The basic laws of sound propagation include the laws of its reflection and refraction at the boundaries of various media, as well as the diffraction of sound and its scattering in the presence of obstacles and inhomogeneities in the medium and at the interfaces between media.

The sound propagation distance is influenced by the sound absorption factor, that is, the irreversible transfer of sound wave energy into other types of energy, in particular, into heat. An important factor is also the direction of radiation and the speed of sound propagation, which depends on the medium and its specific state.

Acoustic waves propagate from a sound source in all directions. If a sound wave passes through a relatively small hole, then it propagates in all directions, and does not go in a directed beam. For example, street sounds penetrating through an open window into a room are heard at all its points, and not just against the window.

The nature of the propagation of sound waves at an obstacle depends on the ratio between the dimensions of the obstacle and the wavelength. If the dimensions of the obstacle are small compared to the wavelength, then the wave flows around this obstacle, propagating in all directions.

Sound waves, penetrating from one medium to another, deviate from their original direction, that is, they are refracted. The angle of refraction can be greater or less than the angle of incidence. It depends on which medium the sound comes from. If the speed of sound in the second medium is greater, then the angle of refraction will be greater than the angle of incidence, and vice versa.

Encountering an obstacle on its way, sound waves are reflected from it according to a strictly defined rule - the angle of reflection is equal to the angle of incidence - the concept of echo is associated with this. If sound is reflected from several surfaces at different distances, multiple echoes occur.

Sound propagates in the form of a diverging spherical wave that fills an ever larger volume. As the distance increases, the oscillations of the particles of the medium weaken, and the sound dissipates. It is known that in order to increase the transmission distance, sound must be concentrated in a given direction. When we want, for example, to be heard, we put our hands to our mouths or use a mouthpiece.

Diffraction, that is, the bending of sound rays, has a great influence on the range of sound propagation. The more heterogeneous the medium, the more the sound beam is bent and, accordingly, the shorter the sound propagation distance.

sound propagation

Sound waves can propagate in air, gases, liquids and solids. Waves do not form in airless space. This can be easily seen from a simple experiment. If an electric bell is placed under an airtight cap from which the air is evacuated, we will not hear any sound. But as soon as the cap is filled with air, sound occurs.

The speed of propagation of oscillatory motions from particle to particle depends on the medium. In ancient times, warriors put their ears to the ground and thus discovered the enemy's cavalry much earlier than it appeared in sight. And the famous scientist Leonardo da Vinci wrote in the 15th century: “If you, being at sea, lower the hole of the pipe into the water, and put the other end to your ear, you will hear the noise of ships very distant from you.”

The speed of sound in air was first measured in the 17th century by the Milan Academy of Sciences. A cannon was installed on one of the hills, and an observation post was located on the other. The time was recorded both at the moment of the shot (by flash) and at the moment of sound reception. From the distance between the observation post and the gun and the time of origin of the signal, the speed of sound propagation was no longer difficult to calculate. It turned out to be equal to 330 meters per second.

In water, the speed of sound propagation was first measured in 1827 on Lake Geneva. Two boats were one from the other at a distance of 13847 meters. On the first, a bell was hung under the bottom, and on the second, a simple hydrophone (horn) was lowered into the water. On the first boat, at the same time as the bell was struck, gunpowder was set on fire, on the second observer, at the moment of the flash, he started a stopwatch and began to wait for the sound signal from the bell to arrive. It turned out that sound travels more than 4 times faster in water than in air, i.e. at a speed of 1450 meters per second.

Sound propagation speed

The higher the elasticity of the medium, the greater the speed: in rubber50, in air330, in water1450, and in steel - 5000 meters per second. If we, who were in Moscow, could shout so loudly that the sound would reach Petersburg, then we would be heard there only in half an hour, and if the sound propagated over the same distance in steel, it would be received in two minutes.

The speed of sound propagation is influenced by the state of the same medium. When we say that sound travels in water at a speed of 1450 meters per second, this does not mean at all that in any water and under any conditions. With an increase in temperature and salinity of water, as well as with an increase in depth, and, consequently, hydrostatic pressure, the speed of sound increases. Or take steel. Here, too, the speed of sound depends both on the temperature and on the qualitative composition of the steel: the more carbon it contains, the harder it is, the faster sound travels in it.

Encountering an obstacle on its way, sound waves are reflected from it according to a strictly defined rule: the angle of reflection is equal to the angle of incidence. Sound waves coming from the air are almost completely reflected upwards from the surface of the water, and sound waves coming from a source in the water are reflected downwards from it.

Sound waves, penetrating from one medium to another, deviate from their original position, i.e. are refracted. The angle of refraction can be greater or less than the angle of incidence. It depends on the medium from which the sound penetrates. If the speed of sound in the second medium is greater than in the first, then the angle of refraction will be greater than the angle of incidence and vice versa.

In air, sound waves propagate in the form of a diverging spherical wave, which fills an ever larger volume, as the particle vibrations caused by sound sources are transferred to the air mass. However, as the distance increases, the oscillations of the particles weaken. It is known that in order to increase the transmission distance, the sound must be concentrated in a given direction. When we want to be heard better, we put our palms to our mouths or use a horn. In this case, the sound will be attenuated less, and the sound waves will propagate further.

As the wall thickness increases, sonar at low mid frequencies increases, but the “insidious” coincidence resonance, which causes sonar suffocation, begins to appear at lower frequencies and captures a wider area of ​​them.

Propagation of sound in water

SPEARFISHING

Propagation of sound in water .

Sound travels five times faster in water than in air. The average speed is 1400 - 1500 m / s (the speed of sound propagation in air is 340 m / s). It would seem that audibility in the water is also improving. In fact, this is far from the case. After all, the strength of sound does not depend on the speed of propagation, but on the amplitude of sound vibrations and the perceiving ability of the hearing organs. In the cochlea of ​​the inner ear is the organ of Corti, which consists of auditory cells. Sound waves vibrate the eardrum, auditory ossicles, and the membrane of the organ of Corti. From the hair cells of the latter, perceiving sound vibrations, nervous excitation goes to the auditory center, located in the temporal lobe of the brain.

A sound wave can enter the inner ear of a person in two ways: by air conduction through the external auditory canal, eardrum and auditory ossicles of the middle ear, and through bone conduction - vibration of the skull bones. On the surface, air conduction predominates, and under water, bone conduction. This is confirmed by a simple experience. Cover both ears with the palms of your hands. On the surface, audibility will deteriorate sharply, but this is not observed under water.

So, underwater sounds are perceived mainly by bone conduction. Theoretically, this is explained by the fact that the acoustic resistance of water approaches the acoustic resistance of human tissues. Therefore, the energy loss during the transition of sound waves from water to the bones of the human head is less than in air. Air conduction under water almost disappears, since the external auditory canal is filled with water, and a small layer of air near the eardrum weakly transmits sound vibrations.

Experiments have established that bone conduction is 40% lower than air conduction. Therefore, the audibility under water in general deteriorates. The range of audibility with bone conduction of sound depends not so much on the strength as on the tone: the higher the tone, the farther the sound is heard.

The underwater world for a person is a world of silence, where there are no extraneous noises. Therefore, the simplest sound signals can be perceived under water at considerable distances. A person hears a blow on a metal canister immersed in water at a distance of 150-200 m, the sound of a rattle at 100 m, a bell at 60 m.

Sounds made underwater are usually inaudible on the surface, just as sounds from the outside are not heard underwater. To perceive underwater sounds, you must at least partially dive. If you enter the water up to your knees, you begin to perceive a sound that has not been heard before. As you dive, the volume increases. It is especially well audible when immersing the head.

To give sound signals from the surface, it is necessary to lower the sound source into the water at least half, and the sound strength will change. Orientation under water by ear is extremely difficult. In air, sound arrives in one ear 0.00003 seconds earlier than in the other. This allows you to determine the location of the sound source with an error of only 1-3 °. Under water, the sound is simultaneously perceived by both ears and therefore there is no clear, directional perception. Orientation error is 180°.

In a specially set experiment, only individual light divers after long wanderings and. searches went to the location of the sound source, which was 100-150 m from them. It was noted that systematic training for a long time makes it possible to develop the ability to quite accurately navigate by sound underwater. However, as soon as the training stops, its results are nullified.

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Sound is understood as elastic waves lying within the limits of audibility of the human ear, in the range of oscillations from 16 Hz up to 20 kHz. Oscillations with a frequency below 16 Hz called infrasound, over 20 kHz-ultrasound.

Water is denser and less compressible than air. In this regard, the speed of sound in water is four and a half times greater than in air, and is 1440 m/s Sound vibration frequency (nude) is related to the wavelength (lambda) by the relationship: c= lambda-nu. Sound propagates in water without dispersion. The speed of sound in water varies depending on two parameters: density and temperature. A change in temperature by 1° entails a corresponding change in the speed of sound by 3.58 m per second. If we follow the speed of sound propagation from the surface to the bottom, it turns out that at first, due to a decrease in temperature, it quickly decreases, reaching a minimum at a certain depth, and then, with depth, it begins to increase rapidly due to an increase in water pressure, which, as is known, increases by about 1 atm for every 10 m depths.

Starting from a depth of approximately 1200 m, where the temperature of the water remains practically constant, the change in the speed of sound is due to the change in pressure. “At a depth of approximately 1200 m (for the Atlantic), there is a minimum value for the speed of sound; at greater depths, due to the increase in pressure, the speed of sound increases again. Since sound rays are always bent towards the areas of the medium where their speed is the lowest, they are concentrated in the layer with the minimum speed of sound” (Krasilnikov, 1954). This layer, discovered by Soviet physicists L.D. Rozenberg and L.M. Brekhovskikh, is called the "underwater sound channel". Sound entering the sound channel can propagate over long distances without attenuation. This feature must be kept in mind when considering the acoustic signaling of deep-sea fish.

Sound absorption in water is 1000 times less than in air. Sound source in the air with a power of 100 kW in the water can be heard at a distance of up to 15 km; sound source in water 1 kW heard at a distance of 30-40 km. Sounds of different frequencies are absorbed differently: high-frequency sounds are most strongly absorbed and low-frequency sounds are the least absorbed. The low absorption of sound in water made it possible to use it for sonar and signaling. Water spaces are filled with a large number of different sounds. The sounds of water bodies of the World Ocean, as the American hydroacousticist Wenz (1962) showed, arise in connection with the following factors: tides, currents, wind, earthquakes and tsunamis, industrial human activity and biological life. The nature of the noise created by various factors differs both in the set of sound frequencies and in their intensity. On fig. Figure 2 shows the dependence of the spectrum and pressure level of the sounds of the World Ocean on the factors that cause them.

In different parts of the World Ocean, the composition of noise is determined by different components. In this case, the bottom and shores have a great influence on the composition of sounds.

Thus, the composition and intensity of noise in different parts of the World Ocean are extremely diverse. There are empirical formulas that show the dependence of the intensity of sea noise on the intensity of the factors that cause them. However, for practical purposes, ocean noise is usually measured empirically.

It should be noted that among the sounds of the World Ocean, industrial sounds created by man are the most intense: the noise of ships, trawls, etc. According to Shane (1964), they are 10-100 times more intense than other sounds of the World Ocean. However, as can be seen from Fig. 2, their spectral composition is somewhat different from the spectral composition of sounds caused by other factors.

When propagating in water, sound waves can be reflected, refracted, absorbed, diffracted, and interfered.

Encountering an obstacle on its way, sound waves can be reflected from it in the case when their wavelength (lambda) less than the size of the obstacle, or go around (diffract) it in the case when their wavelength is greater than the obstacle. In this case, one can hear what is happening behind the obstacle without seeing the source directly. Falling on an obstacle, sound waves in one case can be reflected, in another case they can penetrate into it (be absorbed by it). The value of the energy of the reflected wave depends on how strongly the so-called acoustic impedances of the media “p1c1” and “p2c2” differ from each other, on the interface of which sound waves fall. Under the acoustic resistance of the medium is meant the product of the density of the given medium p and the speed of sound propagation With in her. The greater the difference in the acoustic impedance of the media, the greater part of the energy will be reflected from the separation of the two media, and vice versa. In the case of, for example, sound falling from the air, rs which 41, into the water, rs which is 150,000, it is reflected according to the formula:

In connection with the above, sound penetrates much better into a solid body from water than from air. From air to water, sound penetrates well through bushes or reeds protruding above the water surface.

In connection with the reflection of sound from obstacles and its wave nature, the addition or subtraction of the amplitudes of sound pressures of the same frequencies that have come to a given point in space can occur. An important consequence of such addition (interference) is the formation of standing waves upon reflection. If, for example, the tuning fork is brought into oscillation, bringing it closer and further away from the wall, one can hear the increase and decrease in the volume of the sound due to the appearance of antinodes and nodes in the sound field. Usually, standing waves are formed in closed containers: in aquariums, pools, etc., with a source sounding for a relatively long time.

In the real conditions of the sea or other natural reservoir, during the propagation of sound, numerous complex phenomena are observed that arise in connection with the heterogeneity of the aquatic environment. A huge influence on the propagation of sound in natural reservoirs is exerted by the bottom and interfaces (water - air), temperature and salt heterogeneity, hydrostatic pressure, air bubbles and planktonic organisms. The water-air interface and the bottom, as well as the inhomogeneity of the water, lead to the phenomena of refraction (curvature of sound rays), or reverberation (multiple reflection of sound rays).

Water bubbles, plankton and other suspended matter contribute to sound absorption in the water. Quantification of these numerous factors has not yet been developed. It is necessary to take them into account when setting up acoustic experiments.

Let us now consider the phenomena that occur in water when sound is emitted in it.

Imagine a sound source as a pulsating sphere in infinite space. The acoustic energy emitted by such a source is attenuated inversely with the square of the distance from its center.

The energy of the resulting sound waves can be characterized by three parameters: speed, pressure and displacement of oscillating water particles. The last two parameters are of particular interest when considering the auditory abilities of fish, so we will dwell on them in more detail.

According to Harris and Berglijk (Harris a. Berglijk, 1962), pressure wave propagation and displacement effects are presented differently in the near (at a distance of less than one wavelength of sound) and far (at a distance of more than one wavelength of sound) acoustic field.

In the far acoustic field, the pressure attenuates inversely with the distance from the sound source. In this case, in the far acoustic field, the displacement amplitudes are directly proportional to the pressure amplitudes and are interconnected by the formula:

where R - acoustic pressure in dynes/cm 2 ;

d- particle displacement value in cm.

In the near acoustic field, the dependence between the pressure and displacement amplitudes is different:

where R-acoustic pressure in dynes/cm 2 ;

d - displacement of water particles in cm;

f - oscillation frequency in hz;

rs- acoustic resistance of water equal to 150,000 g/cm2 sec 2 ;

lambda is the wavelength of sound in m; r - distance from the center of the pulsating sphere;

i= SQR i

It can be seen from the formula that the displacement amplitude in the near acoustic field depends on the wavelength, sound, and distance from the sound source.

At distances smaller than the wavelength of the sound in question, the displacement amplitude decreases inversely with the square of the distance:

where BUT is the radius of the pulsating sphere;

D- increase in the radius of the sphere due to pulsation; r is the distance from the center of the sphere.

Fish, as will be shown below, have two different types of receivers. Some of them perceive pressure, while others perceive the displacement of water particles. The above equations are therefore of great importance for the correct assessment of fish responses to underwater sources of sound.

In connection with the emission of sound, we note two more phenomena associated with emitters: the phenomenon of resonance and directivity of emitters.

The emission of sound by the body occurs in connection with its vibrations. Each body has its own oscillation frequency, determined by the size of the body and its elastic properties. If such a body is brought into oscillation, the frequency of which coincides with its own frequency, the phenomenon of a significant increase in the amplitude of the oscillation occurs - resonance. The use of the concept of resonance makes it possible to characterize certain acoustic properties of fish emitters and receivers. Sound radiation into water can be directional or non-directional. In the first case, sound energy propagates predominantly in a certain direction. A graph expressing the spatial distribution of the sound energy of a given sound source is called its directivity diagram. The directivity of the radiation is observed in the case when the diameter of the emitter is much larger than the wavelength of the emitted sound.

In the case of omnidirectional radiation, sound energy diverges uniformly in all directions. This phenomenon occurs when the wavelength of the emitted sound exceeds the diameter of the emitter lambda>2A. The second case is most typical for low-frequency underwater radiators. Typically, the wavelengths of low-frequency sounds are much larger than the dimensions of the underwater emitters used. The same phenomenon is typical for fish emitters. In these cases, the radiation patterns of the emitters are absent. In this chapter, only some of the general physical properties of sound in the aquatic environment have been noted in connection with the bioacoustics of fish. Some more specific questions of acoustics will be considered in the relevant sections of the book.

In conclusion, let us consider the sound measurement systems used by various authors. Sound can be expressed by its intensity, pressure, or level of pressure.

The intensity of sound in absolute units is measured either by a number erg / sec-cm 2, or W / cm 2. At the same time 1 erg/sec=10 -7 Tue.

Sound pressure is measured in bars.

There is a relationship between the intensity and pressure of sound:

which can be used to convert these values ​​from one to another.

No less often, especially when considering the hearing of fish, due to the huge range of threshold values, sound pressure is expressed in relative logarithmic decibel units, db. If the sound pressure of one sound R, and the other R o, then they consider that the first sound is louder than the second by kdb and calculate it according to the formula:

In this case, most researchers take the threshold value of human hearing equal to 0.0002 as the zero reading of the sound pressure P o bar for frequency 1000 Hz.

The advantage of such a system is the possibility of a direct comparison of the hearing of humans and fish, the disadvantage is the difficulty of comparing the results obtained for the sound and hearing of fish.

The actual values ​​of sound pressure created by fish are four to six orders of magnitude higher than the accepted zero level (0.0002 bar), and the threshold levels of hearing of various fish lie both above and below the conditional zero count.

Therefore, for the convenience of comparing the sounds and hearing of fish, American authors (Tavolga and Wodinsky, 1963, etc.) use a different frame of reference.

The sound pressure of 1 bar, which is 74 db higher than previously accepted.

Below is an approximate ratio of both systems.

The actual values ​​in the American reference system are marked with an asterisk in the text.

Hydroacoustics (from Greek. hydro- water, acusticococcus- auditory) - the science of phenomena occurring in the aquatic environment and associated with the propagation, emission and reception of acoustic waves. It includes the development and creation of hydroacoustic devices intended for use in the aquatic environment.

The history of development

Hydroacoustics is a science that is rapidly developing at the present time, and undoubtedly has a great future. Its appearance was preceded by a long path of development of theoretical and applied acoustics. We find the first information about the manifestation of human interest in the propagation of sound in water in the notes of the famous Renaissance scientist Leonardo da Vinci:

The first distance measurements by means of sound were made by the Russian researcher Academician Ya. D. Zakharov. On June 30, 1804, he flew in a balloon for scientific purposes, and in this flight he used the reflection of sound from the earth's surface to determine the flight altitude. While in the basket of the ball, he shouted loudly into the downward horn. After 10 seconds, a distinctly audible echo came. From this, Zakharov concluded that the height of the ball above the ground was approximately 5 x 334 = 1670 m. This method formed the basis of radio and sonar.

Along with the development of theoretical issues in Russia, practical studies were carried out on the phenomena of the propagation of sounds in the sea. Admiral S. O. Makarov in 1881 - 1882 proposed to use a device called a fluctometer to transmit information about the speed of the current under water. This marked the beginning of the development of a new branch of science and technology - hydroacoustic telemetry.

Scheme of the hydrophonic station of the Baltic Plant, model 1907: 1 - water pump; 2 - pipeline; 3 - pressure regulator; 4 - electromagnetic hydraulic shutter (telegraph valve); 5 - telegraph key; 6 - hydraulic membrane emitter; 7 - board of the ship; 8 - tank with water; 9 - sealed microphone

In the 1890s at the Baltic Shipyard, on the initiative of Captain 2nd Rank M.N. Beklemishev, work began on the development of hydroacoustic communication devices. The first tests of a hydroacoustic transmitter for underwater communication were carried out at the end of the 19th century. in the experimental pool in the Galernaya harbor in St. Petersburg. The vibrations emitted by it were well heard for 7 miles on the Nevsky floating lighthouse. As a result of research in 1905. created the first hydroacoustic communication device, in which a special underwater siren controlled by a telegraph key played the role of a transmitter, and a carbon microphone, fixed from the inside on the ship's hull, served as a signal receiver. The signals were recorded by the Morse apparatus and by ear. Later, the siren was replaced by a membrane-type emitter. The efficiency of the device, called a hydrophonic station, has increased significantly. Sea trials of the new station took place in March 1908. on the Black Sea, where the range of reliable signal reception exceeded 10 km.

The first serial stations for sound underwater communication designed by the Baltic Shipyard in 1909-1910. installed on submarines "Carp", "Gudgeon", "Sterlet", « Mackerel" and " Perch» . When installing stations on submarines, in order to reduce interference, the receiver was located in a special fairing towed astern on a cable-cable. The British came to a similar decision only during the First World War. Then this idea was forgotten, and only at the end of the 1950s it was again used in different countries when creating noise-resistant sonar ship stations.

The impetus for the development of hydroacoustics was the First World War. During the war, the Entente countries suffered heavy losses in the merchant and navy due to the actions of German submarines. There was a need to find means to combat them. They were soon found. A submarine in a submerged position can be heard by the noise generated by the propellers and operating mechanisms. A device that detects noisy objects and determines their location was called a noise direction finder. The French physicist P. Langevin in 1915 suggested using a sensitive receiver made of Rochelle salt for the first noise direction finding station.

Fundamentals of hydroacoustics

Features of the propagation of acoustic waves in water

Components of an echo occurrence event.

The beginning of comprehensive and fundamental research on the propagation of acoustic waves in water was laid during the Second World War, which was dictated by the need to solve the practical problems of the navies and, first of all, submarines. Experimental and theoretical work was continued in the postwar years and summarized in a number of monographs. As a result of these works, some features of the propagation of acoustic waves in water were identified and refined: absorption, attenuation, reflection and refraction.

The absorption of acoustic wave energy in sea water is caused by two processes: the internal friction of the medium and the dissociation of salts dissolved in it. The first process converts the energy of an acoustic wave into thermal energy, and the second process, being converted into chemical energy, brings the molecules out of equilibrium, and they decay into ions. This type of absorption increases sharply with an increase in the frequency of the acoustic vibration. The presence of suspended particles, microorganisms and temperature anomalies in the water also leads to the attenuation of the acoustic wave in the water. As a rule, these losses are small, and they are included in the total absorption, however, sometimes, as, for example, in the case of scattering from the wake of a ship, these losses can be up to 90%. The presence of temperature anomalies leads to the fact that the acoustic wave enters the zones of the acoustic shadow, where it can undergo multiple reflections.

The presence of water-air and water-bottom interfaces leads to the reflection of an acoustic wave from them, and if in the first case the acoustic wave is completely reflected, then in the second case the reflection coefficient depends on the bottom material: it poorly reflects the muddy bottom, well - sandy and rocky . At shallow depths, due to the repeated reflection of an acoustic wave between the bottom and the surface, an underwater sound channel arises, in which the acoustic wave can propagate over long distances. Changing the value of the speed of sound at different depths leads to the curvature of the sound "rays" - refraction.

Refraction of sound (curvature of the path of the sound beam)

Sound refraction in water: a - in summer; b - in winter; on the left - change in speed with depth. The speed of sound propagation varies with depth, and the changes depend on the time of year and day, the depth of the reservoir, and a number of other reasons. Sound rays emerging from a source at a certain angle to the horizon are bent, and the direction of the bend depends on the distribution of sound velocities in the medium: in summer, when the upper layers are warmer than the lower ones, the rays bend downward and are mostly reflected from the bottom, while losing a significant portion of their energy ; in winter, when the lower layers of water maintain their temperature, while the upper layers cool, the rays bend upward and are repeatedly reflected from the surface of the water, with much less energy being lost. Therefore, in winter, the sound propagation distance is greater than in summer. The vertical sound velocity distribution (VSDS) and the velocity gradient have a decisive influence on the propagation of sound in the marine environment. The distribution of the speed of sound in different regions of the World Ocean is different and varies with time. There are several typical cases of VRSZ:

Scattering and absorption of sound by inhomogeneities of the medium.

Propagation of sound in underwater sound. channel: a - change in the speed of sound with depth; b - path of rays in the sound channel. The propagation of high-frequency sounds, when the wavelengths are very small, is influenced by small inhomogeneities, usually found in natural reservoirs: gas bubbles, microorganisms, etc. These inhomogeneities act in two ways: they absorb and scatter the energy of sound waves. As a result, with an increase in the frequency of sound vibrations, the range of their propagation is reduced. This effect is especially noticeable in the surface layer of water, where there are the most inhomogeneities. Scattering of sound by heterogeneities, as well as irregularities in the surface of the water and the bottom, causes the phenomenon of underwater reverberation that accompanies the sending of a sound pulse: sound waves, reflecting from a combination of heterogeneities and merging, give a tightening of the sound pulse, which continues after its end. The limits of the range of propagation of underwater sounds are also limited by the own noises of the sea, which have a dual origin: some of the noises arise from the impact of waves on the surface of the water, from the sea surf, from the noise of rolling pebbles, etc.; the other part is associated with marine fauna (sounds produced by hydrobionts: fish and other marine animals). Biohydroacoustics deals with this very serious aspect.

Distance of propagation of sound waves

The range of propagation of sound waves is a complex function of the radiation frequency, which is uniquely related to the wavelength of the acoustic signal. As is known, high-frequency acoustic signals are rapidly attenuated due to strong absorption by the aquatic environment. Low-frequency signals, on the contrary, are capable of propagating in the aquatic environment over long distances. So an acoustic signal with a frequency of 50 Hz is capable of propagating in the ocean for distances of thousands of kilometers, while a signal with a frequency of 100 kHz, typical for side-scan sonar, has a propagation range of only 1-2 km. The approximate ranges of modern sonars with different frequencies of the acoustic signal (wavelength) are given in the table:

Areas of use.

Hydroacoustics has received wide practical application, since no effective system has yet been created for transmitting electromagnetic waves under water at any significant distance, and therefore sound is the only possible means of communication under water. For these purposes, sound frequencies from 300 to 10,000 Hz and ultrasounds from 10,000 Hz and above are used. Electrodynamic and piezoelectric emitters and hydrophones are used as emitters and receivers in the sound region, and piezoelectric and magnetostrictive ones are used in the ultrasonic region.

The most significant applications of hydroacoustics are:

• To solve military problems;
• Maritime navigation;
• Sound underwater communication;
• Fish-searching reconnaissance;
• Oceanological research;
• Areas of activity for the development of the wealth of the bottom of the oceans;
• Use of acoustics in the pool (at home or in a synchronized swimming training center)
• Marine animal training.

Notes

Literature and sources of information

LITERATURE:

• V.V. Shuleikin Physics of the sea. - Moscow: "Nauka", 1968. - 1090 p.
• I.A. Romanian Fundamentals of hydroacoustics. - Moscow: "Shipbuilding", 1979. - 105 p.
• Yu.A. Koryakin Hydroacoustic systems. - St. Petersburg: "Science of St. Petersburg and the naval power of Russia", 2002. - 416 p.

Sound travels through sound waves. These waves pass not only through gases and liquids, but also through solids. The action of any waves is mainly in the transfer of energy. In the case of sound, transport takes the form of minute movements at the molecular level.

In gases and liquids, a sound wave shifts molecules in the direction of its movement, that is, in the direction of the wavelength. In solids, sound vibrations of molecules can also occur in the direction perpendicular to the wave.

Sound waves propagate from their sources in all directions, as shown in the figure to the right, which shows a metal bell periodically colliding with its tongue. These mechanical collisions cause the bell to vibrate. The energy of vibrations is imparted to the molecules of the surrounding air, and they are pushed away from the bell. As a result, pressure increases in the air layer adjacent to the bell, which then spreads in waves in all directions from the source.

The speed of sound is independent of volume or tone. All sounds from the radio in the room, whether loud or soft, high or low, reach the listener at the same time.

The speed of sound depends on the type of medium in which it propagates and on its temperature. In gases, sound waves travel slowly because their rarefied molecular structure does little to oppose compression. In liquids, the speed of sound increases, and in solids it becomes even faster, as shown in the diagram below in meters per second (m/s).

wave path

Sound waves propagate in air in a manner similar to that shown in the diagrams to the right. Wave fronts move from the source at a certain distance from each other, determined by the frequency of the bell's oscillations. The frequency of a sound wave is determined by counting the number of wavefronts that pass through a given point per unit time.

The sound wave front moves away from the vibrating bell.

In uniformly heated air, sound travels at a constant speed.

The second front follows the first at a distance equal to the wavelength.

The sound intensity is maximum near the source.

Graphic representation of an invisible wave

Sound sounding of the depths

A beam of sonar beams, consisting of sound waves, easily passes through ocean water. The principle of operation of sonar is based on the fact that sound waves bounce off the ocean floor; this device is usually used to determine the features of the underwater relief.

Elastic solids

Sound propagates in a wooden plate. The molecules of most solids are bound into an elastic spatial lattice, which is poorly compressed and at the same time accelerates the passage of sound waves.