Glossary
Short definitions of the noise metrics, thresholds, and procedural terms used across this site, each with a permanent link.
What this page is for
Articles on this site link individual terms directly to their entries below. Each entry gives the shortest definition that is still accurate, notes the boundary a reader is most likely to cross wrongly, and points to the fuller treatment in the mmalc X Machina article The sound we measure, the harm we don't.
Some of what causes trouble in aircraft-noise documents is not a term at all. It is a distinction between two things that share a name, or a step in a process that does not lead where it appears to lead. Those have no headword and so cannot be entries. Five recur often enough to state here:
- Measurement, threshold, and remedy are three separate stages. Clearing one does not clear the next.
- A metric, a policy threshold, and an enforcement limit are three different things. Much of the public confusion about aircraft-noise regulation begins when they are treated as interchangeable.
- Having an impact formally recognized is not the same as having a right to make it stop.
- Single-event measures and cumulative measures answer different questions. A change in one does not imply a change in the other.
- The word "noise" collapses four questions: what physically occurred, how it is experienced in context, what consequences the exposure produces, and when the law or society treats the interference as unreasonable.
Two regulatory instruments are best explained the same way, because a one-sentence stub for either would mislead.
14 CFR Part 150 establishes the federal framework for airport noise-compatibility planning. It works to prevent incompatible land uses from being introduced into high-noise areas, and it can also support mitigation for incompatible uses that already exist. What it does not give an individual resident is a private enforcement right. For an individual property, being within the 65 DNL contour is ordinarily the basic federal eligibility threshold for mitigation funding rather than an entitlement to it, and additional program requirements still apply before measures such as sound insulation, or in some cases acquisition, can receive FAA funding. Fuller treatment: The sound we measure, the harm we don't.
The National Environmental Policy Act (NEPA) is the federal law requiring agencies to consider the environmental effects of proposed actions before making decisions. FAA Order 1050.1G sets out FAA's procedures for carrying out that review. The review is largely procedural: it can require FAA to identify an effect, analyze it, disclose it, consider objections, and in some circumstances undertake a more demanding review. A large noise increase can therefore matter a great deal to the process without giving the person underneath it a right to a quieter outcome. Fuller treatment: The sound we measure, the harm we don't.
Noise measurement
Decibel (dB)
The decibel expresses sound pressure level as a ratio against a reference pressure of 20 micropascals, conventionally close to the threshold of human hearing around 1 kHz. The scale is logarithmic, not linear: an increase of 3 dB represents approximately twice the acoustic energy, and an increase of 10 dB represents ten times the acoustic energy.
Decibels do not add arithmetically. Two independent sources each producing 50 dB together produce about 53 dB, not 100 dB. Nor does 0 dB mean silence — it means the measured sound pressure equals the reference level. Perceived loudness is a further step removed: an increase of about 10 dB is often experienced as roughly a doubling of loudness, but that is an approximation that depends on frequency, level, duration, the listener, and the character of the sound.
Fuller treatment: The sound we measure, the harm we don't, appendix.
A-weighting (dBA)
An A-weighting filter modifies the measured spectrum to approximate the frequency sensitivity of human hearing, strongly reducing the contribution of very low frequencies and also reducing some high-frequency components. The result is written dBA or dB(A). FAA uses A-weighted sound levels in its principal community-noise metrics.
Weighting is a form of compression, so two sounds can carry the same dBA value while sounding very different. A-weighting can be less informative for sounds dominated by strong low-frequency content, conspicuous tones, or impulses, which is why some noise standards apply separate corrections for those characteristics.
Fuller treatment: The sound we measure, the harm we don't, appendix.
Lmax
Lmax is the maximum sound level measured during a specified observation period, subject to the meter's time weighting and settings. It answers one question: how loud did it get.
It is a single-event measure, and it says little about how long that level lasted or how often it happened.
Fuller treatment: The sound we measure, the harm we don't, appendix.
Sound Exposure Level (SEL)
SEL takes all of the acoustic energy in a discrete event — one aircraft flyover, for example — and normalizes it to a one-second reference period. It is useful for comparing the total acoustic energy of individual events of different durations.
SEL is not the maximum level of the event, and it should not be read as though a listener experienced the SEL value continuously for one second.
Fuller treatment: The sound we measure, the harm we don't, appendix.
Leq
Leq is the level of a hypothetical constant sound containing the same acoustic energy as the actual fluctuating sound over a specified period. It incorporates both level and duration: a brief loud event contributes more energy than a quieter one, and a long sequence of moderate events can accumulate substantial exposure.
Because decibels are logarithmic, Leq is not an ordinary arithmetic average of the displayed decibel numbers. And two periods with the same Leq can still be experienced very differently, since the metric does not preserve how the energy was distributed among events.
Fuller treatment: The sound we measure, the harm we don't, appendix.
Day-Night Average Sound Level (DNL, or Ldn)
DNL is a cumulative 24-hour energy metric, and FAA's principal metric for community aircraft-noise exposure. It combines the acoustic energy of events across the day and applies a 10 dB weighting to noise occurring between 10 p.m. and 7 a.m. before the equivalent daily level is calculated. For airport analysis it is commonly expressed for an average annual day, so a year's pattern of operations is represented as exposure on a single representative day. Ldn is the notation EPA used for the same metric that FAA now calls DNL. In FAA aircraft-noise analysis, DNL ordinarily represents the aircraft contribution rather than folding in existing ambient sound from roads, wind, wildlife, and other sources.
The nighttime weighting is a mathematical penalty intended to reflect greater sensitivity to nighttime noise, not a claim that an aircraft becomes 10 dB louder at night. DNL is neither a peak nor a count: a substantial number of individually conspicuous overflights can still produce a cumulative value well below 65 DNL, depending on the sound exposure level of each event.
Fuller treatment: The sound we measure, the harm we don't, sections 3 and 5, and appendix.
Community Noise Equivalent Level (CNEL)
CNEL is a 24-hour energy-average metric closely related to DNL, and the metric California commonly uses, alongside Ldn, in General Plan Noise Elements. Airport land-use planning in California also makes extensive use of it. CNEL keeps DNL's 10 dB weighting for noise between 10 p.m. and 7 a.m. and adds an evening weighting from 7 p.m. to 10 p.m., commonly described as 5 dB; more precisely, the threefold weighting is equivalent to about 4.77 dB.
The evening and nighttime additions are penalties in the calculation, not measurements showing that the physical sound became louder. DNL and CNEL are often numerically close, but they are not identical and should not be silently substituted for one another.
Fuller treatment: The sound we measure, the harm we don't, section 4 and appendix.
Statistical sound levels (L10, L50, L90)
Statistical sound levels describe the level exceeded for a stated proportion of the measurement period. L10 is exceeded 10 percent of the time and tends to reflect the louder part of the sound environment. L50 is the median level. L90 is exceeded 90 percent of the time and is often used as an indicator of the underlying or residual background level.
Whether L90 is the appropriate definition of "ambient" depends on the regulatory context, and "ambient" itself can be defined in different ways. Short-term background measurements should not be casually compared with annual DNL or CNEL values, which are different metrics answering a different question.
Fuller treatment: The sound we measure, the harm we don't, appendix.
N-above
An N-above metric counts the number of events exceeding a chosen level — the number of aircraft events above 60 dBA, for example. It describes recurrence directly, which a cumulative average does not.
It requires choosing a threshold, and a fixed threshold recreates the baseline problem: an event at 55 dBA may be very prominent in a 30 dBA environment while disappearing into a much louder urban background.
Fuller treatment: The sound we measure, the harm we don't, appendix.
N-Above-Ambient+10
A proposed metric that would count events exceeding the ambient level by a stated amount, rather than measuring them only against a fixed absolute threshold. It was put forward by community commenters during FAA's Noise Policy Review.
Its status matters: it remains a proposal submitted to FAA, not an adopted FAA metric and not an FAA scientific finding.
Fuller treatment: The sound we measure, the harm we don't, section 5.
Thresholds and regulatory terms
Terms in this section describe how noise numbers are used in federal and state processes. Later articles will add procedural and regulatory entries here — ANCA, Part 161, CEQA, Environmental Assessment, operation, enplanement, and the departure procedures RREHD ONE and CHRLY8 among them. Part 150 and NEPA are covered in the standing note above.
65 DNL
65 DNL is a policy threshold applied to the DNL metric, and it performs several different jobs. It is the residential compatibility boundary under Part 150. It is ordinarily the basic federal eligibility threshold for mitigation funding for an individual property. And it is the reference level in FAA's environmental-review significance test, under which an increase of 1.5 dB or more is significant at a noise-sensitive area already exposed at or above DNL 65, or where the increase causes the area to reach DNL 65.
None of those functions is a lawful exposure ceiling. 65 DNL is not a legal exposure limit, not a per-flight or per-house limit an airline could breach, and not a private enforcement right. A house exposed to DNL 66 does not establish that anyone has violated anything.
Fuller treatment: The sound we measure, the harm we don't, section 5.
SID (Standard Instrument Departure)
A published departure route that aircraft leaving an airport are expected to fly, defined by named waypoints, headings, and altitude constraints rather than by controller instruction. A single airport may have several, used for different runways and different directions of travel. RNAV SIDs, the type at issue at STS, are flown using satellite navigation rather than ground-based beacons.
A SID specifies where aircraft are expected to fly, not where they are permitted to fly. Controllers can and do vector aircraft off a published procedure, so the track flown on any given day may differ from the published route.
CATEX (Categorical Exclusion)
A determination that a proposed federal action belongs to a class of actions the agency has already found does not individually or cumulatively have a significant environmental effect, and so requires neither an environmental assessment nor an environmental impact statement. It is a finding that no further environmental review is needed, not a review reaching that conclusion. A categorical exclusion can be defeated by extraordinary circumstances, which the agency must consider before applying it. The three 2024 STS departure procedures were approved under a categorical exclusion.
Health and evidence
This section will grow as articles cite the health literature. Terms such as "highly annoyed," the Schultz Curve, and FAA's Neighborhood Environmental Survey belong here.
Disability-Adjusted Life Year (DALY)
A DALY is a standard public-health measure combining years of life lost to premature death with years lived in less than full health. It allows burdens of very different kinds — premature mortality, sleep disturbance, annoyance — to be expressed in a single unit and compared.
Fuller treatment: The sound we measure, the harm we don't, section 2.