UNCERTAINTY OF AIRCRAFT NOISE MEASUREMENTS: EVALUATION FOR AN AIRCRAFT NOISE MONITORING NETWORK

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1 UNCERTAINTY OF AIRCRAFT NOISE MEASUREMENTS: EVALUATION FOR AN AIRCRAFT NOISE MONITORING NETWORK Chrstophe Rosn Acostcs Department, Aéroports de Pars 03 Aérogare Sd CS90055, Orly aérogare cedex, France ABSTRACT Generally, an arcraft nose montorng network comples wth Class templates from the standard IEC 667- abot specfcatons on sond level meters. In addton, some specfc standards sch as ISO 0906 or NF EN 3-90 provde ways to evalate ncertanty. Aéroports de Pars operates 50 nose montorng systems on several arports. Specfcally at Pars Charles de Galle arport, the nose management s based on a nose bdget ndex calclated from measred maxmm nose level of all arcraft movements on the arport. In ths context, t s mportant to know exactly the nstrments metrology n order to exceed the standards templates and to assess ts own ncertanty. To do ths, t s necessary to know all the parameters whch can nflence the measre. The objectve s to mprove the robstness of measrements. Ths artcle lsts all the nflence factors and presents an ncertanty calclaton sng specfc vales, standards vales or docmented vales. Ths approach s consstent wth the French standard draft abot the evalaton of the ncertantes of envronmental nose. INTRODUCTION Generally, the ncertanty n acostcs that goes along wth a measre s set to 3dB. Laboratores wshng to be accredted accordng to the referental ISO 705 mst comply wth the specfc reqrements and evalate the ncertantes n measrement. However, there s no standard of reference or gde to assess or calclate ncertanty n envronmental acostcs. Informatons on ncertantes are ncreasngly reqested by cstomers, especally by resdents n the case of nose montorng arond arports. Frthermore, n accordance wth French reglatons, the IGMP ndex (measred and calclated from the weghted sond levels LA,eq,s,max of each arcraft overflght) specfc to Pars Charles de Galle arport s calclated each year. Ths global ndex s sed to pt a pper lmt to the nmber of movements n the arport. The lmt s set by the average of three years consdered nosy. For several years, the Laboratory try to mprove the robstness of the calclaton (ncreasng the operang rate, mprovng the detecton of flyng arcraft, mproved detecton of arcraft nose by a method ncldng ado mltvaldaton arcraft recognton, ncreasng the correlaton rate...). It s now necessary to know the ncertanty srrondng the vale of ths ndex. Chrstophe.rosn@adp.fr

2 . GENERALITIES ABOUT THE UNCERTAINTIES The ncertanty calclaton s done wth the followng standards: ISO/IEC Gde 98-3:008. Uncertanty of measrement - Part 3: Gde to the expresson of ncertanty n measrement (GUM:995). ISO/IEC 667-:000: Electroacostcs Sond level meters Part Specfcatons ISO 0906:009: Acostcs Unattended montorng of arcraft sond n the vcnty of arports.. Measrand The prpose of the measre s not to measre nose level at sorce bt to measre the nose level receved on the grond by local resdents. It s therefore ndependent of the propagaton condtons. As a reslt, the physcal qantty beng measred, also called measrand s the sond pressre level of the fne layer of ar n contact wth the membrane of the mcrophone, expressed n dba.. Ishkawa dagram Ths method allocates the ncertantes of the varos parameters nto fve famles accordng to ther sorces: Method of measrement operators (Man) measrng devce (Machne) Envronnement prodct to be measred (Materal). It can be otlned as an Ishkawa dagram, also called "fshbone dagram". Ths dagram s shown n Fgre. Method Man Machne Uncertanty of measrement Envronment Materal Fgre - Ishkawa dagram Frst, we classfy the dfferent sorces of ncertanty n each category of famles (Table ). Method Man Machne Envronment Materal calbraton, lnearty temperatre calbraton wth fttng drft of eqpment hmdty procedres A-weghted flter resolton of eqpment electromagnetc capactaton tolerance freqency response felds sensblty bandwth pressre Table - Orgn of ncertanty for the elementary vale measrement the sond

3 .3 Prncple of ncertanty calclaton To perform the calclaton of the ncertanty assocated wth a measrement, t s necessary to defne the standard ncertanty of each nflence affectng a measre to calclate the overall ncertanty from the standard ncertantes, and defne the ncertanty expanded wth a confdence ndex. In the prevos secton, the varables that affect the measrement of elementary levels were lsted. The measrand s the LAeqs level. It s calclated from two LAeq,0.5s (elementary vale measred by a montorng nose staton). Consder the followng hypothess : L Aeq,0.5s =μ + e + e + wth μ the rght vale, not known and e, e errors arond the tre vale. It s also assmed that the average vale of possble errors s centered on 0 (no bas correcton of the measred vale). In ths scheme, error s noted e : δ 0 e Standard ncertanty s defned as : = δ k Wth k vale whch depend on the probablty dstrbton. For a rectanglar probablty dstrbton, standard ncertanty s : 3 In a dfferent scheme, the standard ncertanty de to +e or e error calclated wth a rectanglar probablty s : -e 0 +e 3 After calclatng the standard ncertantes, the combned standard ncertanty of the elementary vale s sed. Combned standard ncertanty s defned as : = =

4 Expanded ncertanty s calclated wth the k coverage factor,to defne a confdence. k= for a 95% confdence k=.58 for a 99% confdence k=3 for a 99.7% confdence U = k. CALCULATION OF UNCERTAINTY. Combnaton from LAeq,0.5s to LAeq,s The evalaton of the ncertanty concerns the elementary vale second stored n database. Eqpment delvers a measre every 0.5 second. Ths elementary vale reslts of a calclaton made from two vales LAeq,0.5s. The nt vale stored n a database, and commncated to the cstomer and wth all ndcators are constrcted s the LAeq,s. It s recalclated by the central server from LAeq0, 5s measred onste by each nose montorng eqpment. The translaton of LAeq,0.5second to Laeq,second occrs n two consectve energetcally smmng vales LAeq,0.5second. W and W are the energy assocated wth the two LAeq,0.5second and Laeq,second.The mathematcal expresson of LAeqsecond s as follows: W W LAeq s 0 log (4) wth : W P P 0 ; W P P 0 (5) and P 0 the reference pressre level : P 0 =.0-5 Pa. 0 W 0 and L Aeq 0.5s The expresson (4) can be smplfed, whch leads to: L 0 logw W 3 0 W 0 (6) L Aeq 0.5s Aeq s (7) Calclaton of ncertanty n L aeqs As we saw above, the LAeq,0.5s are sed to calclate LAeq,s, whch s the elementary vale sed to calclate varos ndcators of arcraft nose. Also, ths calclaton leads to ncertanty abot the LAeq,s vale. Ths ncertanty s determned from the formla (7) whch allow to calclate the LAeq,s. The varances combnaton law (see formla (3)) provdes the analytcal expresson: Aeqs Aeqs L W W Aeqs L L W W (8) 4

5 Dervatves are L Aeqs W L L et W Aeqs W L Aeqs W. The calclaton s detaled only for one dervatve: 0 log W W Aeqs 0 W ln 0 W W 3 (9) Wth the second dervatve, the calclaton s the same: LAeq s 0 W ln 0 W W (0) It remans to calclate the standard ncertanty and W. For ease of readng, W and W are wrtten W wth =;. The varances combnaton law carres ot to the followng mathematcal expresson: W P W () P P0 P0 W P P Now, expressons et P have ncertantes becase they are based on L aeq0.5s (formla 0 P0 (6)). The combned ncertanty assocated wth LAeq,0.5s takes part n the calclaton of the combned ncertanty on LAeq,s. Ths acton reslted n: W L ) W ( Aeq 0. 5s () L Aeq 0.5s After several steps of calclaton, we fnd W ln 0 0 L Aeq 0.5s 0 0 L Aeq 0.5s However, for one gven type of staton, ncertantes L aeq. 5s L L. Aeq 0.5s Aeq 0.5s (3). 0 are eqal. So we can wrte 5

6 It s now possble to gve the fnal expresson of the combned ncertanty of the vale of each Laeqs from expressons (8), (9), (0) and (3). After smplfcatons, the fnal reslt s wrtten as below : L Aeq 0.5s Aeq 0.5s 5 5 L 0 0 L Aeqs L Aeq 0..5s (4) W W Ths expresson shows that the ncertanty LAeq,s depends on the dfference of 0.5second consectve levels. For vales of LAeq,0.5s dentcal or very smlar, the ncertanty s less than LAeq,s on LAeq,0.5s. However, for vales of LAeq,0.5s havng a very bg dfference, especally the extreme vales of the dynamc range of statons (30-40 db), we see that the ncertanty Laeqs s eqal to the ncertanty LAeq,0.5s. L L Aeqs Aeq 0. 5s To cover the fll dynamc range, the ncertanty s LAeqs s: In the case of an ncrease: L L Aeq s Aeq 0. 5s. Spectrm of arcraft nose The nose sorce measred by the nose montorng statons s the arcraft flyng-over. Therefore, the spectrm measrement s lmted to an average spectrm of arcraft nose. Bandwdth s 50Hz - 4kHz. It s defned by observng the nflence of each one-thrd octave band on the reconstrcton of the LAeq n dba. 3. INFLUENCING FACTORS In the followng docment, the dfferent ncertanty factors are classfed by famly: calbraton otdoor wndscreen protecton wth antbrd spkes pstonphone calbrator measrement system: analyzer and mcrophone In each famly, nflencng factors are descrbed precsely. 3. Calbraton Ths tem otlnes the varos errors made drng the calbraton phase adjstment wth a nose montorng staton. 3.. Tolerance of calbraton Ths s the permssble devaton between the theoretcal vale and the vale read drng calbraton. The nflence qantty s 0. dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =5.8E Calbraton drft Ths s the extent of the vale allowed for the calbraton drft. Ths ensres that the vales measred by atomatc checks performed every sx hors (nserton voltage and actator) are wthn the template +/-0.5 dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =.9E-0 6

7 3.3 Otdoor wndscreen protecton wth antbrd spkes For these tems of ncertanty, the manfactrer has not provded any nformaton. Experments n an anechoc room of a Natonal Metrology Laboratory were condcted to determne the nflence of the otdoor wndscreen protecton, wth or wthot waterloggng Otdoor wndscreen protecton dry (50Hz-4kHz) It s the nflence of otdoor wndscreen protecton dry (wthot water nsde) on the measre. In bandwdth 50Hz-4kHz, the nflence qantty s 0. dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =.E Otdoor wndscreen protecton wet (50Hz-4kHz) Ths s the nflence of the water nsde the foam of the otdoor wndscreen protecton on the measre. The maxmm water mpregnaton sed corresponds to an eqvalent of 40mm of ran, wth mm/mn rates and 3mm/mn. In bandwdth 50Hz-4kHz, the nflence qantty s 0.5 dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =.9E Pstonphone calbrator The vales sed are from the docmentaton provded by the hardware manfactrer or the vales n reports calbraton of eqpment made n the Natonal Metrology Laboratory Tolerance n reference condtons (calbraton n natonal laboratory) The vale comes from the calbraton report of the pstonphone calbrator. The vale s an expanded ncertanty, the standard ncertanty s therefore half the extended ncertanty vale. =3.5E Drft of pstonphone calbrator Before each calbraton, the nose level generated by the pstonphone s compared to the nose of another pstonphone. The tolerance for ths comparson s 0. dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =5.8E Tolerance on operatng condtons wth the mcrophone Ths s the vale gven n the manfactrer docment based on AtoCorrect temperatre and hmdty accordng to the mcrophone sed. Tolerance s 0. dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =5.8E Measrement system: analyzer and mcrophone 3.5. Resolton The sond level measred LAeq0.5s s gven wth /0th of db (A). The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =5.8E-0 7

8 3.5. Freqency weghtng (A-weghted flter 50Hz-4kHz) The vales come from calbratons certfcates performed by a natonal metrology laboratory for all measrng eqpments. For each freqency band, the mnmm and maxmm devatons are selected. From a spectrm of plane type, the calclaton of overall levels LAeq,max and LAeq,mn s calclated n dba. The nflence qantty s the dfference between the mnmm vale and maxmm vale, t s 0.8 dba. The probablty dstrbton for ths tem s a rectangle probablty dstrbton. =.3E Level lnearty of the measrement system Accordng to IEC 667-, the maxmm vale of the expanded ncertanty s 0.3 db. Ths fxed vale s chosen as the varable nflence. The vale s an expanded ncertanty, the standard ncertanty s therefore half the extended vale. =.5E Drectvty of mcrophone The nose measrement statons are nstalled perpendclar to the overflghts. Accordng to ISO 0906:009, the angle θ s zero or well below 30. Annex B.3. of the ISO 0906 standard specfes that n ths case, the ncertanty s db to the vales of the IEC The probablty dstrbton for ths poston s a rectangle law. =.9E Response to brsts of 4-kHz tones Accordng to IEC 667-, the maxmm vale of the expanded ncertanty s 0.3 db. Ths fxed vale s chosen as the varable nflence. The vale s an expanded ncertanty, the standard ncertanty s therefore half the extended vale. =.5E Temperatre Accordng to IEC 667-, the maxmm vale of the expanded ncertanty s 0.3 db for operatng temperatres between -0 C and +50 C. Ths fxed vale s chosen as the varable nflence. The vale s an expanded ncertanty; the standard ncertanty s therefore half the extended vale. =.5E Hmdty Accordng to IEC 667-, the maxmm vale of the expanded ncertanty s 0.3 db for a relatve hmdty between 5 and 90%. Ths fxed vale s chosen as the varable nflence. The vale s an expanded ncertanty, the standard ncertanty s therefore half the extended vale. =.5E Statc ar pressre Accordng to IEC 667-, the maxmm vale of the expanded ncertanty s 0.3 db for a statc ar pressre between 850 and 080 hpa. Ths fxed vale s chosen as the varable nflence. The vale s an expanded ncertanty, the standard ncertanty s therefore half the extended vale. =.5E Cable The cable that connects the mcrophone to analyzer s consdered as new n each nstallaton ste. The factor of ncertanty assocated wth the cable s consdered neglgble compared wth other ncertantes tems. 8

9 4. SUMMARY TABLE OF UNCERTAINTY CALCULATION The followng table smmarzes all the nflencng factors lsted above: INFLUENCING FACTORS nflence qantty Standard ncertanty Varance ² Calbraton Tolerance of calbraton E Calbraton drft 0.5.9E Otdoor wndscreen protecton wth antbrd spkes Otdoor wndscreen protecton dry (50Hz-4kHz) 0..E Otdoor wndscreen protecton wet (50Hz-4kHz) 0.5.9E Pstonphone calbrator Tolerance n reference condtons (calbraton n natonal laboratory) E Drft of pstonphone calbrator E Tolerance on operatng condtons wth the mcrophone E Measrement system: analyzer and mcrophone Resolton E Freqency weghtng (A-weghted flter 50Hz-4kHz) 0.8.3E Level lnearty of the measrement system 0.3.5E Drectvty of mcrophone.9e Response to brsts of 4-kHz tones 0.3.5E Temperatre 0.3.5E Hmdty 0.3.5E Statc ar pressre 0.3.5E Cable 0 0.0E Table - Smmary table of ncertanty calclaton The combned standard ncertanty s 0.666dBA. The expanded ncertanty U wth confdence level of 95% (k = ) s.3dba. The expanded ncertanty U wth confdence level of 99% (k =,58) s.7dba. 9

10 CONCLUSION In ths paper, the ncertanty calclaton s performed for the elementary vales measred by the arcraft nose montorng network. The ncertanty assessment presented here s an overall ncertanty that can be assocated wth all measrements for the entre stock of eqpment operated by Aéroports de Pars laboratory, regardless of the measrement eqpment sed. For each nflencng factor, the ncertanty s sometmes determned from the vales of the calbraton certfcates measred by a natonal metrology laboratory, or sometmes determned from fxed vales of the normatve docments, or sometmes qantfed from vales provded by the hardware manfactrer. The se of vales from the calbraton certfcates can be sed to redce the standard ncertantes, and therefore n some cases can redce the overall ncertanty attached to a measre. Work mst be contned to lead to an assessment of the ncertantes assocated wth nose ndcators sed n envronmental acostcs. REFERENCES [] Internatonal Standard ISO/IEC 667-:00: Electroacostcs - Sond level meters - Part : Specfcatons. [] Eropean Standard IEC 6094:003: Electroacostcs Sond calbrators, [3] Internatonal Standard ISO/IEC Gde 98-3:008. Uncertanty of measrement -- Part 3: Gde to the expresson of ncertanty n measrement (GUM:995). [4] Internatonal Standard ISO 0906:009: Acostcs Unattended montorng of arcraft sond n the vcnty of arports. [5] French Standard NF S 3-90 NF S 3-90:008: Acostqe - Caractérsaton d brt d'aéronefs perçs dans l'envronnement, AFNOR [6] French Standard NF S 3-00:008: Acostqe - Caractérsaton et mesrage des brts de l'envronnement - Méthode partclère de mesrage, AFNOR [7] Otdoor Mcrophone System Type 4AM, Instrcton Manal. Docment avalable onlne on the GRAS ste, accessed : jne_00.pdf [8] Otdoor Mcrophone System Type 4AM on the GRAS ste, accessed : [9] Pstonphone Type 4AP, Instrcton Manal. Docment avalable onlne on the GRAS ste, accessed : [0] Pstonphone Type 4AP Docment avalable onlne on the GRAS ste, accessed : [] French Standard Pr S 3-5. Gde por l évalaton des ncerttdes de mesrage en acostqe de l envronnement, Pars, French Assocaton for Standardzaton, Pars, France;. to be pblshed n 03. 0