| 摘要 |
To supervise the integrity of an enclosed space one terminal of an ultrasonic transmission link is placed inside the space, and the other terminal outside, so that any breach of the walls enclosing the space causes an increase in the received ultrasonic intensity which initiates an alarm. The frequency used may be as low as 18 kHz. In an installation involving several supervised spaces, each space contains a receiver and a plurality of transmitters are outside all the spaces. Individual transmitters may emit coded radiation.
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