4 kHZ. By contrast cats and dogs can hear sounds as high as 40 kHZ and dolphins and bats can hear sounds at frequencies as high as 160 kHZ.
When it comes to measuring sound pressure level there are several curves used, but the most common is the A-weighted sound pressure level, which has been weighted to reduce the influence of the low and high-frequency extremes. It has been found to correlate well with the human hearing response and with a person's subjective judgment of the loudness of sounds. A-weighting emphasizes speech frequency bands and de-emphasizes the lower and higher frequencies, and is used to measure both environmental noise and industrial noise.
Decibel (dB) is a unit used to measure the intensity of a sound by comparing it with a given level on a logarithmic scale. Sound levels increase logarithmically meaning that an increase by ten (10) dB equates to a 10x increase in the sound energy but is only perceived to be 2x louder by the human ear.
The graphic above uses 60 dB as the baseline measurement as it commonly associated with the loudness of a normal human conversation. By increasing or decreasing the sound energy the chart clearly illustrates the difference in actual sound energy and the level of change perceived by the human ear. For example, an increase from 60 dB to 90 dB results in a sound energy level that is 1,000 times louder but only perceived to be 8 times as loud by the human ear.
Noise is nothing more than unwanted sound, especially one that is loud or unpleasant or that causes a disturbance. The problem with measuring noise is that it's very subjective. Defining annoyance or people's negative reactions to noise is very complex and there isn't a single noise level in studies or lab experiments that can be defined as the threshold of when sound becomes noise for all people. The source of the noise is a critical element as well as non-acoustic factors that predispose someone to find a particular noise source and or level to be annoying.
If no one agrees on a level of noise that triggers annoyance, then how can the FAA and airport's quantify airplane noise? The answer is again complex, but the Federal Government as part of a broader assessment of urban noise formed the Federal Interagency Committee on Urban Noise (FICUN) in 1979. This committee which was comprised of members from the Environmental Protection Agency (EPA), Federal Aviation Administration (FAA), Federal Highway Administration (FHA), Department of Defense (DOD), Housing and Urban Development (HUD), and Veterans Affairs (VA) were tasked with consolidating federal agency land use compatibility guidelines using the Annual Average Day-Night Sound Level (DNL) as the common descriptor of noise levels. The committee and its subsequent report "First Report on the Federal Urban Noise Initiative," established the Federal government's DNL 65 dB standard as the threshold for significant noise and land use compatibility planning.
The term significant noise in the language of the report was not meant to imply that noise levels below 65 DNL are not considered to be impactful or annoying, but simply that the federal appropriations for noise mitigation measures from a noise land use compatibility perspective were established to be DNL 65. The major objective of the FICUN committee was to establish land use planning standards that would limit residential and other non-compatible land use in the 65 DNL or higher noise contour.
From an airport authority and municipal government perspective, the standard and guidelines were meant to inform state and local land use policy and zoning laws to ensure future land use compatibility with airports. All Federal funding linked to aviation noise mitigation from the FAA perspective is centrally tied to the reduction of non-compatible land use around airports. Stated another way the primary goal of every airport's FAA Part 150 Study and award of grant funding under this program is to reduce non-compatible land use, period.
Aircraft Noise Event
Like a thumbprint noise events have their own unique acoustic signature. Airplane noise events have a different signature from cars, ships, trains, crowd noise, and other sources. An airplane noise event's acoustic signature generally follows a bell-shaped curve, with a consistent rise and fall over a relatively long duration. This unique curve enables noise and operations monitoring systems (NOMS) integrated with noise monitoring terminals (NMTs) to determine with a high degree of accuracy the source of a noise event registered by a microphone.
Acoustical triggers are set up in the system that is customized to each individual NMT that takes into account the average ambient noise level and use this information to establish a threshold sound trigger level and minimum duration level during which the sound level must remain above the trigger threshold to be considered an aircraft noise event. A maximum noise event duration also ensures that high decibel events, that persist at a very high noise level for an extended period like running lawnmowers or construction equipment are not registered as airplane noise events.
Sound Exposure Level (SEL)
Sound Exposure Level is the constant sound level that has the same amount of energy in one second as the original noise event over its entire duration. SEL accounts for both the loudness and the duration of the noise event by compressing all the noise energy of the event into one second and is used to measure noise from moving sources like airplanes, trains, and cars. As a general rule, the SEL of an aircraft noise event is usually about 10 dB above the Lmax (maximum noise level) of the event.
Equivalent Continuous Sound Level (Leq)
Leq is the equivalent continuous sound level in A-weighted decibels, equivalent to the total sound energy measured over a stated period of time usually 1, 8, or 24 hours. Equivalent signifies that the total acoustical energy associated with the fluctuating sound (during the specified time period) is equal to the total acoustical energy associated with the steady sound level or Leq for the same specified period of time. Leq is often used to show the relationship between community noise and aircraft noise which combined produces the total Leq.
Statistical Noise Level (Ln)
Ln is a representation of the statistical percentage of time the noise level exceeds a certain decibel level for a defined period of time. Any statistical value between 0.01% and 99.99% can be calculated but common metrics are L10, L50, L90, and L95. For example, if you were to sample noise levels once a second for an hour you would end up with 3,600 individual samples. To derive the L50 you would simply add up all the one-second noise measurements and divide by 3,600. The result would be the decibel level above which the noise level was exceeded for 50% of the total hour measurement period. Ln is often expressed in the following format 70 dBA L10 (18-hour), which indicates the decibel level, the statistical value, and the duration of time during which the noise measurements were collected.
Day-Night Average Sound Level (DNL)
The Day-Night Average Sound Level (DNL) is the Federal Aviation standard for airport noise land use compatibility planning. The requirements for airport noise compatibility planning are stated in: Code of Federal Regulations, Title 14-Aeronautics and Space, Subchapter I-Airports (Parts 150-159), Part 150-Airport Noise Compatibility Planning (14 CFR Part 150).
DNL is defined as the average equivalent A-weighted sound level during a 24-hour day, obtained after the addition of a ten (10) decibel penalty for noise events that occur during nighttime hours from 10 p.m. to 7 a.m. DNL was introduced as a simplified method for predicting the effects on a population of the average long-term exposure to noise. It is an enhancement of the Equivalent Sound Level (Leq) metric with the addition of the 10 dB penalty for nighttime noise. DNL represents daily levels of noise exposure averaged on an annual or daily basis, while Leq represents the equivalent energy noise exposure for a shorter time period, typically one hour.
The 10 dB penalty equates to one (1) daytime aircraft noise event equalling ten nighttime (10) aircraft noise events which effectively doubles the sound energy level of the event which has a much bigger cumulative effect on the DNL average. The idea of the nighttime penalty was developed to recognize that noise events during night time hours tend to be more disruptive and have a higher likelihood of annoyance due to the lower ambient noise levels in the community.
To calculate DNL the Leq level is used as the hourly equivalent sound level. The hourly Leqs are summed for the 15 hours of daylight (7 a.m. to 10 p.m.) and added to the sum of the hourly Leqs for the remaining 9 hours of nighttime with the 10 dB penalty added to nighttime events. The result is the DNL noise level or a 24-hour summary of noise levels for a given location.
Aircraft noise contours are calculated using the FAA's Aviation Environmental Design Tool (AEDT). This tool is the official aircraft noise calculation tool used by airports to produce legal Noise Exposure Maps (NEMs) for FAA review. NEMs are aircraft noise contour maps that quantify non-compatible land use within the 65 DNL contour or higher as defined in 14 CFR 150.21.
Aircraft noise contours do not include community noise level contributions just those by airplanes. The AEDT program generates a grid encompassing the airport study area and the DNL value at each grid point are calculated by the model. Like values are then connected automatically to form a contour. Generally speaking, an airport will model noise contours between the 65-75 DNL values in five-decibel increments.
Community Noise Equivalent Level (CNEL)
Community Noise Equivalent Level (CNEL) is the California state standard for airport noise land use compatibility. It is defined as the average equivalent A-weighted sound level during a 24-hour day, obtained after the addition of a five (5) decibel penalty for noise events that occur during the evening from 7 p.m. to 10 p.m. and during nighttime hours from 10 p.m. to 7 a.m. a ten (10) decibel penalty is assessed for noise events during this period. The FAA recognizes the CNEL standard as an official metric for use in California only.
CNEL is codified in the California Code of Regulations: Title 21. Public Works, Division 2.5. Division of Aeronautics (Department of Transportation), Chapter 6. Noise Standards, Article 1. General, Section 5012.
The California Airport Noise Regulations include a provision requiring airports to submit quarterly and annual reports to State and County agencies to demonstrate compliance with the terms of the regulations. The noise exposure contour map and the associated report indicate the amount of incompatible land use within the applicable CNEL criterion boundary contour. For most airports, the contour of significance is the 65 dBA CNEL contour.
The latest quarterly CNEL noise contour for SJC Airport can be found on the Noise Reports page of the Airport Noise Office website.