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We know that sound travels through the air. That is why we can hear. No sound can exist in a vacuum. But if sound is transmitted through the air, due to the interaction of its particles, will it not be transmitted by other substances? Will be.
Propagation and speed of sound in different media
Sound is not only transmitted by air. Probably everyone knows that if you put your ear to the wall, you can hear conversations in the next room. AT this case sound is transmitted through the wall. Sounds propagate in water and in other media. Moreover, the propagation of sound in various environments happens differently. The speed of sound varies depending on the substance.
Curiously, 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 the medium, and sound waves are transmitted faster in environments with better conductivity.
The density and conductivity of water is greater than that of air, but less than that of metal. Accordingly, the sound is transmitted differently. When moving from one medium to another, the speed of sound changes.
The length of a sound wave also changes as it passes from one medium to another. Only its frequency remains the same. But that's why we can distinguish who specifically speaks even through the walls.
Since sound is vibrations, all the laws and formulas for vibrations and waves are well applicable to sound vibrations. When calculating the speed of sound in air, one should also take into account the fact that this speed depends on the air temperature. As the 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. That is why sounds are well transmitted by the earth. Putting your ear to the ground, you can hear from afar the sound of footsteps, the clatter of hooves, and so on.
In childhood, everyone must have had fun by putting their ear to the rails. The sound of train wheels is transmitted along the rails for several kilometers. To create the reverse effect of sound absorption, soft and porous materials are used.
For example, in order to protect a room from extraneous sounds, or, conversely, in order to prevent sounds from escaping from the room to the outside, the room is treated and soundproofed. The walls, floor and ceiling are upholstered with special materials based on foamed polymers. In such an upholstery, all sounds subside very quickly.
Questions.
1. With what frequency does the human eardrum vibrate when sound reaches it?
The tympanic membrane of the human ear vibrates with the frequency of the sound that has come to it.
2. What kind of wave - longitudinal or transverse - is sound propagating in air? in water?
In air and water, sound travels in longitudinal waves.
3. Give an example showing that a sound wave does not propagate instantly, but at a certain speed.
Most good example- a flash of lightning, and then the thunder that comes after it.
4. What is the speed of sound propagation in air at 20 °C?
The speed of sound propagation in air at 20°C is 343 m/s 2 .
5. Does the speed of sound depend on the medium in which it propagates?
V = 340 m/s. Yes, it depends.
Exercises.
1. Determine the speed of sound in water if a source oscillating with a period of 0.002 s excites waves of 2.9 m in length in water.
2. Determine the length of the 725 Hz sound wave in air, water and glass.
3. One end of a long metal pipe was hit once with a hammer. Will the sound from the impact propagate to the other end of the pipe through the metal? through the air inside the pipe? How many blows will a person standing at the other end of the pipe hear?
The person will hear two hits. One sound will come to him by metal pipe and the other by air.
4. An observer standing near a straight section railway, I saw steam above the whistle of a steam locomotive going in the distance. 2s after the appearance of steam, he heard the sound of a whistle, and after 34 s the locomotive passed by the observer. Determine the speed of the locomotive.
5. The observer moves away from the bell, which is struck every second. At first, the visible and audible beats coincide. Then they stop matching. Then, at some distance of the observer from the bell, the visible and audible strikes coincide again. Explain this phenomenon.
If a sound wave encounters no obstacles in its path, it propagates uniformly in all directions. But not every obstacle becomes an obstacle for her.
Having met an obstacle in its path, the sound can bend around it, be reflected, refracted or absorbed.
sound diffraction
We can talk to a person standing around the corner of a building, behind a tree, or behind a fence, although we cannot see him. We hear it because the sound is able to bend around these objects and penetrate into the area behind them.
The ability of a wave to go around an obstacle is called diffraction .
Diffraction is possible when the wavelength of the sound wave exceeds the size of the obstacle. Low frequency sound waves are quite long. For example, at a frequency of 100 Hz, it is 3.37 m. As the frequency decreases, the length becomes even longer. Therefore, a sound wave easily bends around objects commensurate with it. The trees in the park do not prevent us from hearing the sound at all, because the diameters of their trunks are much smaller than the wavelength of the sound wave.
Due to diffraction, sound waves penetrate through gaps and holes in an obstacle and propagate behind them.
Let us place a flat screen with a hole in the path of the sound wave.
When the sound wave length ƛ much larger than the hole diameter D , or these values are approximately equal, then behind the hole the sound will reach all points of the area that is behind the screen (the area of sound shadow). The outgoing wave front will look like a hemisphere.
If ƛ only slightly smaller than the slot diameter, then the main part of the wave propagates directly, and 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, it is possible different variants its further distribution. Sound can be reflected from the interface, it can go to another medium without changing direction, or it can be refracted, that is, go by changing its direction.
Let's suppose that an obstacle has appeared in the path of the sound wave, the size of which is much larger than the wavelength, for example, a sheer cliff. How will the sound behave? Since it cannot go around this obstacle, it will be reflected from it. Behind the obstacle is acoustic shadow zone .
Sound reflected from an obstacle is called echo .
The nature of the reflection of the sound wave can be different. It depends on the shape of the reflective surface.
reflection is called a change in the direction of a sound wave at the interface between two different environments. When reflected, the wave returns to the medium from which it came.
If the surface is flat, the sound is reflected from it in the same way as a ray of light is reflected in a mirror.
Sound rays reflected from a concave surface are focused at one point.
The convex surface dissipates sound.
The effect of dispersion is given by convex columns, large moldings, chandeliers, etc.
Sound does not pass from one medium to another, but is reflected from it if the densities of the media differ significantly. So, the sound that appeared in the water does not pass into the air. Reflected from the interface, it remains in the water. A person standing on the river bank will not hear this sound. This is due to the large difference in wave resistance of water and air. In acoustics, wave resistance is equal to the product of the density of the medium and the speed of sound in it. Since the wave resistance of gases is much less than the wave resistance of liquids and solids, then hitting the boundary of air and water, the sound wave is reflected.
Fish in the water do not hear the sound that appears above the surface of the water, but they clearly distinguish the sound, the source of which is a body vibrating in the water.
refraction of sound
Changing the direction of sound propagation is called refraction . This phenomenon occurs when sound passes from one medium to another, and the speed of its propagation in these media is different.
The ratio of the sine of the angle of incidence to the sine of the angle of reflection is equal to the ratio of the speeds of sound propagation in media.
where i - angle of incidence,
r is the angle of reflection,
v1 is the speed of sound propagation in the first medium,
v2 is the speed of sound propagation in the second medium,
n is the index of refraction.
The refraction of sound is called refraction .
If the sound wave does not fall perpendicular to the surface, but at an angle other than 90°, then the refracted wave will deviate from the direction of the incident wave.
Sound refraction can be observed not only at the interface between media. Sound waves can change their direction in an inhomogeneous medium - the atmosphere, the ocean.
In the atmosphere, refraction is caused by changes in air temperature, the 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 salinities.
sound absorption
When a sound wave hits a surface, some of its energy is absorbed. And how much energy a medium can absorb can be determined by knowing the sound absorption coefficient. This coefficient shows what part of the energy sound vibrations absorbs 1 m 2 obstacles. It has a value from 0 to 1.
The unit of measure for sound absorption is called sabin . It got its name from the American physicist Wallace Clement Sabin, founder of architectural acoustics. 1 sabin is the energy that is absorbed by 1 m 2 of the surface, the absorption coefficient of which is 1. That is, such a surface must absorb absolutely all the energy of the sound wave.
Reverberation
Wallace Sabin
The property of materials to absorb sound is widely used in architecture. While researching the acoustics of the Lecture Hall, part of the Fogg Museum, Wallace Clement Sabin concluded that there was a relationship between the size of the auditorium, the acoustic conditions, the type and area of sound-absorbing materials, and reverberation time .
Reverb called the process of reflection of a sound wave from obstacles and its gradual attenuation after turning off the sound source. AT indoors sound can bounce off walls and objects multiple times. As a result, various echo signals appear, each of which sounds as if apart. This effect is called reverb effect .
The most important feature of a room is reverberation time , which was introduced and calculated by Sabin.
where V - the volume of the room,
BUT – general sound absorption.
where a i is the sound absorption coefficient of the material,
Si is the area of each surface.
If the reverberation time is long, the sounds seem to "roam" around the room. They overlap each other, drown out the main source of sound, and the hall becomes booming. With a short reverberation time, the walls quickly absorb sounds, and they become deaf. Therefore, each room must have its own exact calculation.
Based on the results of his calculations, Sabin arranged the sound-absorbing materials in such a way that the "echo effect" was reduced. And the Boston Symphony Hall, on which he was an acoustic consultant, is still considered one of the the best halls in the world.
Over long distances, sound energy propagates only along gentle rays, which do not touch the ocean floor all the way. In this case, the limitation imposed by the medium on the range of sound propagation is its absorption in sea water. The main mechanism of absorption is associated with relaxation processes that accompany the violation of the thermodynamic equilibrium between ions and molecules of salts dissolved in water by an acoustic wave. It should be noted that the main role in absorption in a wide range of sound frequencies, magnesium sulphide salt MgSO4 belongs, although in percentage terms its content in sea water is quite small - almost 10 times less than, for example, NaCl rock salt, which nevertheless does not play any noticeable role in sound absorption.
Absorption in sea water, generally speaking, is greater the higher the frequency of the sound. At frequencies from 3-5 to at least 100 kHz, where the above mechanism dominates, the absorption is proportional to the frequency to a power of about 3/2. At lower frequencies, a new absorption mechanism is activated (possibly due to the presence of boron salts in water), which becomes especially noticeable in the range of hundreds of hertz; here, the absorption level is anomalously high and decreases much more slowly with decreasing frequency.
To more clearly imagine the quantitative characteristics of absorption in sea water, we note that due to this effect, sound with a frequency of 100 Hz is attenuated by a factor of 10 on a path of 10 thousand km, and with a frequency of 10 kHz - at a distance of only 10 km (Fig. 2). Thus, only low-frequency sound waves can be used for long-range underwater communications, for long-range detection of underwater obstacles, and the like.
Figure 2 - Distances at which sounds of different frequencies attenuate 10 times when propagating in sea water.
In the region of audible sounds for the frequency range of 20-2000 Hz, the range of propagation under water of medium-intensity sounds reaches 15-20 km, and in the region of ultrasound - 3-5 km.
Based on the values of sound attenuation observed in laboratory conditions in small volumes of water, one would expect much greater ranges. However, in vivo In addition to damping due to the properties of water itself (the so-called viscous damping), its scattering and absorption by various inhomogeneities of the medium also affect.
The refraction of sound, or the curvature of the path of the sound beam, is caused by the heterogeneity of the properties of water, mainly along the vertical, due to three main reasons: changes in hydrostatic pressure with depth, changes in salinity, and changes in temperature due to uneven heating of the water mass by the sun's rays. As a result of the combined action of these causes, the speed of sound propagation, which is about 1450 m / s for fresh water and about 1500 m / s for sea water, changes with depth, and the law of change depends on the season, time of day, depth of the reservoir, and a number of other reasons . Sound rays leaving the source at some angle to the horizon are bent, and the direction of the bend depends on the distribution of sound velocities in the medium. In summer, when the upper layers are warmer than the lower ones, the rays bend down and are mostly reflected from the bottom, losing a significant portion of their energy. On the contrary, in winter, when the lower layers of the water maintain their temperature, while the upper layers cool, the rays bend upward and undergo multiple reflections from the surface of the water, during which much less energy is lost. Therefore, in winter, the sound propagation distance is greater than in summer. Due to refraction, so-called. dead zones, i.e. areas located close to the source in which there is no audibility.
The presence of refraction, however, can lead to an increase in the range of sound propagation - the phenomenon of ultra-long propagation of sounds under water. At some depth below the surface of the water there is a layer in which sound propagates at the lowest speed; above this depth, the speed of sound increases due to an increase in temperature, and below this, due to an increase in hydrostatic pressure with depth. This layer is a kind of underwater sound channel. A beam deviated from the axis of the channel up or down, due to refraction, always tends to get back into it. If a sound source and receiver are placed in this layer, then even sounds of medium intensity (for example, explosions of small charges of 1-2 kg) can be recorded at distances of hundreds and thousands of kilometers. A significant increase in the sound propagation range in the presence of an underwater sound channel can be observed when the sound source and receiver are located not necessarily near the channel axis, but, for example, near the surface. In this case, the rays, refracting downwards, enter the deep layers, where they deviate upwards and come out again to the surface at a distance of several tens of kilometers from the source. Further, the pattern of propagation of rays is repeated, and as a result, a sequence of so-called. secondary illuminated zones, which are usually traced to distances of several hundred km.
The propagation of high-frequency sounds, in particular ultrasounds, when the wavelengths are very small, is influenced by small inhomogeneities that are usually found in natural reservoirs: microorganisms, gas bubbles, etc. These inhomogeneities act in two ways: they absorb and scatter the energy of sound waves. As a result, with an increase in the frequency of sound vibrations, the range of their propagation is reduced. This effect is especially noticeable in the surface layer of water, where there are the most inhomogeneities. Scattering of sound by inhomogeneities, as well as by irregularities in the water surface and the bottom, causes the phenomenon of underwater reverberation that accompanies the sending of a sound impulse: sound waves, reflecting from a combination of inhomogeneities and merging, give a delay of the sound impulse, which continues after its end, similar to reverberation observed in enclosed spaces. Underwater reverberation is a rather significant interference for a number of practical applications of hydroacoustics, in particular for sonar.
The limits of the propagation range of underwater sounds are also limited by the so-called. own noises of the sea, which have a twofold origin. Part of the noise arises from the impact of waves on the surface of the water, from the surf, from the noise of rolling pebbles, etc. The other part is related to the marine fauna; this includes sounds produced by fish and other marine animals.
