Compromise of active and passive methods of vibroacoustic information protection.

Compromise of active and passive methods of vibroacoustic information protection.

A.Kondratyev, O.Klyanchin

Catalog «Security Systems-2005»

Protection of speech data from possible leakage via technical channels is one of the most important tasks of ensuring information security, both in government and commercial structures.

This article is devoted to the most relevant technical channel for leakage of speech information—vibroacoustic.

Any information leakage channel is formed on the basis of three components: a transmitter (source of a dangerous signal), a dangerous signal propagation medium, a receiver (device for collecting information). A vibroacoustic channel is no exception. In this case, the dangerous signal is the speech of the interlocutors, that is, confidential information discussed out loud. The dangerous signal propagation medium is fences made of solid materials, walls, window panes, and pipeline communications. Due to their large size and considerable length, these structures have membrane properties and are subject to strong influence of sound vibrations. Due to their ability to propagate in solid structures, ventilation channels and air ducts, sound vibrations can be transmitted over significant distances from the object of the distance.

Thus, to use a vibroacoustic channel for information leakage, an intruder does not need to penetrate the protected premises; it is enough to gain access to adjacent premises or ventilation systems and install information collection devices there that convert vibrations of the protected structure into electrical ones.

ACTIVE METHODS OF SPEECH INFORMATION PROTECTION

As a rule, the implementation of a set of measures to protect information takes place in premises that are either already in operation or are in the final stages of construction and finishing work. In such harsh conditions, the use of active methods is preferable. However, this approach has a number of disadvantages, in particular:

significant financial costs for the design and creation of active noise suppression systems;
installation of distributed active noise suppression systems, which involves the inevitable change of interior solutions implemented in the premises;
parasitic acoustic noise caused by the physical principles of operation of distributed active noise suppression systems.

Implementation of active methods of information protection

Over the past few years, the market for technical means of protecting vibroacoustic information has been influenced by various trends, which have led to the emergence of a wide variety of this type of devices.

As a rule, a consumer faced with the task of implementing vibroacoustic protection of information faces the problem of choosing a protection system to ensure it. Most buyers are guided by the following criteria: low price, minimum dimensions, and the number of vibration transducers connected to the generator. Unfortunately, with this approach it is not always possible to build a high-quality protection system. The efficiency of any vibroacoustic noise suppression system usually depends on the following main parameters:

efficiency coefficient (EC) of vibration transducers;
the ability to adjust the spectrum of the interference signal in octave bands;
the presence of a performance monitoring system.

The efficiency of vibration transducers is mainly determined by the effective radius of the latter, which affects the required number of these devices and, accordingly, the cost of the system, as well as the level of parasitic acoustic interference. The higher the efficiency, the greater the comfort can be achieved, while meeting the standards for protecting the premises and the fewer transducers may be required. There are two types of transducers on the market: piezoceramic and electromagnetic.

Piezoceramic transducers (Fig. 1) have a higher efficiency, a low level of parasitic acoustic interference and a higher price. Electromagnetic ones are inferior to the first in characteristics, but have a lower price. Usually the difference in the cost of the two types of sensors is about: 10-40%. Moreover, as practice shows, the higher the range, the fewer of them are required. Thus, the cost of a system with piezoceramic transducers is lower than with electromagnetic ones.

Figure 1—Piezoelectric transducer «KVP-8»

It is also worth noting that recently transducers have appeared that do not emit parasitic acoustic interference at all. Such devices are installed at the facility during its construction or major repairs and are mounted directly into the walls. Due to this installation method, it is possible to achieve the best matching of transducers and completely get rid of parasitic acoustic interference.

The ability to adjust the spectrum of the interference signal in octave bands (equalizer) allows you to optimally form an interference signal to meet the requirements for protecting premises, as well as ensuring comfortable work. Today, almost all systems existing on the market have this function. Thus, the final level of parasitic acoustic interference largely depends on the correct setup (adjustment) of the system along with its correctly selected configuration.

Monitoring the efficiency of work allows monitoring compliance with the standards for protecting the premises during operation of vibroacoustic noise suppression devices. This is ensured by installing a branched system for measuring the signal level on enclosing structures. The use of these systems guarantees the solution of the following tasks:

monitoring the operability of converters and the system as a whole;
monitoring compliance with the requirements for protecting premises.

If the first task does not imply serious requirements for the control system and the market already offers devices in which it has been implemented, then to solve the second task it is necessary to use precise measuring devices (noise meters) of 0-1 accuracy class, which significantly increases the cost of the system.

Having considered the main features of vibroacoustic noise suppression systems, we should touch upon their additional functions, which include:

activation of the system by voice (by the presence of a signal);
modular implementation of the system.

Voice activation system (acoustic trigger)

This system is designed to control the vibroacoustic noise suppression system. In essence, the acoustic trigger analyzes the level of acoustic signals in the room and, if it exceeds a preset threshold, activates the vibroacoustic noise suppression system. The use of these systems can be useful in conditions of strong parasitic acoustic interference caused by either incorrect settings or incorrect configuration of the protection system, or high sound permeability of enclosing structures. By activating the protection system only at the time of negotiations, it is possible to reduce the period of exposure of people in the protected room to parasitic acoustic noise.

The appearance of such systems on the market has raised many questions about their effectiveness. The problem is that the guidelines do not specify the minimum level of acoustic signal at which activation should occur. And the question of the most preferable level remains open. When choosing a low threshold, the protection system will be activated by any rustle, for example, the sound of papers being shifted. On the contrary, a high threshold causes the risk that the system will not be activated at the time of discussing classified information.

Modular implementation of a vibroacoustic noise suppression system

This solution assumes that each converter has its own built-in noise generator. In this case, there is no need for a stationary generator, and only power is supplied to the converters. But the small dimensions of the generator cause a number of disadvantages characteristic of it, for example:

no possibility to adjust the signal level by octave bands;
low quality of the interference signal, since it is generated digitally;
small range due to the limited power of the built-in generator;
large dimensions of the converter, which can disrupt the interior design of both the entire room and the windows in particular

PASSIVE METHODS OF SPEECH INFORMATION PROTECTION

The disadvantages that arise when using active protection methods can be minimized, and in some cases completely eliminated by using special approaches. They are based on the use of a wide range of passive methods of speech information protection.

Passive methods are understood as a set of design and construction and installation activities aimed at improving:

enclosing structures (walls, floor, ceiling, windows, doors) of the premises;
engineering systems (supply and exhaust ventilation, heating, air conditioning);
wired systems for various purposes.

By carrying out the above-mentioned works, it is possible to achieve the appropriate levels of sound and vibration insulation and a decrease in the levels of dangerous signals arising due to acoustoelectric transformations, which, in turn, allows for a significant reduction, and in some cases, a complete abandonment of the use of active noise reduction methods.

Taking into account the considerable experience of using passive (construction) protection methods, it is possible to comparatively evaluate the positive and negative aspects of both approaches, as well as the areas of their optimal application.

Development and implementation of a set of passive methods

The development and implementation of passive methods of information protection are carried out by the contractor and include the following set of activities:

analysis of architectural, construction and design documentation for engineering support systems, as well as wired systems for various purposes;
development of a special part of architectural, construction and design documentation for engineering support systems;
author's supervision of construction and installation works for the implementation of a special part of projects;
special instrumental studies of implemented solutions in order to determine the sufficiency of the measures taken.

Analysis of architectural and construction documentation

This event involves identifying potentially dangerous areas in terms of voice information leakage via technical channels. Based on the analysis results, a set of measures for passive protection of certain areas is formed and a special section on the construction and equipment of protected premises is developed.

The development of a special section is mandatory regardless of the complexity and scale of the work performed within the framework of the construction or reconstruction of the facility. Thus, upon completion of the work, the customer receives a complete set of executive documentation for the construction and equipment of the premises, which significantly simplifies the further operation of the facility.

Author's supervision and instrumental control

In addition to the development of a special section on the construction and equipment of protected premises, it is mandatory to carry out author's (technical) supervision of construction and installation works. An important feature of these activities is that it is necessary to convey to the contractors of construction organizations all the nuances of the work with an emphasis on ensuring the required level of sound and vibration insulation of enclosing structures, as well as elements of engineering support systems. In addition to visual control over the progress of construction and installation works, appropriate instrumental control is necessary, carried out at various stages of construction readiness of the facility, appropriate instrumental control should be carried out.

Combination of passive and active methods

In cases where passive methods are insufficient to ensure the required level of protection, active noise suppression methods are used, the use of which, if properly configured, does not worsen the acoustic environment in the protected area.

The design of distributed active noise suppression systems, like other wired systems, involves the development of a cable duct, cable network, generator and terminal equipment. At the construction and reconstruction stage, there is a unique opportunity to design a hidden and serviced cable duct. These works will ultimately allow obtaining a distributed protection system without changing the interior solutions implemented at the facility.

When designing and developing a block of protected premises, it is possible to create a single hardware room—a control center, switching on systems, and signaling their operability, which significantly simplifies the issues of operating all systems

Financial aspect of using passive methods

The development and implementation of passive methods of protecting an object leads to a slight increase in the cost of design and construction and installation works and will not stand out significantly in the cost items on the scale of the entire construction or reconstruction. At the same time, the maximum implementation of passive methods will lead to significant savings on the equipment of active noise protection systems, on their installation, as well as the labor-intensive process of adjustment

Implementation of passive methods

Undoubtedly, passive methods of protection are indispensable when implementing security measures for doorways. In this case, most often for protected premises of low and medium levels, a combination of passive methods is used, consisting in improving the sound insulation of door panels (the use of sufficiently thick — made of solid wood — door panels, as well as soft seals along the contour of their adjoining, the device of thresholds), and active ones in the form of using noise speakers. According to customer requirements, for high-level premises it is often necessary to use only passive methods. Then the above is supplemented by making door panels multi-layered, with the inclusion of sound- and vibration-absorbing layers; sound- and vibration-absorbing materials are used when finishing vestibules. Doorways made in this way allow for sound absorption of at least 60-70 dB, which meets the highest security requirements.

Quite a few technical solutions have been tested within the framework of passive methods, which have demonstrated high efficiency in terms of protection against leakage through ventilation ducts (Fig. 2). length of about 2 m, As practice has shown, existing acoustic filters of various models on the market, with their linear length of about 2 m, provide attenuation of about 40-50 dB, and, consequently, protection along this channel.

Figure 2— Practical implementation of «passive» protection of the ventilation system

Speaking about the protection of premises from leakage through acoustic and vibration channels through boundary structures, it should be noted that until recently there was no wide range of solutions in the field of passive methods. The situation changed with the advent of finishing and at the same time sound-absorbing materials with a very diverse texture and color of surfaces and quite good values of the sound absorption coefficient. Today it is quite possible to design and manufacture not only capital (brick, concrete), but also lightweight boundary structures (partitions, walls) with a degree of sound and vibration insulation that ensures compliance with the requirements for information security in high-importance premises.

The same applies to ceiling and floor slabs. Here, protection through the acoustic channel, as a rule, is not so relevant, and blocking the vibroacoustic channel with active methods turns out to be quite labor-intensive.

A special case is the provision of security via the vibration channel using optical-electronic (laser) means of remote speech listening for window glass. Passive methods, except for the use of external light-proof blinds, do not exist. And their use can be challenged. For this channel, active methods remain the undisputed leader.

ACTIVE AND PASSIVE METHODS IN COMPARISON

Analyzing modern active and passive methods, we can highlight the following pros and cons of each approach.

The advantages of active noise suppression systems are as follows:

relatively low cost of initial implementation;
the ability to fine-tune noise-reducing signals that reduce parasitic noise;
the ability to provide security for virtually any room;
easy transfer of the system to a different level of security.

However, active methods have a number of disadvantages, including:

parasitic acoustic noise (reduced, but not eliminated with professionally performed system setup);
impossibility of complete concealment of protective measures;
necessity to switch on and off the protection system (for the duration of a closed event);
security of the premises is ensured only with normal power supply to the premises;
periodic instrumental verification of efficiency and, possibly, adjustment;
reliability of the system is determined by the reliability of noise generators and noise sensors.

The indisputable advantages of passive methods of information protection are the following:

absence of parasitic acoustic noise in the protected premises;
high temporary reliability and stability of sound and vibration absorption parameters;
complete secrecy of the applied protection measures;
constant protection of the premises for a certain period of time;
the protection of the premises does not depend on the availability of power supply;
increased comfort in the premises (reduction of the overall noise level).

Compared with the advantages of passive methods, they have significantly fewer disadvantages, including the following:

the use of this type of information protection methods in full is possible only during construction or major repairs in the protected premises;
changing the parameters of the protection of the premises, as a rule, is associated with construction work.

CONCLUSION

Thus, when developing various solutions to ensure the protection of speech information from leakage through acoustic and vibroacoustic channels, it is advisable to operate with various methods and means of protection. Nevertheless, we should not forget about a number of other channels of speech information leakage, which are no less dangerous, but this is the topic of another article.

About the authors: Kondratyev A.V., Head of the Special Research Laboratory of the Central Bank of the Russian Federation «MASKOM»; Klyanchin O.S., Head of the Department of Integrated Security Systems, Central Bank of the Russian Federation «MASKOM»