Flyover noise measurements on landing aircraft with a microphone array

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The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under.

Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.

Flyover Noise Measurements on Landing Aircraft with a Microphone Array U. Michel * B. Barsikow J J. Helbig J M. Hellmig § M. Schiittpelz ^

ABSTRACT The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft.



Engine noise during landing approach has decreased so much on some modern aircraft that their noise emission is considerably influenced by airframe noise. The goal of further reductions of the noise levels of landing aircraft requires that the dominating sources of airframe noise during the landing approach are identified, because these are the sources that have to be technically treated for an effective noise reduction. Microphone arrays with the appropriate data processing software have shown their capability of mapping noise sources on fast moving objects with good spatial resolution. The focal position of the analysis can be moved with the speed of the object which increases the integration time for a frequency analysis and eliminates the Doppler frequency shift that would otherwise make a frequency analysis difficult (Barsikow and King [1]). This feature was extensively used in studies of sound sources on high-speed trains (King and Bechert [2], Barsikow et al. [3], Briihl and Schmitz [4] and Barsikow [5]). A linear microphone array was successfully applied 1996 by Michel et al. [6] for a study of airframe and engine noise of a Tornado combat aircraft in high-speed low-level flight. A surprising result of the study was that the noise emission of this combat aircraft into the *DLR, Institut fur Antriebstechnik, Abteilung Turbulenzforschung, Miiller-Breslau-Str. 8, 10623 Berlin, Germany, Email:, Tel. +49 30 310006-26, Fax +49 30 31000639 takustik-data, Kirchblick 9, 14129 Berlin, Germany, Tel. +49 30 80902606, Fax +49 30 80902607 *DLR §akustik-data ^akustik-data °Copyright ©1998 by U. Michel, B. Barsikow, J. Helbig, M. Hellmig, M. Schiittpelz. Published by the Confederation of European Aerospace Societies, with permission.

forward arc is dominated by an exhaust noise source at the nozzle exit and by airframe noise and that airframe noise is louder when the external stores were removed. Based on this experience, a large planar array was developed to enable a two-dimensional mapping of the sound sources on landing commercial aircraft. Over 170 landings were recorded with this array, including all frequently flown modern aircraft types. The eventual results of this study will enable a comparison of the noise emission of the various flap and slat designs, identify the best designs from the standpoint of noise emission and also identify avpidable noise sources on existing aircraft. The test results will not only help improve the prediction schemes for .the various airframe noise sources but will also allow to derive the directivity of engine inlet and exhaust noise during the landing approach. This shall enable a further reduction of engine noise. The main purpose of the study is the identification of the loudest noise sources that must be modified for a reduced noise emission. Only some first results are reported in this paper, mainly to demonstrate the capabilities of the method. This study is part of a German national research program in which it is accompanied by an investigation of airframe noise sources on model and full size wings with high-lift devices in wind tunnels (Dobrzynski et al. [7]).

2 2.1

TEST SETUP Microphone array

A two-dimensional investigation of the sound sources on aircraft requires two-dimensional arrays. The simplest version of such an array is created by crossing two linear arrays at an angle of 90 degrees. Experiences with such arrays on moving sources are described by Barsikow [5]. A particular disadvantage of this arrangement is a cross-shaped sidelobe distribution which was substantially improved with a newsignal processing procedure by Elias [8] and Piet and Elias [9]. A major drawback of classical arrays with constant microphone spacing is the appearance of spatial aliases at high frequencies. The aliasing problem can be solved by using non-redundant planar microphone distributions. A distribution on logarithmic spirals was described by Watts et al [10]. The microphone positions for the flyover tests reported in the following were optimized with an evolution strategy (a random optimization procedure described by Rechenberg [11]) for the largest possible amplification (difference in dB between main lobe and highest side lobe) for a set of frequencies. The resulting distribution of 96 microphones is shown

Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.

in figure 1. The microphones are more densely concentrated in the center of the array. One microphone is in the center and 19 microphone positions repeat every 72 degrees within a circle of 360 degrees. This array was tested on the company airfield of Daimler-Benz Aerospace Airbus in Hamburg with flyover altitudes of about 30 m. The number of condenser microphones was later increased to 111 to enlarge the effective size of the array from S = 7.5 m to 8.1 m for a measuring campaign on the airport of Frankfurt/Main with flyover altitudes between 35 m and 40 m. All microphones were mounted with their preamplifiers lying on top of the plate with a grazing incidence of the microphone diaphragm. The array was placed just outside the localizer protection zone under the approach path. The center of the array was placed 8 m to one side of the extended runway center line. The aim was to look vertically at one wing of the aircraft which means that the other wing was investigated under a slant angle. A response of the array with 96 microphones for a stationary point source with a frequency of 2.8 kHz located at (z,y) = (0.46 m,—0.14 m) about 3 m above the plate is shown in figure 2 (colored figures at the end). The beam width of the main lobe (3 dB down) is about 0.12 m. The amplification of the array is higher than 20 dB for focal positions in the immediate vicinity of the source (radial distance less than 0.6 m). This amplification decreases to about 5 logM dB, where M is the number of microphones, when the distance between array focus and source position increases. This explains the need for a large number of microphones for such an array. The amplification for large distances amounts to about 10 dB for about 100 microphones.

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D = 26 m and S = 7.5 (situation in the Hamburg tests) and Ar ~ 0.55 m and D = 37 m and S = 8.1 (Frankfurt tests).


Flight speed and aircraft position

The aircraft speed above ground was derived from the signals of two optical sensors located with a known separation along the extended runway center line. These sensors consisted of a linear array of light sensitive diodes behind an optical lens. This light sensing array was oriented perpendicularly to the flight direction. The light intensity measured by this device is reduced proportionally to the width of the aircraft when it passes by and generates a voltage pulse on its output which is recorded together with the microphone

signals. The flyover altitude was determined with laser distance meters pointing vertically into the sky. A trig-

ger pulse was generated by these instruments when a target was hit by the low-power laser beam and the measured distance was stored in the instrument for later readout via its serial interface. Three such instruments were used in Frankfurt/Main. One distance

Aircraft identification

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which equals Ar ~ 0.4 m for a frequency of 2 kHz and




The spatial resolution of this array is defined by the ratio of the distance D of the aircraft from the array to the array diameter S and is proportional to the wavelength A of the sound. The resolution in the flyover tests was experimentally found to be Ar ~ 0.7 A D/S

meter was placed about 90 m in front of the array and 6 m to the right of the extended runway centerline. Its trigger pulse was used to start the data acquisition. Two more distance meters were placed 8 m to the right and 8 m to the left of the extended runway centerline to measure the distance of the aircraft over the array. Some flyovers were recorded in the dark after sunset. In these cases the aircraft speed had to be estimated from the trigger pulses of the laser distance meters.

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phone signals. It was shown by Dougherty [12] for a wind-tunnel test that such a procedure eliminates the corresponding side lobes as well. The procedure can be applied repeatedly.




Two TV cameras were used to help identify the aircraft

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-*—Direction of motion [m]

Figure 1: Positions of the 96 microphones of the smaller array that were determined with the evolution strategy. This relatively small dynamic range can be increased substantially if the contributions of known noise sources are numerically removed from the micro-

type. One camera looked vertically and a second was installed about 40 m to the side. A half frame of each camera was recorded with a frame grabber and stored on hard disk. In addition, a listing of all landed aircraft (type and operator) was supplied by Flughafen Frankfurt Main AG, the airport operator. The aircraft types and tail signs were also visually identified. Deutsche Lufthansa AG cooperated by asking their pilots to report tail sign, landing mass and flap setting after landing on a radio frequency borrowed for this purpose from Deutsche Flugsicherung (German ATC).

Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.


Data acquisition and array analysis

A PC-based data acquisition unit was used that was capable of sampling 128 signals with a sampling frequency of 50 kHz and a resolution of 16 bits. The microphone signals were sampled together with the two signals of the optical detectors and the trigger pulses of the three laser distance meters. Three seconds of data were first read into the computer's memory and subsequently stored on hard disk. The sampling was started with the trigger pulse of one of the three laser distance meters. The frames of the cameras were grabbed at the moment of the trigger pulse of one of the other two laser distance meters. The array analysis was carried out in the time domain. The samples p(ten) of the array output for the time steps ten = teo+n*At have to be calculated in the frame of emission time te. At is the time interval for the Fast Fourier Transform (FFT) and can differ from the sampling time interval. The corresponding microphone data have to be interpolated for the n reception times ten + tmn, where the tmn are the propagation times from the source position for the nth time step to the rath microphone. The analysis at emission time results in an analysis with swept focus and eliminates the Doppler frequency shift. The length of the time segment for the analysis must be restricted and was set to the duration in which the emission angle changed from 82.5 to 97.5 degrees relative to the flight direction. This corresponds to a distance of 9.5 meters for a flight altitude of 36 meters and a sampling time of 118 ms yielding about 3000 samples for an FFT with At = 40 /^s for a typical landing speed of 80 m/s. These short measuring times are a considerable disadvantage in comparison to wind-tunnel tests. Overlapping time series with 1024 samples were taken from the available time series. The frequency spectra resulting from the Fourier transform contain 512 frequency values with a frequency spacing of 24.4 Hz. The statistical stability of the resulting spectra can be increased by adding up neighboring frequency bands to one-third octave spectra. The influence of distortions due to the sound propagation through the turbulent atmospheric boundary layer on the array output is not yet known. The distortions reduce the coherence between the various microphone signals of the array and should affect the higher frequencies more than the lower ones. The result is a lowering of the main beam level and a loss of dynamic range.

The reported noise levels are free-field sound pressure levels. 6 dB are subtracted to compensate the pressure doubling on the plate. The normalization is such that the array output of a monopole point source would be identical to the reading of a single microphone.

6 dB below the limit defined by chapter III of ICAO Annex 16. In addition, its engines are rear mounted which gives an undisturbed picture of the airframe noise sources on or under the wing and fuselage. The

aircraft is equipped with flaps but has no slats. The distribution of the A-weighted sound pressure level is shown in figure 3 for the frequency range between 280 Hz and 3500 Hz and an emission angle of 90 degrees. The position of the aircraft silhouette was chosen for a best fit. The region with the loudest noise source is shown in red. The lower threshold in this figure is 18 dB quieter. It can be seen that the noise emission of this aircraft is dominated by two airframe noise sources on the outer part of the wings, noise sources that are 6 dB(A) louder than the engine exhaust.


The noise emission of this aircraft shall now be studied in more detail with the aid of the corresponding onethird-octave bands. The sound emission in the onethird-octave band of 315 Hz is shown in figure 4. It is dominated by the noise of the landing gear and the open wheel wells of this aircraft. The primary engine nozzle can be recognized as a weaker source. The spatial resolution at this low frequency is about 2.6 m. The loudest noise source shown in red is 12 dB quieter than the loudest source in figure 3. The dynamic range is 10 dB in this and the following figures.



RESULTS Noise sources on a regional jet

The noise emission of a regional jet is shown first. This aircraft is especially suited for an airframe noise study because it is very quiet with a landing noise level of

Wake-vortex wing interaction noise

It can be seen in figure 5 that the two sound sources near the wing tips emit in the one-third-octave band of 500 Hz. The narrow-band frequency spectrum of this source position is shown in fig. 6 and indicates that the source is dominated by a tone of 480 Hz which was clearly audible and can easily be mistaken as a turbomachinery tone. The source position is located about 1 m outside the side edge of the flaps in the wing region with the aileron. The A-weighted sound pressure level of this source in this one-third octave band is almost equal to the level shown in figure 3 which indicates that the total noise emission of this aircraft into a direction of 90 degrees relative to the flight direction is dominated by this airframe noise source. A possible origin of this sound source is the interaction of the wake-vortex with the flow over the wing. This will be discussed in section 4. Since most other aircraft do not exhibit these noise sources, it is concluded that they are avoidable which would make the aircraft substantially quieter.



Landing-gear noise

Flap noise

The noise generated by an open gap between the inboard and outboard flaps can be seen in figure 7 where the distribution of the A-weighted sound pressure level is shown for the one-third-octave band of 800 Hz. The loudest sound-pressure level in this figure is 9 dB below the maximum level in figure 3. A flap design without this gap would have avoided this noise source.

Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.

A noise source at the side-edges of the flaps can be observed in all one-third-octave bands for 800 Hz and above. This is shown in figure 8 for the 2000 Hz onethird-octave band. The level of the loudest noise source in this frequency band shown in red is 14 dB below the highest sound pressure level in figures 3 or 5. However, since a similar result can be seen in several one-third octave bands, the contribution of side-edge noise is not necessarily small. Figure 8 demonstrates the spatial resolution of the array analysis. The diameter of the 3 dB- down circle is about 0.5 m. Flap-side edge noise can be more important on aircraft with larger flaps. Such a case is shown in figure 9 in the 1250 Hz one-third-octave band for a narrowbody jet where flap-side edge noise is comparable to engine exhaust noise. 3.5

Slat noise

Slat noise could be identified on some landings. Noise sources along the leading edge of a wing can be seen in figure 10 in the 1250 Hz one-third-octave band. The visible point sources may be related to the slat tracks and to the slat horn. In addition, jet-flap interaction noise can be identified as the most important source in this frequency band.

DISCUSSION AND CONCLUSIONS First results of a study on the airframe noise sources of landing commercial aircraft are reported. The aircraft were investigated with a planar microphone array consisting of 96 and later of 111 microphones that were mounted on an 8 m by 8 m plate on the ground. The investigated aircraft include the Boeing 737, 747, 757,

767, 777, and Airbus A300, A310, A319, A320, A321, A330, and A340 as well as aircraft with tail-mounted engines and several regional aircaft. This will enable a determination of the state of the art concerning airframe noise and engine noise. A number of known airframe noise sources on landing gears, flaps, and slats could be identified despite the presence of engine noise. This study will help define the lowest existing levels of these sources as a guideline for future designs. The influence of different designs of landing gears and flap side edges on the noise emission will be of especial interest. The results will help in the development of prediction methods for the various airframe noise sources. The directivity of the sources will be studied and the variance between different landings of the same aircraft type can be determined. The study is also valuable for the investigation of engine noise during landing approach. It is possible to separate between the noise emitted from the engine inlet and the engine nozzle. The engine speed can be determined from the fan tone in the narrow-band spectra. The influence of different engine mounting (under wing or tail) can also be studied. A surprising result was a loud sound source located on the outer part of the wing of some aircraft which

seems not to be related to the flap side edge. The noise

is concentrated in a narrow frequency range which indicates an instability process as its origin. It is proposed, that the wake vortex is the origin of this source. The flow field in the wake vortex close to the wing of an aircraft may be similar to the Batchelor vortex which is absolutely unstable as shown by Delbende et al. [13]. An explanation that this noise source is only identifiable on a few aircraft types may be that the roll-up of the wake vortex normally occurs some distance behind the wing. If this occurs, for some reason, closer to the trailing edge of the wing, the unsteady flow field of the wake vortex could lead to an unsteady pressure distribution on the wing which acts as a sound source.



The authors thank the German Ministry of Education, Science, Research and Technology for the financial support of this work. This study would not have been possible without the cooperation of Daimler-Benz Aerospace Airbus, Deutsche Lufthansa AG, Flughafen Frankfurt Main AG, and Deutsche Flugsicherung.

REFERENCES [I] B. Barsikow and W. F. King III. On removing the Doppler frequency shift from array measurements of railway noise. J. Sound Vib., 120:190-196, 1988. [2] W. F. King III and D. Bechert. On the sources of wayside noise generated by high-speed trains. J.

Sound Vib., 66:311-332, 1979. [3] B. Barsikow, W. F. King III, and E. Pfizenmaier. Wheel/rail noise generated by a high-speed train investigated with a line array of microphones. J. Sound Vib., 118:99-122, 1987. [4] S. Briihl and K.-P. Schmitz. Noise source localization on highspeed trains using different ar-

ray types. In Proceedings of Internoise 93, pages 1311-1314, 1993. [5] B. Barsikow. Experiences with various configurations of microphone arrays used to locate sound sources on railway trains operated by the DB AG.

J. Sound Vib., 193:283-293, 1996. [6] U. Michel, B. Barsikow, B. Haverich, and M. Schiittpelz. Investigation of airframe and jet noise in high-speed flight with a microphone ar-

ray. AIAA Paper 97-1596, 1997. 3rd AIAA/CEAS Aeroacoustics Conference, Atlanta, Ga, May 1214, 1997. [7] W. Dobrzynski, K. Nagakura, Gehlhar B., and A. Buschmann. Airframe noise studies on wings with deployed high-lift devices. AIAA/CEAS Pa-

per 98-2337, 1998. [8] G. Elias. Source localization with a twodimensional focussed array: optimal signal processing for a cross-shaped array, 1995. Inter-Noise

95, Newport Beach (USA), July 10-12, 1995.

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[9] J.-F. Piet and G. Elias. Airframe noise source localization using a microphone array. AIAA Paper

97-1643, 1997. 3rd AIAA/CEAS Aeroacoustics Conference, Atlanta, Ga, May 12-14, 1997. [10] M. E. Watts, M. Mosher, and M. Barnes. The microphone array phased processing system (MAPPS). AIAA Paper 96-1714, 1996. 2nd AIAA/CEAS Aeroacoustics Conference, State College, Pa, May 6-8, 1996.

[11] I. Rechenberg. Evolutionsstrategie. Frommann Verlag Stuttgart, 1973. [12] R. P. Dougherty. Source location with sparse acoustic arrays: interference cancellation. DNW,

1997. [13] I. Delbende, J.M. Chomaz, and P. Huerre. Absolute/convective instabilities in the batcherlor vortex: a numerical study of the linear impulse response, submitted to JFM, 1997.

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SPL [dB] 79+ 76 to 79 73 to 76 70 to 73 67 to 70 64 to 67 61 to 64 58 to 61 55 to 58 52 to 55






Figure 2: Distribution of the sound-pressure level for a point source with a frequency of 2.8 kHz located about 3 m above the plate. The spatial resolution is about 0.12 m. Note the low sidelobe levels in the vicinity of the source and the higher levels for larger distances.




Figure 4: Distribution of the A-weighted soundpressure level of a regional jet in the 315 Hz one-thirdoctave band. Loudest noise sources shown in red are 12 dB quieter than in figure 3.


Figure 3: Distribution of the A-weighted soundpressure level of a regional jet in the frequency range 280 Hz to 3500 Hz for an emission angle of 90 degrees rel. flight direction. Loudest noise sources are shown in red. Flight speed 76 m/s, altitude 26 m.

Figure 5: Distribution of the A-weighted soundpressure level of a regional jet in the 500 Hz one-thirdoctave band.

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to 2



Frequency [kHz]

Figure 6: Narrow-band spectrum of the dominating sound source near the wing tip.

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Figure 9: Distribution of the A-weighted soundpressure level of a narrow-body jet in the 1250 Hz onethird-octave band. Flap-side edge noise can be seen in addition to engine exhaust noise.


Figure 7: Distribution of the A-weighted soundpressure level of a regional jet in the 800 Hz one-thirdoctave band. The noise source is 9 dB(A) quieter than the loudest source in figure 3. 5 -

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Figure 8: Distribution of the A-weighted soundpressure level of a regional jet in the 2000 Hz onethird-octave band. Loudest noise sources shown in red are 14 dB quieter than in figure 3. Flap-side edge noise can be seen.

Figure 10: Distribution of the unweighted soundpressure level of a wide-body jet in the 1250 Hz onethird-octave band. Noise sources can be seen along the leading edge of the wing. Jet-flap interaction noise is dominating in this frequency range.

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