DE4237721A1 - Identifying traffic through monitored zone - analysing geophone signals corresp. to sound waves triggered by traffic and propagated underground - Google Patents

Abstract

The method involves receiving sound waves, at a geophone (10), stimulated by traffic, coupled into the ground and propagated underground. Corresp. geophone output signals are generated. The categories of the different traffic elements, e.g. pedestrians, light motor vehicles, heavy vehicles are associated with frequency ranges, each defined by an upper frequency. The upper frequencies are staggered and measurement signals are derived by envelope curve detection. The measurement signals levels are compared continuously with the level of a reference signal also derived by envelope curve detection in the background noise frequency region. Measurement signals in a frequency range with levels exceeding the reference signal level are preliminarily categorised as associated with that frequency range. Further test criteria are then applied to confirm final categorisation. USE/ADVANTAGE - Identifying various types of traffic passing through monitored zone. Simple detection and classification.

Description

The invention relates to a method and a device to identify a surveillance area crossing road users in the preamble of Claim 1 and of claim 6 defined genus.

In a known device for identifying Tracked vehicles (DE 38 40 732 A1) the amplitude or the level of the amplified geophone output signal in compared to a comparator with a threshold whose Size of the ground conditions at the geophone site is adjusted. The threshold is exceeded by the The detection criterion for amplitude or level a tracked vehicle.

Another known device for differentiation between wheeled and tracked vehicles (US Pat. No. 4,158,832) a seismic detector in the form of a geophone, which or brought into the ground near a driveway is. This device takes advantage of the fact that most of the energy is one by one Tracked vehicle originating seismic signal from the Beat on the chain links on the driveway. This signal has a strong frequency component that the  Repetition rate corresponds with which the chain links contact the track surface. In contrast to this a wheeled vehicle has no comparable mechanism for Generation of a single strong frequency component, rather occur in that generated by wheeled vehicles large frequency fluctuations. As a result, the device uses the size of the Frequency fluctuations as a criterion for distinguishing between the two different types of vehicles. in the individual, the geophone output signal becomes one Rectangular signal whose frequency is the frequency corresponds to the strongest signal component. This Square wave signal is a differentiator and then fed to an integrator. The DC component of amplified output voltage of the integrator is proportional to the frequency during its alternating component is a measure of the frequency fluctuations in the signal. The The amplitude of the alternating component is therefore characteristic for the type of vehicle that passes the detector. A rough one Amplitude represents a wheeled vehicle, a small one Amplitude a tracked vehicle. The separation is made with the help brought about a comparator that the amplitude of the Alternating component with a predetermined threshold compares.

The invention has for its object a method and a device of the type mentioned above improve that road users different Genus based on the level or the amplitude of the Geophon output signal detected in a simple manner and classified, d. H. be differentiated by categories can. Level and amplitude of the geophone output signal are used as a synonym in the following.  

The task is in a procedure in the preamble of Claim 1 or in a device in the preamble of claim 6 each specified genus According to the invention by the features in the characterizing part of the Claim 1 and claim 6 solved.

The method has the advantage that with relatively simple means detected road users and can be distinguished from each other. The for this required device can be made with a relatively small Implementation of circuit components be and is characterized by a very simple Signal processing off.

Advantageous embodiments of the invention Method or the device according to the invention with expedient refinements and improvements of Invention result from the further claims.

In the method according to the invention measured in different frequency ranges Signal level an identification of road users receive. To keep the false alarm rate low, only a preliminary recognition is made, which is carried out by a test criterion for the respective category of Road users are specifically determined, confirmed must become.

The test criterion for the category "pedestrians" is according to a preferred embodiment of the invention Procedure a number of exceedances of the Reference level by the measurement level in that of the category "Pedestrian" assigned frequency range within a predefined time window used. So that becomes a "Pedestrian" only identified if he has the  Surveillance area with sufficient speed happens.

As a test criterion for the categories "light vehicle" and "Heavy vehicle" is a sufficiently long drop in the Level of the measurement signal (measurement level) in these categories assigned frequency ranges used. In this way the road user is only identified when he the CPA (closest point of approach) has already happened and will be false alarms suppresses the z. B. due to vehicle pitch movements or road bumps can be caused.

To further improve the reliability of the information, an additional test criterion was applied to all categories, by for each category of road users No pre-detection in a category is used with a frequency range next lower upper limit frequency is assigned. So a pedestrian is only identified if in the "Light vehicle" category has not been pre-recognized, and a light vehicle is only identified if No pre-recognition in the "Heavy Vehicle" category is done.

To increase security in pre-detection according to an expedient embodiment of the inventive method, the measurement level before Adjusted comparison with the reference level, d. H. the too adapted to the detecting category. For this, a Category-specific constant value (Adjustment voltage) of the respective measurement signal subtracted. The adjustment voltages are empirical determined. In general, however, the adjustment voltage for the Category "light vehicle" larger than that of the category  "Pedestrian" and smaller than that of the category Adjustment voltage assigned to "heavy vehicle".

An advantageous device for performing the The method according to the invention has a near Monitoring area sunk into the underground Geophon, several low passes, comparators, intermediate storage and a main memory and a test unit. The Low passes are parallel to each other at the exit of the geophone connected and have graduated against each other Time constants. One low pass is at one for each a category of road users (Category low pass) and to one for background noise (Reference low pass) characteristic frequency range customized. The low passes can be used to save components from low-pass elements connected in series be composed so that the same low-pass elements belong to different low passes. Any category low pass a comparator is assigned to which, on the one hand, the Output signal of the assigned category low pass (Measurement signal) and on the other hand the output signal of the Reference low pass is supplied. The comparator gives a pulse representing the pre-detection, if the Level of the output signal of the category low pass (Measurement level) the level of the reference signal (reference level) exceeds. The outputs of the comparators are included a buffer for storing the Comparator pulses connected. All outputs of the Buffer memories are on a main memory connected so that the buffer values on a Write command taken over into the main memory become. The registration command is issued by the test unit triggered, after completing the above described test criteria.  

The invention is based on one in the drawing illustrated embodiment of a device for Identification of road users in more detail below described. The drawing shows a block diagram a device for identifying Road users.

With the one described below Identification procedures are used by road users who pass a defined surveillance area, detected and classified at the same time, d. H. than to belonging to given categories of road users recognized. The method presented here is limited on the identification of pedestrians, wheeled vehicles, hereinafter referred to as light vehicles and tracked vehicles, hereinafter referred to as heavy vehicles, but is another Extension of those to be identified with the method Classes or types of road users possible. So can the method, for example, additionally on the Differentiation between light and heavy Wheeled vehicles and light and heavy tracked vehicles be extended. The procedure assumes that the road users to be identified on surveillance area defined in a road section cross in succession and not at the same time. For Performing the procedure requires a geophone as close as possible to the surveillance area in the ground is sunk. This geophone receives those from Road users triggered in the underground coupling and propagating sound waves and generates a corresponding geophone output signal, the Amplitude or level proportional to the amplitude or the Level of the received sound wave. The Geophone output signal is the starting point of the  Procedure in the following steps is carried out:

First of all, each set category of the identifying road users a frequency range assigned by the upper limit frequency set is. The upper limit frequencies of the different Frequency ranges are graduated against each other. At The definition of the upper limit frequencies is that of the different categories of road users generated typical frequency pattern taken into account. So is the upper limit frequency of the category "light vehicle" assigned frequency range smaller than that Limit frequency of the assigned to the category "pedestrians" Frequency range and greater than the upper limit frequency assigned to the "Heavy Vehicle" category Frequency range. The geophone output signal becomes in each of these frequency ranges by envelope detection derived a signal called measurement signal below. The geophone output signal also becomes one wider frequency range, its upper cut-off frequency very is much smaller than the upper cutoff frequency of the Category "Heavy Vehicle" assigned frequency range and the characteristic of so-called background noise is a subsequent one by envelope detection Reference signal called signal obtained. The levels of the Measurement signals (measurement level) are now continuously with the Level of the reference signal (reference level) compared, and if the reference level is exceeded by a Measuring level in a frequency range becomes a prerecognition of a road user operating in this frequency range assigned category is determined. This Pre-recognition is still specified in a category-specific manner Subjected to test criteria and if these test criteria the preliminary recognition is confirmed  and approved as an identification of the road user. The test criteria are used to determine the false alarm rate at Reduce pre-detection and thus the security of Increase identification. As a test criterion for the Category "pedestrians" is a number of prerogatives predetermined within a predetermined period of time must occur. As a test criterion for the categories "Light vehicles" and "Heavy vehicles" becomes one sufficiently long drop in the measuring level in this Frequency ranges assigned to categories. This drop in the measurement level means that the vehicle the closest point of geophone, the so-called CPA (closest point of approach) has already happened. Lead with it Nodding movements of the vehicle or bumps in the road during the approach of the vehicle, the one Can trigger pre-recognition, not a confirmation the pre-detection and thus to suppress a False alarms.

As an additional test criterion for each category of Road users will still have none Pre-detection in a frequency range with the next lower Limit frequency specified. This is how pre-recognition in the Category "pedestrians" not confirmed, even if in the There is a pre-recognition in the "Light Vehicle" category. Pre-recognition is also carried out in the category "Light vehicle" not confirmed if in the category Heavy vehicle "there is a preliminary recognition.

To carry out the method described above, the monitoring device shown in the drawing in the block diagram has a geophone 10 , to the output of which a plurality of low-pass filters 11 to 14 are connected in parallel with staggered time constants. The low-pass filter 11 with the smallest time constant determines the frequency range assigned to the "pedestrian" category, the low-pass filter 12 with the next larger time constant determines the frequency range assigned to the "light vehicle" category and the low-pass filter 13 with the even larger time constant determines the frequency range assigned to the "heavy vehicle" category . These low-pass filters 11 to 13 are therefore hereinafter referred to as Category low-pass filters 11 to 13. The low pass filter 14 having the greatest time constant is adapted to the background noise characteristic frequency region and is hereinafter referred to Referenztiefpaß fourteenth To reduce the component requirement s, the category low pass 13 and the reference low pass 14 are composed of low pass elements connected in series, so that the category low pass 13 also includes the low pass element of the category low pass 12 and the reference low pass includes the low pass elements of the category low pass 12 and 13 . The outputs 111 to 113 of the category low-pass filters 11 to 13 are each connected to a comparator 18 to 20 via an adder 15 to 17 representing an adjustment unit. In the adders 15 to 17 , a negative DC voltage, the adjustment voltages U1, U2, U3, is added to the output signal of the category low-pass filter 11 to 12 (measurement signal). These adjustment voltages are set on a category-specific basis and determined empirically. The amount of the adjustment voltage U2 is greater than the adjustment voltage U1, but less than the adjustment voltage U3. Of course, it is possible to use a subtractor instead of the adders 15 to 17 and subtract the amount of the adjustment voltages from the measurement signals.

At the output 114 of the reference low pass 14 , the reference signal is available, which is fed to all three comparators 18 to 20 . In each comparator 18 to 20 the amplitude or level of the adjusted measurement signal (measurement level) is continuously compared with the amplitude or level of the reference signal (reference level) and an output pulse is generated when the reference level is exceeded by the measurement level. This output pulse represents the previously mentioned prerecognition. At the output of each comparator 18 to 20 , an intermediate memory 21 to 23 is connected, in which the respective comparator output signals are stored. The outputs of the intermediate memories 21 to 23 are connected to a main memory 24 , in which the intermediate memory values of the intermediate memories 21 to 23 are written as soon as a write command to the main memory 24 is triggered by a test device 25 . The main memory 24 switches on a transmission unit 26 , which issues the memory content of the main memory 24 as a message. When the message is sent, the transmission unit 26 generates a reset signal that the buffer 21 to 23 reaches all the delete or reset inputs R. Since, in addition, the writing of a buffer value into the buffer 23 should trigger an erase signal for the buffer memory 22 and the writing of a buffer value into the buffer memory 22 should trigger an erase signal for the buffer memory 21 , the reset input R of the buffer memories 21 and 22 is an OR gate 27 or 28 upstream. From the OR side 27 connected to the buffer 21 on the output side, one input is connected to the output of the buffer 22 and the other input is connected to the reset output of the transmission unit 26 . From the OR side 28 connected to the buffer 22 on the output side, one input is connected to the output of the buffer 23 and the other input is connected to the reset output of the transmission unit 26 . The reset input R of the buffer store 23 is directly connected to the reset output of the transmission unit 26 .

The test unit 25 is used to test the test criteria already mentioned above to confirm or not confirm the preliminary detections. If the test criteria are met, the test unit 25 triggers a write-in command in the main memory 24 , which takes over the buffer value present in one of the buffers 21 to 23 , which represents a pre-recognition, and thus confirms the pre-recognition. In detail, the test unit 25 has a timer 29 with a predetermined switching time T1, which can be shortened or lengthened via setting inputs E1 and E2. Via a start input S, which is connected to the output of the comparator 18 , the timing element 29 is set, and after the switching time T1 or the modified switching time T1 has elapsed, a signal triggering the write-in command occurs at the activation output A of the timing element 29 Connection line 30 reaches the main memory 24 .

The test unit 25 further comprises a differentiating and adapting unit 31 , which is connected on the input side to the output 112 of the category low pass 12 and on the output side to the two setting inputs E1 and E2 of the timing element 29 . The differentiating and adapting unit 31 detects the level change at the output 112 of the category low pass 12 and opens a time window with the window time T2 when a level drop is first detected. After the time window has closed, ie the window time T2 has elapsed, the level change in the output signal of the category low-pass filter 12 is measured again. If a drop in level is detected, a signal is given to the setting input E1. If a level increase is detected, a signal is given to the setting input E2. In the first case, the switching time of the timer 29 is shortened and extended in the second case.

In order to also meet the test criterion for the “pedestrian” category described at the outset, the buffer store 21 is designed in the form of a counter with a default setting, which counts the pulses output by the comparator 18 , each representing a preliminary recognition. When the preset number of pulses is reached, a buffer value is present at the output of the buffer memory 21 , which is transferred to the main memory 24 as a memory value with the write command.

The function of the device for Identification of road users in one Monitoring area is as follows:

The geophone output signal is low-pass filtered in the low-pass filters 11 to 14 . At the outputs 111 , 112 and 113 of the category low-pass filters 11 , 12 , 13 there is a measurement signal and the reference signal at the output 114 of the reference low-pass filter 14 . In the adders 15 , 16 , the DC adjustment voltage U1, U2, U3, which is a DC voltage, is subtracted from the measurement signal. The amplitude or the level of the measurement signals is thus adapted to the individual categories of road users. In the comparators 18 , 19 , 20 the level of the measurement signals is compared with the level of the reference signal. If, for example, the adjusted measurement level at the input of the comparator 18 exceeds the reference level, a pulse occurs at the output of the comparator 18 , which is counted by the buffer memory 21 designed as a counter. At the same time, this pulse at the output of the comparator 18 starts the timer 29 of the test unit 25 with the switching time T1. Due to the typical step pattern of a pedestrian, the adjusted measurement level will exceed the reference level several times within the switching time T1, so that several pulses occur at the output of the comparator 18 . All of these pulses are counted. If the preset counter reading is reached within the switching time T1, a storage value is present at the output of the buffer store 21 . Once the switch-on time T1 has elapsed, is triggered via the output of the timing element 29, a write command on the main memory 24, and the memory value from the latch 21 is transferred to the main memory 24th The main memory 24 switches on the transmission unit 26 , and this reports the stored value stored in the main memory 24 as identification of a pedestrian. Simultaneously with the issuance of the identification message, the transmitter buffer 26 resets the counter buffer 21 and the process can begin again. If the counter reading in the counter buffer 21 has not been reached when the switch-on time T1 of the timer 29 expires, then no buffer value is present at the output of the counter buffer 21 . When the write command is triggered, no memory value is therefore written into the main memory 24 , and no identification message is sent by the transmission unit 26 .

If, for example, a wheeled vehicle passes the monitoring area, both the adjusted measurement level at the comparator 19 and the adjusted measurement level at the comparator 18 will exceed the reference level. Thus, a pulse representing pre-detection occurs at both the output of the comparator 18 and at the output of the comparator 19 . The counter buffer 21 is increased by one in the counter reading, while a buffer value is written into the buffer 22 , which is present at the output of the buffer 22 and there causes the counter buffer 21 to be reset to the counter reading zero via the OR gate 27 . With the pulse at the output of the comparator 18 , the timer 29 of the test unit 25 is in turn started with the switching time T1. The differentiating and adapting unit 31 differentiates the measurement signal at the output 112 of the category low pass 12 during the entire detection process. If a drop in level is detected for the first time, a time window with window time T2 is opened. When the time window closes, ie the window time T2 expires, the measurement signal at the output 112 of the category low pass 12 is differentiated again. If a drop in level is also detected, the switching time T1 is shortened by the remaining switching time via the input E1, so that the write-in command is triggered immediately. The buffer value representing a pre-detection is thus written from the buffer 22 into the main memory 24 and the pre-detection is thus confirmed as "true". The transmission unit 26 is switched on again and reports the identification of a light vehicle. If the leveling and adaptation unit 31 does not detect a drop in level but rather an increase in level at the time the window is closed, the switching time T1 of the timer 29 is extended by a certain amount. When the extension time has elapsed, the write-in command is then triggered at the main memory 24 and the buffer value at the output of the buffer 22 , if still present, is written. The transmission unit 26 is switched on and reports the identification of a light vehicle.

If the monitoring area is passed by a heavy vehicle, that is to say a tracked vehicle, the adjusted measurement levels on all three comparators 18 to 20 exceed the respective reference level. A pulse representing pre-detection occurs at all three comparators 18 to 20 . The pulses are written into the assigned buffers 21 to 23 and are present at the outputs of the buffers 22 and 23 , while the count in the buffer 21 is increased by one. By means of the buffer value at the output of the buffer 22 , the counter buffer 21 is reset to the counter status "zero" via the OR gate 27 , and the buffer value at the output of the buffer 23 causes the buffer 22 to be deleted via the OR gate 28 . With the pulse at the output of the comparator 18 , the timer 29 has in turn been started. In the same way as described above, the function of the differentiating and adapting unit 31 takes place, so that after the modified switching time T1 the buffer value of the buffer 23 is transferred to the main memory 24 . The main memory 24 switches on the transmission unit 26 and this reports the identification of a tracked vehicle. At the same time, the buffer 23 is deleted.

For military applications it is important that the False alarm rate of the method or device is as small as possible, even under circumstances that do not occur in civil applications. So largely prevent strong air blasts, as in Artillery fires are triggered, false alarms by the  Identification device causes. Such false alarms are possible in that as a result of the acoustic seismic coupling these airborne sound bangs too trigger corresponding seismic impulses that lead to a Trigger the device.

This false alarm problem is avoided by an additional circuit which has a microphone 32 , a preamplifier 36 and a bang detector 33 , which in turn consists of a differentiating element 34 and a threshold switch 35 . If a bang event is detected by the bang detector 33 , the latter sends a blocking signal to the transmitting device 26 , which prevents the output of an identification message.

Claims (17)

1. A method for identifying road users crossing a surveillance area using a geophone arranged at or near the surveillance area for receiving the sound waves triggered by the road users, coupling them into the ground and propagating there, and generating corresponding geophone output signals, characterized in that categories of different road users , e.g. B. pedestrians, light vehicles, heavy vehicles, in each case a frequency range defined by an upper limit frequency is assigned, the upper limit frequencies of the frequency ranges being graduated against one another, that in each of these frequency ranges a measurement signal is obtained from the geophone output signal by envelope detection, that additionally in a background noise characteristic frequency range with a very small upper limit frequency from the geophone output signal by envelope detection a reference signal is obtained that the levels of the measurement signals (measurement level) from the frequency ranges are compared with the level of the reference signal (reference level) that when a reference level is exceeded by a measurement level in one Frequency range, a pre-detection identifying the category assigned to this frequency range is made and that after the fulfillment of category-specific predetermined test criteria, the pre-recognition as a category orie identification is confirmed. 2. The method according to claim 1, characterized in that that as a test criterion for the category "pedestrians" one within a given period of time occurring predetermined number of prerecognitions is used. 3. The method according to claim 1 or 2, characterized characterized in that as a test criterion for the Categories "light vehicle" and "heavy vehicle" sufficiently long drop in the measuring level in the frequency ranges assigned to these categories is used. 4. The method according to claim 2 or 3, characterized characterized that as an additional test criterion for every category of road user No pre-detection in one Frequency range with the next lower upper one Limit frequency assigned category is used. 5. The method according to any one of claims 1 to 4, characterized characterized in that the measurement level before comparison with the reference level by an adjustment level be reduced, the category-specific is.   6. Device for carrying out the method according to one of claims 1 to 5 with at least one geophone sunk at or near the surveillance area into the ground, characterized in that at the output of the at least one geophone ( 10 ) a plurality of low-pass filters ( 11 to 14 ) with one another staggered time constants are connected in parallel, whereby a low-pass filter is adapted to a frequency range characteristic for a road user category (category low-pass filter) and for background noise (reference low-pass filter) such that a comparator ( 18 to 20 ) is assigned to each category low-pass filter ( 11 to 13 ), to which the output signal of the assigned category low-pass filter ( 11 to 13 ) and the output signal of the reference low-pass filter ( 14 ) is supplied and which outputs a pulse representing the pre-detection if the output signal level of the category low-pass filter ( 11 to 13 ) exceeds that of the reference low-pass filter ( 14 ) that the outputs of the comparator en ( 18 to 20 ) each with the input of a buffer ( 21 to 23 ) for storing the comparator pulses and all outputs of the buffer ( 21 to 23 ) are connected to a main memory ( 24 ) and that one of the overwriting of the buffer values in the Main memory ( 24 ) triggering test unit ( 25 ) is provided, which triggers the overwriting after fulfilling the specified test criteria. 7. The device according to claim 6, characterized in that of the category low passes ( 11 to 13 ) assigned to the category "pedestrian" category low pass ( 11 ) the smallest and the category "heavy vehicle" assigned category low pass ( 13 ) the largest time constant and the Category "light vehicle" assigned category low pass ( 12 ) has an intermediate time constant. 8. Apparatus according to claim 6 or 7, characterized in that the test unit ( 25 ) has a timer ( 29 ) with a predetermined switching time (T1) which can be activated via a start input (S) and after the switching time (T1) via a with the main memory enable output (a) connected (24) initiates a write command to the main memory (24). 9. Device according to one of claims 6 to 8, characterized in that for checking the test criterion assigned to the category "pedestrian" the start input (S) of the timing element ( 29 ) is connected to the output of the comparator ( 18 ) which corresponds to the category low pass ( 11 ) is assigned to the "pedestrian" category, and that the buffer ( 21 ) connected downstream of this comparator ( 18 ) is designed as a counter, which outputs a buffer value when a predetermined counter reading is reached. 10. The device according to claim 9, characterized in that for checking the categories "light vehicle" and "heavy vehicle" assigned test criterion, the switching time (T1) of the timing element ( 29 ) can be changed and the timing element has a setting input (E1, E2) for adapting the Switching time (T1) and that the test unit ( 25 ) has a differentiating and adapting unit ( 31 ) connected on the input side to the output of the category low pass ( 12 ) for the category "light vehicle" and on the output side to the setting input (E1, E2) of the timing element ( 29 ) ), which opens a time window (T2) when a level drop is initially detected at the output ( 112 ) of the category low pass ( 12 ) and shortens the switching time (T1) of the timer ( 29 ) at the time the window closes when a level drop is detected and which when detection of a level increase Switching time (T1) of the timer ( 29 ) extended. 11. Device according to one of claims 6 to 10, characterized in that between each category low pass ( 11 to 13 ) and the downstream comparator ( 18 to 20 ), an adjusting member ( 15 to 17 ) is turned on, one of the comparator output signal to the respective Subtracted category coordinated, constantly specified adjustment voltage. 12. The apparatus according to claim 11, characterized in that the amount of adjustment voltage (U2) set on the adjustment member ( 16 ) assigned to the category "light vehicle" is greater than the adjustment voltage (U1.) Set on the adjustment member ( 15 ) assigned to the category "pedestrian" ) and is smaller than the adjustment voltage (U3) set on the adjustment member ( 17 ) assigned to the category "heavy vehicle". 13. Device according to one of claims 6 to 12, characterized in that in order to implement the additional test criterion, the output of each buffer ( 22 , 23 ) is additionally connected to the reset or erase input (R) of that buffer ( 21 , 22 ), which is assigned to a category low pass ( 11 , 12 ) with the next smaller time constant. 14. The apparatus according to claim 13, characterized in that the reset or erase input (R) of all buffers ( 21 to 23 ) is controlled by reading out the main memory ( 24 ). 15. The apparatus according to claim 14, characterized in that the main memory ( 24 ) is followed by a transmission unit ( 26 ) which outputs the memory content taken over by triggering the main memory ( 24 ) as an identification message and with the takeover of the memory content, the buffers ( 21 to 23 ) controls. 16. The apparatus according to claim 15, characterized in that a microphone ( 32 ) is arranged in the vicinity of the geophone ( 10 ) above the floor, the output of which is connected to a bang detector, and in that the bang detector ( 33 ) blocks the identification message of the transmitting unit, when the temporal change in the level of the microphone output signal exceeds a preset value. 17. The apparatus according to claim 16, characterized in that the bang detector ( 33 ) has a differentiating element ( 34 ) and a threshold switch connected downstream ( 35 ).