Protection of the organization's manager's speech information from hidden recording by a visitor.

Protection of speech information of the head of the organization from hidden recording by the visitor.

A. Adamyan

1. Introduction

Speech information is one of the main sources of obtaining data on a person's personal life or the financial, research, and production activities of an organization, i.e. information that is not subject to wide publicity (sometimes even classified). Despite the significantly increased role of automated information systems (AIS), speech information in message flows is still predominant (up to 80% of the entire flow) [1]. A person speaking, the environment for the propagation of acoustic, vibroacoustic, and electromagnetic oscillations, the propagation lines of electrical oscillations, and technical means for detecting and processing said oscillations form a channel for unauthorized access to information that must be protected.

As a result, the protection of speech information is one of the most important tasks in the overall set of measures to ensure the information security of an object.

To intercept it, a person interested in obtaining information (the «enemy») can use a wide range of portable acoustic speech reconnaissance devices that allow intercepting speech information via direct acoustic, vibroacoustic, electroacoustic and optical-electronic (acousto-optical) channels. The main ones include:

• portable radio recording equipment (dictaphones, tape recorders);
• directional microphones;
• electronic stethoscopes;
• electronic devices for intercepting speech information (embedded devices) with microphone and contact type sensors with transmission of information via radio, optical (in the infrared wavelength range) and ultrasonic channels, electrical power networks, telephone communication lines, connecting lines of auxiliary technical equipment or specially laid channels;
• optical-electronic (laser).

This paper examines the issue of protecting speech information from hidden recording by means of the first type of the above-mentioned means — dictaphones. In particular, protecting speech information from hidden recording by a visitor to the organization's manager.

The relevance of this issue is that at present there is an urgent need to ensure the protection of negotiations from hidden recording by electronic recording means. Today, the acquisition and use of hidden means of transmitting information via radio is sharply limited by legislation that came into force in 1996. In this situation, the simplest and most legal way to record speech information is to use dictaphones.

This class of equipment (due to the rapid development of electronics) has become ultra-miniature in size and allows recording large volumes of information.

There are two classes of dictaphones:

• analog (recording on magnetic tape);
• digital (recording on flash memory).

For more effective implementation of methods of protection against hidden recording using voice recorders, it is necessary to know the main features of hidden sound recording, factors affecting the quality of recording information, and typical techniques. These data are briefly discussed in Chapter 2.

The work considers two main areas of protection against unauthorized audio recording that exist today:

• prevention of bringing sound recording devices into controlled areas (includes the use of such means as: metal detectors, non-linear locators, X-ray devices, as well as special devices for determining the presence of working voice recorders);
• recording the fact of using a voice recorder and taking adequate measures (here, several classes of devices are considered that have different effects on voice recorders: on the information carrier itself (magnetic tape), on microphones in the acoustic range, on the electrical circuits of the voice recorder).

The work also provides several typical options for equipping the manager's office with speech information protection devices.

2. Features of hidden sound recording

Knowledge of the features of covert sound recording using voice recorders, factors affecting the quality of information recording, and typical recording techniques will be useful in the process of protecting against unauthorized removal of acoustic information. They will help to pay attention to the behavioral characteristics of people trying to record a conversation, to choose the right place for a confidential conversation, and to take competent countermeasures.

2.1. Factors Affecting the Quality of Sound Recording

Sound recording indoors is characterized by a large amount of acoustic interference, which is associated with the presence of reflected waves from the interior of the room, as well as with the presence of noise created by both people and noise and vibrations penetrating the room from the outside (from the street or from neighboring rooms) [2].

In quiet conditions, even the squeak of a mosquito, the buzz of a fly, the ticking of a clock and other sounds can be heard, but in conditions of noise and interference, even a loud conversation may not be heard. In other words, in conditions of noise and interference, the hearing threshold for receiving a weak sound increases. This increase in the hearing threshold is called acoustic masking. The amount of masking is determined by the amount of increase in the hearing threshold for the received sound signal.

External noises do not exhaust the list of interferences that occur during covert recording of acoustic information. The fact is that a voice recorder camouflaged in clothing records all the noises around it, and first of all those created by the operator himself, since he, as a rule, is located closest to the microphone. For example, people breathe, which means that their clothes are constantly in motion — the belt creaks from the rising and falling diaphragm, the jacket rubs against the shirt, etc. People do not hear this, however, the microphone hidden in the clothes catches everything, and the recorded conversation will be accompanied by incredible background noise.

The biggest inconvenience for a voice recorder is a conversation on the go. Here, everything is making noise: sleeves rubbing as you swing your arms, outerwear, the contents of your pockets (all sorts of keys, change, papers — everything jingles, rustles and creaks). Surrounding noises will also be picked up and recorded. And if in normal life we ​​do not hear them, using these filters given by nature, then when playing back the recording, everything will be recreated in the most inconvenient form.

The factors considered are fundamental when conducting covert sound recording, and they should be taken into account when choosing a place for the microphone of the sound recording device.

2.2. Selecting the microphone type and its installation location

Many modern voice recorders allow you to choose between built-in and remote microphones depending on the recording conditions. Of course, a built-in microphone makes the device more compact and ergonomic. However, its capabilities for covert recording of audio information are significantly limited, since such microphones have rather modest characteristics due to their extremely small size, and their placement is completely determined by the size and camouflage of the entire recording device.

The situation is different with remote acoustic receivers. They are well camouflaged and therefore can be installed in an area that ensures high-quality recording. Particular attention should be paid to the choice of the possible placement and type of such microphones.

When placing remote acoustic receivers, operators usually take into account the following three factors:

• Number of recorded speech sourcesTo record one interlocutor, one-way directional microphones are usually used from a distance of 50-70 cm. Less often, two-way directional microphones (for example, ribbon microphones) are used. However, the minimum distance to the source in this case increases to 80-100 cm, since at a closer distance the recording will «mumble».

  Both two-way and one-way microphones are suitable for recording a dialogue. In the first case, the microphone is placed between the interlocutors, in the latter case, they try to install it so that both objects are symmetrically located relative to the working axis of the acoustic receiver.

  To record a conversation between several interlocutors, one-way microphones with a large difference in sensitivity along the «front-rear» line are often used. They are placed so that the working axis is directed at the interlocutors, and the rear is towards the sources of acoustic interference.

  For recording a scene «at a round table», one-way directional microphones are most often used. Ideally, they are placed in the center in a vertical position with the direction of zero sensitivity downwards.

• Spatial orientation of the microphoneIn general, spatial orientation is determined by the dependence of the microphone sensitivity on the angle between its working axis and the direction to the sound source. For most types of acoustic receivers, an increase in this angle is accompanied by a drop in both the overall sensitivity and, in particular, the sensitivity at high frequencies. Only in some types of microphones, for example, two-way (octal) and, to a lesser extent, one-way microphones, the sensitivity at high frequencies changes when the working axis is rotated from the direction in the same way as the sensitivity at low frequencies. Therefore, microphones are directed with their working axis not at the source only in cases when it is necessary to make the recording of this sound less loud compared to others or to give the sound greater softness and less clarity.
• Distance to the source of the acoustic signalThe distance to the source is determined based on the properties of the room in which the audio recording is made and the properties of the microphone and the source.

  Acoustic processes at each point in the room, as noted above, are fairly well determined by the value of the acoustic ratio. The perception of the source in it depends on the relationship between the distance from the source to the microphone and the radius of the room's reverberation.

  If the distance from the source to the microphone is less than the radius of reverberation, then during playback the apparent dimensions of the sound source are larger than the actual dimensions, and the dimensions of the surrounding space are smaller than the actual dimensions. This creates a general impression of closeness and intimacy of the sound. When the distance of the microphone from the source is greater than the radius of reverberation, on the contrary, the dimensions of the source seem smaller than the actual dimensions, and the surrounding space is larger. The general impression of the sound is volume, airiness, power. When the microphone is located at a distance from the sound source equal to the radius of reverberation, the sound quality during playback is intermediate compared to that described above.

2.3. Means of ensuring the secrecy of operational sound recording

It was noted above that, depending on the model used, the voice recorder may have a built-in or remote microphone. The former is significantly inferior to the latter in technical characteristics, and also has fewer opportunities for hidden use. Therefore, in practice, remote acoustic receivers are more often used.

A remote microphone can be camouflaged as any element of personal belongings. It is often made in the form of a button and inserted into a buttonhole on clothing. And since the buttons are interchangeable, it is enough to simply carry out a general disguise from the proposed assortment. For example, the standard option is a white button on a light shirt. Remote microphones in the form of a pen cap, tie clip and other objects are also widely used (as a rule, they do not arouse any suspicion).

Simpler devices do not have standard camouflage, and due to their small size they can be hidden under clothing or in various objects (a book, a folder, a briefcase). Depending on the type of voice recorder used and the distance from the sound source, microphones can be equipped with an additional amplifier. As a rule, this is done if the microphone is installed at a significant distance from the voice recorder.

Various classes of voice recorders can be used for industrial espionage: from simple analog (with recording on a microcassette) costing from $35, to modern digital voice recorders (with recording on a microchip, on a Smart Media card, on a mini disk, on a Memory Stick card, etc.) costing up to several thousand dollars.

All the main types of portable voice recorders used for industrial espionage purposes generally meet the following technical specifications: frequency range — from 200-300 Hz to 3-5 kHz, flutter coefficient (tape speed fluctuation coefficient) — up to 4%, residual noise level — 30 dB, harmonic coefficient — up to 10%, syllable intelligibility — 60-80% with a confidence level of no worse than 0.9.

«Simple» analog voice recorders include voice recorders such as SONY-909M, SONY-950, NATIONAL-RNZ-36, OLYMPUS-L400. Recording in such devices is performed on a microcassette MS-90, which allows for up to 6 hours of continuous recording. Some voice recorders are equipped with a silent autostop, most have a VOX system (automatic recording start when an acoustic signal source appears — acoustomat), an external microphone and a remote on/off system.

Sometimes, simple products such as OLIMPUS-S928 or SONY-359, which cost between 35 and 100 US dollars, are used for unauthorized recording. However, the recording quality of these models is worse, and products of this class often do not have a jack for connecting an external microphone.

There are also professional products specially designed for hidden audio recording. These include, for example, voice recorders such as UHER CR-1600, UHER CR-1601, MARANTZ PMD-201, MARANTZ PMD-221. The main drawback is the very high price, which can reach several thousand US dollars.

However, the most convenient for unauthorized recording of conversations are digital voice recorders. Modern digital voice recorders have reached super-miniature sizes (while the quality of reproduction is an order of magnitude higher) largely due to the fact that the recording is made on various types of digital media: microchips, cards: Smart Media, Memory Stick, etc.

Among the latest developments, the following can be noted:

Toshiba DMR-420 WE/850WE. Features: one of the smallest to date. Recording is done on a built-in microchip. Recording time — 260/531 min. in high quality mode, in normal mode 3-4 times longer. Improved playback quality, simplified process of connecting to a computer;

Fig. 1. Toshiba DMR-420 WE

Toshiba DMR-SX2. Features: recording is done on a Smart Media card with a capacity of up to 65 MB, with which the recording time reaches as much as 1065 minutes. A special adapter and software are required to connect to a personal computer;

Sony ICD — MS1. Features: recording is done on a Memory Stick card. A 16 MB card allows you to record up to 131 minutes (the maximum card capacity is 64 MB). Simplified process of connecting to a personal computer. Estimated cost is $330.

Olympus VN-90/VN-180. Features: recording — on a built-in microchip. Recording duration — 90/180 minutes. Simple, reliable, inexpensive model. Estimated cost — $80-100.

Fig. 2. Olympus VN-90

Edic (made in Russia). Features: the most interesting of the above-mentioned voice recorders for unauthorized data collection. The smallest in its class — only 8 g. A large number of functions, easy connection to a computer. Recording is made on a microchip. Duration — 70/140 minutes. Estimated price — $125-150).

Fig. 3. Edic voice recorder

It is important to know what problems your unscrupulous competitors face when using voice recorders and how they can solve them:

• Some voice recorders have unpleasant control features — they switch off with a characteristic click of the ejected buttons or turn on the rewind after the end of the tape, which can also result in undesirable consequences. (This applies to a greater extent to analog voice recorders of the previous generation, modern digital voice recorders are free of these shortcomings).
• Another important problem is the recording capacity. Therefore, the person making the hidden audio recording is forced to constantly monitor the conversation time in order not to go beyond the cassette time. This is sometimes very inconvenient. To increase the recording time, some dictaphones, as noted above, use a reduced tape drive speed (for analog dictaphones) or a low-quality recording mode (for digital), but the recording quality, as already noted, deteriorates significantly and sometimes it even becomes problematic to identify the conversation.
• Low quality of sound recording due to various previously mentioned reasons (acoustic interference, etc.).

Sometimes, to save resources of the dictaphone, the voice activation function is used — acoustomat. But here, as with a bug, the beginning of the first phrase is «eaten». If the response threshold is set incorrectly, then whole sentences may be missed.

To improve the intelligibility of speech obtained as a result of covert sound recording, various «cleaning» filters are used. They are especially effective if the information was recorded against a background of powerful, but concentrated in the spectrum interference or specifically «colored» noise.

3. Protection from unauthorized audio recording

As already noted in the introduction, there are two main directions for solving the issue of protecting information from unauthorized recording by means of dictaphones:

• Preventing the bringing of sound recording devices into controlled premises (by means of «detectors» of dictaphones);
• Recording the fact of using a voice recorder and taking adequate measures (devices for suppressing recording of working voice recorders).

Let's consider them in more detail.

3.1. Voice recorder detectors

The first of the above methods can be implemented only if there is a sufficiently powerful security service and very solid financial resources. In accordance with the physical principles used in detection devices, the following types of equipment can be distinguished that are capable of solving these problems: metal detectors; non-linear radars; X-ray devices; special voice recorder detectors.

3.1. 1. Metal detectors

Metal detectors can be used at entrances to premises or during external inspection of persons and items they carry (cases, bags, etc.). These devices are of two types: stationary (arched) and portable.

Due to the limited sensitivity of metal detectors, the reliability of detecting such small objects as modern microcassette and digital voice recorders is insufficient in most cases, especially when open inspection is undesirable or simply impossible. Thus, metal detectors can only be considered as an auxiliary tool in combination with other more effective measures to detect and suppress sound recording devices.

Fig. 4. METOR 160 metal detector

At posts of this type, the control equipment is camouflaged as interior items. Portable metal detectors, covertly placed under the clothes of the control post personnel, are often used as a supplement to the stationary metal detector.

3.1.2 Nonlinear locators

Non-linear radars are capable of detecting voice recorders at significantly greater distances than metal detectors, and in principle can be used to control the carrying of sound recording devices at entrances to premises. However, this raises such issues as the level of safe radiation, response identification, the presence of «dead» zones, compatibility with surrounding systems and electronic equipment.

Fig. 5. Non-linear radar «Rodnik-2K»

Structurally, the device can be made in the form of a classic frame (analogous to a metal detector frame) or installed directly in the doorway of the manager's office.

The task of detecting electronic products (in our case, voice recorders) in the «frame» mode requires the definition of two main criteria for nonlinear locators:

• parameters of the locator transmitter;
• parameters of the locator receiver.

These two parameters also affect two more important operational criteria:

• detection characteristic of the system;
• safety of using non-linear locator for personnel for a long time.

The peculiarity is that these two criteria are antagonistic. The goal of the system user is to detect everything related to radio electronic devices, and this is determined by the detection characteristic. However, achieving the set goal may conflict with the safety of working with open HF radiation. This contradiction can be resolved with the correct selection and evaluation of the parameters of the nonlinear locator itself.

The installation option for a nonlinear locator is shown schematically in Figure 6. To increase the probability of detecting sound recording devices, the antennas are located on both sides. If we assume that the average height of a person is 170 cm and the conventional width is 60 cm, then the irradiated surface will be (170 x 60) x 2 = 20400 sq. cm. In accordance with regulatory documents, the permissible power flux density to which personnel are continuously exposed during the working day should not exceed 10 μW/sq. cm. Knowing the size of the irradiated surface, we can find the maximum power of the radiation source. It should not exceed 0.204 W. This value represents the average power of the maximum permissible radiation.

Fig. 6.

Not a single model of nonlinear locators with continuous emission mode meets the safety requirements for service personnel. On the contrary, all pulse locators, despite the seemingly significant values ​​of the emitted pulse power, fully meet the safety requirements.

Therefore, when deciding on the use of these means of protection, it is necessary to take into account the safety of the manager in whose office this equipment is located and operated.

Of the non-linear locators existing on the Russian market, the following can be distinguished: «Cyclone»; NR-900M1; «Onega-3»; «Rodnik-23».

According to the results of the test conducted in the «Confident» magazine [3], of the above-mentioned devices, the «Cyclone» device looks preferable in terms of its characteristics.

In general, the non-linear locator is an effective means of early detection of sound recording equipment, which can be classified as limited access systems or stationary monitoring systems.

3.1.3 X-ray devices

X-ray devices can reliably detect the presence of voice recorders, but only in items carried. Obviously, the scope of application of these control devices is extremely limited, since they practically cannot be used for personal inspection and covert control. A stationary X-ray scope has the following characteristics:

• maximum overall dimensions of the checked carry-on baggage — 500x400x350 mm;
• power supply from a single-phase current network with a voltage of 220 V;
• power consumption 1500 W.

Fig. 7. Stationary X-ray complex «Premier ST»

The necessity and possibility of their use should be considered in the context of specific tasks and existing local conditions. At the same time, it is worth noting that, contrary to popular negative opinion, modern X-ray equipment creates minimal dose loads on the examined object, which do not affect even film and photo materials. For the best examples of this equipment, the dose is less than 100 microroentgen per examination.

This means of protection is quite expensive and not every organization can afford it.

3.1.4. Special devices for detecting the presence of working voice recorders

There are two operating principles of such devices, based on (a) the effect of detecting acoustic signals and (b) the detection of secondary electromagnetic radiation (SEMR).

The characteristic noise of the tape drive mechanism and clicks when pressing buttons are common phenomena for cassette tape recorders of the 70-80s. Therefore, special techniques were used to mask their operation, from placing the devices near sound sources (such as a clock) to fingering a rosary during a conversation to mask the clicks of the recorder with the clatter of the knuckles. However, these times have sunk into oblivion, since it is practically impossible to detect the acoustic signal from the tape drive mechanism in the vast majority of modern devices with the usual background noise in the room and other interference. And digital recorders are generally completely silent.

Thus, recording side electromagnetic radiation is now the only possible way to detect working recorders.

As a rule, the operation of many voice recorder detectors (especially portable ones) is based on the principle of detecting radiation from the erasure generator — magnetization bias (EBB). However, the following problems arise when such detectors operate:

• the frequency range used is characterized by a large number of sources of powerful magnetic fields (TVs, public transport overhead lines, fluorescent lamps, electric motors of household appliances, etc.), which literally «jam» the radiation of dictaphones much more effectively than they used to jam «foreign» radio stations;
• many of modern foreign-made dictaphones do not have a magnetic field at all. Erasing is provided by a permanent magnet, and magnetization is provided by the so-called «constant component».

Consequently, these devices are practically unsuitable for detecting the most modern means of sound recording.

Theoretically, it is possible to detect side emissions that arise as a result of self-excitation of an electronic device due to parasitic connections in generator and amplifier stages, for example, a microphone amplifier. However, measurements show that the range of possible registration of PEMI of this kind (in the range of 20 kHz — 50 MHz) does not exceed several centimeters for household sound recording devices, and from special devices with a metal case they are not registered at all even by highly sensitive laboratory devices.

There are devices that react to the alternating magnetic field that occurs when electric motors operate. In the laboratory, they work very clearly, but in practice, the main difficulty in implementing them is the presence of a large number of sources of low-frequency magnetic fields, the diversity of spectral portraits of radiation from different types of voice recorders, and low signal levels. True, the metal cases of voice recorders are no longer an obstacle to detecting fields of this type.

As a result of the analysis of this «food for thought», we can conclude that it is objectively difficult to create truly reliable equipment for detecting working sound recording equipment. And yet, attempts to create such devices do not cease, and a number of models are even available for sale.

In general, this equipment includes the following units:

• a low-frequency magnetic antenna, designed as a separate element and placed as close as possible to the intended location of the voice recorder;
• a detector unit that performs the operation of detecting PEMI, with an adjustable response threshold;
• filters that limit the frequency band in which monitoring is carried out; sometimes rejection filters (that is, «closing» certain ranges) are added, tuned to the frequencies of the most powerful sources of local interference (as a rule, they are structurally implemented in the detector unit);
• devices for light (LED scale, pointer indicator, control lamp) and sound (vibration) indication of the presence of PEMI (they are structurally implemented either in the detector block or are placed on a special control panel);
• power supply.

Let's consider some examples of the practical implementation of these means.

At first glance, the best option is the RK 645-SS product, which implements the first direction of combating dictaphones. Flat magnetic antennas are placed along the perimeter of the door. The detection range of a standard sound recording device is up to 1 m. However, a significant drawback is the complete impossibility of detecting switched off dictaphones, that is, if a person enters an office (building) with a non-working dictaphone, and only then turns it on, the system will not record it. Therefore, such a device must be supplemented with others: an arched metal detector and a non-linear locator, and this is already a very, very expensive pleasure.

An interesting domestic development is the PTRD-018 (Portable tape recorder detector). It is designed for covert detection of working magnetic sound recording devices. The device consists of a recording unit and 4 (8 or 16) sensors that are installed permanently (for example, in the table at which the most important negotiations are conducted, or in the armrests of the client's chair).

The sign used to detect a dictaphone is the electromagnetic field created by the working electric motor of the tape drive mechanism. Note that the spectrum of this electromagnetic field lies in the range of very low frequencies, and as a result, even the metal cases of «branded» devices for hidden sound recording do not protect them from detection by this device.

The main obstacle to signal detection by devices of this type is the electromagnetic field of industrial interference both at the main frequencies and at their harmonics (up to the 9th), which significantly limits the use of such devices. In addition, it is fundamentally impossible to detect the use of digital voice recorders.

There are also portable versions of detectors of working voice recorders, which can be used outside the office. An example is the TRD 009V product from CCS. The dimensions of the device allow you to easily place it in your pocket. The alarm signal is a slight vibration of the case. Moreover, the closer you are to the voice recorder, the stronger the vibration.

However, it should be taken into account that in practice such portable systems are ineffective, since their use requires the sensor to be brought as close as possible to the intended location of the voice recorder. You have to literally hug the interlocutor, which is not only inconvenient, but also simply tactless.

3.2. Devices for suppressing recording of working voice recorders

From the materials of the previous subsection it is clear that detecting a voice recorder is a very complex technical task. At the same time, a voice recorder operating for recording can be suppressed, that is, conditions can be created under which recording is impossible. In recent years, various voice recorder suppressors have been used more often, in which both acoustic and electromagnetic interference can be used. The following types of impact on voice recorders exist:

• on the information carrier itself (magnetic tape);
• on microphones in the acoustic range;
• on the electronic circuits of the sound recording device.

3.2.1. Systems for suppressing voice recorders by impacting the information carrier

This method of influencing dictaphones has found application in devices such as a demagnetizing arch, which is installed in the vestibule of the entrance door and creates a powerful, alternating magnetic field (usually with the network frequency or a multiple of it). As a result, objects in the vestibule (including cassettes with recorded information) are demagnetized.

The devices are characterized by high energy consumption and are quite dangerous to health. Therefore, the organization using such systems is obliged to inform visitors about the presence of danger, which is a giveaway factor and leads to the fact that, at the insistence of the client, the conversation can take place outside the walls of this institution.

Due to the above reasons, as well as the fact that these devices do not work on modern digital voice recorders, this class of means is ineffective.

3.2.2. Counteraction systems using the principle of action directly on the microphone itself

These systems can be divided into two groups:

• impact on the microphone in the ultrasonic range in order to overload the microphone amplifier;
• use of an active acoustic interference generator in the speech range.

Ultrasonic suppression systems emit powerful ultrasonic vibrations that are inaudible to the human ear (usually the emission frequency is about 20 kHz), which directly affect both the microphones of voice recorders and acoustic bugs, which is their undoubted advantage. This ultrasonic effect leads to overload of the low-frequency amplifier located immediately after the acoustic receiver. Overloading the amplifier leads to significant distortion of the recorded (transmitted) signals, often to a degree that cannot be deciphered.

For example, the «Zavesa» system, when using two ultrasonic emitters, is capable of suppressing voice recorders and acoustic bugs in a room with a volume of 27 m3. However, ultrasonic suppression systems have an important drawback: their effectiveness is sharply reduced if the microphone of the voice recorder or «bug» is covered with a filter made of a special material or a low-pass filter with a cutoff frequency of 3.4-30;4 kHz is installed in the low-frequency amplifier.

The second group of suppression means, using generators of active acoustic interference in the speech range, is used in limited cases. However, today a large number of active vibroacoustic camouflage have been created, successfully used to suppress means of interception of speech information. We can list some of them: the systems «Zaslon», «Kabinet», «Baron», «Porog-2M», «Fon-V», «Shorokh», VNG-012GL, VNG-006, ANG-2000, NG-101, «Echo», etc. We will consider these means in terms of protection against unauthorized recording on a voice recorder (acoustic interference).

To form vibroacoustic interference, special generators based on vacuum, gas-discharge and semiconductor radio elements are used. In practice, noise oscillation generators have found the widest application.

Along with noise interference, speech-like interference, chaotic pulse sequences, etc. are used for active acoustic masking.

The role of devices that convert electrical oscillations into acoustic oscillations of the speech frequency range is usually performed by small-sized broadband acoustic speakers. They are usually installed indoors in places where acoustic reconnaissance equipment is most likely to be located (in our case, this could be the manager's desk, the conference table. The speakers should be installed in such a way that the places where visitors sit are completely «covered by interference»)

When organizing acoustic masking, it is necessary to remember that acoustic noise can create an additional disturbing factor for employees, for participants in negotiations, create discomfort and irritate the human nervous system, causing various functional deviations, leading to rapid fatigue. Indeed, it is difficult to imagine a confidential conversation between partners accompanied by a noise generator with a power of 75-90 dB.

Optimization of the operating mode of the acoustic noise suppression systems under consideration will reduce the level of side noise and ensure greater comfort when conducting conversations in the protected premises.

In a number of systems, it is possible to regulate the level of the interference signal. In some, it is done manually (Cabinet, VNG-012GL, ANG-2000), and in some, automatically (Zaslon-2M) (depending on the level of the masked speech signal). In the Baron complex, it is possible to independently regulate the level of the interference signal in three frequency ranges (central frequencies: 250, 1000 and 4000 Hz), in the Shorokh-1 system — in five octave bands, and in the VNG-012GL generator — in fifteen one-third octave bands.

Another direction for increasing the comfort of conversations is the optimization of the interference spectrum, ensuring the implementation of the required standards for information protection with a minimum integral interference level.

Acoustic masking systems use noise, «speech-like» and combined interference. The most frequently used types of noise interference are:

1. «White» noise (noise with a constant spectral density in the speech frequency range);
2. «Pink» noise (noise with a tendency for spectral density to fall off by 3 dB per octave toward high frequencies);»pink» noise (noise with a tendency for spectral density to fall off by 3 dB per octave toward high frequencies);
3. Noise with a tendency for spectral density to fall off by 6 dB per octave toward high frequencies;
4. Noise «speech-like» interference (noise with an amplitude spectrum envelope similar to a speech signal).

In acoustic masking systems, interference of the «white» and «pink» noise types is usually used. «Speech-like» interference is formed (synthesized) from speech signals. In this case, interference can be formed both from the hidden signal and from speech fragments (segments) uncorrelated with the hidden signal. It is known that, with the same emitted power, speech-like noises (interference) have a slightly greater masking effect. Additional masking capabilities of such interference, compared to the rest of the above, are explained by their structural proximity to the masked speech signals. As a result, the human auditory mechanism is unable to distinguish a useful speech signal against the background of speech-like interference even at speech/noise ratios higher than when masking with dispersed noise.There are also other rather interesting developments in this area. Let's consider one of them — equipment called CNDS [4] (Confidental Negotions Digital System), which ensures confidentiality of negotiations with a different number of participants both in stationary and portable conditions.

Fig. 8. CNDS

The principle of operation of the device is based on masking the primary carrier of speech information with a continuously emitted sound signal (noise) from acoustic speakers, the frequency band of which coincides with the frequency band of the speech signal. The level of emitted noise is selected in such a way that human speech would sound unintelligible at any point in the manager's office. The noise characteristics must exclude the possibility of noise cleaning of the speech signal intercepted on secondary carriers. The noise cleaning procedure uses a two-channel adaptive filter, to one input of which the room signal (a mixture of the speech signal and noise) is fed, and to the second input — the original emitted noise.

In stationary conditions (in our case — in the manager's office) the number of participants can reach 12 people, the equipment elements can be built into office furniture.

The CNDS equipment includes a digital speech signal processing unit with a built-in microphone, acoustic speakers and a set of headphones for the participants in the negotiations. The digital processing unit generates masking noise and outputs it to the acoustic speakers, simultaneously implementing digital two-channel adaptive filtering to noise-clean the speech signal received by the built-in microphone. The speech signal, «cleaned» from masking noise, is presented to the participants using headphones.

White noise is used as a masking signal. When more effective speech-like interference is used, the negotiators, in addition to the purified speech signal presented to them through headphones, always additionally perceive weakened masking noise due to loose ear pads. If this noise is speech-like, it distracts the hearing, impairing the perception of the purified speech.

Using CNDS during negotiations allows us to obtain the following results:

• the speech of the negotiators, recorded on a dictaphone in the breast pocket of one of the negotiators, is completely unintelligible (only the speech of the dictaphone owner is somewhat intelligible);
• in the signal received from a microphone located behind the speaker at a distance of no more than a meter from his head, speech is not audible

The advantage of acoustic generators is that they suppress any eavesdropping equipment that includes a microphone. The disadvantage is that acoustic interference is audible, it interferes with conversation and unmasks the operation of security equipment. Despite all its disadvantages, this method is a much less expensive, safer and more reliable (!) way to maintain the confidentiality of conversations compared to means of detecting dictaphones. It is advisable to use it sparingly, in cases where maximum security of negotiations is required (in this case, all minor troubles recede into the background).

3.2.3. Systems for suppressing dictaphones by affecting the electronic circuits of the sound recording device

An alternative to acoustic interference is electromagnetic interference, which is generated by special generators.

The Russian market offers a wide variety of similar devices: «Rubezh», «RaMZes», «Shumotron», «Buran», «UPD», «Bastion». These devices differ in their design and circuit design.

The principle of their operation is the same: the «suppressor» of dictaphones is a generator of electromagnetic radiation of sufficiently high power, operating in the microwave range (as a rule, these radio interference generators have a relatively narrow radiation band in order to minimally interfere with radio receiving equipment of various purposes and to maximize the spectral density of the signal). The frequencies at which these devices operate are usually around 1 GHz, although there are exceptions. Power is units of watts, for example, 5-6 watts. Devices with greater power do not meet medical standards.

Fig. 9. LGSh-103 «RaMZes-Double»

Structurally, dictaphone jammers consist of a generator, a power source and an antenna. They emit electromagnetic interference in a direction: usually it is a 60-70 degree cone directed in one direction (the rear lobe of the radiation is practically absent). It is in this zone that dictaphones are suppressed. The directional signal allows to significantly increase the electromagnetic field strength in the suppression zone and reduce interference induced on electronic equipment located outside the suppression zone (office equipment, computers, TVs, etc.). Since the noise signal is induced directly in the input circuits, other eavesdropping equipment that includes microphones is equally well suppressed. As with dictaphone detectors, the degree of shielding of the dictaphone or other eavesdropping device plays an important role, therefore, if dictaphones in plastic cases are suppressed at a distance of up to 5-6 meters, then in metal ones — 1.5-2.5 meters. If the voice recorder is equipped with a remote microphone, the suppression range becomes even greater due to the fact that the connecting cable acts as an antenna receiving radiation from the suppression equipment.

There are mainly two versions of electromagnetic suppressors: portable, mounted in a regular case, and stationary, placed in the negotiation area under the table or in the nearest cabinet.

Recently, a very small device has appeared — «Shumotron-4». Its dimensions are 200x150x50 together with autonomous power supply. The device can be placed in a basket, but it works only for 15-20 minutes, and the suppression zone is 1.5-2 times smaller than that of conventional ones.

The radiating antenna of the device has a strictly defined directionality, so it would be correct to speak not about protecting the premises as a whole, but about creating a certain protected zone (or zones). The interference emitted by these antennas, affecting the elements of the electronic circuit of the recorder, causes noise-like interference in them. As a result, noise is recorded simultaneously with speech, which leads to significant distortion of the recorded information or even to its complete suppression.

The best suppression results are achieved when the recorder is located on the axis of the main lobe of the directivity pattern of the antenna of the device. Therefore, when installing «suppressors», it is important to correctly position the antenna or the device itself (if there is a built-in antenna).

Another point that should be noted. If the recording device is on the owner's body (in a suit, etc.), then it is possible that the speech of the owner of the voice recorder may or may not be recorded, but there will certainly be big problems with recording the speech of the interlocutor. This will happen because the sound pressure acting on the microphone of the recording device, created by the voice of the owner of the voice recorder and his interlocutor are incommensurate in level. It is important for the owner of the suppressor that his speech is not recorded. This is achieved by using the suppression system.

Some types of voice recorders in recording mode (UHER, DICTAPHONE, some models of SONY, PANASONIC, etc.) when entering the suppression zone begin to «make noise» in the acoustic range, thereby revealing the intentions of their owner. So if your interlocutor suddenly makes noise in his pocket when you turn on the voice recorder jammer, it means he wanted to record you, and the precautions taken were not in vain.

Typically, such devices are used in the office, but they can also be used in cars powered by the on-board network; sometimes camouflage in the form of a «case» is used.

The stationary version is usually placed under the table, and the antenna is most often attached to the table top from below or placed directly on the table, which provides an optimal zone. Another problem is that the radiation from one antenna does not provide a suppression zone that covers a normal negotiating table about 3 m long. To expand the suppression zone, a second antenna is installed (Shumotron, Storm, Ramses-double) or even 4 antennas (Shumotron). Two-antenna systems provide a suppression zone along one, wide side of the negotiating table [5].

Much attention should be paid to safety precautions. The electromagnetic field power flux density for the 300-3000 MHz range (the frequency range in which the suppressors under study operate) is standardized by medical and sanitary standards with a threshold of 40 mW/cm2. However, adverse effects on the body can occur not only by a one-time high intensity of radiation, but also by systematic low-intensity irradiation. This must be taken into account when placing the suppressor — its impact on the location of the manager must be minimal.

It is interesting to note that this equipment is equally effective against both kinematic and digital voice recorders. These devices have become the most widespread in practice due to their effectiveness and relatively low price.

Unfortunately, voice recorder jammers have some disadvantages.

First: adverse effects on the human body. Many devices of this class have medical certificates. As a rule, they indicate at what distance and for how long a person can safely be in the main lobe zone. For example, for one of the products at distances of 1.5 m this time is up to 40 minutes a day, at a distance of 2.0 m up to 1 hour, and in the zone of the rear and side lobes the time of stay is not limited. Here we can make the following comparison: cell phones, despite their prevalence, have a greater effect (harmful) on the human body than suppressors.

The second disadvantage: the source of noise electromagnetic radiation interferes with conventional radio-electronic equipment. Radio receivers, active acoustic speakers, conventional telephones, office radiotelephones with analog radio channels, audio and video intercoms, household TVs, computer monitors are most susceptible to the impact. If the suppressors are poorly located, there may be false alarms of security and fire alarms. In fact, almost all of these problems can be solved by proper installation, location relative to other radio-electronic devices and, as already mentioned above, correct location relative to the owner (in our case, the manager).

4. Typical options for equipping the manager’s office with speech information protection equipment

The following typical options for equipping the manager's office with speech information protection tools are classified by the «size» of the organization, its financial capabilities (this is directly related to the amount allocated for protecting speech information from unauthorized recording and, consequently, what tools can be purchased). Of course, there are many other determining factors, and any organization requires an individual approach to this issue. But, since information protection is «an expensive pleasure», the financial issue, in my opinion, is one of the priorities.

When writing these standard options, the main task was to ensure maximum protection of confidential negotiations, regardless of the financial resources of the organization. In richer organizations, this protection may be less noticeable, more comfortable for the participants of the negotiations, easier to manage, but the level of protection should be approximately the same.

Let's consider them in more detail.

1) Organization with small financial resources

Since the financial resources in such companies are limited, they most likely do not have the opportunity to purchase a variety of security tools.

Here you can use the acoustic protection system for confidential negotiations (such as: CNDS, «Echo»). These are inexpensive but very effective means of protection against unauthorized recording on a voice recorder. The disadvantage of this option is, firstly, its «unconcealedness» for the participants in the negotiations, and secondly, the head of the organization must decide for himself: when talking with which visitor to use this means of protection, and in which case it is not necessary. It is more expedient to use this means in negotiations that require maximum confidentiality, safety of information.

2) Organization of «medium power»

The financial resources of these organizations allow using not just one means of protection against unauthorized recording of speech information, but several. In this case, it is more correct to choose multi-faceted means, for example: one means of detecting dictaphones, another — a means of influencing the dictaphone itself. In the doorway of the manager — a hidden metal detector frame or non-linear locators (such as: «Cyclone»; NR-900M1; «Onega-3»; «Rodnik-23») in the form of a metal detector frame. A means of suppressing dictaphones such as «Rubezh», «RaMZes», «Shumotron», «Buran», aimed at the place where the visitor is usually located. And for especially important, confidential negotiations, an acoustic protection system (such as CNDS, «Echo») is also used.

When a visitor passes through a doorway, the manager receives information (for example, on a hidden display) from a metal detector (non-linear radar), and if positive information is received about the presence of electronic equipment, he can discreetly turn on a means of suppressing voice recorders.

3) Large organization

This includes organizations with large financial resources that allow them not to skimp on information security issues. These organizations already have a powerful security service, and some functions for protecting against unauthorized removal of information via voice recorders lie with them (for example: detecting voice recorders). Here we will consider 2 options.

Option 1

A frame metal detector is installed at the checkpoint of the enterprise. Non-linear radars (Cyclone; NR-900M1; Onega-3; Rodnik-23) in the form of a metal detector arch are covertly installed in the doorway of the organization's executive reception area, and a PTRD-018, RK645-55 type voice recorder detector is installed in the doorway of the executive's office. Voice recorder suppression systems of the Rubezh, RamZes, Shumotron, and Buran types are installed under the executive's desk and under the conference table. For especially important negotiations, the CNDS acoustic protection system is also used.

Option 2

Checkpoint — a stationary X-ray complex, installed covertly (for example: the Premier ST complex). In the doorway of the reception area — an arched metal detector (covert). The remaining means are similar to the first option.

In summary, it should be said once again that all the above typical examples are very conditional. This information can only help the head of the organization a little in choosing the means and methods of protecting speech information from unauthorized recording by means of dictaphones.

5. Conclusion

The paper describes the features of covert sound recording using voice recorders, factors affecting the quality of sound recording, typical recording techniques, the main types of voice recorders, and also provides characteristics of some modern models of digital voice recorders. This information can be useful in forming a protection system against the considered means of unauthorized information retrieval.

Two directions of counteracting unauthorized recording are considered:

• detection of voice recorders at an early stage;
• suppression of recording of working voice recorders.

The first group includes the following types of technical means: metal detectors, nonlinear radars, fluoroscopy devices, as well as special voice recorder detectors.

Due to the limited sensitivity of metal detectors, the reliability of detecting such small objects as modern microcassette and digital voice recorders is insufficient in most cases. Thus, metal detectors can only be considered as an auxiliary tool in combination with other more effective measures for detecting and suppressing sound recording devices.

Non-linear radars are capable of detecting voice recorders at significantly greater distances than metal detectors and can be used to control the entry of sound recording devices into premises (when installed as a metal detector arch). The following problems may arise when using these devices: safe radiation level, response identification, presence of «dead» zones, compatibility with surrounding systems and electronic equipment.

X-ray devices can reliably detect the presence of voice recorders, but only in items carried. Obviously, the scope of application of these means of control is extremely limited, since they practically cannot be used for the purposes of personal inspection and hidden control. This means of protection is quite expensive and not every organization can afford it.

Special detectors of voice recorders, with all their advantages, have one significant drawback — a short detection range of voice recorders. The real distance at which a voice recorder can be detected for such devices (for example, TRD-800, SRM-700) is 30-50 cm for plastic voice recorders and 2-10 cm for voice recorders in a metal case. Devices with microprocessor processing (for example, PTRD series devices) allow detecting voice recorders at greater distances, but in any case this distance, as a rule, does not exceed one meter under normal conditions, and in the presence of operating computer equipment in the immediate vicinity, even microprocessor detectors give false signals.

Suppression devices for recording working dictaphones are classified according to the principle of their impact on dictaphones (on the magnetic tape of a cassette recorder, on microphones in the acoustic range, and also on the electronic circuits of a sound recording device). The paper examines the following technical means of this group: a demagnetizing arc, ultrasonic suppression systems, acoustic interference generators, and the so-called «suppressors» of dictaphones (suppression systems by influencing the electronic circuits of a sound recording device).

Degaussing arc: these devices are almost never used in practice due to a number of disadvantages: they are quite harmful to health, are characterized by high energy consumption and minimal efficiency (only dictaphones with recording on a magnetic medium (cassette) are disabled, and these devices do not affect modern digital dictaphones).

Ultrasonic suppression systems are capable of providing reliable suppression of voice recorders and acoustic bugs in a fairly large room. However, ultrasonic suppression systems have an important drawback: their effectiveness is sharply reduced if the microphone of the voice recorder or «bug» is covered with a filter made of a special material or a low-pass filter with a cutoff frequency of 3.4-4 kHz is installed in the low-frequency amplifier.

Acoustic jammers: The advantage of acoustic generators is that they suppress any eavesdropping equipment that includes a microphone. The disadvantage is that acoustic jamming is audible, it interferes with conversation and unmasks the operation of security equipment. Despite all its disadvantages, this method is a much less expensive, safer and more reliable way to maintain the confidentiality of conversations compared to means of detecting dictaphones. It is advisable to use it sparingly, in cases where maximum security of negotiations is required (in this case, all minor troubles recede into the background).

«Suppressors» of dictaphones (systems for suppressing dictaphones by affecting the electronic circuits of the sound recording device): These devices have become most widespread in practice due to their effectiveness and relatively low price. This equipment is equally effective against both kinematic and digital dictaphones. The disadvantages of «suppressors» include: adverse effects on the human body, as well as the fact that the source of noise electromagnetic radiation interferes with conventional electronic equipment. However, with proper installation of the device and compliance with the operating rules, they can be largely avoided.

The work also provides conditional typical examples of equipping the head's office with these means, which were classified depending on the financial resources of the organization. Together with the considered features of covert sound recording using dictaphones, a detailed description of technical means of protecting speech information, the principles of their operation, this work should provide useful information when forming a system of protection against unauthorized removal of information by means of a dictaphone by a visitor to the head of the organization.

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