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TU Berlin

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Dr.-Ing. Carsten Spehr

Former Academic Staff

Active control of sound traveling through a bogie-shroud barrier combination

The aim of this study was to examine the conditions which allow to reduce the sound passing through a gap by an active noise control system exemplified on the bogie-shroud barrier system.
Active control means to superpose the primary sound field with a secondary sound field and thus minimizing an arbitrary target value. One aim of this study was to find out which target value is suitable to abate the sound power radiated into the far-field. By doing this, the influence of the positions of the secondary source and the error microphone were studied and their best positions identified.

The sound propagation through the gap was simulated in cylindrical coordinates. The pressure fields of the primary and secondary source were calculated separately and were superposed afterwards. To model the bogie-shroud and the barrier as boundary conditions in cylindrical coordinates they are assumed as arcs of a circle around zero. The mixed boundary condition problem following from this was studied for stability at a different number of ansatz-functions and conditional equations. The stability was much higher when a quadratic equation system was applied, in contrast to solving over-determined systems with more conditional equations than ansatz functions which led to oscillating solutions.
In the first series of simulations the net radiated power was minimized as the target level. In the next series it was examined whether a global diminution of the radiated sound power is possible with a compact system without error microphones in the far-field. The best results were achieved with the secondary source on the lower edge of the barrier. The best position for the error microphone was the region on the upper part of the gap. The influence of the slot width was studied, too. It could be shown that with an increasing slot width the level as well as the frequency ranges of the improvement decreased.

In the second part the influence of the position of the secondary source was studied analytically. The influence of the modes was analyzed by using the example of a lightly damped rectangular cavity with which the optimal position of the secondary source could be derived. Using the one-dimensional wave equation the influence of the positions of the error microphone and the secondary source could be derived.

A series of experiments was launched to study whether the results of the simulation could be found in reality. The test rig was built of 25 mm thick chipboard. The angle of the shroud was variable and the slot width between the shroud and the barrier in the three examined variants was approx. 215 mm, 370 mm und 525 mm.
The aim of the experiments was to verify the results for the optimal position of the error microphone and the secondary source which meant to prove the possibility of preliminary optimizing of active noise arrangements. The tests were conducted with sinusoidal functions in steps of 12.5 Hz. The filters to adapt the secondary source were calculated by the well known FXLMS-Algorithms. The measurements were controlled by an arrangement without active control.
The sound level was determined on a grid in front of the gap perpendicularly to the barrier and the shroud. Confirming the simulations the experiments showed that the total sound power propagating through the gap could be diminished.
In the simulation as well as in the experiments the position of the secondary source at the lower part of the barrier achieved the highest level of improvement. The frequency range of the improvement of the variant 1 (215 mm gap) reached up to more than 500 Hz. The level of improvement was not correctly predicted by the simulation, which overestimated the results of the experiments. The gap width limits the frequency range of positive improvements. Explicit statements about a cut off frequency are difficult but it was observed that improvements could be achieved as long as the gap width was less than one third of the wavelength.
The best results were achieved with the error microphone centric between the barrier and the shroud. Tough also with the other tested positions in the upper region of the gap improvements of about 10 dB to 20 dB could be observed.

The study showed that optimizing the position of the error microphone and the secondary source is essential for the active noise control and widely influences the achievable improvement. The optimization can be reduced in advance due to analytical or numerical investigation. These results can be adapted to other similar geometries were sound propagates through a gap like machines with a feeding for the material or tilted windows. For a real adoption the filter would need to be adapted to the primary noise.

Dissertation : http://opus.kobv.de/tuberlin/volltexte/2008/1790/
Supervisor : Prof. Dr.-Ing. Michael Möser


J. Kokavecz and C. Spehr (2003). Charakterisierung einer Körperschallquelle am Beispiel einer Servopumpe. Fortschritte der Akustik - DAGA 2003 (Poster)

Merino-Martínez, R. and Sijtsma, P. and Snellen, M. and Ahlefeldt, T. and Antoni, J. and Bahr, C.J. and Blacodon, D. and Ernst, D. and Finez, A. and Funke, S. and Geyer, T.F. and Haxter, S. and Herold, G. and Huang, X. and Humphreys, W.M. and Leclère, Q. and Malgoezar, A. and Michel, U. and Padois, T. and Pereira, A. and Picard, C. and Sarradj, E. and Siller, H. and Simons, D.G. and Spehr, C. (2019). A review of acoustic imaging methods using phased microphone arrays. CEAS Aeronaut J. Springer Science and Business Media LLC, 197-230.


Dr.-Ing. Carsten Spehr
German Aerospace Center (DLR)
Institute of Aerodynamics and Flow Technology
Experimental Methods
Bunsenstraße 10
D-37073 Göttingen

Tel.: +49 551 709-2427
Fax: +49 551 709-2830

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