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Acoustic characterization of the conditions of a water-filled siphon 1000
Pattern recognition has been used and developed as a process of advanced analysis of acoustic signal. Designing a robust pattern recognition system involves three fundamental tasks: signal pre-processing, feature extraction and selection, and finally classifier design and optimization. This paper reports on an application to detect and monitor conditions of a large, water-filled siphon used in underground tunnels. Acoustic signals were collected from 4 hydrophones under various typical siphon conditions and used as input data to study the variation of the acoustic field. The discrete wavelet transform (DWT) was used in feature extraction and K-nearest neighbors (KNN) classification was applied. Subsequently, the system was tested on new unknown data and compared with supervised training samples. The results demonstrated that the acoustic sensors have high reproducibility for collecting signals under operational conditions. The pattern recognition system is also capable of discriminating different pipe conditions but further refinement is needed to improve sensitivity and to compensate for the effect of variable water level and sensor misalignment.
Acoustic data were collected in a siphon which was constructed from 450mm diameter concrete pipes in the Hydraulics Laboratory in the University of Bradford. The siphon was 4.2 m long and 2.0 m high. It was installed on a 500 mm layer of fine sand in an open top box made of 12mm plywood. The siphon was instrumented with four 25 mm hydrophones, 3 of which were installed in the left leg of the siphon. The other hydrophone was installed in the right leg of the siphon 75 mm above the speaker and used as a reference receiver. The source was a 50 mm diameter water resistant speaker in a PVC enclosure which is able to operate underwater. The hydrophones and the speaker were attached securely to two aluminum tubes which were lowered into the opposite legs of the siphon and kept at the same positions in all of the experiments conducted in the siphon. Figure 1 illustrates the equipment used in this experiment. The siphon was filled with clean water to the level of 900 mm below the top of the right vertical pipe (reference water level) in all the experiments except water level test.
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