GB2249207A

GB2249207A - Security alarm

Info

Publication number: GB2249207A
Application number: GB9023432A
Authority: GB
Inventors: Kalvin S Mudhar
Original assignee: Automated Security Holdings Ltd
Current assignee: Automated Security Holdings Ltd
Priority date: 1990-10-27
Filing date: 1990-10-27
Publication date: 1992-04-29
Also published as: GB9023432D0

Abstract

An audible sound 10 generator for a security alarm contained within a housing 18 is described which further includes means for acoustically detecting the injection of a foam into the housing. For this purpose, a low power sound transmitter 20 and a sound receiver 22 accoustically coupled to one another are mounted in the housing. The acoustic coupling between the low power sound transmitter and the sound receiver is modified by the injection of foam into the housing. <IMAGE>

Description

SECURITY ALARM Field of the invention The present invention relates to security alarms.

Background of the invention Most burglar and fire alarms are required to produce a loud sound when an alarm condition is detected and use either a bell or a siren for this purpose. The alarm can be rendered ineffective by tampering with the sound generator to prevent it from operating correctly.

Various techniques have been attempted to silence a sound generator of a security alarm and, for this reason, the sound generator is commonly installed in a housing which is itself fitted with devices to detect tampering. For example, it is known to sound the alarm if any attempt is detected to open the housing while the alarm is set.

In order to interfere with the sound generator without opening its housing, it has been known for criminals to inject a hardening foam into the housing of the sound generator and the present invention is concerned with preventing a sound generator from being immobilised in this way.

Summary of the invention According to the present invention, there is provided an audible sound generator for a security alarm contained within a housing which further includes means for acoustically detecting the injection of a foam into the housing.

The means for acoustically detecting the injection of a foam into the housing may suitably comprise a low power sound transmitter and a sound receiver acou5tically coupled to one another, the acoustic coupling between the low power sound transmitter and the sound receiver being modified by the injection of foam into the housing.

Preferably, the low power sound transmitter may operate in the ultrasonic range so as not to be audible to humans.

The sound emitted from the transmitter may conveniently be modulated for additional security, the modulating signal being used in the demodulation of the received signal.

In use, the low power sound transmitter will produce a sound at all times or at regular intervals which will have a known pitch, amplitude and modulation envelope. If foam is injected into the housing, the sound sensed by the receiver will be severely attenuated and will suffer harmonic distortion. Changes in the amplitude and/or frequency content of the received signal will therefore be sensed when a foam is injected before the foam has a chance to harden and this will initiate an alarm condition. The alarm condition may cause a sound to be emitted from the high power audible sound generator or may be used to summon help in other ways, such as by sounding another audible alarm or through transmission of a prerecorded message using the telephone system.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a section through an audible sound generator of the invention, and Figure 2 is a block diagram of an electronic circuit for detecting the injection of a foam into the sound generator of Figure 1.

In Figure 1, an audible islgn dover sound generator 10 has a gong 12 with a striker 14 and a circuit mounted a circuit board 16. The sound generator 10 is located within a wall mounted housing 18 which protects the generator 10 against the elements and unauthorised tampering. Any attempt to open the housing 18 will create an alarm condition at the remote control unit (not shown) which drives the sound generator 10.

If foam is injected into the housing 18, it will muffle the gong 12 and possibly prevent the striker 14 from contacting the gong 12. It is therefore necessary to safeguard against this form of interference with the sound generator.

To detect the injection of foam, a low power ultrasonic acoustic transmitter 20 formed for example by a piezoelectric crystal is mounted inside the housing 18 and the sound which it emits is sensed by a receiver 22. Care is taken when mounting the transmitter 20 and receiver 22 to ensure that the most of the coupling takes place through the air in the housing 18 rather through the supports of the transmitter 20 and the receiver 22.

As shown in Figure X, an oscillator 40, the output of which is modulated by a modulator 42 and amplified by a power amplifier 44 is used to drive the transmitter 20 so that the latter emits a sound of known pitch and repetition frequency. The sound is sensed by the receiver 22 and fed through a preamplifier 50 to a decoder 52 which also receives reference signals from the oscillator 40 and modulator 42. If a sound is transmitted at any time but not received clearly at the same time, then an alarm condition is initiated and a signal is sent to the central control unit.

An advantage of the invention is that it directly detects the property of the foam which interferes with the opera f the sound generator (the inability of sound to pass without attenuation through the housing) rather than some other property which may not affect the operation of the sound generator. Tampering can therefore be sensed reliably than by, say, detecting interruption of a light path since such detection may not work with a transparent foam and may give a false alarm in the event of smoke which obstructs the light path but does not prevent the sound generator from operating.

Claims

1. An audible sound generator for a security alarm contained within a housing which further includes means for acoustically detecting the injection of a foam into the housing.

2. A sound generator as claimed in claim 1, wherein the means for acoustically detecting the injection of a foam into the housing comprise a low power sound transmitter and a sound receiver acoustically coupled to one another, the acoustic coupling between the low power sound transmitter and the sound receiver being modified by the injection of foam into the housing.

3. A sound generator as claimed in claim 2, wherein the low power sound transmitter operates in the ultrasonic range.

4. A sound generator as claimed in claim 2 or 3, wherein the sound emitted from the transmitter is modulated.

5. A sound generator constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.