... the image, that has gone around the world:

Airplane's noise mirrors at the ground

a)

The photo, that has gone around the world: jet noise reflected at the ground.
First time measurement technology is sensitive enough, to see the ground reflection of noise.
Acoustic interference integral with overlay of an optical image, integration interval 25ms.
Click the image to see a high-speed acoustic movie

Movie: 10.000 images per second, integration interval 2.5ms, 400kB/1,3MB; zipped *.AVI-format
(Sorry, the movies color lookup table is different to that of the image)

b)

The photograph is taken from the measuring position in front of the airplane.
Click the image to hear the noise.

Figures a) and b): Noise emission of a Boing 737-400 in half power position of the left airjet turbine. Measuring distance is 30 meters. First times the image shows mirroring noise emission of the compressor at the ground, the reason why it has gone around the world.
Data: TURB4.CHL, GROS4.INI, 20kSps, interference integral over 500 Samples = 25 msec, 16 channel microphone array 90cm x 90cm.

c)

d)

View to the power supply turbine:
Distance 15m, 50kSps, 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.

Airplanes produce a lot of noise. One of the main problems is the extreme loudness. In the shown measuring position we get amplitudes from the only 1:1-buffered microphones in the range of 15 Vpp (motors in half-power position). Using Measuring microphones with a sensitivity of 50mV/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). Note the strong ground reflexion.
The overflow of an turbojet-airplane shows, that strongest emission occurs in an angle of 30░ to 60░ (typ: quadrupol). Following Lighthill [1] the noise-intensity of the jet grows partially with the eighth power (8th!) of the relative speed to the environmental air (0,7 < M < 1,6). Subsequent developements of turbo-fan and turbo-prop engines try to reduce the speed of the fan by using larger air conducting diameters.
Because of some circumstances it was not possible to map the vector- and the acoustic images very exactly. It was not possible to use the support of the laser positioning device of the array.

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 = 100kPa = 0.1 MPa
1 Pa = 1 N/m▓

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

Mikrofonempfindlichkeit des Messmikrofons MK250:
50 mV/Pa = 50 mV m▓/N = 5kV/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


Very 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