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January 1997

Airplane's Jet Noise Mirrors at the Ground





Airplanes produce a lot of noise. One of the main problems is the extreme loudness. In the measuring position we got amplitudes from the microphones in the range of 15 Vpp (jets in half-power position). Using Measuring microphones Gefell MK250 with a sensitivity of 50 mV/Pa this means, the ac-air pressure in a distance of 30 meters was 0.003 atmospheres or 300 Pa or 143.5 dB(C). To reduce the maximum level, we used felt-pads covering the microphones.

At the airport it was not able, to calculate the images. We went home. At GFaI I found, that 5 of 6 records were overamplified. Only one record was in range. Independent which parameters I changed, this error-free record produced only two vertical noise sources. Where is the second jet? It was frustrating. I was sure, to find two horizontal sources for the two jets. Is there any new and unknown acoustic effect within the interference reconstruction, we did not know? Or do we have an unknown software bug?

After a week of experiments with PSI-Tools I asked our partner at Lufthansa-Technik AG back, why only this single record was in range. The partner looked in his test-table and answered: "In this test the right jet was down, only the left jet was running. In all other tests both jets were running". So the vertical emission could be only the noise reflected at ground! This was one of the best days I ever had! And PSI-Tools got a new function: If a channel is overamplified, now it showed red lines at the horizonal borders of the channel.

Because of certain circumstances it was not possible to map the vector-image and the acoustic image very exactly. It was not possible to use the support of the laser positioning of the array, because it was to light to recognize the laser point. For safety reasons, we were not allowed, to come closer to the airplane.

Details about Figures 1) and 3): Noise emission of a Boing 737-400 in half power position of the left jet. The right jet was stopped. Measuring distance was 30 meters. For the first time we see a mirroring noise emission at the ground. Data: TURB4.CHL, GROS4.INI, 20 kSps, interference integral over 500 Samples = 25 msec, 16 channel reflective microphone array with mics Gefell MK250, 90cm x 90cm, felt pads on the mics to reach the sound level of 143 dB(C).



View to the power supply turbine in Fig.4): Data: Distance 15 m, 50 kSps, interference integral done with maximum-operator over three seconds (PSI-Tools), recorded with a 16 channel microphone array 90cm x 90cm. Data: HILF2.CHL, HILF2EXP.INI, 20kSps.

Jet noise is not our special field of research. To find research activities about noise reduction at airplanes in germany see the link http://www.ba.dlr.de/sm/at/tf/dlr_laerm.html.


Remarks

1 atm = 100000 Pa = 100 kPa = 0.1 MPa
1 Pa = 1 N/m▓

0 dB = 2*10E-5 Pa Wahrnehmungsgrenze
110 dB = 20 Pa Schmerzschwelle

Mikrofonempfindlichkeit des Messmikrofons Gefell MK250:
50 mV/Pa = 50 mV m▓/N = 5 kV/atm
d.h. als Gr÷▀engleichung:

P [Pa] = V [mV] / 50 bzw.
Lp [dB] = 20 lg (u~ / u░) Schalldruckpegel in dB (MK250: u░ = 1ÁV)


Reference

[1] Lighthill, M.J.: On sound generated aerodynamically. Proc. Royal Soc., (A) Part 1: 211(1952) pp. 564-587, Part 2: 222 (1954) pp. 1-31


Thanks to the Airport Schoenefeld GmbH and to Lufthansa Technik AG
for the kind support to get noise records and fotos!


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File created 10:49 21.01.1997
Soft HTML-redesign, avi-conversion to mp4, smart corrections, additional comments 20.10.2020