Noise Measurement:

All sounds come from a sound source.  It takes energy to produce this sound, and this energy is transmitted through the air in sound waves, or tiny, quick oscillations of pressure just above and just below atmospheric pressure.  These oscillations, or sound pressures, impinge on the ear, creating the sound that humans can hear.  The denser the air, the better conductor of sound waves, and the less dense air is a poorer conductor of sound waves. 

Much of the sound generated by an aircraft is the result of moving air.  As air moves through a jet engine, past a propeller, or over a wing, it is deflected and accelerated resulting in turbulence as it mixes with the surrounding air.  These changes in airflow can generate significant sound levels.  There are many different terms used in the discussion of aircraft noise. 

While these sound levels can be measured in many different ways, the Federal Aviation Administration (FAA) provides guidelines for the measurement and presentation of data.  The following acoustic terms are defined in easier to understand terms to help visitors to better understand the information presented.
 

Numbers of People exposed to different noise levels

Noise Definitions:  

Ambient Noise Level
Ambient Noise is the existing background noise level characteristic of an environment..

Attenuation
Attenuation is the reduction of sound energy by the absorption or divergence of sound waves. The amount of attenuation due to absorption varies with the substance through which the sound waves are being transmitted, or propagating. For example, sound energy is absorbed differently by a window than by a wall, or by water than by air. Similarly, attenuation due to divergence varies by the size and shape of the space between source and receiver. For example, if you toss a rock into the lake the resulting waves decrease in height as they spread outward from where the rock entered the water. Generally speaking, through air, noise decreases by six decibels for a doubling of distance when sound emanates from a single point in space. In contrast, noise emanating from a line, such as a train or constant road traffic, decreases by only three decibels per doubling of distance.

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 the human ear.  Frequency is the closeness of the vibrations which means the higher the pitch and vice versa.  Human ears are better equipped to hear mid and high frequencies than low frequencies, thus humans find mid and high frequency noise to be the most annoying.  In general, humans 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 matches the human ears' sensitivity.

Community Noise Equivalent Level (CNEL)
CNEL measurements are a weighted average of sound levels gathered throughout a 24-hour period.  This is essentially a measure of ambient noise.  Different weighting factors apply to day, evening, and nighttime periods. This measurement recognizes that humans are most sensitive to noise in late night hours and are more sensitive during evening hours than in daytime hours.  CNEL depends not only on the noise level of individual aircraft approaches or departures, but also on the number of approaches or departures during the measurement period.

Day-Night Average Sound Level (DNL)
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.  Under Federal Aviation Regulation (FAR) Part 150, the FAA has established Ldn/DNL as the cumulative noise exposure metric for use in airport noise analyses, and has developed recommended guidelines for a noise and land use compatibility evaluation.

dBC
C-weighted decibels adjust sound pressure towards the low frequency end of the spectrum. Although less consistent with human hearing than A-weighting, dBC can be used to consider the impacts of certain low frequency operations.

Decibel (dB)
In sound, decibels measure a scale from the threshold of human hearing, 0 dB, upward towards the threshold of pain, about 120-140 dB.  Because decibels are such a small measure, they are computed logarithmically and cannot be added arithmetically.  An increase of ten dB is perceived by human ears as a doubling of noise. Most sounds in our day-to-day environment have sound pressure levels on the order of 30 to 100 dB.

DNL Contour
The "map" of noise exposure around an airport.  A contour is computed through an FAA model called the Integrated Noise Model (INM), which calculates annual noise exposure. FAA defines significant noise exposure as any area within the 65dB DNL contour; that is the area within an annual average noise exposure of 65 decibels or higher.

Effective Perceived Noise Level (EPNL)
EPNL measurements consist of a frequency weighting scheme considerably more complicated than the A-weighting filter used to determine SELs.  They incorporate a penalty for the presence of pure tones to account for people's increased annoyance with single frequencies, such as the tones emanating from the compressor of turbofan engines. Thus, although specific values must be determined by computer analysis of a signal, EPNL has been adopted for certain specialized uses involving the noise of individual over-flights.

Equivalent-Continuous Sound Level (Leq)
An equivalent-continuous sound level is simply an average level over a stated time period. In other words, if you had a sound level that fluctuated over a defined period of time its equivalent non-fluctuating level is its Leq.

Frequency Weighting
The average human ear senses tone resulting from sound oscillation in frequencies between 20 and 20,000 Hertz (Hz). Because the human ear doesn't respond to all of these frequencies equally well, weightings are applied to more accurately quantify what the ear is actually sensing. The most common weighting used by the airport as directed by FAA Part 150 guidelines, is A-weighting (dBA).
 

Maximum Sound Level (Lmax)
This is the highest level displayed on a sound level meter during a noise event or time period.

Metrics
An acoustic metric is simply a measurement derived through the use of a mathematical formula for a specific purpose. The metrics that are most commonly used in airport noise mitigation are DNL, Leq, SEL, Lmax, Lmin, NLR, and Attenuation. These metrics are A-weighted unless otherwise noted.

Minimum Sound Level (Lmin)
This is the lowest level displayed on a sound level meter during a noise event or time period.

Noise Exposure Map (NEM)
An FAR Part 150 requirement which is prepared by airports to depict current and future noise contours.  NEMs also take into account potential land use changes around airports.

Noise Contour
See DNL Contour.

Noise Level Reduction (NLR)
The Residential Acoustical Treatment Program reduces the transmission of noise into homes. This reduction is known as the noise level reduction. It is the amount, in decibels, that outdoor noise is reduced as it passes from the exterior to the interior of a structure.

Single Event Noise Exposure Level (SENEL)
The noise exposure level of a single aircraft event measured over the time between the initial and final points when the noise level exceeds a predetermined threshold. It is important to distinguish single event noise levels from cumulative noise levels such as CNEL. Single event noise level numbers are generally higher than CNEL numbers, because CNEL represents an average noise level over a period of time, usually a year.

Single Event
Noise generated by a single aircraft overflight.

Sound Exposure level (SEL)
SEL is similar to Leq. The difference between the two is that an SEL represents the entire noise event as though it occurred within a duration of one second. With the SEL metric we can directly compare two noise events having different durations.

Sound Pressure Level (SPL)
The average human ear begins sensing at a pressure of only 20 micropascals and begins to experience pain at approximately 100 pascals.  The ratio between these pressures is greater than a million to one.  Because the human ear perceives pressure changes logarithmically over a large range, the sound pressure level (SPL) is measured in pascals and converted to Decibels to get a more manageable logarithmic scale.  Because of this logarithmic relationship, one cannot simply arithmetically add, subtract, or average decibel levels.  For example if you have two 60 decibel noise sources occurring simultaneously, they combine to equal 63 decibels.  While a three decibel change is generally just perceptible, a ten decibel change, either up or down, is approximately twice or half as loud.

Aviation Definitions:

A

Air Carrier - A commercial airline with published schedules operating at least five round trips per week.

Airport Layout Plan (ALP) - The official, FAA approved map of an airport's facilities

Air Route Traffic Control Center (ARTCC)- A facility providing air traffic control to aircraft on an IFR flight plan within controlled airspace and principally during the enroute phase of flight.

Air Taxi - An aircraft certificated for commercial service available for hire on demand.

Air Traffic Control (ATC)- The control of aircraft traffic, in the vicinity of airports from control towers, and in the airways between airports from control centers

Air Traffic Control Tower (ATCT)- A central operations tower in the terminal air traffic control system with an associated IFR room if radar equipped, using air/ground communications and/or radar, visual signaling and other devices to provide safe, expeditious movement of air traffic.

Altitude MSL - Aircraft altitude measured in feet above mean sea level.

Approach Lighting System (ALS) - Radiating light beams guiding pilots to the extended centerline of the runway on final approach and landing.

Approach Lights - High intensity lights located along the approach path at the end of an instrument runway.  Approach lights aid the pilot in the transition from instrument flight conditions to visual conditions at the end of an instrument approach.

Arrival - The act of landing at an airport.

Arrival Procedure - A series of directions from air traffic control, using fixes and procedures, to guide an aircraft from the enroute environment to an airport for landing.

Arrival Stream - A flow of aircraft following similar arrival procedures.

Auxiliary Power Unit (APU)- A self-contained generator in aircraft producing power for ground operation and for starting the engines.

Avionics - Airborne navigation, communications, and data display equipment required for operation under specific air traffic control procedures.

B

Backblast - Low frequency noise and high velocity air generated by jet engines on takeoff.

Base Leg - A flight path at right angles to the landing runway. The base leg normally extends from the downwind leg to the intersection of the extended runway centerline.

C

Center - See ARTCC.

Commuter Airline - Operator of small aircraft (maximum size of 30 seats) performing scheduled service between two or more points.

D

Decision Height (DH) - The height at which a decision must be made during an instrument approach either to continue the approach or to execute a missed approach.

Departure - The act of an aircraft taking off from an airport.

Departure Procedure - A published IFR departure procedure describing specific criteria for climb, routing, and communications for a specific runway at an airport.

Displaced Threshold - A threshold that is located at a point on the runway other than the physical beginning. Aircraft can begin departure roll before the threshold, but cannot land before it.

Distance Measuring Equipment (DME)- Equipment (airborne and ground) used to measure, in nautical miles, the distance of an aircraft from the DME navigational aid.

Downwind Leg - A flight path parallel to the landing runway in the direction opposite the landing direction.

Duration - The length of time in seconds that a noise event lasts. Duration is usually measured in time above a specific noise threshold.

E

Enroute - The portion of a flight between departure and arrival terminal areas.

F

FAA - The Federal Aviation Administration is the agency responsible for aircraft safety, movement and control.

FAR - Federal Aviation Regulations are the rules and regulations, which govern the operation of aircraft, airways and airmen.

FAR Part 36 - A Federal Aviation Regulation defining maximum noise emissions for aircraft.

FAR Part 150 - A Federal Aviation Regulation governing noise and land use compatibility studies and programs.

FAR Part 91 - A Federal Aviation Regulation governing the phase out of Stage 1 and 2 aircraft as defined under FAR Part 36.

Fix - A geographical position determined by visual references to the surface, by reference to one or more Navaids, or by other navigational methods.

Fleet Mix - The mix of differing aircraft types operated at a particular airport or by an airline.

Flight Plan - Specific information related to the intended flight of an aircraft. A flight plan is filed with a Flight Service Station or Air Traffic Control facility.

G

General Aviation (GA) - Civil aviation excluding air carriers, commercial operators and military aircraft.

Glide Slope - Generally a 3-degree angle of approach to a runway established by means of airborne instruments during instrument approaches, or visual ground aids for the visual portion of an instrument approach and landing.

Global Positioning System (GPS) - A satellite based radio positioning, navigation, and time-transfer system.

Ground Power Unit (GPU) - A source of power, generally from the terminals, for aircraft to use while their engines are off.

Ground Effect - The excess attenuation attributed to absorption or reflection of noise by manmade or natural features on the ground surface.

Ground Track - is the seeming path an aircraft would follow on the ground if its airborne flight path were plotted on the terrain.

H

High Speed Exit Taxiway - A taxiway designed and provided with lighting or marking to define the path of aircraft traveling at high speed from the runway center to a point on the center of the taxiway.

I

Instrument Approach - A series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight conditions from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually.

Instrument Flight Rules (IFR) - Rules and regulations established by the FAA to govern flight under conditions in which flight by visual reference is not safe.

Instrument Landing System (ILS) - A precision instrument approach system which normally consists of a localizer, glide slope, outer marker, middle marker, and approach lights.

Instrument Meteorological Conditions (IMC) - Weather conditions expressed in terms of visibility, distance from clouds, and cloud ceilings during which all aircraft are required to operate using instrument flight rules.

K

Knots - A measure of speed used in aerial navigation. One knot is equal to one nautical mile per hour (1.15 knots = 1 mile).

L

Load Factor - The percentage of seats occupied in an aircraft.

Localizer - A navigational aid that consists of a directional pattern of radio waves modulated by two signals which, when receding with equal intensity, are displayed by compatible airborne equipment as an "on-course" indication, and when received in unequal intensity are displayed as an "off-course" indication.

Localizer Type Directional Aid (LDA) - A facility of comparable utility and accuracy to a localizer, but not part of a complete ILS and not aligned with the runway.

M

Middle Marker (MM) - A beacon that defines a point along the glide slope of an ILS, normally located at or near the point of decision height.

Missed Approach Procedure - A procedure used to redirect a landing aircraft back around to attempt another landing. This may be due to visual contact not established at authorized minimums or instructions from air traffic control, or for other reasons.

Multi-Lateration Sensor - Specialized device that uses three or more sensors to triangulate the Mode A,C,S, transponder responses to find the location, altitude and speed of aircraft.  It uses GPS and advanced timing system to provide flight tracking data on aircraft.

N

National Airspace System (NAS) - The common network of U.S. airspace: air navigation facilities, equipment and services, airports or landing areas, aeronautical charts, information and services, rules, regulations and procedures, technical information, manpower and material.

Nautical Mile (NM) - A measure of distance used in air and sea navigation. One nautical mile is equal to the length of one minute of latitude along the earth's equator. The nautical mile was officially set as 6076.115'.

Navaid - Navigational Aid.

Non-Directional Beacon (NDB) - Signal that can be read by pilots of aircraft with direction finding equipment. Used to determine bearing and can "home" in or track to or from the desired point.

Noise Monitoring Site (NMS) - See RMS

Non-Precision Approach Procedure - A standard instrument approach procedure in which no electronic glide slope is provided.

O

Operation - An arrival, departure or overflight of an aircraft. Every flight requires at least two operations, a take-off and landing.

Outer Marker(OM) - An ILS navigation facility located four to seven miles from the runway on the extended centerline indicating the beginning of final approach.

Overflight - Aircraft originating or terminating outside the metropolitan area that transit the airspace without landing.

P

Precision Approach Path Indicator (PAPI) - An airport lighting facility in the terminal area used under VFR conditions. It is a single row of two to four lights, radiating high intensity red or white beams to indicate whether the pilot is above or below the required runway approach path.

Precision Approach Procedure - A standard instrument approach procedure in which an electronic glide slope is provided, such as an ILS. GPS precision approaches may be provided in the future.

Precision Runway Monitoring (PRM) - A system of high-resolution monitors for air traffic controllers to use in landing aircraft on parallel runways separated by less than 4,300'.

Preferential Runway - The most desirable runway from a noise abatement perspective to be assigned whenever possible.

R

Radar Vectoring - Navigational guidance where air traffic controller issues a compass heading to a pilot.

Reliever Airport - An airport for general aviation and other aircraft which might otherwise use a larger and busier air carrier airport.

Remote Monitoring Site (RMS) - A microphone placed in a community and recorded at San Antonio Airport's Noise Monitoring Office. A network of 12 RMS?s generates data used in preparation of the airport's Noise Exposure Map.

Run-up - A procedure used to test aircraft engines after maintenance to ensure safe operation prior to returning the aircraft to service. The power settings tested range from idle to full power and may vary in duration.

Run-up Locations - Specified areas on the airfield where scheduled run-ups may occur. These locations are sited, so as to produce minimum noise impact in surrounding neighborhoods.

Runway - A long strip of land (usually paved and lighted) used by aircraft to land or to take off.

S

Sequencing Process - Procedure in which air traffic is merged into a single flow, and/or in which adequate separation is maintained between aircraft.

Simultaneous Offset Instrument Approach (SOIA) - An approach system permitting simultaneous Instrument Landing System approaches to airports having staggered but parallel runways. SOIA combines Offset ILS and regular ILS definitions.

Standard Instrument Departure (SID) - An aeronautical chart designed to expedite clearance delivery and to facilitate transition between takeoff and enroute operations.

Standard Terminal Arrival Route (STAR) - A published IFR arrival procedure describing specific criteria for descent, routing, and communications for a specific runway at an airport.

T

Taxiway - A paved strip that connects runways and terminals providing the ability to move aircraft so they will not interfere with takeoffs or landings.

Terminal Airspace - The air space that is controlled by a TRACON.

Terminal Area - A general term used to describe airspace in which approach control service or airport traffic control service is provided.

Terminal Radar Approach Control (TRACON) - An FAA air traffic control service to aircraft arriving and departing or transiting airspace controlled by the facility. TRACONs control IFR and participating VFR flights.

Threshold
1) Specified boundary of a runway
2) baseline noise level above which microphones record a noise event.

Transponder Avionics - radio transmitter that emits a selectable deceit beacon code (Mode A).  This transmitter can also supply altitude (Mode C) and aircraft registration and ownership information (Mode S)

V

Very High Frequency Omni-directional Range (VOR) - A ground based electronic navigation aid transmitting navigation signals for 360 degrees oriented from magnetic north. VOR is the historic basis for navigation in the national airspace system.

Vector - A heading issued to a pilot to provide navigational guidance by radar. Vectors are assigned verbally by FAA air traffic controllers.

Visual Approach - Wherein an aircraft on an IFR flight plan, operating in VFR conditions under the control of an air traffic facility and having an air traffic control authorization, may proceed to destination airport under VFR.

Visual Approach Slope Indicator (VASI) - An airport lighting facility in the terminal area navigation system used primarily under VFR conditions. It provides vertical visual guidance to aircraft during approach and landing, by radiating a pattern of high intensity red and white focused light beams, indicating that the aircraft is above, on, or below the glide path.

Visual Flight Rules (VFR) - Rules governing procedures for conducting flight under visual meteorological conditions, or weather conditions with a ceiling of 1,000 feet above ground level and visibility of three miles or greater. It is the pilot's responsibility to maintain visual separation, not the air traffic controller's, under VFR.

Visual Meteorological Conditions (VMC) - Weather conditions equal to or better than specifications for Visual Flight Rules (VFR).

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