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Chapter3. Implementation 37 Various distances ∆xs, based on the wavelength of the respective TOB, have been tried out. The tests had no immediate effect on the resulting SODIX plot. The second phenomenon, marked by a black box in Fig. 3.9, is similar to a comb filter effect. The periodic SPL fluctuation for frequencies f 4000 Hz for mi- crophones in the first third of the array hints at wave interference in that seg- ment. This occurs most likely due to sound wave reflection from the floor or the microphone frame, resulting in cancellation and amplification of the sound waves. According to the test reports, the whole test chamber as well as the frame were lined with sound absorbers in order to supress acoustic reflections. Analy- sis of the modulation frequency of the interference pattern yields a wavelength of λ ≈ 0,54 m, which suggests a reflection by the array frame, as the wavelength is equal to twice the distance from the microphone top ends to the bar they were mounted on and would therefore result in the observable sinusoidal amplifica- tion and cancelation. The comb filter effect in the raw data does not seem to have a noticeable impact on the result plots. 3.2.2 SODIX Setup This section deals with the various preliminary steps required to configure SODIX for an optimal simulation run. Source position distribution Sect. 2.3.4 explained the necessity of having a suitable source position grid to allow the SODIX routine the localization of noise sources. The grid used origi- nally (which will be referred to as the reference grid) is of equal length for each TOB (ls = 5 m) and uses a spacing of one fourth of the respective wavelength between source points for all TOBs (∆xs = 0.25λ), see Fig. A.1. Therefore, the resulting grid has a very dense source point distribution for the highest frequen- cies. As SODIX calculates the directivity of each source point once per iteration step, the time needed for the calculation of the source directivity increases ex- ponentially for higher frequencies as the number of source points increases in- versely proportional to the wavelength. This section focusses on the attempts made to reduce SODIX computation time by adjusting the density and length of the source position grid, i.e. the number of points. Verification directivity calcu- lations are conducted using the most tenuous assumable conditions within thePDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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