Why does sound travel faster in water than in air? How sound travels in space.

Propagation of sound in water

SPEARFISHING

Propagation of sound in water .

Sound travels five times faster in water than in air. average speed equals 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 bones of the skull. 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 sound waves from water to the bones of the human head is less than in the 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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Most people are well aware of what sound is. It is associated with hearing and is associated with physiological and psychological processes. In the brain, the processing of sensations that come through the hearing organs is carried out. The speed of sound depends on many factors.

Sounds that humans hear

In the general sense of the word, sound is physical phenomenon, which causes an effect on the hearing organs. It has the form of longitudinal waves of different frequencies. Humans can hear sound whose frequency ranges from 16-20,000 Hz. These elastic longitudinal waves, which propagate not only in the air, but also in other media, reaching the human ear, cause sound sensations. People can't hear everything. Elastic waves with a frequency of less than 16 Hz are called infrasound, and above 20,000 Hz - ultrasound. Their human ear cannot hear.

Sound characteristics

There are two main characteristics of sound: loudness and pitch. The first of them is related to the intensity of the elastic sound wave. There is another important indicator. Physical quantity, which characterizes the height, is the oscillation frequency of the elastic wave. In this case, one rule applies: the larger it is, the higher the sound, and vice versa. Another important characteristic is the speed of sound. AT different environments oh it is different. It represents the propagation speed of elastic sound waves. In a gaseous environment, this indicator will be less than in liquids. The speed of sound in solids is the highest. Moreover, for longitudinal waves it is always greater than for transverse ones.

Sound Wave Velocity

This indicator depends on the density of the medium and its elasticity. In gaseous media, it is affected by the temperature of the substance. As a rule, the speed of sound does not depend on the amplitude and frequency of the wave. In rare cases, when these characteristics have an influence, one speaks of the so-called dispersion. The speed of sound in vapors or gases ranges from 150-1000 m/s. In liquid media, it is already 750-2000 m/s, and in solid materials - 2000-6500 m/s. AT normal conditions the speed of sound in air reaches 331 m/s. In ordinary water - 1500 m / s.

The speed of sound waves in different chemical media

The speed of sound propagation in different chemical media is not the same. So, in nitrogen it is 334 m / s, in air - 331, in acetylene - 327, in ammonia - 415, in hydrogen - 1284, in methane - 430, in oxygen - 316, in helium - 965, in carbon monoxide - 338, in carbonic acid - 259, in chlorine - 206 m/s. The speed of a sound wave in gaseous media increases with increasing temperature (T) and pressure. In liquids, it most often decreases with an increase in T by several meters per second. Sound speed (m/s) in liquid media (at 20°C):

Water - 1490;

Ethyl alcohol - 1180;

Benzene - 1324;

Mercury - 1453;

Carbon tetrachloride - 920;

Glycerin - 1923.

The only exception to this rule is water, in which the speed of sound also increases with increasing temperature. It reaches its maximum when this liquid is heated to 74°C. As the temperature rises further, the speed of sound decreases. With an increase in pressure, it will increase by 0.01% / 1 Atm. in salty sea water as temperature, depth, and salinity increase, so does the speed of sound. In other environments, this indicator varies in different ways. So, in a mixture of liquid and gas, the speed of sound depends on the concentration of its components. In an isotopic solid, it is determined by its density and elastic moduli. In unlimited dense environments transverse (shear) and longitudinal elastic waves propagate. Sound speed (m/s) in solids (longitudinal/transverse wave):

Glass - 3460-4800/2380-2560;

Fused quartz - 5970/3762;

Concrete - 4200-5300/1100-1121;

Zinc - 4170-4200/2440;

Teflon - 1340/*;

Iron - 5835-5950/*;

Gold - 3200-3240/1200;

Aluminum - 6320/3190;

Silver - 3660-3700/1600-1690;

Brass - 4600/2080;

Nickel - 5630/2960.

In ferromagnets, the speed of a sound wave depends on the strength of the magnetic field. In single crystals, the speed of a sound wave (m/s) depends on the direction of its propagation:

ruby (longitudinal wave) - 11240;
cadmium sulfide (longitudinal / transverse) - 3580/4500;
lithium niobate (longitudinal) - 7330.

The speed of sound in a vacuum is 0, because it simply does not propagate in such an environment.

Determining the speed of sound

Everything related to sound signals interested our ancestors thousands of years ago. Almost all prominent scientists worked on the definition of the essence of this phenomenon. ancient world. Even ancient mathematicians found that sound is caused by the oscillatory movements of the body. Euclid and Ptolemy wrote about it. Aristotle established that the speed of sound differs by a finite value. The first attempts to determine this indicator were made by F. Bacon in the 17th century. He tried to establish the speed by comparing the time intervals between the sound of a shot and a flash of light. Based on this method, a group of physicists at the Paris Academy of Sciences determined the speed of a sound wave for the first time. Under various experimental conditions, it was 350–390 m/s. The theoretical substantiation of the speed of sound for the first time in his "Principles" was considered by I. Newton. P.S. succeeded in making the correct determination of this indicator. Laplace.

Formulas for the speed of sound

For gaseous media and liquids, in which sound propagates, as a rule, adiabatically, the temperature change associated with tensions and compressions in longitudinal wave, cannot quickly level out for short period time. Obviously, this figure is influenced by several factors. The speed of a sound wave in a homogeneous gaseous medium or liquid is determined by the following formula:

where β is the adiabatic compressibility, ρ is the density of the medium.

In partial derivatives, this value is calculated according to the following formula:

c 2 \u003d -υ 2 (δρ / δυ) S \u003d -υ 2 Cp / Cυ (δρ / δυ) T,

where ρ, T, υ are the pressure of the medium, its temperature and specific volume; S - entropy; Cp - isobaric heat capacity; Cυ - isochoric heat capacity. For gaseous media, this formula will look like this:

c 2 = ζkT/m= ζRt/M = ζR(t + 273.15)/M = ά 2 T,

where ζ is the adiabat value: 4/3 for polyatomic gases, 5/3 for monatomic gases, 7/5 for diatomic gases (air); R - gas constant (universal); T- absolute temperature, measured in kelvins; k - Boltzmann's constant; t - temperature in °C; M- molar mass; m- molecular mass; ά 2 = ζR/M.

Determination of the speed of sound in a solid body

In a solid body with homogeneity, there are two types of waves that differ in the polarization of oscillations in relation to the direction of their propagation: transverse (S) and longitudinal (P). The speed of the first (C S) will always be lower than the second (C P):

C P 2 = (K + 4/3G)/ρ = E(1 - v)/(1 + v)(1-2v)ρ;

C S 2 = G/ρ = E/2(1 + v)ρ,

where K, E, G - moduli of compression, Young, shear; v - Poisson's ratio. When calculating the speed of sound in a solid body, adiabatic moduli of elasticity are used.

Speed of sound in multiphase media

In multiphase media, due to the inelastic absorption of energy, the speed of sound is directly dependent on the frequency of vibrations. In a two-phase porous medium, it is calculated using the Biot-Nikolaevsky equations.

Conclusion

The measurement of sound wave speed is used in determining various properties of substances, such as the moduli of elasticity of a solid, the compressibility of liquids and gases. A sensitive method for the determination of impurities is the measurement of small changes in the speed of the sound wave. In solids, the fluctuation of this index makes it possible to study the band structure of semiconductors. The speed of sound is a very important quantity, the measurement of which allows you to learn a lot about a variety of media, bodies and other objects. scientific research. Without the ability to determine it, many scientific discoveries would be impossible.

If a sound wave encounters no obstacles in its path, it propagates uniformly in all directions. But not every obstacle becomes an obstacle for her.

Having met an obstacle in its path, the sound can bend around it, be reflected, refracted or absorbed.

sound diffraction

We can talk to a person standing around the corner of a building, behind a tree, or behind a fence, although we cannot see him. We hear it because the sound is able to bend around these objects and penetrate into the area behind them.

The ability of a wave to go around an obstacle is called diffraction .

Diffraction is possible when the wavelength of the sound wave exceeds the size of the obstacle. Low frequency sound waves are quite long. For example, at a frequency of 100 Hz, it is 3.37 m. As the frequency decreases, the length becomes even longer. Therefore, a sound wave easily bends around objects commensurate with it. The trees in the park do not prevent us from hearing the sound at all, because the diameters of their trunks are much smaller than the wavelength of the sound wave.

Due to diffraction, sound waves penetrate through gaps and holes in an obstacle and propagate behind them.

Let us place a flat screen with a hole in the path of the sound wave.

When the sound wave length ƛ much larger than the hole diameter D , or these values are approximately equal, then behind the hole the sound will reach all points of the area that is behind the screen (the area of sound shadow). The outgoing wave front will look like a hemisphere.

If ƛ only slightly smaller than the slot diameter, then the main part of the wave propagates directly, and not most of diverges slightly to the sides. And in the case when ƛ much less D , the whole wave will go in the forward direction.

sound reflection

If a sound wave hits the interface between two media, it is possible different variants her further dissemination. The sound can be reflected from the interface, it can go to another medium without changing direction, or it can be refracted, that is, go, changing its direction.

Let's suppose that an obstacle has appeared in the path of the sound wave, the size of which is much larger than the wavelength, for example, a sheer cliff. How will the sound behave? Since it cannot go around this obstacle, it will be reflected from it. Behind the obstacle is acoustic shadow zone .

Sound reflected from an obstacle is called echo .

The nature of the reflection of the sound wave can be different. It depends on the shape of the reflective surface.

reflection called a change in the direction of a sound wave at the interface between two different media. When reflected, the wave returns to the medium from which it came.

If the surface is flat, the sound is reflected from it in the same way as a ray of light is reflected in a mirror.

Sound rays reflected from a concave surface are focused at one point.

The convex surface dissipates sound.

The effect of dispersion is given by convex columns, large moldings, chandeliers, etc.

Sound does not pass from one medium to another, but is reflected from it if the densities of the media differ significantly. So, the sound that appeared in the water does not pass into the air. Reflected from the interface, it remains in the water. A person standing on the river bank will not hear this sound. This is due to the large difference in wave resistance of water and air. In acoustics, wave resistance is equal to the product of the density of the medium and the speed of sound in it. Since the wave resistance of gases is much less than the wave resistance of liquids and solids, when it hits the border of air and water, the sound wave is reflected.

Fish in the water do not hear the sound that appears above the surface of the water, but they clearly distinguish the sound, the source of which is a body vibrating in the water.

refraction of sound

Changing the direction of sound propagation is called refraction . This phenomenon occurs when sound passes from one medium to another, and the speed of its propagation in these media is different.

The ratio of the sine of the angle of incidence to the sine of the angle of reflection is equal to the ratio of the speeds of sound propagation in media.

where i - angle of incidence,

r is the angle of reflection,

v1 is the speed of sound propagation in the first medium,

v2 is the speed of sound propagation in the second medium,

n is the index of refraction.

The refraction of sound is called refraction .

If the sound wave does not fall perpendicular to the surface, but at an angle other than 90°, then the refracted wave will deviate from the direction of the incident wave.

Sound refraction can be observed not only at the interface between media. Sound waves can change their direction in an inhomogeneous medium - the atmosphere, the ocean.

In the atmosphere, refraction is caused by changes in air temperature, speed and direction of movement air masses. And in the ocean, it appears due to the heterogeneity of the properties of water - different hydrostatic pressure at different depths, different temperatures and different salinity.

sound absorption

When a sound wave hits a surface, some of its energy is absorbed. And how much energy a medium can absorb can be determined by knowing the sound absorption coefficient. This coefficient shows what part of the energy of sound vibrations is absorbed by 1 m 2 of the obstacle. It has a value from 0 to 1.

The unit of measure for sound absorption is called sabin . It got its name from the American physicist Wallace Clement Sabin, founder of architectural acoustics. 1 sabin is the energy that is absorbed by 1 m 2 of the surface, the absorption coefficient of which is equal to 1. That is, such a surface must absorb absolutely all the energy of the sound wave.

Reverberation

Wallace Sabin

The property of materials to absorb sound is widely used in architecture. While researching the acoustics of the Lecture Hall, part of the Fogg Museum, Wallace Clement Sabin concluded that there was a relationship between the size of the auditorium, the acoustic conditions, the type and area of sound-absorbing materials, and reverberation time .

Reverb called the process of reflection of a sound wave from obstacles and its gradual attenuation after turning off the sound source. In an enclosed space, sound can bounce off walls and objects multiple times. As a result, various echo signals appear, each of which sounds as if apart. This effect is called reverb effect .

most important characteristic premises is reverberation time , which was introduced and calculated by Sabin.

where V - the volume of the room,

BUT – general sound absorption.

where a i is the sound absorption coefficient of the material,

Si is the area of each surface.

If the reverberation time is long, the sounds seem to "roam" around the room. They overlap each other, drown out the main source of sound, and the hall becomes booming. With a short reverberation time, the walls quickly absorb sounds, and they become deaf. Therefore, each room must have its own exact calculation.

Based on the results of his calculations, Sabin arranged the sound-absorbing materials in such a way that the "echo effect" was reduced. And the Boston Symphony Hall, on which he was an acoustic consultant, is still considered one of the finest halls in the world.

Questions.

1. With what frequency does the human eardrum vibrate when sound reaches it?

The human eardrum vibrates with the frequency of the sound that comes to it.

2. What kind of wave - longitudinal or transverse - is sound propagating in air? in water?

In air and water, sound travels in longitudinal waves.

3. Give an example showing that a sound wave does not propagate instantly, but at a certain speed.

The most obvious example is a flash of lightning followed by thunder.

4. What is the speed of sound propagation in air at 20 °C?

The speed of sound propagation in air at 20°C is 343 m/s 2 .

5. Does the speed of sound depend on the medium in which it propagates?

V = 340 m/s. Yes, it depends.

Exercises.

1. Determine the speed of sound in water if a source oscillating with a period of 0.002 s excites waves of 2.9 m in length in water.

2. Determine the length of the 725 Hz sound wave in air, water and glass.

3. One end of a long metal pipe was hit once with a hammer. Will the sound from the impact propagate to the other end of the pipe through the metal? through the air inside the pipe? How many blows will a person standing at the other end of the pipe hear?

The person will hear two hits. One sound will come to him through a metal pipe, and another through the air.

4. Observer standing near straight section railway, I saw steam above the whistle of a steam locomotive going in the distance. 2s after the appearance of steam, he heard the sound of a whistle, and after 34 s the locomotive passed by the observer. Determine the speed of the locomotive.

5. The observer moves away from the bell, which is struck every second. At first, the visible and audible beats coincide. Then they stop matching. Then, at some distance of the observer from the bell, the visible and audible strikes coincide again. Explain this phenomenon.

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/sec. Sound vibration frequency (nude) is related to the wavelength (lambda) by the relation: 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 approximately 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 shown by the American hydroacousticist Wenz (Wenz, 1962), 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. Big influence at the same time, the composition of sounds is affected by the bottom and shores.

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, in 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. How more difference acoustic impedance of media, the greater part of the energy will be reflected from the separation of 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, a tuning fork is brought into oscillation, bringing it closer and further away from the wall, one can hear the increase and decrease in the sound volume 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 relatively long sounding source.

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 heterogeneity 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 Bergeldzhik (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 great importance for the correct assessment of fish responses to underwater sound sources.

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 a 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 significantly more sizes used underwater emitters. The same phenomenon is typical for fish emitters. In these cases, the radiation patterns of the emitters are absent. In this chapter, only some general physical properties sound in aquatic environment in relation to fish bioacoustics. 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.

Sound intensity 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 by 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.