| 摘要 |
PCT No. PCT/SE94/00078 Sec. 371 Date Aug. 4, 1995 Sec. 102(e) Date Aug. 4, 1995 PCT Filed Feb. 2, 1994 PCT Pub. No. WO94/18650 PCT Pub. Date Aug. 18, 1994Method and means for preventing trespassing and/or unauthorized staying in an area outdoors or indoors, e.g. in a building, part of a building, in vehicles, vessels, etc., sound signals with periodically varying frequencies with the variation of the frequencies out of phase are transmitted from several sound generators in such directions that the sound waves from the sound generator meet within the area to be protected.
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