EP2189956B1 - Fire alarm and method for detecting pollution - Google Patents

Description

Technical area

The invention relates to a fire detector, with a sensor that detects smoke that penetrates into a detection range of the detector, the detector has a device with which can be determined whether the ingress of smoke, other aerosols or gas in the detection area is disturbed. In addition, the invention relates to a method by which disturbances of the penetration of smoke, other aerosols or gas can be detected in the Erfassungsberelch the detector.

State of the art

One of the tasks of smoke detectors is to detect and report damage fires or to take immediate action against the spread of fire and smoke. Smoke alarms are used to protect people, buildings and valuables. To ensure that the detectors can always reliably fulfill this safety-related task, the detectors must be regularly checked and maintained. So z. For example, DIN EN 14676 as the standard for smoke detectors provides an annual visual check in which contamination of the smoke inlets of the detector is to be checked. Such a visual inspection requires that the persons performing this inspection have access to the rooms in which the detectors are installed. With the aim of reducing the associated personnel costs and costs, various methods have already become known for contamination
the smoke inlet openings or associated disturbances of the penetration of smoke into the detection area of the detector to recognize automatically.

In the DE 44 09 900 C1 a scattered light smoke detector is described, the measuring light transmitter is aligned with an optical sump. The optical sump is set in mechanical vibration during a smoke measurement with a frequency in the inaudible frequency range. The light transmitted by the transmitter is partially reflected at the sump on a measuring receiver and generates there an alternating signal, while light that is scattered on existing (background) aerosols on the receiver, generates a DC signal. When the smoke inlets Clogging of the detector, no new background aerosol can penetrate into the detection area, while the existing aerosol settles. Therefore, when the aerosol-related background signal disappears, soiling of the smoke inlets is inferred.

In the DE 103 1 688 A1 a smoke detector is described, which is installed in the exhaust duct of an aircraft tollet. In order to detect whether smoke can still penetrate into the smoke detector or whether the smoke inlet openings are dirty, covered or sealed, at least two pressure sensors are used here. One of the pressure sensors is located in the detector, another behind the ventilation grille of the toilet and possibly a third person in the toilet room. If z. As the detector covered, so creates a pressure difference between the pressure sensor in the detector and the sensors outside the detector. This pressure difference is detected and evaluated for the detection of a cover.

The EP 0 503 167 A1 and the JP 222 78 00 describe scattered light smoke detectors with a detection area in a measuring chamber, which is protected by a grid from the entry of insects. Outside the grating a test light source is mounted, which shines through the grating and whose light can be detected inside the detector at a receiver. If the grid becomes dirty, the smoke entering the detection area is disturbed and less light can pass through the grid. The resulting reduced reception of the test light inside the detector is interpreted as an indication of contamination of the insect screen.

In the EP 1 870 866 A1 a scattered light smoke detector is described with a detection area in a measuring chamber, which is surrounded by an impurity filter. On the inside of the pollution filter, through which smoke can penetrate into the detector, a light guide is laid annularly, which collects light that passes through the pollution filter and passes to a receiver. The strength of the light passing through the contaminant filter is evaluated as a measure of contamination of the contaminant filter.

From the EP 1 191 496 is known a scattered light smoke detector whose detection range is outside the detector. The detector has an ultrasonic sensor that monitors the area around the scattering point (detection area). Among other things, the ultrasonic sensor is used to examine the area around the detector for foreign bodies, which impair the flow conditions for fire detection, thus impairing the entry of smoke into the detection area of the detector.

From the JP 02128298 In turn, a smoke detector with a detection area in a measuring chamber and a method has become known with which the blockage of an insect protection net with openings that allow the entry of smoke into the detection area of the detector can be detected. For this purpose, the resonance frequency of a sound body, which is located in the interior of the detector, tuned to the resonant frequency of the measuring chamber with unpolluted insect grid. The resonance frequency of the measuring chamber results from the volume of the measuring chamber and the area and depth of the openings by means of the Helmholtz equations. In the operation of the detector, a blockage of the insect net is to show that the resonant frequency of the measuring chamber relative to the resonance frequency of the sound body shifts and the vibration of the sound body stops.

In order to detect disturbances of the penetration of smoke into the detection range of a smoke detector, the DE 10 2006 023 048 A1 a smoke alarm device with a sensor that optically and / or acoustically scans at least one of the openings in the housing of the detector. For optical scanning, an extinction and / or distance measurement is proposed, while for acoustic 5 sampling only a distance measurement is proposed. For this purpose, the distance sensor is mounted inside the housing and directed towards the housing opening of the smoke detector. If the monitored opening is open, the distance z. B. measured to the next wall, while in a cover of the opening, only the distance is measured to the cover. A cover of the opening can thus be detected.

With this arrangement, it is not possible to detect contamination of a protective grid, since the distance between insect screen and distance sensor is independent of the degree of contamination of the protective grid.

Neither is it possible to detect the contamination of an opening in the housing of a smoke detector in which an insect screen, equivalent to an impurity filter, is mounted immediately in front of the housing openings inside or outside the housing, as is common in many smoke detectors. Also in this case, the distance sensor will always measure only the distance to the insect screen, regardless of whether the grid is dirty, an object is in front of the opening, or the opening is taped.

Object of the invention

The invention is therefore an object of the invention to provide a smoke detector and a method of the type mentioned above, with which even with a mounted directly to the housing openings protection against dirt or insects impairment of the penetration of smoke, other aerosols or gas in the detection area can be recognized, or provide alternatives to the known prior art.

Description of the invention

The object is achieved according to the preambles and the characterizing features of claims 1, 6, 7, 8, 10 and 12 and will be described in more detail below.

The invention is based on the following findings: All components of a smoke detector such. As the housing, the openings, insect protection, circuit board and measuring chamber have different acoustic impedances.

Sound that strikes an interface between two media that have a different acoustic impedance is reflected differently depending on the impedance differences.

The acoustic impedances of individual components depend on their material properties and their geometry.

Based on these findings, the invention relates to a method for detecting disturbances of penetration of fire characteristics such as smoke in a detection range of a fire detector with a housing. In this case, the housing has openings for the occurrence of the fire characteristics in the detection area, which in the interior of the housing z. B. is within a scattered light measuring chamber. With the method according to the invention, dirt or covers of the housing openings and / or insect protection are recognized by, on the one hand, a characteristic acoustic field is generated within the smoke detector by exciting a sound transducer. The characteristic acoustic field is created by superposition of sound waves that are emitted directly from the transducer and sound waves that are at least partially reflected at acoustic interfaces inside and outside of the detector. On the other hand, at least one physical variable of the acoustic field, such as amplitude and / or phase, is measured at at least one location within the acoustic field and the measured value (s) is compared with at least one reference. If in this case the at least one measured value deviates from the reference by a predeterminable amount, contamination or covering of the smoke intrusion openings or of the insect protection is ascertained.

In connection with the present invention are with fire characteristics especially aerosols such. For example, smoke and gases that arise during combustion, meant. However, all other measurable phenomena which occur in the event of a fire, penetrate into the housing of a detector and can be measured in its detection range are also considered fire characteristics. Likewise are to be understood as fire characteristics means which simulate the aforementioned fire characteristics to z. B. to test a fire alarm.

A detection area is an area with which a fire parameter must come into contact or a room area in which a fire parameter must penetrate, so that it can be detected by the corresponding sensor system. For example, in a scattered light smoke detector, the detection area is the space in which the light beam of a measuring light transmitter and the field of view of a scattered light receiver overlap.

The insect screen in turn is a grid or tissue, which is permeable to the measured fire characteristic, but disturbances such. B. insects and coarse particles from the detection area keeps away.

In a method according to the invention, the sound transducer is excited with at least one discrete frequency. In order to achieve the best possible measurement result, the frequency, the location of the transducer and the locations of one or more microphones are mutually matched. The tuning can be experimental or based on a simulation of the characteristic sound field and its interference by contamination or coverage of the fire alarm, z. B. by means of a finite element simulation done. For reasons of symmetry, it may make sense that the transducer is placed in the middle of the housing. A temperature dependence of the characteristic sound field can, for. B. be compensated with a frequency selected in dependence on the temperature, resulting in a reliable detection of contamination over a wide temperature range.

In a further development of the method, the sound transducer is excited with a plurality of different discrete frequencies, wherein for the measured values of the sound field for each frequency a comparison is made with a respective reference belonging to this frequency. The additional frequencies may cause soiling or debris in areas of the housing that are at a single frequency, e.g. B. due to Schallabschattungen, less well recognized, have been detected better. Again, the different measurement frequencies can be selected depending on the temperature.

A variant of the method according to the invention is that the sound transducer with a frequency sweep (frequency sweep) is excited in a defined frequency interval with a sequence of frequencies and a resulting spectrum is measured at least one location in the acoustic field. Deviations of the measured spectrum to a reference spectrum can, for. B. by means of a rule-based analysis and / or pattern recognition z. B. by means of a fuzzy log or neural networks and / or cross-correlation of both spectra are detected. Since a measured spectrum has a characteristic pattern for each measuring location, which shifts substantially unchanged along the frequencies in the case of temperature changes, a further temperature compensation can be dispensed with here, especially if the changes in the pattern are evaluated. In addition, this variant of the method, as well as the variant with a plurality of discrete frequencies, allows conclusions to be drawn about the location or the respective housing opening at which there is a cover or contamination.

The invention also relates to a method for detecting disturbances of the penetration of fire characteristics into a detection area of a fire detector with a housing having openings for the occurrence of the fire characteristics in the detection area, the method detects soiling or covering the housing openings and / or insect protection in which it is not the effects of changes in impedance on a characteristic sound field that are measured, but the acoustic impedances themselves. Accordingly, the acoustic impedances of the complete fire detector and / or the housing openings and / or the insect protection are measured in this method. A contamination or a cover is detected when one or more of the measured acoustic impedances deviates from a reference corresponding to the respective impedance by a predeterminable amount. If z. B. covered a housing opening, then sound waves are reflected more strongly on the cover than without cover. Thus, the acoustic impedance of this opening, but also the acoustic impedance of the entire detector housing changes, which can be approximately represented as a parallel connection of the impedances of all housing openings. Likewise, the reflection behavior of an insect repellent changes when it becomes increasingly polluted.

Methods for measuring the acoustic impedances are known in principle and therefore need not be explained in detail. Nevertheless, here on the DE 103 26 078 A1 from which a method for measuring acoustic impedances is described, in which the resonance frequency of a sound transducer is measured and from which then the acoustic impedance of the medium is surrounded, which surrounds the sound transducer. Since in the method according to the invention the change of the acoustic impedances is used as an indication for a contamination or covering of the housing openings or an insect protection, in a further method the change of the resonance frequency is evaluated. In this method for detecting disturbances of the penetration of fire characteristics in a detection range of a fire detector with a housing having openings for entry into the detection area, for detecting contamination or covers of the housing openings and / or insect repellent at least one resonant frequency of a sound transducer Inside the detector measured. The measured resonance frequencies are compared with a reference frequency corresponding to the respective resonance frequency. If the at least one measured resonant frequency deviates from its reference frequency by a predeterminable amount, then contamination or coverage is detected. Similar to the open air resonance frequency of a loudspeaker shifts the other resonant frequency, when the speaker is installed in a speaker housing, here shifts the resonant frequency of a transducer in the housing of the fire detector when the housing openings and / or insects increasingly clogged by contamination or cover. To measure the resonant frequency of the sounder, it can be used on the method as in the DE 102 26 078 A1 or in the EP1898377 are described.

In all the methods described above, a compensation of temperature dependencies of the measurements can be made by references selected as a function of the temperature from a table. Preferably, this table is completed during the operation of the detector. To create and complete the table measurements in the unpolluted uncovered state z. B. at the factory, during commissioning and during the use phase. All recorded measured values are stored in the table at the currently prevailing temperature as the reference value. Amplitudes or phases are assigned to their respective frequencies. As soon as the detector reaches a new temperature during operation, a new contamination measurement is carried out. If a plausibility check of these values shows that the detector is still not soiled or uncovered, these values are entered as reference values in the table for the respective temperature. If temperatures are reached that already have reference values in the table, then the values already entered are used as references for the contamination detection. For the evaluation of the amplitudes of a characteristic sound field z. B. the rows of the table are assigned different temperatures, while the columns are assigned different frequencies.

The invention also relates to a fire detector, with the above-mentioned method can be performed. A fire detector according to the invention comprises a detection area for fire parameters in a housing which has openings for penetration of the fire characteristics and means for detecting soiling or covering of the housing openings and / or an insect screen. The means for detecting contaminants or covers of the housing openings and / or the insect screen comprise a sounder that generates a characteristic acoustic field preferably in a non-audible frequency range in the housing of the fire detector, at least one microphone at least one suitably chosen location, the changes in amplitude and / or the phase of the characteristic acoustic field detected at each location and a Control and evaluation unit, which closes on exceeding the changes of amplitude and / or phase by a predeterminable amount to a contamination or cover.

In this case, the changes in amplitude and / or the phase of the acoustic field at the location of the measurement are caused by a changed reflection behavior, ie by changes in the acoustic impedance of the housing openings and / or insect protection.

In another embodiment, the means for detecting contaminants or covers of the housing openings and / or insect protection of a fire detector according to the invention comprise a unit for measuring the acoustic impedances of the complete fire detector and / or the housing openings and / or the insect protection. The control and evaluation unit of this fire detector according to the invention detects a contamination or a cover when one or more of the measured acoustic impedances deviates from a corresponding reference by a predeterminable amount. In a further development of this embodiment, in the unit for measuring the acoustic impedances, the acoustic impedance of the complete fire detector is determined based on at least one of the resonant frequencies of the sounder.

In a further embodiment of the fire detector according to the invention, the means for detecting contaminants or covers of the housing openings and / or insect protection comprise a unit for measuring at least one resonance frequency of a sounder and a control and evaluation unit, which compares the measured resonance frequencies with a corresponding reference frequency and at Exceeding recognized deviations from a respective reference frequency by a predeterminable amount detects contamination or coverage.

Brief description of the drawings

Fig. 1

shows a fire detector according to the invention

Fig. 2

shows another inventive fire detector with a different design

Fig. 3a

shows sound spectra of an unpolluted fire alarm

Fig. 3b

shows sound spectra of a fire detector with a cover at one point of the fire alarm

Fig. 3c

shows sound spectra of a fire detector with a cover at a location other than in Fig. 3b

Description of the embodiments

Fig. 1 shows schematically a fire detector (1) according to the invention, whose function with the aid of FIGS. 3a-3c is explained. The FIGS. 3a-3b each show, by way of example, a continuous amplitude spectrum 17 abc in the frequency range from a to t and a line spectrum formed from a plurality of discrete frequencies a-t. A continuous amplitude spectrum 17abc can be measured on the microphone 15 in the fire detector 1 when the sound transducer 8 is excited with a continuous frequency sweep while a discrete line spectrum a-t is measured when the sound transducer 8 is excited at several discrete frequencies a-t. The frequencies a-t are preferably located in a non-audible frequency range for measurement purposes.

Using the example of a scattered light smoke detector 1, a fire detector 1 according to the invention will now be explained. The detector 1 has a housing 2 with openings 10a and 10b, in which a measuring chamber 3 is housed. In the measuring chamber 3 is the detection range for Rauch18, which results from the intersection of a Messlichtkegels emitted from the measuring light transmitter 4, with the field of view of the scattered light receiver 5, the measuring chamber 3 is bounded laterally by a labyrinth 12 through which on the one hand smoke in the measuring chamber 3 and the detection area 18 can penetrate, on the other hand, extraneous light is prevented from penetrating into the measuring chamber 3. Smoke that enters the measuring chamber must first pass through the housing openings 10a, 10b and the insect screen 11. Since it happens again and again that smoke detectors z. B. are masked during renovation and then forgotten to remove the cover or pollute the insect screen 11 over time and then smoke can not penetrate unhindered in the detection area 18, the smoke detector 1 has a device according to the invention with which such covers or dirt can be recognized and displayed. Such a device comprises, in addition to a control and evaluation unit 16 and a microphone 15, a sound generator 8, which is mounted here between the housing 2 and the measuring chamber 3. In this position, he could on the one hand sound as an alarm through the sound outlet opening 7 in the housing outwardly conceivable would be an additional use as a communication interface z. B. in the ultrasonic range - and on the other hand pass through the sound inlet opening 6 inwards into the measuring chamber 3. The sound entering the measuring chamber 3 is reflected therein several times and passes through the labyrinth 12 from the measuring chamber 3 in the surrounding housing 2 and there is also on housing boundaries, the boundary surfaces of the housing openings, not shown 10a 10b, the insect screen 11 is reflected and partially passes back into the measuring chamber 3 in which the microphone 15 is mounted. The superimposition of sound waves, which are emitted directly from the sounder with the reflected sound waves, creates a characteristic sound field, which in some places has larger and at other locations smaller amplitudes and different phase angles compared to the original signal. By changing the reflection behavior z. B. by a cover of the housing openings 10 a 10 b or contamination of the insect screen, the switching field and thus also the amplitude or phase changes at the respective locations. So shows Fig. 3a each exemplary a continuous amplitude spectrum 17a and a discrete line spectrum at, which can be measured with the microphone 15 in the unpolluted uncovered state of the smoke detector 1 with appropriate detector geometry and matching material properties. Spectrum 17a and the in Fig. 3a therefore, amplitudes a - t also serve as references for the detection of soiling or covers. A continuous spectrum 17 ac can be measured if the sound generator 8 is excited with a continuous frequency sweep, while the discrete spectra are absent in the Fig. 3a-c result when the sound generator 8 is excited at the discrete frequencies a-t.

• "Wenn im Frequenzbereich von h - k die Amplituden deutlich kleiner und im Frequenzbereich o - q deutlich größer als im Referenzspektrum sind, dann liegt eine Verschmutzung oder Abdeckung im Bereich der Gehäuseöffnung 10a vor und wenn Im Frequenzbereich von a - c die Amplituden deutlich kleiner und im Frequenzbereich o - q größer als im Referenzspektrum sind, dann liegt eine Verschmutzung oder Abdeckung im Bereich der Gehäuseöffnung 10b vor"
• erkannt werden.

Fig. 3b shows exemplary spectra 17b at how they may result in coverage of the opening 10a during Fig. 3c shows possible spectra 17c, at cover 10b. By comparing the spectra 17b and at Fig. 3b with the reference spectra 17a and at at Fig. 3a shows a significant reduction of the amplitudes in the frequency range of h - k and an increase in the amplitudes in the frequency range o - q. In contrast, a comparison of the reference spectra in Fig. 3a with spectra 17c and at in Fig. 3c a significant reduction in the amplitudes in the frequency range of a - c and an increase in the amplitudes in the frequency range o - q. Accordingly, contaminants or covers in the control and evaluation unit 16 (FIG. Fig. 2 ) by applying the exemplary rule:

• "If the amplitudes are significantly smaller in the frequency range of h - k and significantly larger in the frequency range o - q than in the reference spectrum, then there is contamination or coverage in the area of the housing opening 10a and if the amplitudes in the frequency range of a - c are significantly smaller and in the frequency range o - q are greater than in the reference spectrum, then there is a contamination or cover in the region of the housing opening 10b before "
• be recognized.

The result of the measurement and the comparison is stored in the fire detector 1, transmitted via an interface, not shown, to an external unit and can be displayed acoustically or visually directly on the detector.

If one compares only the frequency range o - q in the Fig. 3a - 3c with one another, one recognizes that the amplitudes of the frequencies o, p and q are greater in the case of contamination than in the clean state. Thus, the frequencies o, p and q are also suitable as a single frequency or as a frequency group for detecting contamination or covering of the housing openings 10 or of the insect screen 11.

The arrangement of the microphone 15 in the measuring chamber as in Fig. 1 it is shown simplified the detection of contamination of the insect screen 11, if this, as is the case with some fire detectors, is mounted directly on or in the measuring chamber. Soiling of the labyrinth could be recognized as well.

Fig. 2 shows an alternative embodiment of a fire detector according to the invention. Here, the measuring chamber 3 with the measuring light transmitter 4 and the scattered light receiver 5 together with the sounder 8, the control and evaluation unit 16 and two opposing microphones 15 a and 15 b are arranged together on a printed circuit board 14 in the housing 2. Again, the sounder 8 preferably acts both as an alarm and as a sounder for the pollution measurement. The contamination measurement is preferably carried out at inaudible frequencies. The use of two microphones at different locations enhances the detection of soils or covers by better detecting sounds that are shadowed by the measuring chamber 3 or other components not shown, by the second microphone. The contamination detection is otherwise performed as described above. The result of the contamination measurement can then be displayed directly on the detector, transmitted via an interface, not shown, to an interrogator, a central office or the like or stored for later retrieval in a memory, not shown.

Since the amplitude changes described above are predominantly due to changes in the acoustic impedances Z _{G / I of} the housing openings 10 and the insect screen 11, the impedance changes in another fire detector 1 according to the invention directly as an indication of contamination or covers of the housing openings 10 and the Insektengittrs 11 of the fire detector 1 evaluated. Such a fire detector according to the invention has a device for measuring the acoustic impedances of the housing openings 10 or of the insect screen 11. For this purpose, the sounder 8 emits an acoustic signal whose reflections are measured by a housing opening 10a, 10b or an insect screen 11. The quotient of the original signal and the reflected signal gives the reflectance r from which the impedance Z _{G / I of} the housing openings 10a, 10b or of the insect screen 11 is calculated via the known acoustic impedance of the air Z ₀ via the following formula. $${\mathrm{Z}}_{\mathit{on}}={\mathrm{Z}}_0\cdot \frac{\mathrm{1}\mathrm{+}\mathit{r}}{\mathrm{1}\mathrm{-}\mathit{r}}$$

Subsequently, the measured acoustic impedances Z _G Z _I in the control and evaluation unit 16 are compared with limit values which are determined as a function of the impedances of an unpolluted or uncovered detector. If Z _{G / I} deviates by a predetermined amount from the value of an uncovered or unpolluted housing opening 10a, 10b or the insect screen 11, then the housing opening 10a, 10b or the insect screen 11 is considered covered or at least so heavily polluted that Smoke no longer undisturbed can penetrate into the detection range 18 of the fire alarm.

The acoustic impedance Z _{M of} the complete fire detector 1 is derived in a further inventive fire detector 1, taking into account the piezoelectric basic equations of the resonant frequency of the sounder 8. As a sound generator 8 a piezoelectric sounder 8 is used with feedback electrode for this purpose, which is connected to the feedback input of the excitation circuit, not shown. Due to the resulting feedback of the sounder 8 is automatically excited via the excitation contacts, via which the sounder 8 is connected to the signal output of the excitation circuit, with one of its resonance frequencies. The resonance frequency is then determined by measuring the period.

In a further embodiment of a fire detector according to the invention, the determination of the acoustic impedance of the detector is dispensed with. For this purpose, in the control and evaluation unit 16, the measured resonant frequency of the sounder 8 is compared with the resonant frequency of an unpolluted uncovered detector 1. In this fire detector 1, a contamination or cover of the housing openings 10 and the insect screen 11 is detected when the measured resonance frequency deviates by a certain amount from the reference frequency at a clean and uncovered detector 1.

Claims (12)

1. Method for detecting disturbances in the penetration of fire characteristics into a detection zone (18) of a fire alarm (1) inside a housing (2) which has openings (10) for the entry of the fire characteristics into the detection zone (18), contaminants or maskings, in particular of the housing openings (10) and/or of an insect guard (11) being detected by the method, characterized by the method steps of:

   a. producing a characteristic acoustic field inside the smoke detector by exciting a sound transducer (8) and by superimposing on sound waves emitted directly by the sound transducer (8) their own reflections at interfaces inside and outside the alarm (1),

   b. measuring at least one physical variable of the acoustic field, in particular amplitude (a-t) and/or phase at at least one location (15) inside the acoustic field in the smoke detector,

   c. comparing the at least one measured value with a reference,

   d. detecting a contaminant or masking of the smoke penetration openings in the alarm when the at least one measured value deviates from the reference by a predeterminable amount.
2. Method according to Claim 1, characterized in that the sound transducer (8) is excited with at least one discrete frequency (a-t).
3. Method according to Claim 1 or 2, characterized in that the sound transducer (8) is excited with a plurality of different discrete frequencies (a-t), and a comparison with a respective reference is carried out for each frequency (a-t).
4. Method according to Claim 1, characterized in that the sound transducer is excited with a frequency sweep in a defined frequency interval, and a resulting spectrum (17a/b/c) is measured at at least one location (15) in the acoustic field.
5. Method according to Claim 3 or 4, characterized in that deviations in the measured continuous or line spectrum (17b, 17c) relative to a reference spectrum (17a) are detected by means of a rule-based analysis and/or a pattern recognition, for example by means of a fuzzy logic or neural network and/or a cross correlation.
6. Method for detecting disturbances in the penetration of fire characteristics into a detection zone (18) of a fire alarm (1) inside a housing (2) which has openings (10) for the entry of the fire characteristics into the detection zone (18), contaminants or maskings, in particular of the housing openings (10) and/or of an insect guard (11) being detected by the method, characterized by the measurement of the acoustic impedances of the complete fire alarm (1) and/or of the housing openings (10) and/or of the insect guard (11), a contaminant or masking being detected when one or more of the measured acoustic impedances deviates from a corresponding reference by a predeterminable amount.
7. Method for detecting disturbances in the penetration of fire characteristics into a detection zone (18) of a fire alarm (1) inside a housing (2) which has openings (10) for the entry of the fire characteristics into the detection zone (18), contaminants or maskings, in particular of the housing openings (10) and/or of an insect guard (11) being detected by the method, characterized in that at least one resonant frequency of a sound transducer (8) in the interior of the alarm (1) is measured and compared with a corresponding reference frequency, and a contaminant or masking being detected when the at least one measured resonant frequency deviates by a predeterminable amount.
8. Fire alarm suitable for carrying out a method according to one of Claims 1 to 5, having a detection zone (18) for fire characteristics in a housing (2) which has openings (10) for the penetration of the fire characteristics, and having means for detecting contaminants or maskings of the housing openings (10) and/or of an insect guard (11), characterized in that the means for detecting contaminants or maskings of the housing openings (10) and/or of the insect guard (11) comprise a sound transducer (8) in the interior of the housing (2), which produces a characteristic acoustic field in the housing (2), at least one microphone (15) at at least one suitably selected location (15) in the housing (2), which detects changes in the amplitude and/or the phase of the characteristic acoustic field at the respective location (15), and a control and evaluation unit (16) which infers a contaminant or masking upon overshooting of the changes in amplitude and/or phase by a predeterminable amount.
9. Fire alarm according to Claim 8, characterized in that the changes in the amplitude and/or the phase of the acoustic field are caused by changes in the acoustic impedance of the housing openings (10) and/or of an insect guard (11).
10. Fire alarm suitable for carrying out a method according to Claim 6, having a detection zone (18) for at least one fire characteristic in a housing (2) which has openings (10) for the penetration of the fire characteristics, and having means for detecting contaminants or maskings of the housing openings (10) and/or of an insect guard (11), characterized in that the means for detecting contaminants or maskings of the housing openings (10) and/or of the insect guard (11) comprise a unit for measuring the acoustic impedances of the complete smoke detector (1) and/or of the smoke penetration openings (10) and/or of the insect guard (11), a contaminant or a masking being determined when one or more of the measured acoustic impedances deviate from a corresponding reference by a predeterminable amount, and a control and evaluation unit (16) which detects a contaminant or masking upon overshooting of the deviation by a predeterminable amount.
11. Fire alarm according to Claim 10, characterized by a device for measuring resonances, the acoustic impedance being determined with the aid of at least one of the resonant frequencies of the sound transducer (8).
12. Fire alarm suitable for carrying out a method according to Claim 7, having a detection zone (18) for at least one fire characteristic in a housing (2) which has openings (10) for the penetration of the fire characteristics, and having means for detecting contaminants or maskings of the housing openings (10) and/or of an insect guard (11), characterized in that the means for detecting contaminants or maskings of the housing openings (10) and/or of the insect guard (11) comprise a sound transducer (8) in the interior of the housing (2), which produces a characteristic acoustic field in the housing (2), a unit for measuring at least one resonant frequency of the sound transducer (8), and a control and evaluation unit (16), which compares the measured resonant frequencies with a corresponding reference frequency and detects a contaminant or masking upon overshooting of known deviations from a respective reference frequency by a predeterminable amount.

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