Environmental Noise Measurement

8/3/2019 Environmental Noise Measurement

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Introduction

This booklet answers many of the basic questions aboutenvironmental noise criteria and environmental noise mea-surements. It gives a brief explanation of the following:See page

What noise sources do we control? .........................................2How do we describe ambient noise? ...................................... 3How do we control noise intrusions? ............... ....................... 4Why are we disturbed by property boundary noise? ........... 5How do we measure dB(A) level? ............................................ 6What is LAeq,T and ~ n ? ............................................................... 7What is Lt.? .................... ............................................................... 8How do we measure impulsive noise? .................................... 9How do we measure transient noise? ................................... 10What is Sound Exposure Level LAE? ...................................... 10How to monitor airport noise ........................................ .......... 11How to identity pure tones ...................................................... 12

See pageOctave band analysis ............................... ... .............................13Field equipment and accessories ...................... ............. ....... 14Permanent outdoor microphone stations ............ ................. 15Applications for tape recorders ............................................ .. 16How to make noise reports ...................................................... 17Legis lation for reducing noise levels today ......................... 18Planning for future noise cont rol ............................................ 19Sound Level Meters ..................................................................20Modular Sound Level Meter ................................ .................... 21Noise Level Analyzer ................................................................ 23Definitions of environmental noise cr ite ria .......................... 23


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BackgroundEnvironmental (formerly community) n o i s ~ control has twobasic objectives 1) to protect us today against noise intrusions which annoy us and disrupt our daily activities and 2)to protect us in the future against increasing noise levelsthat would further reduce the quality of our environment.Noise control activities include legislating noise emissionlimits on equipment sold and used ,in our communities, influencing the location and the construction techniques ofnew highways and buildings, legislating noise emission limits at property boundaries and regulating aircraft flight procedures.The basic unit of measurement for environmental nOise, asrecommended by ISO 1996/1, is the dB(A), but other measuring concepts, based on dB(A), are used to provide single-number criteria for describing fluctuating noise and topredict human reaction to the temporal qualities of noise.BrOel & K j ~ r ' s companion booklet Measuring Sound describes the fundamentals of acoustics and the use of SoundLevel Meters. This booklet explains the specialized criteriaand measuring techniques peculiar to environmental noise.


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Ambient NoiseAmbient noise includes all sounds present in an environment. The ambient noise level may be measured at any moment, but it will vary widely with time, e.g., with the comingand going of trucks and aircraft. Traffic noise is higher atnoon than at midnight and higher still during the morningand evening rush hours. Therefore a single dB(A) measurement says very little about ambient noise.Complete noise histories can, however, be recorded graphically, but, such charts are unwieldy and cannot be condensed to fit easily into reports, furthermore, they are diffi-cult to interpret and describe verbally. They give too muchraw dat.a, a frequent problem in dynamic measurements,which has created the need for simple noise descriptors.One suchdescriptor, recommended by ISO 1996/1, is LAeq,T'the equivalent continuous dB(A) level which has the sameenergy as the original fluctuating noise for the same givenperiod of time T, LAeq,T is an excellent criterion for studyinglong-term trends in ambient noise, However, it does notconvey any measure of environmental noise variationswhich is also an important factor when considering humanresponse. To overcome this ISO 1996/1 recommends measuring percentile levels, LAN,T, i.e. that dB(A) level which isexceeded for N% of a stated time period T. Percentile levels reveal maximum and minimum noise levels, They areused in baseline studies and in environmental impact statements to protect against new highways and new industrialplants degrading the acoustic quality of the environment.


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Noise IntrusionsNoise intrusions are characterized by their transient quality.Typical examples are motorbikes, trucks, aircraft, roaddrills and sirens. Their noise stands out far above all othersounds, and they interrupt without warning such personalactivities as sleep, study, entertainment, relaxation andconversation. Noise intrusions are especially annoying whenthey are needless, e.g. the acceleration of a noisy motorbike or a car operating with a faulty silencer.National standards frequently guarantee the availability ofnew and quieter automotive vehicles by limiting, throughlegislation, their maximum allowable noise emission. It thenbecomes the responsibility of local authorities to set maximum operating noise levels to ensure that silencer systemsare properly maintained, that replacement silencers andtyres preserve the original low noise emission and that thevehicles are not operated in a manner that produces excessive noise. Noise enforcement lim,its are usually set as themaximum allowable dB(A) level created during a vehiclepass-by.Aircraft noise emission is also controlled by national standards, so too is airport noise by regulating flight procedures to protect the maximum number of people from flyover noise, particularly during nighttime hours.


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Property Boundary NoiseProperty boundary regulations protect us against noise intrusions from neighbours, industry, commercial establishments and construction sites.Noise codes are most useful during evenings and weekendsand also during warm seasons when windows are open andthere are outdoor activities. The noise of industry may wellbe masked by traffic noise during weekdays, but in the evenings and on Sundays the same noise can be very offensive.Inappropriate noise also plays a large part in human reaction: we accept our neighbour'S motor mower on Saturdaymorning, but we are terribly upset if he operates it on Sunday afternoon. We are also offended if a discothequespreads noise throughout the neighbourhood becausedoors and windows are left open.Audible pure tones frequently cause even low-level noise tobe intolerable. We find audible pure tones in noise emittedby power transformers, turbine generators and air-movingequipment.Noise emission limits at property boundaries are usually expressed in d8(A). Some noise codes recognize the excessirritation caused by prominent pure tones in noise spectraand legislate also against them.


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Measuring d8(A)The most common measurement in environmental noise isthe dB(A) level. It can be measured with a simple SoundLevel Meter having an A-weighting filter to simulate thesubjective response of the human ear. The dB(A) level isused to report ambient noise and noise intrusions, it is alsoused in computing LAeq T and LAN T Ambient noise should be measured with the Sound LevelMeter switched to its "F" (Fast) time weighting. The rangeof ambient noise fluctuations should also be reported, e.g.54 to 58 dB(A). When measuring noise intrusions, the maximum level should be reported, to assist in this some SoundLevel Meters have a Max-Hold mode in which electroniccircuitry captures and holds on display the highest measured dB(A) level. In automotive noise enforcement, theMax-Hold feature is an advantage because it does not require an experienced operator and the data is more credible in court testimony.Graphic Level Recorders are available for recording thenoise history of transient events and of long-term environmental conditions.


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Equivalent Continuous LevelFor studying long-term trends in environmental noise, it isconvenient to use a single-number descriptor e.g. to definean entire day's noise history. The descriptor most oftenused is LAeq .T, i.e. that continuous dB(A) level which wouldhave produced the same A-weighted sound energy in thesame time T as the actual noise history.The simplest Instrument for measuring LAeq .T is an Integrating Sound Level Meter. It both monitors noise and computes LAeq,T' When reporting LAeq ,T' the period of observation T is frequently understood to be 24 hours unless otherwise stated. Quite often LAeq,T measurements are also required for intermediate periods (normally two to threewithin a 24 hour period) to determine how noise varies withtime and hence community activities.Variations on LAeq,T are Ldn and CNEL. In Ldn measurements10 dB is added to nighttime levels from 2200 to 0700 hoursto account for the fact that people are less tolerant of noiseduring their sleeping hours. CNEL has in addition 5 dB added to evening levels from 1900 to 2200 hours.In most cases the calculation of LAeq,T is based on closely'spaced samples of the instantaneous sound pressure.There is however a method known as "Tsktmsximslpegelnin the Federal Republic of Germany which bases the calculation on the maximum A-weighted sound levels ocurring insuccessive intervals of 3s or 5s. These levels are designated LTm .3 and LTm .5 respectively and can be measured directlyon special Sound Level Meters.A Noise Level Analyzer Is available. for measuring and reporting all the descriptors mentioned above as well as thestatistical data described in the following.


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Statistical Amplitude DistributionHuman response depends greatly upon the range withwhich noise levels vary in a given environment. For a givenLAeq ,T we would find a higher, more steady, level more toler-able than a lower background level with frequent noise in-trusions.The statistical, descriptor for the variation of noise is LAN,T'i.e. that dB(A) level exceeded for N% of the time T. Forexample, LA90 .T is used to estimate the residual backgroundnoise level in the environment whereas LA1 T or LA10 ,T isused to estimate maximum levels. The complete range ofLAN,T levels, known as the cumulative distribution, can bemeasured directly using specially designed Sound LevelMeters or Noise Level Analyzers; these instruments are alsocapable of measuring the corresponding probability distri-bution of the noise levels.LAN,T and LAeq,T values are used to evaluate existing noiseenvironments In order to control the impact of predictablenoise sources such as highways and construction sites.Two environmental noise ratings make use of the descrip-tors mentioned here and on the previous page. The first iscalled the Traffic Noise Index, TN/, and the second is calledthe Noise Pollution Level, NPL. Both ratings can be mea-sured directly using a Noise Level Analyzer.

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Impulsive NoiseImpulsive sounds are greater contributors to human annoy-ance than slower transient sounds even when both producethe same reading on a Sound Level Meter set to its "F"time weighting. The greater annoyance is partly due to theirstartling effect as well as to the fact that the human earresponds faster than the circuitry in the the Sound LevelMeter and therefore perceives a higher "reading" beforethe sound begins to decay.Some national standards for measuring environmentalnoise require the use of Sound Level Meters also equippedwith an "I" (Impulse) time weighting to evaluate sourcessuch as pile drivers, forge hammers and punch presses allof which emit impulsive noise. In the "I" mode the rise timeof the circuitry is about 4 times faster than in the "F" mode.This simulates the response time of the human ear. Thecircuitry also incorporates a hold feature which capturesand holds the maximum displayed level for as long as re-qired by the operator.In the Federal Republic of Germany LTm.3 and LTm.5 are quiteoften measured using the "I" time weighting.

" I " Impulse Characteristic" ............. - ..,.,..., A~ : ! : : I = I = I = f : ~ ( : : ' \ : ~ ~ s : c : = = = : : ! : : > Time .


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Transient NoiseThe most commonly encountered transient noise levelscome from vehicle pass-bys and aircraft flyovers. In nearlyall cases the maximum d8(A)* level of a transient needs tobe measured. However, this does not say anything aboutthe acoustic energy of the transient which is an importantsubjective consideration. To overcome this ISO 1996/1 recommends measuring the Sound Exposure Level (SEL), LAe,which is a measure of the acoustic energy in a transientnoise. SEL is defined as that level which, lasting for onesecond, has the same acoustic energy as the transientnoise.The SELs of a series of unrelated transient noise eventscan be compared with each other because they are a measure of acoustic energy: furthermore they can be combinedon an energy basis like normal sound levels and then quitesimply be converted to an LAeq,T value acting over anyspecified time T.SEL can be measured directly on certain Integrating SoundLevel Meters and on a Noise Level Analyzer. The latter canalso be used to monitor environmental noise levels andprint out periodic reports, at preselected time intervals, ofvarious environmental noise ratings

Most flyover noise Is measured In dB(A). However, some regulations require the use of dB(O). The O-welghtlng filter amplifies spectral components between 1 and 10 kHz -o account for the presence of annoyingpure tones In je t noise spectra.

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Airport Noise MonitoringWith the multitude of flight operations around even medi um-sized airports, it is not possible to answer complaintsor reprimand violators without documentin.9 the time andlevel of excessive flyover noise. This necessitates a systemfor monitoring airport noise automatically.The first requirement is one or more outdoor noise monitoring terminals (NMTs) each equipped with a microphone unitwhich can operate year-round under all weather conditions.The NMTs must transmit noise data to a computer basedcentral processing station at the airport and also be capable of remote calibration.The central processing station must, for each NMT, detectall noise levels which exceed certain thresholds and legallimits, identify the corresponding NMTs, report all dataabove the threshold and report the time of day of the maximum dB(A) level of the event (normally with "S" timeweighting). Quite often the system will also be required tocalculate and report the SEL and the LAeq T of the event aswell as overall statistics, i.e. selected values of LANT foreach NMT.Airport noise monitoring enables airport authorities to identify and caution offending pilots and airlines. Equally important, airport noise monitoring can provide well-documenteddata which airport authorities can use to discuss presentconditions and long-term trends with local residents as partof their p r o ~ r a m to improve community relations.


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Identifying Pure TonesWhen pure tones are present in a nOise spectrum, the dB(A)level is not adequate to predict human response becausepure tones, especially at high frequencies, are much 'moreannoying than a broad band noise of the same level.Pure tones are often present in sounds emitted by industrialequipment such as blowers, electrical generators, highspeed machinery and power transformers. Although ourears can detect prominent pure tones in a noise spectrum,it is not possible to confirm or quantify them with a SoundLevel Meter. alone; One-Third Octave Band Filters must beused.Some environmental noise regulations limit the emission ofpure tones at property boundaries. One noise code confirms the presence of a pure tone if its one-third octaveband level exceeds the arithmetic average of the two adjacent bands by (1) 15 dB for bands centered on 25 Hz to125Hz, (2) 8 dB for bands centered on 160 Hz to 400 Hz and(3) 5 dB for bands centered on 500 Hz to 10kHz. If a puretone is confirmed, then its one-third octave band level mustbe at least 10 dB below the specified octave band limit forbroad band noise.Field measurements are made with a Sound Level MeterFitted with a set of One-Third Octave Band Filters.


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Octave Band AnalysisCommunity authorities must often work with local industryin evaluating various proposals for noise control, e.g. will aproposed noise barrier or enclosure be effective in shielding nearby residents? They must also work with buildingcontractors to ensure that apartments are insulated fromoutdoor noise as well as from each other.Octave band frequency analysis is required to investigate 'anoise source and to predict the necessary insulation characteristics of noise barriers and enclosures and to measurenoise reduction between common walls of adjacent apartments. Octave band analysis is also invaluable when an existing noise control system must be redesigned becausethis will assist in defining the minimum modifications necessary to enable the system to meet required specifications.Octave band analysis is performed using a Sound Level ,Meter fitted with a set of Octave Band Filters.In cases where noise levels are steady, Noise Rating curves(NR-curves) are sometimes used with octave band data todetermine the annoyance level of environmental noise. Inkeeping with the characteristics of human hearing, NRcurves put more weight on higher frequencies. When theoctave band spectrogram of a particular noise is superimposed on these curves, the rating number for the noise as awhole Is obtained by noting where the spectrogram penetrates the highest numbered curve; in the example shownthe Noise Rating is 80 .

Octave Band Pressure Level

31 .5 83 125250500 1 k 2 4 8Frequency Hz


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Field Equipment and AccessoriesWhen measuring noise outdoors, precautions should betaken against wind which can create extraneous noise as Itpasses over the microphone. A suitable windscreen may befitted over the microphone to prevent this. Windscreensshould be used whenever there is a noticeable breeze orwhen standard procedures require them. They also shieldthe microphone from dust and dirt.When outdoor measurements have to be made for a day ormore under humid conditions then use should be made ofback vented microphones fitted with a dehumidifier acces-sory. If measurements are to be made in the rain, then arain cover should be mounted on the microphone. The elec-tronics, which in most cases will be a Noise Level Analyzer,must be placed in a suitable weatherproof shelter and con-nected to the microphone assembly via an extension cable.The effects of other environmental conditions can be con-sidered negligible.An acoustics calibrator is required to verify that the chainof sound measuring instrumentation is measuring properlyand accurately. Calibrations are usually performed beforeand after each day's measurements or whenever standardprocedures require them.


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Permanent Outdoor Microphone UnitsA permanent Outdoor Microphone Unit must withstand therigours of outdoor conditions for long periods withoutmaintenance. It must be free of the effects of temperature,humidity, snow, rain and air pollution; and it must deterbirds from perching on, and soiling, the microphone assembly.The various components and protective devices of a permanent outdoor microphone unit must be designed as asystem to preserve the frequency .response and omnidirectional characteristics of a Sound Level Meter .The built-in electronics must be capable of driving longlines, if necessary, back to a data acquisition station. Theymust also have a built-in calibrator which can be remotelyactuated from the data acquisition station.


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Tape RecordersThe role of Tape Recorders has changed over the years.They are seldom necessary anymore for temporary signalstorage because analysis equipment such as Noise LevelAnalyzers, Modular Sound Level Meters, Integrating SoundLevel Meters, Graphic Level Recorders and Octave andOne-Third Octave Band Filters are all available as compactbattery-operated instruments for direct measurements inthe field .Tape Recorders are essential, however, when you wish toanalyze data exhaustively by various methods and whenyou wish to store today's data for evaluation to possiblenew criteria in the future . Tape Recorders are also valuablein court testimony for demonstrating annoying sounds.Portable Instrumentation Tape Recorders usually have twoor four channels and use Sound Level Meters for their inputand operate from internal batteries.


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The Measurement ReportEnvironmental noise data is often used in legal proceedings. In fact, you should always collect data with the assumption that it may some day have to be examined incourt testimony. Therefore it is of utmost importance thatthe conditions of the measurement be carefully documentedin a formal measurement report.The following information should be reported.1.Model, manufacturer and serial number of each instrument used.2. Date of last laboratory calibration.3. Statement of on-site calibration verification before andafter each series of measurements.4. Time and frequency weighting networks used.5. location of mlcophone and description of area.6. Time and date of measurement.7. Weather conditions.8. Residual noise level, If measuring noise intrusions.9. Names of observers.

10. Description of the measurement and all instrument read-Ings.A comprehensive and carefully documented formal reporthas the best chance in any legal proceedings or in anynoise control negotiations settled out of court.

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Reducing Noise Levels TodayAn early step in any noise control program is to have atechnically qualified person draft a noise code that followssuccessful codes in similar sized communities and can beenforced within the allowable budget of manpower andmoney. The noise limits must not be higher than prevailinglocal or national codes. And the code should provide forsuccessively lower limits in future years as technologicalimprovements and other noise control forces are reflectedin quieter products. Lastly, there should be adequate plansfor training personnel who will be Involved in enforcing thenoise code.The code should cover vehicle noise, industrial and residen-tial boundary noise and any other noise considered to be apublic nuisance. The noise code must define the measure-ment location with respect to the sound source, the maxi-mum allowable noise levels and the Sound Level Meter timeand frequency weightings. If the community has an airport,restrictions should be set on sideline (lateral) and flyovernoise.The proposed legislation should be accompanied by favor-able publicity so the public will view It as a means to im-prove the community and not as an interference with theirfreedom of action.


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Planning for the FutureLand-use planning is a major factor in preventing futurenoise problems. For example, land-use planning can prevent the building of single-family homes in areas where anairport is expected to expand in the future or where a newmotorway is planned.Building codes can require adequate acoustic insulation toshield apartment dwellers from their neighbours and fromhighway and aircraft noise.Boundary noise limits and zoning controls give communitiesthe authority -to control the noise impact of new industryand commercial establishments.At higher levels of government, noise emission limits onnew vehicles and other powered equipment are a great helpin lowering ambient noise levels in the future.Finally, a well-organized program for periodic environmentalnoise studies will prove whether or not the battle againstnoise is successful.


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Sound Level MetersMost environmental noise investigations begin with measurements using a Sound Level Meter. The scope of themeasurements will usually decide what type of instrumentshould be used and with which standards it must comply. Awide variety of Sound Level Meters is available coveringrequirements for simple noise surveys, where only dB(A)levels need to be measured, to elaborate measurementswhere Equivalent Continuous Levels (LAeqr ), Sound Exposure Levels (SEL), maximum, minimum, impulse and peaklevels are required. Furthermore, there are Sound Level Meters available for national standards wh ich differ from international standards.Octave and One-Third Octave Band Filter sets are availablefor the three more elaborate instruments shown here in thebottom row.

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Modular Sound Level MeterThe Sound Level Meters discussed on the previous pagecan be considered as dedicated instruments with unchange-able capabilities. A Modular Sound Level Meter, on the oth-er hand, can undergo electronic "firmware" changes to suitdifferent measurement requirements. This is done by select-ing an appropiate application module having a particularset of instructions which can be loaded into the firmware ofthe instrument. A corresponding change of front panel isalso made possible.Three appropiate application modules, designated 1, 2, and3, provide capabilities which convert the instrument to thefollowing:1. Precision Integrating Sound Level Meter capable ofmeasuring LAeqT and SEL2. Statistical Analyzer capable of measuring LAN T(1 N 99) and probability distribution3. "Taktmaximal" Precision Integrating Sound Level Metercapable of measuring LAeqT and SEL according to Ger-man standards

In addition the application modules also provide the instru-ment with other useful capabilities usually found on elabo-rate dedicated Sound Level Meters.


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Noise Level AnalyzerIt is widely recognised that the annoying element of noisedepends not only on frequency content (A-weighting) andenergy content (LAeq T SEL) but also on its fluctuating nature. Statistical analyses are therefore necessary to determine noise levels which are exceeded for given percentages of the measurement duration. Le . percentile levelsLANTThe Noise Level Analyzer is a portable instrument designedfor the measurement and analysis of environmental noise.airport noise. traffic noise and any other noise where statistical analysis is useful. The instrument is also capable ofmeasuring Equivalent Continuous Level (LAeq T). Sound Exposure Level (SEL). NOise Pollution Level (NPL). TrafficNoise Index (TN/). Day-Night Average Sound Level (ONL)and Community Noise Equivalent Level (CNEL) over required intervals and produce print-outs containing all results and plots of statistical data.The instrument performs noise measurements according toIEC 651 . ANSI 1.4 and German UTaktmax;mar standards.

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Glossary of termsThe following terms are used for describing environmentalnoise.A-weighted Sound Pressure Level, L"A: A measure of noiselevels, in dB(A), using the A-(frequency) weighted network.A-weighted sound pressure levels correlate well with subjective loudness.Equivalent Continuous A-Weighted Sound Pressure Level,LAeq,T: That constant level in dB(A) which, lasting for as longas a given A-weighted noise event, Le. for a period of timeT, has the same amount of acoustic energy as the givenevent.Sound Exposure Level (SEL), LAE : That constant level indB(A) which, lasting for one second, has the same amountof acoustic energy as a given A-weighted noise event.Day-Night Average Sound Level (DNL), Ldn : A 24-hourequivalent continuous level in dB(A) where 10 dB is addedto nighttime noise levels from 2200 hours to 0700 hours.Community Noise Equivalent Level (CNEL), Lden : A 24-hourequivalent continuous level in dB(A) where 5 dB is added toevening noise levels from 1900 hours to 2200 hours and10 dB is added to nighttime noise levels from 2200 hours to0700 hours.Percentile Level, LAN,T: That noise level in dB(A) exceededfor N% of the measurement time T.

Noise Pollution Level (NPL), LNP: A variation on LAeq.rwhich accounts for short term variability in noise level. LNPis defined as;LNP = LAeq ,T + 2,56 (JWhere er is the standard deviation of the dB(A) levels. For agaussian distribution of dB(A) levels the term 2,56 er can bereplaced by (L A10 ,T - LA90 ,T)Traffic Noise Index, TNI: Introduced as a descriptor of roadtraffic noise, it is defined as follows :TNt = 4 (L A10 ,T - LA90 ,T) + LA90 ,T - 30Perceived Noise Level, LpN : A complex rating based onone-third octave band data used to certify aircraft types forflyover noise. An approximation is given by adding13 ( 3) dB to the measured A-weighted noise level.Effective Perceived Noise Level, LEPN : This is the result ofapplying tone and duration corections to the PerceivedNoise Level based on one-third octave band data.Noise Exposure Fo'recast, NEF: A complex criteria for predicting future noise impact of airports. The computationconsiders the effective perceived noise level of each typeof aircraft, flight profile, number of flights, time of day, etc.Generally used in plots of equal NEF contours around airports for zoning control.

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We hope this booklet has served as an informative introduction toenvironmental noise measurements. If you have other questionsabout measurement techniques, instrumentation or standards used.in your own country, contact your local BrOel & Kjcer representativeor contact us directly.BrOeJ & Kjatr2850 NatrumDenmark


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Environmental Noise Measurement

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