| 摘要 |
PROBLEM TO BE SOLVED: To reduce reverberations in a cylindrical case with bottom by sticking a piezoelectric element on the inner bottom face of the cylindrical case and filling the entire internal space of the cylindrical case with a polyurethane foam for absorbing reverberation in the compression state of a specified volume ratio, thereby minimizing the reduction of a reflecting wave receiving level. SOLUTION: A polyurethane foam 11 has a hole through which a lead wire 12a is inserted. A filling material 8 made of an elastic material, such as silicone rubber is injected so as to enclose a space between a board 13 and the cylindrical case 2 with bottom. The board 13 has a hole, through which the lead wire 12a is inserted. One terminal of the read wire 12a is soldered to an electrode face of the piezoelectric element 1. A lead wire 12b takes electrical continuity with the cylindrical case 2 via a conductive adhesive 10. The polyurethane foam 11, whose foaming magnification factor is 25-100, is compressed at the volume ratio of 50-90% and packed in an internal space of the case 2.
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