This invention relates to apparatus for detecting movement of air or changes in air pressure (sometimes called sub-sound or infra-sound because of the very low frequencies, typically in the region of 0.1 to 5Hz, which are involved) . Such apparatus may be used to detect the breaching of a closed environment. The closed environment may be, for example, a closed and empty vehicle such as an aircraft or spacecraft or submarine or even a road vehicle or a closed room within a building, for example a secure laboratory or a bank vault. Breaching of the closed environment may occur when, for example, a person or animal tries to gain access to or tries to exit the closed environment or when a normally secure door, window or hatch fails or the closed environment is otherwise breached by, for example, failure of a seal or a fault. Conventionally, when an aircraft is parked at an airfield or is otherwise closed and empty (for example when the aircraft is being towed), the hatches or doors, although closed, are usually unlocked and are simply sealed by tape so that an intrusion can only be detected by detecting removal of the tape.

In the interests of improved security, efforts have been made to devise apparatus which can be used on a parked (or otherwise closed and empty) aircraft to provide a reliable indication that the closed environment has been breached by, for example, an intrusion. Most attempts to devise such apparatus rely on the use of microphones to detect sub-sound or infra-sound resulting from the attempt by an intruder to open an aircraft hatch. Microphones generally use a tensioned diaphragm which is fixedly mounted at itε periphery to the

microphone housing. In the case of an electrostatic or electret microphone, the diaphragm may be mounted between two conductive plates so that movement or vibration of the diaphragm in response to air movements are detected as changes in capacitance. The sensitivity and characteristics of such microphones tend, however, to vary. Moreover, the tension within the diaphragm is liable to change with the ambient conditions, for example with changes in temperature, so resulting in undesired changes in the response characteristics of the microphone.

It is an aim of the present invention to provide an apparatus for detecting the breaching of a closed environment which should be capable of producing more reproducible and/or sensitive results than previously proposed apparatus.

In one aspect, the present invention provides apparatus having a diaphragm movable as a whole in response to changes in air pressure or air movements resulting from, for example, the breaching of a closed environment.

In apparatus embodying the invention, allowing the diaphragm or diaphragms to move as a whole in response to air movements means that the sensor should be unaffected by changes such as temperature, etc. The diaphragm may be sealed to a housing to define a closed chamber behind the diaphragm which should enable a good response at the frequencies desired to be detected which are very low and in the region of a few Hz, for example 1.75Hz.

In another aspect, the present invention provides apparatus having separate diaphragms responsive to changes in air pressure or air movements and to vibrations and/or noise. The separate diaphragms may independently sense changes in air pressure or air

movement and vibration or may provide outputs which are subsequently processed to provide respective signals indicating changes in air pressure or air movement and vibrations. In another aspect the present invention provides sensing means for sensing movement of air within an environment, the sensing means comprising a transducing means for generating a signal in response to air movement, the transducing means comprising a rigid diaphragm mounted within a housing by mounting means allowing the whole diaphragm to move as a body relative to the housing with the mounting means sealing the diaphragm to the housing so as to define behind the diaphragm a closed chamber which is unaffected by movements of air outside the housing, the diaphragm thereby being operative to move in response to air movements resulting from the breaching of the closed environment. Processing means responsive to the movement of the diaphragm may be provided for generating an alarm.

The diaphragm may be provided by a dynamic, generally a moving coil, loudspeaker having a substantially conical diaphragm and a chamber behind the diaphragm may be sealed to extend the normal frequency response range of the loudspeaker beyond 3OHz down to about 1Hz. The mouth of the conical diaphragm may be about 3 to 4 inches (6 to 10 cm) in diameter. A base loudspeaker may be used because of its sensitivity in the relevant frequency range. Where a closed chamber is provided behind the diaphragm, the chamber may be provided with air bleeding means, for example a small hole of about 0.5mm (or possibly even smaller) in diameter, to enable the air pressure within the closed chamber to be allowed to acclimatise to changes in the ambient pressure, for

example if the aircraft is parked at an airport at high or low altitude.

Where the diaphragm is sealed to a housing, a deformable porous foam member sealed by an air- impermeable sealing means such as a grease or oil may be used to form the seal. As another possibility, an expansible bellows arrangement or any suitable mounting means which seals the coupling of the diaphragm to its housing against the entrance of air yet also allows the diaphragm to move as a whole relative to the housing may be used.

The sensing means may comprise a further diaphragm which may be disposed to face in a direction opposite to that of the diaphragm to enable a signal to be provided indicating whether or not the sensing means is subject to vibration. Such a signal may be used to inhibit the generation of an alarm when a signal indicating vibration above a given threshold is derived.

The use of two diaphragms enables the apparatus to determine whether an environment is subject to vibrations and enables the apparatus to avoid giving a false alarm in the event of such vibration. This may be of particular advantage where the apparatus is to be used on a parked aircraft because, for example, the take-off and landing nearby of a large jet such as Boeing 747 may induce severe vibrations in the parked aircraft. In addition, this should enable the apparatus to be unaffected by atmospheric conditions such as thunderstorms which may result in vibrations that, in the absence of the further diaphragm, might result in a false alarm and also prevents deliberate attempts to induce a false alarm by generating loud noises.

In another aspect, the present invention provides sensing means for sensing movement of air, the sensing means comprising two transducing means each comprising

a diaphragm mounted within a housing by mounting means allowing the diaphragm to move relative to the housing, the transducing means being arranged so that front surfaces of the two diaphragms face in opposite directions. Processing means may be provided for deriving signals representing any air movement within the environment and any vibration exerted on the environment from the transducing means and an alarm generated when a signal indicating movement of air but no signal indicating vibration above a given threshold is derived. Compensation can thus be provided for the effects of vibrations, for example vibrations resulting from severe weather conditions or the take-off or landing of other aircraft or other loud noises, so helping to avoid giving a false alarm.

Where two diaphragms are provided, at least one of them may be sealed to a housing so as to define behind the diaphragm a closed chamber unaffected by movements of air. If two diaphragms are provided, then the two diaphragms may form part of respective identical transducing means and signals from the two transducing means summed to provide a signal representing any sub- sound and subtracted the signals to provide a signal indicating any vibrations induced in the closed environment. As another possibility, one of the transducing means may have air vents or apertures enabling movements in air to be communicated to both sides of the diaphragm so that the diaphragm does not respond to such air movements but only to other influences such as vibrations or audio signals. This enables separate transducing means to be used to detect air movement and vibrations.

If two diaphragms are provided, they may be axially aligned and opposed to one another so as to be spaced

apart by an air gap which is sufficiently large to avoid having any affect on the measurements being made. This should enable more accurate measurements to be made because it ensures that the same air mass or volume is sensed by both diaphragms.

The giving of an alarm may be inhibited if an audio signal deriving means derives an audio indicating signal above a given threshold. This enables the possibility of a false alarm to be avoided in the event of loud noises, for example thunderclaps or the noise of an aircraft taking off or landing.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a diagrammatic part-sectional view through one example of a sensing means embodying the invention for use in an apparatus in accordance with the present invention;

Fig. 2 is a view similar to Fig. 1 of another example of a sensing means for use in an apparatus in accordance with the invention;

Fig. 3 shows a block diagram of one example of processing means for an apparatus in accordance with the invention for processing signals supplied by the sensing means;

Fig. 4 is a view similar to Fig. 1 of another example of sensing means for use in an apparatus in accordance with the invention;

Fig. 5 is a block diagram similar to Fig. 3 of one example of processing means suitable for use in an apparatus in accordance with the present invention for processing signals from the sensing means shown in Fig.

4;

Fig. 6 shows a block diagram of another example of processing means suitable for use in an apparatus in

accordance with the present invention for processing output signals from the sensing means shown in Fig. 4;

Fig. 7 is a schematic block diagram of one example of an apparatus in accordance with the invention; Fig. 8 illustrates diagrammatically one example of an operation panel for an apparatus in accordance with the invention;

Figs. 9A to 9C show a flowchart for illustrating operation of an apparatus in accordance with the present invention;

Fig. 10 illustrates a very diagrammatic view of apparatus in accordance with the invention;

Fig. 11 illustrates very diagrammatically the placement of apparatus in accordance with the invention in an enclosed environment;

Fig. 12 is a diagrammatic view, with part of the housing cut away, of another embodiment of apparatus in accordance with the invention;

Fig. 13 is a schematic view of one end of the housing of the apparatus shown in Fig. 12 to show one possible display;

Fig. 14 is a block diagram similar to Fig. 3 of another example of processing means suitable for use in processing signals from the sensing means shown in Fig. 1 or Fig. 2;

Fig. 15 shows a block diagram of another example of processing means suitable for use in processing output signals from the sensing means shown in Fig. 4;

Figs. 16a to lβh show graphs of voltage against time indicating representative signals produced at various points in the processing means shown in Fig. 14; and

Fig. 17 illustrates very diagrammatically the use of apparatus embodying the invention for enabling sensing of seismic activity, for example. It should, of course, be understood that the

drawings are not to scale and that like components are referred to by like reference numerals throughout.

Referring now to the drawings, Fig. 1 shows a part- sectional view of one example of sensing means 1 in accordance with the present invention suitable for use in an apparatus for detecting the breaching of a closed environment.

As shown in Fig. 1, the sensing means 1 comprises a first transducing means 2 having a housing 3 within which a rigid diaphragm 4 is mounted so as to enable the diaphragm 4 to move as a body relative to the housing 3. In the example shown in Fig. 1, the rigid diaphragm 4 comprises a rigid cone having its periphery coupled to a flange 2a of the housing 2 by means of a foam mounting member 5 which is sufficiently flexible or deformable to allow the cone 4 to move relative to the housing 3. The cone 4 has a rear extension 4a carrying a moving or voice coil 6. The voice coil 6 is surrounded by either, as shown, an electromagnet 7 or a permanent magnet. The cone 4, voice coil 6 and electromagnet 7 form a conventional dynamic base loudspeaker. Indeed, a commercially available loudspeaker such as the Audax AT080M0 manufactured by Audax a French company of Chateau-du-Loir, near le Mans, France 72500 may be used. The transducing means 2 shown in Fig. 1 differs from the commercially available base loudspeaker in that the housing 2, conical diaphragm 4 and mounting means 5 define a chamber 8 which is completely sealed apart from a small air bleed which enables the pressure within the chamber 8 to follow or acclimatise to the ambient pressure. The small air bleed may be provided by the air-permeable foam mounting means 5. However, as the characteristics of the foam mounting means are not reproducible, it is preferred that the foam mounting means is sealed or made non air-permeable by a coating

5a of grease or the like and that a small air bleed hole 3a is formed in the housing 3. Typically, the air bleed hole should have a diameter less than about 0.5mm. Any other suitable air bleeding arrangement which allows for the pressure within the closed chamber to acclimatise to the ambient pressure may be used. The sealing of the chamber 8 extends the frequency sensitivity of the transducing means 2 beyond the normal 3OHz roll off frequency of the conventional loudspeaker to enable detection of sub-sound or infra-sound in the range of for example 0.1 to 5Hz or at least to enable detection of frequencies down to about 0.5-lHz. The air bleed provided to the chamber 8 by either the air bleed hole 3a or the air permeability of the oam mounting means 5a enables the quasi-DC or extremely low frequency (below 0.05Hz, typically) response of the transducing means 2 to go to zero and avoids changes in the overall ambient air pressure affecting the response of the transducing means 2. A completely sealed chamber 8 could be used if, for example, a separate sensor was provided to detect the ambient air pressure enabling the apparatus to compensate for changes in the response of the loudspeaker with the ambient air pressure. The sensing means 1 shown in Fig. 1 also comprises a second transducing means 20 which is identical to the transducing means 2 apart from the fact that the housing 30 of the second transducing means 20 is formed with apertures 31 so that the chamber 8a is not sealed. The apertures 31 may be formed in any desired pattern but their total area should be equivalent to the open area at the front of the conical diaphragm 4 so that any changes in air pressure are transmitted equally to the front and the back of the conical diaphragm 4 so that conical diaphragm 4 of the transducing means 20 is not

responsive to changes in air pressure.

The first and second transducing means 2 and 20 are arranged so that the front faces of the cones 4 face in opposite directions. As shown in Fig. 1, the cones 4 are axially aligned and the transducing means 2 and 20 are spaced apart by an air gap 9 by, for example, spacer bars 10 (two of which are shown) which couple the housings 3 and 30 together at specific points around the periphery of the housing so as to allow free movement of air into the air gap between the two transducing means 2 and 20. The spacing or separation of the first and second transducing means 2 and 20 should be sufficiently large to avoid the gap affecting the measurements being made. Typically, the separation of the first and second transducing means 2 and 20 should be at least 1cm. The maximum separation between the first and second transducing means 2 and 20 is determined by the desirability for the first and second transducing means to respond to the same air mass and may, of course, be constrained by the size requirements for the sensing means 1. In practice, the separation between the first and second transducing means 2 and 20 may be about 3cm. However, the air gap may, as indicated above, be larger or smaller than 3cm. Fig. 2 shows another example of a sensor la in accordance with the present invention. The sensor la shown in Fig. 2 differs from that shown in Fig. 1 in that the positions of the first and second transducing means 2' and 20' are reversed so that the second transducing means 20' is placed on top of the first transducing means 20. Of course, the positions of the first and second transducing means shown in Fig. 1 could also be reversed. In addition, the conical diaphragms of the first and second transducing means 2' and 20' shown in Fig. 2 are coupled to their respective housings 3 by flexible rubber

or plastics material bellows arrangements 50 rather than the foam mounting means 5 shown in Fig. 1.

As shown in Fig. 2, the first and second transducing means 2' and 20' may each be provided with a perforated front grill 11 as conventionally used for loudspeakers. Such grills could also be provided in the sensing means 1 shown in Fig. 1. The air inlet area of the apertures Ila of the grills 11 should be equal to the area of the apertures 31 in the housing 3 of the second transducing means 20 so as to ensure pressure equalisation on both sides of the conical diaphragm 4 in the second transducing means 20 and to match the air inlets of the first and second transducing means 2' and 20'.

Because the chamber 8 of the first transducing means 2 or 2' is sealed, the conical diaphragm 4 of the first transducing means 2 or 2' responds to movements of air or changes in air pressure caused by the breaching of a closed environment within which the sensing means 1 or la is located by, for example, a person trying to gain access to or exit from the closed environment. However, because the second transducing means 20 or 20' has a non- sealed chamber 8a, the conical diaphragm 4 of the second transducing means 20 or 20' does not respond to such air movements or changes in air pressure but only to audible sounds and vibrations occurring in the direction of the axis of the conical diaphragm. The second transducing means 20 or 20a thus enables separate detection of vibrations and can be used to avoid the sensing means 1 or la giving a false alarm if, for example, the enclosed environment within which the sensing means 1 or la is located is subject to vibrations from an external source such as, in the case where the sensing means is located on an aircraft, the take-off or landing of another aircraft nearby, or atmospheric disturbances such as thunderstorms or even a deliberate attempt to generate

a false alarm by making a loud noise outside the closed environment.

In operation of the sensing means 1 or la, axial movement of the conical diaphragms 4 results in an electrical signal in the corresponding voice coil 6 with a frequency equal to that of the frequency of vibration of the conical diaphragm. Fig. 3 illustrates one example of processing means 40 for processing signals from the output leads 6a, 6b of the moving coils 6. In the interests of simplicity, the first and second transducing means 2 and 20 are shown only schematically in Fig. 3. It will, of course, be appreciated that the processing means shown in Fig. 3 may be used with either the sensing means 1 or the sensing means la. As shown in Fig. 3, the output from the voice coil 6 of the first transducing means 1 is coupled to the input of an amplifier 41 of a first processing arrangement 40a. The amplifier 41 may be of any suitable conventional form. Generally, the amplifier 41 will be an operational amplifier. The output 41a of the amplifier 41 is supplied to a first band-pass filter 42 which is arranged to pass frequencies in the range of from l-3Hz. Reference may be made to any suitable text book on electronics for suitable forms for the filter 42. For example, the second edition of "The Art of Electronics" by Horowitz and Hill published by the Press Syndicate of the University of Cambridge describes various suitable forms of active filters, such as Butterworth filters. The output 42a of the first filter 42 is coupled to the input of a second amplifier 43 which may be, for example, a variable gain amplifier of known construction. The output 43a of the variable gain amplifier 43 is supplied to the input of a second filter 44. The second filter 44 is a sharp filter designed to pass the frequency which it is desired to detect, in this

example 1.75Hz plus or minus 0.25-0.5Hz. The chapter entitled "Active Filters and Oscillators" in the second edition of Horowitz and Hill describes examples of suitable circuits for forming the sharp filter 44. For example, the sharp filter 44 may be a so-called "biquad" filter as shown in Fig. 5.2 and as described at pages 278 and 279 of the second edition of Horowitz and Hill. Other suitable forms of state-variable filters or other band pass filters of the required frequency may be used. The output of the sharp filter 44 is supplied to a full wave rectifying circuit 45 of conventional form. The rectified output is then supplied to an amplitude demodulator 46 which smooths the rectified output using a lower frequency signal. The amplitude demodulator 46 is coupled to a peak detector 47 which again may be of conventional form. The peak detector 47 detects the maximum voltage of the amplitude demodulated signal supplied by the amplitude demodulator 46. The detected peak is then supplied to a threshold circuit which may be, for example, a Schmitt circuit or a conventional comparator which provides a high output signal only when the peak exceeds a given voltage. The threshold detector 48 thus provides an output signal OPl when the amplitude of vibration of the conical diaphragm 4 at the frequency of 1.75Hz exceeds a threshold value determined by the threshold circuit 48.

The second transducing means 20 is coupled to a second processing arrangement 40b which is matched to the processing arrangement 40a of the first transducing means 2 and so comprises an amplifier 41', a first filter 42', a variable gain amplifier 43', a sharp, that is a narrow band pass, filter 44', a full wave rectifier 45', an amplitude demodulator 46', a peak detector 47' and a threshold circuit 48. The second processing arrangement outputs a high output signal 0P2 when the amplitude of

the vibration of the conical diaphragm 4 of the second transducing means 20 at the detection frequency, in this case 1.75Hz, exceeds a threshold value. As the second transducing means 20 does not respond to changes in air movement or air pressure but only to vibrations, a high output signal OP2 indicates that the sensing means 1 is subject to vibrations above a given threshold at a frequency of 1.75Hz, for example, where the sensing means 1 is mounted on a parked aircraft, due to, for example, atmospheric effects such as thunderstorms and like or the take-off or landing of a large plane such as a Boeing 747, or other external loud noises.

The output from the voice coil 6 of one of the first and second transducing means 2 and 20 is also coupled to a third processing arrangement 40c which provides an output signal when the associated transducing means detects an audio signal. The third processing arrangement is similar to the first and second processing arrangements but differs in that only a single filter 42a is provided. The filter 42a is similar to the filters 42 and 42' but is designed to pass frequencies in the range of from 1.5-500Hz so as to enable detection of low frequency audio signals such as may be caused by atmospheric effects or passing aircraft. The third processing arrangement provides a high output signal OP3 when the amplitude of vibration of the conical diaphragm 4 of the associated transducing means 2 or 20 within the frequency pass band of the filter 42a exceeds a given amplitude as determined by the threshold circuit 48". Preferably, the third processing arrangement is coupled to the first transducing means 2 because, as indicated above, the first transducing means 1 is more sensitive, in the frequency range concerned, than the second transducing means 20. At least the amplifiers 41, 41' and 41" and the

filters 42, 42' and 42a may be mounted within a suitable housing 401 as shown in Fig. 1 onto the body of the sensor 1. This should enable signal losses and introduction of noise due to electrical connections to be reduced. Of course, if desired and if space permits, all of the processing means 40 may be provided within the housing 401.

Fig. 4 illustrates another example of a sensing means lb in accordance with the present invention. In the example shown in Fig. 4, the two transducing means 2" each have a sealed rear chamber 8. As shown in Fig. 4, the construction of the first and second transducing means 2" is similar to that of the first transducing means 2' shown in Fig. 2. However, of course, the construction of the transducing means 2" may be similar to that of the transducing means 2 shown in Fig. 1.

Fig. 5 illustrates one example of processing means for processing the output signals from the voice coil 6 of the sensing means lb shown in Fig. 4. The third or audio processing arrangement 40c shown in Fig. 5 is the same as that shown in Fig. 3. The first and second processing arrangements 40'a and 40'b however differ from those shown in Fig. 3. Thus, as shown in Fig. 5, the outputs of the sharp filters 44 and 44' of the first and second processing arrangements are supplied to respective adding and subtracting circuits 60 and 61 of conventional form. The summed and subtracted signals are then processed as described above with reference to Fig. 3 by full wave rectifying circuits 45, 45', amplitude modulators 46, 46', peak detectors 47, 47' and threshold detectors 48, 48' to provide output signals OPl and OP2. Because the conical diaphragms 4 of the two transducing means 2' are vibrating in the opposite directions, the signal resulting from the processing following the subtracting circuit 61 provides a signal

0P2 when vibration occurs at the detection frequency, 1.75Hz in this example, while the signal resulting from the further processing of the signals supplied by the summing circuit 60 provides a high output signal OPl when movement of air or a pressure change occurs as a result of a person trying to gain entry into the enclosed space within which the sensing means is situated.

Fig. 6 illustrates an alternative arrangement for processing the output signals from the sensing means lb. The processing means shown in Fig. 6 differs from that shown in Fig. 5 merely by virtue of the fact that the outputs of the filters 44 and 44' are summed after amplitude modulation. This arrangement does, of course, require further full wave rectifying and amplitude modulating circuits 45 and 46 because subtraction has to occur prior to rectification.

As another possibility, the addition and subtraction may occur before the filtering and amplification or at any suitable point in the filtering and amplification chain 41 to 44.

In other respects, the processing means shown in Fig. 6 is the same as that shown in Fig. 5.

Fig. 7 is a block diagram of one example of an intruder detection apparatus 100 using the sensing means 1, la or lb described above. The apparatus 100 is powered from a rechargeable, battery 101 so enabling the apparatus to operate as a stand-alone piece of equipment. The battery 101 is coupled to a power supply unit 102 via a battery charger 103 and a power control unit 104, all of known form. The power supply unit 102 is capable of being coupled to any suitable mains voltage MV to enable recharging of the battery 101 via the power control unit 104 and the battery charger 103. The battery charger 103 is arranged to adapt the rate of charging of the battery to the actual state of charge of the battery as is known

in the art.

The power control unit 104 comprises conventional voltage regulation and level shifting circuitry and supplies three output voltages on lines 104A, 104B and 104C.

The power supply lines 104A and 104B provide the positive and negative power supply rails, typically ±5 volts, for the sensing means 1 and the processing arrangement. For simplicity in Fig. 7, the processing arrangement is illustrated simply as a block 400 providing three outputs OPl, 0P2 and 0P3 which are supplied to a microprocessor 105. The third power supply line 104C (again typically providing 5 volts) from the power control circuit 104 is used to power the microprocessor 105. This enables the power control circuit 104 to disconnect the power supply to the power supply lines 104A and 104B if the battery voltage drops below a certain desired minimum level while still allowing power supply to the microprocessor 105 so that the status of the apparatus can be indicated. The microprocessor 105 is associated with a storage device 106 which may be of any suitable conventional form, for example the storage device 106 may be a hard disc drive. The apparatus 100 is also provided with a keypad 107 for inputting instructions to the microprocessor 105. A display 108, for example an LCD display is provided for enabling display of instructions input to the microprocessor 105 and also of data stored in the memory 106. As will be explained below, the microprocessor 105 also receives input signals from an entry/exit key 109 and a tilt switch 110. The apparatus may also be provided with suitable radio transmitters or transceivers 111 and 112 for supplying signals to a radio pager and a base station, respectively. Of course, suitable

transceivers may be provided for supplying signals via any appropriate communication link, for example a telephone or computer communication link. The microprocessor 105 also controls operation of a number of LEDs 113-116 for providing a visual indication of a low battery voltage, entry into the enclosed space in which the apparatus 100 is situated, exit from the enclosed space and the fact that an unauthorised entry into the enclosed space has occurred. The microprocessor may also control a siren 117 or other similar device for providing an audible signal.

Fig. 8 indicates very schematically the operation panel 100a of the apparatus. As shown in Fig. 8 the display 108 may be controlled by the microprocessor 105 so as to provide an indication 108a of the current battery charging level.

As indicated above, the apparatus 100 is designed to be a portable standalone piece of equipment which operates independently of the power supply to the environment within which the apparatus is to be located.

Figure 10 shows a very diagrammatic perspective view of apparatus 100 in accordance with the invention. As shown the apparatus 100 has a case 120 which is generally formed of metal or a plastics material and has the shape of a briefcase or attache case. The case 120 has a carrying handle 120 to facilitate transport of the apparatus to and from the closed environment within which it is to be located in operation. The control panel 100a shown in Figure 8 is mounted into, as shown, a side wall of the case 120. The case 120 also has a number of air inlets, in the example shown two air inlets 120b and 120c each of which communicates with the air gap 9 between the transducing means of the sensor 1. The air inlets 120b and 120c may communicate with the air gap 9 by means of air pipes 121 as shown in phantom lines in Figure 10.

The remaining components of the apparatus 100 are mounted within the case 120, for example within a foam mounting block (not shown). Figure 10 illustrates a first block 1020 which represents a common housing for the power supply unit 102, battery charger and power control unit 104, a second block 1010 which represents the location of the rechargeable battery and a third block 1050 which represents the printed circuit board or boards carrying the processing arrangement 400, the microprocessor 105 and associated circuitry. If desired, a separate printed circuit board may be provided for the driving circuit for the display 107. The case 120 has a socket 122 for enabling connection to a mains power supply MV as discussed above. The case 120 will usually be locked or otherwise secured in a manner which allows the case only to be opened for maintenance, repair or replacement of the operational components of the apparatus. Of course, any other suitable housing arrangement for the apparatus 100 may be used. The operation of the apparatus 100 will now be described with reference to the flowchart shown in Figs. 9A to 9C.

Figure 11 shows very schematically a closed environment 200 (which may be the passenger cabin of an aircraft) having at least one door or hatch 200a and within which the apparatus 100 is located.

The apparatus 100 is carried by its case handle 120a and placed at a suitable location within the passenger cabin 200 of an aircraft by an authorised keyholder either of the airline or of the airport security. The authorised keyholder then activates the apparatus by inserting an entry/exit key (not shown) into the entry/exit key slot 109. As another possibility, the authorised keyholder may be required to key in a code via the keypad 107. Once the microprocessor detects the

entry of the key at step S2 in Fig. 9A, the microprocessor then determines at step SI whether or not a signal indicating incorrect orientation of the sensing means 1 is being received from the tilt switch 110 which, where the sensing means 1 is in a separate housing, is, of course, mounted in the housing of the sensing means 1. If the microprocessor detects that the orientation is not correct, a message may be displayed on the LCD screen 108 at step S3 and an audible warning given by means of the siren 117.

Once the sensing means 1 has been correctly oriented, the microprocessor checks at step S4 the current battery voltage and displays it on the LCD screen as indicated at step S5. The microprocessor 105 will also check, as shown in Fig. 9A at step S6 whether the battery voltage is low and, if so, may light the battery low warning LED and display on the LCD display 108 a message indicating that the apparatus cannot be alarmed until the battery has been recharged. Assuming that the battery voltage is satisfactory then the microprocessor may check at step S8 for any inputs via the keypad 107 from the authorised keyholder and then execute any input instructions at step S9, for example alter the date or time stored in the memory 106. To reduce even further the possibility of an unauthorised person tampering with the apparatus, the microprocessor 105 may be programmed only to allow inputs via the keypad 107 to alter the date or time setting etc. when the apparatus is not in use and is plugged into a mains power supply, for example to recharge the battery, at the base station.

When the microprocessor subsequently detects at step S10 that an arm/disarm key has been removed, the exit LED 115 begins to flash to warn the authorised keyholder that he should exit the aircraft as the apparatus will shortly

become armed. The microprocessor 105 may also display a warning message to the authorised user on the LCD screen 108.

Assuming that the authorised user does not reinsert the arming key, the microprocessor 105 then waits for the preset time period, for example thirty seconds, at step Sll to enable authorised keyholder to exit the aircraft and ensure all hatches 200a are closed. The apparatus 100 is now in a condition in which any subsequent entry into the aircraft will be detected and an alarm given, unless the arm/disarm key is inserted to deactivate the apparatus 100. In a preferred embodiment, the apparatus 100 will be arranged to give a first signal indicating entry into the closed environment and then, if the arm/disarm key is not inserted, a second signal indicating that the entry into the closed environment is unauthorised. During its operational condition, the microprocessor 105 will monitor, as shown at step S12 in Figure 9B, the battery voltage and provide a warning at step S12a if the battery voltage is low. The warning preferably will take the form of an alert signal sent via the radio transceivers 111 and 112 to both the base station and a radio pager carried by either an employee of the airline or a member of the airport security responsible for the aircraft.

As illustrated in Fig. 9B at steps S13 and S13a, the microprocessor may optionally transmit the current status of the apparatus to the base station 111 at predetermined times, for example every hour. In addition, the microprocessor may, as shown by steps S14 and S15, periodically store data regarding the status of the apparatus in the memory 106.

Once the apparatus has been armed, the microprocessor continuously monitors, at steps S16 to S18, the output lines OPl, OP2 and OP3 from the

processing arrangement to see if a high signal has been received.

In the example shown, the microprocessor 105 first checks at step SI6 to see whether the signal is an audio signal, that is whether the signal OP3 is high. If the answer is yes, the microprocessor resets, at step S17, the count CT of its internal or an external counter (not shown) and continues to monitor for other signals.

If no audio signal is detected at step S16, the microprocessor then checks at step S18 whether a high output signal OP2 indicating a vibration signal has been received. If the answer is yes, the microprocessor again resets the counter and continues to monitor for further output signals OPl, OP2 and OP3. When the microprocessor detects at step S19 that a high output signal OPl has been received indicating the presence of sub-sound, it increments at step S20 the count CT of the counter and then checks at step S21 whether the count of the counter has exceeded a predetermined value CTc. If the answer is yes, the microprocessor activates the entry LED at step S22, provides a signal, preferably to the base station and/or radio pager indicating that the aircraft has been entered, and then checks at step S23 to see if the arming key has been inserted. If the answer at step S23 is yes, the microprocessor then returns to step S8. If, however, the arming key is not inserted, the microprocessor determines that the entry is unauthorised and supplies a second signal via the radio transceivers 111 and 112 to alert the base station and the authorised keyholder or user on his radio pager. The status of the entry LED may be altered. For example on entry the entry LED may flash and upon detection of an unauthorised entry the entry LED may be lit continuously or may flash at a different rate. As another possibility, separate LED for

indicating entry and a subsequent alarm may be provided. Where desired, and if the siren 117 does not present too big a drain on the battery, the siren 117 may also be activated. If the count has not reached the predetermined value, then the microprocessor continues to monitor for signals until the count reaches the predetermined value when the alarm will be given indicating the occurrence of an intrusion. As will be appreciated from the above, the counter is only incremented if a signal is received which indicates air movement or a change in air pressure resulting from breaching of the closed environment, for example entry into or exit from the aircraft. Any vibrations or audible signals due to severe weather conditions or the take-off or landing nearby of large airplanes is avoided because these signals are separately detected by the second and third processing arrangements and used to reset rather than increment the counter. The provision of the radio transceivers to supply signals to a base station and a radio pager enables appropriate personnel to be alerted immediately to the fact that an intrusion has occurred. However, if desired for, for example, costs reasons, these could be omitted so that the apparatus simply provides an indication that the closed environment has been breached (by lighting the entry LED) when it is later inspected by authorised personnel.

In the apparatus described above, high output signals OPl to OP3 are provided when a peak exceeds a given threshold. However, alternatively or additionally the rate of change of the outputs from the amplitude modulators 46, 46' and 46" may be determined by using a simple differentiation circuit and a high output signal OPl, OP2 or OP3 provided when the rate of change exceeds

a given threshold. This may give a more accurate detection of a breach if the closed environment. As another possibility, the rate of change may be determined by using the microprocessor to control the gain of the variable gain amplifiers 43, 43' and 43a. The gain of these amplification stages may also be adjusted as required for the characteristics of a particular aircraft.

Generally, the power control unit will operate to disconnect the power supply to the sensing means 1 if the battery voltage drops below a predetermined level, for example 8 to 7 volts. The use of a separate power supply line 104C for the microprocessor 105 enables the microprocessor to detect the situation and to, for example, transmit warning signals to the base station or radio pager indicating that the battery needs recharging.

Figure 12 shows a modified version of the apparatus with part of the housing or casing 120 cut away to show the internal layout. The apparatus differs from that described above in that the sensor 1 and associated air pipes 121a are arranged so that the air pipes 121a communicate with air inlet ports 121'b and 121'c provided at a surface of the casing 120 which is designed to stand on the aircraft floor. Feet 120b are provided on the casing 120 to allow air to enter the air inlet ports. Filters (not shown) may be provided in the air inlet ports 120'b and 120'c to prevent ingress of dirt or other foreign matter.

The apparatus shown in Fig. 12 is basically functionally identical to that described above except that, in this case, arming of the system and other control functions are enabled by use of an electronic key card unit 1070 of conventional form into which an appropriately programmed electronic key card is inserted. As shown, the electronic key card unit 1070 is

located immediately below a display panel 1080 which may again be a liquid crystal display panel. The display panel and associated control buttons 107a to 107d enable arming and disarming of the apparatus and other control functions by means of a menu-based system which will be described in more detail below.

The battery 1010, power supply unit 1020, microprocessor 105 storage device 106 and associated control circuitry in the form of a processor unit 1050 and a radio transceiver 1060 are provided as modular units separately housed within the casing 120, for example within apertures in a foam support. In this case, a single radio transceiver 1060 is provided enabling communication with either a single pager or a base station, which may then transmit signals to a number of pages, as discussed above with reference to Figure 9. Figure 12 also shows a conventional mains plug socket 1090 for enabling recharging of the battery and a pc port, for example a conventional parallel or serial interface port, 2000 for enabling data stored by the microprocessor on the data storage device 106 to be downloaded or transferred to another device such as personal computer. Electrical connection between the various components of the apparatus is illustrated only very diagrammatically in Fig. 12 but will in fact correspond closely to that shown in Fig. 7 with power supply to the display 1080, card reader 1070 and sensor 1 being provided via the processor unit 1050 to enable the microprocesor to switch off the power to those components when required.

The sensor 1 shown in Fig. 12 may be of the form shown in Fig. 1, 2 or 4. Where the sensor 1 is of the form shown in Fig. 1 or Fig. 2, then the processing means which forms part of the microprocessor unit 1050 may be of the type described above with reference to Fig. 3.

Similarly, where the sensor is of the form shown in Fig. 4, then the processing means may be of the type shown in Fig. 5 or Fig. 6.

The apparatus shown in Figs. 12 and 13 differs from that shown in Figs. 8 and 10 in that the apparatus is operated using an electronic key card. This has several advantages. In particular, it enables individual key cards to be uniquely identified so enabling the apparatus to determine which particular key card was used to access the apparatus. Moreover, it enables different key cards to have different levels of authorisation so that certain key cards, for example those of the security personnel, can only arm or disarm the apparatus while other key cards may enable the user to change operational conditions within the apparatus, for example to enable changing of the entry and exit times allowed for an operator to gain access to the apparatus when situated on an aircraft and then to locate the aircraft once the apparatus has been activated or armed. Such a key card may also be used to enable the preset settings of the variable gain amplifiers 43a to be adjusted.

As indicated above, the radio tranceiver 1060 enables signals to be supplied to a base station and/or to radio pagers. Further key cards may be provided to enable a particular apparatus to be associated with particular uniquely numbered pagers or groups of pagers. In addition, a further key card may be provided to enable the apparatus to be shut down so as to prevent any electromagnetic interference while the apparatus is being transported by air.

Fig. 13 shows the summary display which is provided by the LCD display screen 1080 when the apparatus is plugged in and being charged. This summary shows a number of selectable display windows or menus. The display window 1081 indicates the sensor level, that is

the preset selected for the variable gain amplifiers 43a which should be set to the optimum for the particular aircraft with which the apparatus is to be used. The display window 1082 indicates at "EXT" the exit time and at "ENT" the entry time set for the apparatus. Display window 1083 indicates at "LOG" the approximate amount of available data storage memory within the apparatus and at "BAT" the current charging level of the battery.

The display window 1084 provides information relating to the identity of the apparatus and indicates its group number, its unique serial number and the number of any pagers associated with it.

The display window 1085 indicates the status of related pagers and base station. These are shown as being off because the apparatus is not armed.

The display window 1086 indicates the current time and date.

Beneath the display window 1086 are four small display windows 1087 each aligned with a respective function key 107a to 107d. The small display windows 1087 indicate the functions of the function buttons 107a to 107d. It may, of course, be possible to adapt the LCD display screen 1080 for touch sensitive input in which case the control buttons 107a to 107d could be omitted and functions activated simply by touching the appropriate ones of the display windows 1087.

The control buttons 107b and 107c enable a user to scroll up and down between the various windows 1081 to 1087 while the control button 107d enables the selected menu to be entered and the control button 107a accepts any valid settings and returns to the summary screen.

From the summary screen, either of the function buttons 107a and 107d is used to access a start menu which displays a number of windows each of which when selected enables a user to access further subwindows.

The start window may display a "time" window allowing the user to access submenus to change the entry and exit times, to set the date and clock time, a "sensor" window which allows the user to access submenus to change the sensor level, a "display" window allowing the user to access submenus to change the contrast and backlighting of the display, for example, a "log" window to allow a user to access submenus to show the history of events held in the internal log of the apparatus, a "pager/base" menu to allow the user to access submenus to send test messages to a pager/base station, to switch the pager/base on/off and to allow changes to the pager beeper and a "summary" window to return the user to the summary screen. One or more of these windows and associated sub-menus may be available to a user depending upon the authorisation code carried by his key card.

Fig. 14 illustrates a further modified form of processing means which may be used in any apparatus in accordance with the present invention where a sensor of the type shown in Fig. 1 or Fig. 2 is used.

The processing means shown in Fig. 14 has three processing arrangements 40"a, 40"b and 40"c like the processing means shown in Fig. 3. Identical or substantially similar components are identified by the same reference numerals in Figs. 3 and 14.

The third processing arrangement 40"c differs from the third processing arrangement 40c shown in Fig. 3 merely by the fact that the variable gain amplifier 43 is replaced by a fixed gain amplifier similar to the amplifier 41".

The first and second processing arrangements 40"a and 40"b have the same components as the first and second processing arrangements of Fig. 3 up to the peak detectors 47 and 47'. The output of the peak detector 47' of the second

processing arrangement 40"b is supplied to a threshold detector 48' to provide the output 0P2 as described above with reference to Fig. 3. However, in addition, the outputs of both the peak detectors 47 and 47' are supplied to a summing circuit 60' and a subtracting circuit 61'. The output of the subtracting circuit 61 is supplied by a first differentiating circuit 62 of known form to a variable gain amplifier 43a similar to the variable gain amplifiers 43 and 43' and thence via a line £ to a further summing circuit 60". The output of the differentiating circuit 62 is also supplied to another similar differentiating circuit 62a. The output of the differentiating circuit 62a is supplied via another variable gain amplifier 43a and a line E to the further summing circuit 60". The output from the subtracting circuit 61' is also supplied via a line E directly to the summing circuit 60". The output of the summing circuit 60' is supplied via a low pass filter 63 which passes signals in the frequency range DC to 0.01 Hz, an invertor 64 and a further variable gain amplifier 43a to the further summing circuit 60". The output G of the further summing circuit 60" is supplied via an amplifier 41a to the threshold detector 48 to provide the output OPl. As will be appreciated from the above, the outputs OP2 and OP3 are derived in a manner similar to that described with reference to Fig. 3. The output OPl provides a signal which is a combination of the difference between the outputs of the peak detectors 47 and 47' which effectively indicates the velocity of any air movement, plus the first and second time differentials of that signal minus the output of the summing circuit 60 which represents a combination of vibration and ambient air movement. The inventors have found that this combination of signals enables precise

and accurate detection of air movements resulting from the opening of an aircraft door or hatch.

The variable gain amplifiers 43 and 43' are ganged together so as to always have the same gain and the gain of these amplifiers is adjustable under the control of the microprocessor 105. The gains of the variable gain amplifiers 43a are arranged so as to be adjustable, under control of the microprocessor 105 (see Fig. 7), between a number of different preset levels. This is advantageous because different aircraft will produce different characteristic signals when a hatch or door is open. The ability to change the gains of the variable gain amplifier 43a between different preset levels thus enables the apparatus to be optimised for different types of aircraft.

Typically, for a Boeing 747 aircraft, the ratio between the outputs is:

E'. E'. E = 1:2:0.25

where E is the direct output from the subtracting circuit 61, E is the output of the variable gain amplifier 43a connected to the first differentiation circuit 62 and E is the output of the variable gain amplifier 43a connected to the second differentiating circuit 62a. Fig. 15 shows processing means similar to that shown in Fig. 14 but suitable for use with the sensor shown in Fig. 4. The third processing arrangement 40'c of Fig. 15 corresponds to that of Fig. 5 except that the variable gain amplifier 43a of Fig. 5 is replaced by a fixed gain amplifier 41' as in the arrangement of Fig. 14. The first and second processing arrangements 40"'a and 40"'b of Fig. 15 correspond to the first and second processing arrangements 40'a and 40'b of Fig. 5 up to the peak detectors 47 and 47'. Thereafter, the processing means shown in Fig. 15 corresponds to that shown in Fig. 14

except that the output of the invertor 64 is not coupled via the associated variable gain amplifier 43a to the summing circuit 60' but is used as a signal to control the gain of the variable gain amplifiers 43 and 43' in known manner to provide automatic gain control. This provides an alternative way of adjusting the output signal OPl to compensate for vibration and ambient air movement.

It will, of course, be appreciated that Fig. 14 could be modified to use the automatic gain control circuit shown in Fig. 15 and that Fig. 15 could, alternatively, be modified to remove the feedback path and to couple the output of the amplifier 43a associated with the invertor 64 to the summing circuit 60' in the manner shown in Fig. 14.

In use, the apparatus once fully charged is generally placed in an upright position extending along the main passenger aisle of a fully powered down aircraft having all of its hatches (except that required for exit by the operator) secured. Once the apparatus has been armed and the operator has exited the aircraft, the apparatus monitors continuously for any breaches of the hatches as discussed above with reference to Figure 9.

Figs. 16a to 16h are graphs of voltage against time showing the waveform outputs at various points in the processing means shown in Fig. 14 when the apparatus was situated and armed in the passenger compartment of a Boeing 747 parked at an airfield experiencing relatively calm weather conditions with only a light breeze blowing. The waveforms resulted from the opening of a small hatch a long distance from the sensor 1. Comparative sensitivity of the waveforms is such that each division on the y axis V in Fig. 16a, c, e and g represents 100 millivolts while each division on the y axis in Fig. 16b, d, f and h represents 40 millivolts. Each division on

the x or time axis t represents one second.

Fig. 16a shows the output A of the filter 42. Figs. 16a and 16b show the outputs A and B, respectively, of the filters 42 and 42' and thus represent the sensed air movement and vibrations. Figs. 16c and 16d represent the outputs C and D of the peak detectors 47 and 47' and thus the air movement envelope and vibration envelope, respectively. Fig. 16e shows the output E of the subtracting circuit 61' and thus represents the air movement signal with vibration removed while Fig. 16f represents the output E of the first differentiating circuit 62 and thus shows the differentiated air movement signal. Fig. 16g represents the output G of the summing circuit 60" and thus represents the combination of the output of the subtracting circuit 61' with the outputs £ and E of the variable gain amplifiers 46a associated with the first and second differentiating circuits 62 and 62a minus the output from the variable gain amplifier 43a coupled to the invertor 64. Fig. 16h represents the output OPl of the threshold detector 48.

As can be seen by comparing Fig. 16a to lβh, the processing means shown in Fig. 14 enables the opening of the small hatch to be detected and isolated from surrounding draft noise (Fig. 16a) and vibration (Fig. 16b) to provide a clear output signal OPl (Fig. lβh) for supply to the microprocessor 105 to enable the microprocessor to determine, as discussed above with reference to Figs. 9a to 9c, whether an alarm should be generated. The inventors have found that the amplitude modulation envelope produced by an intrusion is very reproducible for a given aircraft and therefore, as an alternative to using peak threshold detection or the combination discussed with reference to Figs. 14 and 15, a conventional correlation circuit may be provided to

compare the shape of the detected amplitude modulated envelope with that of the amplitude modulated envelope expected for that aircraft.

As another possibility, frequency modulation may be used in conjunction with amplitude modulation to enable more information to be extracted from the incoming signal.

It will be appreciated that the second transducing means only detects vibration in one perpendicular plane. If desired, of course, three mutually orthogonal sensing means could be provided to enable detection of vibration in any direction.

Various modifications and changes will be apparent to the person skilled in the art. Apparatus and sensing means in accordance with the invention may be used in any situation where it is desired to detect the breach of a closed environment. The closed environment may be, for example, a closed and empty vehicle other than an aircraft, for example a spacecraft or submarine or even a road vehicle or even a closed room or a closed suite of rooms within a building, for example a secure laboratory or a bank vault. Breaching of the closed environment may occur when, for example, a person or animal tries to gain access to or tries to exit the closed environment or when a normally secure door, window or hatch fails or a bulkhead or seal cracks or fails. Where it is not likely that vibrations will give rise to false alarms, the second transducing means may be omitted and reliance made simply upon the signals from the first transducing means. Similarly, if the particular conditions in which the apparatus is situated mean that false alarms resulting from audio signals are unlikely, then the audio signal processing arrangement may be omitted. The apparatus may also be used in sealed pipe lines and the like to monitor for breaches in the sealed

pipe line.

The fact that the apparatus is battery operated and portable is particularly advantageous where the apparatus is intended to be used in a vehicle such as an aircraft and enables the apparatus to be easily removed and kept in a secure place during operational use of the vehicle. This may be of particular advantage in the event of a subsequent accident involving the vehicle as data stored in the apparatus may be of assistance in determining the reasons for the accident, for example the data may help eliminate the possibility of sabotage by an unauthorised entry while the aircraft (or other vehicle) was parked.

As another possibility, if it is not desired that the apparatus be completely standalone, then the apparatus may be designed to be operated by a mains power supply with, possibly, a back-up battery to ensure continued operation in the event of a mains power supply failure. A mains operated version of the apparatus may be suitable for use in buildings, for example in bank vaults and secure laboratories.

It will of course be appreciated that movable diaphragms other than conical diaphragms may be used and that the frequency of sub-sound to which the apparatus is sensitive may be adjusted to meet the particular use to which the apparatus is to be put.

The sensor 1 of Fig. 1 (or the sensor la of Fig. 2 or the sensor lb of Fig. 4) may be used for purposes other than detecting the breaching of a closed environment. For example, the sensor may be associated with appropriatelymodified processing means and software to enable detection of vibrations or audible signals of a given frequency only in the absence of air movements, for example. Also, apparatus embodying the invention may be used to monitor or log particular frequencies in a particular environment by, for example, supplying the

output signals OPl, OP2 and OP3 of the processing means of any one of Figures 3, 5, 6, 14 or 15 to appropriate conventional data logging devices.

As another possibility, a sensor embodying the invention may be used to enable detection of seismic shock waves. Fig. 17 illustrates very schematically one arrangement in which the sensor 1, la or lb may be used to detect seismic shock waves. As shown in Fig. 17, the sensor 1 is mounted so as to be in communication with the interior of a sealed chamber 202 securely fixed to a concrete base 203 embedded in the surface 204 of the earth. As shown in Fig. 17, the sensor 1 may be mounted within the chamber 202. Alternatively, so as to facilitate maintenance of the sensor 1, the sensor may be mounted in a separate housing detachably secured to the chamber 202 and communicating therewith via an air passageway. Signals representing air movement resulting from seismic shock waves, related vibrations and extraneous external noise may be derived in a manner similar to that described above with reference to Figs. 3 to 9C with suitable modification to enable, for example, seismic shock waves and related vibrations to be distinguished from surrounding traffic noise or the like. For the reasons given above, at least first stages of amplification and filtering for the signals derived from the sensor will generally be provided at the sensor. The remainder of the processing circuitry may be provided separately adjacent the chamber 202. Alternatively, it may be possible to provide for transmission via microwave or RF of data signals to a remote base station at which further processing, logging of the processed signals and generation of suitable alarm signals indicating the presence of seismic activity may be derived.