Sources of different sounds. Sound waves
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 together 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 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 big 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 huge amount 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.
Sound, as we remember, is elastic longitudinal waves. And waves are generated by oscillating objects.
Examples of sound sources: an oscillating ruler, one end of which is clamped, oscillating strings, a speaker membrane.
But oscillating objects do not always generate sound audible to the ear - if the frequency of their oscillations is below 16 Hz, then they generate infrasound, and if more than 20 kHz, then ultrasound.
Ultrasound and infrasound - from the point of view of physics, are the same elastic vibrations of the medium as ordinary sound, but the ear is not able to perceive them, since these frequencies are too far from the resonant frequency eardrum(the membrane simply cannot vibrate with such frequency).
High frequency sounds are felt as thinner, low frequency sounds as bassier.
If the oscillatory system makes harmonic vibrations same frequency, its sound is called in a clear tone. Usually sound sources produce sounds of several frequencies at once - then the lowest frequency is called main tone, and the rest are called overtones. Overtones are determined timbre sound - it is because of them that we can easily distinguish a piano from a violin, even when their fundamental frequency is the same.
Volume sound is a subjective sensation that allows us to compare sounds as “louder” and “less loud.” Volume depends on many factors - frequency, duration, individual characteristics listener. But most of all it depends on sound pressure, which is directly related to the vibration amplitude of the object that produces the sound.
The unit of measurement for loudness is called dream.
IN practical problems usually use a quantity called volume level or sound pressure level. This value is measured in belah [B] or, more often, in decibels [dB].
This value depends logarithmically on the sound pressure - that is, an increase in pressure 10 times increases the volume level by 1 dB.
The sound of flipping through a newspaper is approximately 20 dB, an alarm clock is 80 dB, the sound of an airplane taking off is 100-120 dB (on the verge of pain).
One of unusual applications sound (more precisely ultrasound) is echolocation. You can make a sound and measure the time it takes for the echo to come. The greater the distance to the obstacle, the greater the delay will be. Typically this method of measuring distances is used underwater, but bats apply it directly in the air.
The echolocation distance is determined as follows:
2r = vt, where v is the speed of sound in the medium, t is the delay time to the echo, r is the distance to the obstacle.
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, which fluctuate, 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 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- These are elastic waves that cause a person to feel the sensation of sound.
A sound wave can travel a wide variety of distances. Gunfire can be heard at 10-15 km, the neighing of horses and barking dogs - at 2-3 km, and whispers only at 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). In this case, water hammer occurs. 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 meet the bottom, iceberg or other solid, they are reflected and give 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 dissecting 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. At external influences caused by mechanical vibration or sound wave at frequencies of 4-8 Hz, a person feels movement internal organs, at a frequency of 12 Hz - an attack of seasickness.
· Highest intensity infrasonic vibrations create machines and mechanisms that have surfaces large sizes, performing low-frequency mechanical vibrations (infrasound of mechanical origin) or turbulent flows of gases and liquids (infrasound of aerodynamic or hydrodynamic origin).
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 substances and bodies that produce sound.
Sources of sound - oscillating bodies
Next. 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 completely 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.
Ears are a very thin and delicate instrument, given to us by nature, so we should take care of it, since replacements and analogues are in human body does not exist.
Questions.
1. Tell about the experiments depicted in Figures 70-73. What conclusion follows from them?
In the first experiment (Fig. 70), a metal ruler clamped in a vice makes a sound when it vibrates.
In the second experiment (Fig. 71), one can observe vibrations of the string, which also produces sound.
In the third experiment (Fig. 72), the sound of a tuning fork is observed.
In the fourth experiment (Fig. 73), the vibrations of the tuning fork are “recorded” on a smoked plate. All these experiments demonstrate the oscillatory nature of the appearance of sound. Sound occurs as a result of vibrations. In the fourth experiment this can also be clearly observed. The tip of the needle leaves a trace in the form of a sine wave. In this case, sound does not appear from nowhere, but is generated by sound sources: a ruler, a string, a tuning fork.
2. How common property do all sound sources have?
Any sound source necessarily vibrates.
3. Mechanical vibrations of what frequencies are called sound vibrations and why?
Sound vibrations are mechanical vibrations with frequencies from 16 Hz to 20,000 Hz, because in this frequency range they are perceived by humans.
4. What vibrations are called ultrasonic? infrasonic?
Vibrations with frequencies above 20,000 Hz are called ultrasonic, and with frequencies below 16 Hz - infrasonic.
5. Tell us about measuring the depth of the sea using echolocation.
Exercises.
1. We hear the sound of the flapping wings of a flying mosquito. but no flying bird. Why?
The oscillation frequency of the wings of a mosquito is 600 Hz (600 beats per second), a sparrow is 13 Hz, and human ear perceives sounds from 16 Hz.
