Requirements for interference generating 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 following should be noted as reasons that force developers of special equipment to improve equipment of this class:
— the appearance of numerous publications on methods and means of covert control of speech information using sensors installed outside protected premises, which indicate the danger of this method of covert control;
— the practical need to ensure one's own safety in conditions where premises are located 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 for situations where it is necessary to quickly equip a room in which it should be possible to safely conduct confidential negotiations;
— the ability to manage the protection status and ensure control of the established protection parameters.
In the presence of a fairly stable demand for protection equipment, manufacturers are looking for additional technical solutions that allow more effectively solving the problem of neutralizing leakage channels.
In most cases, this task is reduced to purely technical issues, 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 efficiency 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 security for an object in the specific conditions of its deployment.
The level of security achieved when using protection equipment can ultimately be expressed by a set of standard technical requirements, the implementation of which allows special equipment to be classified as a certain class.
The lack of regulated standard requirements for protection equipment applicable to typical situations does not allow the consumer to clearly determine the compliance of the purchased equipment for a set of their own specific conditions.
The presence of a certificate from the State Technical Commission of Russia essentially means that the declared technical device is indeed a generator of vibration or acoustic interference of a certain frequency range and, in certain situations, can be used to suppress speech signals.
At the same time, the limits of the device's applicability, its capabilities for protecting the required range of leakage channels are not defined at all, there is no organic connection with regulatory requirements for protection and hardware for assessing the degree of compliance with protection standards.
It is assumed that the consumer must solve all these problems independently or by engaging relevant specialists who can integrate protection equipment in specific situations.
This situation is explained by the fact that the specific embodiment as a means of protecting 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 as a block diagram.
Fig. 1. Generalized diagram of the use of active protection systems
The diagram in Fig. 1 shows the possible components and components that determine the effectiveness of active protection, taking into account all influencing factors.
The initial speech acoustic signal (ISS) in the room Lo 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 Lo process 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.
In a room, which for an acoustic signal is a linear system with some transient characteristic hп, a complex picture of the spatial-frequency distribution of the acoustic signal is formed, depending on the architectural-acoustic parameters (AAP) — the size of the room, the types of acoustic surface cladding, the presence of upholstered furniture and interior items in the room, even the humidity and temperature of the air.
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 acoustic field pattern changes significantly when the sound source moves, i.e. 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.
Vibration leakage channels are created when acoustic vibrations of the air environment are converted into mechanical vibrations of solid media, which are the enclosing building structures of the premises and engineering and technical communications.
Both types of leakage channels represent a single type of mechanical vibrations that have the obvious property of spreading in space, thereby creating the prerequisites for signal penetration at a significant distance from the original room.
All possible vibration and acoustic leakage channels (VALC) 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 that act in the leakage channels.
It should be noted that in the protected premises there already exists 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 premises, 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 premises, and even more so when receiving speech at a distance from the signal source.
The room's own noise, like the speech signal, creates an acoustic field determined by the room's parameters and, like the speech signal, penetrates into the information leakage channel.
In addition to them, other types of noise and interference operate in the channel, independent of the room's noise, which are created by various noise sources (NS) that are generally 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, in the location of the possible location of the speech signal reception sensors Di, there is an additive mixture of the speech signal Lсi(t) and noise determined by the totality of the 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.
Considering the random and non-stationary nature of signals and interference, the speech reception quality indicator against a background of noise is a function of time and should tend to be stationary over a significant averaging period, which places strict demands on the assessment of the initial efficiencies of leakage channels in areas with a relatively uniform structure.
Active protection is used in situations where the initial efficiency of leakage channels exceeds a certain required standard value, and consists of creating additional interfering interference Ni, statistically independent of existing noise.
It is obvious that active protection should be used only in cases where the inherent 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 a room based on the inherent interference of a channel is the ability to create interference with the required spectral distribution and a high degree of stationarity (constancy) over time.
However, the problems of practical application of active protection methods and equipment are not limited to these 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 (VACL) into the original room, causing additional interfering acoustic noise No.
The same noise penetrates 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, and the situation is aggravated by a comparison of the initial and resulting acoustic environment after installation of noise protection equipment, which is not in favor of the latter.
To objectively assess the degree of impact of disturbing acoustic noise on a person, one can be guided by the sanitary standards in force in the country for permissible levels of acoustic noise in various types of premises and the time of their impact on a person.
However, it is necessary to remember that a person's subjective negative assessment of the disturbing impact of noise occurs at levels lower than those regulated by standards, especially with their long-term impact.
Similarly, noise penetrating into adjacent rooms reduces the comfort of work in them, and given that these rooms may not belong to the organization implementing the 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 rooms of an outside organization without their consent.
Consequently, the possibilities of active protection are limited in principle and are completely determined by the properties of vibration and acoustic leakage channels and parasitic channels of signal and noise propagation.
The problem of complex 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 residual side noise in the protected and adjacent rooms does not exceed sanitary standards
No(t, f) < Nsan(t, f), Nп(t, f) < Nsan(t, f),
where f is the frequency; j 0 is the maximum standard parameter for the protection indicator; Nsan(t, f) is the sanitary standards.
Various functionals can be considered as protection indicators, from simple ones such as the signal-to-noise ratio to complex ones such as the estimated speech intelligibility, which are more adequate to the problem being solved.
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 protective measures appears to be quite complex.
Moreover, the overall degree of fulfillment of the above requirements depends on the values that determine the attenuation of signals and interference, to a greater extent than on the value of additional protective interference.
This can be seen in a simple example.
Let the attenuation value of the speech signal in the leakage channel be equal to A, then the level of interference that must be created in the channel is equal to the value N = Lc*A/q, where Lc is the level of the speech signal 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, considering the leakage channel to be symmetrical, using the formula Nп = N*A.
Then the permissible level of interference 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 the consideration of 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 for the optimal solution of some typical protection problem;
— interference signals in the emission channels must be independent to eliminate the possibility of reducing the protective properties when using interference correlation in covert information control systems;
— the emission channels must allow regulation of emission power levels and interference signal spectra separately for different emission channels, which helps to 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 are applicable to various situations, taking into account the features of the developed equipment.
Currently existing devices for creating vibroacoustic interference do not fully satisfy all of the specified requirements, which leads to the need for the consumer to carry out additional procedures for setting the protection parameter, and most importantly, to solve the problem of the need to create interference of different levels and spectrums (it is obvious, for example, that the levels and spectrum of interference for noise pollution of a capital wall and window glazing must be different).
The products Baron and Shorokh-1, which include vibration-type emitters and provide the ability to connect acoustic emitters, which, however, are not included in the delivery set, meet the requirements for the range of emitters to the greatest extent.
Both products have independent emission channels (4 and 3), which allows for a variety of types of structures and communications that need to be jammed, and the interference spectrum in octave frequency bands and the overall interference level in the generation channels can be adjusted.
The Baron product includes a device for monitoring the degree of protection.
However, it should be noted that the main emphasis in these products is on creating vibration interference mainly on concrete structures, communications and window glazing without taking into account the requirements for protecting acoustic channels.
On a positive note, it should be noted that the documentation for the Shorokh product offers specific recommendations for typical installation of vibration and acoustic emitters in a room.
The most basic issue of the assembly during development is the problem of choosing the number of emitters of different types or independent emission channels designed to create interference in different types of leakage channels.
A reasonable approach in such a situation is to focus on a standard or a series of standard solutions determined by the practical needs of customers.
For example, a standard room can be considered to be a medium-sized room with an average number of openings and communications, or an average standard meeting room with the corresponding design features.
The experience of each developer will allow him to compile a minimum sufficient list of emitters that allows solving the problem of protecting a typical room, and taking into account possible variations, rooms close to the average typical room in other situations.
The use of non-traditional types of interference for noise suppression of speech, as proposed in the Baron product, does not have a scientific basis and is based rather on intuitive assumptions, and not on a large volume of statistical articulation tests.
The signals of broadcasting stations, which are proposed to be used for masking speech, have a significantly non-stationary nature depending on the type of transmission (speech, music), and cases of significant time pauses, in which the degree of protection will be significantly reduced, are not excluded.
The very idea of using such interference is apparently based on attempts to reduce the effect of the interfering impact of secondary acoustic noise on others, rightly believing that extraneous speech or music to a lesser extent reduces the comfort of the working conditions of personnel in noise.
However, this problem should be solved as shown above by using passive protection measures, and not by using non-stationary interference, the protective properties of which have not been fully studied.
According to information from developers, the IKMC-1 company is preparing to manufacture the VNK-012GL vibroacoustic protection hardware and software complex, which was developed to the maximum possible extent taking into account the above-mentioned requirements for equipment:
— the equipment generates 5 independent interference channels, 4 of which are intended for connecting vibration emitters and one for connecting acoustic emitters;
— the number of vibration emitters and acoustic emitters is designed to protect a typical room with a floor area of 25 — 30 m2;
— spectrum and interference level adjustment in each channel independently in octave or one-third octave frequency bands;
— includes its own test signal source for adjusting the equipment;
— setting protection parameters using built-in testing equipment and ongoing monitoring of them, which is especially important when temporarily equipping protected premises;
— the ability to monitor absolute values of acoustic noise levels to assess their compliance with sanitary standards;
— a software method for configuring equipment and setting its initial parameters, allowing the use of statistical methods for assessing protective properties, easily managing interference characteristics and monitoring the current state of protection, forming databases for various protection objects, creating protocols based on the results of work;
— the developer offers not only equipment, but a set of measures taking into account the capabilities of passive protection, aimed at solving the problem of protecting the customer's facility.
The conducted review of the main tactical requirements imposed on vibroacoustic noise suppression equipment showed that the developers have sufficient reserves for improving the equipment taking into account typical or specific conditions of the deployment of objects and their architectural and acoustic features.
The next direction of equipment development should be the creation of typical technical requirements for individual components and equipment as a whole, which will standardize the equipment, allowing developers to improve in the further development of their equipment in the areas of minimizing total costs, and consumers to receive special equipment with technical characteristics, the combination of which allows solving the problem of protecting their own facilities with minimal costs.
Kargashin V.L.
Candidate of Technical Sciences
Senior Researcher
Source: magazine «Special Equipment»