BIOMECHANICAL MECHANISMS OF ACTION OF LOW-FREQUENCY ACOUSTIC VIBRATIONS ON A PERSON PDF Free Download
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ISSN 1812-5123. Russian Journal of Biomechanics. 2020. Vol. 24, No. 2: 188-200
© Ivanov N.I., Zinkin V.N., Slivina L.P., 2020
Nikolay I. Ivanov, Ph.D., Professor, Head of Department of Ecology and Life Safety, Saint-Petersburg
Valeriy N. Zinkin, Ph.D., Professor, Senior Scientist of Research and Testing Center for Aerospace Medicine
and Military Ergonomics, Saint-Petersburg
Lyudmila P. Slivina, Ph.D., Professor, Head of Department of Hygiene, Volgograd
DOI: 10.15593/RJBiomech/2020.2.09
BIOMECHANICAL MECHANISMS OF ACTION OF LOW-FREQUENCY
ACOUSTIC VIBRATIONS ON A PERSON
N.I. Ivanov1, V.N. Zinkin2, L.P. Slivina3
1 D.F. Ustinov Baltic State Technical University "Voenmeh", 1-ya Krasnoarmeyskaya Street, 190005, Saint-
Petersburg, Russian Federation
2 Central Research Institute of the Air Force of the Ministry of Defense of the Russian Federation, e-mail:
nyshtaev.vfb@rambler.ru
3 Volgograd State Medical University, 400131, Volgograd, 1 Pavshikh Bortsov Square, Russian Federation
Abstract. The aim of this work to substantiate the mechanisms of action of low-frequency
acoustic vibrations on humans, caused by mechanical interaction of acoustic oscillations
with the anatomical structures of the body. One of the modern features of industrial noise
is the domination noise spectrum of the low frequency noise and infrasound with high
intensity. To characterize these ranges, the scientific literature uses the term low-frequency
acoustic oscillations, due to similar physical properties and biological effects on the human
body and animals. It is proven that infrasound has negative effects on many organs
(respiratory, hearing, vision, etc.) and human systems (central and autonomic nervous,
etc.) and leads to the development of diseases, including professional (sensorineural
hearing loss, vestibulopathy, autonomic disorders). On this basis, infrasound is included in
the list of harmful production factors. A number of biological effects is formed by a direct
mechanical interaction of acoustic oscillations with the anatomical structures of the human
body. The length of the acoustic wave and its intensity are decisive parameters for the
formation of a reaction in the human body. The interaction of low frequency acoustic
oscillations with the anatomical structures of the body must be considered as the interaction
of two mechanical systems that leads to the development of various physical effects in
tissues and organs (diffraction, resonance, elastic waves, cavitation, etc.). The latter are
the basis of the direct action of acoustic oscillations and lead to anatomical damage of the
tissues, a conformational disturbance of cellular structures and macromolecules, activation
of receptors (mechano-, proprio, vestibuloadaptive, etc.). Biomechanical effect of low-
frequency acoustic oscillations should be taken into account in the normalization of low
frequency sound range, and determining ways and means of protection from low-frequency
acoustic oscillations not only the organ of hearing and the head and internal organs.
Key words: low-frequency acoustic oscillations, mechanisms of action, wavelength, sound
pressure level, sound conductivity, resonance, protection.
INTRODUCTION
Noise takes a leading place among the unfavorable factors of the working environment, and its
effect leads to a decrease in working capacity, an increase in general and occupational
morbidity. Despite the large amount of clinical and experimental data on the effect of noise on
humans and animals, the widespread prevalence of noise in industry and transport, an increase
in economic losses due to an increase in the incidence of people in “noise” professions, the lack
of effectiveness of noise protection equipment and noise prevention measures pathology,
Biomechanical mechanisms of action of low-frequency acoustic vibrations on a person
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the expansion of the negative impact of noise on the environment and the population necessitate
the continuation of research on the prevention of noise pathology [12, 14, 31, 34].
The changes taking place in the country's economic complex in recent years, the
instability of production and financing, and the lack of economic interest among employers in
reducing occupational diseases and occupational injuries have contributed to maintaining an
unsatisfactory state of working conditions in the Russian Federation. Over the past two decades,
there has been an increase in the share of workers employed in harmful and dangerous working
conditions in all types of economic activity. The number of workers employed in harmful and
hazardous working conditions that do not meet sanitary and hygienic standards amounted to
32.8% of the total number of workers in industry. More than 3.5 million people are exposed to
industrial noise with a level exceeding the maximum permissible level. Unsatisfactory working
conditions, the impact of harmful production factors on the body of workers, are the main
reason for the formation of occupational diseases in them [22].
Despite the measures taken to combat noise, this does not lead to a decrease in the
incidence rate; therefore, the level of economic losses from noise continues to remain at a high
level. This situation requires constant monitoring of the prevention of the harmful effects of
noise and its improvement [21].
Throughout evolution, a person was exposed to various types of mechanical vibrations,
including acoustic, which led to the formation of specific structures in the form of
mechanoreceptors (tactile and auditory receptors, proprio-, baro- and vestibuloreceptors) to
perceive external signals from the environment. Depending on the frequency of acoustic
vibrations, infra-, ultra- and sound-ranges are distinguished. This classification is based on the
perception of sound by the human ear, and this separation is conditional. Infrasound is
commonly understood as acoustic vibration with a frequency below 16–20 Hz. The sound range
includes acoustic vibration 20–20000 Hz, perceived by the human ear as tonal signals. By the
predominance of the predominance of acoustic energy in one or another part of the spectrum,
the noise is divided into low-frequency (before 250 Hz), mid-frequency (500–1000 Hz) and
high-frequency (2000–8000 Hz). Ultrasound includes acoustic vibrations over 16000 Hz, which
the person does not perceive in the ear.
In recent decades, the scientific literature on the adverse biological effects of noise uses
the term low-frequency acoustic oscillations. As a rule, this range includes acoustic vibration
with a frequency below 250 Hz, that is, it includes infrasound and low-frequency. A number of
authors justify the isolation of this range by the presence of close physical similarity of a number
of parameters (long wavelength, low attenuation, etc.) and similar biological effects (auricular
and extraauricular) when exposed to humans and animals [1, 15, 37].
The history of the study of the biological effects of infrasound is estimated at several
decades. The greatest contribution to the study of the effects of noise and infrasound was made
by domestic scientists (E.T. Andreeva-Galanina, N.I. Karpova, V.G. Artamonova,
N.F. Izmerov, G.A. Suvorov, V.I. Svidovy et al.) [17]. It has been shown that infrasound has
an adverse effect on many organs and systems of a person. Based on the data obtained, it was
included in the list of harmful production factors [Order of the Ministry of Health and Social
Development of 08.16.2004 No. 83] and a list of occupational diseases caused by its action was
determined [Order of the Ministry of Health and Social Development of 04.04.2004 No. 417n].
At the same time, it should be noted that scientific publications on the mechanism of
action of infrasound are not enough, and they are fragmented in nature [13, 15, 23, 32].
Purpose of work: to substantiate the mechanisms of action of low-frequency acoustic
oscillation on humans, due to the mechanical interaction of acoustic vibration with the
anatomical structures of the body.
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FEATURES OF MODERN PRODUCTION NOISE
Production noise generated during the operation of machinery, vehicles, and production
equipment is acoustic vibration in a wide range from infrasound to ultrasonic. Currently, the
share of industrial enterprises that do not meet the sanitary and epidemiological requirements
for noise level is 31.3%, and the share of jobs is 24.63 %. Sound levels generated by production
equipment reach 92–95 dBA or more, and noise from specialized vehicles used in these
industries reaches over 87 dBA (with the norm of 80 dBA SanPiN 2.2.4.3359–16).
The main reasons for exceeding noise levels in the workplace are the imperfection of
technological processes, design shortcomings of technological equipment, their physical wear
and failure to perform scheduled repairs, insufficient responsibility of employers and
production managers for the state of labor conditions and safety [11].
There is a clear tendency to increase the contribution of low-frequency and infrasound
components in the production noise spectrum (in Table 1). The results of acoustic
measurements show that if the air noise levels are about 90–100 dBA, then we can expect the
presence of infrasound 100–107 dB with the sound pressure level [13, 15, 35, 37].
The most unfavorable acoustic parameters include the sound level, especially over
100 dBA, time fluctuations (non-constant and pulsed noise), long and continuous action during
the working shift, long experience with noise, the predominance of high frequencies in the
spectrum (2000–6000 Hz) and the presence of noise. It is shown that the combination of a
number of these parameters is accompanied by an increase in adverse effects from noise [3, 36,
38–41].
Based on the above, modern features of industrial noise include:
– widespread noise in industrial facilities;
– fairly high proportion of industrial enterprises that do not meet the sanitary and
epidemiological requirements for noise level;
– combined effect of adverse acoustic parameters;
– frequent occurrence in the noise spectrum of the share of low-frequency and
infrasound frequencies of high intensity (the sound pressure level over 100 dB).
Table 1
Sources of low-frequency and infrasound vibrations at industrial facilities
Name
The sound pressure
levels, dB
Maximum of the energy
spectrum, Hz
Motor transport
93–120
4–31.5
Railway transport
92–127
8–50
Cargo river and sea vessels
110–130
8–45
Hydrofoils and hovercraft
100–130
6–10
Turbojet aircraft
105–135
16–125
Piston aircraft
95–110
50–250
Helicopters
100–120
8–45
Metallurgical industry
95–108
8–31.5
Gas and oil industry
92–123
8–63
Aviation industry
90–132
10–150
Mining and construction industry
98–123
10–45
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AURA EFFECTS OF LOW-FREQUENCY ACOUSTIC OSCILLATIONS
Noise, being a general biological stimulus, affects all organs and systems of the body.
When exposed to noise, certain physiological changes develop, which depend on specific
conditions: the level and nature of noise, the duration of its exposure, individual characteristics
of a person and other factors that can not always be taken into account. Under certain conditions,
the effect of noise leads to the development of pathology. Physiological and pathological effects
caused by the influence of noise are usually divided into two groups: specific (aural)
manifestations that occur in the hearing organ, and non-specific (extraaural) manifestations that
occur in other organs and systems.
The basis for the perception of sound by the human ear is the length of the acoustic wave
(λ). It was found that the sensitivity of the ear decreases with a decrease in the frequency of
sound. To approximate the results of objective measurements to subjective perception, standard
values of sound level correction are used (see Table 2).
From Table 2, it follows that in the low-frequency region (< 250 Hz), the value of ΔLA
reaches significant values (from 8.6 dB to 42 dB), thereby showing a decrease in the
significance of the auditory analyzer in the formation of the human body's response to the action
of low frequencies of the audio range. From our point of view, this was the basis for establishing
sufficiently high values of the maximum permissible levels of sound pressure for octave bands
with an average geometric frequency of 31.5 Hz (107 dB) and 63 Hz (95 dB) in accordance
with SanPiN 2.2.4.3359–16.
Table 2
Standard correction value ΔLA [6]
Frequency, Hz
16
31.5
63
125
250
500
1000
2000
4000
8000
Correction, dB
80
42
26.3
16.1
8.6
3.2
0
–1.2
–1.0
1.1
For a long time, there was an opinion that acoustic vibration in the infrasonic part of the
spectrum lie beyond the limits of human auditory perception. It is found that the sound is not
perceived as pure tones, but as a combination of auditory and tactile sensations, and is
accompanied by a feeling of pressure (pulsation) in the middle ear, tickling and massage of the
eardrum. Apparently, the perception of infrasound due to harmonics resulting from
deformations in the middle and inner ear. Confirmation can be the data of theoretical and
experimental studies that have shown the ability of low-frequency acoustic oscillations to cause
a single-stage displacement of the entire column of snail fluid. The "rhythmic functional
pulsation of the nuclei" of the auditory receptor cells observed by many authors not only in the
apical part, but also in other parts of the spiral organ, further inclines to this point of view [8,
13, 15, 20].
The low-frequency acoustic oscillations of hearing thresholds are set: for 100 Hz they
are about 40 dB, and for 1 Hz – 140 dB. Long-term effect low-frequency sound and lead to an
increase of the threshold of hearing, mostly in the ranges of low and medium frequencies
(125–500Hz) in contrast to medium and high frequency noises, the action of which lead to an
increase of the threshold of audibility in a range of 4-6 kHz. This should be taken into account
due to the fact that the maximum of speech frequencies is in the low and medium frequencies
(500–2000 Hz), so the development of hearing disorders in humans in the low and medium
ranges can lead to difficulties in understanding speech, which is a prognostically unfavorable
factor in social terms [35, 37].
A feeling of excessive pressure in the ear under the action of low-frequency acoustic
oscillations appeared at an ultrasound of 130 dB, further increase led to the appearance of
hyperemia of the eardrum in a person, and at an ultrasound of more than 150 dB, some subjects
experienced a feeling of pain in the ear. This threshold should be considered as a criterion for
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the limit of endurance of a mechanical system when exposed to infrasound. Mechanical damage
to the human hearing organ is expected to occur at levels above 160 dB.
To a certain extent, the human ear is protected from damage due to the acoustic reflex
of the middle ear, or the reflex of the stapes. Normally, this reflex is triggered at 75–90 dB
ultrasound and has its own latent reaction period of 25–100 ms. Therefore, it will show its effect
faster with longer pulses (low-frequency) than short ones (high-frequency). The contraction of
the stirrup muscle increases the impedance of the conducting mechanisms, which results in an
attenuation (reduction) of approximately 15 to 20 dB at low and medium frequencies.
Studies of individuals who were long-term exposed to the influence of low-frequency
acoustic oscillations in the production environment at a level of 100 to 130 dB, showed the
presence of anatomical changes in the eardrum, as well as the appearance of a permanent shift
in the threshold of audibility.
Thus, these data indicate that high-intensity of low-frequency acoustic oscillations can
have an adverse direct effect on the hearing organ up to its mechanical damage.
The transmission of sound to the structures of the hearing organ is carried out in two
ways-air and bone. In the latter case, at a low-frequency of the stimulus, the skull oscillates as
a whole with an increase in the frequency of vibrations of its individual parts in the opposite
phase. This inertial type of bone conduction is observed when sounds with a relatively large λ
are active, when the size of the head (the distance from ear to ear is about 30 cm) becomes less
than λ, which corresponds to a frequency of less than 1000 Hz. When the acoustic wave
frequency over 1000 Hz the skull varies not in phase and different in its parts, so there is a
compression of the entire labyrinth and the perilymph capsule exposed to the pressure affecting
the position of the stapes and the membrane window of the cochlea. This type of bone sound
transmission is called compression, and it is in contrast to the inertial type due to differences in
the mobility of windows, which is important in the diagnostic interpretation of the results of
audiometry in bone sound transmission. Regardless of the type of bone transmission pathway,
this leads to the appearance of a traveling wave on the basilar membrane, followed by the
development of auditory reception. If at low sound levels, the transmission of the stimulus due
to bone conduction is small, then at high levels it increases, exacerbating the harmful effect on
the person. Normal bone conduction thresholds are approximately 35–40 dB higher than air
ones [27].
In accordance with SanPiN 2.2.4.3359–16 at sound levels above the maximum
permissible level (80 dBA) in the workplace, it is mandatory to apply anti-noise to protect the
hearing organ, which block the air channel of acoustic vibrations. At high noise levels, you
already need to protect all the bone structures of the head with an anti-noise helmet. The
threshold value of its application should be the sound level above 115 dBA (sound pressure
level 80 dB + bone conduction threshold 35 dB). At the same time, the presence in the noise
spectrum of low-frequency acoustic oscillations sound pressure level over 90–100 dB is the
basis for anti-noise head protection.
EXTRAORALLY EFFECTS OF LOW-FREQUENCY ACOUSTIC OSCILLATIONS
The influence of the length of the acoustic wave λ (diffraction effect)
An important place in the interaction of the acoustic vibration and the human body is
occupied by λ. Table 3 shows the ratio of λ to various human anthropometric data.
Table 3 shows that the height of a person and the human torso (chest and abdomen) are
comparable with the frequency of 250 Hz and below, and the human head – from 1000 Hz and
below. The commensurability of the length of the incident wave with the obstacle in the path
of its propagation allows you to circumvent the obstacle without changing the parameters of
the wave, that is diffraction occurs, which makes it possible to simultaneously affect the head
and body a person. This phenomenon provides uniform compression and discharge of the entire
Biomechanical mechanisms of action of low-frequency acoustic vibrations on a person
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surface of the human body, followed by the excitation of surface tissues, followed by the
formation of wave processes in internal organs and structures. This phenomenon provides a
mechanism for the passage of low-frequency acoustic oscillations into the human body.
Table 3
The ratio of λ to various human anthropometric data
The studied parameter
Infrasound (2–20 Hz)
Low-frequency sound
(31.5–250 Hz)
λ = 170–17 m
λ = 11–1.4 m
The ratio of the wavelength / human
height (1.8 m)
100–10
6–1
The ratio of wave length / human torso
(0.8–1.0 m)
212–21
14–2
Wavelength ratio / human head (0.3 m)
700–70
40–5
As can be seen, large λ and its commensurability with anthropometric indicators ensures
the similarity between low-frequency sound and their effect on human and, hence, allows to
predict the close nature of the overall responses, the formation of which is due to mechanical
interaction of the human body and λ [32, 33].
Resonance
The resonance theory, which was based on the coincidence of the resonant frequencies
of the human body with the frequency of the incident λ [9], is widely used in works on the
action of infrasound. At the same time, the authors give resonant frequencies for the body as a
whole and for individual organs, obtained under the action of a general vibration. However, in
the case of low-frequency acoustic oscillations, the system is more rigid than in the case of
general vibration, and the main resonance of the “chest/abdominal cavity” system is noted in
the frequency range of 40–60 Hz instead of 4–8 Hz in the case of vibration (Table 4).
From the data of Table 4, it follows that the resonant frequencies for a person are in the
frequency range of the low-frequency acoustic oscillations.
To confirm the above data, we conducted research in the laboratory. Direct
measurements of continuous acoustic oscillation parameters (90–135 dB ultrasound frequency
below 100 Hz) and pulse parameters with a maximum spectrum in the range of 20–30 Hz were
performed by placing bioobjects inside the chamber and inserting an acoustic sensor into the
chest and skull cavity. Studies have shown that acoustic vibration, regardless of the method of
generation, frequency of repetition, pressure amplitude and pulse duration, freely penetrate into
the chest cavity and the skull cavity of animals and at the same time practically does not change
the shape and spectral composition of the wave incident on the bioobject. The coefficient of
transformation of the acoustic signal (parameter outside/parameter inside) fluctuated in the
ranges of 0.78–1.45 in drugged animals. There were no differences in the passage of these
acoustic vibration between a dog and a rabbit.
The thorax of animals and humans should be considered as a closed small shell, which
is not an obstacle to the penetration of low-frequency acoustic oscillations regardless of the
method of their generation and does not change the amplitude, time and spectral characteristics
of the incident wave. There were no interspecific differences in the passage of low-frequency
acoustic oscillations into the chest, as well as resonance. The main route of low-frequency
acoustic oscillations penetration into the chest cavity is its outer surface.
An increase in the volume of the lungs leads to a deterioration in the penetration of
acoustic waves into the chest. It is necessary to take into account the fact that for waves of small
amplitude (P± < 10 kPa), the effect of respiration on the resulting chest pressure is significant.
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The resulting oscillation inside the chest cavity, in addition to the parameters of the incident
wave, depends on the amplitude and phase of the pressure caused by breathing. It follows that
the vibrations of the chest cavity reach the maximum amplitude when the maximum amplitudes
of these phenomena coincide. In the case of a discharge inside the chest, the amplitude of the
incident wave will decrease by the amount of discharge. From this it follows that waves of small
amplitude of pulse of low-frequency acoustic oscillations significantly influence the breathing
phase on the passage of the incident wave, and for continuous ones – the volume of the lungs.
The study of the passage of low-frequency acoustic oscillations into the skull cavity of
animals showed that the measured parameters (amplitude, time, and spectral) of the oscillatory
process in the skull almost completely coincide with similar parameters of the incident wave,
regardless of the method of generation (pulsed or continuous). Also, there were no interspecies
differences in the passage of low-frequency acoustic oscillations into the cranial cavity and no
resonance phenomena were found. The main route of low-frequency acoustic oscillations
penetration into the skull appears to be the vessels and spinal canal. There was no resonance in
the cavities and interspecific differences in the passage of acoustic vibration.
Thus, the chest and skull should be considered as a closed shell, which is not an obstacle
to the penetration of the low-frequency acoustic oscillations.
The resonance (fr. resonance, from lat. resono “I respond”) is a frequency-selective
response of an oscillatory system to a periodic external influence, which manifests itself in a
sharp increase in the amplitude of stationary vibrations when the frequency of the external
influence coincides with certain values characteristic of this system. The concept of resonance
has its own characteristics depending on the field of application (electronics, microwave, optics,
etc., including acoustics). In the scientific literature, attempts are periodically made to use the
term resonance in relation to biological objects under the influence of electromagnetic pulses
and mechanical vibrations. In this case, it is necessary to use this term more correctly, for
example, “biological resonance”. At the same time, it is necessary to take into account that
biological tissues and organs in most cases have a high heterogeneity, contain a large amount
of liquid (blood, lymph, water), have sufficient elasticity and plasticity, there are cyclical
fluctuations in shape and volume in the space-time continuum of life, etc. The presence of these
factors has a serious negative impact on the quality of biological structures and creates
difficulties for the occurrence of acoustic resonance.
Table 4
The main resonant frequencies of the human body
Organs and parts of the human body
Resonance frequency, Hz
Whole body
3–6; 4–8; 5–12
Internal organs
10–100
Chest/abdomen
40–60
Head
8–27
Chest
2–12; 5–8; 4–8
Abdominal cavity
2–14; 3–4
Eyeball
12–27
Propagation of elastic waves
The diffraction ability of the low-frequency acoustic oscillations leads to the fact that a
person is subjected to uniform excessive variable pressure with a frequency of falling λ and
elastic waves are formed in various structures of the body. At low ultrasound frequencies (up
to 100 dB), only the human ear perceives the action of λ due to the deformation (displacement)
of the eardrum.
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When the ultrasonic frequency exceeds 100 dB, the vibrational velocity of the particles
reaches 0.01 m/s, which leads to the excitation of mechonoreceptors. Arousal of these receptors
forms a person's sense of vibration of the body, head, and internal organs. Therefore, when a
person is exposed to high-level acoustic fields, the term "air vibration" is often used [17]. With
an ultrasound of more than 140 dB, it is possible to predict the development of pain, primarily
in the eardrums and internal organs [32, 37].
In laboratory tests, it was found that the threshold of vibrotactical sensitivity in humans
corresponds to their level of 100-110 dB [16]. An increase in the ultrasound frequency of more
than 125 dB causes a feeling of "air vibration" of the entire body [7, 17]. This is proved by
experimental studies with testers, when the exposed chest, abdomen and arms were exposed to
acoustic oscillation radiation (frequency 125 and 350 Hz at 125 dB ultrasound), while the ears
and head were reliably protected by a helmet. After exposure, there was an increase in the
threshold of vibro-tactile sensitivity, as well as the level of blood pressure and a deterioration
in the indicators of psychophysiological tests. In addition, these individuals complained of
fatigue, headache, sleep disturbances, and a feeling of vibration of certain parts of the body and
internal organs. The latter phenomenon was explained by activation of intero-and
proprioceptors. The role of mechanoreceptors in the perception of low-frequency acoustic
oscillations has been convincingly proved by studies performed on genetically deaf people or
mice and on animals with a bilaterally destroyed hearing organ. As a result, a number of
physiological indicators were found to be deteriorating, including physical performance. When
ultrasound is more than 140 dB, pain is observed, primarily in the eardrums and internal organs.
In laboratory experimental studies on rats, a change in the pulse activation of the sciatic
nerve was shown, which indicated the direct perception of low-frequency acoustic oscillation
mechanoreceptors [4].
Thus, the data obtained strongly show that low-frequency acoustic oscillations have a
direct effect on mechanoreceptors, which is the basis for the formation of biological effects.
A low rate of propagation of elastic waves in tissues at a low-frequency leads to the
appearance of waves that are commensurate with the size of cells or cellular organelles. The
magnitude of deformation shifts during the propagation of acoustic vibration depends not only
on the ultrasonic frequency, but also is inversely dependent on the frequency. At low
frequencies (below 250 Hz) and high sound pressure levels, it reaches a few millimeters, and at
high – a few centimeters. This can cause synchronized conformational fluctuations of
macromolecules, and therefore lead to changes in the size and shape of cellular organelles.
Deformational shifts in biological structures can lead to structural disorders in the form of tissue
and visceral injuries. In addition, this is facilitated by the fact that the human body consists of
tissues that have a large difference in mechanical properties [5] (Table 5), so at the interface of
tissues with a high density difference (air/tissue, air/liquid, liquid/tissue), increased mechanical
stresses will occur during the passage of acoustic vibration through the tissues, up to structural
damage to organs (lungs, brain, heart) and vessels, especially small ones [32].
Table 5
The value of the speed of sound and the specific density of human biological tissues
Biological tissue
Speed of sound, m/s
Specific density, kg/m3
Bone
3300
1920
Blood
1590
1048–1066
Skin
1610
1093–1121
Water
1500
1000
Lungs
70
260
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Vestibular effects
The impact of low-frequency acoustic oscillations along with changes in the auditory
analyzer, lead also to violations of the vestibular apparatus. A certain explanation for this is the
close anatomical location of the ear labyrinth and the spiral organ and, apparently, finding the
natural frequency range of these organs in the range of 2–20 Hz. In laboratory conditions, the
effect of low-frequency acoustic oscillations above the level of 100 dB, according to some
authors, caused the subjects to experience subjective sensations (dizziness, nausea, balance
disorders) in a state with objective signs (decreased statokinetic balance, nystagmus), which
strongly indicated a violation of the function of the vestibular analyzer. According to other
sources, these vestibular disorders appear only when the ultrasound is 140–155 dB. In addition,
the presence of functional shifts of nucleic acids in the receptor cells of semicircular channels
in Guinea pigs after single and multiple exposure to infrasound (8 and 16 Hz at 90–120 dB
ultrasound) indicated the possibility of depletion of energy and plastic resources in them by the
type of fatigue [8, 20].
Clinical examination of workers who were systematically exposed to the influence of
low-frequency sound in industrial conditions revealed that they had increased excitability of
the vestibular apparatus.
Some authors explain the mechanism of excitation of receptor structures of the
vestibular analyzer by the energy of low-frequency acoustic oscillations, which cause fluid
movement and the formation of vortex flows in the labyrinth. Others associate this effect with
resonant excitation of cells, structures of the inner ear, and possibly otoliths as a result of the
direct action of acoustic vibration on them. In addition, these disorders may also have a central
origin due to violations of cortical-subcortical and stem interaction, thus creating conditions for
the formation of somatic and vegetative reflexes [8, 10, 17].
CONCLUSIONS
Based on field and laboratory studies, it is shown that low-frequency acoustic
oscillations cause not only auditory (aural) effects in humans and animals, but also have a
general effect (extraaural). The latter are inherent in acoustic oscillations with a frequency
below 170–200 Hz and a frequency range above 100 dB. The impact of low-frequency acoustic
oscillations on various organs must first be considered from the perspective of interaction of
mechanical systems, since they, penetrating into the human body, have a direct impact on all
organs. From our point of view, the lungs are in the most unfavorable conditions:
– the presence of air communication with the environment makes it possible for low-
frequency acoustic oscillations to enter the lungs not only through the surface of the body, but
also through large airways;
– the airiness of the organ and the complex morphological structure of tissues of
different densities create significant acoustic heterogeneity and lead to deformations at the
interface of media with different acoustic impedances;
– the large external and huge internal surface of the lungs in comparison with other
organs allows them to absorb a large amount of sound energy;
– matching the natural frequency of the thorax and lungs frequency range of the low-
frequency acoustic oscillations gives the possibility of resonance phenomena in the body under
the action of external mechanical vibrations, which can be a leading cause of structural defects.
As a result of experimental work, it was shown that the parameters of the low-frequency
acoustic oscillations (sound pressure level, frequency, pulse length and area) measured outside
and inside the chest and skull of animals differ slightly. The data obtained suggest that low-
frequency acoustic oscillation has a direct effect on the lungs, brain, and other internal organs.
It is known that when acoustic waves propagate in the medium, mechanical deformations occur
that propagate at a speed that depends on the elastic properties and density of the medium.
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When an acoustic wave propagates, the elastic strain energy is transferred. Based on the results
of research by foreign and domestic authors [5, 24], we can distinguish three groups of tissues
in the human and animal bodies that differ in acoustic properties. The first group includes bone
tissue (similar in its acoustic properties to a solid body, for example, metal), in which the speed
of the longitudinal acoustic wave reaches 2500–4500 m·s–1.
The second group of biological tissues (skin, spleen, muscle tissue, brain, liver, kidneys)
and liquids (blood, liquor) have acoustic properties similar to water, where the speed of sound
is equal to 1500 m·s–1. The maximum deviation from the average value is only 7–10 % for these
tissues, which causes the passage of an acoustic wave through their border with low losses.
The lungs were included in the third group of tissues, the acoustic wave velocity in them
is much less than in the first two groups, and according to different authors is less than 70 m×s-
1 due to anatomical and morphological features of the respiratory organ. The lungs are a
complex structure, where 10% of their volume is occupied by large airways and blood vessels
and 90 % by the parenchyma, which consists of alveoli, alveolar passages and sacs, capillaries,
arterioles and venules [10, 18, 25, 26].
As a result of the high content of air in the lungs, they have a much lower density
(0.26 g·ml–1) than other tissues. The speed of sound in them D.A. Rice /1983/ proposed to
determine by the formula:
С = В/p, (1)
where C is the speed of sound, m·s–1; B is the volume stiffness, Pa; p is the density, kg·m–3.
Due to the low speed of sound in the lung tissue, the wavelength of the acoustic signal
at moderately high frequencies (~ 1 kHz) may be commensurate with the size of cells or cellular
organelles. This can lead to synchronized conformational fluctuations of macromolecules, and
therefore to changes in the size and shape of cellular organelles, that is, to damage cells.
In addition, the pulmonary parenchyma is a complex material with high acoustic
heterogeneity. Thus, the elastic frame and vascular bed have a similar acoustic impedance-
144.000 kg·m–2·s–1, and the air-only 440 kg· m–2·s–1 [5]. In addition, acoustic waves with
different speeds can propagate through the lungs in several ways: the large Airways, blood
vessels, and parenchyma. A large difference in the speed of sound and acoustic impedances in
the lungs themselves, as well as between the lungs and nearby anatomical formations, leads to
a very strong scattering of the acoustic wave at the border with a high nonlinear load-stress ratio
and the appearance of waves of reflection, refraction and focusing, or, depending on the
intensity of the acoustic field, cavitation, which creates conditions for mechanical damage to
the lungs.
Noteworthy are the results of experimental studies that proved the appearance of
cavitation in liquids when irradiated with low-frequency acoustic oscillations (10–200 Hz). As
a rule, there are two types of cavitation bubbles-small spherical and large deformed bubbles.
The movement and splitting of bubbles of the first type is accompanied by the transformation
of acoustic energy into the energy of plastic deformation or the occurrence of chemical reactions
with the formation of highly aggressive radical products of H20 splitting. The activity of the
second type of bubbles may be associated with the appearance of acoustic microflows, high
shear stresses and mechanical failures [19].
There are several hypotheses for the origin and development of cavitation in liquids and
tissues. When acoustic vibrations of low intensity propagate in a real liquid, the microbubbles
present in it begin to pulsate in phase with the field. The occurrence of chemical reactions and
mechanical damage is associated with an increase in the intensity of acoustic vibration. At some
point, called the threshold of developed cavitation, any one (or more) of the listed effects occurs
spontaneously. It was found that at the intensity of acoustic fields < 150 dB, 1–2 bubbles of the
second type may be present in the volume under study, and at the intensity of 160 dB, the
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number of small pulsating bubbles may exceed the number of large bubbles by 1–2 orders of
magnitude or more.
From the above it follows that low-frequency acoustic oscillations, freely penetrating
into the chest, have a direct effect on the lungs. Morphological features of the respiratory organ
contribute to the spread of these acoustic vibration in it in the form of elastic oscillatory waves
at a low speed, which can increase the conformational vibrations of macromolecules up to cell
damage. Acoustic inhomogeneity of the lungs in comparison with other organs, including the
brain, leads to higher strain loads under the action of low-frequency acoustic oscillations. It is
known that the deformation can be elastic (reversible) or plastic (irreversible). Therefore, in
one case, the organ or its elements, reacting to the deformation of the acoustic impact, retain
their structure (physical structure), in the other case, their destruction occurs. In a complex
heterogeneous system, such as the lungs, we can expect the manifestation of various types of
deformation, which is determined primarily by acoustic features, and these, in turn, are due to
the mechanical properties of tissues (strength, elasticity, elasticity), both inside the organ and
between the organ and its environment. Plastic deformation should be observed, first, at the
interface of the medium tissue/air, that is, on the subpleural surface of the lungs and in the
alveoli, and secondly, in the mechanically weakest areas. It is known that in organs that have a
"vascular-connective tissue" skeleton, such is the vascular and primarily the capillary bed. With
increasing intensity of the acoustic field, the probability of not only deformation, but also
cavitation increases, which leads to even greater destruction of the pulmonary parenchyma.
With prolonged (chronic) action of acoustic fields of moderate intensity, elastic deformation
prevails, which leads to constant mechanical stress in various parts of the lungs, accumulation
of residual deformation and gradual destruction of the lung framework, which leads to the
development of emphysema [32, 37, 41].
Thus, it is proved that low-frequency acoustic oscillations change the biomechanical
state of a living system, causing acoustic stress in heterogeneous media and deformation in
organs as a whole and (or) its individual sections. In this case, the biological effect of low-
frequency acoustic oscillations exposure is determined by both physical criteria (intensity,
duration, frequency) and design features (structure) of the irradiated organs. The presence of so
many really interacting elements that determine the effect of acoustic interaction requires in
each case a detailed account of them for an objective assessment of the conditions and
forecasting the effect of low-frequency acoustic oscillations on the body. Mechanical damage
causes a violation of the integrity of the structure of tissue, cells, subcellular and intercellular
structures. Specific cell damage is usually accompanied (or followed by) and general non-
specific manifestations of damage (violation of permeability, damage to the lipid components
of cell and subcellular membranes, changes in the activity of intracellular enzymes, etc.).
Direct evidence of confirmation of the above mechanisms of action is experimental
work, which shows that low-frequency acoustic oscillations can cause morphofunctional
disorders in a number of organs (brain, heart, liver, etc.) up to structural damage to tissues,
blood vessels [1, 2, 28, 29].
The stated provisions allow us to assert: 1) acoustic vibrations of infrasound and low-
frequency sound range have similarities in the mechanisms of action on large biological objects
(man, dog, etc.);
2) biomechanical mechanisms of action of infrasound and low frequency noise should
be taken into account in the hygienic regulation of noise and infrasound;
3) low-frequency acoustic oscillations cause adverse aural and extraaural effects, it
requires the development of special types of body and head protection – extraaural anti-noise
(anti-noise vest and anti-noise helmet). The reason for their use is the presence of low-frequency
acoustic oscillation sound pressure level in the production noise spectrum above 90–100 dB
[31, 37].
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Received 23 March 2018
