The circular cylinder pipe is extensively used for the supplement of the gas. The leakage of this gas induces the catastrophic problem when it leases into open area in the city without any monitoring. This paper investigated theoretically the propagation of the acoustic wave of the circular cylinder pipe containing the gas in a pipe for the development of the leakage monitoring system. The acoustic wave is propagated through the waveguide of the circular pipe with the characteristics acoustically coupled by the gas contained in a cylinder and the shell. However, as a special case, the acoustic waves in a metal pipe containing gas are corresponded closely to the uncoupled in-vacuo shell waves and to the rigid-wall duct fluid waves. In this case, the dominant acoustic energy can be estimated at the frequencies in which coincidence between the shell modes and the acoustic modes occurs. In the paper, the characteristics of the dominant waves are theoretically investi-gated and analyzed experimentally with a long steel pipe. The measured data is clearly analyzed by the continuous wavelet transform and by spectral density analysis. Finally, a bandwidth of the frequency for a good estimation of the correlation would be suggested through the consideration of the coherence function between two sensors.

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