Abstract: A method of, and apparatus for, monitoring sound-conducting media wherein sound vibrations are transmitted to and absorbed by the sound-conducting medium. The frequency of the transmitted sound vibrations periodically pass through a predetermined frequency range. During an evaluation period the course of any one of the amplitude or phase of the received sound vibrations are compared with the course of the amplitude or phase during one of the preceding evaluation periods, and when there occur predetermined deviations of both amplitude courses or both phase courses from one another there is triggered an alarm signal.

Abstract: A method of, and apparatus for, monitoring sound-conducting media wherein sound vibrations are transmitted to and absorbed by the sound-conducting medium. The frequency of the transmitted sound vibrations periodically pass through a predetermined frequency range. During an evaluation period the course of any one of the amplitude or phase of the received sound vibrations are compared with the course of the amplitude or phase during one of the preceding evaluation periods, and when there occur predetermined deviations of both amplitude courses or both phase courses from one another there is triggered an alarm signal.

Abstract: When applying light gates in burglar alarm systems and the like, in which pulsed sources provide pulsed beams of light (which may be UV, visible, or IR radiation), an evaluation circuit is provided coupled to the pulse source which detects coincidence of radiation received when it is being transmitted and also radiation received when none is transmitted, providing an output signal if: (a) no radiation is received when radiation is transmitted, or (b) radiation is received when none is being transmitted, so that phasing of transmitted radiation, as received, with respect to the transmitter becomes an additional recognition factor.