We perceive the sounds, while away from their sources. Usually the sound comes to us through the air. Air is an elastic medium transmitting sound.
If you remove the sound transmission environment between the source and receiver, then the sound will not be distributed and, therefore, the receiver will not perceive it. We will demonstrate it on experience.
Position the air pump-alarm clock under the bell (Fig. 80). While the bell is air, the sound of the call is heard clear. When pumping air from under the bell, the sound gradually weakens and finally becomes sabbard. Without the transmitting medium oscillation, the call plate can not spread, and the sound does not reach our ear. Let's let down the air and hear the ringing again.
Fig. 80. Experience proving that in space where there is no real medium, the sound does not apply
Well carried out the sounds of elastic substances, such as metals, wood, liquid, gases.
We put one-way wooden board for one end, and we will take away to another end. Having attached ear to the board, heard the clock.
We will be tied to a metal spoon beep. The end of the ramp is applied to the ear. Hitting a spoon, hear strong sound. Even stronger sound heard if the beep replaced the wire.
Soft and porous bodies - bad sound conductors. To protect any room from the penetration of foreign sounds, the walls, the floor and the ceiling are paved in layers from sound-absorbing materials. Ascellies use felt, pressed tube, porous stones, various synthetic materials (for example, foam), made on the basis of foamed polymers. The sound in such layers quickly fades.
Liquids are well conducted sound. Fishes, for example, hear steps and voices on the shore, it is known to be experienced fishermen.
So, the sound applies to any elastic medium - solid, liquid and gaseous, but cannot be distributed in space where there is no substance.
Source fluctuations create an elastic wave of sound frequency in its surrounding medium. The wave, reaching the ear, affects the eardrum, causing it to fluctuate with the frequency corresponding to the frequency of the sound source. The trembling of the eardrum is transmitted by means of a hearing nerve ending system, annoy them and those cause a feeling of sound.
Recall that only longitudinal elastic waves can exist in gases and liquids. The sound in the air, for example, is transmitted by longitudinal waves, i.e., alternating condenses and air plows coming from the sound source.
A sound wave, like any other mechanical waves, spreads in space not instantly, but at a certain speed. This can be seen, for example, watching from afar for shooting from a gun. First we see the fire and smoke, and then after a while hearing the sound of the shot. Smoke appears at the same time when the first sound oscillation occurs. Measuring the time lapse T between the moment of sound occurrence (the moment of smoke appearance) and the moment when it comes to the ear, you can determine the speed of sound propagation:
Measurements indicate that the speed of sound in air at 0 ° C and normal atmospheric pressure is 332 m / s.
The speed of sound in the gases is the greater, the higher their temperature. For example, at 20 ° C, the speed of sound in the air is 343 m / s, at 60 ° C - 366 m / s, at 100 ° C - 387 m / s. It is explained by the fact that the elasticity of gases increases with an increase in temperature, and the greater the elastic force arising in the medium during its deformation, the greater the mobility of the particles and the faster the oscillations from one point to the other are transmitted.
The speed of the sound depends on the properties of the medium in which the sound is distributed. For example, at 0 ° C, the sound speed in hydrogen is 1284 m / s, and in carbon dioxide - 259 m / s, since hydrogen molecules are less massive and less inert.
Currently, the speed of sound can be measured in any environment.
Molecules in liquids and solid bodies are located closer to each other and interact more stronger than gases molecules. Therefore, the speed of sound in liquid and solid media is greater than in gaseous.
Since the sound is a wave, then to determine the speed of sound, in addition to the formula V \u003d S / T, you can use the formulas known to you: V \u003d λ / T and V \u003d Vλ. When solving problems, the speed of sound in air is usually considered to be 340 m / s.
Questions
- What is the purpose of the experience depicted in Figure 80? Describe how this experience is carried out and which conclusion from it follows.
- Can the sound spread in gases, liquids, solid bodies? Answers confirm the examples.
- What bodies do the sound better - elastic or porous? Give examples of elastic and porous tel.
- What wave is a longitudinal or transverse - is a sound propagating in the air; in water?
- Give an example showing that the sound wave is not spreading instantly, but at a certain speed.
Exercise 30.
- Can the sound of a strong explosion on the moon be heard on earth? Justify the answer.
- If one of the ends of the thread bind over one half of the soap mix, then with the help of such a phone you can talk even with a whisper, being in different rooms. Explain the phenomenon.
- Determine the speed of sound in water, if the source, hesitating with a period of 0.002 s, excites the wave in the water with a length of 2.9 m.
- Determine the length of the sound wave with a frequency of 725 Hz in the air, in water and in glass.
- On the end of the long metal pipe once hit by a hammer. Will the sound from the blow spread to the second end of the pipe for metal; By air inside the pipe? How many blows will hear a man standing from the other end of the pipe?
- Observer, standing near the straight line of the railway, saw a couple over the whistle of the walkway away the locomotive. After 2 seconds after the appearance of a couple, he heard the whistle sound, and after 34, the locomotive passed by the observer. Determine the speed of the locomotive.
Underwater hunting
Distribution of sound in water .
The sound spreads in water five times faster than in the air. The average speed is 1400-1500 m / s (the speed of sound propagation in the air is 340 m / s). It would seem that hearing in water is also improved. In fact, this is not so. After all, the power of the sound depends not on the rate of propagation, but from the amplitude of sound oscillations and the perceiving ability of the hearing organs. In the snail of the inner ear there is a Cortium organ consisting of hearingly cells. Sound waves scroll the eardrum, auditory bones and a corti-organ membrane. From the hair cells of the latter, perceive sound oscillations, the nervous excitement goes to the auditory center located in the temporal lobe of the brain.
The sound wave can fall into the inner ear of the person in two ways: air conduction through the outer hearing pass, the eardrum and hearing bones of the middle ear and through bone conductivity - the vibration of the bones of the skull. The surface is dominated by air, and under water bone conductivity. This is convinced of a simple experience. Close the hands of both ear palms. On the surface, the audibility will deteriorate sharply, this is not noted under the water.
So, under water, sounds are perceived mainly by bone conduction. It is theoretically explained by the fact that the acoustic resistance of water is approaching the acoustic resistance of human tissues. Therefore, energy losses in the transition of sound waves from water in the bone of human head is less than in the air. The air conduction under water almost disappears, since the outer hearing pass is filled with water, and a small air layer near the eardrum sweeping poorly transfers sound oscillations.
The experiments found that bone conductivity is 40% lower than air. Therefore, audibility under water is generally worse. The distance of hearingness during bone conductivity of the sound depends not so much from the strength, but from the tonality: the higher the tone, the further hear the sound.
The underwater world for a person is the 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 to a metal canister immersed in water, at a distance of 150-200 m, a ratchet sound is 100 m, a bell tler - by 60 m.
The sounds published under water are usually not heard on the surface, just as the sounds are not audible under water. To perceive underwater sounds, you must at least partially immerse yourself. If you enter the water on your knees, you start to perceive the sound, which before that was not heard. As the volume is immersed, the volume increases. Especially well heard when immersing the head.
To feed sound signals from the surface, it is necessary to omit the sound source to the water at least half, and the sound of the sound will change. Orientation under water for hearing is extremely difficult. In the air, the sound comes into one ear earlier than 0.00003 seconds than to another. This allows you to determine the location of the sound source with a mistake of only 1-3 °. Under the water, the sound is simultaneously perceived by both ears and therefore not a clear, directional perception occurs. An error in the orientation is 180 °.
In a specially delivered experience only individual light divers after long wandering and. The search was published to the location of the sound source, which was 100-150 meters from them. It is noted that systematic training for a long time allow you to develop the ability to quite accurately navigate the sound under water. However, as soon as the training stops, its results are reduced to no.
Over long distances, sound energy spreads only along the gentle rays, which on the entire path do not touch the bottom of the ocean. In this case, the restriction imposed by the medium for the range of sound propagation is the absorption of it in sea water. The main mechanism of absorption is associated with relaxation processes accompanying the violation of the acoustic wave of thermodynamic equilibrium between ions and molecules dissolved in water salts. It should be noted that the main role in the absorption in a wide range of sound frequencies belongs to the MGSO4 sulfur salt, although in the percentage of its content in sea water is quite small - almost 10 times less than, for example, a NAS1 stone salt, which nevertheless does not play Any noticeable role in sound absorption.
Absorption in seawater, generally speaking, the greater the higher the frequency of the sound. At frequencies from 3-5 to at least 100 kHz, where the above mechanism dominates the above, absorption is proportional to the frequency of about 3/2. At lower frequencies, a new absorption mechanism is included (it is possible, it is associated with the presence of boron salts in water), which becomes particularly noticeable in the HOTER band; Here the absorption level is abnormally high and significantly slowly falls with a decrease in frequency.
To simply imagine the quantitative characteristics of the absorption in seawater, we note that due to this effect, the sound with a frequency of 100 Hz is weakened 10 times on the way at 10 thousand km, and with a frequency of 10 kHz - at a distance of only 10 km (drawing 2). Thus, only low-frequency sound waves can be used for long-range communication, for long-range discovery of underwater obstacles, etc.
Figure 2 - distances on which the sounds of different frequencies are fucked by 10 times when distributed in seawater.
In the area of \u200b\u200baudible sounds for the frequency range of 20-2000 Hz, the range of distribution under water of the sounds of medium intensity reaches 15-20 km, and in the ultrasound area - 3-5 km.
If you proceed from the values \u200b\u200bof the attenuation of the sound observed in the laboratory conditions in small volumes of water, it would be possible to expect much more long distances. However, in natural conditions, in addition to attenuation, due to the properties of the water itself (t. N. viscous attenuation), it is also affected by its scattering and absorption of various inhomogeneities of the medium.
The sound refraction, or the curvature of the sound beam path, is caused by the inhomogeneity of the properties of water, mainly vertically, due to the three main reasons: changes in hydrostatic pressure with depth, changes in saline and temperature change due to the unequal warming of the mass of water by sunlight. As a result of the cumulative action of these reasons, the speed of sound propagation, which is about 1450 m / s for fresh water and about 1500 m / s for sea, varies with depth, and the law of change depends on the time of year, time of the day, the depth of the reality and a number of other reasons. . Sound rays that came out of the source at some angle to the horizon are bent, and the direction of bending depends on the distribution of sound velocities in the medium. In the summer, when the upper layers of the warmeter of the lower, the rays bend down the book and mostly reflected from the bottom, while losing a significant proportion of its energy. On the contrary, in winter, when the lower layers of water retain their temperature, while the upper layers are cooled, the rays bend upwards and undergo repeated reflections from the surface of the water at which much less energy is lost. Therefore, in winter, the range of sound distribution is greater than in the summer. Due to refraction, t. N 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 the ultralow propagation of sounds under water. At some depth under the surface of the water there is a layer in which the sound applies to the lowest speed; Above this depth, the speed of sound increases due to an increase in temperature, and below - due to an increase in hydrostatic pressure with depth. This layer is a kind of underwater audio channel. The beam, rejected from the axis of the channel up or down, as a result of refractive, always seeks to get into it back. If you place the source and receiver sound in this layer, then even the sounds of medium intensity (for example, the explosions of small charges in 1-2 kg) can be recorded at distances hundreds and thousands of km. A significant increase in the range of sound propagation in the presence of an underwater audio channel can be observed when the source is located and the audio receiver is not necessarily near the channel axis, and, for example, at the surface. In this case, the rays, refraining the book, enter the deep-sea layers, where they deflect the upstairs and come back to the surface at a distance of several tens of km from the source. Next, the pattern of the spread of the rays is repeated and the resulting sequence of this is formed. Secondary illuminated zones, which are usually traced until the distance of several hundred km.
The spread of high frequency sounds, in particular ultrasounds, when the wavelengths are very small, the small inhomogeneities are influenced, usually available in natural reservoirs: microorganisms, bubbles of gases, etc. These inhomogeneities act two ways: they absorb and dispel the energy of sound waves. As a result, with an increase in the frequency of sound oscillations, their distribution range is reduced. This effect is especially noticeable in the surface layer of water, where the most inhomogeneities. Sound scattering with inhomogeneities, as well as irregularities of the surface of the water and the bottom causes the phenomenon of underwater reverb, accompanying the parcel of the sound pulse: sound waves, reflecting from the totality of heterogeneities and merging, give a tightening of the sound pulse, continuing after its end, like a reverberation observed in closed rooms. Underwater reverb is a fairly significant interference for a number of practical applications of hydroacoustics, in particular for hydrolycation.
The limits of the distribution of underwater sounds are still limited and the so-called. Own sea noise having twofold origin. A part of the noise occurs from the shocks of the waves on the surface of the water, from the marine surrum, from the noise of the pebbles rolling and the like. The other part is associated with sea fauna; This includes sounds produced by fish and other marine animals.
The sound is one of the components of our life, and a person hears him everywhere. In order to consider this phenomenon in more detail, you must first figure out with the concept itself. For this, it is necessary to refer to the encyclopedia, where it is written that "sound is elastic waves, propagating in any elastic medium and create mechanical oscillations in it." Speaking easier language - this is hearing fluctuations in any environment. From what it is, and the main characteristics of the sound depend. First of all, the speed of propagation, for example, in water is different from another medium.
Any sound analogue has certain properties (physical characteristics) and qualities (the reflection of these signs in human sensations). For example, duration, duration, height frequency, grades, and so on.
The speed of sound in water is significantly higher than, let's say in the air. Consequently, it spreads faster and much further heard. This occurs due to the high molecular density of the aquatic environment. It is 800 times more denser than air and steel. It follows that the propagation of sound is largely depends on the medium. Turn to specific numbers. So, the speed of sound in water is equal to 1430m / s, in the air - 331.5 m / s.
Low-frequency sound, for example, noise that manufactures a working ship engine is always heard a little earlier than the ship appears in the visibility zone. Its speed depends on several things. If the water temperature rises, then, naturally, the speed of sound in water increases. The same thing happens with an increase in the salinity of water and pressure, which grows with an increase in the depth of the water space. A special role for the speed can be such a phenomenon as thermoclinic. These are places in which water layers are found in different temperatures.
Also in such places are different (due to the difference in temperature mode). And when the waves of sound pass through such a single layer, they lose most of their strength. Faced with thermoclinic, the sound wave is partly, and sometimes completely, it is reflected (the degree of reflection depends on the angle under which the sound falls), after which, on the other side of this place, a shadow zone is formed. If we consider an example when the sound source is located in the aqueous space above the thermocline, then it is not yet necessary to hear something that is difficult, but almost impossible.
Which are published above the surface, in the water itself are never heard. And on the contrary, occurs when under a water layer: he does not sound over it. A bright example is modern divers. Their rumor is much reduced due to the fact that water affects and the high speed of sound in water reduces the quality of the direction of the direction, from where it moves. This is the most dull with the stereo ability of sound perception.
Under the layer of water, they go into the human ear more than any bone of the cranial box of the head, and not as in the atmosphere, through the drummers. The result of such a process becomes its perception at the same time with both ears. The human brain is not capable of distinguishing the places where the signals come from, and in what intensity. The result is the emergence of consciousness that the sound rolling from all sides at the same time, although this is not so.
In addition to the above, sound waves in the aqueous space have such qualities as absorption, divergence and dispersion. The first is when the power of sound in salt water is gradually coming out due to the friction of the aqueous medium and the salts are in it. Divergence is manifested in removing sound from its source. It seems to dissolve in space as light, and in the end its intensity drops significantly. And oscillations are disappeared completely due to dispersion on all sorts of obstacles, inhomogeneities of the medium.
This lesson highlights the theme "Sound Waves". In this lesson, we will continue to study acoustics. First we repeat the definition of sound waves, then consider their frequency bands and get acquainted with the concept of ultrasound and infrasound waves. We will also discuss the properties inherent in sound waves in various environments, and learn what characteristics are inherent. .
Sound waves -these are mechanical oscillations that, spreading and interacting with the hearing organ, are perceived by a person (Fig. 1).
Fig. 1. Sound wave
The section that works in physics by these waves is called acoustics. The profession of people whom in common people are called "hearaks" - acoustics. A sound wave is a wave propagating in an elastic medium, it is a longitudinal wave, and when it spreads in an elastic medium, compression and discharge alternate. It is transmitted over time by the distance (Fig. 2).
Fig. 2. Spreading a sound wave
Sound waves include such oscillations that are carried out with a frequency of 20 to 20,000 Hz. For these frequencies correspond to the wavelengths of 17 m (for 20 Hz) and 17 mm (for 20,000 Hz). This range will be called audio sound. These wavelengths are shown for air, the speed of propagation of the sound in which is equal to.
There are still such ranges that are engaged in acoustics - infrasound and ultrasound. Infrasound are those that have a frequency less than 20 Hz. And ultrasound are those that have a frequency of more than 20,000 Hz (Fig. 3).
Fig. 3. Sound Wave Ranges
Each educated person must navigate in the sound wave frequency range and know that if he goes to the ultrasound, then the picture on the computer screen will be built with a frequency of more than 20,000 Hz.
Ultrasound -these are mechanical waves similar to sound, but having a frequency of 20 kHz to a billion hertz.
Waves having a frequency of more than a billion hertz, called hyperwich.
Ultrasound is used to detect defects in cast details. On the studied part guide the stream of short ultrasound signals. In those places where there are no defects, the signals pass through the part without registering the receiver.
If there is a crack, an air cavity or other inhomogeneity, the ultrasonic signal is reflected from it and returning, falls into the receiver. Such a method is called ultrasonic flaw detection.
Other examples of ultrasound use are ultrasound, ultrasound devices, ultrasound therapy.
Infrasound - Mechanical waves similar to sound, but having a frequency of less than 20 Hz. They are not perceived by the human ear.
Natural sources of infrasound waves are storm, tsunami, earthquakes, hurricanes, volcanic eruptions, thunderstorms.
Infrasound is also important waves that are used for surface oscillations (for example, to destroy some large objects). We launch infrase in the soil - and the soil crushes. Where is it used? For example, on diamond prima, where they take the ore, in which there are diamond components, and crush into small particles to find these diamond splashes (Fig. 4).
Fig. 4. Application of infrasound
The speed of the sound depends on the conditions of the medium and temperature (Fig. 5).
Fig. 5. The speed of propagation of the sound wave in various environments
Note: In the air, the speed of the sound is equal to, with the speed increases by. If you are a researcher, you can use such knowledge. You may even come up with some temperature sensor that will fix the temperature difference by changing the sound of the sound in the medium. We already know that the more denser the environment, the more serious interaction between the particles of the medium, the faster the wave is spread. In the past, we discussed this on the example of dry and air of wet air. For water, the speed of propagation of sound. If you create a sound wave (knocking on the charter), then the speed of its propagation in water will be 4 times more than in the air. By water, the information will reach faster 4 times than by air. And in steel and faster: 
(Fig. 6).
Fig. 6. Speed \u200b\u200bwave propagation rate
You know from the epics that Ilya Muromets enjoyed (and all the warriors and ordinary Russian people and boys from the RVS Gaidar), enjoyed a very interesting way to detect the object, which is approaching, but is still far away. The sound that he publishes when moving is not yet heard. Ilya Muromets, surrendering to the Earth, can hear it. Why? Because the solid land is transmitted with a greater speed, it means that it will make it faster to the ear of Ilya Muromets, and he will be able to prepare for the meeting of the enemy.
The most interesting sound waves are musical sounds and noises. What objects can create sound waves? If we take the source of the wave and the elastic environment, if we force the sound source to fluctuate harmoniously, then we will have a wonderful sound wave, which will be called musical sound. These sources of sound waves can be, for example, strings guitar or piano. It may be a sound wave, which is designed in the gap of the air pipe (organ or pipe). From music lessons you know notes: up, re, mi, fa, salt, la, si. In acoustics, they are called tones (Fig. 7).
Fig. 7. Musical tones
All items that can produce tones will feature. What do they differ? They differ in the wavelength and frequency. If these sound waves are created by non-harmoniously sounding bodies or are not associated with a general one orchestral play, then such a number of sounds will be called noise.
Noise - Disorder fluctuations in various physical nature, characterized by the complexity of the temporary and spectral structure. The concept of noise is household and there is physical, they are very similar, and therefore we introduce it as a separate important object of consideration.
Go to quantitative estimates of sound waves. What are the musical sound waves characteristics? These characteristics apply exclusively on harmonic sound oscillations. So, sound volume. What determines the volume of the sound? Consider the propagation of the sound wave in time or fluctuations in the sound wave source (Fig. 8).
Fig. 8. Sound volume
At the same time, if we added not a lot of sound to the system (they knocked quietly by the piano key, for example), it will be a quiet sound. If we are loud, raising your hand high, call this sound, knocking down the key, get a loud sound. What does it depend on? The quiet sound of the amplitude of oscillations is less than that of a loud sound.
Next important characteristics of musical sound and any other - height. What does the height of the sound depend on? The height depends on the frequency. We can force the source to fluctuate often, and we can make it fluctuate not very quickly (that is, to make fewer fluctuations per unit of time). Consider the scan on the time of high and low sound of one amplitude (Fig. 9).
Fig. 9. Sound height
You can make an interesting conclusion. If a person sings the bass, then he has a sound source (these are voice ligaments) fluctuates several times more slowly than a person who sings soprano. In the second case, voice ligaments fluctuate more often, therefore, the foci of compression and discharge in the spread of the wave causes.
There is another interesting characteristic of sound waves, which physicists are not learning. it timbre. You know and easily distinguish the same musical play, which is performed on a balalaica or a cello. What is the difference between these sounds or is this execution? We were asked at the beginning of the experiment of people who extract sounds, to make them about the same amplitude, so that the volume of sound is the same. It is like in the case of an orchestra: if there is no discharge of a tool, everyone played approximately equally, in the same force. So the timbre of balalaiki and cello differs. If we painted the sound, which is removed from one tool, from the other, with the help of charts, they would be the same. But you easily feature these sound tools.
Another example of the importance of the timbre. Imagine two singers who finish the same musical university with identical teachers. They studied equally well on the five. For some reason, one becomes an outstanding performer, and the other all his life is dissatisfied with his career. In fact, this is determined exclusively by their tool, which causes vocal fluctuations in the medium, i.e., they differ in Tembre voices.
Bibliography
- Sokolovich Yu.A., Bogdanova GS Physics: Handbook with examples of solving problems. - 2nd edition reddished. - X.: Vesta: Publishing House "Rocky", 2005. - 464 p.
- Pryrickin A.V., Godnik E.M., Physics. 9 cl.: Textbook for general formation. institutions / A.V. Pryrickin, E.M. Genther. - 14th ed., Stereotype. - M.: Drop, 2009. - 300 p.
- Internet portal "eduspb.com" ()
- Internet portal "msk.edu.ua" ()
- Internet portal "class-fizika.narod.ru" ()
Homework
- How does the sound apply? What can serve as a sound source?
- Can the sound spread in space?
- Is every wave that has reached the human hearing organ is perceived by him?
