Ambient Noise Intruder Alarm

This invention relates to syystems for providing an alarm when an intruder enters a closed space being supervised. The invention particularly aims to provide an intruder alarm apparatus which consumes a minimal amount of energy when in "watch" condition, that is to say, whilst the integrity of the closed space remains unbroken.

Known intruder alarms include systems which employ a transmitter and a receiver and means for issuing an alarm indication when the signal received at the receiver rises above or falls below a threshold level. The transmitter emits continuously throughout the watch periods, or at least for the major portions of such watch periods. Accordingly, these systems consume considerable energy during the watch periods. Also, the atmosphere is polluted with a considerable quantity of energy from the transmitter, for example ultra-sound if the apparatus uses ultrasonic waves.

In accordance with this invention, there is provided an intruder alarm apparatus for producing an alarm when an intruder enters a closed space being supervised, comprising a first and second receivers of ambient or environmental noise, and a comparator arranged to provide an alarm when the integrity of the closed space is broken so that the difference between the outputs of the two receivers falls below or rises above a threshold level. The receivers may be disposed respectively inside and outside the closed space, to respond when said difference falls below said level. Alternatively, the first and second receivers may be disposed within the closed space and spaced several paces apart. If an intruder has entered the closed space, and then comes closer to one receiver than the other, the outputs of the two receivers will differ and the alarm is produced when this difference exceeds the threshold.

Energy is saved during the watch condition because there is no transmitter to drive. Thus the system is entirely passive. The ambient or environmental noises relied upon may be of any region and may be in any frequency range, whether audible or not, propagated by air.

Embodiments of this invention will now be described, by way of examples only, with reference to the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an intruder alarm apparatus supplied to an enclosed space;

FIGS. 2A and 2B comprise a series of graphs for explaining the operation of the apparatus of FIG. 1; and

FIGS. 3 and 4 are schematic diagrams of second and third embodiments, respectively, of the invention.

Referring to FIG. 1, a wall W encloses a space S which is to be supervised. First and second receivers R.sub.1 and R.sub.2 are placed respectively inside and outside the space S and their outputs are applied to a differential amplifier A acting as a comparator.

The noise N which is naturally present in the atmosphere reaches the external receiver R.sub.2 directly, but only reaches the internal receiver R.sub.1 through the wall W, and is greatly attenuated. However, if the wall W is branched by an intruder, then the ambient noise reaching the internal receiver R.sub.1 is no longer attenuated so much. The amplifier A is responsive to the difference between the outputs from the two receivers and is arranged to provide an output alarm signal AS when the difference .DELTA.S between the outputs S.sub.1 and S.sub.2 falls below a threshold value, the alarm signal energizing means E emitting an audible or visual alarm.

FIG. 2a shows how variations in the intensity of the ambient noise N are followed synchronously by the outputs S.sub.1 and S.sub.2 of the respective receivers R.sub.1 and R.sub.2, assuming that the integrity of the closed space is maintained. The difference .DELTA.S remains constant.

FIGS. 2b shows the corrresponding variations occurring when the wall W is breached. The reduction in .DELTA.S occurring at the time T of break-in produces the alarm signal. The disabling of either receiver may also produce an alarm signal, by producing a change in .DELTA.S beyond preset limits.

The apparatus may include two or more receivers. These may respond preferentially on specific frequencies, whether sonic or ultrasonic, being chosen in dependence upon the transmission characteristics of the particular material of which wall W is made. Thus, FIG. 4 shows a third receiver R.sub.N and a comparator A.sub.N for comparing its output with the output of receiver R.sub.2 and any alarm signal from comparator A.sub.N is supplied to means E.

The or each differential amplifier may include integrators to establish average noise levels, so that only differences between the average noise levels will be responded to by the amplifier. Thus, transient noise bursts will not cause false alarms, for example where a noise burst is delayed by the wall and reaches the internal receiver after the external receiver. Integrators I.sub.1 and I.sub.2 are shown for amplifier A.

The radio or tape player of a car may be adapted to form this type of alarm apparatus.

If there is insufficient ambient noise, artificial noise may be transmitted, perhaps at a preferred frequency.

It will be appreciated that the receiver R.sub.1 need not be fully enclosed.

In an alternative arrangement shown in FIG. 3, two receivers R.sub.1 and R.sub.2 are disposed within a space S to be protected, spaced several paces apart. An intruder coming closer to one receiver will cause a change in the output thereof, the other received remaining stable. It is unlikely that the intruder will follow such a path as to always cause equal changes in the outputs of the two receivers. An alarm is given when the outputs of the two receivers, as compared by the comparator, indicate that the noise intensities received by the respective receivers differ by greater than a threshold value. Preferably the system operates on high frequencies so that small noise changes will cause significant output signal variations at the respective receivers.