Impedance Variations Due to a Single Cylindrical Hole in a Rubber Sheet
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The absorption of acoustic energy by a rubber sheet containing a single cylindrical hole was reported in reference 1. In that case, the hole was sited so that the top of the hole was located in the middle of the rubber sheet and the bottom of the hole was against an infinitely hard surface upon which the sheet was mounted. There were two resonant systems, the first being the walls of the holes vibrating radially as the thickness of the rubber varied and the second being the motion of the top of the hole normal to the hard surface, similar to a drum-head. We have extended this work to the case where the hole entends through the rubber sheet and is capped with a steel plate. The additional mass of the steel plate causes the resonant frequency of the rubber-steel-hole system to be at a much lower frequency than the rubber alone and also eliminates the drum-head resonance. We have analyzed this system using a dedicated, time dependent finite element program and have also verified the results of the finite element program by use of a small water-filled guided wave tube. The results show that the impedance is a useful parameter which is both sensitive to the geometry of the entire system and is a good descriptor of the resonant system.