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B-1 APPENDIX B. WYLE 2010 B777 IAD TAXI NOISE MEASUREMENTS AND DATA PROCESSING In order to characterize the noise emissions of Boeing 777 aircraft while taxiing, acoustic and video measurements made on taxiway E at Dulles International Airport (IAD) in July 2010 were utilized. Three cameras were collocated with pairs of microphones opposite the terminal as shown in Figure B-1. The sensors are at the red balloons labeled W, C, and E for West, Center, and East, respectively. A schematic of the sensors relative to the taxiway center line can be seen in Figure B-2. A picture of the equipment at the center location is shown in Figure B-3. The two microphones are 1.5 m and 4.0 m above the ground. The video camera is near the base of the tripod mounted on the pelican case. FIGURE B-1 Location of sensors at Dulles International Airport. Sensor Locations Taxi Line
B-2 FIGURE B-2 Schematic of sensors relative to taxiway centerline.
B-3 FIGURE B-3. Photo of equipment deployed at center location. The microphone signals were run into Larson Davis Model 831 sound level meters set to record one-third octave band time histories at a 1-second time resolution. The AC output of the meters were run into Edirol R-09HR solid state recorders set to continuously record 24-bit wave files at a sampling rate of 44100 samples per second. The spectral analyses of the Edirol recordings are the basis of the noise characterization of the Boeing 777. The video camera recorded both visible light and infra-red into a 4-channel DVR. All video cameras were hooked up to one DVR. The audio signal from a center-channel microphone was also run into the DVR audio input. The marker poles in Figure B-3 were used to determine the location and speed of the taxiing aircraft upon playback of the video. An example still shot of a Boeing 777 recorded is shown in Figure B-4. By using the operations data provided by the airport along with the time the data was taken, the airframe-engine combination could be found from records of the US aircraft fleet. The Boeing 777 pictured in Figure B-4 had PW4077 engines made by Pratt & Whitney. All the Boeing 777s studied in this memo had either PW4077 or PW4090 model engines. In order to determine Noise emissions of the engines, one-third octave band time histories were calculated from the recordings. A time resolution of 0.25 seconds was used. The speed of the aircraft was determined from the video using the time displayed and the marker poles. The spectral time histories were synchronized to the position of the aircraft by the time displayed in the video. The time resolution of the aircraftâs position was increased by using the frame count of the video along with the displayed time. Microphones Video Camera Marker Poles
B-4 FIGURE B-4 Example video of aircraft taxiing in front of center camera. By using the meteorological data and the taxi speeds, the spectral time histories were used to determine the free-field emissions from each of the 777s pass-bys using the Acoustic Repropagation Technique (Hobbs et al., 2010a). Figure B-5 shows the 1 kHz one-third octave band depropagated levels to a distance of 1000 feet for all the pass bys measured. FIGURE B-5 Depropagated data from all microphones for 1 kHz one-third octave band. Each line in Figure B-5 represents the depropagated levels from a single microphone. Reasons for the variation in levels include: variations in thrust; number of engines running; shielding of the far engine if it is the only one running. Because of proximity of each engine to the microphones, the difference in measured levels from spherical spreading would be approximately 4 dB. The depropagated levels were averaged together as a function of the angle from the nose of the aircraft (Theta). An 11 term Fourier Transform was used to fit the data using the method outlined by Abuelma'atti. Figure B-6 shows the average and fitted curve for the band displayed in Figure B-5. THETA dB 0 30 60 90 120 150 180 40 45 50 55 60 65 70 Marker Poles
B-5 The data was fit from an angle of 15 degrees off the nose to 165 degrees. Data outside this range either does not exist because the aircraft was never in line with the microphones or was below the signal to noise ratio cutoff because the aircraft was too far away. FIGURE B-6 Directivity of the average repropagated 1 kHz one-third octave band levels (Solid Line) compared to Fourier curve fit (Dashed Line) The resulting depropagated levels were reduced to a format used in the Advanced Acoustic Model. This model was used to predict sound levels from taxi pass bys as a function of distance from the aircraft. The same was done with the Boeing 777 noise characterizations made by ANOPP for just the engine components as noise sources. The A-weighted sound exposure levels for a modeled taxi pass by at 16 knots for a set of distances are compared in Figure B-7. As can be seen in Figure B-7, the source characterization of the Boeing 777 made from the Dulles Taxi measurements is compare well with both the Pratt & Whitney engine and General Electric engine ANOPP sources. FIGURE B-7 Sound exposure level versus distance modeled in the Advance Acoustic Model for source characterization of the Boeing 777 measured at IAD (diamond), ANOPP engine- components-only (no suppression) of the Pratt & Whitney engine (square) and the General Electric engine (triangle). THETA dB 0 30 60 90 120 150 180 40 45 50 55 60
B-6 In conclusion, the taxi measurements result in a source characterization that closely matches the ones made in the ANOPP estimations made by Georgia Tech. The individual pass-by measurements indicate a significant variation of noise emissions from the taxiing aircraft. Sources of the variation would be better understood if it was known what the actual engine thrust was during the measurement as well as which engines were running.
