Sound sources. Sound vibrations
Man lives in a world of sounds. Sound for humans is a source of information. He warns people about danger. Sound in the form of music, birdsong gives us pleasure. We enjoy listening to a person with a pleasant voice. Sounds are important not only for humans, but also for animals, for which good sound detection helps them survive.
Sound – these are mechanical elastic waves propagating in gases, liquids, and solids.
Reason for the sound - vibration (oscillations) of bodies, although these vibrations are often invisible to our eyes.
Sound sources
- physical bodies that vibrate, i.e. tremble or vibrate at a frequency
from 16 to 20,000 times per second. The vibrating body can be solid, for example, a string
or the earth's crust, gaseous, for example, a stream of air in wind musical instruments
or liquid, for example, waves on water.
Volume
Loudness depends on the amplitude of vibrations in the sound wave. The unit of sound volume is 1 Bel (in honor of Alexander Graham Bell, the inventor of the telephone). In practice, loudness is measured in decibels (dB). 1 dB = 0.1B.
10 dB – whisper;
20–30 dB
– noise standards in residential premises;
50 dB– medium volume conversation;
80 d B
– the noise of a running truck engine;
130 dB– pain threshold
Sound louder than 180 dB can even cause eardrum rupture.
High sounds represented by high-frequency waves - for example, birdsong.
Low sounds These are low-frequency waves, such as the sound of a large truck engine.
Sound waves
Sound waves- These are elastic waves that cause a person to experience sound.
A sound wave can travel a wide variety of distances. Gunfire can be heard for 10-15 km, the neighing of horses and barking dogs - for 2-3 km, and whispers for only a few meters. These sounds are transmitted through the air. But not only air can be a conductor of sound.
By placing your ear to the rails, you can hear the sound of an approaching train much earlier and at a greater distance. This means that metal conducts sound faster and better than air. Water also conducts sound well. Having dived into the water, you can clearly hear the stones knocking against each other, the noise of the pebbles during the surf.
The property of water - it conducts sound well - is widely used for reconnaissance at sea during war, as well as for measuring sea depths.
A necessary condition for the propagation of sound waves is the presence of a material medium. In a vacuum, sound waves do not propagate, since there are no particles there that transmit the interaction from the source of vibration.
Therefore, due to the lack of atmosphere, complete silence reigns on the Moon. Even the fall of a meteorite on its surface is not audible to the observer.
In each medium, sound travels at different speeds.
Speed of sound in air- approximately 340 m/s.
Speed of sound in water- 1500 m/s.
Speed of sound in metals, steel- 5000 m/s.
In warm air, the speed of sound is greater than in cold air, which leads to a change in the direction of sound propagation.
FORK
- This U-shaped metal plate, the ends of which can vibrate after being struck.
Published tuning fork the sound is very weak and can only be heard at a short distance.
Resonator- a wooden box on which a tuning fork can be attached serves to amplify the sound.
In this case, sound emission occurs not only from the tuning fork, but also from the surface of the resonator.
However, the duration of the sound of a tuning fork on a resonator will be shorter than without it.
E X O
A loud sound, reflected from obstacles, returns to the source of sound after a few moments, and we hear echo.
By multiplying the speed of sound by the time elapsed from its origin to its return, you can determine twice the distance from the sound source to the obstacle.
This method of determining the distance to objects is used in echolocation.
Some animals, such as bats,
also use the phenomenon of sound reflection using the echolocation method
Echolocation is based on the property of sound reflection.
Sound - running mechanical wave on and transfers energy.
However, the power of simultaneous conversation of all people on the globe is hardly more than the power of one Moskvich car!
Ultrasound.
· Vibrations with frequencies exceeding 20,000 Hz are called ultrasound. Ultrasound is widely used in science and technology.
· The liquid boils when an ultrasonic wave passes through (cavitation). This causes water hammer. Ultrasounds can tear pieces off the surface of metal and crush solids. Ultrasound can be used to mix immiscible liquids. This is how emulsions in oil are prepared. Under the influence of ultrasound, saponification of fats occurs. Washing devices are designed on this principle.
· Widely used ultrasound in hydroacoustics. Ultrasounds of high frequency are absorbed very weakly by water and can spread over tens of kilometers. If they encounter the bottom, iceberg or other solid body in their path, they are reflected and produce an echo of great power. An ultrasonic echo sounder is designed on this principle.
In metal ultrasound spreads practically without absorption. Using the ultrasonic location method, it is possible to detect the smallest defects inside a part of large thickness.
· The crushing effect of ultrasound is used for the manufacture of ultrasonic soldering irons.
Ultrasonic waves, sent from the ship, are reflected from the sunken object. The computer detects the time the echo appears and determines the location of the object.
· Ultrasound is used in medicine and biology for echolocation, for identifying and treating tumors and some defects in body tissues, in surgery and traumatology for cutting soft and bone tissues during various operations, for welding broken bones, for destroying cells (high power ultrasound).
Infrasound and its impact on humans.
Vibrations with frequencies below 16 Hz are called infrasound.
In nature, infrasound occurs due to the vortex movement of air in the atmosphere or as a result of slow vibrations of various bodies. Infrasound is characterized by weak absorption. Therefore, it spreads over long distances. The human body reacts painfully to infrasonic vibrations. Under external influences caused by mechanical vibration or sound waves at frequencies of 4-8 Hz, a person feels the movement of internal organs, and at a frequency of 12 Hz - an attack of seasickness.
· Highest intensity infrasonic vibrations create machines and mechanisms that have large surfaces that perform low-frequency mechanical vibrations (infrasound of mechanical origin) or turbulent flows of gases and liquids (infrasound of aerodynamic or hydrodynamic origin).
There are a lot of people around us sound sources: musical and technical instruments, human vocal cords, sea waves, wind and others. Sound or, in other words, sound waves– these are mechanical vibrations of the medium with frequencies of 16 Hz – 20 kHz(see § 11-a).
Let's consider experience. By placing the alarm clock on a pad under the bell of the air pump, we will notice that the ticking will become quieter, but will still be audible. Having pumped out the air from under the bell, we will stop hearing the sound at all. This experiment confirms that sound travels through air and does not travel in a vacuum.
The speed of sound in air is relatively high: it ranges from 300 m/s at –50°C to 360 m/s at +50°C. This is 1.5 times faster than the speed of passenger aircraft. Sound travels much faster in liquids, and even faster in solids. In a steel rail, for example, the speed of sound is » 5000 m/s.
Take a look at the graphs of air pressure fluctuations at the mouth of a person singing the sounds “A” and “O”. As you can see, the vibrations are complex, consisting of several vibrations superimposed on each other. At the same time, clearly visible main fluctuations, the frequency of which is almost independent of the spoken sound. For a male voice this is approximately 200 Hz, for a female voice - 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 the 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 blocked by trees: sounds with wavelengths of 1–2 m easily bend around tree trunks whose diameter is less than a meter.
Let us carry out an experiment confirming that the sources of sound are indeed oscillating bodies.
Let's take the device fork– a metal slingshot mounted on a box without a front wall for better radiation of sound waves. If you hit the ends of the slingshot of a tuning fork with a hammer, it will produce a “clean” sound called musical tone(for example, the note “A” of the first octave with a frequency of 440 Hz). Let us move a sounding tuning fork towards a light ball on a string, and it will immediately bounce to the side. This happens precisely because of the frequent vibrations of the ends of the tuning fork slingshot.
The reasons on which the frequency of vibration of a body depends are its elasticity and size. The larger the body size, 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, but also how it will capture sounds and respond to them depends on elasticity and size. The phenomenon of a sharp increase in the amplitude of oscillations when the frequency of an external influence coincides with the natural frequency of the body is called resonance (Lat. “reasonably” - I respond). Let's do an experiment to observe resonance.
Let's place two identical tuning forks side by side, turning them towards each other on those sides of the boxes where there are no walls. Let's hit the left tuning fork with a hammer. In a second we'll drown it out with our hands. We will hear the sound of the second tuning fork, 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 its own vibrations.
Before you understand what sound sources there are, think about what sound is? We know that light is radiation. Reflecting from objects, this radiation reaches our eyes, and we can see it. Taste and smell are small particles of bodies that are perceived by our respective receptors. What kind of animal is this sound?
Sounds are transmitted through the air
You've probably seen how the guitar is played. Perhaps you can do this yourself. Another important thing is the sound the strings make in a guitar when you pluck them. That's right. But if you could place a guitar in a vacuum and pluck the strings, you would be very surprised that the guitar would not make any sound.
Such experiments were carried out with a wide variety of bodies, and the result was always the same: no sound could be heard in airless space. The logical conclusion follows that sound is transmitted through the air. Therefore, sound is something that happens to particles of air and sound-producing bodies.
Sources of sound - oscillating bodies
Further. As a result of a wide variety of numerous experiments, it was possible to establish that sound arises due to the vibration of bodies. Sources of sound are bodies that vibrate. These vibrations are transmitted by air molecules and our ear, perceiving these vibrations, interprets them into sensations of sound that we understand.
It's not difficult to check. Take a glass or crystal goblet and place it on the table. Tap it lightly with a metal spoon. You will hear a long thin sound. Now touch the glass with your hand and knock again. The sound will change and become much shorter.
Now let several people wrap their hands around the glass as completely as possible, along with the stem, trying not to leave a single free area, except for a very small place for hitting with a spoon. Hit the glass again. You will hardly hear any sound, and the one that will be will be weak and very short. What does this mean?
In the first case, after the impact, the glass oscillated freely, its vibrations were transmitted through the air and reached our ears. In the second case, most of the vibrations were absorbed by our hand, and the sound became much shorter as the vibrations of the body decreased. In the third case, almost all vibrations of the body were instantly absorbed by the hands of all participants and the body hardly vibrated, and therefore made almost no sound.
The same goes for all other experiments you can think of and conduct. Vibrations of bodies, transmitted to air molecules, will be perceived by our ears and interpreted by the brain.
Sound vibrations of different frequencies
So sound is vibration. Sound sources transmit sound vibrations through the air to us. Why then do we not hear all the vibrations of all objects? Because vibrations come in different frequencies.
The sound perceived by the human ear is sound vibrations with a frequency of approximately 16 Hz to 20 kHz. Children hear sounds of higher frequencies than adults, and the ranges of perception of different living creatures generally vary greatly.
The ears are a very thin and delicate instrument given to us by nature, so we should take care of it, since there is no replacement or analogue in the human body.
Sound is caused by mechanical vibrations in elastic media and bodies, the frequencies of which lie in the range from 20 Hz to 20 kHz and which the human ear can perceive.
Accordingly, this mechanical vibration with the indicated frequencies is called sound and acoustic. Inaudible mechanical vibrations with frequencies below the sound range are called infrasonic, and with frequencies above the sound range they are called ultrasonic.
If a sounding body, for example an electric bell, is placed under the bell of an air pump, then as the air is pumped out the sound will become weaker and weaker and finally stop completely. The transmission of vibrations from the sounding body occurs through the air. Let us note that during its oscillations, the sounding body alternately compresses the air adjacent to the surface of the body, and, on the contrary, creates a vacuum in this layer. Thus, the propagation of sound in the air begins with fluctuations in air density at the surface of the vibrating body.
Musical tone. Volume and pitch
The sound that we hear when its source performs a harmonic oscillation is called musical tone or, for short, tone.
In any musical tone we can distinguish two qualities by ear: volume and pitch.
The simplest observations convince us that the tones of any given pitch are determined by the amplitude of the vibrations. The sound of a tuning fork gradually fades after striking it. This occurs along with the damping of oscillations, i.e. with a decrease in their amplitude. By hitting the tuning fork harder, i.e. By giving the vibrations a larger amplitude, we will hear a louder sound than with a weak blow. The same can be observed with a string and in general with any source of sound.
If we take several tuning forks of different sizes, it will not be difficult to arrange them by ear in order of increasing pitch. Thus, they will be arranged in size: the largest tuning fork gives the lowest sound, the smallest one gives the highest sound. Thus, the pitch of a tone is determined by the frequency of vibration. The higher the frequency and, therefore, the shorter the period of oscillation, the higher the sound we hear.
Acoustic resonance
Resonance phenomena can be observed in mechanical vibrations of any frequency, in particular in sound vibrations.
Let's place two identical tuning forks next to each other, with the holes of the boxes on which they are mounted facing each other. Boxes are needed because they amplify the sound of tuning forks. This occurs due to resonance between the tuning fork and the columns of air enclosed in the box; hence the boxes are called resonators or resonant boxes.
Let's hit one of the tuning forks and then muffle it with our fingers. We will hear how the second tuning fork sounds.
Let's take two different tuning forks, i.e. with different pitches, and repeat the experiment. Now each of the tuning forks will no longer respond to the sound of another tuning fork.
It is not difficult to explain this result. The vibrations of one tuning fork act through the air with some force on the second tuning fork, causing it to perform its forced vibrations. Since tuning fork 1 performs a harmonic oscillation, the force acting on tuning fork 2 will change according to the law of harmonic oscillation with the frequency of tuning fork 1. If the frequency of the force is different, then the forced oscillations will be so weak that we will not hear them.
Noises
We hear a musical sound (note) when the vibration is periodic. For example, this kind of sound is produced by a piano string. If you hit several keys at the same time, i.e. make several notes sound, then the sensation of musical sound will remain, but the difference between consonant (pleasant to the ear) and dissonant (unpleasant) notes will clearly appear. It turns out that those notes whose periods are in the ratio of small numbers are consonant. For example, consonance is obtained with a period ratio of 2:3 (fifth), 3:4 (quanta), 4:5 (major third), etc. If the periods are related as large numbers, for example 19:23, then the result is dissonance - a musical, but unpleasant sound. We will move even further away from the periodicity of oscillations if we hit many keys at the same time. The sound will already be noise-like.
Noise is characterized by a strong non-periodicity of the oscillation shape: either it is a long oscillation, but very complex in shape (hissing, creaking), or individual emissions (clicks, knocks). From this point of view, noises should also include sounds expressed by consonants (hissing, labial, etc.).
In all cases, noise vibrations consist of a huge number of harmonic vibrations with different frequencies.
Thus, the spectrum of a harmonic vibration consists of one single frequency. For a periodic oscillation, the spectrum consists of a set of frequencies - the main one and its multiples. In consonant consonances we have a spectrum consisting of several such sets of frequencies, with the main ones being related as small integers. In dissonant consonances, the fundamental frequencies are no longer in such simple relationships. The more different frequencies there are in the spectrum, the closer we come to noise. Typical noises have spectra in which there are extremely many frequencies.
This lesson covers the topic “Sound Waves”. In this lesson we will continue to study acoustics. First, let's 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 of sound waves in different media and learn what their characteristics are. .
Sound waves – these are mechanical vibrations that, spreading and interacting with the organ of hearing, are perceived by a person (Fig. 1).
Rice. 1. Sound wave
The branch of physics that deals with these waves is called acoustics. The profession of people who are popularly called “listeners” is acousticians. 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 discharge alternate. It is transmitted over time over a distance (Fig. 2).
Rice. 2. Sound wave propagation
Sound waves include vibrations that occur with a frequency from 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 called audible sound. These wavelengths are given for air, the speed of sound in which is equal to .
There are also ranges that acousticians deal with - infrasonic and ultrasonic. Infrasonic are those that have a frequency of less than 20 Hz. And ultrasonic ones are those that have a frequency greater than 20,000 Hz (Fig. 3).
Rice. 3. Sound wave ranges
Every educated person should be familiar with the frequency range of sound waves and know that if he goes for an ultrasound, the picture on the computer screen will be constructed 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 being 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, an 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, 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 infrasound 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 breaks up. Where is this used? For example, in diamond mines, where they take ore that contains diamond components and crush it 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. Speed of sound wave propagation in various media
Please note: in air the speed of sound at is equal to , and 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 medium. We already know that the denser the medium, the more serious the interaction between the particles of the medium, the faster the wave propagates. In the last paragraph we discussed this using the example of dry air and moist air. For water, the speed of sound propagation is . 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. By water, information will reach 4 times faster than by air. And in steel it’s even faster: 
(Fig. 6).
Rice. 6. Sound wave propagation speed
You know from the epics that Ilya Muromets used (and all the 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 moving is not yet audible. Ilya Muromets, with his ear to the ground, can hear her. Why? Because sound is transmitted over solid ground at a higher speed, which means it will reach Ilya Muromets’ ear 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 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 may be a sound wave that is created in the air gap of a 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 produce tones will have features. How are they different? They differ in wavelength and frequency. If these sound waves are not created by harmoniously sounding bodies or are not connected into some kind of common orchestral piece, then such a quantity of sounds will be called noise.
Noise– random oscillations of various physical natures, characterized by the complexity of their temporal and spectral structure. The concept of noise is both domestic and physical, they are very similar, and therefore we introduce it as a separate important object of consideration.
Let's move 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. How is sound volume determined? Let us consider the propagation of a sound wave in time or the oscillations of the source of the sound wave (Fig. 8).
Rice. 8. Sound volume
At the same time, if we did not add a lot of sound to the system (we hit a piano key quietly, for example), then there will be a quiet sound. If we loudly raise our hand high, we cause this sound by hitting the key, we get a loud sound. What does this depend on? A quiet sound has a smaller vibration amplitude than a loud sound.
The next important characteristic of musical sound and any other sound is height. What does the pitch of sound depend on? The height depends on the frequency. We can make the source oscillate frequently, or we can make it oscillate not very quickly (that is, perform fewer oscillations per unit time). Let's consider the time sweep of a high and low sound of the same amplitude (Fig. 9).
Rice. 9. Pitch
An interesting conclusion can be drawn. If a person sings in a bass voice, then his sound source (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, and therefore more often cause pockets of compression and discharge in the propagation of the wave.
There is another interesting characteristic of sound waves that physicists do not study. This timbre. You know and easily distinguish the same piece of music performed on a balalaika or cello. How are these sounds or this performance different? At the beginning of the experiment, we asked people who produce sounds to make them of approximately the same amplitude, so that the volume of the sound is the same. It’s like in the case of an orchestra: if there is no need to highlight any instrument, everyone plays approximately the same, at the same strength. So the timbre of the balalaika and cello is different. If we were to draw the sound produced 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 university with the same teachers. They studied equally well, with straight A's. For some reason, one becomes an outstanding performer, while the other is dissatisfied with his career all his life. In fact, this is determined solely by their instrument, which causes vocal vibrations in the environment, i.e. their voices differ in timbre.
Bibliography
- Sokolovich Yu.A., Bogdanova G.S. Physics: a reference book with examples of problem solving. - 2nd edition repartition. - X.: Vesta: publishing house "Ranok", 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 travel? What could be the source of sound?
- Can sound travel through space?
- Is every wave that reaches a person’s hearing organ perceived by him?
