Sound propagation is observed to be dense. School encyclopedia

This lesson covers the topic "Sound Waves". In this lesson, we will continue to study acoustics. First, we will repeat the definition of sound waves, then consider their frequency ranges and get acquainted with the concept of ultrasonic and infrasonic waves. We will also discuss the properties inherent in sound waves in different environments and find out what characteristics they have. .

Sound waves -these are mechanical vibrations, which, spreading and interacting with the organ of hearing, are perceived by a person (Fig. 1).

Figure: 1. Sound wave

The section that deals with these waves in physics is called acoustics. The profession of people who are called "rumors" in the common people is acoustics. A sound wave is a wave propagating in an elastic medium, it is a longitudinal wave, and when it propagates in an elastic medium, compression and depression alternate. It is transmitted over time over a distance (Fig. 2).

Figure: 2. Propagation of a sound wave

Sound waves include those vibrations that occur with a frequency of 20 to 20,000 Hz. For these frequencies, the corresponding wavelengths are 17 m (for 20 Hz) and 17 mm (for 20,000 Hz). This range will be referred to as audible sound. These wavelengths are given for air, in which the speed of sound propagation is.

There are also such ranges that acoustics deal with - infrasonic and ultrasonic. Infrasound are those that have a frequency less than 20 Hz. And ultrasonic ones are those that have a frequency of more than 20,000 Hz (Fig. 3).

Figure: 3. Ranges of sound waves

Every educated person should be guided in the frequency range of sound waves and know that if he goes to ultrasound, 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 waves, but with a frequency from 20 kHz to a billion hertz.

Waves with a frequency of more than a billion hertz are called hypersound.

Ultrasound is used to detect defects in cast parts. A stream of short ultrasonic signals is directed to the part to be examined. In those places where there are no defects, the signals pass through the part without being registered by the receiver.

If there is a crack, air cavity or other inhomogeneity in the part, then the ultrasonic signal is reflected from it and, returning, enters the receiver. This method is called ultrasonic flaw detection.

Other examples of ultrasound applications are ultrasound machines, ultrasound machines, and ultrasound therapy.

Infrasound - mechanical waves, similar to sound waves, but having a frequency of less than 20 Hz. They are not perceived by the human ear.

Natural sources of infrasonic waves are storms, tsunamis, earthquakes, hurricanes, volcanic eruptions, and thunderstorms.

Infrasound is also an important wave that is used to vibrate the surface (for example, to destroy some large objects). We launch infrasound into the soil - and the soil is crushed. Where is this used? For example, in diamond mines, where ore is taken in which there are diamond components, and crushed into small particles to find these diamond inclusions (Fig. 4).

Figure: 4. Application of infrasound

The speed of sound depends on environmental conditions and temperature (Fig. 5).

Figure: 5. The speed of propagation of a sound wave in various media

Note: in air, the speed of sound at is, at, the speed increases by. If you are a researcher, then this knowledge may be useful to you. You may even come up with some kind of temperature sensor that will record temperature differences by changing the speed of sound in the environment. We already know that the denser the medium, the more serious the interaction between the particles of the medium, the faster the wave propagates. We discussed this in the last paragraph using the example of dry air and humid air. For water, the speed of sound propagation. If you create a sound wave (knock on a tuning fork), then the speed of its propagation in water will be 4 times higher than in air. Information will reach 4 times faster by water than by air. And even faster in steel: (fig. 6).

Figure: 6. The speed of propagation of a sound wave

You know from the epics that Ilya Muromets used (and all heroes and ordinary Russian people and boys from Gaidar's RVS), used a very interesting method of detecting an object that is approaching, but is still far away. The sound it makes when driving is not yet heard. Ilya Muromets, leaning his ear to the ground, can hear it. Why? Because sound is transmitted at a higher speed on solid ground, which means that it will quickly reach the ear of Ilya Muromets, and he will be able to prepare to meet the enemy.

The most interesting sound waves are musical sounds and noises. What objects can create sound waves? If we take a wave source and an elastic medium, if we make the sound source vibrate harmoniously, then we will have a wonderful sound wave, which will be called musical sound. These sources of sound waves can be, for example, the strings of a guitar or piano. This can be a sound wave that is created in the gap of an air pipe (organ or pipe). From music lessons, you know the notes: do, re, mi, fa, sol, la, si. In acoustics, they are called tones (Fig. 7).

Figure: 7. Musical tones

All objects that can emit tones will have features. How do they differ? They differ in wavelength and frequency. If these sound waves are created by non-harmonious sounding bodies or are not connected into a common orchestral piece, then such a number of sounds will be called noise.

Noise - random vibrations of various physical nature, characterized by the complexity of the temporal and spectral structure. The concept of noise is everyday and there is physical, they are very similar, and therefore we introduce it as a separate important object of consideration.

We pass on to quantitative estimates of sound waves. What are the characteristics of musical sound waves? These characteristics apply exclusively to harmonic sound vibrations. So, sound volume... What determines the sound volume? Consider the propagation of a sound wave in time or the oscillation of a sound wave source (Fig. 8).

Figure: 8. Sound volume

At the same time, if we added not very much sound to the system (for example, hit a piano key softly), then there will be a quiet sound. If we raise our hand loudly and call this sound by hitting the key, we will get a loud sound. What does it depend on? A quiet sound has a lower vibration amplitude than a loud sound.

The next important characteristic of musical sound and any other is height... What does the pitch of the sound depend on? The pitch depends on the frequency. We can make the source oscillate frequently, or we can make it oscillate not very quickly (that is, make fewer oscillations per unit time). Consider the time sweep of high and low sound of the same amplitude (Fig. 9).

Figure: 9. Sound pitch

An interesting conclusion can be drawn. If a person sings in bass, then his sound source (these are the vocal cords) vibrates several times slower than that of a person who sings soprano. In the second case, the vocal cords vibrate more often, therefore, more often cause foci of compression and vacuum in the propagation of the wave.

There is another interesting characteristic of sound waves that physicists do not study. it timbre... You know and easily distinguish one and the same piece of music, which is performed on the balalaika or on the cello. What is the difference between these sounds or this performance? At the beginning of the experiment, we asked people who extract sounds to make them of approximately the same amplitude, so that the sound volume was the same. It's like in the case of an orchestra: if you don't need to select an instrument, everyone plays about the same, with the same strength. So the timbre of balalaika and cello is different. If we were to draw the sound that is extracted from one instrument, from another, using diagrams, they would be the same. But you can easily distinguish these instruments by their sound.

Another example of the importance of timbre. Imagine two singers who graduate from the same music college with the same teachers. They studied equally well at fives. For some reason, one becomes an outstanding performer, while the other is dissatisfied with his career all his life. In fact, this is determined exclusively by their instrument, which causes just vocal vibrations in the environment, that is, their voices differ in timbre.

List of references

1. Sokolovich Yu.A., Bogdanova G.S. Physics: a handbook with examples of problem solving. - 2nd edition redistribution. - X .: Vesta: Ranok publishing house, 2005. - 464 p.
2. Peryshkin A.V., Gutnik E.M., Physics. 9th grade: textbook for general education. institutions / A.V. Peryshkin, E.M. Gutnik. - 14th ed., Stereotype. - M .: Bustard, 2009 .-- 300 p.

1. Internet portal "eduspb.com" ()
2. Internet portal "msk.edu.ua" ()
3. Internet portal "class-fizika.narod.ru" ()

Homework

1. How does sound propagate? What could be the source of the sound?
2. Can sound propagate in space?
3. Is every wave that reaches a human hearing organ is perceived by it?

Have you ever thought about the fact that sound is one of the brightest manifestations of life, action, movement? And also that each sound has its own "face"? And even with our eyes closed, not seeing anything, we can only guess by the sound what is happening around. We can distinguish the voices of acquaintances, hear rustling, rumbling, barking, meowing, etc. All these sounds are familiar to us from childhood, and we can easily identify any of them. Moreover, even in absolute silence, we can hear each of the listed sounds with our inner ear. Imagine it as if in reality.

What is sound?

Sounds perceived by the human ear are one of the most important sources of information about the world around us. The noise of the sea and wind, the singing of birds, the voices of people and the cries of animals, thunder, sounds moving with the ear, make it easier to adapt to changing external conditions.

If, for example, a stone fell in the mountains, and there was no one nearby who could hear the sound of its fall, was there a sound or not? The question can be answered both positively and negatively equally, since the word "sound" has a double meaning. Therefore, it is necessary to agree. Therefore, it is necessary to agree on what to consider as sound - a physical phenomenon in the form of propagation of sound vibrations in the air or a listener's sensation. essence is a cause, the second is an effect, while the first concept of sound is objective, the second is subjective.In the first case, sound is really a stream of energy flowing like a river stream. Such sound can change the environment through which it passes, and itself is changed by it In the second case, by sound we mean those sensations that arise in the listener when a sound wave is applied to the brain through the hearing aid. Hearing a sound, a person can experience various feelings. A wide variety of emotions evokes in us the complex complex of sounds that we call music. Sounds form the basis of speech, which serves as the main means of communication in human society. Finally, there is such a form of sound as noise. The analysis of sound from the point of view of subjective perception is more difficult than with an objective assessment.

How to create sound?

Common to all sounds is that the bodies that generate them, that is, the sources of sound, oscillate (although most often these oscillations are invisible to the eyes). For example, the sounds of the voices of people and many animals arise as a result of vibrations of their vocal cords, the sound of wind musical instruments, the sound of a siren, the whistle of the wind, and thunder are caused by the vibrations of the air masses.

Using the ruler as an example, you can literally see with your eyes how sound is born. What movement does the ruler make when we secure one end, pull back the other, and release it? We will notice that he seemed to tremble, hesitated. Based on this, we conclude that the sound is created by short or long vibration of some objects.

The source of sound can be not only vibrating objects. The whistle of bullets or projectiles in flight, the howling of the wind, the roar of a jet engine are born from ruptures in the air stream, which also cause its rarefaction and compression.

Also, sound vibrational movements can be noticed using a device - a tuning fork. It is a curved metal rod mounted on a stem on a resonator box. If you hit the tuning fork with a hammer, it will sound. The oscillation of the tuning fork branches is imperceptible. But they can be detected if a small ball suspended on a thread is brought to the sounding tuning fork. The ball will periodically bounce, which indicates the vibrations of the cameron branches.

As a result of the interaction of the sound source with the surrounding air, air particles begin to contract and expand in time (or "almost in time") with the movements of the sound source. Then, due to the properties of air as a fluid, vibrations are transmitted from some air particles to others.

To an explanation of the propagation of sound waves

As a result, vibrations are transmitted through the air over a distance, that is, a sound or acoustic wave propagates in the air, or, simply, sound. Sound, reaching a person's ear, in turn, excites vibrations of his sensitive areas, which are perceived by us in the form of speech, music, noise, etc. (depending on the properties of sound dictated by the nature of its source).

Sound wave propagation

Is it possible to see how the sound "runs"? In transparent air or in water, vibrations of particles are invisible in themselves. But you can easily find an example that will tell you what happens when sound propagates.

A necessary condition for the propagation of sound waves is the presence of a material environment.

In a vacuum, sound waves do not propagate, since there are no particles transmitting the interaction from the source of oscillations.

Therefore, on the moon, due to the lack of atmosphere, complete silence reigns. Even the fall of a meteorite on its surface is not audible to the observer.

The speed of propagation of sound waves is determined by the speed of transmission of interactions between particles.

The speed of sound is the speed of propagation of sound waves in the medium. In a gas, the speed of sound turns out to be on the order of (more precisely, somewhat less) the thermal speed of molecules and therefore increases with increasing gas temperature. The greater the potential energy of interaction between the molecules of a substance, the greater the speed of sound, therefore the speed of sound in a liquid, which, in turn, exceeds the speed of sound in a gas. For example, in sea water the speed of sound is 1513 m / s. In steel, where transverse and longitudinal waves can propagate, their propagation speed is different. Shear waves propagate at a speed of 3300 m / s, and longitudinal waves at a speed of 6600 m / s.

The speed of sound in any environment is calculated by the formula:

where β is the adiabatic compressibility of the medium; ρ is the density.

Sound wave propagation laws

The basic laws of sound propagation include the laws of its reflection and refraction at the boundaries of different media, as well as the diffraction of sound and its scattering in the presence of obstacles and inhomogeneities in the medium and at the interfaces between media.

The sound absorption distance is influenced by the sound absorption factor, that is, the irreversible transfer of the sound wave energy into other types of energy, in particular, into heat. An important factor is also the direction of radiation and the speed of sound propagation, which depends on the environment and its specific state.

Acoustic waves propagate from the sound source in all directions. If a sound wave passes through a relatively small hole, then it propagates in all directions, and does not go in a directed beam. For example, street sounds entering a room through an open window can be heard at all points, not just against the window.

The propagation of sound waves near an obstacle depends on the ratio between the size of the obstacle and the wavelength. If the dimensions of the obstacle are small compared to the wavelength, then the wave flows around this obstacle, propagating in all directions.

Sound waves, penetrating from one medium to another, deviate from their original direction, that is, they are refracted. The angle of refraction can be greater or less than the angle of incidence. It depends on which medium the sound is entering from. If the speed of sound in the second medium is greater, then the angle of refraction will be greater than the angle of incidence, and vice versa.

When they meet an obstacle on their way, sound waves are reflected from it according to a strictly defined rule - the angle of reflection is equal to the angle of incidence - this is related to the concept of echo. If sound reflects off multiple surfaces at different distances, multiple echoes occur.

Sound propagates in the form of a diverging spherical wave, which fills an ever larger volume. With increasing distance, the vibrations of the particles of the medium weaken, and the sound scatters. It is known that to increase the transmission distance, sound must be concentrated in a given direction. When we want, for example, to be heard, we put our hands to our mouth or use a mouthpiece.

Diffraction, that is, the bending of sound beams, has a great influence on the sound propagation distance. The more heterogeneous the medium, the more the sound beam is bent and, accordingly, the shorter the sound propagation distance.

Sound properties and characteristics

The main physical characteristics of sound are vibration frequency and intensity. They also affect the auditory perception of people.

The period of oscillation is the time during which one complete oscillation occurs. An example is a swinging pendulum, when it moves from the extreme left position to the extreme right and returns back to its original position.

The oscillation frequency is the number of complete oscillations (periods) in one second. This unit is called hertz (Hz). The higher the vibration frequency, the higher the sound we hear, that is, the sound has a higher pitch. According to the accepted international system of units, 1000 Hz is called kilohertz (kHz), and 1,000,000 is called megahertz (MHz).

Frequency distribution: audible sounds - within 15Hz-20kHz, infrasounds - below 15Hz; ultrasounds - within 1.5 (104 - 109 Hz; hypersound - within 109 - 1013 Hz.

The human ear is most sensitive to sounds with a frequency of 2000 to 5000 kHz. The greatest hearing acuity is observed at the age of 15-20 years. Hearing deteriorates with age.

The concept of wavelength is associated with the period and frequency of oscillations. The length of a sound wave is the distance between two successive thickening or rarefaction of the medium. For example, waves propagating on the surface of the water is the distance between two crests.

Sounds also differ in timbre. The main tone of the sound is accompanied by minor tones, which are always higher in frequency (overtone). Timbre is a quality characteristic of a sound. The more overtones are superimposed on the main tone, the "juicier" the sound is musically.

The second main characteristic is the vibration amplitude. This is the largest deviation from the equilibrium position during harmonic vibrations. For example, with a pendulum - its maximum deviation to the extreme left position, or to the extreme right position. The vibration amplitude determines the intensity (strength) of the sound.

The strength of sound, or its intensity, is determined by the amount of acoustic energy flowing in one second through an area of \u200b\u200bone square centimeter. Consequently, the intensity of acoustic waves depends on the magnitude of the acoustic pressure created by the source in the medium.

Loudness is in turn related to the intensity of the sound. The higher the intensity of the sound, the louder it is. However, these concepts are not equivalent. Loudness is a measure of the strength of the auditory experience caused by sound. Sound of the same intensity can create different hearing perceptions in different people. Each person has his own threshold of hearing.

A person ceases to hear sounds of very high intensity and perceives them as a feeling of pressure and even pain. This sound power is called the pain threshold.

Effects of sound on human hearing organs

Human hearing organs are able to perceive vibrations with a frequency of 15-20 hertz to 16-20 thousand hertz. Mechanical vibrations with the indicated frequencies are called sound or acoustic (acoustics - the study of sound) The human ear is most sensitive to sounds with a frequency of 1000 to 3000 Hz. The greatest hearing acuity is observed at the age of 15-20 years. Hearing deteriorates with age. In a person under 40 years of age, the greatest sensitivity is in the region of 3000 Hz, from 40 to 60 years old - 2000 Hz, over 60 years old - 1000 Hz. In the range of up to 500 Hz, we are able to distinguish a decrease or increase in the frequency of even 1 Hz. At higher frequencies, our hearing aids become less susceptible to such small frequency changes. So, after 2000 Hz, we can distinguish one sound from another only when the difference in frequency is at least 5 Hz. With a smaller difference, the sounds will seem the same to us. However, there are almost no rules without exception. There are people with unusually fine hearing. A gifted musician can perceive the change in sound by only a fraction of the vibration.

The outer ear consists of the auricle and the auditory canal, connecting it to the eardrum. The main function of the outer ear is to determine the direction to the sound source. The ear canal, a two centimeter long tube tapering inward, protects the inner parts of the ear and acts as a resonator. The ear canal ends with the eardrum, a membrane that vibrates with sound waves. It is here, on the outer border of the middle ear, that the transformation of objective sound into subjective one takes place. Behind the eardrum there are three small bones connected to each other: the hammer, the incus and the stirrup, with the help of which vibrations are transmitted to the inner ear.

There, in the auditory nerve, they are converted into electrical signals. The small cavity, where the malleus, incus and stirrup are located, are filled with air and connected to the oral cavity by the Eustachian tube. Thanks to the latter, the same pressure is maintained on the inner and outer sides of the eardrum. Usually, the Eustachian tube is closed, and opens only when there is a sudden change in pressure (when yawning, swallowing) to equalize it. If a person's Eustachian tube is closed, for example, due to a cold, then the pressure does not equalize, and the person feels pain in the ears. Further, vibrations are transmitted from the eardrum to the oval window, which is the beginning of the inner ear. The force acting on the tympanic membrane is equal to the product of pressure and the area of \u200b\u200bthe tympanic membrane. But the real ordinances of hearing begin with the oval window. Sound waves propagate in the liquid (perilymph) that the cochlea is filled with. This organ of the inner ear, shaped like a cochlea, is three centimeters long and is divided by a septum into two parts along its entire length. Sound waves reach the septum, bend around it, and then propagate towards almost the same place where they first touched the septum, but from the other side. The cochlear septum consists of a basic membrane, which is very thick and taut. Sound vibrations create wave-like ripples on its surface, while the crests for different frequencies lie in completely defined areas of the membrane. Mechanical vibrations are converted into electrical vibrations in a special organ (organ of Corti), located above the top of the main membrane. A tectorial membrane is located above Corti's organ. Both of these organs are immersed in a liquid - endolymph and separated from the rest of the cochlea by Reisner's membrane. The hairs growing from the organ, Corti almost penetrate the tectorial membrane, and when sound occurs, they touch - the sound is transformed, now it is encoded in the form of electrical signals. The skin and bones of the skull play a significant role in enhancing our ability to perceive sounds, due to their good conductivity. For example, if you put your ear to the rail, then the movement of an approaching train can be detected long before it appears.

Effect of sound on the human body

Over the past decades, the number of all kinds of cars and other sources of noise has sharply increased, the proliferation of portable radios and tape recorders, often turned on at high volume, and the passion for loud popular music. It is noted that in cities every 5-10 years the noise level increases by 5 dB (decibel). It should be borne in mind that for the distant ancestors of man, noise was an alarm signal, indicated the possibility of danger. At the same time, the sympathetic-adrenal and cardiovascular systems, gas exchange were quickly activated, and other types of metabolism also changed (blood sugar and cholesterol levels increased), preparing the body for fight or flight. Although in modern man this function of hearing has lost such practical significance, the "vegetative reactions of the struggle for existence" have survived. So, even a short-term noise of 60-90 dB causes an increase in the secretion of pituitary hormones, stimulating the production of many other hormones, in particular, catecholamines (adrenaline and norepinephrine), increases the work of the heart, constricts blood vessels, and increases blood pressure (BP). At the same time, it was noted that the most pronounced increase in blood pressure is observed in patients with hypertension and those with a hereditary predisposition to it. Under the influence of noise, the activity of the brain is disrupted: the nature of the electroencephalogram changes, the acuity of perception, mental performance decreases. There was a deterioration in digestion. Prolonged exposure to noisy environments is known to lead to hearing loss. Depending on their individual sensitivity, people evaluate noise differently as unpleasant and disturbing. At the same time, music and speech of interest to the listener, even at 40-80 dB, can be transferred relatively easily. Usually, the ear perceives vibrations in the range of 16-20,000 Hz (vibrations per second). It is important to emphasize that unpleasant consequences are caused not only by excessive noise in the audible range of vibrations: ultra- and infrasound in the ranges that are not perceived by human hearing (above 20 thousand Hz and below 16 Hz) also causes nervous tension, malaise, dizziness, changes in the activity of internal organs, especially the nervous and cardiovascular systems. It has been established that the incidence of hypertension is clearly higher among residents of areas located near major international airports than in a quieter area of \u200b\u200bthe same city. Excessive noise (above 80 dB) affects not only the hearing organs, but also other organs and systems (circulatory, digestive, nervous, etc.) the vital processes are disrupted, energy metabolism begins to prevail over plastic, which leads to premature aging of the body.

With these observations and discoveries, methods of purposeful influence on a person began to appear. It is possible to influence the mind and behavior of a person in various ways, one of which requires special equipment (technotronic techniques, zombies.).

Soundproofing

The degree of noise insulation of buildings is primarily determined by the permissible noise standards for premises of this purpose. The normalized parameters of constant noise at the design points are the sound pressure levels L, dB, octave frequency bands with geometric mean frequencies of 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz. For approximate calculations, it is allowed to use sound levels LA, dBA. The normalized parameters of unstable noise at the design points are equivalent sound levels LA eq, dBA, and maximum sound levels LA max, dBA.

Allowable sound pressure levels (equivalent sound pressure levels) are standardized by SNiP II-12-77 "Protection against noise".

It should be borne in mind that the permissible levels of noise from external sources in the premises are established subject to the provision of standard ventilation of the premises (for residential premises, wards, classrooms - with open vents, transoms, narrow window sashes).

Airborne sound insulation refers to the attenuation of sound energy as it travels through an enclosure.

The normalized parameters of sound insulation of the enclosing structures of residential and public buildings, as well as auxiliary buildings and premises of industrial enterprises are the airborne sound insulation index of the enclosing structure Rw, dB and the index of the reduced level of impact noise under the ceiling.

Noise. Music. Speech.

From the point of view of the perception of sounds by the hearing organs, they can be divided mainly into three categories: noise, music and speech. These are different areas of sound phenomena that have information specific to a person.

Noise is a haphazard combination of a large number of sounds, that is, the fusion of all these sounds into one discordant voice. It is believed that noise is a category of sounds that disturbs or annoys a person.

Humans can only withstand a certain amount of noise. But if an hour or two passes, and the noise does not stop, then tension, nervousness and even pain appear.

Sound can kill a person. In the Middle Ages, there was even such an execution when a person was seated under a bell and began to beat him. Gradually the bell ringing killed the person. But that was in the Middle Ages. In our time, supersonic aircraft have appeared. If such a plane flies over the city at an altitude of 1000-1500 meters, then the glass in the houses will burst.

Music is a special phenomenon in the world of sounds, but, unlike speech, it does not convey precise semantic or linguistic meanings. Emotional satiety and pleasant musical associations begin in early childhood, when the child still has verbal communication. Rhythms and tunes connect him to his mother, and singing and dancing are an element of communication in games. The role of music in human life is so great that in recent years, medicine has ascribed healing properties to it. With the help of music, you can normalize biorhythms, ensure the optimal level of activity of the cardiovascular system. But one has only to remember how the soldiers go into battle. From time immemorial, the song has been an indispensable attribute of a soldier's march.

Infrasound and ultrasound

Is it sound that we cannot hear at all? So what if we don't hear? Are these sounds inaccessible to anyone or nothing else?

For example, sounds with a frequency below 16 hertz are called infrasound.

Infrasound is elastic vibrations and waves with frequencies below the range of frequencies audible to humans. Usually 15-4 Hz is taken as the upper limit of the infrasonic range; such a definition is arbitrary, since with sufficient intensity, auditory perception also arises at frequencies of a few Hz, although the tonal nature of the sensation disappears and only individual oscillation cycles become distinguishable. The lower frequency limit of infrasound is undefined. Currently, the area of \u200b\u200bhis study extends down to about 0.001 Hz. Thus, the infrasonic frequency range covers about 15 octaves.

Infrasonic waves propagate in air and water, as well as in the earth's crust. Infrasounds also include low-frequency vibrations of large-sized structures, in particular vehicles and buildings.

And although our ears do not "catch" such vibrations, somehow a person still perceives them. At the same time, we have unpleasant and sometimes disturbing sensations.

It has long been noticed that some animals experience a sense of danger much earlier than humans. They react in advance to a distant hurricane or an impending earthquake. On the other hand, scientists have found that catastrophic events occur in nature, infrasound - low-frequency air vibrations. This gave rise to the hypothesis that animals, thanks to their keen sense, perceive such signals earlier than humans.

Unfortunately, infrasound is generated by many machines and industrial plants. If, say, it arises in a car or plane, then after some time the pilots or drivers are seized with anxiety, they get tired faster, and this can be the cause of the accident.

They make noise in infrasonic machines, and then it is harder to work on them. And everyone around will have hard times. It is no better if the ventilation infrasound in a residential building is buzzing. It seems to be inaudible, but people are annoyed and can even get sick. To get rid of infrasonic adversity allows a special "test" that any device must pass. If it "fills" in the infrasound zone, then it will not receive a pass to people.

What is a very high sound called? Such is the squeak that is inaccessible to our ears? This is ultrasound. Ultrasound - elastic waves with frequencies from approximately (1.5 - 2) (104 Hz (15 - 20 kHz) to 109 Hz (1 GHz); the frequency wave region from 109 to 1012 - 1013 Hz is usually called hypersound. By frequency, ultrasound is conveniently divided into 3 ranges: low frequency ultrasound (1.5 (104 - 105Hz), medium frequency ultrasound (105 - 107Hz), high ultrasound frequency range (107 - 109Hz). Each of these ranges is characterized by its own specific features of generation, reception, propagation and application ...

By its physical nature, ultrasound is elastic waves, and in this it does not differ from sound, therefore the frequency boundary between sound and ultrasonic waves is conditional. However, due to higher frequencies and, consequently, short wavelengths, there are a number of features of the propagation of ultrasound.

Due to the small wavelength of ultrasound, its nature is determined, first of all, by the molecular structure of the medium. Ultrasound in a gas, and in particular in air, propagates with great attenuation. Liquids and solids are, as a rule, good ultrasound conductors, with much less attenuation.

The human ear is unable to perceive ultrasonic waves. However, many animals accept it freely. These are, among other things, dogs so familiar to us. But dogs, alas, cannot "bark" with ultrasound. But bats and dolphins have an amazing ability to both emit and receive ultrasound.

Hypersound is elastic waves with frequencies from 109 to 1012 - 1013 Hz. By its physical nature, hypersound is no different from sound and ultrasonic waves. Due to the higher frequencies and, therefore, lower than in the ultrasound region, the wavelengths become much more significant interactions of hypersound with quasiparticles in the medium - with conduction electrons, thermal phonons, etc. Hypersound is also often presented as a flow of quasiparticles - phonons.

The frequency range of hypersound corresponds to the frequencies of electromagnetic oscillations of the decimeter, centimeter and millimeter ranges (the so-called ultra-high frequencies). The frequency of 109 Hz in air at normal atmospheric pressure and room temperature should be of the same order of magnitude as the free path of molecules in air under the same conditions. However, elastic waves can propagate in a medium only if their wavelength is noticeably greater than the mean free path of particles in gases or greater than the interatomic distances in liquids and solids. Therefore, hypersonic waves cannot propagate in gases (in particular in air) at normal atmospheric pressure. In liquids, the attenuation of hypersound is very large and the propagation range is small. Hypersound propagates relatively well in solids - single crystals, especially at low temperatures. But even in such conditions, hypersound is only able to travel a distance of 1, maximum 15 centimeters.

Sound is mechanical vibrations propagating in elastic media - gases, liquids and solids, perceived by the hearing organs.

With the help of special devices, you can see the propagation of sound waves.

Sound waves can harm human health and vice versa, help to treat ailments, it depends on the type of sound.

It turns out that there are sounds that are not perceived by the human ear.

List of references

Peryshkin A.V., Gutnik E.M. Physics Grade 9

Kasyanov V.A.Physics Grade 10

Leonov A. And "I know the world" Det. encyclopedia. Physics

Chapter 2. Acoustic noise and its impact on humans

Purpose: To study the effect of acoustic noise on the human body.

Introduction

The world around us is a wonderful world of sounds. The voices of people and animals, music and the sound of the wind, birdsong are heard around us. People transmit information through speech, and with the help of hearing, they perceive it. For animals, sound is of no less importance, but in some ways and more, because their hearing is sharper.

From the point of view of physics, sound is mechanical vibrations that propagate in an elastic medium: water, air, solid, etc. The ability of a person to perceive sound vibrations, listen to them, is reflected in the name of the doctrine of sound - acoustics (from the Greek akustikos - audible, auditory). The sensation of sound in our hearing organs occurs with periodic changes in air pressure. Sound waves with a large amplitude of change in sound pressure are perceived by the human ear as loud sounds, with a small amplitude of change in sound pressure - as quiet sounds. The sound volume depends on the vibration amplitude. The volume of the sound also depends on its duration and on the individual characteristics of the listener.

Sound vibrations of high frequency are called high-pitched sounds, sound vibrations of low frequency are called low-pitched sounds.

Human hearing organs are able to perceive sounds with a frequency ranging from about 20 Hz to 20,000 Hz. Longitudinal waves in a medium with a pressure change frequency of less than 20 Hz are called infrasound, with a frequency of more than 20,000 Hz - ultrasound. The human ear does not perceive infrasound and ultrasound, that is, it does not hear. It should be noted that the indicated boundaries of the sound range are arbitrary, since they depend on the age of people and the individual characteristics of their sound apparatus. Typically, with age, the upper frequency limit of perceived sounds decreases significantly - some older people can hear sounds with frequencies not exceeding 6,000 Hz. Children, on the other hand, can perceive sounds whose frequency is slightly higher than 20,000 Hz.

Some animals hear vibrations with frequencies greater than 20,000 Hz or less than 20 Hz.

The subject of the study of physiological acoustics is the organ of hearing itself, its structure and action. Architectural acoustics studies the propagation of sound in rooms, the effect of size and shape on sound, the properties of materials that cover walls and ceilings. This refers to the auditory perception of sound.

There is also musical acoustics, which examines musical instruments and the conditions for their best sounding. Physical acoustics is concerned with the study of sound vibrations themselves, and recently it has also embraced vibrations that lie beyond the limits of audibility (ultrasound). She widely uses a variety of methods to convert mechanical vibrations into electrical ones and vice versa (electroacoustics).

Historical reference

The study of sounds began in ancient times, since a person is characterized by an interest in everything new. The first observations on acoustics were made in the 6th century BC. Pythagoras established a relationship between the pitch and the long string or pipe that produces sound.

In the 4th century BC, Aristotle was the first to correctly imagine how sound propagates in the air. He said that a sounding body causes compression and rarefaction of air, the echo was explained by the reflection of sound from obstacles.

In the 15th century, Leonardo da Vinci formulated the principle of the independence of sound waves from various sources.

In 1660, in the experiments of Robert Boyle, it was proved that air is a conductor of sound (sound does not propagate in a vacuum).

In 1700-1707 published a memoir by Joseph Saver on acoustics, published by the Paris Academy of Sciences. In these memoirs, Saver examines a phenomenon that is well known to organ designers: if two pipes of an organ emit simultaneously two sounds, only slightly different in height, then periodic amplifications of sound are heard, similar to drum rolls. Saver explained this phenomenon by the periodic coincidence of the vibrations of both sounds. If, for example, one of the two sounds corresponds to 32 vibrations per second, and the other to 40 vibrations, then the end of the fourth vibration of the first sound coincides with the end of the fifth vibration of the second sound, and thus the sound is amplified. From organ pipes, Saver moved on to the experimental study of string vibrations, observing the nodes and antinodes of vibrations (these names, which still exist in science, were introduced by him), and also noticed that when the string is excited, along with the main note, other notes sound, length whose waves are ½, 1/3, ¼ ,. from the main one. He called these notes the highest harmonic tones, and this name was destined to remain in science. Finally, Saver was the first to try to determine the boundary of the perception of vibrations as sounds: for low sounds he indicated the boundary at 25 vibrations per second, and for high sounds - 12 800. Then, Newton, based on these experimental works of Saver, gave the first calculation of the wavelength of sound and came to the conclusion, now well known in physics, that for any open pipe the wavelength of the emitted sound is twice the length of the pipe.

Sound sources and their nature

Common to all sounds is that the bodies that generate them, that is, the sources of sound, vibrate. Everyone is familiar with the sounds that arise when the skin stretched over the drum, the waves of the sea surf, the branches swayed by the wind. They are all different from each other. The "color" of each individual sound strictly depends on the movement due to which it occurs. So if the vibrational motion is extremely fast, the sound contains vibrations of high frequency. A less rapid oscillatory motion creates a lower frequency sound. Various experiments indicate that any sound source necessarily vibrates (although most often these vibrations are not noticeable to the eye). For example, the sounds of the voices of people and many animals arise as a result of vibrations of their vocal cords, the sound of wind musical instruments, the sound of a siren, the whistle of the wind, and thunder are caused by the vibrations of the air masses.

But not every vibrating body is a source of sound. For example, an oscillating weight suspended on a thread or spring does not make a sound.

The frequency at which the oscillation repeats is measured in hertz (or cycles per second); 1Hz is the frequency of such a periodic oscillation, the period is 1s. Note that it is frequency that is the property that allows us to distinguish one sound from another.

Studies have shown that the human ear is able to perceive mechanical vibrations of bodies as sound, occurring with a frequency of 20 Hz to 20,000 Hz. At very fast, more than 20,000 Hz or very slow, less than 20 Hz, we do not hear sound vibrations. That is why we need special instruments to register sounds outside the frequency range perceived by the human ear.

If the speed of the oscillatory motion determines the frequency of the sound, then its magnitude (size of the room) is the loudness. If such a wheel is rotated at high speed, a high frequency tone will be generated, a slower rotation will generate a tone of a lower frequency. Moreover, the finer the teeth of the wheel (as shown by the dotted line), the weaker the sound, and the larger the teeth, that is, the more they force the plate to deviate, the louder the sound. Thus, we can note another characteristic of sound - its loudness (intensity).

It is impossible not to mention such a property of sound as quality. Quality is closely related to structure, which can vary from overly complex to overly simple. The pitch of the tuning fork, supported by the resonator, has a very simple structure, since it contains only one frequency, the magnitude of which depends solely on the design of the tuning fork. In this case, the sound of a tuning fork can be both strong and weak.

Complex sounds can be created, for example, multiple frequencies contain the sound of an organ chord. Even the sound of a mandolin string is quite complex. This is due to the fact that the stretched string vibrates not only with the fundamental (like a tuning fork), but also with other frequencies. They generate additional tones (harmonics), the frequencies of which are an integer number of times higher than the frequency of the fundamental tone.

The concept of frequency is inappropriate to apply in relation to noise, although we can talk about some areas of its frequencies, since it is they that distinguish one noise from another. The noise spectrum can no longer be represented by one or several lines, as in the case of a monochromatic signal or a periodic wave containing many harmonics. He is depicted as a whole strip

The frequency structure of some sounds, especially musical ones, is such that all overtones are harmonic with respect to the fundamental tone; in such cases, the sounds are said to have a pitch (determined by the frequency of the fundamental). Most of the sounds are not so melodic, they do not have an integer ratio between frequencies inherent in musical sounds. These sounds are similar in structure to noise. Therefore, summarizing what has been said, we can assert that the sound is characterized by loudness, quality and pitch.

What happens to the sound after it occurs? How does it reach, for example, our ear? How does it spread?

We perceive sound with the ear. Between the sounding body (sound source) and the ear (sound receiver) there is a substance that transmits sound vibrations from the sound source to the receiver. Most often, this substance is air. Sound cannot propagate in an airless space. As waves cannot exist without water. Experiments confirm this conclusion. Let's consider one of them. A bell is placed under the bell of the air pump and turned on. Then they begin to pump out air with a pump. As the air becomes thinner, the sound becomes weaker and weaker and, finally, almost completely disappears. When I begin to let the air in under the bell again, the sound of the bell becomes audible again.

Of course, sound propagates not only in the air, but also in other bodies. This can also be verified by experience. Even a sound as faint as the ticking of a pocket watch lying on one end of the table can be clearly heard by putting your ear to the other end of the table.

It is well known that sound is transmitted over long distances over the ground and especially over railroad tracks. By putting your ear to the rail or to the ground, you can hear the sound of a far-reaching train or the sound of a galloping horse.

If we, while under water, hit a stone on a stone, then we will clearly hear the sound of the impact. Consequently, sound propagates in water as well. Fish hear footsteps, and the voices of people on the shore, this is well known to fishermen.

Experiments show that different solids conduct sound in different ways. Elastic bodies are good conductors of sound. Most metals, wood, gases, and liquids are elastic bodies and therefore conduct sound well.

Soft and porous bodies are poor sound conductors. When, for example, a watch is in a pocket, it is surrounded by soft fabric, and we do not hear its ticking.

By the way, the propagation of sound in solids is related to the fact that the experiment with a bell placed under a bell seemed not very convincing for a long time. The fact is that the experimenters did not sufficiently isolate the bell, and the sound was heard even when there was no air under the bell, since the vibrations were transmitted through all possible connections of the installation.

In 1650, Athanasius Kirch'er and Otto Gücke, on the basis of an experiment with a bell, concluded that air was not needed for sound propagation. And only ten years later, Robert Boyle convincingly proved the opposite. Sound in air, for example, is transmitted by longitudinal waves, that is, by alternating condensations and rarefaction of air, coming from the sound source. But since the space around us, in contrast to the two-dimensional surface of water, is three-dimensional, then sound waves propagate not in two, but in three directions - in the form of diverging spheres.

Sound waves, like any other mechanical waves, propagate in space not instantly, but with a certain speed. The simplest observations make it possible to verify this. For example, during a thunderstorm, we first see lightning and only after a while we hear thunder, although the vibrations of the air, which we perceive as sound, arise simultaneously with the flash of lightning. The fact is that the speed of light is very high (300,000 km / s), so we can assume that we see a flash at the moment of its occurrence. And the sound of thunder, which was formed simultaneously with lightning, requires quite perceptible time for us to travel the distance from the place of its occurrence to the observer standing on the ground. For example, if we hear thunderclaps more than 5 seconds after we saw lightning, then we can conclude that the thunderstorm is at a distance of at least 1.5 km from us. The speed of sound depends on the properties of the medium in which the sound travels. Scientists have developed various ways to determine the speed of sound in any environment.

The speed of sound and its frequency determine the wavelength. Observing waves in a pond, we notice that the diverging circles are sometimes smaller and sometimes larger, in other words, the distance between the crests of the waves or the troughs of the waves can be different depending on the size of the object due to which they arose. By keeping our hand low enough above the surface of the water, we can feel every splash passing by us. The greater the distance between the following waves, the less often their crests will touch our fingers. Such a simple experience allows us to conclude that in the case of waves on the water surface, for a given wave propagation speed, a higher frequency corresponds to a smaller distance between wave crests, that is, shorter waves, and, conversely, to a lower frequency, longer waves.

The same is true for sound waves. The fact that a sound wave passes through a certain point in space can be judged by the change in pressure at this point. This change completely repeats the vibration of the membrane of the sound source. A person hears sound because the sound wave exerts varying pressure on the eardrum of his ear. As soon as the crest of the sound wave (or high pressure area) reaches our ear. We feel the pressure. If the areas of increased pressure of the sound wave follow each other quickly enough, then the eardrum of our ear also fluctuates quickly. If the crests of the sound wave lag significantly behind each other, then the eardrum will oscillate much more slowly.

The speed of sound in air is surprisingly constant. We have already seen that the frequency of sound is directly related to the distance between the crests of the sound wave, that is, there is a certain relationship between the frequency of sound and wavelength. We can express this relationship as follows: the wavelength is equal to the speed divided by the frequency. It can be said in another way: the wavelength is inversely proportional to the frequency with a coefficient of proportionality equal to the speed of sound.

How does sound become audible? When sound waves enter the ear canal, they vibrate the eardrum, middle ear, and inner ear. Once in the fluid filling the cochlea, the air waves affect the hair cells inside the organ of Corti. The auditory nerve transmits these impulses to the brain, where they are converted into sounds.

Measuring noise

Noise is an unpleasant or undesirable sound, or a set of sounds that interfere with the perception of useful signals, disturb the silence, have a harmful or irritating effect on the human body, and reduce its working capacity.

In noisy areas, many people develop symptoms of noise sickness: increased nervous excitability, fatigue, high blood pressure.

Noise level is measured in units

Expressing the degree of pressure sounds - decibels. This pressure is not perceived indefinitely. The noise level of 20-30 dB is practically harmless to humans - it is a natural background noise. As for loud sounds, here the permissible limit is approximately 80 dB. A sound of 130 dB already causes a painful sensation in a person, and 150 dB becomes unbearable for him.

Acoustic noise - random sound vibrations of different physical nature, characterized by a random change in amplitude, frequency.

When a sound wave propagates, consisting of thickening and rarefaction of air, the pressure on the tympanic membrane changes. The unit of pressure is 1 N / m2 and the unit of sound power is 1 W / m2.

The threshold of audibility is the minimum sound volume that a person perceives. It is different for different people, and therefore, conventionally, the sound pressure equal to 2x10 "5 N / m2 at 1000 Hz, corresponding to a power of 10" 12 W / m2, is considered to be the hearing threshold. It is with these quantities that the measured sound is compared.

For example, the sound power of engines during takeoff of a jet plane is 10 W / m2, that is, it exceeds the threshold by 1013 times. It is inconvenient to operate with such large numbers. It is said about sounds of different loudness that one is louder than the other, not so many times, but by so many units. The loudness unit is called Bel - after the inventor of the telephone A. Bela (1847-1922). Loudness is measured in decibels: 1 dB \u003d 0.1 B (Bel). A visual representation of how sound intensity, sound pressure and volume level are related.

The perception of sound depends not only on its quantitative characteristics (pressure and power), but also on its quality - frequency.

The sound of the same strength at different frequencies differs in volume.

Some people cannot hear high frequency sounds. So, in the elderly, the upper limit of sound perception is reduced to 6000 Hz. They do not hear, for example, the squeak of a mosquito and the trills of a cricket, which emit sounds with a frequency of about 20,000 Hz.

The famous English physicist D. Tyndall describes one of his walks with a friend in the following way: "The meadows on both sides of the road were teeming with insects, which for my hearing filled the air with their sharp buzzing, but my friend did not hear anything of this - the music of insects flew beyond the boundaries of his hearing" !

Noise levels

Loudness - the level of energy in a sound - measured in decibels. Whispering equates to about 15 dB, the rustle of voices in a student audience reaches about 50 dB, and street noise in heavy traffic is about 90 dB. Noises above 100 dB can be unbearable to the human ear. Noises on the order of 140 dB (such as the sound of a jet taking off) can be painful to the ear and damage the eardrum.

For most people, hearing acuity dulls with age. This is due to the fact that the ear bones lose their original mobility, and therefore vibrations are not transmitted to the inner ear. In addition, ear infections can damage the eardrum and negatively affect the function of the bones. If you have any hearing problems, you should see a doctor immediately. Some types of deafness are caused by damage to the inner ear or auditory nerve. Hearing impairment can also be caused by constant noise exposure (for example, in the factory floor) or sudden and very loud sound bursts. Be very careful when using personal stereo players, as excessive volume can also lead to deafness.

Acceptable noise in rooms

Regarding the noise level, it is worth noting that such a concept is not ephemeral and unregulated in terms of legislation. So, in Ukraine to this day, the Sanitary Standards of Permissible Noise in the premises of residential and public buildings and on the territory of residential development, adopted in the days of the USSR, are in force. According to the specified document, in residential premises, the noise level must not exceed 40 dB during the day and 30 dB at night (from 22:00 to 8:00).

Noise often carries important information. A car or motorcycle racer listens carefully to the sounds that the engine, chassis and other parts of a moving vehicle emit, because any extraneous noise can be a harbinger of an accident. Noise plays an essential role in acoustics, optics, computer technology, and medicine.

What is noise? It is understood as disorderly complex vibrations of various physical nature.

The noise problem arose a long time ago. Already in ancient times, the sound of wheels on the cobblestone pavement caused insomnia in many.

Or maybe the problem arose even earlier, when the neighbors in the cave began to quarrel over the fact that one of them knocked too loudly while making a stone knife or ax?

Noise pollution of the environment is growing all the time. If in 1948, when examining residents of large cities, when asked whether they were bothered by the noise in their apartment, 23% of the respondents answered affirmatively, then in 1961 - already 50%. In the last decade, the noise level in cities has increased 10-15 times.

Noise is a type of sound, although it is often referred to as "unwanted sound." At the same time, according to experts, the noise of a tram is estimated at 85-88 dB, a trolleybus - 71 dB, a bus with an engine with a capacity of more than 220 hp. from. - 92 dB, less than 220 hp from. - 80-85 dB.

Scientists from Ohio State University have concluded that people who are regularly exposed to loud sounds are 1.5 times more likely than others to develop acoustic neuroma.

An acoustic neuroma is a benign tumor that causes hearing loss. Scientists examined 146 patients with acoustic neuroma and 564 healthy people. All of them were asked questions about how often they have to deal with loud sounds no less than 80 decibels (traffic noise). The questionnaire took into account the noise of appliances, motors, music, children's screaming, noise at sports events, in bars and restaurants. Study participants were also asked if they were using hearing protectors. Those who regularly listened to loud music had a 2.5-fold increased risk of developing an acoustic neuroma.

Those who were exposed to technical noise - 1.8 times. For people who regularly listen to children's screams, the noise in stadiums, restaurants or bars - 1.4 times. When using hearing protection, the risk of developing an acoustic neuroma is no higher than in people who are not exposed to noise at all.

Human exposure to acoustic noise

The impact of acoustic noise on a person is different:

A. Harmful

Noise leads to a benign tumor

Prolonged noise adversely affects the hearing organ, stretching the eardrum, thereby decreasing the sensitivity to sound. It leads to a breakdown in the activity of the heart, liver, depletion and overstrain of nerve cells. Sounds and noises of high power affect the hearing aid, nerve centers, and can cause pain and shock. This is how noise pollution works.

Artificial noises, technogenic. They are the ones that negatively affect the human nervous system. One of the worst urban noises is the noise of motor vehicles on major highways. It irritates the nervous system, so a person is tormented by anxiety, he feels tired.

B. Favorable

Useful sounds include foliage noise. The splash of waves has a calming effect on our psyche. The quiet rustle of foliage, the murmur of a stream, a light splash of water and the sound of the surf are always pleasant to a person. They calm him down, relieve stress.

C. Therapeutic

The therapeutic effect on a person with the help of the sounds of nature originated among doctors and biophysicists who worked with astronauts in the early 80s of the twentieth century. In psychotherapeutic practice, natural noises are used in the treatment of various diseases as an aid. Psychotherapists also use the so-called "white noise". This is a kind of hiss, vaguely reminiscent of the sound of waves without splashing water. Doctors believe that "white noise" is soothing and soothing.

Effect of noise on the human body

But is it only the hearing organs that suffer from noise?

Students are encouraged to find out by reviewing the following statements.

1. Noise causes premature aging. In thirty cases out of a hundred, noise reduces the life expectancy of people in large cities by 8-12 years.

2. Every third woman and every fourth man suffer from neuroses caused by an increased level of noise.

3. Diseases such as gastritis, stomach and intestinal ulcers are most common in people living and working in noisy environments. In pop musicians, stomach ulcers are an occupational disease.

4. A sufficiently strong noise within 1 min can cause changes in the electrical activity of the brain, which becomes similar to the electrical activity of the brain in patients with epilepsy.

5. Noise depresses the nervous system, especially with repetitive action.

6. Under the influence of noise, there is a persistent decrease in the frequency and depth of breathing. Sometimes there is arrhythmia of the heart, hypertension.

7. Under the influence of noise, carbohydrate, fat, protein, salt metabolism changes, which manifests itself in a change in the biochemical composition of the blood (the level of sugar in the blood decreases).

Excessive noise (above 80 dB) affects not only hearing organs, but also other organs and systems (circulatory, digestive, nervous, etc.), vital processes are disrupted, energy metabolism begins to prevail over plastic, which leads to premature aging of the body ...

NOISE PROBLEM

A large city is always accompanied by traffic noise. Over the past 25-30 years, in large cities of the world, noise has increased by 12-15 dB (i.e., the noise volume has increased 3-4 times). If an airport is located within a city, as is the case in Moscow, Washington, Omsk and a number of other cities, then this leads to multiple exceeding the maximum permissible level of sound stimuli.

Still, road transport is the leading source of noise in the city. It is he who causes noise up to 95 dB on the main streets of cities on the scale of the sound level meter. Noise level in living rooms with closed windows facing highways is only 10-15 dB lower than outside.

The noise of cars depends on many reasons: the brand of the car, its serviceability, the speed of movement, the quality of the road surface, the engine power, etc. The noise from the engine increases sharply at the moment of its starting and warming up. When the car is moving at the first speed (up to 40 km / h), the engine noise is 2 times higher than the noise generated by it at the second speed. When the vehicle is braked sharply, the noise also increases significantly.

The dependence of the state of the human body on the level of environmental noise has been revealed. Certain changes in the functional state of the central nervous and cardiovascular systems caused by noise were noted. Ischemic heart disease, hypertension, increased blood cholesterol levels are more common in people living in noisy areas. Noise significantly disrupts sleep, reducing its duration and depth. The period of falling asleep increases by an hour or more, and after waking up, people feel fatigue and headache. Over time, all this turns into chronic overwork, weakens the immune system, promotes the development of diseases, and reduces efficiency.

It is now believed that noise can shorten a person's life expectancy by almost 10 years. There are more and mentally ill people due to the increasing sound stimuli, especially the noise affects women. In general, the number of hearing impaired people in cities has increased, and headaches and increased irritability have become the most common occurrences.

NOISE POLLUTION

High power sound and noise affects the hearing aid, nerve centers and can cause pain and shock. This is how noise pollution works. The quiet rustle of foliage, the murmur of a stream, bird voices, a light splash of water and the sound of the surf are always pleasant to a person. They calm him down, relieve stress. It is used in medical institutions, in psychological relief rooms. Natural noises of nature are becoming more rare, disappear altogether or are drowned out by industrial, traffic and other noises.

Prolonged noise adversely affects the hearing organ, reducing the sensitivity to sound. It leads to a breakdown in the activity of the heart, liver, exhaustion and overstrain of nerve cells. Weakened cells of the nervous system cannot sufficiently coordinate the work of various body systems. Hence, violations of their activities arise.

We already know that 150 dB noise is fatal to humans. Not for nothing in the Middle Ages there was an execution under the bell. The hum of the bell ringing tormented and slowly killed.

Each person perceives noise differently. Much depends on age, temperament, health, environmental conditions. Noise has an accumulative effect, that is, acoustic stimuli, accumulating in the body, depress the nervous system more and more. Noise has a particular harmful effect on the neuropsychic activity of the body.

Noises cause functional disorders of the cardiovascular system; has a harmful effect on the visual and vestibular analyzers; reduce reflex activity, which often causes accidents and injuries.

The noise is insidious, its harmful effect on the body is carried out invisibly, imperceptibly, crashes in the body are not immediately detected. In addition, the human body is practically defenseless against noise.

Increasingly, doctors talk about noise sickness, predominant damage to the hearing and nervous system. The source of noise pollution can be an industrial plant or transport. Heavy dump trucks and trams are especially noisy. Noise affects the human nervous system, and therefore noise protection measures are taken in cities and at enterprises. Railroad and tram lines and the roads along which freight transport passes should be removed from the central parts of cities to sparsely populated areas and green spaces should be created around them that absorb noise well. Airplanes should not fly over cities.

SOUNDPROOFING

Sound insulation helps to avoid the harmful effects of noise

Reducing the noise level is achieved through construction and acoustic measures. In external enclosing structures, windows and balcony doors have significantly less sound insulation than the wall itself.

The degree of noise insulation of buildings is primarily determined by the permissible noise standards for premises of this purpose.

FIGHTING ACOUSTIC NOISE

The MNIIP acoustics laboratory is developing the sections "Acoustic ecology" as part of the project documentation. Projects on sound insulation of premises, noise control, calculations of sound reinforcement systems, acoustic measurements are being carried out. Although in ordinary rooms, people increasingly want acoustic comfort, - good protection against noise, intelligible speech and the absence of so-called. acoustic phantoms - negative sound images formed by some. In structures intended for additional fight against decibels, at least two layers alternate - "hard" (drywall, gypsum fiber). Also, the acoustic design must occupy its own modest niche inside. Frequency filtering is used to combat acoustic noise.

CITY AND GREEN PLANTS

If you protect your home from noise by trees, then it will be useful to find out that sounds are not absorbed by foliage. Striking the trunk, the sound waves break, heading down towards the soil, which is absorbed. Spruce is considered the best guardian of silence. Even on the most busy highway, you can live in peace if you protect your home by a number of green trees. And it would be nice to plant chestnuts nearby. One adult chestnut tree clears space up to 10 m high, up to 20 m wide and up to 100 m long from automobile exhaust gases. At the same time, unlike many other trees, the chestnut decomposes the toxic substances of gases with almost no harm to its “health”.

The importance of greening city streets is great - dense plantings of bushes and forest belts protect from noise, reducing it by 10-12 dB (decibel), reduce the concentration of harmful particles in the air from 100 to 25%, reduce the wind speed from 10 to 2 m / s, reduce concentration of gases from cars up to 15% per unit volume of air, make the air more humid, lower its temperature, that is, make it more acceptable for breathing.

Green spaces also absorb sound, the higher the trees and the denser they are, the less sound is heard.

Green spaces in combination with lawns, flower beds have a beneficial effect on the human psyche, soothe the eyesight, nervous system, are a source of inspiration, and increase people's efficiency. The greatest works of art and literature, the discoveries of scientists, were born under the beneficial influence of nature. This is how the greatest musical creations of Beethoven, Tchaikovsky, Strauss and other composers, pictures of the wonderful Russian landscape painters Shishkin, Levitan, works of Russian and Soviet writers were created. It is no coincidence that the Siberian Scientific Center was founded among the green spaces of the Priobsky Bor. Here, in the shade of the city noise, surrounded by greenery, our Siberian scientists successfully conduct their research.

Greenery is high in cities such as Moscow, Kiev; in the latter, for example, there are 200 times more plantings per inhabitant than in Tokyo. In the capital of Japan, over 50 years (1920-1970), about half of "all green areas located within" a radius of ten kilometers from the center were destroyed. In the USA, almost 10 thousand hectares of central city parks have been lost over the five years.

← Noise has a harmful effect on human health, first of all, hearing, the state of the nervous and cardiovascular systems deteriorate.

← Noise can be measured using special devices - sound level meters.

← It is necessary to combat the harmful effects of noise by controlling the noise level, as well as by using special measures to reduce the noise level.

\u003e\u003e Physics: Sound in different environments

An elastic medium is required for sound propagation. In a vacuum, sound waves cannot propagate, since there is nothing to vibrate there. This can be verified by simple experience. If we place an electric bell under a glass bell, then as 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 a while we hear thunder rolls (Fig. 52). This delay arises due to the fact that 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 the bullet fired from the Kalashnikov machine gun (PK) decreases at a distance of 800 m. The muzzle velocity is 825 m / s, which is significantly higher than the speed of sound in the air. Therefore, a person who has heard the sound of a shot or the whistle of a bullet need not worry: this bullet has already passed him. The bullet overtakes 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 an increase in air temperature, it increases, and with a decrease, 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. So, 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 generally 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, gunpowder was set on fire and at the same time they struck a bell, lowered into the water. The sound of this bell with the help of a special horn, also lowered into the water, was caught on another boat, which was 14 km from the first. The time interval between a flash of light and the arrival of a sound signal was used to determine the speed of sound in water. 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 passes further. When sound passes from air to 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 higher. The hearing aid of fish responds to this very thing. Therefore, for example, screams and noises above the surface of the water are a sure way to scare away marine life. A person who is 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 the sound pressure increased during the transition from air to water, now, on the contrary, it decreases sharply. 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 on the border between water and air gave our ancestors a reason to consider the underwater world as a "world of silence". Hence the expression: "He is like a fish." However, Leonardo da Vinci also suggested listening to underwater sounds, putting your ear to the oar, lowered into the water. Using this method, you can make sure that the fish are actually quite chatty.

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 along the rail, the other through the air.

Ground 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 leading to a tunnel to the walls or not. Putting their ear to the ground, they also watched the approach of the enemy cavalry.

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

1. Why during a thunderstorm we first see lightning and only then hear thunder? 2. What determines the speed of sound in gases? 3. Why does a person standing on the bank of a river not hear sounds arising under water? 4. Why were blind people often the "rumors" who in ancient times watched the enemy's earthworks?

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

S.V. Gromov, N.A. Homeland, Physics grade 8

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If the sound wave does not meet obstacles in its path, it propagates evenly 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, 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 do not 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 is possible 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 3.37 m. As the frequency decreases, the length becomes even greater. Therefore, the sound wave easily bends around objects that are commensurate with it. Trees in the park do not prevent us from hearing sound at all, because the diameters of their trunks are much less than the length of the sound wave.

Due to diffraction, sound waves penetrate through slots and holes in an obstacle and propagate behind them.

We place a flat screen with a hole in the path of the sound wave.

In the case when the sound wavelength ƛ much larger than the bore diameter D , or these values \u200b\u200bare approximately equal, then behind the hole the sound will reach all points of the area that is behind the screen (the area of \u200b\u200bthe sound shadow). The front of the outgoing wave will appear as a hemisphere.

If ƛ only slightly smaller than the slit diameter, then the main part of the wave propagates directly, and a small part slightly diverges 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 variants of its further propagation are possible. Sound can be reflected from the interface, can pass into another medium without changing direction, or it can be refracted, that is, go by changing its direction.

Suppose there is an obstacle in the path of the sound wave, the size of which is much larger than the wavelength, for example, a sheer rock. How will the sound behave? Since he cannot go around this obstacle, he will be reflected from him. Behind the obstacle is acoustic shadow area .

The 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 media. When reflected, the wave returns to the environment from which it came.

If the surface is flat, sound bounces off it just like a beam of light is reflected in a mirror.

Sound beams reflected from a concave surface are focused at one point.

The convex surface scatters sound.

The diffuse effect is produced 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 appears in the water does not pass into the air. Reflecting 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, the wave resistance is equal to the product of the density of the medium by the speed of sound in it. Since the wave resistance of gases is much less than the wave resistance of liquids and solids, the sound wave is reflected when it hits the border of air and water.

Fish in 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.

Sound refraction

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 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 the media.

where i - angle of incidence,

r - angle of reflection,

v 1 Is the speed of sound propagation in the first medium,

v 2 Is the speed of sound propagation in the second medium,

n Is the refractive index.

The refraction of sound is called refraction .

If the sound wave falls not 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 a heterogeneous environment - 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 salinity.

Sound absorption

When a sound wave meets 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 what part of the energy of sound vibrations is absorbed by 1 m 2 of the obstacle. It has a value between 0 and 1.

The unit of measurement for sound absorption is called sabin ... It got its name from the name of the American physicist Wallace Clement Sabin, founder of architectural acoustics. 1 sabin is the energy 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 newly built Fogg Museum, Wallace Clement Sabin came to the conclusion that there is a relationship between the size of the hall, acoustic conditions, the type and area of \u200b\u200bsound-absorbing materials and reverberation time .

Reverb the process of reflection of a sound wave from obstacles and its gradual attenuation after turning off the sound source is called. In an enclosed space, sound can be reflected multiple times from walls and objects. As a result, various echoes are generated, each of which sounds as if in isolation. This effect is called reverb effect .

The most important characteristic of a room is reverberation time which Sabin entered and calculated.

where V - the volume of the room,

AND - general sound absorption.

where a i Is the sound absorption coefficient of the material,

S i - the area of \u200b\u200beach surface.

If the reverberation time is long, the sounds seem to "roam" 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.

From his calculations, Sabin positioned the sound-absorbing materials in such a way that the "echo effect" was reduced. And the Boston Symphony Hall, for 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 seen and only after a while thunder rumbles are heard. This delay arises due to the fact that the speed of sound in air is much less than the speed of light coming from lightning. It is curious to remember in which environment the sound propagates the fastest, and where it does not propagate 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 deduced the final formula for its determination. The speed of sound depends on temperature: with an increase in air temperature, it increases, and with a decrease, 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 generally greater than the speed of sound in air. The experiments on determining the speed were first carried out on Lake Geneva in 1826. The two physicists boarded the boats and traveled 14 km. On one boat, gunpowder was set on fire and at the same time they struck a bell, lowered into the water. The sound of the bell with the help of a special horn, also lowered into the water, was caught on another boat. The time interval between a flash of light and the arrival of a sound signal was used to determine the speed of sound in water. At a temperature of + 8 °, it turned out to be approximately 1440 m / s. People working in underwater structures confirm that coastal sounds are clearly audible under water, and fishermen know that fish swim away at the slightest suspicious noise on the coast.

The speed of sound in solids is greater than in liquids and gases. For example, if you put your ear to a rail, then after hitting the other end of the rail, a person will hear two sounds. One of them will "come" up to the ear along the rail, the other - through the air. Ground 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, the cavalry was rushing or not. By the way, thanks to this, people who have lost their hearing are sometimes able to dance to the music, which reaches their auditory nerves not through the air and outer ear, but through the floor and bones.

The speed of sound is the speed of propagation of elastic waves in a medium, both in longitudinal (in gases, liquids or solids) and in transverse, shear (in solids), is 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.