3.2 Strategies to improve the results It is clear that one cannot be happy with the results of figure 5. Let us try to improve them. From eq. (18) it is apparent that the problems certainly arise in the inversion of A C jj jj H H , namely when this difference is small. To try to increase this difference one will change our structure by adding point masses, so to change the behavior of sub-structure A and C, while B remains unchanged. 3.2.1 Adding mass to sub-structure A A mass of 35 grams has been added to nodes 1, 3, 5 and 15, 17, 19. The new results for B 11 H and B 12 H are shown in figure 6. Fig. 6 B 11 H and B 12 H with added masses in sub-structure A It can be observed from figure 6 that the disturbance observed between 1000-1200 Hz in figure 5 moves into the range 8001000 Hz, generally improving the results. 3.2.2 Adding mass to sub-structure B An alternative could be to add mass to the sub-structure B itself. Adding 35 grams at co-ordinates j = 9 and 11, leads to the results presented in figure 7. Fig. 7 B 11 H and B 12 H with added masses of 35 grams in sub-structure B As sub-structure B is altered, its natural frequency (identified in figure 5) changes to the left. However, the disturbances remain in the area of 1000-1200 Hz and one can conclude that they are caused caused by sub-structure A. So maybe one can improve the results by adding some more mass to sub-structure B, to cause the natural frequency to deviate from the disturbance. Let us now add masses of 70 grams. The results are displayed in figure 8. 200 400 600 800 1000 1200 1400 1600 1800 2000 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Receptance [dB] (ref. 1 m/N) Frequency [Hz] Hbjj11 Hb2jj11-1% 200 400 600 800 1000 1200 1400 1600 1800 2000 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Receptance [dB] (ref. 1 m/N) Frequency [Hz] Hbjj12 Hb2jj12-1% 200 400 600 800 1000 1200 1400 1600 1800 2000 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Receptance [dB] (ref. 1 m/N) Frequency [Hz] Hbjj11 Hb2jj11-1% 200 400 600 800 1000 1200 1400 1600 1800 2000 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Receptance [dB] (ref. 1 m/N) Frequency [Hz] Hbjj12 Hb2jj12-1% 389
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