The mode though the vibration frequency remains almost the same as shown in Fig. 8. In low frequencies the both side of each panel are nodal points however the side lines of a panels become loop points in frequency region above 130Hz. Vibration spectra of center line and edge lines of a panel are compared in Fig.9. Also the over-all levels of the two are almost the same, the vibration levels of the center line are higher than that of edge lines by 5dB〜12dB in frequencies 20Hz〜 160Hz band, it corresponds to the results of modal analysis. The vibration levels on a ceiling might be predicted more rationally by dividing the energy flow into few steps. For example, the dividing points are the top and the bottom of a hanger, reinforced portions of a ceiling panel, and center parts a panel, because the mechanism of the vibration drop in each step is simplified. In Fig. 10, the vibration levels of the ceiling are compared with hanger type. The vibration levels of the ceiling with any hanger must fall on between the levels of bolt connection and of no connection. The level difference is at most 12dB in all frequency ranges and this is the limit that isolation hangers can reduce. It is rather a small value than expected.
If the intervals of hangers become smaller than this experiment, the vibration levels that can be reduced by a hanger design might be expanded
Fig. 9 Difference of Vibration Spectra depending on Measuring Points
Fig. 10 Averaged acceleration Levels of the Ceiling Panels for Four Hangers
4. CONCLUSION
The main results of the experiment by using a well-isolated floating floored cabin mode] are briefly listed below.
(1) The reducible vibration levels of a ceiling by a isolation hanger is at most 12dB in all frequency ranges.
(2) Experimental modal analysis is effective to obtain fine vibration modes of a ceiling structure which is composed of very loosely connected panels.
(3) In order to predict the vibration levels of a ceiling, it will be useful to divide the vibration level drop into few steps to make easy to understand the mechanism of vibration drop.
Acknowledgment
The authors would acknowledge ASK Co. Ltd for their helpful support.
References
1) Harano et al "Effects of Vibration Reduction in Cabins with a New Floating Lining System at Trial Run. Proceedings of Inter-noise98 (1998)
2) Harano et al "Vibration Isolation Characteristics of Silicon-Based Isolator" Proceedings of 1999 Meeting The Institute of Noise Control Engineering of Japan
