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.
Sound is one of the components of our life, and people hear it everywhere. To consider this phenomenon in more detail, we first need to understand the concept itself. To do this, you need to turn to the encyclopedia, where it is written that “sound is elastic waves, propagating in any elastic medium and creating mechanical vibrations in it.” Speaking more in simple language- These are audible vibrations in any environment. The main characteristics of the sound depend on what it is. First of all, the speed of propagation, for example, in water differs from other environments.
Any sound analogue has certain properties(physical features) and qualities (reflection of these features in human sensations). For example, duration-duration, frequency-pitch, composition-timbre, and so on.
The speed of sound in water is much higher than, say, in air. Consequently, it spreads faster and is heard much further. This happens due to the high molecular density of the aquatic environment. It is 800 times denser than air and steel. It follows that the propagation of sound largely depends on the medium. Let's look at specific numbers. Thus, the speed of sound in water is 1430 m/s, in air - 331.5 m/s.
Low-frequency sound, for example, the noise produced by a running ship's engine, is always heard somewhat earlier than the ship appears in the visual range. Its speed depends on several things. If the temperature of the water increases, then, naturally, the speed of sound in the water increases. The same thing happens with an increase in water salinity and pressure, which increases with increasing water depth. Such a phenomenon as thermoclines can have a special role on speed. These are places where layers of water of different temperatures occur.
Also in such places it is different (due to the difference in temperature conditions). And when sound waves pass through such layers of different densities, they lose most of their strength. When a sound wave hits a thermocline, it is partially, or sometimes completely, reflected (the degree of reflection depends on the angle at which the sound falls), after which a shadow zone forms on the other side of this place. If we consider an example when a sound source is located in a body of water above the thermocline, then below it it will be not only difficult, but almost impossible to hear anything at all.
Which are heard above the surface, are never heard in the water itself. And the opposite happens when under the water layer: above it it does not sound. A striking example of this is modern divers. Their hearing is greatly reduced due to the fact that water affects their high speed sound in water reduces the quality of determining the direction from which it is moving. This dulls the stereophonic ability to perceive sound.
Under the layer of water enters the human ear most of all through the bones cranium heads, and not as in the atmosphere, through the eardrums. The result of this process is its perception by both ears simultaneously. At this time, the human brain is not able to distinguish between the places where the signals come from and in what intensity. The result is the emergence of consciousness that the sound seems to roll in from all sides at the same time, although this is far from the case.
In addition to what is described above, sound waves in water have such qualities as absorption, divergence and dispersion. The first is when the strength of sound in salt water gradually fades away due to friction of the aquatic environment and the salts in it. Divergence is manifested in the distance of sound from its source. It seems to dissolve in space like light, and as a result its intensity drops significantly. And the oscillations disappear completely due to dispersion by all sorts of obstacles and inhomogeneities of the environment.
Hydroacoustics (from Greek hydor- water, acousticoc- auditory) - the science of phenomena occurring in aquatic environment and related to the propagation, radiation and reception of acoustic waves. It includes issues of development and creation of hydroacoustic devices intended for use in the aquatic environment.
History of development
Basics of hydroacoustics
Features of the propagation of acoustic waves in water
Components of an echo event.
Beginning of comprehensive and basic research on the propagation of acoustic waves in water was initiated during the Second World War, which was dictated by the need to solve practical problems navies and primarily submarines. Experimental and theoretical works were continued in post-war years and summarized in a number of monographs. As a result of these works, some features of the propagation of acoustic waves in water were identified and clarified: absorption, attenuation, reflection and refraction.
Absorption of acoustic wave energy in sea water is caused by two processes: internal friction environment and dissociation of salts dissolved in it. The first process converts the energy of an acoustic wave into heat, and the second, transforming into chemical energy, removes molecules from an equilibrium state, and they disintegrate into ions. This type of absorption increases sharply with increasing frequency of acoustic vibration. The presence of suspended particles, microorganisms and temperature anomalies in water also leads to attenuation of the acoustic wave in water. As a rule, these losses are small and are included in the total absorption, but sometimes, as, for example, in the case of scattering from the wake of a ship, these losses can amount to up to 90%. The presence of temperature anomalies leads to the fact that the acoustic wave falls into acoustic shadow zones, where it can undergo multiple reflections.
The presence of interfaces between water - air and water - bottom leads to the reflection of an acoustic wave from them, and if in the first case the acoustic wave is completely reflected, then in the second case the reflection coefficient depends on the bottom material: a muddy bottom reflects poorly, sandy and rocky ones reflect well. . At shallow depths, due to multiple reflections of the acoustic wave between the bottom and the surface, an underwater sound channel appears, in which the acoustic wave can propagate to long distances. Changing the speed of sound at different depths leads to bending of sound “rays” - refraction.
Sound refraction (curvature of the sound beam path)
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Refraction of sound in water: a - in summer; b - in winter; on the left is the change in speed with depth. The speed of sound propagation changes with depth, and changes depend on the time of year and day, the depth of the reservoir and a number of other reasons. Sound rays emerging from a source at a certain angle to the horizon are bent, and the direction of bending 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. ; in winter, when the lower layers of water maintain their temperature, while the upper layers cool, the rays bend upward and are repeatedly reflected from the surface of the water, while significantly less energy is lost. Therefore, in winter the range of sound propagation is greater than in summer. The vertical distribution of sound speed (VSD) and the velocity gradient have a decisive influence on the propagation of sound in the marine environment. The distribution of sound speed in different areas of the World Ocean is different and changes over time. There are several typical cases of VRSD: |
Dispersion and absorption of sound by inhomogeneities of the medium.
Propagation of sound in underwater sound. channel: a - change in the speed of sound with depth; b - ray path in the sound channel. The propagation of high-frequency sounds, when the wavelengths are very small, is influenced by small inhomogeneities usually found in natural bodies of water: gas bubbles, microorganisms, etc. These inhomogeneities act in two ways: they absorb and dissipate energy 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 dispersion 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 pulse: sound waves, reflecting from a set of inhomogeneities and merging, give rise to a prolongation of the sound pulse, which continues after its end. The limits of the propagation range of underwater sounds are also limited by the natural noise of the sea, which has a dual origin: part of the noise arises from the impacts of waves on the surface of the water, from the sea surf, from the noise of rolling pebbles, etc.; the other part is associated with marine fauna (sounds produced by hydrobionts: fish and other marine animals). Biohydroacoustics deals with this very serious aspect. |
Sound wave propagation range
The propagation range of sound waves is complex function radiation frequency, which is uniquely related to the wavelength of the acoustic signal. As is known, high-frequency acoustic signals quickly attenuate due to strong absorption by the aquatic environment. Low-frequency signals, on the contrary, are capable of propagating over long distances in the aquatic environment. Thus, an acoustic signal with a frequency of 50 Hz can propagate in the ocean over distances of thousands of kilometers, while a signal with a frequency of 100 kHz, typical for side-scan sonar, has a propagation range of only 1-2 km. The approximate operating ranges of modern sonars with different acoustic signal frequencies (wavelengths) are given in the table:
Areas of use.
Hydroacoustics has received wide practical application, since it has not yet been created effective system transmission of electromagnetic waves under water over any significant distance, and sound is therefore the only possible means connections underwater. For these purposes, sound frequencies from 300 to 10,000 Hz and ultrasound from 10,000 Hz and above are used. Electrodynamic and piezoelectric emitters and hydrophones are used as emitters and receivers in the sound domain, and piezoelectric and magnetostrictive ones in the ultrasonic domain.
The most significant applications of hydroacoustics:
- To solve military problems;
- Marine navigation;
- Sound communication;
- Fishing exploration;
- Oceanological research;
- Areas of activity for the development of the resources of the bottom of the World Ocean;
- Using acoustics in the pool (at home or in a synchronized swimming training center)
- Training sea animals.
Notes
Literature and sources of information
LITERATURE:
- V.V. Shuleikin Physics of the sea. - Moscow: “Science”, 1968. - 1090 p.
- I.A. Romanian Basics of hydroacoustics. - Moscow: “Shipbuilding”, 1979 - 105 p.
- Yu.A. Koryakin Hydroacoustic systems. - St. Petersburg: “Science of St. Petersburg and the sea power of Russia”, 2002. - 416 p.
Sound travels through sound waves. These waves travel 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 However, sound vibrations of molecules can also occur in a 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 the radio in the room, whether loud or quiet, high tone or low, reach the listener simultaneously.
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 this point per unit of 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 travels five times faster in water than in air. The average speed is 1400 - 1500 m/sec (the speed of sound in air is 340 m/sec). It would seem that audibility in water also improves. In fact, this is far from the case. After all, the strength of sound does not depend on the speed of propagation, but on the amplitude of sound vibrations and the perceptive ability of the hearing organs. In the snail inner ear The organ of Corti is located and consists of auditory cells. Sound waves vibrate the eardrum auditory ossicles and the membrane of the organ of Corti. From the hair cells of the latter, which perceive sound vibrations, nervous excitement goes to the auditory center located in the temporal lobe of the brain.
A sound wave can enter a person's inner ear in two ways: air conduction through the external auditory canal, eardrum and ossicles of the middle ear and through bone conduction- vibrations of the skull bones. On the surface, air conduction predominates, and under water, bone conduction predominates. Simple experience convinces us of this. Cover both ears with the palms of your hands. On the surface, audibility will deteriorate sharply, but under water this is not observed.
So, under water, sounds are perceived primarily through bone conduction. Theoretically, this is explained by the fact that the acoustic resistance of water approaches the acoustic resistance of human tissue. Therefore, the energy loss during the transition of sound waves from water to the bones of a person’s head is less than in air. Air conduction almost disappears under water, since the external auditory canal is filled with water, and a small layer of air near eardrum weakly transmits sound vibrations.
Experiments have shown that bone conductivity is 40% lower than air conductivity. Therefore, audibility under water generally deteriorates. The range of audibility with bone conduction of sound depends not so much on the strength as on the tonality: the higher the tone, the farther the sound is heard.
The underwater world for humans is a world of silence, where there are no extraneous noises. Therefore, the simplest sound signals can be perceived under water at considerable distances. A person hears a blow on a metal canister immersed in water at a distance of 150-200 m, a rattle at 100 m, a bell at 60 m.
Sounds made underwater are usually inaudible on the surface, just as sounds from outside are inaudible underwater. To perceive underwater sounds, you must be at least partially immersed. If you enter the water up to your knees, you begin to perceive a sound that was not heard before. As you dive, the volume increases. It is especially audible when the head is immersed.
To send sound signals from the surface, you must lower the sound source into the water at least halfway, and the sound strength will change. Orientation underwater by ear is extremely difficult. In air, sound arrives in one ear 0.00003 seconds earlier than in the other. This allows you to determine the location of the sound source with an error of only 1-3°. Under water, sound is simultaneously perceived by both ears and therefore clear, directional perception does not occur. The error in orientation can be 180°.
In a specially staged experiment, only individual light divers after long wanderings and... searches went to the location of the sound source, which was located 100-150 m from them. It was noted that systematic training over a long time makes it possible to develop the ability to quite accurately navigate by sound under water. However, as soon as the training stops, its results are nullified.
