Sound propagation. Parameters that characterize the sound field. Parameters characterizing the sound field Physical quantities characterizing the sound field
In the environment. The concept "Z. P." it is usually used for areas whose dimensions are of the order of or more than the length of the sound. waves. With energetic. side Z. p. is characterized by sound density. energy (the energy of the vibrating process per unit of volume); in those cases when it occurs in the sound field, it is characterized by the intensity of sound.
The picture of Z. p. In the general case depends not only on the acoustic. power and directivity characteristics of the emitter - the sound source, but also from the position and sv-in the boundaries of the medium and the interfaces decomp. elastic media, if any. In an unrestricted homogeneous environment, the Z. p. Of a single source is field of a traveling wave. Microphones, hydrophones, etc. are used to measure the salary level; it is desirable to have their dimensions small in comparison with the wavelength and with the characteristic dimensions of the field inhomogeneities. In the study of Z. n. Are also used decomp. methods of visualization of sound fields. Study of Z. p. Decomp. emitters are produced in damped chambers.
Physical encyclopedic dictionary. - M .: Soviet encyclopedia. . 1983 .
SOUND FIELD
The set of space-time distributions of quantities characterizing the considered sound disturbance. The most important of them: sound pressure p, vibrational particle v, vibrational displacement of particlesx , relative change in density (so-called acoustic) s \u003d dr / r (where r is a medium), adiabatic. temperature change d T, accompanying compression and rarefaction of the medium. When introducing the concept of 3. n. Environment is considered as continuous and the molecular structure of a substance is not taken into account. 3.p. Is studied either by methods geometric acoustics, or based on wave theory. pressure satisfies the wave ur-tion
And with the known R you can determine the remaining characteristics of 3.p. by f-lam: 
Where from - speed of sound, g \u003d c p/c V - ratio of heat capacity at constant. pressure to heat capacity at constant. volume, and - coefficient. thermal expansion Wednesday. For harmonious. 3.p.wave ur-tion turns into Helmholtz ur-tion: D R+k 2 R \u003d 0, where k \u003dw / c - the wave number for the frequency w, and the expressions for v and x take the form:
In addition, 3. p. Must satisfy the boundary conditions, that is, the requirements that impose on the values \u200b\u200bcharacterizing the 3. p., Physical. properties of boundaries - surfaces that limit the environment, surfaces that limit obstacles placed in the environment, and interfaces decomp. Wednesday For example, on an absolutely rigid boundary, the components vibrate. speed v n must vanish; the sound pressure should vanish on the free surface; on the border characterized acoustic impedance, p / v n should be equal to the specific acoustic. boundary impedance; at the interface between two media, the quantities R and v n on both sides of the surface must be equal in pairs. In real liquids and gases, there is an add. boundary condition: vanishing of the tangent vibrate. velocity at a rigid boundary or equality of tangent components at the interface between two media. p \u003d p (x6 ct), running along the axis x in the positive ("-" sign) and negative ("+" sign) directions. In a flat wave p / v\u003d br fromwhere r from - wave impedance Wednesday. Put it in places. sound pressure direction fluctuate. speed in a traveling wave coincides with the direction of wave propagation, in places it is negated. pressure - opposite to this direction, and in places where the pressure vanishes. speed also vanishes. Harmonic. flat has the form: p=p 0 cos (w t-kx +j) ,
Where R 0 and j 0 - respectively, the wave amplitude and its beginning. at the point x \u003d 0. In media with dispersion of the speed of sound, the speed is harmonic. waves from\u003d w / k frequency dependent. 2) Oscillation in limiting areas of the environment in the absence of ext. influences, for example. 3. p., Arising in a closed volume at a given start. conditions. Such 3. p. Can be represented in the form of a superposition of standing waves characteristic of a given volume of the medium. 3) 3. p. Arising in unlimited. environment at a given start. conditions - values R and v in a certain beginning. moment in time (for example, 3. p. arising after the explosion) .4) 3. p. radiation generated by oscillating bodies, jets of liquid or gas, collapsing bubbles, and other natures. or arts. acoustic emitters (see. Sound emission). The simplest radiation in the form of the field are as follows. Monopole - a spherically symmetric diverging wave; for harmonious. radiation, it has the form: p \u003d -irwQexp ( ikr) / 4p r, where Q -
the productivity of a source (for example, the rate of change in the volume of a pulsating body, small compared to the wavelength), placed in the center of the wave, and r - distance from the center. The amplitude of the sound pressure for monopole radiation varies with distance as 1 / r, and 
in the non-wave zone ( kr<<1) v varies with distance as 1 / r 2, and in wave ( kr\u003e\u003e 1) - as 1 / r... Phase shift j between R and v decreases monotonically from 90 ° at the center of the wave to zero at infinity; tg j \u003d 1 / kr... Dipole radiation - spherical. a divergent wave with an "eight" directivity characteristic of the form:
Where F - the force applied to the medium at the center of the wave, q is the angle between the direction of the force and the direction to the observation point. The same radiation is created by a sphere of radius a<
Measurement of parameters 3. p. Produce decomp. sound receivers: microphones - for air, hydrophones - for water. When studying the fine structure 3.p .
you should use receivers whose dimensions are small compared to the wavelength of the sound. Visualization of sound fields possible by observation diffraction of light by ultrasound, Toepler's method ( shadow method), by electronic-optical method. transformations, etc. Lit .: Bergman L .. Ultrasound and its application in science and technology, trans. with it., 2nd ed., M .. 1957; R e in to and S. N. N., A course of lectures on the theory of sound, M., 1960; Isakovich M.A., General, M., 1973. M. A. Isakovich.
Physical encyclopedia. In 5 volumes. - M .: Soviet encyclopedia. Chief Editor A.M. Prokhorov. 1988 .
See what "SOUND FIELD" is in other dictionaries:
The region of space in which sound waves propagate. The concept of a z.p. is usually used for areas located far from the sound source, the dimensions of which are significantly larger than the wavelength (λ) of sound. Equation describing ... ... Encyclopedia of technologyFizikos terminų žodynas
sound field Encyclopedia "Aviation"
sound field - sound field - an area of \u200b\u200bspace in which sound waves propagate. The concept of Z. p. Is usually used for areas located far from the sound source, the dimensions of which are significantly larger than the wavelength λ of the sound. The equation,… … Encyclopedia "Aviation"
The region of space in which sound waves propagate, i.e., acoustic vibrations of particles of an elastic medium (solid, liquid or gaseous) that fill this area occur. Z. p. Is fully determined if for each of it ... ... Great Soviet Encyclopedia
The area of \u200b\u200bspace into which the sound propagates. waves ... Natural science. encyclopedic Dictionary
sound field of reflected waves (with acoustic logging) - - Topics Oil and Gas EN secondary sound field ... Technical translator's guide
The sound * field is understood as that limited area of \u200b\u200bspace in which the hydroacoustic message spreads. A sound field can exist in any elastic medium and is the vibrations of its particles that arise as a result of the influence of external disturbing factors. A distinctive feature of this process from any other ordered motion of particles of the medium is that at small perturbations, the propagation of waves is not associated with the transfer of the substance itself. In other words, the vibrations of each particle occurs relative to the position that it occupied before the disturbance.
An ideal elastic medium in which the sound field propagates can be represented as a set of absolutely rigid elements of it, interconnected by elastic bonds (Figure 1.1). The current state of an oscillating particle of this medium is characterized by its displacement U relative to the equilibrium position, oscillatory speed vand frequency hesitation. The vibrational velocity is determined by the first time derivative of the particle displacement and is an important characteristic of the process under consideration. Typically, both parameters are harmonic functions of time.
Particle 1
(Fig. 1.1), shifted by the value U from its equilibrium position, through elastic bonds, it affects the surrounding particles, forcing them to also move. As a result, the disturbance introduced from the outside begins to spread in the considered environment. If the law of particle displacement change 1
is defined by the equality 
Where U m Is the vibration amplitude of the particle, and w - vibration frequency, then the law of motion of others i - th particles can be represented as:
where U mi - vibration amplitude i - oh particles, y i- the phase shift of these oscillations. With distance from the source of excitation of the medium (particles 1 ) values \u200b\u200bof vibration amplitudes U mi energy dissipation will decrease, and phase shifts y idue to the limited speed of propagation of excitation - to increase. Thus, under sound field you can also understand the totality of vibrating particles of the medium.
If, in a sound field, we select particles that have the same phase of vibration, we get a curve or surface, which is called wave front... The front of the wave is constantly moving away from the source of disturbance at a certain speed, which is called wave front propagation speed, wave propagation speedor simply speed of sound in a given environment. The vector of the indicated velocity is perpendicular to the surface of the wave front at the point under consideration and determines the direction sound beamalong which the wave propagates. This speed depends significantly on the properties of the environment and its current state. In the case of a sound wave propagating in the sea, the speed of sound depends on the temperature of the water, its density, salinity, and a number of other factors. So, with an increase in temperature by 1 ° C, the speed of sound increases by about 3.6 m / s, and with an increase in depth by 10 m, it increases by about 0.2 m / s. On average, under sea conditions, the speed of sound can vary in the range of 1440 - 1585 m / s. If Wednesday anisotropic, i.e. having different properties in different directions from the center of the disturbance, then the speed of propagation of the sound wave will also be different, depending on these properties.
In general, the speed of propagation of a sound wave in a liquid or gas is determined by the following expression:
(1.2)
where TO Is the bulk modulus of the medium, r 0 - the density of the undisturbed medium, its static density. The bulk modulus is numerically equal to the stress that occurs in the medium during its unit relative deformation.
An elastic wave is called longitudinalif the vibrations of the particles under consideration occur in the direction of wave propagation. The wave is called transverse,if particles vibrate in planes perpendicular to the direction of wave propagation.
Transverse waves can arise only in a medium that has form elasticity, i.e. able to resist shear deformation. Only solid bodies have this property. Longitudinal waves are associated with the volumetric deformation of the medium, so they can propagate both in solids, and in liquid and gaseous media. The exceptions to this rule are superficial waves formed on the free surface of a liquid or on the interfaces of immiscible media with different physical characteristics. In this case, liquid particles simultaneously perform longitudinal and transverse vibrations, describing elliptical or more complex trajectories. The special properties of surface waves are explained by the fact that gravity and surface tension play a decisive role in their formation and propagation.
In the process of oscillations in a disturbed medium, zones of increased and decreased pressure and density in relation to the equilibrium state arise. Pressure 
where is its instantaneous value in the sound field, and is the static pressure of the medium in the absence of excitation, is called sound and is numerically equal to the force with which the wave acts on a unit area, set perpendicular to the direction of its propagation. Sound pressure is one of the most important characteristics of the state of the environment.
To assess the change in the density of the medium, a relative value is used, called seal c, which is determined by the following equality:
(1.3)
where r 1 -the instantaneous value of the density of the medium at the point of interest to us, and r 0 -its static density.
All of the above parameters can be determined if some scalar function is known, called potential j of the vibrational speed.In accordance with the Helmholtz theorem, this potential fully characterizes acoustic waves in liquid and gaseous media and is associated with the vibrational velocity v by the following equality:
. (1.4)
Longitudinal sound wave is called flatif its potential j and other related quantities characterizing the sound field depend only on time and one of their Cartesian coordinates, for example, x(Figure 1.2). If the mentioned quantities depend only on time and distance r from some point about space called the center of the wave, longitudinal sound wave is called spherical... In the first case, the wave front will be a line or plane, in the second - an arc or a section of a spherical surface.
In elastic media, when considering processes in sound fields, the principle of superposition can be used. So, if a system of waves, determined by the potentials j 1 ... j n, then the potential of the resulting wave will be equal to the sum of the indicated potentials:
(1.5)
However, when considering processes in powerful sound fields, one should take into account the possibility of manifestation of nonlinear effects, which can make the use of the principle of superposition inadmissible. In addition, at high levels of disturbance to the medium, the elastic properties of the medium can be radically violated. So, in a liquid medium, ruptures filled with air can occur, its chemical structure can change, etc. In the model presented earlier (Fig. 1.1.), This will be equivalent to breaking the elastic bonds between the particles of the medium. In this case, the energy spent on creating oscillations will practically not be transferred to other layers, which will make it impossible to solve a particular practical problem. The described phenomenon is called cavitation.
From an energy point of view, the sound field can be characterized by a stream of sound energyor sound power Pwhich are determined by the amount of sound energy Wpassing through a given surface per unit of time:
(1.6)
Sound power related to area sthe surface under consideration determines intensity sound wave:
(1.7) In the last expression, it is assumed that the energy is uniformly distributed over the site s.
Often, to characterize the sound environment, the concept is used sound energy density, which is defined as the amount of sound energy per unit volume of the elastic medium.
Let us examine the relationship between the individual parameters of the sound field.
1.3 Equation of continuity of medium
The continuity equation of the medium connects the velocity potential and its compaction. In the absence of discontinuities in the medium, the law of conservation of mass takes place, which can be written in the following form:
where W 1 and r 1Is the volume and density of the liquid in the sound field, and W 0 and r 0 - the same parameters in the absence of disturbance. This law says that in a continuous linear medium, a change in volume causes such a change in the density of the medium that their product, corresponding to the mass of the volume under consideration, always remains constant.
In order to introduce into consideration the compaction of the medium, we subtract from the left and right sides of equality (1.8) the product W 0 r 1... As a result, we will have:
(1.9)
It is accepted here that 
This assumption is possible due to the fact that in the ultrasonic frequency range the variations in the volume and density of the liquid are insignificant in relation to their absolute value and the replacement in the denominator of equality (1.9) of the quantity r 1 on r 0 practically does not affect the analysis result.
Let be ρ 1 \u003d 1.02 g / cm 3, and ρ 0
\u003d 1.0 g / cm 3. Then
and 
... The relative error of the accepted assumptions is 
.
Let us express the relative volumetric deformation of the medium, represented by the left side of equality (1.9), in terms of the partial displacements of the liquid particles and take into account that the right side of this equality determines the compaction of the medium. Then we will have:
(1.10)
where U x, U yand U z - displacement of the particles of the medium along the corresponding axes of the orthogonal coordinate system.
We differentiate the last equality in time:
Here v x, v yand v z- components of the vibrational speed along the same axes. Considering that
(1.12)
(1.13) where Ñ is the Hamiltonian operator, which determines the spatial differentiation:
(1.14)
| Important! |
Oscillatory motion equation
The equation of oscillatory motion connects the velocity potential and sound pressure. To derive this equation, we select in the sound field an elementary volume oscillating along the axis oh(Fig. 1.3.) In accordance with Newton's law, you can write:
(1.15)
where F -force acting on the allocated volume in the direction of the axis oh,
m- the mass of a given volume, j - acceleration of volume movement along the same axis . If we denote the pressures acting on the face of the selected volume by p 1 and p 2, and accept that\u003e, then the force F can be defined by the following equality:
(1.16)
where 
Substituting expression (1.16) into equality (1.15) and taking into account that 
and acceleration 
and also carrying out the passage to the limit to infinitesimal quantities, we find:
(1.17)
Taking into account that 
and 
we finally get:
. (1.18)
The last equation does not contain coordinates and therefore is valid for a wave of any shape.
The equation of state of the environment
The equation of state of the medium as applied to the ultrasonic field, in which all processes proceed practically without changing the temperature, expresses the relationship between the pressure and the density of the medium. In an ideal fluid, in which there are no viscous friction forces, the sound pressure rproportional to the hardness of the medium TO and its compaction c: 
However, if the medium is real, then there are viscous friction forces in it, the magnitude of which is proportional to the viscosity of the medium and the rate of change in the state of the medium, in particular, the rate of change in its compaction. Therefore, the expression that determines the pressure in a viscous medium will acquire a component that depends on these factors:
(1.19)
where L is the coefficient of proportionality. As a result of experiments, an estimate of this coefficient was found, which allowed the final expression that determines the state of the environment to be written in the form:
(1.20) where h is the coefficient of dynamic (Newtonian) viscosity of the medium. The resulting equation is valid for any waveform.
Wave equation
The wave equation determines the law of variation of the velocity potential. To derive this equation, we substitute expression (1.20) for the state of the medium into equality (1.18). As a result, we get:
(1.21)
In order to represent the compaction of the medium in terms of the velocity potential, we differentiate expression (1.21) in time:
(1.22)
Taking into account dependence (1.13), obtained from the condition of continuity of the medium and equality (1.2), we write down the desired wave equation in the final form:
(1.23)
If the wave is plane and propagates, for example, along the axis oh, then the velocity potential will depend only on the coordinate xand time. In this case, the wave equation takes on a simpler form:
(1.24) Solving the obtained equations, it is possible to find the law of variation of the velocity potential and, as a consequence, any parameter characterizing the sound field.
Analysis of the main parameters of the sound field
Let us first determine the parameters characterizing a plane harmonic wave. To do this, we find a solution to equation (1.24), which is a second-order linear differential equation and, therefore, has two roots. The indicated roots represent two processes j 1 (x, t)and j 2 (x, t)defining waves that travel in opposite directions. In an isotropic medium, the parameters of the sound field at points equidistant from the radiation source are the same, which allows us to restrict ourselves to finding only one solution, for example, for a wave j 1propagating in the positive direction of the axis oh.
Since the indicated particular solution is a function of the current coordinate and time, we will look for it in the following form:
where 
- wave frequency, m Is the sought coefficient that determines the dependence of the velocity potential on the spatial coordinates, 
- wave number, 
... Calculating the necessary derivatives of j 1 and substituting them into equation (1.24), we find:
(1.26) Solving the last equality with respect to m and taking into account that the wave that decays with distance from the source of the disturbance corresponds to its negative value, we will have:
(1.27)
In an ultrasonic field, the second term in parentheses of expression (1.27) is much less than unity, which allows us to expand this expression in a power series, limiting ourselves to two terms:
(1.28)
Substituting the found value m into equality (1.25) and introducing the notation
(1.29)
find the final expression for the velocity potential j 1:
Private solution for potential j 2 can be found similarly to the considered case:
Let's use the obtained expressions to determine the main parameters of the sound field.
The sound pressure in the zone of propagation of a positively directed wave is determined by the following equality:
(1.32)
where 
.
If we turn to equality (1.4) and take into account that in the ultrasonic field \u003e\u003e and, then the expression for the vibrational velocity can be written in the following form:
where 
The obtained expressions show that changes in the current values \u200b\u200bof the sound pressure and the vibrational velocity occur in phase, as a result of which in the places of compaction of the medium the vector of the vibrational velocity coincides in the direction with the propagation velocity of the wave front, and in places of discharge it is opposite to it.
Let us find the ratio of sound pressure and vibrational velocity, which is called specific acoustic resistance:
(1.34)
Specific acoustic resistance is an important characteristic of a medium that affects many parameters of the processes that take place in it.
Sound wave propagation
When creating hydroacoustic devices, one of the most important tasks is the correct choice of radiation parameters: the carrier frequency of the sending signal, the method of signal modulation and its energy characteristics. The range of propagation of the wave, the features of its reflection and passage through various interfaces between media with different physical properties, the possibility of separating the signal from the accompanying interference.
As noted above, one of the main energy characteristics of a hydroacoustic signal is its intensity. The expression that defines this parameter can be found from the following considerations. Let us consider a certain elementary section of the wave front with an area that, while oscillating, in time shifts relative to the initial position by an amount 
This shift will be opposed by forces 
internal interaction. To overcome these forces, work will be spent.The power required to ensure the oscillations under consideration is defined as the work expended per unit of time:
(1.35)
where T - wave period. In turn, the intensity is determined by the power spent on movement single the area of \u200b\u200bthe wave front and, therefore, will be equal to:
(1.36)
Substituting equalities (1.32) and (1.33) into the resulting expression, we find:
Considering that 0.5 
- signal intensity in the immediate vicinity of the emitter, then the law of intensity variation with distance from the source will be determined by the following equality:
(1.38)
The last formula was obtained by the English physicist and mathematician Stokes and bears his name. It shows that with distance from the radiation source, the intensity of the sound wave decreases exponentially. Moreover, as follows from expression (1.29), the damping index and proportional to the square of the oscillation frequency of the emitted wave. This imposes certain restrictions on the choice of carrier frequencies of the bursts, especially for long-range sensing.
However, using the Stokes formula, it is not always possible to obtain a correct estimate of the attenuation of a sound wave. Thus, experiments show that sound waves in a marine environment decay much faster than it follows from the above expression. This phenomenon is due to the difference in the properties of the real environment from the idealized one, usually considered in the theoretical solution of problems, as well as the fact that the marine environment is an inhomogeneous liquid, which includes living organisms, air bubbles and other impurities.
In practice, various empirical formulas are usually used to determine the law of change in the intensity of a sound wave. So, for example, at its frequencies lying in the range of 7.5 - 60 kHz, the value of the coefficient and in decibels per kilometer (dB / km) can be estimated using the following relationship:
, (1.39)
and the law of intensity variation at distances from the vibrator not exceeding 200 km, with an error of up to 10%, is determined by the equality:
(1.40)
In the case of a spherical wave, the intensity
. (1.41)
From the last expression it follows that the wave is largely weakened due to the expansion of its front with increasing distance r.
An ultrasonic wave propagates in a straight line during its motion in a homogeneous isotropic medium. However, if the medium is inhomogeneous, then the trajectory of the sound beam is curved, and under certain conditions, the signal can be reflected from the intermediate layers of the aqueous medium. The phenomenon of the bending of sound beams due to the inhomogeneity of the marine environment is called refraction of sound... Sound refraction can have a significant impact on the accuracy of hydroacoustic measurements, so the degree of its influence in most cases must be evaluated.
When a ray propagates towards the bottom, it passes, as a rule, three zones on its way: an isothermal (constant temperature) surface zone, a temperature jump zone, characterized by a sharp negative temperature gradient, and a bottom isothermal zone (Figure 1.4). The thickness of the jump zone can be several tens of meters. When a sound wave passes through the shock layer, strong refraction and a significant decrease in sound intensity are observed. The decrease in intensity is due to the divergence of the rays due to sharp refraction at the upper boundary of the jump layer, as well as their reflection from this layer. The extreme rays of the split beam form the sound shadow zone.
| Figure 1.4. |
arise standing wave. A feature of a standing wave is that all of its points vibrate with the same phase, forming through gaps equal to a quarter of the wavelength of vibrations, antinodes in which the amplitude of vibrations is maximum, and nodes in which there are no vibrations at all. A standing wave practically does not transfer energy.
Reflection and refraction of sound waves
When a wave hits the interface between two media, particles of the medium belonging to this interface are excited. In turn, the oscillations of the boundary particles give rise to wave processes, both in the medium of the incident wave and in the adjacent medium. The first wave is called reflectedand the second is refracted... Corners 
and (Figure 1.5) between the normal to the interface and the direction of the rays are called angles fall, 
reflectionsand refractions, respectively. According to Descartes's laws, there are equalities:
(1.42)
If on the path of the beam propagation there are several interfaces between the media, then the equality will be true:
(1.43)
The quantity is called snell's constant... Its value does not change along the sound beam.
The energy ratios in the incident, reflected and refracted rays are determined using the coefficients ANDand IN reflection and refraction, respectively. The indicated coefficients are determined by the following equalities:
(1.44)
It can be shown that in environments with the same acoustic impedances, sound energy is completely transferred from one environment to another. When there is a large difference in the acoustic impedances of the media, practically all of the incident energy is reflected from the interface between the media.
The considered regularities take place when the dimensions of the reflecting surface exceed the wavelength of the incident radiation. If its wavelength is larger than the dimensions of the reflecting surface, then, as a rule, the wave is partially reflected from the obstacle (scattered), and partially bends around it. The phenomenon of a wave bending around an obstacle is called diffraction of sound... Diffraction also occurs for objects that are larger than the wavelength of vibrations, but in this case the phenomenon appears only at the edges of the reflecting surface. An acoustic shadow zone is formed behind the obstacle, in which there are no sound vibrations. At the same time, the picture of the sound field in front of the obstacle is complicated due to the interaction of the incident, reflected and diffracting waves. The sound wave can be reflected from numerous objects scattered in sea \u200b\u200bwatersuch as air bubbles, plankton, particles of solid floating substances, etc. In this case, the reflected signal is called a signal surround reverb... It is perceived by the radiation receiver as an oscillating echo at the end of the signal transmission. At the beginning, this echo can be quite high, and then quickly decays.
Reverb can occur due to the scattering of sound by flat surfaces that have small irregularities compared to the wavelength. Most often such surfaces are the bottom or surface of the sea. This reverberation is called bottom or superficial, respectively.
... Basic principles of hydroacoustic sounding
Almost all hydroacoustic navigation devices used in the transport fleet operate in the mode of active sensing of water space. The development of devices implementing this mode requires:
§ determining the requirements for probing radiation based on the content of the problem being solved;
§ determining the requirements for the receiving and transmitting antennas;
§ analysis of the propagation conditions of the probing signal and assessment of the nature of the received signal;
§ development of requirements for the input units of the system that perform the primary transformation of the received signal;
§ determination of the composition of the receiving path, which transforms the primary information to the form necessary for its display or further use by other devices or systems;
§ determination of the composition of devices for displaying and recording information;
§ formulation of requirements for the output signal of a hydroacoustic device from the side of other devices working with it.
As mentioned above, the probe radiation can be continuous or pulsed. Continuous radiation at the same signal amplitudes has the highest average power, which can turn out to be a decisive advantage when probing areas that are sufficiently remote from the radiation source. A higher average power of the emitted signal allows not only to increase the level of the received reflected signal, but also often to avoid the phenomenon of cavitation. Most often this type of radiation is used in Doppler systems for measuring ship speed.
If it is necessary to measure the distances to reflecting objects, continuous radiation must be pre-modulated in a special way. Proper choice of modulation and processing of the received signal allows you to create the most accurate measurement systems. However, it should be borne in mind that in the case under consideration, the received signal is usually accompanied by a fairly significant interference arising from the volumetric reverberation.
Pulsed radiation is characterized by the shape of the pulse, its duration T and (Fig. 1.6), frequency or pulse repetition period. Most often, rectangular pulses are used (Fig. 1.6.a), which are the most energy-saturated. In the recent past, the exponential form was widely used (Fig. 2.6, b) due to the fact that it was easier to implement technically. Solving individual problems may require the creation of pulses with a more complex shape of their envelopes.
The duration of the pulse is of great importance, since it, together with its amplitude, determines the power contained in it, and, consequently, the maximum sensing range. In addition, the range resolution depends on the pulse duration, i.e. is the minimum range difference that can be measured by the system. Indeed, due to the fact that the impulse is a carrier of single information, all changes in the range within its spatial extent will not be recorded by the system. Taking into account that the pulse travels twice the distance - to the reflector and back, the resolution of the system will be equal to half the spatial pulse length:
(1.45)
In practice, the pulse duration is most often in the range from 10 -5 from up to 10 -3 from.
The pulse repetition rate is usually chosen so that, in any operating range, a subsequent pulse is emitted only after the reflected one has been received. In other words, the period t p pulse repetition must satisfy the inequality: 
Where 
- the maximum sensing range in the working range, - the average speed of sound in water, usually taken equal to 1500 m / s... This approach creates conditions for the use of one antenna as receiving and transmitting. In some cases, the pulse repetition rate can be selected from other considerations.
It is very important when forming the requirements for the probing signal to choose the correct carrier frequency of the radiation. Signal attenuation, its reflection from the interfaces between media and various objects, as well as the trajectory of the wave front, largely depend on it. Decreasing the carrier frequency, as a rule, requires increasing the size of the antenna devices, but contributes to an increase in the sensing range.
Formulating the basic requirements for the antenna system, it is necessary:
§ determine the number of antennas and their layout on the ship;
§ choose the best degree of radiation directivity;
§ choose the type of element that converts electrical energy into mechanical energy and vice versa, as well as the type of antenna;
§ determine how to install antennas on board.
The number of antennas used and their layout is determined by the nature of the problem being solved, as well as the presence or absence of their redundancy in order to increase the reliability of the system. Each antenna can be independently mounted on board the ship, or all antennas are combined into one antenna unit, which is usually installed in a clinket. Such a block can contain up to 20 or more antennas, which in this case are more appropriate to call vibrators.
The required degree of radiation directivity is also dictated by the nature of the problem being solved.
Ferromagnetic and piezoceramic vibrators are used as converters of electrical energy into mechanical and vice versa, the principle of operation of which is discussed below.
general characteristics receiving and transmitting antennas
Ferromagnetic converters of electrical energy into mechanical energy use the effect of magnetostriction. The essence of this effect lies in the fact that when the magnetic state of a product made of ferromagnetic material changes, some change in its dimensions occurs. The sample is deformed, and this deformation increases with increasing intensity of its magnetization. If we take a bar core as a sample, provide it with a winding and supply it with alternating current, then the length of the core will change periodically. The electrical energy spent on its magnetization is converted into the energy of mechanical vibrations, capable of exciting a sound field in an elastic medium, in which the considered rod is placed.
There is also the opposite effect. If the core is made of a ferromagnetic material with some residual magnetization, deform somewhat, i.e. change its internal tension, then the strength of the magnetic field associated with it will also change. In this case, the change in the magnetic field will be
Zthe sound field manifests itself in the form of kinetic energy of vibrating material bodies, sound waves in media with an elastic structure (solids, liquids and gases). The process of propagation of vibrations in an elastic medium is called wave... The direction of propagation of the sound wave is called sound beam, and the surface connecting all adjacent points of the field with the same oscillation phase of the particles of the medium is wave front... In solids, vibrations can propagate both in the longitudinal and transverse directions. In the air only longitudinal waves.
Free sound fieldis called a field in which the direct sound wave dominates, and the reflected waves are absent or negligible.
Diffuse sound field- it is such a field, at each point of which the density of sound energy is the same and in all directions of which the same energy flows are distributed within a unit of time.
Sound waves are characterized by the following basic parameters.
Wavelength - equal to the ratio of the speed of sound (340 m / s - in air) to the frequency of sound vibrations. Thus, the wavelength in air can vary from 1.7 cm (for f \u003d 20,000 Hz) up to 21 m (for f \u003d 16 Hz).
Sound pressure - is defined as the difference between the instantaneous pressure of the sound field at a given point and the statistical (atmospheric) pressure. Sound pressure is measured in Pascals (Pa), Pa \u003d N / m 2. Physical analogs are electrical voltage, current.
Sound intensity - the average amount of sound energy passing per unit of time through a unit of surface perpendicular to the direction of wave propagation. Intensity is measured in units of W / m 2 and represents the active component of the power of sound vibrations. The physical counterpart is electrical power.
In acoustics, measurement results are usually displayed in the form of relative logarithmic units. A unit called Bel (B) is used to assess the auditory experience. Since Bel is a rather large unit, a smaller value was introduced - decibel (dB) equal to 0.1 B.
Sound pressure, sound intensity is expressed in relative acoustic levels:
,
Zero values \u200b\u200bof acoustic levels correspond to generally accepted 
and W / m 2 at a harmonic sound vibration with a frequency of 1000 Hz. The values \u200b\u200bgiven correspond to approximately the minimum values \u200b\u200bfor producing an auditory sensation (absolute threshold of hearing).
Conditions for measuring the characteristics of microphones. Acoustic measurements have a number of specific features... Thus, the measurement of some characteristics of electroacoustic equipment must be carried out in free field conditions, i.e. when there are no reflected waves.
In ordinary rooms, this condition is impracticable, and it is difficult and not always possible to carry out measurements in the open air. First, in the open air it is difficult to avoid reflections from surfaces such as the ground. Secondly, the measurement in this case depends on atmospheric conditions (wind, etc.) and can lead to large errors, not to mention a number of other inconveniences. Third, in the open air it is difficult to avoid the influence of extraneous (industrial, etc.) noises.
Therefore, to carry out measurements in a free field, special sound-damped chambers are used, in which there are practically no reflected waves.
Measuring microphone characteristics in a jammed chamber... To measure the sensitivity of a microphone in a free field, one should first measure the sound pressure at the point where the test microphone will be placed, and then place it at this point. But since there is practically no interference in the chamber, and the distance of the microphone from the loudspeaker is taken equal to 1 - 1.5 m (or more) with a radiator diameter of not more than 25 cm, the measuring microphone can be placed near the microphone under test. The diagram of the measuring setup is shown in Fig. 4. Sensitivity is determined over the entire nominal frequency range. Setting the required pressure on the sound pressure meter (sound level meter), measure the voltage developed by the test microphone, and determine its axial sensitivity.
E OC = U M / P(mV / Pa)
Sensitivity is determined either by the no-load voltage or by the voltage at the rated load. As a rule, the module of the internal resistance of the microphone at a frequency of 1000 Hz is taken as the nominal load.
Fig. 4. Functional diagram of microphone sensitivity measurement:
1 - tonal or white noise generator; 2 - octave filter (one-third octave); 3 - amplifier; 4 - damped chamber; 5 - acoustic emitter; 6 - test microphone; 7 - measuring microphone; 8 - millivoltmeter; 9 - millivoltmeter, graduated in pascals or decibels (sound level meter).
Sensitivity levelis defined as the sensitivity expressed in decibels relative to a value equal to 1.
Standard sensitivity level (in decibels) is defined as the ratio of the voltage developed at the nominal load impedance at a sound pressure of 1 Pa to the voltage corresponding to the power \u003d 1 mW and calculated by the formula:
where is the voltage (V) developed by the microphone at the nominal load resistance (Ohm) at a sound pressure of 1 Pa.
Frequency response Microphone is called the dependence of the microphone sensitivity on frequency at constant values \u200b\u200bof sound pressure and microphone supply current. The frequency response is taken by smoothly changing the generator frequency. The obtained frequency response is used to determine its unevenness in the nominal and operating frequency ranges.
Directional characteristic The microphone is removed according to the same scheme (Fig. 4), and depending on the task, either at several frequencies, using a tone generator, or for a noise signal in one-third octave bands, or for a given frequency band, using an appropriate band-pass filter instead of one-third octave filters.
To measure the directivity characteristics, the test microphone is mounted on a rotary disc with a dial. The disc is rotated manually or automatically, synchronously with the recording table. The characteristic is taken in one plane passing through the working axis of the microphone, if it is a body of rotation around its axis. For other forms of the microphone, the characteristic is taken for specified planes passing through the working axis. The angle of rotation is measured between the working axis and the direction to the sound source. The directivity characteristic is normalized relative to the axial sensitivity.
Sound field is an area of \u200b\u200bspace in which sound waves propagate, that is, acoustic vibrations of particles of an elastic medium (solid, liquid or gaseous) that fill this area occur. The concept of a sound field is usually used for areas that are on the order of or greater than the sound wavelength.
On the energy side of the sound field, it is characterized by the density of sound energy (the energy of the oscillatory process per unit volume) and the intensity of sound.
The vibrating surface of a body is a radiator (source) of sound energy, which creates an acoustic field.
Acoustic field called the area of \u200b\u200belastic medium, which is a means of transmission of acoustic waves. The acoustic field is characterized by:
· sound pressure p zv, Pa;
· acoustic impedance z A, Pa * s / m.
The energy characteristics of the acoustic field are:
· intensity I, W / m 2;
· sound power W,W is the amount of energy passing per unit time through the surface surrounding the sound source.
An important role in the formation of the acoustic field is played by directivity characteristic of sound emission Ф , i.e. angular spatial distribution of sound pressure generated around the source.
All the listed values \u200b\u200bare interrelated and depend on the properties of the environment in which the sound propagates.
If the acoustic field is not limited by the surface and extends almost to infinity, then such a field is called a free acoustic field.
IN limited space (eg indoors) the propagation of sound waves depends on the geometry and acoustic properties of surfaces located in the path of propagation of the waves.
The formation of a sound field in a room is associated with the phenomena reverberation and diffusion.
If a sound source begins to act in the room, then at the first moment of time we have only direct sound. When the wave reaches the sound-reflecting barrier, the field pattern changes due to the appearance of reflected waves. If an object is placed in the sound field, the dimensions of which are small in comparison with the length of the sound wave, then practically no distortion of the sound field is observed. For effective reflection, it is necessary that the dimensions of the reflecting obstacle be greater than or equal to the length of the sound wave.
A sound field in which a large number of reflected waves with different directions arise, as a result of which the specific density of sound energy is the same throughout the field, is called diffuse field.
After the source of sound radiation stops, the acoustic intensity of the sound field decreases to zero level for an infinite time. In practice, it is believed that a sound is completely attenuated when its intensity drops 10 6 times from the level that exists at the moment it is turned off. Any sound field as an element of an oscillating medium has its own characteristic of sound attenuation - reverberation ("sound").
Sound- psychophysiological sensation caused by mechanical vibrations of particles of an elastic medium. Sound vibrations correspond to the frequency range in the range of 20 ... 20,000 Hz. Oscillation with frequency less than 20 Hz is called infrasonic, and more than 20,000 Hz - ultrasonic... The impact on a person of infrasonic vibrations causes unpleasant sensations. In nature, infrasonic vibrations can occur during sea waves, vibrations of the earth's surface. Ultrasonic vibrations are used for medicinal purposes in medicine and in radio electronic devices such as filters. Excitation of sound causes an oscillatory process that changes the pressure in an elastic medium, in which alternating compression and rarefaction layerspropagating from a sound source in the form of sound waves. In liquid and gaseous media, the particles of the medium vibrate relative to the equilibrium position in the direction of wave propagation, i.e. waves are longitudinal. Transverse waves propagate in solids, since the particles of the medium vibrate in the direction perpendicular to the wave propagation line. The space in which sound waves propagate is called the sound field.... A distinction is made between a free sound field, when the influence of the enclosing surfaces reflecting sound waves is small, and a diffuse sound field, where at each point the sound power per unit area is the same in all directions. The propagation of waves in the sound field occurs at a certain speed, which is called speed of sound... Formula (1.1)
c \u003d 33l√Т / 273, where Т is the temperature on the Kelvin scale.
In the calculations, c \u003d 340 m / s is taken, which approximately corresponds to a temperature of 17 ° C at normal atmospheric pressure. The surface connecting adjacent points of the field with the same oscillation phase (for example, points of concentration or rarefaction) is called wave front. The most common sound waves are with sphericaland plane wave fronts... The front of a spherical wave has the shape of a sphere and is formed at a short distance from the sound source if its dimensions are small compared to the length of the emitted wave. The front of a plane wave has the shape of a plane perpendicular to the direction of propagation of the sound wave (sound beam). Waves with a flat front are formed at large distances from the sound source compared to the wavelength. The sound field is characterized by sound pressure, oscillatory speed, sound intensity and sound energy density.
Sound pressure is the difference between the instantaneous value of the pressure p am at a point in the medium when a sound wave passes through it and the atmospheric pressure p as at the same point, i.e. p \u003d p as - p am. The SI unit of sound pressure is newton per square meter: 1 N / m 2 \u003d 1 Pa (pascal). Real sound sources create even at the most loud sounds sound pressures are tens of thousands of times less than normal atmospheric pressure.
Oscillatory speed represents the speed of oscillation of particles of the medium about their rest position. Oscillatory speed is measured in meters per second. This speed should not be confused with the speed of sound. The speed of sound is a constant value for a given environment, the vibrational speed is variable. If the particles of the medium move in the direction of wave propagation, then the vibrational velocity is considered positive, with the reverse movement of the particles - negative. Real sound sources, even at the loudest sounds, cause oscillatory speeds several thousand times less than the speed of sound. For a plane sound wave, the vibrational velocity formula has the form (1.2)
V \u003d p / ρ · s, where ρ is the air density, kg / m 3; s - speed of sound, m / s.
The product ρ s for given atmospheric conditions is a constant value, it is called acoustic impedance.
Sound intensity - the amount of energy passing per second through a unit of area perpendicular to the direction of propagation of the sound wave. Sound intensity is measured in watts per square meter (W / m 2).
Sound energy density is the amount of sound energy per unit volume of the sound field: ε \u003d J / c.
4. Control questions
Glossary
Literature