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A new method to more accurately estimate the impact mass when an elastic ball is impacting on a surface of an elastic plate is suggested. Conventionally, frequency ratio (FR) and center frequency techniques have been widely used for a mass estimation. However, these methods do not work when the mounting effect of a sensor is dominant and/or the operating background noise becomes high. Thus a new technique to eliminate these effects by using a time-frequency analysis is attempted and verified through an experiment. It is revealed that the proposed method is valid for estimating the center frequency of an impact response signal easily even in a noisy environment, thus making a mass estimation of an impact source on a plate type structure more successful than the conventional techniques.

The bells cast during the Silla dynasty were hung over various types of hollows or cavities in the ground. The sound pipe, called the eumtong, which opens through the top, appears in most of the bells. Another characteristic of these bells is that they ring with a very long beating frequency, sometimes for more than 3 s. The King Songdok bell (‘‘Emille’’ bell) cast during the Silla dynasty (771 A.D.) is the most famous one among these. Its height is 3.66 m, the diameter of bell mouth is 2.23 m, and its mass is about 20 000 kg. A very extensive experiment was designed and conducted. To visualize how the bell radiates sound, a cylindrical acoustic holograph was attempted. The total measurement points for the holograph are 32×12; 32 microphones at every 30 degree. The acceleration signals were also measured extensively at many points, which were 11×12. The acoustic characteristics of the inside cavity of the bell were also analyzed. This paper explains the role of the acoustic cavity in the ground, the sound pipe, and the source of long beating. Sound visualization based on the holographic measurement will demonstrate how its sound radiates in space.

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