However fairly large discrepancy between two values is still observed. The calculation by equivalent mass agreed fairly well in former experiment²⁾. In this case, the vibration levels of the beam might be affected by the energy transmitted at the edges of the ceiling. More simplified experimental apparatus ought to be employed to obtain isolation effects of hangers.

　

Table 2 Resonant Freq. of two lsolators

Fig. 5 Comparison of Measured and Calculated Vibration Reduction of Silicon Isolator.

　

3.2 Vibration Features of the Ceiling

Vibration level drop between the beam and the ceiling are compared with each hanger type in Fig. 6.

These Vibration drop curves should be the same because the beam and ceiling structure is identical.

The vibration levels of the panels are higher than that of the beam in frequencies lower than 250Hz except at 50HZ band, and become lower by about 20dB in the ranges above 250Hz.

The increase in vibration levels of panels in low frequency is caused by two reasons. One reason is that the energy transmitted from the floating floor to the ceiling is large in low frequency ranges and the other is that a flexible structure has lower vibration modes than a rigid structure. The vibration drop about 20dB in high frequency ranges is due partially to the diffusion of vibration energy to panels and partially to the energy dissipation by the damping of the panels. Theses values depend on the distance from the beam and the panels and the way they are connected. The glass wool layers attached to the panels seem to contribute adding damping to the panels.

The lowest vibration mode of the ceiling when no hanger was connected is illustrated in Fig. 7. The mode is that the edge lines become the node points changes significantly when a soft isolator is connected in the center of the ceiling, even and the central part vibrates like a drum.

　

Fig. 6 Vibration Difference between the Beam and the Panels.

Fig. 7 The lowest Vibration Mode of the Ceiling without a Hanger.

Fig. 8 The lowest Vibration Mode of the Ceiling with a Silicon Hanger.

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