Sea water is an acoustically inhomogeneous medium. Heterogeneity sea water is to change the density with depth, the presence of gas bubbles, suspended particles and plankton in the water. Therefore, the distribution acoustic oscillations (sound) in sea water is a complex phenomenon that depends on the distribution of density (temperature, salinity, pressure), sea depth, nature of the soil, the state of the sea surface, water turbidity with suspended impurities of organic and inorganic origin and the presence of dissolved gases.
Sound in a broad sense is the oscillatory motion of particles of an elastic medium, propagating in the form of waves in a gaseous, liquid or solid medium; in a narrow sense - a phenomenon subjectively perceived by a special sense organ of man and animals. A person hears sound with a frequency of 16 Hz to 16-20 × 10 3 Hz . The physical concept of sound covers both audible and inaudible sounds. Sound below 16 Hz called infrasound , above 20 × 10 3 Hz - ultrasound ; the highest frequency acoustic vibrations in the range from 10 9 to 10 12 -10 13 Hz belong to hypersound.
The propagation of sound in water is a periodic compression and rarefaction of water in the direction of the sound wave. The rate of transmission of vibrational motion from one water particle to another is called the speed of sound.
The theoretical formula for the speed of sound for liquids and gases is: с = , where α is the specific volume, γ= 
- the ratio of the heat capacity of water at constant pressure c p to the heat capacity of water at constant volume c v , approximately equal to one, k is the true coefficient of compressibility of sea water.
With an increase in water temperature, the speed of sound increases both due to an increase in the specific volume and due to a decrease in the compressibility coefficient. Therefore, the influence of temperature on the speed of sound is the greatest in comparison with other factors. When the salinity of water changes, the specific volume and compressibility coefficient also change. But the corrections for the speed of sound from these changes have different signs. Therefore, the effect of salinity change on the speed of sound is less than the effect of temperature. Hydrostatic pressure affects only the vertical change in the speed of sound; with depth, the speed of sound increases.
The speed of sound does not depend on the strength of the sound source.
According to the theoretical formula, tables have been compiled that make it possible to determine the speed of sound from the temperature and salinity of water and correct it for pressure. However, the theoretical formula gives values of the speed of sound that differ from those measured by an average of ±4 m·s -1 . Therefore, in practice, empirical formulas are used, of which the formulas are most widely used Del Grosso and W. Wilson, providing the smallest errors.
The error in the speed of sound, calculated by the Del-Grosso formula, does not exceed 0.5 m·s -1 for waters with a salinity greater than 15‰ and 0.8 m·s -1 for waters with a salinity of less than 15‰.
Wilson's formula, proposed by him in 1960, gives higher accuracy than Del Grosso's formula. It is built on the principle of constructing the Bjerknes formula for calculating the conditional specific volume in situ and has the form:
c = 1449.14 + δс p + δc t + δc s + δс stp ,
where δc p is the correction for pressure, δc t is the correction for temperature, δc s is the correction for salinity, and δc stp is the combined correction for pressure, temperature and salinity.
The root-mean-square error in calculating the speed of sound using the Wilson formula is 0.3 m·s -1 .
In 1971, another formula was proposed for calculating the speed of sound from the measured values of T, S and P and slightly different correction values:
c = 1449.30 + δс p + δc t + δc s + δс stp ,
When measuring depths with an echo sounder, the speed of sound averaged over the layers is calculated, which is called the vertical speed of sound. It is determined by the formula with stp 
,
where c i is the average speed of sound in a layer of thickness h i .
The speed of sound in sea water at a temperature of 13 0 C, a pressure of 1 atm and a salinity of 35‰ is 1494 m s -1; as already mentioned, it increases with increasing temperature (3 m s -1 per 1 0 C), salinity (1.3 m s -1 per 1 ‰) and pressure (0.016 m s -1 per 1 m of depth) . It is about 4.5 times the speed of sound in the atmosphere (334 m s -1). The average sound speed in the World Ocean is about 1500 m s -1 , and the range of its variability is from 1430 to 1540 m s -1 on the ocean surface and from 1570 to 1580 m s -1 at depths of more than 7 km.
Sound travels 4.5 times faster in sea water than in air. The speed of its propagation depends on temperature, salinity and pressure. With an increase in any of these factors, the speed of sound increases.
How is the speed of sound measured?
It can be calculated by knowing temperature, salinity and depth - the three main characteristics measured at oceanographic stations. For many years this method was the only one. AT last years the speed of sound in sea water began to be measured directly. Sound speed meters work on the principle of measuring the length of time for which a sound pulse travels a certain distance.
How far can sound travel in the ocean?
Sound vibrations from an underwater explosion produced by the Columbia University research vessel Vema in 1960 were recorded at a distance of 12,000 miles. A depth charge was detonated in an underwater sound channel off the coast of Australia, and after about 144 minutes, sound vibrations reached Bermuda, that is, almost the opposite point of the globe.
What is an audio channel?
This is a zone in which the speed of sound first decreases with depth to a certain minimum, and then increases due to an increase in pressure. Excited in this zone sound waves they cannot get out of it, as they return to the axis of the channel by bending. Once in such a channel, sound can travel thousands of miles.
What is SOFAR?
It's an abbreviation English words"sound fixing and ranging" (detection of sound sources and measuring the distance to them). The SOFAR system uses a sound channel at depths of 600 - 1200 m. By notches from several receiving stations, it is possible to determine the location of the sound source in this channel with an accuracy of 1 mile. During the Second World War, with the help of this system, it was possible to save many pilots shot down over the sea. Their planes had small bombs that exploded under pressure when they reached the depth of the sound channel.
What is a sonar?
Sonar works on the same principle as radar, but instead of radio waves, it uses sound (acoustic) waves. Sonar can be active or passive. An active system emits sound vibrations and receives a reflected signal, or echo. To determine the distance, one must take half the product of the speed of sound and the time elapsed between the emission of a sound pulse and the reception of a reflected signal. The passive system works in listening mode, and it can only determine the direction in which the sound source is located. Sonar is used for submarine detection, navigation, finding schools of fish, and for determining depth. In the latter case, the sonar is a conventional echo sounder.
What is refraction and reflection of sound waves?
Due to differences in the density of sea water, sound waves in the ocean do not propagate in a straight line. Their direction is bent due to a change in the speed of sound in water. This phenomenon is called refraction. In addition, sound energy is scattered on suspensions and marine organisms, reflected from the surface and the bottom and scattered on them, and, finally, is attenuated when propagating through the water column.
What causes the sounds of the sea?
Sea noise includes the sounds of waves and surf, noise caused by precipitation, seismic and volcanic activity, and finally the sounds made by fish and other marine organisms. Noises caused by the movement of the vessel, the operation of mechanisms that extract minerals, as well as noise generated during underwater and surface oceanographic works that occur outside the platforms themselves and measuring equipment, are also considered marine noise.
Waves, tides, currents
Why do waves arise?
Those waves; which we are accustomed to seeing on the surface of the water, are formed mainly by the action of the wind. However, waves can also be caused by other causes: underwater earthquakes or underwater volcanic eruptions. Tides are also waves.
Sound is one of the components of our life, and a person hears it everywhere. In order to consider this phenomenon in more detail, we first need to understand the concept itself. To do this, you need to turn to the encyclopedia, where it is written that “sound is elastic waves, propagating in some elastic medium and creating in it mechanical vibrations". Speaking more plain language are the audible vibrations in a medium. The main characteristics of the sound depend on what it is. First of all, the speed of propagation, for example, in water is different from another medium.
Any audio analogue has certain properties(physical features) and qualities (reflection of these signs in human sensations). For example, duration-duration, frequency-pitch, composition-timbre, and so on.
The speed of sound in water is much higher than, say, in air. Therefore, it spreads faster and is much farther audible. This happens because of the high molecular density of the aqueous medium. It is 800 times denser than air and steel. It follows that the propagation of sound depends largely on the medium. Let's look at specific numbers. So, the speed of sound in water is 1430 m/s, in air - 331.5 m/s.
Low-frequency sound, such as the noise that a ship's engine makes, is always heard a little before the ship enters the field of view. Its speed depends on several things. If the temperature of the water rises, then naturally the speed of sound in the water rises. The same happens with an increase in water salinity and pressure, which increases with increasing depth of the water space. Such a phenomenon as thermal wedges can have a special role on speed. These are places where layers of water of different temperatures meet.
Also in such places it is different (due to the difference in temperature conditions). And when sound waves pass through such layers of different density, they lose most of their strength. Faced with a thermocline, the sound wave is partially, and sometimes completely, reflected (the degree of reflection depends on the angle at which the sound falls), after which, on the other side of this place, a shadow zone is formed. If we consider an example when a sound source is located in the water space above the thermocline, then it will be almost impossible to hear something even lower.
Which are published above the surface, are never heard in the water itself. And vice versa happens when under the water layer: it does not sound above it. bright volume an example is modern divers. Their hearing is greatly reduced due to the fact that water affects and the high speed of sound in water reduces the quality of determining the direction from which it is moving. This dulls the stereophonic ability to perceive sound.
Under a layer of water, they enter the human ear most of all through the bones of the cranium of the head, and not, as in the atmosphere, through the eardrums. The result of this process is its perception simultaneously by both ears. The human brain is not able at this time to distinguish the places where the signals come from, and in what intensity. The result is the emergence of consciousness that the sound, as it were, rolls from all sides at the same time, although this is far from being the case.
In addition to the above, sound waves in the water space have such qualities as absorption, divergence and scattering. The first is when the strength of sound in salt water gradually disappears due to the friction of the aquatic environment and the salts in it. Divergence is manifested in the removal of sound from its source. It seems to dissolve in space like light, and as a result, its intensity drops significantly. And fluctuations completely disappear due to scattering on all sorts of obstacles, inhomogeneities of the medium.
Over long distances, sound energy propagates only along gentle rays, which do not touch the ocean floor all the way. In this case, the limitation imposed by the medium on the range of sound propagation is its absorption in sea water. The main mechanism of absorption is associated with relaxation processes that accompany the violation of the thermodynamic equilibrium between ions and molecules of salts dissolved in water by an acoustic wave. It should be noted that the main role in absorption in a wide range of sound frequencies belongs to the magnesium sulphide salt MgSO4, although its percentage in sea water is quite small - almost 10 times less than, for example, rock salt NaCl, which nevertheless does not play any significant role in the absorption of sound.
Absorption in sea water, generally speaking, is greater the higher the frequency of the sound. At frequencies from 3-5 to at least 100 kHz, where the above mechanism dominates, the absorption is proportional to the frequency to a power of about 3/2. At lower frequencies, a new absorption mechanism is activated (possibly due to the presence of boron salts in water), which becomes especially noticeable in the range of hundreds of hertz; here, the absorption level is anomalously high and decreases much more slowly with decreasing frequency.
To more clearly imagine the quantitative characteristics of absorption in sea water, we note that due to this effect, sound with a frequency of 100 Hz is attenuated by a factor of 10 on a path of 10 thousand km, and with a frequency of 10 kHz - at a distance of only 10 km (Fig. 2). Thus, only low-frequency sound waves can be used for long-range underwater communications, for long-range detection of underwater obstacles, and the like.
Figure 2 - Distances at which sounds of different frequencies attenuate 10 times when propagating in sea water.
In the region of audible sounds for the frequency range of 20-2000 Hz, the range of propagation under water of sounds of medium intensity reaches 15-20 km, and in the region of ultrasound - 3-5 km.
Based on the values of sound attenuation observed in laboratory conditions in small volumes of water, one would expect much greater ranges. However, under natural conditions, in addition to damping due to the properties of water itself (the so-called viscous damping), its scattering and absorption by various inhomogeneities of the medium also affect.
The refraction of sound, or the curvature of the path of the sound beam, is caused by the heterogeneity of the properties of water, mainly along the vertical, due to three main reasons: changes in hydrostatic pressure with depth, changes in salinity, and changes in temperature due to uneven heating of the water mass by the sun's rays. As a result of the combined action of these causes, the speed of sound propagation, which is about 1450 m / s for fresh water and about 1500 m / s for sea water, changes with depth, and the law of change depends on the season, time of day, depth of the reservoir, and a number of other reasons . Sound rays leaving the source at some 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 down and are mostly reflected from the bottom, losing a significant portion of their energy. On the contrary, in winter, when the lower layers of the water maintain their temperature, while the upper layers cool, the rays bend upward and undergo multiple reflections from the surface of the water, during which much less energy is lost. Therefore, in winter, the sound propagation distance is greater than in summer. Due to refraction, so-called. dead zones, i.e. areas located close to the source in which there is no audibility.
The presence of refraction, however, can lead to an increase in the range of sound propagation - the phenomenon of ultra-long propagation of sounds under water. At some depth below the surface of the water there is a layer in which sound propagates at the lowest speed; above this depth, the speed of sound increases due to an increase in temperature, and below this, due to an increase in hydrostatic pressure with depth. This layer is a kind of underwater sound channel. A beam deviated from the axis of the channel up or down, due to refraction, always tends to get back into it. If a sound source and receiver are placed in this layer, then even sounds of medium intensity (for example, explosions of small charges of 1-2 kg) can be recorded at distances of hundreds and thousands of kilometers. A significant increase in the sound propagation range in the presence of an underwater sound channel can be observed when the sound source and receiver are located not necessarily near the channel axis, but, for example, near the surface. In this case, the rays, refracting downwards, enter the deep layers, where they deviate upwards and come out again to the surface at a distance of several tens of kilometers from the source. Further, the pattern of propagation of rays is repeated, and as a result, a sequence of so-called. secondary illuminated zones, which are usually traced to distances of several hundred km.
The propagation of high-frequency sounds, in particular ultrasounds, when the wavelengths are very small, is influenced by small inhomogeneities that are usually found in natural reservoirs: microorganisms, gas bubbles, 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 pronounced in surface layer water, where the most inhomogeneities. Scattering of sound by inhomogeneities, as well as by irregularities in the water surface and the bottom, causes the phenomenon of underwater reverberation that accompanies the sending of a sound pulse: sound waves, reflecting from a combination of inhomogeneities and merging, give a tightening of the sound pulse, which continues after its end, similar to reverberation observed in enclosed spaces. Underwater reverberation is a rather significant interference for a number of practical applications of hydroacoustics, in particular for sonar.
The limits of the propagation range of underwater sounds are also limited by the so-called. own noises of the sea, which have a twofold origin. Part of the noise arises from the impact of waves on the surface of the water, from the surf, from the noise of rolling pebbles, etc. The other part is related to the marine fauna; this includes sounds produced by fish and other marine animals.