The basic laws of sound propagation include the laws of its reflection and refraction at the boundaries of various media, as well as the diffraction of sound and its scattering in the presence of obstacles and inhomogeneities in the medium and at the interfaces between media.
The range of sound propagation is influenced by the sound absorption factor, that is, the irreversible transition of sound wave energy into other types of energy, in particular heat. An important factor is also the direction of radiation and the speed of sound propagation, which depends on the medium and its specific state.
From a sound source, acoustic waves propagate in all directions. If a sound wave passes through a relatively small hole, then it spreads in all directions, and does not travel in a directed beam. For example, street sounds penetrating through an open window into a room are heard at all points, and not just opposite the window.
The nature of the propagation of sound waves near an obstacle depends on the relationship between the size of the obstacle and the wavelength. If the size of the obstacle is small compared to the wavelength, then the wave flows around this obstacle, spreading in all directions.
Sound waves, penetrating from one medium to another, deviate from their original direction, that is, they are refracted. The angle of refraction may be greater or less than the angle of incidence. It depends on which medium the sound penetrates into which. If the speed of sound in the second medium is greater, then the angle of refraction will be greater than the angle of incidence, and vice versa.
When you encounter an obstacle on your way, sound waves are reflected from it according to a strictly defined rule - the angle of reflection equal to angle falling - the concept of echo is connected with this. If sound is reflected from several surfaces at different distances, multiple echoes occur.
Sound travels in the form of a diverging spherical wave that fills an increasingly larger volume. As the distance increases, the vibrations of the particles of the medium weaken and the sound dissipates. It is known that to increase the transmission range, sound must be concentrated in a given direction. When we want, for example, to be heard, we put our palms to our mouths or use a megaphone.
Diffraction, that is, the bending of sound rays, has a great influence on the range of sound propagation. The more heterogeneous the medium, the more the sound beam is bent and, accordingly, the shorter the sound propagation range.
Sound propagation
Sound waves can travel in air, gases, liquids and solids. Waves do not arise in airless space. This is easy to verify from simple experience. If an electric bell is placed under an airtight cap from which the air has been evacuated, we will not hear any sound. But as soon as the cap is filled with air, a sound occurs.
The speed of propagation of oscillatory motions from particle to particle depends on the medium. In ancient times, warriors put their ears to the ground and thus detected the enemy's cavalry much earlier than it appeared in sight. And the famous scientist Leonardo da Vinci wrote in the 15th century: “If you, being at sea, lower the hole of a pipe into the water, and put the other end of it to your ear, you will hear the noise of ships very distant from you.”
The speed of sound in air was first measured in the 17th century by the Milan Academy of Sciences. A cannon was installed on one of the hills, and an observation post was located on the other. The time was recorded both at the moment of the shot (by flash) and at the moment the sound was received. Based on the distance between the observation point and the gun and the time of origin of the signal, the speed of sound propagation was no longer difficult to calculate. It turned out to be equal to 330 meters per second.
The speed of sound in water was first measured in 1827 on Lake Geneva. The two boats were located 13,847 meters apart from each other. On the first, a bell was hung under the bottom, and on the second, a simple hydrophone (horn) was lowered into the water. On the first boat, gunpowder was set on fire at the same time as the bell was struck; on the second, the observer started the stopwatch at the moment of the flash and began to wait for the sound signal from the bell to arrive. It turned out that sound travels more than 4 times faster in water than in air, i.e. at a speed of 1450 meters per second.
Speed of sound
The higher the elasticity of the medium, the greater the speed: in rubber 50, in air 330, in water 1450, and in steel - 5000 meters per second. If we, who were in Moscow, could shout so loudly that the sound would reach St. Petersburg, then we would be heard there only after half an hour, and if the sound propagated over the same distance in steel, then it would be received in two minutes.
The speed of sound propagation is influenced by the state of the same medium. When we say that sound travels in water at a speed of 1450 meters per second, this does not mean that in any water and under any conditions. With increasing temperature and salinity of water, as well as with increasing depth, and therefore hydrostatic pressure, the speed of sound increases. Or let's take steel. Here, too, the speed of sound depends on both the temperature and the qualitative composition of the steel: the more carbon it contains, the harder it is, and the faster sound travels in it.
When meeting an obstacle on their way, sound waves are reflected from it in a strictly a certain rule: The angle of reflection is equal to the angle of incidence. Sound waves coming from the air will be almost completely reflected upward from the surface of the water, and sound waves coming from a source located in the water will be reflected downward from it.
Sound waves, penetrating from one medium to another, deviate from their original position, i.e. refracted. The angle of refraction may be greater or less than the angle of incidence. It depends on what medium the sound penetrates into. If the speed of sound in the second medium is greater than in the first, then the angle of refraction will be greater than the angle of incidence and vice versa.
In the air, sound waves propagate in the form of a diverging spherical wave, which fills an increasingly larger volume, as particle vibrations caused by sound sources are transmitted to the air mass. However, as the distance increases, the vibrations of the particles weaken. It is known that to increase the transmission range, sound must be concentrated in a given direction. When we want to be heard better, we put our palms to our mouths or use a megaphone. In this case, the sound will be attenuated less, and the sound waves will travel further.
As the wall thickness increases, soundlocation at low middle frequencies increases, but the “insidious” coincidence resonance, which causes the strangulation of soundlocation, begins to manifest itself at lower frequencies and covers a wider area.
We know that sound travels through the air. That's why we can hear. No sounds can exist in a vacuum. But if sound is transmitted through the air, due to the interaction of its particles, will it not also be transmitted by other substances? Will.
Propagation and speed of sound in different media
Sound is not transmitted only by air. Probably everyone knows that if you put your ear to the wall, you can hear conversations in the next room. IN in this case sound is transmitted by the wall. Sounds travel in water and other media. Moreover, sound propagation occurs differently in different environments. The speed of sound varies depending on the substance.
It is curious that the speed of sound propagation in water is almost four times higher than in air. That is, fish hear “faster” than we do. In metals and glass, sound travels even faster. This is because sound is a vibration of a medium, and sound waves travel faster in better conductive media.
The density and conductivity of water is greater than that of air, but less than that of metal. Accordingly, sound is transmitted differently. When moving from one medium to another, the speed of sound changes.
The length of the sound wave also changes as it passes from one medium to another. Only its frequency remains the same. But this is precisely why we can discern who exactly is speaking even through walls.
Since sound is vibrations, all laws and formulas for vibrations and waves are well applicable to sound vibrations. When calculating the speed of sound in air, it should also be taken into account that this speed depends on the air temperature. As temperature increases, the speed of sound propagation increases. At normal conditions the speed of sound in air is 340,344 m/s.
Sound waves
Sound waves, as is known from physics, propagate in elastic media. This is why sounds are well transmitted by the earth. By placing your ear to the ground, you can hear the sound of footsteps, clattering hooves, and so on from afar.
As a child, everyone probably had fun putting their ear to the rails. The sound of train wheels is transmitted along the rails for several kilometers. To create the reverse sound absorption effect, soft and porous materials are used.
For example, in order to protect a room from extraneous sounds, or, conversely, to prevent sounds from escaping from the room to the outside, the room is treated and soundproofed. The walls, floor and ceiling are covered with special materials based on foamed polymers. In such upholstery all sounds fade away very quickly.
If a sound wave does not encounter obstacles in its path, it propagates evenly in all directions. But not every obstacle becomes a barrier for her.
Having encountered an obstacle in its path, 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 sound is able to bend around these objects and penetrate into the area behind them.
The ability of a wave to bend around an obstacle is called diffraction .
Diffraction occurs when the sound wavelength exceeds the size of the obstacle. Low frequency sound waves are quite long. For example, at a frequency of 100 Hz it is equal to 3.37 m. As the frequency decreases, the length becomes even greater. Therefore, a sound wave easily bends around objects comparable to it. The trees in the park do not interfere with our hearing of sound at all, because the diameters of their trunks are much smaller than the length of the sound wave.
Thanks to diffraction, sound waves penetrate through cracks and holes in an obstacle and propagate behind them.
Let's place a flat screen with a hole in the path of the sound wave.
In the case where the sound wavelength ƛ much larger than the hole diameter D , or these values are approximately equal, then behind the hole the sound will reach all points in the area that is behind the screen (sound shadow area). The front of the outgoing wave will look like a hemisphere.
If ƛ is only slightly smaller than the diameter of the slit, then the main part of the wave propagates straight, and small part 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, different options for its further propagation are possible. Sound can be reflected from the interface, can move to another medium without changing direction, or can be refracted, that is, move, changing its direction.
Suppose an obstacle appears in the path of a 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 .
The sound reflected from an obstacle is called echo .
The nature of the reflection of the sound wave may 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, 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. Thus, sound that appears in water does not transfer 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 explained by the large difference in the wave impedances of water and air. In acoustics, wave impedance 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 significantly less than the wave resistance of liquids and solids, when a sound wave hits the boundary of air and water, it is reflected.
Fish in water do not hear the sound appearing above the surface of the water, but they can 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 travels from one medium to another, and its speed of propagation in these environments 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 – angle of reflection,
v 1 – speed of sound propagation in the first medium,
v 2 – speed of sound propagation in the second medium,
n – refractive index.
The refraction of sound is called refraction .
If a 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.
Refraction of sound can be observed not only at the interface between media. Sound waves can change their direction in a heterogeneous medium - the atmosphere, the ocean.
In the atmosphere, refraction is caused by changes in air temperature, speed and direction of movement of 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 encounters a surface, part 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 how much of the energy of sound vibrations is absorbed by 1 m2 of obstacle. It has a value from 0 to 1.
The unit of measurement 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 surface, the absorption coefficient of which is 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 studying 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 hall, the acoustic conditions, the type and area of sound-absorbing materials and reverberation time .
Reverberation call the process of reflection of a sound wave from obstacles and its gradual attenuation after the sound source is turned off. In an enclosed space, sound can be reflected repeatedly from walls and objects. As a result, various echo signals arise, each of which sounds as if separately. This effect is called reverberation effect .
The most important characteristic of the room is reverberation time , which Sabin entered and calculated.
Where V – volume of the room,
A – general sound absorption.
Where a i – sound absorption coefficient of the material,
S i - area of each surface.
If the reverberation time is long, the sounds seem to “wander” around the hall. 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 dull. Therefore, each room must have its own exact calculation.
Based on his calculations, Sabin arranged the sound-absorbing materials in such a way that the “echo effect” was reduced. And the Boston Symphony Hall, on the creation of which he was an acoustic consultant, is still considered one of the best halls in the world.
Sound travels through sound waves. These waves pass not only through gases and liquids, but also through solids. The action of any waves consists mainly in the transfer of energy. In the case of sound, transfer takes the form of minute movements at the molecular level.
In gases and liquids, a sound wave moves molecules in the direction of its movement, that is, in the direction of the wavelength. In solids sound vibrations molecules can also occur in the direction perpendicular to the wave.
Sound waves travel from their sources in all directions, as shown in the picture to the right, which shows a metal bell periodically colliding with its tongue. These mechanical collisions cause the bell to vibrate. The energy of vibrations is transmitted to the molecules of the surrounding air, and they are pushed away from the bell. As a result, pressure increases in the layer of air adjacent to the bell, which then spreads in waves in all directions from the source.
The speed of sound is independent of volume or tone. All sounds from a radio in a room, whether loud or soft, high pitched or low pitched, reach the listener at the same time.
The speed of sound depends on the type of medium in which it travels and its temperature. In gases, sound waves travel slowly because their rarefied molecular structure offers little resistance to compression. In liquids the speed of sound increases and in solids it becomes even faster, as shown in the diagram below in meters per second (m/s).
Wave path
Sound waves travel through air in a manner similar to that shown in the diagrams to the right. The wave fronts move from the source at a certain distance from each other, determined by the frequency of the bell's vibrations. The frequency of a sound wave is determined by counting the number of wave fronts passing through a given point per unit time.
The sound wave front moves away from the vibrating bell.
In uniformly heated air, sound travels at a constant speed.
The second front follows the first at a distance equal to the wavelength.
The sound intensity is greatest close to the source.
Graphic representation of an invisible wave
Sound sounding of depths
A sonar beam of sound waves easily passes through ocean water. The principle of sonar is based on the fact that sound waves are reflected from the ocean floor; This device is usually used to determine underwater terrain features.
Elastic solids
Sound travels in a wooden plate. The molecules of most solids are bound into an elastic spatial lattice, which is poorly compressed and at the same time accelerates the passage of sound waves.
Sound in water is absorbed hundreds of times less than in air. However, audibility in aquatic environment much worse than in the atmosphere. This is explained by the peculiarities of human perception of sound. In the air, sound is perceived in two ways: the transmission of air vibrations to the eardrums of the ears (air conduction) and the so-called bone conduction, when sound vibrations are perceived and transmitted to the hearing aid by the bones of the skull.
Depending on the type of diving equipment, the diver perceives sound in water with a predominance of either air or bone conduction. The presence of a volumetric helmet filled with air allows you to perceive sound through air conduction. However, a significant loss of sound energy is inevitable as a result of sound reflection from the surface of the helmet.
When descending without equipment or in equipment with a tight-fitting helmet, bone conduction predominates.
A feature of sound perception under water is also the loss of the ability to determine the direction of the sound source. This is due to the fact that the human hearing organs are adapted to the speed of sound in the air and determine the direction of the sound source due to the difference in the time of arrival of the sound signal and the relative sound pressure level perceived by each ear. Thanks to the device auricle a person in the air is able to determine where the sound source is - in front or behind, even with one ear. In water, everything happens differently. The speed of sound propagation in water is 4.5 times greater than in air. Therefore, the difference in the time of reception of the sound signal by each ear becomes so small that it becomes almost impossible to determine the direction of the sound source.
When using a hard helmet as part of the equipment, the possibility of determining the direction of the sound source is completely excluded.
Biological effects of gases on the human body
The question of the biological effects of gases was not raised by chance and is due to the fact that the processes of gas exchange during human breathing under normal conditions and so-called hyperbaric conditions (i.e. under high blood pressure) are significantly different.
It is known that the ordinary atmospheric air that we breathe is unsuitable for breathing by pilots in high-altitude flights. It also finds limited use in the breathing of divers. When descending to depths of more than 60 m, it is replaced by special gas mixtures.
Let us consider the basic properties of gases, which, as in pure form, and in mixture with others, are used for the breathing of divers.
The composition of air is a mixture of various gases. The main components of air are: oxygen - 20.9%, nitrogen - 78.1%, carbon dioxide - 0.03%. In addition, the air contains small quantities of argon, hydrogen, helium, neon, and water vapor.
The gases that make up the atmosphere, according to their effect on the human body, can be divided into three groups: oxygen - is constantly consumed to “maintain all life processes; nitrogen, helium, argon, etc. - do not participate in gas exchange; carbon dioxide - at increased concentrations for harmful to the body.
Oxygen(O2) is a colorless, tasteless and odorless gas with a density of 1.43 kg/m3. It is of utmost importance for humans as a participant in all oxidative processes in the body. During the process of breathing, oxygen in the lungs combines with hemoglobin in the blood and is distributed throughout the body, where it is continuously consumed by cells and tissues. An interruption in supply or even a decrease in its supply to tissues causes oxygen starvation, accompanied by loss of consciousness, and in severe cases - cessation of vital activity. This condition can occur when the oxygen content in the inspired air decreases during normal pressure below 18.5%. On the other hand, when the oxygen content in the inhaled mixture increases or when breathing under pressure exceeding the permissible limit, oxygen exhibits toxic properties - oxygen poisoning occurs.
Nitrogen(N) - colorless, odorless and tasteless gas with a density of 1.25 kg/m3, is the main part atmospheric air by volume and mass. IN Normal conditions physiologically neutral, does not take part in metabolism. However, as the pressure increases with increasing depth of the diver's immersion, nitrogen ceases to be neutral and at depths of 60 meters or more exhibits pronounced narcotic properties.
Carbon dioxide(CO2) is a colorless gas with an acidic taste. It is 1.5 times heavier than air (density 1.98 kg/m3), and therefore can accumulate in the lower parts of closed and poorly ventilated rooms.
Carbon dioxide is formed in tissues as the end product of oxidative processes. A certain amount of this gas is always present in the body and is involved in the regulation of breathing, and the excess is carried by the blood to the lungs and removed with exhaled air. Amount excreted by a person carbon dioxide mainly depends on the degree physical activity and functional state of the body. With frequent, deep breathing (hyperventilation), the carbon dioxide content in the body decreases, which can lead to respiratory arrest (apnea) and even loss of consciousness. On the other hand, an increase in its content in the respiratory mixture beyond the permissible level leads to poisoning.
Of the other gases that make up air, the one that is most used by divers is helium(Not). It is an inert gas, odorless and tasteless. Having a low density (about 0.18 kg/m3) and a significantly lower ability to cause narcotic effects when high pressures, it is widely used as a nitrogen substitute for the preparation of artificial breathing mixtures during descents to great depths.
However, the use of helium in respiratory mixtures leads to other undesirable phenomena. Its high thermal conductivity, and therefore increased heat transfer from the body, requires increased thermal protection or active heating of divers.
Air pressure. It is known that the atmosphere surrounding us has mass and exerts pressure on the surface of the earth and all objects located on it. The atmospheric pressure measured at sea level is balanced in tubes with a cross section of G cm2 by a column of mercury 760 mm high or water 10.33 m high. If this mercury or water is weighed, their mass will be equal to 1.033 kg. This means that “normal atmospheric pressure is 1.033 kgf/cm2, which in the SI system is equivalent to 103.3 kPa *.(* In the SI system, the unit of pressure is pascal (Pa). If conversion is necessary, the following ratios are used: 1 kgf/cm1 = 105 Pa = 102 kPa = =* 0.1 MPa.).
However, in the practice of diving calculations, it is inconvenient to use such precise units of measurement. Therefore, the unit of pressure measurement is taken to be a pressure numerically equal to 1 kgf/cm2, which is called the technical atmosphere (at). One technical atmosphere corresponds to a pressure of 10 m of water column.
When air pressure increases, it is easily compressed, reducing its volume in proportion to the pressure. Compressed air pressure is measured by pressure gauges, which indicate overpressure , i.e. pressure above atmospheric. The unit of excess pressure is designated ati. The sum of excess and atmospheric pressure is called absolute pressure(ata).
Under normal earthly conditions, air presses evenly on a person from all sides. Considering that the surface of the human body is on average 1.7-1.8 m2, the air pressure force exerted on it is 17-18 thousand kgf (17-18 tf). However, a person does not feel this pressure, since 70% of his body consists of practically incompressible liquids, and in the internal cavities - the lungs, middle ear, etc. - it is balanced by the backpressure of the air located there and communicating with the atmosphere.
When immersed in water, a person is exposed to excess pressure from a column of water above him, which increases by 1 ati every 10 m. A change in pressure can cause painful sensations and compression, to prevent which the diver must be supplied with breathing air at a pressure equal to the absolute pressure of the environment.
Since divers have to deal with compressed air or gas mixtures, it is appropriate to recall the basic laws that they obey and provide some formulas necessary for practical calculations.
Air, like other real gases and gas mixtures, to a certain approximation, obeys physical laws that are absolutely valid for ideal gases.
DIVING EQUIPMENT
Diving equipment is a set of devices and products worn by a diver to ensure life and work in the aquatic environment for a given period of time.
Diving equipment is fit for purpose if it can provide:
human breathing when performing work under water;
isolation and thermal protection from exposure cold water;
sufficient mobility and stable position under water;
safety during diving, surfacing and during work;
reliable connection with the surface.
Depending on the tasks to be solved, diving equipment is divided:
by depth of use - for equipment for shallow (medium) depths and deep-sea;
according to the method of providing a breathing gas mixture - autonomous and hose;
according to the method of thermal protection - for equipment with passive thermal protection, electrically and water heated;
according to the method of insulation - for equipment with water-gas-proof wetsuits of the “dry” type and permeable ones of the “wet” type.
The most complete understanding of the functional features of the operation of diving equipment is given by its classification according to the method of maintaining the composition of the gas mixture necessary for breathing. Here are the equipment:
ventilated;
with an open breathing pattern;
with a semi-closed breathing pattern;
with a closed breathing pattern.
