Methodological Aspects of Industrial and Transport Noise Monitoring

Abstract

The chapter outlines the methodological aspects of monitoring industrial and transport noise, including the main physical characteristics, features of sources, measuring instruments, features of hygienic regulation of industrial and transport noise, means and methods of protection against it. It is shown that industrial facilities and most modes of transport are sources of high-intensity noise, the spectrum of which is dominated by frequencies of the low-frequency infrasonic range. The close physical nature of these ranges contributes to the propagation of such noise with low attenuation, and they have good penetrating power, so most noise protection devices are ineffective. This requires careful medical supervision of persons working in such conditions, improvement of means and methods of protection against industrial and transport noise.

1. Introduction

In accordance with modern concepts, noise and infrasound are classified as harmful and dangerous physical factors, the impact of which causes a decrease in efficiency and reliability of activity, and a long or short cumulative effect causes the development of a number of diseases [1, 2, 3].

To date, a large amount of data has been accumulated on the adverse effects of noise on humans. The nature of this influence depends on the sound level, the duration of exposure and the spectral composition of the noise [4, 5, 6]. The hearing organ is the critical organ of the body when exposed to noise. It is generally accepted that the most harmful effect on the organ of hearing is provided by noise, the spectrum of which is dominated by high frequencies of the sound range (from 1 to 8 kHz). In the clinical picture, along with hearing impairment, pathology of the cardiovascular and nervous systems is often found, which made it possible to form the concept of “noise disease” [7].

The physical characteristics of infrasound are well studied by acousticians, however, hygienists and occupational pathologists have long been limited in their research by the lack of reliable and affordable measuring equipment. Therefore, the history of the study of infrasound as a factor in the environment and production environment is relatively short. The first publications devoted to the action of infrasound appeared in the period 1970–1980. During this period, reports appeared in the scientific literature about the high biological efficiency of infrasound [1, 7, 8]. The first hygienic standards for infrasound in the USSR were adopted only in 1981, while the first noise standards for its limitation in workplaces were adopted in 1956. Subsequently, a large number of publications appeared, which reflect the point of view of hygienists on the problem of infrasound effects on humans [1, 7].

Since 2004, infrasound has been included in the list of harmful and hazardous production factors in Russia. The critical organs under the influence of infrasound include not only the organ of hearing, but also the vestibular analyzer, the central and autonomic nervous system, the circulatory and respiratory organs [9, 10, 11, 12]. The presence of several organs and systems in the clinical picture allows us to speak about the separation of infrasound pathology into a separate nosological form [1].

There are reports that low-frequency noise can have a harmful effect not only on the organ of hearing, but also on other human organs and systems. Its biological effect has a certain similarity with the effect of infrasound on the human body. An analysis of industrial and transport noise shows that its spectrum is dominated by low frequencies of the audible and infrasound ranges. Close physical parameters and biological effects allowed a number of authors to introduce the terms “low-frequency acoustic oscillations”, “infrasonic disease” and “vibroacoustic disease” [1, 7].

2. Specificity of industrial and transport noise monitoring

2.1 Physical characteristics

Depending on the frequency, acoustic vibrations are divided into infrasonic, sonic and ultrasonic. According to their physical nature, the acoustic vibrations of these ranges are the same, and their separation is somewhat arbitrary and is associated with the physiological feature of the human auditory analyzer. It is believed that a person hears sounds with frequencies from 16 Hz to 20 kHz. The area of sound frequencies or acoustic vibrations of the air in the infra-, ultra-, and hypersonic ranges is not perceived by the human ear. It should be noted that modern regulatory documents give a slightly different frequency gradation for infrasound. Infrasound is commonly understood as acoustic vibrations with a frequency below 22 Hz. At high levels of sound pressure (SPL) infrasound (over 120 dB), a person has a feeling of pulsation, pressure, and even pain in the eardrum. The physical features of infrasound include a long wavelength and low absorption in the atmosphere and the resulting ability of infrasound to propagate over long distances from the source without significant loss of energy. It should be kept in mind that sound propagates spherically and the decrease in sound pressure is inversely proportional to the square of the distance from the source [13, 14].

The audio frequency range includes acoustic vibrations from 20 Hz to 20 kHz, which are perceived by the human ear. Noise is a disorderly combination of sounds of different strength and frequency. According to the predominance of acoustic energy in one or another part of the spectrum, noise is divided into low-frequency (up to 500 Hz), medium-frequency (from 500 to 1000 Hz) and high-frequency (from 1000 to 8000 Hz).

2.2 Industrial sources of noise and infrasound

The noise generated during the operation of modern production equipment, the operation of machinery and vehicles, is acoustic vibrations in a wide frequency spectrum: from infrasonic to ultrasonic ranges.

The use of various mechanisms and machines in production activities, an increase in their power and dimensions have led to a change for the worse in the acoustic situation at the workplaces of personnel. There is a tendency to increase the contribution of low-frequency components, including infrasound, to the industrial noise spectrum. Production low-frequency noise and infrasound are generated during the cyclic movement of large surfaces, during shock excitation of structures, reciprocating and rotational movement of large masses with a repetition rate of cycles of no more than 20 per second, with the rapid movement of large volumes of liquid and air. In “pure” form, infrasound practically does not occur in a production environment: as a rule, its “companions” are high-intensity noise and general vibration [1].

The spectra of most industrial and transport noises contain low-frequency noise and high-level infrasound. The results of acoustic measurements show that if airborne noise levels are about 90–100 dBA, then the presence of infrasound with a SPL of 100–107 dB can be expected [15].

Acoustic measurements at the enterprises of the metallurgical industry near blast furnaces and steel-smelting furnaces showed the presence of SPL of 95–108 dB at frequencies of 8–31.5 Hz. In the gas and oil industry, sources of low-frequency noise and infrasound are air and reciprocating compressors, ventilation installations, pipelines, and so on. SPLs from 92 to 123 dB in octave bands of 8–63 Hz were registered at workplaces. The maximum SPL in octaves of 4–31.5 Hz during the operation of ventilation units and air conditioning systems is 98–100 dB, during the operation of compressor units – 92–123 dB at frequencies of 8–16 Hz and diesel units 111–123 dB at frequencies of 8– 63 Hz (Figures 1–4) [1, 7, 8].

Figure 1.

Sources of low-frequency noise and infrasound in industry and transport (abscissa—Sound pressure levels, dB).

Figure 2.

The upper limits of the range of the maximum energy spectrum of the noise: Along the abscissa axis—The noise source (1—Metallurgical industry, 2—Gas and oil industry, 3—aviation industry, 4—Mining and construction industry, 5—Spacecraft, 6—Turbojet aircraft, 7—Piston aircraft, 8—Helicopters, 9—Motor transport, 10—Railway transport, 11—Cargo river and sea vessels, 12—Hydrofoils and hovercraft) along the ordinate axis—Noise frequency, Hz.

Figure 3.

The lower limits of the range of the maximum energy spectrum of the noise: Along the abscissa axis—The noise source (legend—See Figure 2).

Figure 4.

The upper and lower limits of the range of the maximum energy spectrum of the noise: Along the abscissa axis—The noise source (legend—See Figure 2).

There are a large number of noise sources in the aviation industry, especially at the stages of testing individual units and components and engines. At the workplaces of aviation specialists, SPLs reach 132 dB in the high-frequency and mid-frequency ranges. The highest noise levels were noted at the workplaces of motor test stations (SPL 120–132 at frequencies of 50–150 Hz) [8, 16, 17, 18].

The main sources of noise in the mining and construction industries are compressors, diesel and ventilation units, vibration platforms, etc. At workplaces, SPL at frequencies of 10–45 Hz is 98–123 dB. In the noise spectrum of vibratory platforms with a high load capacity, SPL in the octave bands of 2–16 Hz is about 100 dB, cranes – 8–16 Hz (79–94 dB), hammers and presses – 8–31.5 Hz (108–114 dB) [7].

Jet engines of rockets and airplanes are powerful sources of low-frequency noise and infrasound. When launching rockets of some types, the highest SPL (150 dB or more) are determined at frequencies of 10–12.5 Hz. During takeoff of turbojet aircraft of the TU-154 type with a total noise in the cabins of about 100 dBA, the infrasound levels are 80 dB at a frequency of 4 Hz and 90 dB at a frequency of 20 Hz. In helicopter cockpits, the highest SPLs are 110–120 dB at a frequency of 28 Hz, which corresponds to the rotational speed of the propeller blades. When servicing aircraft with running main and auxiliary engines and ground equipment, aviation specialists at their workplaces are exposed to noise with an SPL of 100–120 dB in octave bands from 2 to 31.5 Hz [8]. Ground vehicles are also significant sources of low-frequency noise and infrasound. Thus, acoustic oscillations with a SPL of 93–110 dB in the range of 8–31.5 Hz are especially characteristic of the cabs of heavy trucks and busses. With fully open windows, an increase in SPL is noted at frequencies of 2–6 Hz. The speed of traffic has a great influence on acoustic performance [19, 20].

In railway transport, the sources of low-frequency noise and infrasound are the power plants of diesel locomotives and electric locomotives, compressor and ventilation units, and aerodynamic flows at high speeds. Railway workers in their workplaces are exposed to noise with SPLs of 92–115 dB at frequencies of 8–50 Hz. Locomotive crews are in the most unfavorable conditions, at the workplaces of which infrasound reaches SPL from 100 to 115 dB. The presence of open windows during the movement of rolling stock leads to an increase in SPL and a shift in the spectrum to the region of low-frequency noise and infrasound, especially at high speeds [7, 21, 22].

Sources of low-frequency noise and infrasound on sea and river vessels are power plants, diesel generators, propellers, ship ventilation and air conditioning systems, etc. Metal hull structures have high sound conductivity, which contributes to the spread of noise throughout the vessel. At workplaces, the seafarers are exposed to noise with an SPL of 100–130 dB at frequencies of 8–45 Hz. The highest noise levels (up to 100 dBA) are observed in the power compartments of ships, which, as a rule, are 30–40 dB higher than in other habitable spaces. During the operation of hydrofoils and hovercraft, SPLs in the region of 6–10 Hz reach 100–130 dB [7].

2.3 Means of measurement and hygienic regulation of industrial and transport noise

Acoustic measurements are mainly carried out for research purposes, designed to reduce the level of influencing noise, for hygienic monitoring, to comply with the requirements of sanitary standards, as well as for predictive risk assessment of the development of diseases associated with acoustic exposure [1, 7, 23, 24, 25].

For hygienic standardization according to the temporal characteristics of noise, gradation is introduced into three categories [1]:

• constant noise, the sound level of which does not change by more than 5 dBA during an 8-hour working day or during the measurement in the averaging mode of the sound level meter S (slow);

• intermittent noise, the sound level of which changes by more than 5 dBA during an 8-hour working day, a work shift or during a measurement when measured with the averaging time constant of the sound level meter S (slowly);

• impulse noise, consisting of the infrasound of one or more sound events, each with a duration of less than 1 s, while the sound levels L_p,AImax and L_p,ASmax, measured respectively with time corrections I (impulse) and S (slow), differ by at least 7 dB.

The current standards for hygienic regulation of noise at the workplace of personnel use the following indicators:

1. sound pressure level, L_p, dB is 10 decimal logarithms of the ratio of the square of the sound pressure to the square of the reference sound pressure equal to 20 μPa;

2. the equivalent sound pressure level, L_p,eqT, dB is 10 decimal logarithms of the ratio of the square of the sound pressure to the square of the reference sound pressure over a given time interval;

3. A-weighted sound level (A sound level) in dBA is 10 decimal logarithms of the ratio of the squared rms sound pressure, measured using standardized frequency weighting A, to the square of the reference sound pressure. To determine the nature of the noise, sound levels A are measured with time corrections S (slow, τ = 1 s), F (fast, τ = 125 ms), I (impulse, τ = 40 ms);

equivalent sound level with frequency correction A (equivalent sound level A), L_p,Aeq,T, dBA—10 decimal logarithms of the ratio of the square of the root-mean-square sound level A to the square of the reference sound pressure at a given time interval, which is calculated by the formula:

$$L_{p,\mathit{Aeq},T}=10\mathit{lg}\left[\frac{\underset{t_1}{\overset{t_2}{\int }}{P}_A^2\left(t\right)\mathit{dt}}{T{P}_0^2}\right],$$E1

equivalent sound level A for a work shift—L_p,Aeq,8h, dBA, equivalent sound level A, measured or calculated for 8 hours of a work shift, taking into account corrections for impulse and tonal noise, which is calculated by the formula:

$$L_{p,\mathit{Aeq},8h}=10\mathit{lg}\left(\frac{1}{T_0}\sum _i{T}_i{10}^{0,1\left(L_{p,\mathit{Aeq},\mathit{Ti}}+K_i\right)}\right),$$E2

where T₀ is the standard duration of the work shift (8 hours, if the duration of the work shift is different from 8 hours, T₀ is taken equal to the actual duration of the work shift with a total work duration of 40 hours per week); T_i is the duration of the i-th noise exposure interval, h; L_p,Aeq,Ti is the equivalent sound level or sound pressure measured at the i-th noise exposure interval, dBA; K_i is a correction for the nature of the noise, equal to 5 dB in the case of tonal and (or) impulse noise (applied when L_p,Aeq,Ti > 75 dBA, in all other cases K = 0 dB is assumed);

1. the maximum sound level A, L_p,Amax, dBA is the highest sound level measured over a given time interval with standard time correction;

2. the time correction function is a standard exponential function of time for the square of the instantaneous sound pressure in a time averaging operation (international standard). Sound level meters use standard time corrections S (slow, f = 1 s), F (fast, f = 125 ms), I (impulse, f = 40 ms), they are also called averaging time constants;

Peak C-weighted sound level (C sound level), L_p,Cpeak, dBS, is 10 decimal logarithms of the ratio of the square of the peak sound pressure, measured using standardized frequency equalization, to the square of the reference sound pressure.

It should be noted that in addition to the equivalent sound level, when describing and characterizing short-term sounds or noises, in the international practice of acoustic measurements (not standardized in Russia), the exposure parameter of a separate noise phenomenon (event) is often used [15]. The equivalent sound exposure level is the level that, when fixed for a time interval of 1 s, produces sound energy in dBA that is identical to the energy of an actual transient sound or noise. This level, in dBA, is calculated using the formula:

$$L_{\mathit{AE}}=10\left(\frac{1}{t_0}{\int }_{t_1}^{t_2}\frac{P_A^2\left(t\right)}{P_0^2}\mathit{dt}\right),$$E3

where t₀ is the standard duration equal to 1 s.

For the theory and practice of hygienic regulation of the noise factor, acoustic parameters characterizing the average sound level over a long period of time are of undoubted significance. A long time interval consists of infrasound series of basic intervals. A day or some other time period characterizing a certain technological cycle of human activity is used as a base interval. To quantify the noise factor, the average sound level over a long time interval is determined—the average value of the equivalent sound level over a long time interval for a series of basic time intervals enclosed within a long time interval:

$$L_{\mathit{\text{AeqLT}}}=10\mathit{lg}\left(\frac{1}{N}\sum _{i=1}^N{10}^{0,1{\left(L_{\mathit{Aeq},T}\right)}_i}\right),$$E4

where N is the number of basic time intervals, (L_Aeq,T)_i is the equivalent SPL in the i-th basic time interval.

Another parameter often used for the purposes of hygienic regulation and health risk assessment is the noise dose—the total energy accumulated during exposure. The noise dose is proportional to the equivalent (in terms of energy) sound pressure recorded on the frequency correction scale “A” and the action time, measured in Pa²h or Pa²s. Dose—acoustic energy during the duration of the noise, determined by the formula:

$$\mathit{\text{Dose}}={\int }_0^T{P}_A^2\left(t\right)\mathit{dt},$$E5

where Р_А is the instantaneous value of sound pressure on the A scale of the sound level meter, Pa; T is the measurement time, h.

Sometimes it is more convenient to use the relative value of the noise dose (Dsh) in fractions of the permissible value (dimensionless value):

$$D_{\mathit{sh}}=\frac{\mathit{\text{Dose}}}{{\mathit{\text{Dose}}}_{\mathit{dop}}};{\mathit{\text{Dose}}}_{\mathit{dop}}=P_{\mathit{\text{dopA}}}^2{T}_{\mathit{dop}},$$E6

where Р_{dop А}, Pa—the permissible value of sound pressure on the A scale of the sound level meter, corresponding to the maximum permissible noise level of 80 dBA, T_dop—the established duration of the working day.

From a physical point of view, the noise dose reflects the amount of energy transferred and thus can serve as a measure of exposure. Therefore, such a hygienic aspect of its application is important, such as the rigor of calculating the noise estimate in comparison with the current system of normalizing its level.

Dose estimation has an undeniable advantage: it takes into account the transferred energy during the noise action, which allows one to estimate the noise load and correlate it with the biological effects caused. In this regard, its use helps to identify qualitative-quantitative relationships of the fundamental dose-effect ratio.

The noise dose (Dsh) is related to the equivalent L_eq, the maximum permissible sound levels L_dop and the corresponding sound pressure values P_ekv, P_dop by the relation:

$$D_{\mathit{sh}}={10}^{\frac{\left(L\mathit{eq}-L\mathit{dop}\right)}{10}}={10}^{\frac{\left(L\mathit{izm}\mathrm{А}+10lg\left(\frac{\mathrm{Тс}}{\mathrm{То}}\right)-L\mathit{dop}\right)}{10}}={\left(P_{\mathit{eq}}/P_{\mathit{dop}}\right)}^2=\mathit{\text{Dose}}/{\mathit{\text{Dose}}}_{\mathit{dop}}.$$E7

The transition from sound levels measured in dB to the root mean square value of the sound pressure and its square was carried out according to the following formulas:

$$P_{\mathit{\text{Aizm}}}=2\times {10}^{\left[L/20\right]-5},$$E8

$${\mathrm{Р}}_{А\mathit{izm}}^2=4\times {10}^{\left[L/10\right]-10}$$E9

Based on the physical concept of noise dose, the expression for the total dose per shift has the form:

$$D_{\mathit{sh}}=\sum _i{D}_{\mathit{shi}},$$E10

where D_shi—partial doses of noise; i is an index denoting stages of work or basic intervals.

The dose approach to determining the acoustic load is also convenient to use to calculate the integral estimate of the entire impact. It is necessary to additionally determine the actual and relative noise dose for working hours, rest periods and sleep, i.e. for all periods of life. The relative noise dose, separately for working hours, rest periods and sleep is determined by the formula:

$$\mathit{\text{ODsh}}=D_{\mathrm{otn}}={\mathrm{Dsh}}_{\text{fact}}/{\mathrm{Dsh}}_{\mathrm{dop}}=\frac{{\sum }_{i=1}^n{P}_i^2{t}_i}{{\mathit{Dsh}}_{\mathit{dop}}},$$E11

where Р_i—sound pressure in Pa; t_i is the duration of a given noise level (working time, rest, sleep), D_rel is the relative noise dose, Dsh_fact is the actual noise dose, Dsh_dop—permissible noise dose.

The total relative average daily noise dose is determined as the sum of the ratios of the real noise load for three periods of life (work, rest and sleep) to the permissible one according to the formula:

$$\mathit{\text{ODsh}}=\frac{{\mathit{Dsh}}_{\mathit{job}}}{{\mathit{Dsh}}_{\mathit{dop}.\mathit{job}}}+\frac{{\mathit{Dsh}}_{\mathit{\text{relaxation}}}}{{\mathit{Dsh}}_{\mathit{dop}.\mathit{\text{relaxation}}}}+\frac{{\mathit{Dsh}}_{\mathit{\text{dream}}}}{{\mathit{Dsh}}_{\mathit{dop}.\mathit{\text{dream}}}}.$$E12

It can be noted that for research purposes, to build the “dose-effect” dependence, the noise dose measured over the entire frequency range on the “Linear” scale is used.

The normalized indicators of noise at workplaces are:

• equivalent sound level A for a work shift, the normative equivalent sound level at workplaces is 80 dBA;

• maximum sound levels A, measured with time corrections S and I, maximum sound levels A, measured with time corrections S and I, should not exceed 110 dBA and 125 dBA, respectively.

• peak sound level C should not exceed 137 dBC.

Exceeding any normalized parameter is considered to be exceeding the maximum permissible levels.

For certain sectors (sub-sectors) of the economy, an equivalent noise level at workplaces from 80 to 85 dBA is allowed, subject to the confirmation of an acceptable risk to the health of workers based on the results of calculations of the assessment of the occupational risk to the health of workers, as well as the implementation of a set of measures aimed at minimizing the risks to the health of workers.

If the noise level in the workplace exceeds 80 dBA, the employer must carry out an assessment of the health risk of workers and confirm the acceptable health risk.

Work in conditions of exposure to an equivalent noise level above 85 dBA is not allowed.

The impact of noise on a person depends on the intensity, frequency composition and duration of its action, as well as on the location of the person and the nature of the work. Noise with a level of 30–40 dBA at night can cause anxiety, insomnia; at 50–60 dBA, if a person is engaged in mental work, a load is created on the nervous system, and a harmful psychological effect is observed. Sound levels up to 70 dBA already cause certain physiological reactions and can lead to changes in the body. Noise, the US of which reaches 80–90 dBA, affects hearing, causing its deterioration, and high sound levels cause the development of a specific occupational disease—industrial sensorineural hearing loss.

In the current standards for hygienic regulation of infrasound at workplaces, the following indicators are used:

Total infrasound sound pressure level (total infrasound level): sound pressure level in the frequency range 1.4–22 Hz, can be directly measured with an appropriate band pass filter or obtained by energy summing the sound pressure levels in the octave frequency bands 2, 4, 8, 16 Hz.

Equivalent sound pressure level, L_p,eq,T, dB—10 decimal logarithms of the ratio of the square of the sound pressure to the square of the reference sound pressure over a given time interval. Equivalent sound pressure levels for a shift in octave frequency bands are determined by the formula:

$$L_{p,1/1,\mathit{eq},8h}=10\mathit{lg}\left(\frac{1}{T_0}\sum _i{T}_i{10}^{0,1L_{p,1/1,\mathit{eq},T_i}}\right),$$E13

where T₀ is the standard duration of the work shift (8 hours). If the duration of the work shift is different from 8 hours, T₀ is taken equal to the actual duration of the work shift with a total work duration of 40 hours per week. T_i is the duration of the i-th interval of infrasound exposure, h; L_p,1/1,eq,Ti is the equivalent sound pressure level measured in the i-th interval, dB in the octave band.

The equivalent total infrasound level for a work shift is determined by the formula:

$$L_{p,\mathit{ZI},\mathit{eq},8h}=10\mathit{lg}\left(\sum _{i=1}^n{T}_i{10}^{0,1L_{p,\mathit{ZI},\mathit{eq},T_i}}\right),$$E14

where T₀ is the standard duration of the work shift (8 hours); T_i is the duration of the i-th interval of infrasound exposure, h; L_p,ZI,eq,8h—replaceable equivalent total infrasound level; L_p,ZI,eq,Ti is the equivalent total infrasound level measured at the i-th interval of its impact.

The maximum sound pressure level L_p,max, dB is the highest value of the sound pressure level measured over a given time interval with standard time correction (time constant).

The normalized parameters of infrasound are:

• equivalent sound pressure levels for a working shift in octave frequency bands 2, 4, 8, 16 Hz—L_p,1/1,eq,8h, dB;

• equivalent total infrasound level per shift—L_p,ZI,eq,8h, dB—should not exceed 100 dB;

• maximum total infrasound level, measured with time correction S (slow)—should not exceed 120 dB.

To obtain an approximate assessment of the severity of infrasound, you can use the overall sound level measured on the “Lin” scale, and the express indicator—the difference between the sound levels measured on the “Lin” and “A” scales. The greater this difference, the more significant the contribution of low-frequency and infrasonic components in the spectrum of the studied noise. At values of the indicator from 6 to 10 dB, it is considered that there are signs of the presence of infrasound, at 11–20 dB, infrasound is moderately pronounced; 21–30 dB—expressed; more than 30 dB—significant infrasound.

The hygienic standard refers to the quantitative and qualitative values of indicators established by research that characterize the safety of environmental factors for human health. When regulating noise and infrasound, a multi-level approach was used, depending on the nature of human activity. The existing sanitary rules establish the maximum permissible levels at workplaces, permissible levels in residential and public premises and in residential areas. The determination of the maximum permissible levels at the workplace should be made taking into account the intensity and severity of labor activity [7, 8].

The regulation of normative noise values is based on preventing the development of irreversible hearing impairment. At the same time, for infrasound, the approach of its general impact on a person is used, taking into account the reaction of the hearing organ. This has led to the fact that at present there are significant differences in the values of standard SPL in octave bands with geometric mean frequencies of 16 Hz (85 dB) in the infrasonic range and 31.5 Hz (107 dB) and 63 Hz (95 dB) in the low-frequency audio range. It has been established that low-frequency noise can have a harmful effect not only on hearing, but also on other human systems, and its biological effect is similar to the effect of infrasound on a person. Therefore, studies are needed to clarify the nature of the frequency dependence of biological effects on the conditional boundary between the infrasonic and sonic ranges [24, 26, 27, 28].

The above examples show the need for further research to justify the equally effective values of the weighting coefficients under the influence of low-frequency noise and infrasound [29, 30].

2.4 Working conditions

Working conditions are a combination of factors of the labor process (severity and tension) and the working environment (physical, chemical, and biological factors) in which human activities are carried out. The classification of working conditions is based on the principle of grading the deviation of the parameters of these factors from the current hygienic standards [1, 7, 8, 24, 31].

When evaluating working conditions associated with the action of several harmful factors, the summation effect is taken into account depending on the number of factors and the severity of their harmfulness.

In addition, if there is both noise and infrasound at the workplace, working conditions should be rated one step higher. The legitimacy of this approach is due to the fact that these two factors can have a harmful effect on the same critical organs and systems (“targets”), which leads to the summation and potentiation of their adverse effects.

2.5 Medical aspects

For a long time it was believed that infrasound lies beyond the limits of auditory perception. It has now been established that they are perceived not as pure tones, but as a combination of auditory and tactile sensations, which is manifested by a feeling of pulsation in the tympanic membrane and middle ear. The hearing thresholds for infrasound have been established: for 100 Hz they are about 40 dB, and for 1 Hz −140 dB [1, 31].

Long-term action of low-frequency noise and infrasound leads to an increase in the threshold of hearing, mainly in the low and medium frequency ranges. Considering that the maximum of speech frequencies is in these areas, these disorders are prognostically unfavorable in social terms, which is especially pronounced in socio-professional groups of the population exposed to aircraft noise [7, 32, 33, 34, 35, 36, 37].

A questionnaire survey of workers exposed to low-frequency noise and infrasound in production and transport for a long time revealed a complex of unpleasant subjective sensations in the majority [1, 7, 8, 24, 31]. Complaints, depending on their genesis, can be conditionally divided into the following groups:

1. cochlear: a feeling of congestion, pressure, pulsation and pain in the ears, hearing impairment;

2. vestibular: dizziness, imbalance; mechanical: sensation of vibration of the chest and abdominal wall, soft palate, internal organs, cough, shortness of breath, blurred vision; psychological: anxiety, unreasonable feeling of fear, decreased mood, apathy, problems with concentration and memory;

3. neurovegetative: fatigue, general malaise, drowsiness, irritability, sleep disturbances, headache, dizziness, loss of appetite, tachycardia, fluctuations in blood pressure.

The variety of complaints indicates the involvement of many organs and systems in the formation of the subjective perception of low-frequency noise and infrasound [1, 7, 24, 31].

The presence of harmful factors, having an adverse effect on the body of workers, leads to an increase in the level of chronic and general, production-related and occupational morbidity [7].

The impact of noise with a low-frequency and infrasound component is accompanied by an increase in the general morbidity and diseases characteristic of the action of noise and infrasound. This indicates the summation of adverse effects in the combined influence of noise and infrasound. In the structure of morbidity, diseases of the organs of hearing, respiration, blood circulation, digestion, skin and subcutaneous tissue, and the nervous system predominate, and the leading place among them is occupied by sensorineural hearing loss and arterial hypertension [1, 7, 8, 24, 31].

The diseases identified in specialists are related to working conditions based on quantitative assessments of occupational risk, which makes it possible to classify diseases of the organ of hearing as occupational diseases, and diseases of the respiratory organs, eyes, digestion, nervous system, circulatory organs and skin as occupationally caused diseases [31].

At the end of the last century, an understanding was formed that exposure to harmful factors (including noise and infrasound) can lead to the development of occupational diseases. Russia has created a system of medical support for people who are exposed to harmful factors at work. The basis of the complex of medical measures is the conduct of preliminary examinations when applying for a job and periodic medical examinations. In particular, it provides for the passage of persons exposed to noise and infrasound at work, periodic medical examinations (at least once every 2 years) with the obligatory participation of an otolaryngologist and a neurologist, laboratory tests of auditory and vestibular analyzers [1, 7, 8, 31]. According to the indications, an examination and examination of the connection of the disease with the profession is carried out in the conditions of a specialized authorized medical organization.

One of the promising directions for ensuring the acoustic safety of personnel is the implementation of personalized acoustic monitoring technologies based on the use of real-time acoustic hazard indicators [38, 39]. Realized with the help of personalized acoustic monitoring and medical control system, the accumulation of factual information about the influence of acoustic factors on a person, an adequate and reliable quantitative description of the patterns of changes in health and performance open up new opportunities for testing, correcting and justifying management decisions aimed at maintaining health and prolonging professional longevity. Workers exposed to industrial and traffic noise [38, 39].

An important aspect of industrial and transport noise monitoring is noise mapping [40, 41, 42]. The construction of noise maps, especially those that provide objective monitoring of noise pollution in real conditions with a possible determination of the potential effectiveness of the implementation of a set of measures aimed at reducing noise pollution, is an effective tool to support the adoption of appropriate management decisions [43, 44].

2.6 Methods and means of protection against industrial and transport noise

When choosing means and methods of protection against low-frequency noise and infrasound, it must be borne in mind that: there are actually no specialized means of protection against infrasound; in industrial conditions, infrasound is often combined with intense noise; most personal protective equipment designed to protect the organ of hearing is ineffective at frequencies below 500 Hz (sound attenuation does not exceed 10 dB).

The variety of applied methods and means of protection against industrial and transport noise necessitates the development of special information-measuring systems for their qualimetry [45, 46]. Expanding the spectrum of industrial and traffic noise to the low-frequency and infrasonic region, as well as taking into account the parameters of impulse noise, requires a new solution of acoustic problems that have not had a theoretical solution so far. So, there are still no exact methods for solving the propagation of a sound wave over remote distances, there are only probabilistic-statistical approaches applicable exclusively to a specific area and relief where acoustic measurements were made [47].

For timely decision-making on protection from industrial and traffic noise, it is necessary to have objective and comprehensive information about the characteristics of the noise environment [48, 49]. Therefore, the creation of an effective system of dosimetric control, that is, monitoring of noise levels, is one of the urgent tasks of measuring systems for qualimetry of means and methods of noise protection.

The functional tasks of the complex of information-measuring systems for qualimetry of means and methods of protection against industrial and transport noise are: (1) reducing the level of personnel noise dose to regulated limits based on a set of design, technical and health measures; and (2) creation of an effective system of dosimetric control, which makes it possible to quickly register an increase in the level of acoustic exposure to a person who is exposed to noise.

When choosing personal protective equipment, you should be guided by the following.

1. In the presence of noise, the spectrum of which is dominated by medium and high frequencies, and the SPL of low-frequency noise and infrasound do not exceed the maximum permissible levels, it is necessary to use anti-noise (headphones, earbuds and a helmet) designed to protect the hearing organ. When choosing anti-noise, it should be taken into account that at the noise level:

   1. up to 100 dBA—you need to use headphones or earbuds;

   2. 100–110 dBA—a combination of headphones with earbuds;

   3. 110–125 dBA—anti-noise helmets, vests, suits.

2. When exposed to infrasound with levels exceeding the maximum permissible levels, and intense noise, it is necessary to protect not only the hearing organ, but also the central and autonomic nervous systems, the cardiovascular system, and the respiratory system. This is achieved by special noise protection equipment—anti-noise helmet, vest and suit [8, 50].

Special noise protection equipment is a new class of technical personal protective equipment designed to protect a person from the extracochlear effects of infrasound and low frequencies of the sound range. Industrial samples of headphones and experimental samples of anti-noise helmets and vests, which reduce the level of acoustic energy in the low-frequency range, have been developed [8].

An important role in ensuring protection against low-frequency noise and infrasound in the workplace belongs to measures to optimize the conditions of professional activity—the use of collective protective equipment, reducing the length of stay in the noise zone, alternating periods of work and rest, etc. It is necessary to use the alternation of work periods for maintenance of production equipment (“active period of acoustic load”), with periods not related to the maintenance of noise sources (“passive period of acoustic load”). In the latter case, it is important to create comfortable acoustic conditions and carry out rehabilitation measures [8].

Thus, the use of protective equipment is necessary for the effective prevention of occupational morbidity, and hence the reduction of economic losses in production.

3. Conclusion

Industrial facilities and most modes of transport are sources of high-intensity noise, the spectrum of which is dominated by low-frequency infrasonic frequencies. The close physical nature of these ranges contributes to the propagation of such noise with low attenuation, and they have good penetrating power, so most noise protection devices are ineffective. High (more than 100 dBA) noise levels at workplaces of industrial facilities and transport require measurements in the infrasound range as well.

Studies of low-frequency acoustic oscillations as a factor in the production environment have not been completed. Their “targets” are the central nervous and autonomic nervous systems, auditory and vestibular analyzers, respiratory organs, etc. With prolonged exposure, they contribute to the development of occupational diseases. The simultaneous action of low-frequency noise and infrasound (this situation is typical for industrial conditions and vehicles) leads to an aggravation of infrasound pathology, which requires more careful medical monitoring of people working in such conditions, and improvement of means and methods of protection against industrial and transport noise.

Acknowledgments

This work was supported by a grant from the President of the Russian Federation for the state support of leading scientific schools of the Russian Federation (grant NSh-122.2022.1.6).

Conflict of interest

The authors declare no conflict of interest.