Sound (or acoustic) waves are elastic waves propagating in a medium with frequencies in the range of 16-20,000 Hz. Waves of these frequencies, affecting the human hearing system, cause the sensation of sound. Waves with v< 16 Гц (ннфразвуковые) и v >20 kHz (ultrasound) is not perceived by the human hearing organs.
Sound waves in gases and liquids can only be longitudinal, since these media are elastic only with respect to compression (tension) deformations. In solids, sound waves can be both longitudinal and transverse, since solids have elasticity with respect to compression (tension) and shear deformations.
Sound intensity (or sound power) is a quantity determined by the time-average energy transferred by a sound wave per unit time through a unit area perpendicular to the direction of propagation of the wave:
The SI unit of sound intensity is watt per meter squared (W/m2).
The sensitivity of the human ear varies for different frequencies. In order to cause a sound sensation, the wave must have a certain minimum intensity, but if this intensity exceeds a certain limit, then the sound is not heard and causes only a painful sensation. Thus, for each oscillation frequency there is a minimum (hearing threshold) and a maximum (pain threshold) sound intensity that can cause sound perception. In Fig. 223 shows the dependence of the thresholds of audibility and pain on the frequency of sound. The area located between these two curves is the audibility area.
If sound intensity is a quantity that objectively characterizes the sound new process, then the subjective characteristic of sound associated with its intensity is the loudness of the sound, which depends on frequency. According to the physiological Weber-Fechner law, as sound intensity increases, loudness increases logarithmically. On this basis, an objective assessment of sound volume is introduced based on the measured value of its intensity:
where I 0 is the sound intensity at the threshold of audibility, taken for all sounds to be 10 -12 W/m 2 . The value L is called the sound intensity level and is expressed in bels (in honor of the inventor of the Bell telephone). Usually they use units that are 10 times smaller - decibels (dB).
The physiological characteristic of sound is the volume level, which is expressed in phon (phon). The volume for a sound at 1000 Hz (the frequency of a standard pure tone) is equal to 1 phon if its intensity level is 1 dB. For example, noise in a subway car at high speed corresponds to “90 von, and a whisper at a distance of 1 m corresponds to” 20 von.
Real sound is a superposition of harmonic oscillations with a large set of frequencies, i.e. sound has an acoustic spectrum that can be continuous (oscillations of all frequencies are present in a certain interval) and lined (oscillations of certain frequencies separated from each other are present).
In addition to volume, sound is characterized by pitch and timbre. Sound pitch is the quality of sound determined by a person subjectively by ear and depending on the frequency of the sound. As the frequency increases, the pitch of the sound increases, i.e. the sound becomes “higher”. The nature of the acoustic spectrum and the distribution of energy between certain frequencies determines the uniqueness of the sound sensation, called the timbre of sound. Thus, different singers playing the same note have a different acoustic spectrum, that is, their voices have a different timbre.
The source of sound can be any body that vibrates in an elastic medium with a sound frequency (for example, in stringed instruments, the source of sound is a string connected to the body of the instrument).
By oscillating, a body causes vibrations of adjacent particles of the medium with the same frequency. The state of oscillatory motion is successively transmitted to particles of the medium that are increasingly distant from the body, i.e., a wave propagates in the medium with an oscillation frequency equal to the frequency of its source, and with a certain speed depending on the density and elastic properties of the medium. The speed of propagation of sound waves in gases is calculated by the formula
(158.1)
where R is the molar gas constant, M is the molar mass, g = C p /C v is the ratio of the molar heat capacities of the gas at constant pressure and volume, T is the thermodynamic temperature. From formula (158.1) it follows that the speed of sound in a gas does not depend on pressure r gas, but increases with increasing temperature. The greater the molar mass of a gas, the lower the speed of sound. For example, at T = 273 K, the speed of sound in air (M = 29×10 -3 kg/mol) v = 331 m/s, in hydrogen (M = 2×10 -3 kg/mol) v = 1260 m/s . Expression (158.1) corresponds to experimental data.
When propagating sound in the atmosphere, it is necessary to take into account a number of factors: wind speed and direction, air humidity, the molecular structure of the gaseous medium, the phenomena of refraction and reflection of sound at the boundary of two media. In addition, any real medium has viscosity, so sound attenuation is observed, i.e., a decrease in its amplitude and, consequently, the intensity of the sound wave as it propagates. The attenuation of sound is largely due to its absorption in the medium, associated with the irreversible transition of sound energy into other forms of energy (mainly thermal).
For room acoustics great value has sound reverberation - the process of gradual attenuation of sound in enclosed spaces after its source is turned off. If the rooms are empty, then the sound fades slowly and a “boominess” of the room is created. If sounds fade quickly (when sound-absorbing materials are used), then they are perceived as muffled. Reverberation time is the time during which the sound intensity in a room is attenuated by millions and its level by 60 dB. The room has good acoustics if the reverberation time is 0.5-1.5 s.
The specific sensation that we perceive as sound is the result of the impact on the human hearing aid of the oscillatory movement of an elastic medium - most often air. Vibrations in the medium are excited by a sound source and, propagating through the medium, reach the receiving apparatus - our ear. Thus, the infinite variety of sounds we hear is caused by oscillatory processes that differ from each other in frequency and amplitude. One should not confuse two sides of the same phenomenon: sound as a physical process is a special case of oscillatory motion; as a psycho-physiological phenomenon, sound is a specific sensation, the mechanism of its occurrence has now been studied in some detail.
Speaking about the physical side of the phenomenon, we characterize sound by its intensity (strength), its composition and the frequency of the oscillatory processes associated with it; Having in mind sound sensations, we talk about volume, timbre, and pitch.
In solids, sound can propagate both in the form of longitudinal and transverse vibrations. Since liquids and gases do not have shear elasticity, it is obvious that in gaseous and liquid media sound can propagate only in the form of longitudinal vibrations. In gases and liquids, sound waves represent alternating condensations and rarefactions of the medium, moving away from the sound source at a certain speed characteristic of each medium. The surface of a sound wave is the geometric location of particles of the medium that have the same oscillation phase. The surfaces of sound waves can be drawn, for example, so that between the surfaces of adjacent waves there is a layer of condensation and a layer of rarefaction. The direction perpendicular to the surface of the wave is called a ray.
Sound waves in a gaseous medium can be photographed. For this purpose, a
a photographic plate onto which a beam of light from an electric spark is directed from the front so that these rays from an instantaneous flash of light fall on the photographic plate after passing through the air, ambient source sound. In Fig. 158-160 show photographs of sound waves obtained using this method. The sound source was separated from the photographic plate by a small screen on a stand.
In Fig. 158, but it is clear that the sound wave has just come out from behind the screen; in Fig. 158, b the same wave was filmed a second time a few thousandths of a second later. The wave surface in this case is a sphere. In the photograph, the image of the wave is obtained in the form of a circle, the radius of which increases over time.
Rice. 158. Photograph of a sound wave at two points in time (a and b). Reflection of a sound wave (c).
In Fig. 158, c shows a photograph of a spherical sound wave reflected from a flat wall. Here you should pay attention to the fact that the reflected part of the wave seems to come from a point located behind the reflecting surface at the same distance from the reflecting surface as the sound source. It is well known that echo is explained by the phenomenon of reflection of sound waves.
In Fig. 159 shows the change in the wave surface when a sound wave passes through a lens-shaped bag filled with hydrogen. This change in the surface of the sound wave is a consequence of the refraction (refraction) of sound rays: at the interface of two media, where the speed of the waves is different, the direction of propagation of the wave changes.
Rice. 160 reproduces a photograph of sound waves, in the path of which a screen with four slits is placed. Passing through the slits, the waves bend around the screen. This phenomenon of waves bending around encountered obstacles is called diffraction.
The laws of propagation, reflection, refraction and diffraction of sound waves can be derived from Huygens' principle, according to which every particle vibrated
the environment can be considered as a new center (source) of waves; the interference of all these waves produces the wave actually observed (the application of Huygens' principle will be explained in the third volume using the example of light waves).
Sound waves carry with them a certain amount of motion and, as a result, exert pressure on the obstacles they encounter.
Rice. 159. Refraction of a sound wave.
Rice. 160. Diffraction of sound waves.
To explain this fact, let us turn to Fig. 161. In this figure, the dotted line shows a sinusoid of displacements of particles of the medium at some point in time during the propagation of longitudinal waves in the medium. The velocities of these particles at the moment in time under consideration will be represented by a cosine wave, or, what is the same, a sinusoid ahead of the displacement sinusoid by a quarter of a period (solid line in Fig. 161). It is not difficult to imagine that condensations of the medium will be observed where, at a given moment, the displacement of particles is zero or close to zero and where the speed is directed in the direction of wave propagation. On the contrary, rarefaction of the medium will be observed where the displacement of particles is also zero or close to zero, but where the particle velocity is directed in the direction opposite to the propagation of waves. So, in condensations the particles move forward, in rarefactions they move backward. But in
Rice. 161. In the condensations of a passing sound wave, particles move forward,
There are a larger number of particles in condensed layers than in rarefaction. Thus, at any moment of time in traveling longitudinal sound waves, the number of particles moving forward is slightly greater than the number of particles moving backward. As a result, the sound wave carries with it a certain amount of motion, which is manifested in the pressure that the sound waves exert on the obstacles they encounter.
Sound pressure was studied experimentally by Rayleigh and Pyotr Nikolaevich Lebedev.
Theoretically, the speed of sound is determined by Laplace’s formula [§ 65, formula (5)]:
where K is the modulus of all-round elasticity (when compression is performed without heat inflow and loss), density.
If the compression of a body is carried out while maintaining the temperature of the body constant, then for the elasticity modulus the values obtained are smaller than in the case when the compression is carried out without the influx and release of heat. These two values of the modulus of comprehensive elasticity, as proven in thermodynamics, are related in the same way as the heat capacity of a body at constant pressure to the heat capacity of a body at constant volume.
For gases (not too compressed), the isothermal modulus of all-round elasticity is simply equal to the gas pressure. If, without changing the temperature of the gas, we compress the gas (increase its density) by a factor, then the pressure of the gas will increase by a factor. Consequently, according to Laplace's formula, it turns out that the speed of sound in a gas does not depend on the density of the gas.
From the gas laws and Laplace's formula it can be deduced (§ 134) that the speed of sound in gases is proportional to the square root of the absolute temperature of the gas:
where is the acceleration of gravity, the ratio of the heat capacities and the universal gas constant.
At C, the speed of sound in dry air is equal; at average temperatures and average humidity, the speed of sound in air is considered equal; the speed of sound in hydrogen at is equal to
In water the speed of sound is in glass in iron
It should be noted that shock sound waves caused by a shot or explosion, at the beginning of their path, have a speed
significantly superior normal speed sound in a given environment. A shock sound wave in the air caused by a strong explosion can have a speed near the sound source that is several times higher than the normal speed of sound in air, but already at a distance of tens of meters from the explosion site, the speed of propagation of the wave decreases to a normal value.
As already mentioned in § 65, sound waves of different lengths have almost the same speed. The exception is those frequency regions that are characterized by particularly rapid attenuation of elastic waves as they propagate in the medium under consideration. Typically, these frequencies lie far beyond audibility (for gases at atmospheric pressure, these are frequencies on the order of vibrations per second). Theoretical analysis shows that the dispersion and absorption of sound waves are associated with the fact that the redistribution of energy between the translational and vibrational movements of molecules requires some, albeit short, time. This causes the long waves (waves in the audio range) to travel somewhat slower than the very short "inaudible" waves. Thus, in carbon dioxide vapor at atmospheric pressure, sound has speed, while very short, “inaudible” waves propagate at speed
A sound wave propagating in a medium can have different shapes, depending on the size and shape of the sound source. In the most technically interesting cases, the sound source (emitter) is some vibrating surface, such as, for example, a telephone membrane or a loudspeaker diffuser. If such a sound source emits sound waves into open space, then the shape of the wave depends significantly on the relative dimensions of the emitter; The emitter, whose dimensions are large compared to the length of the sound wave, emits sound energy in only one direction, namely in the direction of its oscillatory motion. On the contrary, a radiator of small size relative to the wavelength emits sound energy in all directions. The shape of the wave front in both cases will obviously be different.
Let's consider the first case first. Let's imagine a rigid flat surface of a sufficiently large size (compared to the wavelength), performing oscillatory movements in the direction of its normal. Moving forward, such a surface creates a condensation in front of itself, which, due to the elasticity of the medium, will spread in the direction of the displacement of the emitter). Moving back, the emitter creates a vacuum behind itself, which will move in the medium following the initial condensation. During a short-term oscillation of the emitter, we will observe a sound wave on both sides of it, characterized by the fact that all particles of the medium located at an equal distance from the radiating surface of the average density of the medium and the speed of sound c:
The product of the average density of the medium and the speed of sound is called acoustic resistance environment.
Acoustic resistance at 20°C
(see scan)
Let us now consider the case of spherical waves. When the dimensions of the emitting surface become small compared to the wavelength, the wave front is noticeably curved. This happens because the vibration energy spreads in all directions from the emitter.
The phenomenon can be best understood by the following simple example. Let's imagine that a long log fell onto the surface of the water. The resulting waves travel in parallel rows on both sides of the log. The situation is different when a small stone is thrown into the water, and the waves propagate in concentric circles. The log is relatively large
with the wavelength on the water surface; The parallel rows of waves emanating from it represent a visual model of plane waves. The stone is small in size; the circles diverging from the place of its fall give us a model of spherical waves. When a spherical wave propagates, the surface of the wave front increases in proportion to the square of its radius. At a constant power of the sound source, the energy flowing through each square centimeter of a spherical surface of radius is inversely proportional. Since the energy of oscillations is proportional to the square of the amplitude, it is clear that the amplitude of oscillations in a spherical wave should decrease as the inverse of the first power of the distance from the sound source. The spherical wave equation therefore has the following form:
Occurring in gaseous, liquid and solid media, which, when reaching the human hearing organs, is perceived by him as sound. The frequency of these waves ranges from 20 to 20,000 vibrations per second. Let us present formulas for a sound wave and consider its properties in more detail.
Why does a sound wave appear?
Many people wonder what a sound wave is. The nature of sound lies in the occurrence of disturbance in an elastic medium. For example, when a pressure disturbance in the form of compression occurs in a certain volume of air, this region tends to spread in space. This process causes air to compress in areas adjacent to the source, which also tend to expand. This process covers an increasingly large and most of space until it reaches some receiver, for example, the human ear.
General characteristics of sound waves
Let's consider the questions of what a sound wave is and how it is perceived by the human ear. The sound wave is longitudinal; when it enters the concha of the ear, it causes vibrations of the eardrum with a certain frequency and amplitude. You can also imagine these fluctuations as periodic changes in pressure in a microvolume of air adjacent to the membrane. First it increases relative to normal atmospheric pressure, and then decreases, obeying the mathematical laws of harmonic motion. The amplitude of changes in air compression, that is, the difference between the maximum or minimum pressure created by a sound wave with atmospheric pressure is proportional to the amplitude of the sound wave itself.
Many physical experiments have shown that the maximum pressure that the human ear can perceive without harming it is 2800 µN/cm 2 . For comparison, let's say that atmospheric pressure near the earth's surface is 10 million μN/cm2. Considering the proportionality of pressure and amplitude of oscillations, we can say that the latter value is insignificant even for the strongest waves. If we talk about the length of the sound wave, then for a frequency of 1000 vibrations per second it will be a thousandth of a centimeter.
The weakest sounds create pressure fluctuations of the order of 0.001 μN/cm 2, the corresponding amplitude of wave oscillations for a frequency of 1000 Hz is 10 -9 cm, while the average diameter of air molecules is 10 -8 cm, that is, the human ear is an extremely sensitive organ.
Concept of sound wave intensity
From a geometric point of view, a sound wave is an oscillation a certain shape, from the physical point of view, the main property of sound waves is their ability to transfer energy. The most important example of wave energy transfer is the sun, whose emitted electromagnetic waves provide energy to our entire planet.
The intensity of a sound wave in physics is defined as the amount of energy transferred by the wave through a unit surface area that is perpendicular to the propagation of the wave, and per unit time. In short, the intensity of a wave is its power transferred through a unit area.
The strength of sound waves is usually measured in decibels, which are based on a logarithmic scale, convenient for practical analysis of the results.
Intensity of different sounds
The following scale in decibels gives an idea of the value of the different and the sensations it causes:
- the threshold of unpleasant and uncomfortable sensations starts at 120 decibels (dB);
- a riveting hammer creates a noise of 95 dB;
- high-speed train - 90 dB;
- street with heavy traffic - 70 dB;
- the volume of a normal conversation between people is 65 dB;
- a modern car moving at moderate speeds creates a noise level of 50 dB;
- average radio volume - 40 dB;
- quiet conversation - 20 dB;
- tree foliage noise - 10 dB;
- The minimum threshold of human sound sensitivity is close to 0 dB.
The sensitivity of the human ear depends on the frequency of sound and is maximum for sound waves with a frequency of 2000-3000 Hz. For sound in this frequency range, the lower threshold of human sensitivity is 10 -5 dB. Higher and lower frequencies than the specified interval lead to an increase in the lower sensitivity threshold in such a way that a person hears frequencies close to 20 Hz and 20,000 Hz only at an intensity of several tens of dB.
As for the upper threshold of intensity, after which the sound begins to cause inconvenience for a person and even pain, it should be said that it is practically independent of frequency and lies in the range of 110-130 dB.
Geometric characteristics of a sound wave
A real sound wave is a complex oscillatory packet of longitudinal waves, which can be decomposed into simple harmonic vibrations. Each such oscillation is described from a geometric point of view by the following characteristics:
- Amplitude is the maximum deviation of each section of the wave from equilibrium. The designation A is adopted for this quantity.
- Period. This is the time during which a simple wave completes its complete oscillation. After this time, each point of the wave begins to repeat its oscillatory process. The period is usually denoted by the letter T and measured in seconds in the SI system.
- Frequency. This is a physical quantity that shows how many oscillations a given wave makes per second. That is, in its meaning it is a quantity reciprocal to the period. It is designated f. For the frequency of a sound wave, the formula for determining it through a period is as follows: f = 1/T.
- The wavelength is the distance it travels in one oscillation period. Geometrically, wavelength is the distance between the two nearest maxima or two nearest minima on a sine curve. The oscillation length of a sound wave is the distance between the nearest areas of air compression or the nearest places of its rarefaction in the space where the wave moves. It is usually denoted by the Greek letter λ.
- The speed of propagation of a sound wave is the distance over which the compression region or rarefaction region of the wave propagates per unit time. This value is denoted by the letter v. For the speed of a sound wave, the formula is: v = λ*f.
The geometry of a pure sound wave, that is, a wave of constant purity, obeys the sinusoidal law. In the general case, the formula for a sound wave has the form: y = A*sin(ωt), where y is the coordinate value of a given point on the wave, t is time, ω = 2*pi*f is the cyclic frequency of oscillations.
Aperiodic sound
Many sound sources can be considered periodic, for example, the sound from musical instruments such as a guitar, piano, flute, but there is also large number sounds in nature that are aperiodic, that is, sound vibrations change their frequency and shape in space. Technically, this type of sound is called noise. Vivid examples of aperiodic sound are city noise, sea noise, sounds from percussion instruments, for example, from a drum, and others.
Sound wave propagation medium
Unlike electromagnetic radiation, whose photons do not need any material medium for their propagation, the nature of sound is such that it requires a certain medium for its propagation, that is, according to the laws of physics, sound waves cannot propagate in a vacuum.
Sound can travel in gases, liquids and solids. The main characteristics of a sound wave propagating in a medium are the following:
- the wave propagates linearly;
- it propagates equally in all directions in a homogeneous medium, that is, sound diverges from the source, forming an ideal spherical surface.
- Regardless of the amplitude and frequency of sound, its waves propagate at the same speed in a given medium.
Speed of sound waves in various media
The speed of sound propagation depends on two main factors: the medium in which the wave travels and the temperature. In general, the following rule applies: the denser the medium is, and the higher its temperature, the faster sound moves in it.
For example, the speed of propagation of a sound wave in air near the surface of the earth at a temperature of 20 ℃ and a humidity of 50% is 1235 km/h or 343 m/s. In water at a given temperature, sound moves 4.5 times faster, that is, about 5735 km/h or 1600 m/s. As for the dependence of the speed of sound on temperature in the air, it increases by 0.6 m/s with an increase in temperature for every degree Celsius.
Timbre and tone
If a string or metal plate is allowed to vibrate freely, it will produce sounds of varying frequencies. It is very rare to find a body that produces a sound of one specific frequency; usually the sound of an object has a set of frequencies in a certain interval.
The timbre of a sound is determined by the number of harmonics present in it and their respective intensities. Timbre is a subjective value, that is, it is the perception of a sounding object by a specific person. Timbre is usually characterized by the following adjectives: high, brilliant, sonorous, melodic, and so on.
Tone is a sound sensation that allows it to be classified as high or low. This value is also subjective and cannot be measured by any instrument. Tone is associated with an objective quantity - the frequency of the sound wave, but there is no clear connection between them. For example, for a single-frequency sound of constant intensity, the tone increases as the frequency increases. If the frequency of the sound remains constant and its intensity increases, then the tone becomes lower.
Shape of sound sources
According to the shape of the body that performs mechanical vibrations and thus generates waves of three main types:
- Point source. It produces spherical sound waves that decay rapidly with distance from the source (approximately 6 dB if the distance from the source doubles).
- Line source. It creates cylindrical waves, the intensity of which decreases more slowly than from a point source (for each increase in distance by two times relative to the source, the intensity decreases by 3 dB).
- Flat or two-dimensional source. It generates waves only in a certain direction. An example of such a source would be a piston moving in a cylinder.
Electronic sound sources
To create a sound wave, electronic sources use a special membrane (speaker), which performs mechanical vibrations due to the phenomenon electromagnetic induction. Such sources include the following:
- players of various discs (CD, DVD and others);
- cassette recorders;
- radios;
- TVs and some others.
Sound is mechanical vibrations that propagate in an elastic material medium primarily in the form of longitudinal waves.
In a vacuum, sound does not propagate, since sound transmission requires a material medium and mechanical contact between particles of the material medium.
In a medium, sound travels in the form of sound waves. Sound waves are mechanical vibrations that are transmitted in a medium using its conditional particles. Conventional particles of a medium mean its microvolumes.
Basic physical characteristics of an acoustic wave:
1. Frequency.
Frequency sound wave is the magnitude equal to the number of complete oscillations per unit time. Indicated by the symbol v (nude) and measured in hertz. 1 Hz = 1 count/sec = [ s -1 ].
The sound vibration scale is divided into the following frequency intervals:
· infrasound (from 0 to 16 Hz);
· audible sound (from 16 to 16,000 Hz);
· ultrasound (over 16,000 Hz).
The frequency of a sound wave is closely related to its inverse quantity – the period of the sound wave. Period A sound wave is the time of one complete vibration of the particles of the medium. Designated T and is measured in seconds [s].
According to the direction of vibration of the particles of the medium carrying the sound wave, sound waves are divided into:
· longitudinal;
· transverse.
For longitudinal waves, the direction of vibration of the particles of the medium coincides with the direction of propagation of the sound wave in the medium (Fig. 1).
For transverse waves, the directions of vibration of the particles of the medium are perpendicular to the direction of propagation of the sound wave (Fig. 2).
Rice. 1 Fig. 2
Longitudinal waves propagate in gases, liquids and solids. Transverse - only in solids.
3. Shape of vibrations.
According to the shape of vibrations, sound waves are divided into:
· simple waves;
complex waves.
The graph of a simple wave is a sine wave.
The graph of a complex wave is any periodic non-sinusoidal curve .
4. Wavelength.
Wavelength is the quantity equal to the distance over which a sound wave travels in a time equal to one period. It is designated λ (lambda) and is measured in meters (m), centimeters (cm), millimeters (mm), micrometers (µm).
The wavelength depends on the medium in which the sound travels.
5. Sound wave speed.
Sound wave speed is the speed of sound propagation in a medium with a stationary sound source. Denoted by the symbol v, calculated by the formula:
The speed of the sound wave depends on the type of medium and temperature. The speed of sound is highest in solid elastic bodies, less in liquids, and lowest in gases.
air, normal atmospheric pressure, temperature - 20 degrees, v = 342 m/s;
water, temperature 15-20 degrees, v = 1500 m/s;
metals, v = 5000-10000 m/s.
The speed of sound in air increases by about 0.6 m/s with an increase in temperature of 10 degrees.
LECTURE 3 ACOUSTICS. SOUND
1. Sound, types of sound.
2. Physical characteristics sound.
3. Characteristics of auditory sensation. Sound measurements.
4. Passage of sound across the interface.
5. Sound methods research.
6. Factors determining noise prevention. Noise protection.
7. Basic concepts and formulas. Tables.
8. Tasks.
Acoustics. In a broad sense, it is a branch of physics that studies elastic waves from the lowest frequencies to the highest. In a narrow sense, it is the study of sound.
Sound in a broad sense is elastic vibrations and waves propagating in gaseous, liquid and solid substances; in a narrow sense, a phenomenon subjectively perceived by the hearing organs of humans and animals.
Normally, the human ear hears sound in the frequency range from 16 Hz to 20 kHz. However, with age, the upper limit of this range decreases:
Sound with a frequency below 16-20 Hz is called infrasound, above 20 kHz -ultrasound, and the highest frequency elastic waves in the range from 10 9 to 10 12 Hz - hypersound.
Sounds found in nature are divided into several types.
Tone - it is a sound that is a periodic process. The main characteristic of tone is frequency. Simple tone created by a body vibrating according to a harmonic law (for example, a tuning fork). Complex tone is created by periodic oscillations that are not harmonic (for example, the sound of a musical instrument, the sound created by the human speech apparatus).
Noise is a sound that has a complex, non-repeating time dependence and is a combination of randomly changing complex tones (the rustling of leaves).
Sonic boom- this is a short-term sound impact (clap, explosion, blow, thunder).
A complex tone, as a periodic process, can be represented as a sum of simple tones (decomposed into component tones). This decomposition is called spectrum.
The acoustic spectrum of a tone is the sum of all its frequencies, indicating their relative intensities or amplitudes.
The lowest frequency in the spectrum (ν) corresponds to the fundamental tone, and the remaining frequencies are called overtones or harmonics. Overtones have frequencies that are multiples of the fundamental frequency: 2ν, 3ν, 4ν, ...
Typically, the largest amplitude of the spectrum corresponds to the fundamental tone. It is this that is perceived by the ear as the pitch of the sound (see below). Overtones create the “color” of sound. Sounds of the same pitch created by different instruments are perceived differently by the ear precisely because of the different relationships between the amplitudes of the overtones. Figure 3.1 shows the spectra of the same note (ν = 100 Hz) played on a piano and a clarinet.
Rice. 3.1. Spectra of piano (a) and clarinet (b) notes
The acoustic spectrum of noise is continuous.
