The article examines the characteristics of sound phenomena in the atmosphere: the speed of sound propagation in the air, the influence of wind and fog on the propagation of sound.
Longitudinal vibrations of matter particles, propagating through the material medium (air, water and solids) and reaching the human ear, cause sensations called sound.
Atmospheric air always contains sound waves of varying frequencies and strengths. Some of these waves are created artificially by humans, and some of the sounds are of meteorological origin.
Sounds of meteorological origin include thunder, the howling of the wind, the hum of wires, the noise and rustling of trees, the “voice” of the sea, the sounds of solid and liquid precipitation falling on the earth’s surface, the sounds of the surf off the coast of seas and lakes, and others.
The speed of sound propagation in the atmosphere is affected by air temperature and humidity, as well as wind (direction and its strength). On average, the speed of sound in the atmosphere is 333 m/s. As air temperature increases, the speed of sound increases slightly. Changes in absolute air humidity have less effect on the speed of sound.
The speed of sound in air is determined by Laplace's formula:
(1),
where p is pressure; ? - air density; c? - heat capacity of air at constant pressure; cp is the heat capacity of air at constant volume.
Using the gas equation of state, it is possible to obtain a number of dependences of the speed of sound on meteorological parameters.
The speed of sound in dry air is determined by the formula:
c0 = 20.1 ?T m/s, (2)
and in humid air:
с0 = 20.1 ?ТВ m/s, (3)
where TV = the so-called acoustic virtual temperature, which is determined by the formula TV = T (1+ 0.275 e/p).
When the air temperature changes by 1°, the speed of sound changes by 0.61 m/s. The speed of sound depends on the value of the ratio e/p (the ratio of humidity to pressure), but this dependence is small, and, for example, when the elasticity of water vapor is less than 7 mm, neglecting it gives an error in the speed of sound not exceeding 0.5 m/sec.
At normal pressure and T = 0 °C, the speed of sound in dry air is 333 m/sec. In humid air, the speed of sound can be determined by the formula:
c = 333 + 0.6t + 0.07e (4)
In the temperature range (t) from -20° to +30°, this formula gives an error in the speed of sound of no more than ± 0.5 m/sec. From the above formulas it is clear that the speed of sound increases with increasing temperature and air humidity.
The wind has a strong influence: the speed of sound in the direction of the wind increases, against the wind it decreases. The presence of wind in the atmosphere causes the sound wave to drift, which gives the impression that the sound source has shifted. The speed of sound in this case (c1) is determined by the expression:
c1 = c + U cos ?, (1)
where U is the wind speed; ? — the angle between the wind direction at the observation point and the observed direction of sound arrival.
Knowing the speed of sound propagation in the atmosphere is of great importance when solving a number of problems in studying the upper layers of the atmosphere using the acoustic method. Using the average speed of sound in the atmosphere, you can find out the distance from your location to the point where thunder occurs. To do this, you need to determine the number of seconds between the visible flash of lightning and the moment the sound of thunder arrives. Then you need to multiply the average speed of sound in the atmosphere - 333 m/sec. for the resulting number of seconds.
Sound speed- the speed of propagation of elastic waves in a medium: both longitudinal (in gases, liquids or solids) and transverse, shear (in solids). It is determined by the elasticity and density of the medium: as a rule, the speed of sound in gases is less than in liquids, and in liquids it is less than in solids. Also, in gases, the speed of sound depends on the temperature of a given substance, in single crystals - on the direction of wave propagation. Usually does not depend on the frequency of the wave and its amplitude; in cases where the speed of sound depends on frequency, we speak of sound dispersion.
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Already in ancient authors there is an indication that sound is caused by the oscillatory movement of the body (Ptolemy, Euclid). Aristotle notes that the speed of sound has a finite value, and correctly imagines the nature of sound. Attempts to experimentally determine the speed of sound date back to the first half of the 17th century. F. Bacon in the New Organon pointed out the possibility of determining the speed of sound by comparing the time intervals between a flash of light and the sound of a gunshot. Using this method, various researchers (M. Mersenne, P. Gassendi, W. Derham, a group of scientists from the Paris Academy of Sciences - D. Cassini, J. Picard, Huygens, Roemer) determined the value of the speed of sound (depending on the experimental conditions, 350- 390 m/s). Theoretically, the question of the speed of sound was first considered by I. Newton in his “Principles”. Newton actually assumed that sound propagation is isothermal, and therefore received an underestimate. The correct theoretical value for the speed of sound was obtained by Laplace.
Calculation of speed in liquid and gas
The speed of sound in a homogeneous liquid (or gas) is calculated by the formula:
c = 1 β ρ (\displaystyle c=(\sqrt (\frac (1)(\beta \rho ))))In partial derivatives:
c = − v 2 (∂ p ∂ v) s = − v 2 C p C v (∂ p ∂ v) T (\displaystyle c=(\sqrt (-v^(2)\left((\frac (\ partial p)(\partial v))\right)_(s)))=(\sqrt (-v^(2)(\frac (C_(p))(C_(v)))\left((\ frac (\partial p)(\partial v))\right)_(T))))Where β (\displaystyle \beta )- adiabatic compressibility of the medium; ρ (\displaystyle \rho )- density; C p (\displaystyle C_(p))- isobaric heat capacity; C v (\displaystyle C_(v))- isochoric heat capacity; p (\displaystyle p), v (\displaystyle v), T (\displaystyle T)- pressure, specific volume and temperature of the medium; s (\displaystyle s)- entropy of the medium.
For solutions and other complex physical and chemical systems (for example, natural gas, oil), these expressions can give a very large error.
Solids
In the presence of interfaces, elastic energy can be transferred through surface waves of various types, the speed of which differs from the speed of longitudinal and transverse waves. The energy of these oscillations can be many times greater than the energy of body waves.
The observer used a watch to note the time elapsed between the appearance of the flash and the moment when the sound was heard. The time it took the light to travel this distance was neglected. In order to eliminate the influence of the wind as much as possible, there was a cannon and an observer on each side, and each cannon fired at approximately the same time.
The average value of two time measurements was taken, and based on it. It turned out to be approximately equal to 340 ms -1. The big disadvantage of this method of measurement was that the gun was not always at hand!
Many examinees describe a similar method. One student stands on one side of the football field with a starting pistol, and the other stands on the other side with a stopwatch. The distance between them is carefully measured with a tape measure. The student starts the stopwatch when he sees smoke coming from the barrel and stops it when he hears the sound. The same is done when they switch places to compensate for the effects of the wind. Then the average time is determined.
Since sound travels at 340 ms -1 , a stopwatch will likely not be accurate enough. It is preferable to operate in centiseconds or milliseconds.
Measuring the speed of sound using echo
When a short sharp sound, such as a clap, is produced, the wave impulse can be reflected by a large obstacle, such as a wall, and heard by an observer. This reflected impulse is called an echo. Let's imagine that a person stands at a distance of 50 m from the wall and makes one clap. When the echo is heard, the sound has traveled 100 m. Measuring this interval with a stopwatch will not be very accurate. However, if a second person holds a stopwatch and the first person claps, then the time for a large number of echo sounds can be obtained with reasonable accuracy.
Suppose that the distance at which the clapping person is in front of the wall is 50 m, and the time interval between the first and one hundred and first clap is 30 s, then:
sound speed= distance traveled / time of one clap = 100m: 30 / 100 s = 333 ms -1
Measuring the speed of sound using an oscilloscope
A more sophisticated way to directly measure the speed of sound is to use an oscilloscope. The loudspeaker emits pulses at regular intervals, and they are recorded by a cathode ray oscilloscope (see figure). When a pulse is received by the microphone, it will also be recorded by the oscilloscope. If the timing characteristics of the oscilloscope are known, the time interval between two pulses can be found.The distance between the loudspeaker and microphone is measured. The speed of sound can be found using the formula speed = distance / time.
Speed of sound in various media
The speed of sound is higher in solids than in liquids and higher in liquids than in gases. Past experiments on Lake Geneva have shown that the speed of sound in water is significantly higher than in air. In fresh water, the speed of sound is 1410 ms -1, in sea water - 1540 ms -1. In iron, the speed of sound is approximately 5000 ms -1.
By sending sound signals and noting the time interval before the arrival of the reflected signal (echo), it is possible to determine the depth of the sea and the location of schools of fish. During the war, high-frequency sounders were used to detect mines. Bats in flight use a special form of echo to detect obstacles. The bat emits a high-frequency sound that bounces off an object in its path. The mouse hears the echo, locates the object and avoids it.
The speed of sound in air depends on atmospheric conditions. The speed of sound is proportional to the square root of pressure divided by density. Changes in pressure do not affect the speed of sound in air. This is because an increase in pressure entails a corresponding increase in density and the ratio of pressure to density remains constant.
The speed of sound in air (as in any gas) is affected by temperature changes. The laws for gases indicate that the ratio of pressure to density is proportional to . Thus, the speed of sound is proportional to √T. It is easier to break the sound barrier at higher altitudes because the temperature is lower there.
The speed of sound is affected by changes in humidity. The density of water vapor is less than the density of dry air at the same pressure. At night, when humidity rises, sound travels faster. Sounds are heard more clearly on a quiet, foggy night.
This is partly due to the increased humidity, and partly because in these conditions there is usually a temperature inversion, in which sounds are refracted in such a way that they do not dissipate.
SOUND SPEED- speed of propagation of an elastic wave in the medium. Determined by the elasticity and density of the medium. For running without changing shape with speed With in the direction of the axis X, sound pressure R can be represented in the form p = p(x - - ct), Where t- time. For plane harmony, waves in a medium without dispersion and SZ. expressed in terms of frequency w and k Floy c = w/k. With speed With the harmonic phase propagates. waves, so With called also phase S. z. In media in which the shape of an arbitrary wave changes during propagation, harmonic. the waves nevertheless retain their shape, but the phase velocity turns out to be different for different frequencies, i.e. sound dispersion.In these cases the concept is also used group velocity. At large amplitudes, nonlinear effects appear (see. Nonlinear acoustics), leading to a change in any waves, including harmonic ones: the speed of propagation of each point of the wave profile depends on the pressure at this point, increasing with increasing pressure, which leads to distortion of the wave shape.
Speed of sound in gases and liquids. In gases and liquids, sound propagates in the form of volumetric compression-discharge waves. If the propagation process occurs adiabatically (which, as a rule, is the case), i.e., the change in temperature in the sound wave does not have time to level out even after 1 / 2 , period the heat from the heated (compressed) areas does not have time to move to the cold (rarefied) areas, then S. z. equal to
, Where R is the pressure in the substance, is its density, and the index s shows that the derivative is taken at constant entropy. This S. z. called adiabatic. Expression for S. z. can also be written in one of the following forms: Where TO hell - adiabatic. modulus of all-round compression of matter, - adiabatic. compressibility,
- isothermal compressibility, = - the ratio of heat capacities at constant pressure and volume.In bounded solids, in addition to longitudinal and transverse waves, there are other types of waves. Thus, along the free surface of a solid body or along its boundary with another medium, they propagate surface acoustic waves, the speed of which is less than the speed of body waves characteristic of a given material. For plates, rods and other solid acoustic materials. waveguides are characteristic normal waves The speed of which is determined not only by the properties of the substance, but also by the geometry of the body. So, for example, S. z. for a longitudinal wave in a rod with a st, the transverse dimensions of which are much smaller than the wavelength of sound, different from the S. z. in an unrestricted environment with l(Table 3):
Methods for measuring S.z. can be divided into resonant, interferometric, pulsed and optical (see. Diffraction of light by ultrasound).Naib. Measurement accuracy is achieved using pulse-phase methods. Optical methods make it possible to measure S. z. at hypersonic frequencies (up to 10 11 -10 12 Hz). Accuracy abs. measurements S. z. on the best equipment approx. 10 -3%, while the accuracy is relative. measurements of the order of 10 -5% (for example, when studying the dependence With on temperature or magnetic fields or the concentration of impurities or defects).
Measurements of S. z. are used to define plurals. properties of matter, such as the ratio of heat capacities for gases, compressibility of gases and liquids, elastic moduli of solids, Debye temperature, etc. (see. Molecular acoustics). Determination of small changes in S. z. is sensitive. method of fixing impurities in gases and liquids. In solids, the measurement of S. z. and its dependence on different factors (temperature, magnetic field, etc.) allows you to study the structure of matter: the band structure of semiconductors, the structure of the Fermi surface in metals, etc.
Lit.: Landau L. D., L i f sh i c E. M., Theory of Elasticity, 4th ed., M., 1987; them, Hydrodynamics, 4th ed., M., 1988; Bergman L., and its application in science and technology, trans. from German, 2nd ed., M., 1957; Mikhailov I. G., Solovyov V. A., Syrnikov Yu. P., Fundamentals of molecular acoustics, M., 1964; Tables for calculating the speed of sound in sea water, L., 1965; Physical acoustics, ed. W. Mason, trans. from English, vol. 1, part A, M., 1966, ch. 4; t. 4, part B, M., 1970, ch. 7; Kolesnikov A.E., Ultrasonic measurements, 2nd ed., M., 1982; T r u e l l R., E l b a u m Ch., Ch i k B., Ultrasonic methods in solid state physics, trans. from English, M., 1972; Acoustic crystals, ed. M. P. Shaskolskoy, M., 1982; Krasilnikov V.A., Krylov V.V., Introduction to physical acoustics, M., 1984. A. L. Polyakova.
Today, many new settlers, when furnishing an apartment, are forced to carry out additional work, including soundproofing their home, because... The standard materials used make it possible to only partially hide what is going on in your own home, and not to be interested in the communication of your neighbors against your will.
In solids, it is affected at least by the density and elasticity of the substance resisting the wave. Therefore, when equipping premises, the layer adjacent to the load-bearing wall is made soundproof with “overlaps” at the top and bottom. It allows you to reduce decibels sometimes by more than 10 times. Then basalt mats are laid, and plasterboard sheets are placed on top, which reflect the sound outward from the apartment. When a sound wave “flies up” to such a structure, it is attenuated in the insulator layers, which are porous and soft. If the sound is strong, the materials that absorb it may even heat up.
Elastic substances, such as water, wood, and metals, transmit well, so we hear the beautiful “singing” of musical instruments. And some peoples in the past determined the approach of, for example, horsemen, by putting their ear to the ground, which is also quite elastic.
The speed of sound in km depends on the characteristics of the medium in which it propagates. In particular, the process can be affected by its pressure, chemical composition, temperature, elasticity, density and other parameters. For example, in a steel sheet a sound wave travels at a speed of 5100 meters per second, in glass - about 5000 m/s, in wood and granite - about 4000 m/s. To convert speed to kilometers per hour, you need to multiply the figures by 3600 (seconds per hour) and divide by 1000 (meters per kilometer).
The speed of sound in km in an aquatic environment is different for substances with different salinities. For fresh water at a temperature of 10 degrees Celsius it is about 1450 m/s, and at a temperature of 20 degrees Celsius and the same pressure it is already about 1490 m/s.
A salty environment is characterized by a obviously higher speed of sound vibrations.
The propagation of sound in air also depends on temperature. With a value of 20 for this parameter, sound waves travel at a speed of about 340 m/s, which is about 1200 km/h. And at zero degrees the speed slows down to 332 m/s. Returning to our apartment insulators, we can learn that in a material such as cork, which is often used to reduce external noise levels, the speed of sound in km is only 1800 km/h (500 meters per second). This is ten times lower than this characteristic in steel parts.
A sound wave is a longitudinal vibration of the medium in which it propagates. When, for example, the melody of a piece of music passes through some obstacle, its volume level decreases, because changes. At the same time, the frequency remains the same, thanks to which we hear a woman’s voice as a woman’s, and a man’s as a man’s. The most interesting place is where the speed of sound in km is close to zero. This is a vacuum in which waves of this type almost do not propagate. To demonstrate how this works, physicists place a ringing alarm clock under a hood from which the air is pumped out. The thinner the air, the quieter the bell is heard.
