Sound waves travel fastest in gases. School encyclopedia. Graphic representation of an invisible wave

Over long distances, sound energy travels only along gentle rays that do not touch the ocean floor along the entire path. In this case, the limitation imposed by the environment on the range of sound propagation is its absorption in sea water. The main mechanism of absorption is associated with relaxation processes accompanying the disturbance by an acoustic wave of the thermodynamic equilibrium between the ions and molecules of salts dissolved in water. It should be noted that the main role in absorption in a wide range of sound frequencies belongs to the magnesium sulfur salt MgSO4, although in percentage terms its content in sea water is very small - almost 10 times less than, for example, NaCl rock salt, which nevertheless does not play a role any significant role in sound absorption.

Absorption in sea water, generally speaking, is greater the higher the sound frequency. At frequencies from 3-5 to at least 100 kHz, where the above mechanism dominates, absorption is proportional to frequency to the power of about 3/2. At lower frequencies, a new absorption mechanism is activated (possibly due to the presence of boron salts in water), which becomes especially noticeable in the range of hundreds of hertz; here the level of absorption is anomalously high and falls significantly more slowly with decreasing frequency.

To more clearly imagine the quantitative characteristics of absorption in sea water, we note that due to this effect, sound with a frequency of 100 Hz is attenuated 10 times over a path of 10 thousand km, and with a frequency of 10 kHz - at a distance of only 10 km (Figure 2). Thus, only low-frequency sound waves can be used for long-distance underwater communication, long-range detection of underwater obstacles, etc.

Figure 2 - Distances at which sounds of different frequencies attenuate 10 times when propagating in sea water.

In the region of audible sounds for the frequency range 20-2000 Hz, the propagation range of medium-intensity sounds under water reaches 15-20 km, and in the ultrasound region - 3-5 km.

Based on the sound attenuation values ​​observed in laboratory conditions in small volumes of water, one would expect significantly greater ranges. However, under natural conditions, in addition to attenuation caused by the properties of water itself (the so-called viscous attenuation), its scattering and absorption by various inhomogeneities of the medium also affect it.

Refraction of sound, or curvature of the path of a sound beam, is caused by heterogeneity in the properties of water, mainly vertically, due to three main reasons: changes in hydrostatic pressure with depth, changes in salinity and changes in temperature due to unequal heating of the water mass by the sun's rays. As a result of the combined effect of these reasons, the speed of sound propagation, which is about 1450 m/sec for fresh water and about 1500 m/sec for sea water, changes with depth, and the law of change depends on the time of year, time of day, depth of the reservoir and a number of other reasons. . Sound rays emerging from the source at a certain angle to the horizon are bent, and the direction of the bend depends on the distribution of sound speeds in the medium. In summer, when the upper layers are warmer than the lower ones, the rays bend downwards and are mostly reflected from the bottom, losing a significant share of their energy. On the contrary, in winter, when the lower layers of water maintain their temperature, while the upper layers cool, the rays bend upward and undergo multiple reflections from the surface of the water, during which much less energy is lost. Therefore, in winter the range of sound propagation is greater than in summer. Due to refraction, so-called dead zones, i.e. areas located close to the source in which there is no audibility.

The presence of refraction, however, can lead to an increase in the range of sound propagation - the phenomenon of ultra-long-range propagation of sounds under water. At some depth below the surface of the water there is a layer in which sound travels at the lowest speed; Above this depth, the speed of sound increases due to an increase in temperature, and below this depth, due to an increase in hydrostatic pressure with depth. This layer is a kind of underwater sound channel. A beam that has deviated from the axis of the channel up or down, due to refraction, always tends to fall back into it. If you place the source and receiver of sound in this layer, then even sounds of medium intensity (for example, explosions of small charges of 1-2 kg) can be recorded at distances of hundreds and thousands of km. A significant increase in the range of sound propagation in the presence of an underwater sound channel can be observed when the sound source and receiver are located not necessarily near the channel axis, but, for example, near the surface. In this case, the rays, refracting downward, enter deep-sea layers, where they are deflected upward and exit again to the surface at a distance of several tens of kilometers from the source. Next, the pattern of ray propagation is repeated and as a result a sequence of so-called rays is formed. secondary illuminated zones, which are usually traced to distances of several hundred km.

The propagation of high-frequency sounds, in particular ultrasounds, when the wavelengths are very small, is influenced by small inhomogeneities usually found in natural bodies of water: microorganisms, gas bubbles, etc. These inhomogeneities act in two ways: they absorb and scatter the energy of sound waves. As a result, as the frequency of sound vibrations increases, the range of their propagation decreases. This effect is especially noticeable in the surface layer of water, where there are most inhomogeneities. The scattering of sound by inhomogeneities, as well as uneven surfaces of water and the bottom, causes the phenomenon of underwater reverberation, which accompanies the sending of a sound impulse: sound waves, reflecting from a set of inhomogeneities and merging, give rise to a prolongation of the sound impulse, which continues after its end, similar to the reverberation observed in enclosed spaces. Underwater reverberation is a fairly significant interference for a number of practical applications of hydroacoustics, in particular for sonar.

The range of propagation of underwater sounds is also limited by the so-called. the sea's own noises, which have a dual origin. Some of the noise comes from the impact of waves on the surface of the water, from the sea surf, from the noise of rolling pebbles, etc. The other part is related to marine fauna; This includes sounds made by fish and other marine animals.

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 a 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.

Interesting facts: where does sound travel faster?

During a thunderstorm, a flash of lightning is first visible and only after a while the rumble of thunder is heard. This delay occurs because the speed of sound in air is much less than the speed of light coming from lightning. It’s interesting to remember in which medium sound travels fastest, and where it doesn’t travel at all?

Experiments and theoretical calculations of the speed of sound in air have been undertaken since the 17th century, but only two centuries later the French scientist Pierre-Simon de Laplace derived the final formula for its determination. The speed of sound depends on temperature: as air temperature increases, it increases, and as air temperature decreases, it decreases. At 0° the speed of sound is 331 m/s (1192 km/h), at +20° it is already 343 m/s (1235 km/h).

The speed of sound in liquids is usually greater than the speed of sound in air. Experiments to determine speed were first carried out on Lake Geneva in 1826. Two physicists got into boats and drove away for 14 km. On one boat they set fire to gunpowder and at the same time struck a bell lowered into the water. The sound of the bell was picked up on another boat using a special horn, also lowered into the water. Based on the time interval between the flash of light and the arrival of the sound signal, the speed of sound in water was determined. At a temperature of +8° it turned out to be approximately 1440 m/s. People working in underwater structures confirm that shore sounds can be clearly heard underwater, and fishermen know that fish swim away at the slightest suspicious noise on the shore.

The speed of sound in solids is greater than in liquids and gases. For example, if you put your ear to the rail, then after hitting the other end of the rail the person will hear two sounds. One of them will “come” to the ear by rail, the other by air. The earth has good sound conductivity. Therefore, in ancient times, during a siege, “listeners” were placed in the fortress walls, who, by the sound transmitted by the earth, could determine whether the enemy was digging into the walls or not, whether the cavalry was rushing or not. By the way, thanks to this, people who have lost their hearing are sometimes able to dance to music that reaches their auditory nerves not through the air and the outer ear, but through the floor and bones.

The speed of sound is the speed of propagation of elastic waves in a medium, both longitudinal (in gases, liquids or solids) and transverse, shear (in solids), determined by the elasticity and density of the medium. The speed of sound in solids is greater than in liquids. In liquids, including water, sound travels more than 4 times faster than in air. The speed of sound in gases depends on the temperature of the medium, in single crystals - on the direction of wave propagation.

>>Physics: Sound in various media

For sound to propagate, an elastic medium is required. In a vacuum, sound waves cannot propagate, since there is nothing there to vibrate. This can be verified by simple experience. If we place an electric bell under a glass bell, then as the air is pumped out from under the bell, we will find that the sound from the bell will become weaker and weaker until it stops completely.

Sound in gases. It is known that during a thunderstorm we first see a flash of lightning and only after some time we hear the rumble of thunder (Fig. 52). This delay occurs because the speed of sound in air is much less than the speed of light coming from lightning.

The speed of sound in air was first measured in 1636 by the French scientist M. Mersenne. At a temperature of 20 °C it is equal to 343 m/s, i.e. 1235 km/h. Note that it is to this value that the speed of a bullet fired from a Kalashnikov machine gun (PK) decreases at a distance of 800 m. The initial speed of the bullet is 825 m/s, which significantly exceeds the speed of sound in air. Therefore, a person who hears the sound of a shot or the whistle of a bullet need not worry: this bullet has already passed him. The bullet outruns the sound of the shot and reaches its victim before the sound arrives.

The speed of sound depends on the temperature of the medium: with increasing air temperature it increases, and with decreasing air temperature it decreases. At 0 °C, the speed of sound in air is 331 m/s.

Sound travels at different speeds in different gases. The greater the mass of gas molecules, the lower the speed of sound in it. Thus, at a temperature of 0 °C, the speed of sound in hydrogen is 1284 m/s, in helium - 965 m/s, and in oxygen - 316 m/s.

Sound in liquids. The speed of sound in liquids is usually greater than the speed of sound in gases. The speed of sound in water was first measured in 1826 by J. Colladon and J. Sturm. They carried out their experiments on Lake Geneva in Switzerland (Fig. 53). On one boat they set fire to gunpowder and at the same time struck a bell lowered into the water. The sound of this bell, using a special horn, also lowered into the water, was captured on another boat, which was located at a distance of 14 km from the first. Based on the time interval between the flash of light and the arrival of the sound signal, the speed of sound in water was determined. At a temperature of 8 °C it turned out to be approximately 1440 m/s.

At the boundary between two different media, part of the sound wave is reflected, and part travels further. When sound passes from air into water, 99.9% of the sound energy is reflected back, but the pressure in the sound wave transmitted into the water is almost 2 times greater. The hearing system of fish reacts precisely to this. Therefore, for example, screams and noises above the surface of the water are a sure way to scare away marine life. A person who finds himself under water will not be deafened by these screams: when immersed in water, air “plugs” will remain in his ears, which will save him from sound overload.

When sound passes from water to air, 99.9% of the energy is reflected again. But if during the transition from air to water the sound pressure increased, now, on the contrary, it sharply decreases. It is for this reason, for example, that the sound that occurs under water when one stone hits another does not reach a person in the air.

This behavior of sound at the boundary between water and air gave our ancestors the basis to consider the underwater world a “world of silence.” Hence the expression: “Mute as a fish.” However, Leonardo da Vinci also suggested listening to underwater sounds by putting your ear to an oar lowered into the water. Using this method, you can make sure that the fish are actually quite talkative.

Sound in solids. The speed of sound in solids is greater than in liquids and gases. If you put your ear to the rail, you will hear two sounds after hitting the other end of the rail. One of them will reach your ear by rail, the other by air.

The earth has good sound conductivity. Therefore, in the old days, during a siege, “listeners” were placed in the fortress walls, who, by the sound transmitted by the earth, could determine whether the enemy was digging into the walls or not. Putting their ears to the ground, they also monitored the approach of enemy cavalry.

Solids conduct sound well. Thanks to this, people who have lost their hearing are sometimes able to dance to music that reaches their auditory nerves not through the air and the outer ear, but through the floor and bones.

1. Why during a thunderstorm do we first see lightning and only then hear thunder? 2. What does the speed of sound in gases depend on? 3. Why doesn’t a person standing on the river bank hear sounds arising under water? 4. Why were the “hearers” who in ancient times monitored the enemy’s excavation work often blind people?

Experimental task . Place your wristwatch on one end of a board (or long wooden ruler) and place your ear on the other end. What do you hear? Explain the phenomenon.

S.V. Gromov, N.A. Rodina, Physics 8th grade

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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 this case, the 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 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. Under 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.