The effect of vibration on the human body. Physical characteristics of vibration

Vibration– oscillatory motion of a material point or mechanical system. The reason for the excitation of vibration is the unbalanced force effects arising during the operation of machines and units, kinematic excitation during movement vehicles along an uneven path, etc.

The main physical parameters of vibration are:

Frequency f 0, Hz;

Oscillation period T, s;

Vibration displacement amplitude A, m;

Amplitude of oscillatory velocity V, m/s;

Amplitude of oscillatory acceleration W, m/s 2.

These parameters depend on the following:

The base frequency of the limiting spectrum for general vibration is 63 Hz, for local vibration it is 125 Hz

The hygienic characteristics of vibration, which determine its impact on humans, are the root-mean-square values ​​of vibration velocity and its logarithmic levels. Vibration is estimated by the logarithmic equation of vibration velocity in decibels.

The logarithmic level of vibration velocity is determined by the expression: (3)

where: V 0 – threshold value of vibration velocity equal to 5 10 –8 m/s.

The threshold value of vibration velocity is the value of vibration velocity at which a person barely begins to feel the effect of vibration.

The logarithmic level of vibration acceleration is calculated using the formula: , dB (4)

where W o is the threshold value of vibration acceleration, W o =3 10 –4, m/s 2.

Vibration classification

According to the method of transmission to a person, vibrations are divided into general, transmitted through supporting surfaces to the body of a person sitting or standing man, and local, transmitted through human hands.

In the direction of action, vibration occurs - acting along the axes of the orthogonal coordinate system X, Y, Z - for general vibration, where Z - vertical axis, and A" and U- horizontal axes; acting along the entire orthogonal coordinate system X p, Y p, Z p - for local vibration, where the X p axis coincides with the axis of the grip areas (handle, steering wheel, etc.), and the Z p axis lies in the plane formed by the X„ axis and the direction of supply or application of force. General vibration, according to the source of its occurrence, is divided into transport vibration, which occurs as a result of movement across the terrain; transport and technical, which appears during the operation of machines performing a technological operation in a stationary position or when moving through a specially prepared part of the production premises or industrial site; technological, which occurs during the operation of stationary machines or. transmitted to workplaces that do not have sources of vibration.

43.passage of a sound wave through an obstacle

Sound waves when meeting an obstacle, they are reflected and partially refracted. Part of the refracted energy is absorbed in the barrier material. The remaining part of the sound energy penetrates the barrier (Fig. 11.2). The number of reflections and refractions of energy depends on the frequency of vibrations, the angle of incidence of the wave front on the obstacle and the physical properties of the enclosing structures.

The ability of materials and structures to absorb sound energy is characterized by the sound absorption coefficient a, which is equal to the ratio of sound energy absorbed by the material E potl, to the incident sound energy 4,a D:

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Rice. 11.2. patterns of reflection, absorption and transmission of sound energy when meeting an obstacle (E ppd - incident sound energy: E neg - sound energy reflected by the barrier; E absorb - sound energy passed beyond the barrier)

Soundproofing.

Sound insulation – the use of soundproof barriers along the paths of airborne noise. The noise reduction effect is achieved by reflecting sound waves from soundproofing barriers. Sound absorption is achieved by covering the enclosing surfaces of the room with special porous materials, which reduce the reflection of sound waves from the surfaces they encounter along their propagation paths. Sound energy, entering the pores of sound-absorbing materials, turns into heat as a result of repeated reflection from the pore walls. Porous and loose materials most intensively convert the energy of sound vibrations into heat, which are used for
: obtaining a high sound-absorbing effect.

45 Sound absorption.

For sound absorption, the ability of building materials and structures to dissipate the energy of sound vibrations is used. When sound waves fall on a sound-absorbing surface made of porous material (for example, foam), a significant part of the acoustic energy is spent on causing the air in the pores to vibrate, which causes it to heat up. In this case, the kinetic energy of sound vibrations is converted into thermal energy, which is dissipated in the surrounding space.

Porous and loose materials most intensively convert the energy of sound vibrations into heat, which are used to obtain a high sound-absorbing effect.

Vibration isolation.

Vibration isolation protection is one of the effective ways to protect workplaces, equipment and building structures from vibrations caused by the operation of machines and mechanisms. Vibration isolation is a method of vibration protection that consists in reducing the transmission of vibration from the excitation source to the protected object using devices (vibration isolators) placed between them

To create vibration-proof machines during their design, methods are used that reduce vibration parameters by influencing the excitation source, and for machines with a built-in workplace, additional vibration methods established by GOST 12.4.046-78 When designing technological processes and industrial buildings and structures, machines with the lowest values ​​of vibration characteristics parameters recorded workplaces (zones) where workers may be exposed to vibrations; a machine placement scheme has been developed taking into account the creation of minimum vibration levels at workplaces; calculations (estimates) of expected vibration levels at workplaces were made; construction solutions for foundations and ceilings for installing machines were selected to ensure hygienic vibration standards at workplaces; the necessary means of vibration protection of machines or the operator’s workplace were selected and calculated, allowing, together with construction solutions, to ensure hygienic vibration standards at workplaces.

Spring vibration isolators are effective at low frequencies, rubber ones - at high frequencies (more than 30 Hz).

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Noise is a set of sounds of varying frequency and intensity (strength) arising as a result of the oscillatory movement of particles in elastic media (solid, liquid, gaseous).
The process of propagation of oscillatory motion in a medium is called a sound wave, and the region of the medium in which sound waves propagate is called a sound field.
There are impact, mechanical, and aerohydrodynamic noise. Impact noise occurs during stamping, riveting, forging, etc.
Mechanical noise occurs during friction and beating of units and parts of machines and mechanisms (crusher, mills, electric motors, compressors, pumps, centrifuges, etc.).
Aerodynamic noise occurs in apparatus and pipelines at high speeds of movement of air, gas or liquid and with sudden changes in the direction of their movement and pressure.
Basic physical characteristics of sound:
– frequency f (Hz),
– sound pressure P (Pa),
– intensity or sound power I (W/m2),
– sound power? (W).
Speed ​​of propagation of sound waves in the atmosphere at 20°C is equal to 344 m/s.
The human hearing organs perceive sound vibrations in the frequency range from 16 to 20,000 Hz. Vibrations with a frequency below 16 Hz (infrasounds) and with a frequency above 20,000 (ultrasounds) are not perceived by the hearing organs.
As sound vibrations propagate in the air, areas of rarefaction and high pressure periodically appear. The pressure difference in disturbed and undisturbed media is called sound pressure P, which is measured in pascals (Pa).
The propagation of a sound wave is accompanied by the transfer of energy. The amount of energy transferred by a sound wave per unit time through a unit surface oriented perpendicular to the direction of propagation of the wave is called intensity or sound power I and is measured in W/m2.
The product is called the specific acoustic resistance of the medium, which characterizes the degree of reflection of sound waves when passing from one medium to another, as well as the soundproofing properties of materials.
Minimum sound intensity which is perceived by the ear is called the hearing threshold. The standard comparison frequency is 1000 Hz. At this frequency, the hearing threshold is I 0 = 10-12 W/m 2, and the corresponding sound pressure P 0 = 2*10 -5 Pa. Maximum sound intensity, at which the hearing organ begins to experience pain, is called the pain threshold, equal to 10 2 W/m 2, and the corresponding sound pressure P = 2 * 10 2 Pa.
Since changes in sound intensity and sound pressure audible by humans are enormous and amount to 10 14 and 10 7 times, respectively, it is extremely inconvenient to use absolute values ​​of sound intensity or sound pressure to evaluate sound.
For the hygienic assessment of noise, it is customary to measure its intensity and sound pressure not in absolute physical quantities, but in logarithms of the ratios of these quantities to a conditional zero level corresponding to the hearing threshold of a standard tone with a frequency of 1000 Hz. These logarithms of ratios are called intensity and sound pressure levels, expressed in bels (B). Since the human hearing organ is capable of distinguishing a change in the sound intensity level by 0.1 bels, then for practical use a unit 10 times smaller is more convenient - decibel(dB).
The sound intensity level L in decibels is determined by the formula

L=10Lg(I/I o) .

Since the sound intensity is proportional to the square of the sound pressure, this formula can also be written in the form^

L=10Lg(P 2 /P o 2)=20Lg(P/P o), dB.

Using a logarithmic scale to measure noise levels allows you to fit a large range of I and P values ​​into a relatively small interval of logarithmic values ​​from 0 to 140 dB.
Sound pressure threshold P 0 corresponds to the hearing threshold L = 0 dB, the pain threshold is 120-130 dB. Noise, even when it is small (50-60 dB), creates a significant load on the nervous system, having a psychological impact. When exposed to noise of more than 140-145 dB, the eardrum may rupture.
Total sound pressure level L created by several sound sources with the same sound pressure level Li, are calculated by the formula

L=L i +10Lg n , dB,

where n is the number of noise sources with the same sound pressure level.
So, for example, if noise is created by two identical noise sources, then their total noise is 3 dB greater than each of them separately.
Based on the level of sound intensity, it is still impossible to judge the physiological sensation of the loudness of this sound, since our hearing organ is unequally sensitive to sounds of different frequencies; sounds of equal strength, but of different frequencies, seem unequally loud. For example, a sound with a frequency of 100 Hz and a strength of 50 dB is perceived as equally loud as a sound with a frequency of 1000 Hz and a strength of 20 dB. Therefore, to compare sounds of different frequencies, along with the concept of sound intensity level, the concept of loudness level with a conventional unit - background - was introduced. One background is the sound volume at a frequency of 1000 Hz and an intensity level of 1 dB. At a frequency of 1000 Hz, volume levels are assumed to be equal to sound pressure levels.
In Fig. Figure 1 shows curves of equal loudness of sounds obtained from the results of studying the properties of the hearing organ to evaluate sounds of different frequencies according to the subjective sensation of loudness. The graph shows that our ear has the greatest sensitivity at frequencies of 800-4000 Hz, and the least at 20-100 Hz.

Typically, noise and vibration parameters are assessed in octave bands. An octave is taken as the bandwidth, i.e. frequency interval in which the highest frequency f 2 is twice as large as the lowest f 1 . The geometric mean frequency is taken as the frequency characterizing the band as a whole. Geometric mean frequencies of octave bands standardized by GOST 12.1.003-83 " Noise. General safety requirements"and are 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz with corresponding cutoff frequencies of 45-90, 90-180, 180-355, 355-710, 710-1400, 1400-2800, 2800- 5600, 5600-11200.
The dependence of quantities characterizing noise on its frequency is called the frequency spectrum of noise. For the convenience of physiological assessment of the impact of noise on humans, low-frequency (up to 300 Hz), mid-frequency (300-800 Hz) and high-frequency (above 800 Hz) noise are distinguished.
GOST 12.1.003-83 and SN 9-86 RB 98 " Noise at work places. Maximum permissible levels"classifies noise according to the nature of the spectrum and the duration of its action.
By the nature of the spectrum:
– broadband, if it has a continuous spectrum more than one octave wide,
– tonal, if the spectrum contains pronounced discrete tones. In this case, the tonal nature of noise for practical purposes is established by measurement in one-third octave frequency bands (for a one-third octave band, the sound pressure level in one band exceeds the neighboring ones by at least 10 dB.
According to time characteristics:
– constant, the sound level of which changes over time by no more than 5 dB over an 8-hour working day,
– unstable, the sound level of which changes over time by more than 5 dB over an 8-hour working day.
Variable noises are divided into:
fluctuating in time, the sound level of which continuously changes over time;
intermittent, the sound level of which changes stepwise (by 5 dB or more);
pulse, consisting of one or more sound signals, each lasting less than 1 s.
The greatest danger to humans is tonal, high-frequency and intermittent noise.
According to the method of propagation, ultrasound is divided into:
– airborne (airborne ultrasound);
– spread by contact upon contact with solid and liquid media (contact ultrasound).
The ultrasonic frequency range is divided into:
– low-frequency oscillations (1.12*10 4 - 1*10 5 Hz);
– high-frequency (1*10 5 - 1*10 9 Hz).
The sources of ultrasound are production equipment in which ultrasonic vibrations are generated to perform the technological process, technical control and measurements, as well as equipment during the operation of which ultrasound arises as an accompanying factor.
Characteristics of air ultrasound at the workplace in accordance with GOST 12.1.001 " Ultrasound. General safety requirements" and SN 9-87 RB 98 " Airborne ultrasound. Maximum permissible levels in workplaces" are sound pressure levels in one-third octave bands with geometric mean frequencies 12.5; 16.0; 20.0; 25.0; 31.5; 40.0; 50.00; 63.0; 80.0; 100.0 kHz.
Characteristics of contact ultrasound in accordance with GOST 12.1.001 and SN 9-88 RB 98 " Ultrasound transmitted by contact. Maximum permissible levels in workplaces" are peak vibration velocity values ​​or vibration velocity levels in octave bands with geometric mean frequencies 8; 16; 31.5; 63; 125; 250; 500; 1000; 2000; 4000; 8000; 16000; 31500 kHz.
Vibrations- these are vibrations of solid bodies - parts of apparatus, machines, equipment, structures, perceived by the human body as shocks. Vibrations are often accompanied by an audible noise.
By mode of transmission per person vibration is divided into local And general.
General vibration is transmitted through supporting surfaces to the body of a standing or sitting person. The most dangerous frequency of general vibration lies in the range of 6-9 Hz, since it coincides with the natural frequency of vibration of the human internal organs, which can result in resonance.
Local (local) vibration transmitted through human hands. Local vibration can also include vibration that affects the legs of a sitting person and the forearms in contact with vibrating surfaces of work tables.
Sources of local vibration transmitted to workers can be: hand-held machines with an engine or hand-held power tools; controls of machines and equipment; hand tools and workpieces.
General vibration Depending on the source of its occurrence, it is divided into:
general vibration of category 1 - transport, affecting a person at the workplace in self-propelled and trailed machines, vehicles when driving on terrain, roads and agricultural backgrounds;
general vibration of category 2 – transport and technological, affecting people at workplaces in machines moving on specially prepared surfaces of production premises, industrial sites, and mine workings;
general vibration of category 3 - technological, affecting a person in the workplace near stationary machines or transmitted to workplaces that do not have sources of vibration.
General category 3 vibration is divided into the following types by location:
3a – at permanent workplaces of industrial premises of enterprises;
3b - at workplaces in warehouses, canteens, household, duty rooms and other auxiliary production premises, where there are no machines that generate vibration;
3c - at workplaces in administrative and service premises of the plant management, design bureaus, laboratories, training centers, computer centers, health centers, office premises and other premises of mental workers.
According to time characteristics, vibration is divided into:
– a constant for which the spectral or frequency-corrected normalized parameter during the observation time (at least 10 minutes or the technological cycle time) changes by no more than 2 times (6 dB) when measured with a time constant of 1 s;
– non-constant vibration, for which the spectral or frequency-corrected normalized parameter during the observation time (at least 10 minutes or technological cycle time) changes by more than 2 times (6 dB) when measured with a time constant of 1 s.
Main parameters characterizing vibration:
– frequency f (Hz);
– displacement amplitude A (m) (the magnitude of the largest deviation of the oscillating point from the equilibrium position);
– oscillatory speed v (m/s); oscillatory acceleration a (m/s 2).
As with noise, the entire spectrum of vibration frequencies perceived by humans is divided into octave bands with geometric mean frequencies of 1, 2, 4, 8, 16, 32, 63, 125, 250, 500, 1000, 2000 Hz.
Since the range of changes in vibration parameters from threshold values ​​at which it is not dangerous to actual ones is large, it is more convenient to measure the invalid values ​​of these parameters, and the logarithm of the ratio of the actual values ​​to the threshold ones. This value is called the logarithmic level of the parameter, and its unit of measurement is decibel (dB).

The cause of vibrations is the unbalanced force effects that occur during the operation of machines and units. In some cases, their sources are reciprocating moving parts (crank mechanism in engines and compressors, striker in hand hammers, vibration mechanisms for compacting concrete and asphalt-concrete mixtures, vibratory rammers, vibroforming units in foundries, units for forging welded joints etc.); in other cases, unbalanced rotating masses (hand-held electric and pneumatic grinders, cutting tools of machine tools, etc.). Sometimes vibrations are created by impacts of parts (gears of the gearbox, bearing units, couplings, etc.).

The presence of imbalance in all cases leads to the appearance of unbalanced centrifugal forces, causing vibration. The cause of the imbalance may be inhomogeneity of the material of the rotating body, a mismatch between the center of mass of the body and the axis of rotation, deformation of parts due to uneven heating during hot and cold landings, etc.

The main parameters characterizing vibration occurring according to a sinusoidal law are: displacement amplitude xm - the magnitude of the greatest deviation of the oscillating point from the equilibrium position; amplitude of the oscillatory speed vm - the maximum value of the speed of the oscillating point; amplitude of oscillatory acceleration am - the maximum of the acceleration values ​​of the oscillating point; period of oscillation T - the time interval between two successive identical states of the system; frequency f in hertz, related to the period by the known relation f = 1/T.

The displacement in the case of sinusoidal oscillations is determined by the formula x=xm sin (wt + φ), where w is the circular frequency (w = 2πf); φ—initial phase. In most occupational safety problems, the initial phase is not important and may not be taken into account.

The relationship between displacement, velocity and acceleration is given by the following expressions: v = x = jwx; a = x = v = -w2x, where j = √-1 operator for rotating the oscillation vector by an angle π/2 in time.

In the general case, a physical quantity characterizing vibration (for example, oscillatory speed) is some function of time: v = v (t). Mathematical theory shows that such a process can be represented as a sum of indefinitely lasting sinusoidal oscillations with different periods and amplitudes. In the case of a periodic process, the frequencies of these components are multiples of the fundamental frequency of the process: fn = nf1, where n = 1, 2, 3, ..., f1 is the fundamental frequency of the process, and the amplitudes of the harmonics are determined using the known Fourier series expansion formulas. If the process does not have a certain period (random or short-term single processes), then the number of such sinusoidal components becomes infinitely large, and their frequencies are distributed continuously, while the amplitudes are determined by the expansion according to the Fourier integral formula.

Thus, the spectrum of a periodic or quasiperiodic oscillatory process is discrete (Fig. 27a), and the spectrum of a random or short-term single process is continuous (Fig. 27, b). Most often, the fundamental oscillation frequency due to the operation of the drive is most clearly expressed in the discrete spectrum. If the process is the addition of several periodic processes, the frequencies of the individual components in its spectrum may not be multiples of each other, i.e., a quasiperiodic process takes place (Fig. 27, a). If the process is the sum of several periodic and random processes, its spectrum is mixed, that is, it is depicted in the form of continuous and discrete spectra superimposed on each other (Fig. 27, c).

Rice. 27. Vibration spectra: a - discrete; b - solid; in - mixed

In matters of labor protection, due to the specific properties of the sense organs, the effective values ​​of the parameters characterizing vibration are decisive. Thus, the effective value of the oscillatory velocity is the root mean square of the instantaneous velocity values ​​during the averaging time

Thus, to characterize vibration, the spectra of the effective values ​​of the parameters or the mean squares of the latter are used. When assessing the total impact of oscillations of various frequencies or individual sources on a person, it should be borne in mind that when adding incoherent oscillations, the resulting oscillatory speed (acceleration, displacement) is found by energy summation of the powers of individual components of the spectrum (or individual sources) or, what is the same thing, summation of mean squares, where n is the number of components in the spectrum.

In accordance with this, the resulting effective value of the process is determined by the expression

The image of a continuous spectrum requires a mandatory reservation about the width Δf of the elementary frequency bands to which the image belongs. If f1 is the lower limit frequency of a given frequency band, f2 is the upper limit frequency, then the geometric mean is taken as the frequency characterizing the band as a whole

frequency fсг=√f1f2

In the practice of vibroacoustic research, the entire range of vibration frequencies is divided into octave ranges. In the octave range, the upper limit frequency is twice the lower frequency f2/f2 = 2.

Vibration analysis can also be performed in one-third octave frequency bands. In a third octave .

The geometric mean frequencies of the octave vibration frequency bands are standardized and are: 1, 2, 4, 8, 16, 32, 63, 125, 250, 500, 1000, 2000 Hz.

Considering that the absolute values ​​of the parameters characterizing vibration vary over a very wide range, the concept of parameter level is used in the practice of vibroacoustic research.

The level of a parameter is the logarithmic ratio of the absolute value of the parameter to a certain value of it, selected as a reference point (reference or threshold value). Levels are measured in decibels (dB).

Vibration rate level (dB)

where the mean square of the oscillatory velocity v2 is taken in the corresponding frequency band; v0 - reference or threshold value of oscillatory velocity (m/s), selected by international agreement:

v0 = 5*10-8.

When comparing two oscillatory processes characterized by vibration velocity levels Lv1 and Lv2 (dB), we respectively have the expression for the difference of these equations

Spectra of vibrational velocity levels are the main characteristics of vibration.

There are general and local (local) vibrations. General vibration causes shaking of the whole body, local vibration involves vibrations from other types of equipment. Those who work with hand-held mechanized electric and pneumatic tools (cleaning welds, trimming castings, riveting, grinding, etc.) are exposed to local vibration. In some cases, a worker may be simultaneously exposed to general and local vibration (combined vibration), for example, when working on road construction machines and vehicles.

General vibrations with a frequency of less than 0.7 Hz (rolling), although unpleasant, do not lead to vibration disease. The human body and its individual internal organs move in this case as a single whole, without experiencing mutual movements. The consequence of such vibration is seasickness, which occurs due to disruption of the normal functioning of the balance organs.

Various internal organs and individual parts of the body (for example, the head or heart) can be considered as oscillatory systems with a certain concentrated mass, interconnected by “springs” with certain elastic properties and the inclusion of parallel resistances. It is obvious that such a system has a number of resonances, the frequencies of which (subjective perception of vibrations) also depend on the position of the worker’s body (“standing” or “sitting”).

Resonance at frequencies of 4-6 Hz corresponds to vibrations of the shoulder girdle, hips (in the “standing” position), and head relative to the base (in the “standing” position); at frequencies of 25-30 Hz - head relative to shoulders (sitting position). For most internal organs, natural frequencies lie in the range of 6-9 Hz. Vibrations of workplaces with the indicated frequencies are very dangerous, as they can cause mechanical damage and even rupture of these organs. Systematic exposure to general vibrations in the resonant or near-resonance zone can be the cause of vibration disease - persistent disorders of the physiological functions of the body, caused primarily by the effect of vibrations on the central nervous system. These disorders manifest themselves in the form of headaches, dizziness, poor sleep, decreased performance, poor health, and cardiac dysfunction.

Local vibration causes vascular spasms, which, starting from the end phalanges of the fingers, spread to the entire hand, forearm and cover the vessels of the heart. As a result, a disruption of the peripheral blood supply occurs - a deterioration in the blood supply to the extremities. At the same time, the effect of vibration on nerve endings, muscle and bone tissue is observed, which is expressed in impaired skin sensitivity, ossification of muscle tendons, pain and salt deposits in the joints of the hands and fingers, which leads to deformations and decreased mobility of the joints. All of these changes increase in the cold season and decrease in the warm season. At the same time, disturbances in the activity of the central nervous system are observed, as with general vibration.

Vibration disease belongs to the group of occupational diseases, effective treatment of which is possible only in the early stages, and restoration of impaired functions proceeds very slowly, and in especially severe cases, irreversible changes occur in the body, leading to disability.

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