GB656399A - Improvements in or relating to the detection of movements by means of a system of stationary acoustic waves - Google Patents

Abstract

656,399. Burglar and like alarms. CHARLIN, A. M. B. Sept. 11, 1947, No. 24989. Convention date, Oct. 1, 1946. [Class 118 (i)] Apparatus for detecting movements in an enclosed space comprises an alarm operated by current derived from the modulation produced by the disturbance created in a stationary field of acoustic waves by such movements. A microphone 3 in the enclosed space to be protected receives sound waves from a loudspeaker 2 energized at an almost inaudible frequency by an oscillator 1. Owing to the reflections from the walls and from objects within the space a field of acoustic standing waves is set up and the amplitude of the microphonic current depends upon the position of the microphone within that field. Any appreciable movement within the space will cause a series of fluctuations in the microphone output, i.e. will modulate the "carrier wave due to the oscillator 1. This modulated output is amplified at 4 and detected at 5, any A.C. component appearing in the rectified output being amplified at 8 and rectified at 9 where it serves to energize a relay 10 and so energize an alarm bell 11. In the absence of any disturbance in the protected space the output of the rectifier 5 is a steady D.C. and no energizing current therefore reaches the alarm relay 10. The output from the rectifier 5 also serves to keep energized an auxiliary alarm relay 6 which if the oscillator or amplifier power supply has been cut, e.g. by a burglar, releases and connects an independent power supply 13 to an auxiliary alarm bell 7. Several oscillator, loud-speaker, microphone systems may be connected to one amplifying and alarm system, switch means enabling each microphone to be isolated in turn in order to determine which is causing the alarm to operate.