Intermittent noises arising from a siphon pipe attached to a toilet bowl are investigated experimentally. The see-through model of a drained system is used to study the relation between noises and flow patterns near the entrance of the siphon pipe. We observed the different manner of the formation of large bubbles for different values of sucking velocity in the siphon pipe. The overall value of the noise in the sucking period increases with increasing the draining velocity. When the draining velocity is constant, the largest noise is generated at completion of the large bubble attached on the wall. The intermittent noises in the early stage of the period are found to be caused by small bubbles separated from the tail of the large bubble. The small bubbles are torn off because of shear stress of the main flow acting on the bubble surface. Another intermittent noises arising in the last stage is caused by periodic formation of the large bubble near the entrance. The detailed process of the bubble formation in the siphon pipe and the noises are discussed in this paper.

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