This lesson covers the topic “ Sound waves". In this lesson, we will continue our study of acoustics. First, we will repeat the definition of sound waves, then we will 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 various environments, and find out what characteristics they have. .
Sound waves - these are mechanical vibrations, which, propagating and interacting with the organ of hearing, are perceived by a person (Fig. 1).
Rice. 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 relaxation alternate. It is transmitted over time over a distance (Fig. 2).
Rice. 2. Propagation of a sound wave
Sound waves include those vibrations that are carried out 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).
Rice. 3. Ranges of sound waves
Each educated person must navigate 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 apparatus ultrasound examination, ultrasound machines, 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).
Rice. 4. Application of infrasound
The speed of sound depends on environmental conditions and temperature (Fig. 5).
Rice. 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 and air 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 greater than in air. Information will travel 4 times faster by water than by air. And even faster in steel: 
(fig. 6).
Rice. 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 the RVS Gaidar) used very in an interesting way 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 it will reach the ear of Ilya Muromets faster, 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 a sound source vibrate harmoniously, then we will have a wonderful sound wave, which will be called a musical sound. These sources of sound waves can be, for example, the strings of a guitar or grand 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).
Rice. 7. Musical tones
All objects that can emit tones will have special 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.
Moving on to quantitative assessments sound waves. What are the characteristics of musical sound waves? These characteristics apply exclusively to harmonic sound vibrations. So, sound volume... What determines the volume of a sound? Consider the propagation of a sound wave in time or the oscillation of a sound wave source (Fig. 8).
Rice. 8. Sound volume
At the same time, if we added not very much sound to the system (for example, tapped a piano key softly), then there will be a quiet sound. If we raise our hand loudly, we 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 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 often, 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).
Rice. 9. Sound pitch
An interesting conclusion can be drawn. If a person sings in bass, then his source of sound (these are the vocal cords) oscillates several times slower than that of a person who sings soprano. In the second case, the vocal cords vibrate more often, therefore, more often they cause foci of compression and vacuum in the propagation of the wave.
There is one more interesting characteristic sound waves that physicists do not study. This timbre... You know and easily distinguish the same piece of music, which is performed on the balalaika or on the cello. What is the difference between these sounds or is 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 for grades. 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.
Bibliography
- 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.
- 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.
- Internet portal "eduspb.com" ()
- Internet portal "msk.edu.ua" ()
- Internet portal "class-fizika.narod.ru" ()
Homework
- How does sound propagate? What could be the source of the sound?
- Can sound propagate in space?
- Is every wave that reaches a human hearing organ is perceived by it?
There are so many around us sound sources: musical and technical instruments, human vocal cords, sea waves, wind and others. Sound or otherwise sound waves- these are mechanical vibrations of the medium with frequencies of 16 Hz - 20 kHz(see § 11-a).
Consider experience. Placing the alarm clock on a pillow under the bell of the air pump, we will notice that the ticking will become quieter, but it will still be audible. Having pumped out the air from under the bell, we will cease to hear the sound at all. This experience confirms that sound travels through air and does not travel in a vacuum.
The speed of sound in air is relatively high: it lies in the range from 300 m / s at –50 ° С to 360 m / s at + 50 ° С. This is 1.5 times more than the speed of passenger aircraft. In liquids, sound propagates much faster, and in solids- even faster. In a steel rail, for example, the speed of sound is »5000 m / s.
Take a look at the graphs of air pressure fluctuations near the mouth of a person singing the sounds "A" and "O". As you can see, the oscillations are complex, consisting of several oscillations superimposed on each other. At the same time, one can clearly see basic vibrations, the frequency of which is almost independent of the pronounced sound. For a male voice, this is approximately 200 Hz, for a female voice, it is 300 Hz.
l max = 360 m / s: 200 Hz "2 m, l min = 300 m / s: 300 Hz" 1 m.
So, the sound wavelength of a voice depends on the air temperature and the fundamental frequency of the voice. Recalling our knowledge of diffraction, we will understand why people’s voices can be heard in the forest, even if they are obscured by trees: sounds with wavelengths of 1–2 m easily go around tree trunks whose diameter is less than a meter.
Let us make an experiment confirming that the sources of sound are indeed vibrating bodies.
Let's take the device fork- a metal slingshot mounted on a box without a front wall for better emission of sound waves. If you hit the ends of the tuning fork slingshot with a hammer, it will make a "clear" sound called musical tone(for example, the note "A" of the first octave with a frequency of 440 Hz). Let's move the sounding tuning fork to a light ball on the thread, and it will immediately bounce to the side. This is precisely because of the frequent vibrations of the ends of the tuning fork slingshot.
The reasons on which the frequency of body vibrations depends are its elasticity and size. How bigger size the body, the lower the frequency. Therefore, for example, elephants with large vocal cords emit low-frequency sounds (bass), and mice, whose vocal cords are much smaller, emit high-frequency sounds (squeak).
Not only how the body will sound depends on elasticity and size, but also how it will pick up sounds - respond to them. The phenomenon of a sharp increase in the amplitude of oscillations when the frequency of the external influence coincides with the natural frequency of the body is called resonance (Latin "reasonably" - I respond). Let's make an experiment to observe resonance.
We place two identical tuning forks next to each other, turning them towards each other by those sides of the boxes where there are no walls. Let's hit the left tuning fork with a hammer. In a second, we will drown it out with our hand. We will hear that the second tuning fork sounds, which we did not strike. They say that the right tuning fork resonates, that is, it captures the energy of sound waves from the left tuning fork, as a result of which it increases the amplitude of natural vibrations.
Sound sources.
Sound vibrations
Lesson summary.
1.Organizational moment
Hello guys! Our lesson has a wide practical application in everyday practice. Therefore, your answers will depend on observation in life and on the ability to analyze your observations.
2. Repetition of basic knowledge.
Slides 1, 2, 3, 4, 5 are displayed on the projector screen (Appendix 1).
Guys, before you is a crossword puzzle, having solved which you will learn the key word of the lesson.
1st snippet: name the physical phenomenon
2nd snippet: name the physical process
3rd snippet: name the physical quantity
4th snippet: name the physical device
R | Z | N | ||||||||
V | ||||||||||
Have | ||||||||||
TO |
Pay attention to the highlighted word. This is the word "SOUND", it is the key word of the lesson. Our lesson is about sound and sound vibrations. So, the topic of the lesson is “Sound Sources. Sound vibrations". In the lesson, you will learn what is the source of sound, what sound vibrations are, their occurrence and some practical applications in your life.
3. Explanation of the new material.
Let's do the experiment. The purpose of the experiment: to find out the causes of the occurrence of sound.
Experience with a metal ruler(Appendix 2).
What have you observed? What conclusion can be drawn?
Conclusion: a vibrating body creates sound.
Let's carry out the following experiment. The purpose of the experiment: to find out whether sound is always created by an oscillating body.
The device that you see in front of you is called fork.
Experience with a tuning fork and a tennis ball hanging from a string(Appendix 3) .
You hear the sound that the tuning fork makes, but the tuning fork does not vibrate. To make sure that the tuning fork vibrates, we carefully move it to the shady balls suspended on a thread and see that the vibrations of the tuning fork were transmitted to the ball, which came into periodic motion.
Conclusion: sound is generated by any vibrating body.
We live in an ocean of sounds. Sound is created by sound sources. There are both artificial and natural sources of sound. TO natural sources sound include vocal cords (Appendix 1 - slide number 6). The air we breathe comes out of the lungs through Airways into the larynx. The larynx contains the vocal cords. Under the pressure of the exhaled air, they begin to oscillate. The role of the resonator is played by the mouth and nose, as well as the chest. For articulate speech, in addition to the vocal cords, the tongue, lips, cheeks, soft palate and epiglottis are also needed.
Other natural sources of sound include the buzzing of a mosquito, a fly, a bee ( wings flutter).
Question:due to which the sound is created.
(The air in the ball is compressed under pressure. Then, it expands abruptly and creates a sound wave.)
So, sound is created not only by an oscillating, but also by a sharply expanding body. Obviously, in all cases of the appearance of sound, the air layers move, that is, a sound wave arises.
The sound wave is invisible, it can only be heard, and also registered by physical devices. To register and study the properties of a sound wave, we will use a computer, which is currently widely used by physicists for research. A special research program is installed on the computer, and a microphone is connected, which picks up sound vibrations (Appendix 4). Look at the screen. On the screen you see graphical representation sound vibration. What is given schedule? (sinusoid)
Let's experiment with a tuning fork with a feather. We hit the tuning fork with a rubber mallet. Students see tuning fork vibrations, but they do not hear sound.
Question:Why are there vibrations, but you can't hear the sound?
It turns out guys, the human ear perceives sound ranges in the range from 16 Hz to Hz, this is an audible sound.
Listen to them through a computer and catch the change in the frequencies of the range (Appendix 5). Pay attention to how the appearance of the sinusoid changes when the frequency of sound vibrations changes (the period of the oscillations decreases, and therefore the frequency increases).
There are inaudible sounds to the human ear. These are infrasound (oscillation range less than 16 Hz) and ultrasound (more than Hz range). You can see the diagram of the frequency ranges on the board, draw it in a notebook (Appendix 5). Investigating infra and ultrasounds, scientists have discovered a lot interesting features these sound waves. About these interesting facts your classmates will tell us (Appendix 6).
4. Consolidation of the studied material.
To consolidate the studied material in the lesson, I propose to play the TRUE-WRONG game. I read the situation, and you raise the sign with the inscription, TRUE or FALSE, and explain your answer.
Questions. 1. Is it true that the source of sound is any vibrating body? (right).
2. Is it true that the music sounds louder in a crowded hall than in an empty one? (false, since the empty hall acts as a resonator of oscillations).
3. Is it true that a mosquito flaps its wings faster than a bumblebee? (true, because the sound produced by the mosquito is higher, therefore, the frequency of oscillation of the wings is also higher).
4. Is it true that the vibrations of a sounding tuning fork decay more quickly if its leg is placed on a table? (true, since the vibrations of the tuning fork are transmitted to the table).
5. Is it true that the bats see with sound? (right, because bats emit ultrasound and then listen to the reflected signal).
6. Is it true that some animals "predict" earthquakes using infrasound? (it is true, for example, elephants feel an earthquake in a few hours and are extremely excited at the same time).
7.Is it true that infrasound causes mental disorders in people? (right, in Marseille (France) next to scientific center a small factory was built. Shortly after its launch in one of the scientific laboratories discovered strange phenomena. After staying in her room for a couple of hours, the researcher became absolutely stupid: he could hardly solve even a simple problem).
And in conclusion, I suggest that you get the keywords of the lesson from the cut letters, by rearranging.
KVZU - SOUND
RAMTNOKE - CAMERTON
TRAKZUVLU - ULTRASOUND
FRAKVZUNI - INFRASOUND
OKLABEINYA - VIBRATIONS
5. Summing up the lesson and homework.
Lesson summary. In the lesson, we found out that:
That any body that vibrates creates sound;
Sound travels in the air in the form of sound waves;
Sounds are audible and inaudible;
Ultrasound is an inaudible sound whose vibration frequency is higher than 20 kHz;
Infrasound is inaudible sound with a vibration frequency below 16Hz;
Ultrasound is widely used in science and technology.
Homework:
1.§34, exercise. 29 (Peryshkin class 9)
2. Continue the reasoning:
I hear the sound: a) flies; b) a dropped object; c) thunderstorms, because….
I do not hear the sound: a) from a climbing dove; b) from an eagle soaring in the sky, because ...
The branch of physics dealing with sound vibrations is called acoustics.
The human ear is designed so that it perceives vibrations with a frequency of 20 Hz to 20 kHz as sound. Low frequencies (sound from a bass drum or pipe organ) are perceived by the ear as bass notes. The whistle or squeak of a mosquito corresponds to high frequencies. Vibrations below 20 Hz are called infrasound, and with a frequency over 20 kHz - ultrasound. A person does not hear such vibrations, but there are animals that hear infrasounds emanating from crust before the earthquake. Hearing them, the animals leave the dangerous area.
In music, acoustic frequencies correspond to but there. The "A" note of the main octave (key C) corresponds to a frequency of 440 Hz. The "A" note of the next octave corresponds to a frequency of 880 Hz. And so all the other octaves differ in frequency by exactly two times. Within each octave, 6 tones or 12 semitones are distinguished. Each tone has a frequency of yf2~ 1.12 different from the frequency of the previous tone, each semitone differs from the previous one in "$ 2. We see that each next frequency differs from the previous one not by some Hz, but by the same number of times. Such a scale is called logarithmic, since the equal distance between the tones will be exactly on the logarithmic scale, where not the value itself is deposited, but its logarithm.
If the sound corresponds to one frequency v (or with = 2tcv), then it is called harmonic, or monochromatic. Purely harmonic sounds are rare. Sound almost always contains a set of frequencies, that is, its spectrum (see Section 8 of this chapter) is complex. Musical vibrations always contain the main tone cco = 2p / T, where T is the period, and a set of overtones 2 (Oo, Zco 0, 4coo, etc. A set of overtones indicating their intensities in music is called timbre. Different musical instruments, different singers playing the same note have different timbre. This gives them different colors.
An admixture of non-multiple frequencies is also possible. In classical European music, this is considered dissonant. However, it is used in modern music. Even a slow movement of any frequencies in the direction of increasing or decreasing is used (ukulele).
In non-musical sounds, any combination of frequencies in the spectrum and their change over time are possible. The spectrum of such sounds can be continuous (see Section 8). If the intensities for all frequencies are approximately the same, then such a sound is called "white noise" (the term is taken from optics, where White color- the set of all frequencies).
The sounds of human speech are very complex. They have a complex spectrum that changes rapidly over time with the uttering of one sound, a word, and an entire phrase. This gives speech sounds different intonations and accents. As a result, you can tell one person from another by voice, even if they say the same words.
Let's move on to considering sound phenomena.
The world of the sounds around us is diverse - the voices of people and music, the singing of birds and the buzzing of bees, thunder during a thunderstorm and the sound of a forest in the wind, the sound of passing cars, airplanes and other objects.
Pay attention!
The sources of sound are vibrating bodies.
Example:
Let's fix an elastic metal ruler in a vice. If its free part, the length of which is selected in a certain way, is set in oscillatory motion, then the ruler will emit a sound (Fig. 1).
Thus, the oscillating ruler is the source of the sound.
Consider the image of a sounding string, the ends of which are fixed (Fig. 2). The blurry outline of this string and the apparent thickening in the middle indicate that the string is wobbling.
If the end of the paper strip is brought closer to the sounding string, then the strip will bounce from the jolts of the string. As long as the string vibrates, a sound is heard; stop the string and the sound stops.
Figure 3 shows a tuning fork - a curved metal rod on a stem, which is attached to the resonator box.
If you hit the tuning fork with a soft hammer (or draw a bow over it), the tuning fork will sound (Fig. 4).
Let us bring a light ball (glass bead) suspended by a string to the sounding tuning fork - the ball will bounce off the tuning fork, indicating the vibrations of its branches (Fig. 5).
To "record" the vibrations of a tuning fork with a small (of the order of \ (16 \) Hz) natural frequency and a large vibration amplitude, you can screw a thin and narrow metal strip with a tip at the end to the end of one of its branches. The tip must be bent down and lightly touch the smoked glass plate lying on the table. When the plate moves quickly under the vibrating branches of the tuning fork, the tip leaves a trace on the plate in the form of a wavy line (Fig. 6).
The wavy line drawn by a point on the plate is very close to a sinusoid. Thus, we can assume that each branch of the sounding tuning fork performs harmonic oscillations.
Various experiments indicate that any sound source necessarily vibrates, even if these vibrations are invisible 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, rustling of leaves, and the rolling of thunder are caused by fluctuations in the air masses.
Pay attention!
Not every oscillating body is a source of sound.
For example, an oscillating weight suspended on a thread or spring does not make a sound. The metal ruler will also stop sounding if you lengthen its free end so that the frequency of its oscillations becomes less \ (16 \) Hz.
The human ear is able to perceive mechanical vibrations with a frequency ranging from \ (16 \) to \ (20,000 \) Hz (usually transmitted through air) as sound.
Mechanical vibrations, the frequency of which lies in the range from \ (16 \) to \ (20,000 \) Hz are called sound.
The indicated boundaries of the sound range are conditional, since they depend on the age of people and individual characteristics their hearing aid. Usually, with age, the upper frequency limit of perceived sounds decreases significantly - some older people can hear sounds with frequencies not exceeding \ (6000 \) Hz. Children, on the other hand, can perceive sounds, the frequency of which is slightly higher \ (20,000 \) Hz.
Mechanical vibrations, the frequency of which exceeds \\ (20,000 \\) Hz, are called ultrasonic, and vibrations with frequencies less than \\ (16 \\) Hz - infrasonic.
Ultrasound and infrasound are as widespread in nature as sound waves. Dolphins, bats and some other living creatures emit them and use them for their "negotiations".
