Problems of active protection of vibroacoustic channels.
Kargashin Viktor Leonidovich, Candidate of Technical Sciences
PROBLEMS OF ACTIVE PROTECTION OF VIBRAO-ACOUSTIC CHANNELS
Source: magazine «Special Equipment»
In recent years, the development of devices for creating vibration and acoustic interference, designed to ensure the protection of speech information from its leakage from premises through building structures and engineering communications, has been intensively developing. The reasons that force developers of special equipment to improve the equipment of this class include:
— the appearance of numerous publications on methods and means of covert control of speech information using sensors installed outside the protected premises, which indicate the danger of this method of covert control;
— the need to ensure one's own safety in conditions of the location of premises in the same building with third-party organizations, when there are enclosing structures and utility lines to which there is no controlled access outside the protected facility;
— the need for government agencies to comply with the requirements and recommendations for the protection of restricted areas, which are regulated by the State Technical Commission of Russia;
— the relative cheapness of active protective measures compared to passive measures, especially when it is necessary to quickly temporarily equip a room in which the safety of confidential negotiations must be ensured;
— the ability to manage the protection status and ensure control of the set parameters.
In the presence of a fairly stable demand for protection equipment, manufacturers are searching for additional technical solutions that allow more effectively solving the problem of neutralizing leakage channels. In most cases, this task is reduced to problems of a purely technical nature, mainly to improving the means of excitation of vibration noise signals in massive building structures. Without denying the importance of such a technical problem (increasing the coefficient of electroacoustic conversion of emitters and, as a consequence, increasing the efficiency of the equipment as a whole), we note that from the consumer's standpoint, the practical aspect of protection is also significant, which is associated with ensuring a sufficient level of safety of the facility in the specific conditions of its deployment.
The level of safety achieved when using protection equipment can ultimately be expressed by a set of technical requirements, the implementation of which allows special equipment to be classified as a certain class. The absence of regulated standard requirements for protection equipment as applied to typical situations does not allow the consumer to clearly determine the compliance of the purchased equipment with the set of their own specific conditions. It is assumed that the consumer must solve all these problems independently or by involving appropriate specialists who can integrate protection equipment in specific situations. This situation is explained by the fact that a specific embodiment as a means of protection of equipment for creating vibroacoustic interference is acquired only in combination with a priori unknown characteristics of the protected leakage channels, which are specific for each protected object.
In order to analyze the significant influence of the protected object on the degree of compliance of the equipment with certain tactical, technical, design, ergonomic and economic indicators, let us consider a generalized situation of using active vibroacoustic noise suppression systems, which is shown in Fig. 1 in the form of a block diagram.
The diagram shows possible components that determine the effectiveness of active protection, taking into account all the influencing factors. The initial speech acoustic signal (ISS) in the room Lо should be considered as a signal of the direct acoustic field, i.e. undistorted by the influence of the acoustic properties of the room itself. The signal properties of the process Lо are not deterministic, since they depend on such characteristics as the voice characteristics of the speaker, the specifics of the negotiations (conversation, speech, telephone conversation), the presence of sound amplification devices in the room, the characteristics of the transmitted speech message, the language in which the information is transmitted, the number of speakers. In this regard, the issue of creating valid tests (speech equivalent) by their type and volume level is an independent problem.
Fig. 1. Generalized scheme of application of active protection systems
In a room, which is a linear system for an acoustic signal with some transition characteristic hп, a complex pattern of spatial-frequency distribution of the acoustic signal is formed, depending on the architectural-acoustic parameters (AAP) — the size of the room, types of acoustic surface cladding, the presence of upholstered furniture and interior items in the room, even air humidity and its temperature. It is this acoustic signal, which is a complex composition of direct and diffuse (uniform in space) field components, that creates signals in information leakage channels of both vibration and acoustic types, and the pattern of the acoustic field changes significantly when the sound source moves, that is, it can be non-stationary in time.
Acoustic leakage channels are created when mechanical vibrations of the air environment pass through structures with weakened sound insulation values or in the complete absence of such, such as for air ducts such as ventilation systems. Vibrational leakage channels are created when acoustic vibrations of the air environment are converted into mechanical vibrations of solid media, which are enclosing building structures of premises and engineering and technical communications. Both types of leakage channels represent a single type of mechanical vibrations that have an obvious property of propagating in space, thereby creating the prerequisites for signal penetration at a significant distance from the original room. All possible vibration and acoustic leakage channels (VAKU) in the room have their own transient linear characteristics hi, which determine the spectral-energy parameters of the speech signal in the leakage channels. In a real room, the number of leakage channel types N practically corresponds to the number of homogeneous sections of enclosing structures and communications — each enclosing element (partition, window, door, column, ceiling) and each communication (pipeline, air duct, cable lines) represent an individual channel, the signals in which are correlated, but significantly different in power and its frequency distribution.
The efficiency of leakage channels depends not only on the degree of linear distortion of the original speech signal, but also on various types of interference and noise acting in the leakage channels. It should be noted that in the protected room there is already some acoustic interference that can lead to distortion of speech perception — room noise (RN) Nш(t), for example, broadcast signals, conversations of other people, ventilation noise, air conditioner noise, street noise. Depending on the acoustic properties of the room, primarily its size, these noises can significantly reduce the quality of speech perception even when the information sensor is located directly in the interior of the room, and even more so when receiving speech at a distance from the signal source. The intrinsic noise of the room, like the speech signal, creates an acoustic field determined by the parameters of the room and, like the speech signal, penetrates the information leakage channel. In addition to them, other types of noise and interference act in the channel, independent of the room noise, which are created by various noise sources (NS). In general, they are correlated with each other, since one noise source can create interference in both the acoustic and vibration channels. Each of the noise sources creates interference in the leakage channel with individual spectral characteristics Nсi(t).
Thus, at the location of the possible location of speech signal reception sensors Di, there is an additive mixture of the speech signal Lсi(t) and noise determined by the totality of room noise and leakage channels. Consequently, each of the secret information monitoring sensors provides its own value of the quality of the received information, which can be characterized by some objective indicator, for example, speech intelligibility, signal/noise ratio. Given the random and non-stationary nature of signals and interference, the speech reception quality indicator against the background of noise is a function of time and should tend to be stationary only over a significant averaging period, which imposes strict requirements on the assessment of the initial efficiencies of leakage channels in areas with a relatively homogeneous structure.
Active protection is used in situations where the initial efficiency of leakage channels exceeds some required standard value, and consists of creating additional interfering interference Ni, statistically independent of existing noise. Obviously, active protection should be used only for cases where the intrinsic interference has an insufficient power level or an unsuccessful spectral distribution, i.e. the equipment should generate interference in places where covert monitoring sensors may be located in cases of weak natural interference or the presence of a powerful speech signal in the channel. The advantage of using active protection compared to focusing on the protection of the premises based on the intrinsic interference of the channel is the ability to create interference with the required spectral distribution and a high degree of stationarity (constancy) in time.
However, the problems of practical application of active protection methods and equipment are not limited to the above aspects. The appearance of additional vibration and acoustic interference in leakage channels significantly changes the noise situation in the protected and adjacent rooms. Interference penetrates back through vibroacoustic leakage channels (VALC) into the original room, leading to the appearance of additional interfering acoustic noise No. in it. The same noises penetrate through parasitic vibroacoustic channels (PVAC) into adjacent rooms, including extraneous ones, creating noise Nпi in them.
Acoustic noise penetrating into the protected premises disrupts the working comfort of the protected person (group of persons), especially if the noise level No exceeds the room's own noise Nш. Moreover, the situation is aggravated by a comparison of the initial acoustic environment and the one obtained after the installation of noise protection equipment, which is not in favor of the latter. For an objective assessment of the degree of impact of disturbing acoustic noise on a person, one can be guided by the sanitary standards in force in the country, the permissible levels of acoustic noise in premises of various types and the time of their impact on a person. However, it is necessary to remember that a subjective negative assessment by a person of the disturbing influence of noise occurs at levels lower than those regulated by standards, especially with their long-term exposure. Similarly, noise penetrating into adjacent premises reduces the comfort of work in them, and given that these premises may not belong to the organization providing protection, the situation may lead to more serious measures of influence from third-party organizations or state supervisory bodies. A practical problem is the impossibility of measuring the level of additional noise in the premises of an outside organization without their consent.
Consequently, the capabilities of active protection are fundamentally limited and are completely determined by the properties of vibration and acoustic leakage channels and parasitic signal and noise propagation channels. The problem of comprehensive protection of the channels under consideration can be formulated as follows: it is necessary to ensure some minimum required value of the functional characterizing the protection indicator
j [Lci(t, f), Nш(t, f), Ni(t, f), Nci(t, f)] < j 0
provided that the secondary residual noise in the protected and adjacent rooms does not exceed sanitary standards
No(t, f) < Nсан(t, f), Nп(t, f) < Nсан(t, f),
where f is the frequency;
j 0 — the maximum standard parameter for the protection indicator;
Nсан(t, f) — sanitary standards.
Various functionalities can be considered as a protection indicator, from simple ones (such as the signal-to-noise ratio) to complex ones (such as the calculated speech intelligibility), which are more adequate to the task at hand.
Considering that each of the signal and noise processes depends on the acoustic properties of the protected premises, leakage channels and parasitic channels, the task of implementing protection measures looks quite complex. Moreover, the overall degree of fulfillment of the above requirements depends on the values determining the attenuation of signals and interference, to a greater extent than on the value of additional protective interference.
This can be illustrated by a simple example. Let the attenuation value of the speech signal in the leakage channel be equal to A, then the interference level that must be created in the channel is equal to N = Lc*A/q, where Lc is the speech signal level in the protected room; q is the signal-to-interference ratio required for protection. The level of interference penetrating back into the room can be determined as a first approximation, assuming the leakage channel to be symmetrical, using the formula Nп = N*A. Then the permissible interference level that can be created in the leakage channel must satisfy the condition Nд = LcA2/q < Nсан. It follows that the hardware capabilities of vibration and acoustic interference generators are limited by two types of standards (q, Nсан), the level of the original speech signal (Lc) and the amplitude-frequency characteristics of the leakage channels (A) to the second power. Consequently, increasing passive protection by 2 times (3 dB) leads to weakening the requirements by 4 times (6 dB), and changing the interference power leads only to a corresponding proportional change in the protection indicator.
From examining the generalized scheme of leakage channels, the following important conclusions can be drawn, which are necessary for improving the equipment for generating vibration and acoustic interference:
- the equipment set must include both vibration and acoustic emitters, designed for installation in typical conditions;
- the total number of emitters of both types must be sufficient to optimally solve a certain typical protection problem, taking into account the structural elements of the premises (pipelines, windows, doors, ceilings, etc.);
- interference signals in the emission channels must be independent to exclude the possibility of reducing the protective properties when using interference correlation in covert information control systems;
- emission channels must allow regulating the emission power levels and interference signal spectra separately for different emission channels, which helps minimize energy costs;
- the equipment must allow installation and control by the main protection parameter;
- the equipment must allow setting the protection level taking into account the requirements for the permissible level of residual acoustic noise;
- the developer must offer a set of passive protection measures that is applicable to various situations, taking into account the features of the developed equipment.