1、3 1176 00167 9381NASA Technical Memorandum 81968NASA-TM-81968 198100153271LATERALATTENUATIONOFHIGH-BY-PASSRATIOENGINEDAIRCRAFTNOISEWILLIAML,WILLSHIRE,JR,APRIL1981 _O_l _I:,_ENationalAeronautics and t.i_:_;b“,_,“,_,_;_.:?,Space Administration _,_4_,_i,_ ,/!1_Langley ResearchCenterHampton,Virginia2366
2、5Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SUMMARY A flight experiment was conducted to investigate the lateral attenuationof high-by-pass ratioengined airplanes. A B-747 was flown at low altitudeso over the ends of two microphone arrays. One a
3、rray covering a lateral distanceof 1600 m Consisted of 14 m_crophones positioned over grass. The second arraycovered a lateral distance of 1200 m and consisted of 6 microphones positionedover a concrete runway. Sixteen runs were flown at altitudes ranging from30 to 960 m.The acoustic information rec
4、orded in the field has been reduced toone-third-octave band spectral time histories and synchronized with trackingand weather information. Lateral attenuation as a functionof elevation anglehas been calculated in overall, A-weighted, tone-corrected perceived noiselevel, and effective perceived noise
5、 level units.The B-747 results are compared with similar results for a turbojet-powered T-38 airplane and the SAE-recommendedlateral attenuation predictionprocedure. Less lateral attenuation was measured for the B-747 than for theT-38. The B-747 lateral attenuation values also fell below the SAE cur
6、ve.The B-747 source spectra have considerable energy in the low and high fre-quencies. The low frequency content was not strongly affected by groundeffects or atmospheric absorption and became dominant in the measured spectra.The amount of lateral attenuation (in terms of integrated metrics) of apar
7、ticular noise source appears to depend strongly on the spectral contentof the noise source and the frequency dependence of the ground effects.INTRODUCTIONLateral attenuation of aircraft noise is a significant factor in calcu-lating the community noise exposure around airports. The FAA, other interna
8、-tional air transportation authorities, and community planners require soundtechnical methods upon which to base noise contour calculations. Therefore,the SAE A-21 Aircraft Noise Committee in the past year collected all theexisting available data on lateral attenuation of aircraft to develop aninter
9、im method of lateral attenuation prediction which has been publishedas an Aerospace Information Report (ref. I). One of the key sets of datawas the T-38 experiment (refs. 2 and 3) which contains over 400 lateralattenuation measurements. At the same time, the committee recognized thatin the available
10、 lateral attenuation data base, very few data points wereassociated with high-by-pass-ratio engines. The B-747 lateral attenuationexperiment was undertaken to fill this gap in the available lateral attenuationdata base.The purpose of this report is to present the lateral attenuation resultsof the B-
11、747 experiment in terms of integrated metrics. The experiment,N1- ZYg6ZProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-including data reduction, is discussed in the next section of this report,followed by sections on the data analysis and results. Th
12、e final sectionsof the paper include discussions of the results and concluding remarks.EXPERIMENTTest Site and Aircraft The flight experiment was conducted on November 2, 1980, at NASAWallopsFlight Center, Wallops Island, Virginia. The test aircraft was an AmericanAirlines Boeing 747-123 (tail numbe
13、r N9666) with JT9D-3A engines. The engineshad fixed inlet lips and no splitters. The test airplane was flown ataltitudes of 30, 60, 120, 240, 480, and 960 m over the ends of two microphonearrays. One array covering a lateral distance of 1600 m consisted of 14microphones positioned over grass. The se
14、cond array covered a lateraldistance of 1200 m and consisted of 6 microphones positioned over a concreterunway. The majority, of the microphones were supported 1.2 m above theground plane.A photograph of the experimental site is given as figure I. The flightpath was above runway 10-28 from the west
15、toward the east. The microphonearrays were positioned along runway 04-22 and the grassy area between runway04-22 and its taxiway. A detailed description of the runways and the grassyareas is given in reference 3. A diagram of the microphone layout ispresented as figure 2 which includes the microphon
16、e numbers and heights.The coordinates of the microphones are given in Table I in the data reductioncoordinate system (defined in the data reduction section).Figure 3 is a photograph of the American Airlines B-747 on the groundat Wallops and figure 4 is a photograph of the airplane flying at 30 m ove
17、rthe microphone arrays. The airplane was tracked with a laser tracker. Thelaser reflector housing equipped with a backup C-band radar transponder andbattery pack was mounted underneath the port wing tip near the trailingedge of the wing. Figure 5 consists of close-up photographs of the assembledlase
18、r reflector housing with the transponder and the battery pack. Theinstallation of the laser housing which weighed 29.9 Ibs (with the trans-ponder and battery pack) is shown in figure 6.Sixteen runs were completed; the run conditions are listed in Table II.The airplane was flown with all four engines
19、 at a constant engine pressureratio with the landing gear, gear doors, and flaps deployed. The engine andairplane conditions as recorded in the cockpit are given in Table III.Weather information was gathered from four weather stations positionedalong the microphone arrays. One of the four stations w
20、as mounted on amechanical traversing arm to accurately measure weather parameter profilesup to a height of 2 m. A retrievable balloon was used to raise a fifthweather station to make higher weather parameter profiles with less spatialresolution.2Provided by IHSNot for ResaleNo reproduction or networ
21、king permitted without license from IHS-,-,-Acoustic Data ReductionThe acoustic data collected for the 16 runs have been reduced toone-third-octave band spectral time histories and synchronized with the tracking and weather information. The acoustic data were reduced with anaveraging time of I/4 sec
22、ond. The frequency range of the one-third-octaveband analysis was from 20 Hz to I0 kHz. The recorders were turned on inpthe field for each run when the airplane was incoming and just beyond twomiles from the intersection of runways 04-22 and 10-28. Turning the recorderson early provided ambient meas
23、urement for each microphone for each run. Eachmicrophone system (see figure 7) was tested before going into the field toinsure that the equipment operated within the manufacturers specifications.A pre- and a post-calibration was performed on each system in the field.The pre-calibration involved an a
24、mplitude and a pink noise calibration.The post-calibration was a check on the stability of the amplitude calibra-tion. In the data reduction process, the pink noise calibration was used toinsure a flat frequency response of the record/playback systems. The acousticdata were not corrected to standard
25、 day weather conditions.Tracking Data ReductionThe laser tracker lost track of the airplane for a brief perioddue tothe close proximity of the flight path to the tracker, coupled with the slewrate of the tracker pedestal. The possibility of this occurring was known(refs. 2 and 3), but design require
26、ments of the experiment excluded otherpossible microphone/flight path layouts. The average gap per run wasII seconds. During the tracking data reduction, straight lines were com-puted to fill the gaps and a 21-point sliding average was performed on eachflight track.Tracking information was recorded
27、every I/I0 second. The tracking datawere transformed from spherical coordinates referenced to the laser trackerto Cartesian coordinates referenced to the intersection of runways 04-22and 10-28. The Cartesian coordinate system, referred to as the data reductioncoordinate system, is illustrated in fig
28、ure 8. The tracking point on theairplane was changed during the data reduction from the laser housing mountedon the port wing tip to the geometric center of the four engine exhausts.Weather Data ReductionThe weather data from the five weather stations have been organizedand transformed where needed
29、into a consistent set of data in metric units.Weather data from the balloon weather station are illustrated in figure 9.Profiles of temperature, relative humidity, and wind speed are given inthe figure. The data were taken at approximately the time of run 8. Thewind was generally from the northwest
30、during the test.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-DATAANALYSISThe method used to calculate lateral attenuation is a comparative methodin which a one-third-octave band spectrum from a particular microphone, calledthe measurement micropho
31、ne, is compared for the same run with a one-third-octave band spectrum from a second microphone, referred to as the referencemicrophone, positioned underneath the flight path. The reference spectrum isbrought to the same slant range in terms of spherical spreading and atmosphericabsorption as the me
32、asurement spectrum. The two spectra to be compared areselected from their respective microphone time histories using a sourcedirectivity angle criterion. The angle criterion insures that both spectraare emitted with the same source emission angle and propagated over thesame surface. Lateral attenuat
33、ion is defined as the difference between thereference spectrum and the measurement spectrum.Single Spectra AnalysisFor overall, A-weighted, and tone corrected perceived noise levelanalysis, the one-third-octave band spectra emitted with an emission direc-tivity angle of 122.5 deg, (referenced to the
34、 forward inlet direction, seefig. I0) are selected to be the reference and measurement spectra. Becauseof the oblique angle between the microphone arrays and the flight track in theexperiment, the sound emitted from the airplane at an emission angle of122.5 deg propagated parallel to the microphone
35、arrays. The portions ofthe microphone signals selected with the 122.5 deg angle criterion have thesame source origin and propagated over the same surface. For a particularmeasurement microphone and run, a receive time is calculated for the micro-phone for the sound emitted at 122.5 deg. The two one-
36、third-octave bandspectra located on either side of this time in the microphone time historyare averaged. The average spectrum has an effective averaging time of one-half second and is integrated to form the desired metric. For the same run,a reference microphone, usually microphone I, is chosen and
37、an average one-third-octave band spectrum is obtained in the same manner. Before integratingto form the same metric, spherical spreading and atmospheric absorptioncorrections are applied to bring the reference spectrum to the same slantrange as the measurement spectrum. Atmospheric absorption correc
38、tions arecalculated using the American National Standards Institute (ANSI) standardmethod for the determination of molecular absorption (ref. 4). The measure-ment value is subtracted from the reference value to form the lateralattenuation result. A positive lateral attenuation value denotes anattenu
39、ation.Spectral Time History AnalysisFor effective perceived noise level, EPNL, analysis, the measurementvalue is calculated directly from the measurement microphone time history.The corresponding reference EPNLvalue for the same run is calculated in thefollowing manner: The emission angles for each
40、one-third-octave band spectraof the measurement microphone time history are calculated. Each one-third-octave band spectrum in the measurement time history is replaced with a4Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-spectrum from the reference
41、 microphone for the same emission angle. Thereference microphone spectra are corrected to the same slant range as themeasurement spectra they replace. In this manner a reference time historycongruous to the measurement time history is formed and is used in the reference EPNLcalculation. The measurem
42、ent EPNLvalue is subtracted fromthe reference EPNLvalue to yield a lateral attenuation measurement.“ In order to make the reference EPNLcalculation more efficient, averagereference spectra are used. Instead of replacing the spectra of a particularmeasurement time history with spectra from the refere
43、nce microphone for thesame run, the measurement spectra are replaced with the average referencespectra. The average reference spectra are formed by averagingspectra formicrophone 1 for the first ten runs. To represent the source emission anglesfrom 0 to 180 degrees, 22 average spectra are used. The
44、average referencespectrum of the 22 closest to the emission angle of a particular measurementspectrum is used to replace that measurement spectrum to form the referencetime history.ResultsOverall sound pressure level, SPL, A-weighted sound pressure level, LA,tone corrected perceived noise level, PNL
45、T, and EPNLaverage results asa function of elevation angle are given in figures 11-14 respectively. Dataare given for all the 1.2 m microphones positioned over grass (microphonesI, 3, 4, 6, 7, 8, I0, II, and 13) for runs 1-12. The data for individualmicrophones for similar runs are averaged. For exa
46、mple, the results formicrophone 4 for runs I, 2, 7, and 8, all 30 m runs, are averaged to form asingle data point. The results for runs 3, 4, 9, and I0 (60 m runs), forruns 5 and 6 (120 m runs), and for runs II and 12 (240 m runs) are alsoaveraged. The reference microphone for all the results was mi
47、crophone I,the closest 1.2 m microphone positioned over grass to the airplane flighttrack. The data plotted in figures 11-14 are listed in Table IV along withthe average results for the I0 m microphones positioned over grass, micro-phones 5, 9, and 12. Also included in Table IV are the individual re
48、ferenceand measurement integrated metric values. Measurement EPNLvalues formicrophone 20 for all the runs are listed in Table V. The elevation anglesand slant ranges used in the figures and the tables are the closest approachvalues.In figures 15 - 18 the results of least-squares fits of the datapres
49、ented in figures II - 14 are given and compared with similar T-38 excessground attenuation experimental results. The least-squares fits of theB-747 and the T-38 results are second order and constrained to be 0 at 90 deg.The least-squares fit coefficients for the B-747 results are given in TableVI. No overall T-38 results have been calculated, therefore none are givenin figure 15. The long range (SAE AIR 1751) lateral attenuation
