TECHNICAL FIELD

[001] The present invention relates generally to the field of security, in particular to the detection of unwanted bodies, and more in particular, to intrusion and fire detection.

BACKGROUND OF THE INVENTION

[002] Traditionally security systems are dependent on sensors. Even though their functionality and mode of operation changes from system to system, all of them require some form of information acquisition. Let it be vision, sound, pressure, contact or temperature, some form of data is collected through sensors and analyzed in order to be able to detect whether an unauthorized entity has intruded into controlled premises, or whether an unwanted fire has started.

[003] Regarding intrusion detection, intrusion systems can be classified according to the manner in which data is collected, such as point sensors, line, planar or sector. Point sensors usually involve a contact sensor emitting an alarm once the contact is broken. These types of sensors have the drawback that in case the existence of the alarm is previously known, they can be easily detected by physical inspection of the entry elements and eventually bypassed.

[004] Line sensors work on the principle that a communication link on a straight path is disrupted. These types of alarms are activated when the path from the transmitter to receiver is interrupted by the act of intrusion. Similar to point sensors, it is possible to detect these types of line sensors with simple measurement devices and several methods can bypass the line of measurement by not disturbing the previously detected path. Also, a large number of line sensors, formed together in a plurality of lines, is required to provide a workable system which can mitigate this simple bypassing, resulting in overly complex and costly systems.

[005] Planar sensors are based on the principle that intruders are detected once the change in different variables, such as pressure or temperature, is measured on an entire surface. Pressure sensitive surfaces are a simple example of this type of system, usually integrating piezoelectric elements distributed homogeneously on a surface. Planar sensors have the drawback that they are very costly to install as their installation has to be planned prior to the construction of the space to be secured. On the other hand, if planar sensors are to be installed in existing premises, they often require such a large amount of reconstruction of the existing environment that costs become prohibitive. [006] Sector sensors are based on the principle of visual inspection of the environment or measuring global aspects of the environment, for example, room temperature, in a sector- partitioned environment. Security cameras or thermal cameras can aid in visual inspection of a certain zone whereas the temperature of the room can be monitored by a battery of temperature sensors propagated around the space to be monitored. However sector sensors suffer the drawback that they have limited visibility, and contain zones hidden from the sensor ^' s detecting ability. In the case of vision cameras, an intruder hiding behind an object or obstacle cannot be detected. Thermal cameras, or temperature sensors, can easily be bypassed by correct temperature camouflage of the intruder. This can be achieved by regulating the body temperature or the environment's temperature through different methods. Hence manual inspection, such as a physical inspection by a security guard, to aid in resolving ambiguous unknown hidden zones is mostly compulsory.

[007] Volumetric alarms are based on the principle of detecting disturbances not of a line or a plane, but of a three dimensional volume. Hence they can be considered a special case of sector sensing. For example the area of interest can be flooded by either microwave or infrared radiation. Any movement in the zone disturbs the field and sets off an alarm. However these sensors also suffer the drawback of blind spots, as the radiation is blocked by obstacles in an environment preventing any movement from affecting the radiation field. They are also particularly insensitive to slow movements in the sense that they do not detect a disturbance by a slow moving body. Furthermore, they can also be affected by external electromagnetic or light fields. Hence an intruder here can also try to flood the protected zone by an independent radiation source in order to dissipate changes to its presence, thereby effectively camouflaging himself.

[008] Regarding fire detection, automatic fire sensors can take different forms depending on the physical property associated to the fire detected. Heat detectors respond to the increase of the temperature associated to the fire, and have the inconvenient of the thermal lag, the time it takes the temperature to increase at the location of the detector from the start of the fire: detection is not possible until the moment in which temperature has increased a substantial amount, a moment in which the fire might already be in an advanced state.

[009] Smoke detectors and fire gas detectors are devices detecting the smoke or combustion gases associated to the fire source. Similarly to heat detectors, smoke and fire gas detectors have the limitation that the fire can only be detected when a substantial amount of smoke or gas has reached the detector.

[0010] Flame detectors use optical sensors to detect flames. Flame detectors have the inconvenient that only those combustion processes which produce flame can be detected.

[0011 ] Security and thermal cameras can also be used, combined with appropriate computer algorithms, to detect the visible or thermal effects of fire. However cameras suffer the drawback that they have limited visibility, and contain zones hidden from the sensor ^' s detecting ability.

[0012] Therefore, due to the usage of widely differing technologies, current security units which aim at acting as both intrusion as well as fire alarms have increasing complexity and therefore correspondingly high costs, both for the units as well as the finally implemented on- premise security installation.

[0013] Therefore a need exists to effectively solve the aforementioned problems by providing an improved intruder and fire alarm which can effectively detect the presence of unauthorized intruders and unwanted fire sources without the aforementioned inconveniences.

SUMMARY

[0014] It is therefore an object of the present invention to provide solutions to the above mentioned problems. In particular it is an object of the invention to effectively detect the presence of unwanted bodies in a secured zone. In the preferred embodiments, such bodies comprise any body which, due to its inherent characteristics, such as size, or composition, or reflectivity, or density, causes a change in the premise's sonic impulse response. In one embodiment such unwanted body can be an unauthorized intruder. In another embodiment, such unwanted body can be a body of heated air, indicating a fire.

[0015] In particular, it is the object of the present invention to provide a system for effectively detecting the presence of unwanted bodies in a secured zone.

[0016] It is another object of the present invention to provide a method for effectively detecting the presence of unwanted bodies in a secured zone.

[0017] It is another object of the present invention to provide a computer readable medium comprising instructions, once executed on a computer, for performing a method for effectively detecting the presence of unwanted bodies in a secured zone.

[0018] The invention provides methods and devices that implement various aspects, embodiments, and features of the invention, and are implemented by various means. The various means may comprise, for example, hardware, software, firmware, or a combination thereof, and these techniques may be implemented in any single one, or combination of, the various means.

[0019] For a hardware implementation, the various means may comprise processing units implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. [0020] For a software implementation, the various means may comprise modules (for example, procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by a processor. The memory unit may be implemented within the processor or external to the processor.

[0021 ] Various aspects, configurations and embodiments of the invention are described. In particular the invention provides methods, apparatus, systems, processors, program codes, computer readable media, and other apparatuses and elements that implement various aspects, configurations and features of the invention, as described below.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0022] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements in the different drawings. Corresponding elements may also be referenced using different characters.

[0023] FIG. 1 depicts an unwanted body detection system according to one embodiment of the invention.

[0024] FIG. 2 depicts a diagram of the different parts of a sonic impulse response used by the unwanted body detector.

[0025] FIG. 3 depicts a method of unwanted body detection according to one embodiment of the invention.

[0026] FIG. 4 depicts a secured zone's plot of the presence parameter according to one aspect of the invention.

[0027] FIG. 5 depicts a secured zone's plot of the presence parameter according to another aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] FIG. 1 depicts a system 100 for detecting the presence of unwanted bodies according to one embodiment of the invention. A protection, or secured, zone 190 is defined as any zone, region, premise, structure, or area in which a controlled access is desired, and in which the presence of unwanted bodies is to be detected. The invention provides an intrusion detecting system 1 10 which detects the presence of unwanted bodies within protection area 190. Such bodies comprise any body which, due to its inherent characteristics, such as size, or composition, or reflectivity, or density, causes a change in the premise's sonic impulse response. In one embodiment such unwanted body can be an unauthorized intruder. In another embodiment, such unwanted body can be a body of heated air, indicating a fire. 23. In yet other embodiments the unwanted body may also comprise a person, or a machine, or a body of heated air, or air current, or any other form which causes a change in the sonic impulse response of the secured zone.

[0029] The body detecting system 1 10, comprises among others, an emitter 120, a receiver 130, a processing unit 160 and an alarm actuator 170. A first signal processing unit 140 might be found necessary in order to adapt a test signal to be transmitted by emitter 120 to the properties of the emitter and the emission environment 190. Such signal processing unit may comprise an amplifier, or a digital to analog converter with a sampling frequency at least twice the highest frequency used, among others. Likewise a second signal processing unit 150 might be found necessary in order to adapt the measured signal produced by receiver 130 to the properties of the rest of the receiver. Such signal processing unit may comprise a filter, amplifier, or noise reduction means, or an analog to digital converter with a sampling frequency at least twice the highest frequency used, among others.

[0030] The principle of operation of the unwanted body alarm is based on a sound field being transmitted until the entire protection zone is flooded. Once flooded, the sound field is measured in order to determine any changes within the protected environment. In situations wherein there are supposed to be no changes, any determined change indicates the presence of at least one unwanted body. Such a scenario can be given for example in an office space, or hangar, which is supposed to be void of workers, moving machinery, or fire, at night time. In case an intruder enters the protected zone, or in the event of a fire, the intruder alarm will generate an alarm signal notifying of the unwanted body detection.

[0031 ] The determination as to whether there have been any changes within the protected environment is based on determining variations in the impulse response of the monitored environment, which acts as an acoustic fingerprint of the protected zone. Hence, based on the transmitted and received sound, a bandlimited impulse response is computed periodically. Every room has its own acoustical fingerprint, which is defined by an impulse response. Any disturbance of the propagated sound field from one sampling instant to the next reflects itself as a change in the room's impulse response, hence providing an immediate indication of whether a trespass has taken place or not. If the impulse response does not change from one sampling instant to the next this indicates that there has been no change in air body in the protected environment. Therefore the intruder alarm of the invention solves the previously mentioned shortcomings as it can measure the changes in entire rooms in an almost instantaneous way even though these changes do not occur in the line of sight of the sensors. It also provides a less complex and more economical solution to combined intruder and fire detection, as a same and single technology is used for detecting both types of unwanted bodies, together with a corresponding single installation.

[0032] An impulse response refers to the reaction of any dynamic system in response to some external sudden change or trigger. Hence the emission of an audible or ultrasonic sound and its corresponding reception enables the sampling of the acoustic image of the protected space periodically to monitor the changes in the room. The room, which corresponds to the dynamic system, is excited by a predefined sound, which corresponds to the external change, to measure the response of the room. At the moment of intrusion since an intruder is added to the content of the room, these contents change and so does the corresponding room's acoustic fingerprint. The amount of change in the room ^' s impulse response is measured or determined over time to detect the intrusion. Similarly, In the event of a fire, thermal fluctuations and convective currents associated to the fire source causes the acoustics to rapidly fluctuate.

[0033] FIG. 2 depicts the different parts of an impulse response, as a function of time. Once a sound field is propagated and flooded in a premise, the measured reaction, caused by the different mechanisms in which the sound propagates throughout the environment, can be classified into three different types. In a first type, direct sound 210 represents the direct arrival of the signal from the emitter to the receiver though a straight path connecting both. An intruder or fire will only affect the direct sound if it happens to be in the line of sight between the source and the emitter.

[0034] In a second type, early reflections 220 represent the arrival of first reflections of the sound with the walls of the room. The wave fronts caused by these reflections can still be distinguished individually. An intruder or fire in the vicinity of the source or the receiver will probably affect the early reflections, but these will be possibly unaffected by a receiver or fire in an obstructed or hidden area of the secured zone.

[0035] In a third type, late reflections 203 represent the multiple reflections of the sound with the different room walls. The wave fronts caused by these reflections can no longer be distinguished individually. They constitute the late reverberation, which floods the entire protection zone. The late reverberation is affected by any change in the acoustics of the secured zone, like the one produced by a fire or a hidden intruder, even if this change happens to be in an obstructed area of the secured zone. Therefore monitoring the late reverberation is key to protect the entire secured zone, including any obstructed area not in the line of sight of the emitter and the receiver or its vicinity.

[0036] The impulse response, with its three different parts ,is the most general description of the acoustic fingerprint of a given space. Almost any other quantity commonly used in acoustics, such as reverberation time, the direct-to-reverberant ratio and early decay time, can be derived from the impulse response.

[0037] The unwanted body detection system has several benefits over the previously mentioned implementations. The entire room is filled with the sensing sound field. Hence the unwanted body detection system has no blind spots and can detect out-of-sight bodies. It also has a straightforward and inexpensive installation and can combine two different applications in one device. Hence the intrusion detection system is an economic and not complex system. [0038] Furthermore, working as an intrusion detection system, an intruder will be detected no matter from what entry point the trespass takes place. Hence the intrusion detection system is independent of the point of entry. Also, it detects very small and very slow movements. Hence the intrusion detection system is sensitive and has fast reactivity.

[0039] Additionally, working as a fire detection system, fire sources can be detected when a convection flow has started but temperature has not increased yet, smoke has not reached the detector yet and flame has not been produced yet. Therefore the fire detection system can detect fires at the very early states.

[0040] Although the unwanted body detection system operates also with also with audible sound, the inventors have identified ultrasonic sound frequencies to have particular advantages well suited for the objective of intrusion detection. Such ultrasonic frequencies are those above the upper limit of the human hearing range, typically established at 20 kHz. Firstly, ultrasonic sounds are not audible, which decreases the risk of disturbance caused to others, such as neighbours in a building.

[0041 ] Secondly, ultrasonic frequencies are easily blocked by windows, walls and other separation means, thereby isolating the sound field from others in the vicinity, preventing overspill from one protection zone to the other. Additionally, since there are comparatively few sources of ultrasonic noise, the probability of interference is very low.

[0042] Thirdly, due to the higher frequencies, ultrasonic sound pulses can be shorter, which increases the temporal resolution of the alarm. This has the advantage of fast reactivity even to very small fluctuations in the sound field.

[0043] Finally, sound fields emitted in the ultrasonic range reflect very efficiently from surfaces. Hence they propagate very well throughout the entire monitored zone, cover all three- dimensions, as well as hidden places. This effect prevents the existence of blind spots. Additionally, it has been identified that ultrasonic sound field is also easily affected by the presence of any type of entity, be a human, or a smaller object such as a metallic or plastic robot, drone, etc. Therefore any slight displacement by such an entity will cause significant disturbances in the sound field image resulting in fast reaction detection. Since even the slowest of movements causes a detectable change, the intruder alarm has high sensitivity.

[0044] For safety and practicality reasons, usually ultrasonic signals comprised between 40 and 100 kHz are preferred. Staying close to the 20 kHz limit can be considered as too close to the hearing range. On the other hand, ultrasonic signals above 100 kHz are easily absorbed by the air due to non-linear absorption and considered impractical for this application.

[0045] FIG. 3 depicts a method 300 for effectively detecting at least one unwanted body according to one embodiment of the invention. The steps of the method can be understood in combination with the detector of FIG. 1 . In step 310 the ultrasonic emitter 120, or emission means, transmits an ultrasonic sound. These sound waves have been converted, by the transducer in the ultrasonic receiver, from an electric signal generated and passed on from the processing unit 160, or processing means. As mentioned, optionally the signal may be further processed by signal processing means 140 if necessary before emission as a sound.

[0046] The inventors have identified the logarithmic sine seep method as a particularly advantageous method for measuring the impulse response of a room. The logarithmic sine sweep is based on the emission of a sound with an exponentially time-growing frequency sweep from a given lower frequency f1 to a given upper frequency f2. This method involves emitting a band-limited logarithmic sine sweep signal through a loudspeaker and recording the resulting propagated field through a microphone. The recorded, or received, signal is convolved with the inverse of the transmitted signal generating the room's impulse response. As an example, for a particular environment tested, it was found that the sweep bandwidth could be 4 kHz centered around the transducer resonance, the sweep duration 0.1 s and the time interval between sweeps 0.01 s.

[0047] The logarithmic sine sweep method, besides exhibiting a very good signal-to-noise ratio, has the advantage that the frequency interval can be easily controlled (band limited impulse response). These characteristics are particularly useful for the present application wherein ultrasonic sounds are used to flood a particular zone.

[0048] Ultrasonic emitters typically show a resonant behaviour around a given ultrasonic frequency. Therefore the sine sweep lower and upper frequencies f1 and f2 are chosen so that the frequency intervals are placed around the resonance of the emitter. The length of the sweep is chosen to be a compromise between the need of a good signal-to-noise ratio (which necessitates longer sweeps) and the requirement that the acoustic of the secured zone has to remain stable within one measurement (which necessitates shorter sweeps).

[0049] Once these upper and lower bounds are determined, the sine sweep emission takes place. In order to allow for full propagation of the sound throughout the protected environment, a predefined propagation time interval T1 is allowed to lapse before the sound field is sampled. Propagation of the sound field also comprises the sound waves which have been reflected on any type of surface present in the room. In step 320 a determination is made whether the predefined time interval T1 has lapsed.

[0050] If not, the method allows another predefined waiting time interval T2 to lapse and returns to step 320. In one aspect waiting time interval T2 can be a function of propagation time interval T1 . For example, it could be a factor either smaller or larger than unity of T1 . In one example, T2 = 0.5 T1 . In another example, T2 = 1 .5 T1 . In this manner the waiting interval is linked to the natural propagation of sound waves, resulting in an optimized waiting time which is long enough to ensure correct propagation, but not too long to render the method insensitive to fast or small fluctuations. [0051 ] In case the result of decision step 320 is positive, the receiver 130, or receiving means, is activated, or allowed, to start receiving 330, or sensing, the propagated and reflected sound waves. These sound waves are converted to an electric signal by the transducer in the ultrasonic receiver and passed on to the processing unit 160, or processing means. As mentioned, optionally the signal may be further processed by signal processing means 1 50 if necessary. The processing unit is coupled to a memory 180, or memory means, for storing the received sampled data.

[0052] In step 340 the processor 160 determines the monitored space's impulse response corresponding to sampled interval time T1 . Once determined, the processor compares the generated impulse response with the last impulse response sampled prior to time interval T1 . A presence parameter P is determined reflecting the degree of change, or difference, between both impulse responses: smaller values of P indicate similar impulse responses, and hence, no presence of unwanted bodies; larger values of P indicate dissimilar impulse responses, and hence, a probable presence of unwanted bodies.

[0053] In step 350 this presence parameter is compared to a predetermined tolerance threshold TH. If it is determined that the value of the presence parameter is within the tolerance threshold, it is concluded 370 that the change is not sufficiently large to indicate a valid presence detection and the alarm is not triggered. On the other hand if it is determined that the value of the presence parameter is equal to or above the tolerance threshold, it is concluded 360 that the change is sufficiently large to indicate a valid presence detection and the alarm can be triggered by the alarm actuator 170, or alarm generating means.

[0054] To minimize the risk of false positives, a certain amount of consecutive values above the threshold might be required. In one aspect, three consecutive presence parameter values above the threshold are a good indicator of a valid alarm. The alarm can be a visual alarm, an audible alarm such as a siren, or any other means of indication, such as a message transmission over a fixed or wireless communication link, or an automatic call to the police.

[0055] The unwanted body detection system is configured to measure the impulse response periodically in time and compares the impulse response measurement with the immediate previous impulse response measurement. This way, the alarm can adapt automatically to slow changes of the acoustics of the secured zone due to environmental or atmospheric phenomena such as normal temperature or humidity changes. This is advantageous in that the alarm does not need any initial calibration with respect to the acoustics of the secured space.

[0056] In one aspect of the invention the predetermined tolerance threshold TH is a user- defined parameter enabling the user of the alarm system to preset a sensitivity level for the alarm from a set of possible levels. If the tolerance threshold is set to be high, the alarm becomes less sensitive to variations of the sound field by a changing body. This can be advantageous when a certain degree of change, or movement, needs to be tolerated, for example, with house pets staying at home once the owners leave, or also of factories wherein certain machinery keeps functioning even at night time, or premises wherein an air draught is naturally existent and has to be accounted for.

[0057] On the other hand, if the tolerance threshold is set to be low, the alarm becomes more sensitive to variations of the sound field by a changing body. This can be advantageous when absolutely no minor fluctuation can be tolerated in a highly secured environment, and even the smallest disturbance should trigger the alarm. This can be necessary for example in jewelry stores, or high-end museums. Therefore in this aspect intrusion detection is enabled with user-defined alarm sensitivity.

[0058] In another aspect of the invention the predetermined tolerance threshold is automatically determined. The predetermined tolerance threshold can be set as the mean of the data measured in the intruder-free zone plus a predetermined multiple of the standard deviation of the same data set. In this aspect, the threshold parameter can be, for example, the mean of data plus two times the standard deviation.

[0059] In yet another aspect of the invention the predetermined tolerance threshold is programmed based on different factors, such as time of day. The processor may be programmed to activate a higher tolerance threshold during the day time, and a lower one during a night time. It may also be programmed to set different thresholds depending on whether the monitored zone is an office which is closed due to vacation time. Therefore these different aspects define tolerance thresholds which might be either user-defined or preset. They also define tolerance threshold which might be either fixed or variable. Overall, a programmable or self-adaptive, and flexible body detection is provided.

[0060] Hence the method 300 may be repeated iteratively in a periodic fashion with varying settings as just explained. Once this operation is repeated at different time intervals, a differential profile is generated by determining the cross-correlation of two measurements The correlation C between two consecutive impulse responses can be determined based on the expression:

where y1 and y2 are two consecutive impulse related functions uniquely describing the acoustical fingerprint of the secured zone, and T is the duration of the measurement. The correlation C is defined between -1 and 1 . The correlation C measures the degree of similarity or dissimilarity between the two acoustical fingerprints. A correlation close to one, or unity, means that the two acoustical fingerprints are very similar. A correlation close to zero, or null, means that the two acoustical fingerprints are very distinct. A correlation close to negative one, or -1 , would mean that the two acoustical fingerprints are very similar in absolute value but have opposite signs.

[0061 ] The presence parameter is defined as:

P = 1 c

Although in principle the presence parameter can have a range from 0 to 2, in practical situations the intrusion parameter adopts a value ranging from 0 to 1 , with a value close to 0 indicating similar acoustical fingerprints and thus absence of any unwanted bodies, such as intrusion or fire.

[0062] In one aspect of the invention, the acoustical fingerprint y is simply the measured impulse response:

y(t) = iR(f)

[0063] In this aspect of the invention, the acoustical fingerprint is equally sensitive to all parts of the impulse response, namely the direct sound, the early reflections and the late reverberation. In yet another aspect of the invention, the acoustical fingerprint y corresponds to a time-restricted version of the impulse response containing only the late reverberation, and not the first direct sound and the first reflections. This improves the detection capabilities of the alarm regarding obstructed and shadow zones.

[0064] In yet another aspect of the invention, the acoustical fingerprint y is proportional to the logarithm of the impulse response: y(t) = 20 log ₁₀ (|IR(f)| + .

where epsilon is a regularization parameter (a small number) to avoid possible infinite values of the logarithm. This version has the advantage of being particularly sensitive to smaller values of the impulse response, like the values at the late reverberation, without ignoring completely the information from the direct sound and first reflections, thereby improving the detection capabilities of the alarm.

[0065] In yet another aspect of the invention, the acoustical fingerprint y is proportional to the logarithm of the impulse response and its sign: y(t) = 20[log ₁₀ (|IR(f)| + - log ₁₀ (.?)] sign [IR(i)] ™s version has the further advantage of being sensitive not only to the absolute value of the impulse response but also to the phase, thus further improving the detection capabilities of the alarm.

[0066] The presence parameter P is, in constant conditions, close to zero. When an event occurs inside the room the presence parameter suddenly increases. Due to the sensitivity of the unwanted body detection system, the presence parameter P fluctuates over time simply due to natural occurring air turbulence. In order to take this effect into account, improved values are obtained by averaging several consecutive measurements (for example, averaging five consecutive measurements).

[0067] FIG. 4 depicts an example profile of a particular environment after multiple impulse response sampling instants. The vertical axis corresponds to the values of the presence parameter P, whereas the horizontal axis corresponds to time. As can be seen from this profile, the presence parameter P is small throughout the multiple measurements, and also within the predefined tolerance.

[0068] FIG. 5 depicts another example profile with an unwanted body producing a change at two different moments in time. This could be, in an example, an intruder making an intrusion at two different time instants. As can be seen from the profile, the presence parameter exhibits two significant peaks (circled zones), or regions of turbulence. Each peaking region represents the changing acoustics of the secured space due to the change produced by the unwanted body.

[0069] In one aspect of the invention the ultrasonic transducers are piezoelectric ultrasonic sensors. Piezoelectric sensors have the advantage that they provide an ultrasonic field in existence of electricity, hence they emit sound in the same manner as loudspeakers do. However they also produce electricity in the presence of recorded pressure, such as microphones. In one aspect only the emitter 120 is of piezoelectric type. In another aspect only the receiver 130 is of piezoelectric type. In yet another aspect both emitter and receiver are of piezoelectric type.

[0070] It has been found that different protection zones can have widely differing dimensions, as well as construction materials. Wall openings, such as doors and windows, and their sizes, also affect the final impulse response. Hence the basic impulse response can change considerably from environment to environment. Therefore in one aspect of the invention a testing, or learning, mode is provided, wherein the device emits at different frequencies and obtains samples at different time intervals.

[0071 ] In one aspect of the invention, the processor comprises an adaptive learning algorithm which adapts the periodicity of the sound emissions and corresponding received samples or measurements. The signal processing means may also be controlled by processor to emit at different frequencies or amplitudes. Once the differential profile between different impulse responses reaches a constant, it can be determined that the sound waves propagate sufficiently to fill up most of the monitored zone. Therefore this learning mode enables the automatic adjustment, or tuning, of the alarm to changing environments. Since ultrasounds are inherently high frequency, and corresponding short wavelength, waves, the periodicity of the measurements can be very high, enabling performing more refined measurements both in time and space, with a very high granularity. [0072] In one aspect of the invention, either the emitter, or the receiver, or both, can be omnidirectional devices. It has been found that omnidirectional sound sources fill up the room more homogeneously. Likewise, an omnidirectional sound receiver is more effective at capturing the maximum sound field propagated throughout the monitored environment. Omnidirectional sound sources are also smaller in dimensions, hence they occupy less space and are harder to identify by potential intruders.

[0073] Furthermore, it is to be understood that the embodiments, realisations, and aspects described herein may be implemented by various means in hardware, software, firmware, middleware, microcode, or any combination thereof. Various aspects or features described herein may be implemented, on one hand, as a method or process or function, and on the other hand as an apparatus, a device, a system, or computer program accessible from any computer- readable device, carrier, or media. The methods or algorithms described may be embodied directly in hardware, in a software module executed by a processor, or a combination of the two.

[0074] The various means may comprise software modules residing in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.

[0075] The various means may comprise logical blocks, modules, and circuits may be implemented or performed with a general purpose processor, a digital signal processor (DSP), and application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine.

[0076] The various means may comprise computer-readable media including, but not limited to, magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips, etc.), optical disks (e.g., compact disk (CD), digital versatile disk (DVD), etc.), smart cards, and flash memory devices (e.g., EPROM, card, stick, key drive, etc.). Additionally, various storage media described herein can represent one or more devices and/or other machine-readable media for storing information. The term "machine-readable medium" can include, without being limited to, various media capable of storing, containing, and/or carrying instruction(s) and/or data. Additionally, a computer program product may include a computer readable medium having one or more instructions or codes operable to cause a computer to perform the functions described herein.

[0077] What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination, or permutation, of components and/or methodologies for purposes of describing the aforementioned embodiments. However one of ordinary skill in the art will recognize that many further combinations and permutations of various embodiments are possible within the general inventive concept derivable from a direct and objective reading of the present disclosure. Accordingly, it is intended to embrace all such alterations, modifications and variations that fall within scope of the appended claims.