Sound. Basic characteristics of the sound field

The space in which sound travels is called the sound field. The characteristics of the sound field are divided into linear and energy.

Linear sound field characteristics:

1. sound pressure;

2. mixing of particles of the medium;

3. speed of oscillation of particles of the medium;

4. acoustic resistance of the environment;

Energy characteristics of the sound field:

1. strength (intensity) of sound.

1. Sound pressure is the additional pressure that occurs when sound passes through a medium. It is an additional pressure to the static pressure in the medium, for example, to the atmospheric pressure of the air. Indicated by the symbol R and is measured in units:

P = [N/m2] = [Pa].

2. The displacement of particles of the medium is a value equal to the deviation of the conditional particles of the medium from the equilibrium position. Indicated by the symbol L, measured in meters (cm, mm, km), L = [m].

3. The speed of vibration of particles of the medium is the speed of displacement of particles of the medium relative to the equilibrium position under the influence of a sound wave. Indicated by the symbol u and is calculated as the displacement ratio L In time t during which this shift occurred. Calculated by the formula:

Unit of measurement [m/s], in non-system units cm/s, mm/s, µm/s.

4. Acoustic resistance is the resistance that a medium provides to an acoustic wave passing through it. Formula for calculation:

Unit: [Pa s/m].

In practice, another formula is used to determine acoustic impedance:

Z=p*v. Z-acoustic impedance,

p is the density of the medium, v is the speed of the sound wave in the medium.

Of the energy characteristics, only one is used in medicine and pharmacy - the strength or intensity of sound.

The strength (intensity) of sound is a value equal to the amount of sound energy E, passing per unit of time t per unit area S. Indicated by the symbol I. Formula for calculation: I=E/(S t) Units: [J/s m2]. Since a Joule per second is equal to 1 Watt, then

I = [ J/s m 2 ] = [ W/m2].

Psychophysical characteristics of sound.

Psychophysics is the science of the connection between objective physical influences and the resulting subjective sensations.

From the point of view of psychophysics, sound is a sensation that occurs in the auditory analyzer when mechanical vibrations act on it.

Psychophysically, sound is divided into:

The tones are simple;

The tones are complex;

Idle tone is a sound corresponding to a sinusoidal harmonic mechanical vibration of a certain frequency. Graph of a simple tone - a sine wave (see 3. Waveform).

Complex tone- this is a sound consisting of a different (multiple) number of simple tones. The complex tone graph is a periodic non-sinusoidal curve (see 3. Waveform).

Noise - This is a complex sound, consisting of a large number of simple and complex tones, the number and intensity of which changes all the time. Low-intensity noises (the sound of rain) calm the nervous system, while high-intensity noises (the operation of a powerful electric motor, the operation of city transport) tire the nervous system. Combating noise is one of the tasks of medical acoustics.

Psychophysical characteristics of sound:

Pitch

Sound volume

Sound timbre

Pitch is a subjective characteristic of the frequency of an audible sound. The higher the frequency, the higher the pitch.

Sound volume - This is a characteristic that depends on the frequency and strength of sound. If the sound strength does not change, then with an increase in frequency from 16 to - 1000 Hz, the volume increases. At a frequency from 1000 to 3000 Hz it remains constant; with a further increase in frequency, the volume decreases and at frequencies above 16,000 Hz the sound becomes inaudible.

Loudness (loudness level) is measured using a unit called "phon". The loudness in the background is determined using special tables and graphs called “isoacoustic curves”.

Sound timbre- this is the most complex psychophysical characteristic of perceived sound. Timbre depends on the number and intensity of simple tones included in a complex sound. A simple tone has no timbre. There are no units for measuring the timbre of sound.

Logarithmic units of sound measurements.

Experiments have established that large changes in the strength and frequency of sound correspond to minor changes in volume and pitch. Mathematically, this corresponds to the fact that the increase in the sensation of height and volume occurs according to logarithmic laws. In this regard, logarithmic units began to be used for sound measurements. The most common units are "bel" and "decibel".

Bel is a logarithmic unit equal to the decimal logarithm of the ratio of two homogeneous quantities. If these quantities are two different sound intensities I 2 and I 1, then the number of bels can be calculated using the formula:

N B =log(I 2 /I 1)

If the ratio of I 2 to I 1 is 10, then N B = 1 white, if this ratio is 100, then 2 whites, 1000 - 3 whites. For other ratios, the number of bels can be calculated using logarithm tables or using a microcalculator.

A decibel is a logarithmic unit equal to a tenth of a bel.

Indicated by dB. Calculated by the formula: N dB =10·lg(I 2 /I 1).

The decibel is a more convenient unit for practice and therefore is used more often in calculations.

An octave is a logarithmic unit of medical acoustics that is used to characterize frequency intervals.

An octave is an interval (band) of frequencies in which the ratio of the higher frequency to the lower frequency is two.

Quantitatively, the frequency interval in octaves is equal to the binary logarithm of the ratio of two frequencies:

N OCT =log 2 (f 2 /f 1). Here N is the number of octaves in the frequency interval;

f 2, f 1 - boundaries of the frequency interval (extreme frequencies).

One octave is obtained when the frequency ratio is two: f 2 / f 1 =2.

In medical acoustics, standard octave frequency boundaries are used.

Within each interval, average rounded octave frequencies are given.

The frequency boundaries of 18 - 45 Hz correspond to the average octave frequency - 31.5 Hz;

the frequency boundaries of 45-90 Hz correspond to an average octave frequency of 63 Hz;

boundaries 90-180 Hz - 125 Hz.

The sequence of average octave frequencies when measuring hearing acuity will be the following frequencies: 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz.

In addition to white, decibel and octave in acoustics The logarithmic unit "decade" is used. The frequency interval in decades is equal to the decimal logarithm of the ratio of two extreme frequencies:

N dec =log(f 2 /f 1).

Here N decades is the number of decades in the frequency interval;

f 2, f 1 - boundaries of the frequency interval.

One decade is obtained when the ratio of the extreme frequencies of the interval is equal to ten: f 2 / f 1 = 10.

In scale terms, a decade is equal to white, but is used only in acoustics, and only to characterize the frequency ratio.

Conditions for human perception of sound.

Elastic waves propagating in continuous media are called sound waves. Actually sound are called waves whose frequencies lie within the range of perception by the human organ of hearing. The sensation of sound occurs in a person if his hearing aid is exposed to waves with a frequency of approximately 16 to 20,000 Hz. Waves with a frequency lying outside these boundaries are not audible, since they do not create auditory sensations. Elastic waves with a frequency below 16 Hz are called infrasound, and with a frequency of 20,000 Hz up to 10 8 -10 9 Hz- ultrasound. The field of physics that studies how sound waves are excited, how they propagate, and how they interact with a medium is called acoustics.

The general principles of vibrational and wave types of mechanical motion that we obtained in previous chapters are also applicable to the study of acoustic phenomena. However, a number of special issues related to the peculiarities of sound perception and its technical use led to the separation of acoustics into a special field of physics.

For the occurrence and propagation of sound waves, the presence of an elastic medium (solid body, air, water) is necessary. To verify this, let's place a regular electric bell under an air bell. Until the air is pumped out from under the bell, the bell can be clearly heard. As the air is pumped out, the sound weakens and finally disappears altogether. The air environment under the bell becomes so rarefied that it can no longer transmit sound vibrations. The rarefaction must be such that the gas molecules are separated from each other at distances greater than the distances at which the forces of molecular interaction appear. Then the molecules that have received a certain amount of motion from the bell hammer cannot transfer it directionally to neighboring molecules, but are scattered during random collisions, which are exchanged in thermal motion.

As we have seen, the occurrence of waves is possible if the medium provides elastic resistance to deformation and has inertia.

A solid body resists both longitudinal deformations - tension and compression, and shear. Therefore, in a solid body, sound waves can be both longitudinal and transverse. In liquids and gases that do not offer shear resistance under normal conditions, sound waves are only longitudinal.

Sound waves in a medium are created by an oscillating body. For example, the vibration of a telephone membrane creates successive compressions and rarefactions in the adjacent layer of air, spreading in all directions.

To study the state of the medium in which a sound wave propagates, you can resort to the method that we used when studying the movement of a liquid. At each point in space filled with a medium in a state of sound motion, periodic changes occur: a) the position of the particle relative to the equilibrium one, b) the speed of displacement of the particle, c) the magnitude of pressure (compression and rarefaction) relative to their average value existing in an undisturbed medium. The change in pressure in this case is called redundant or sound pressure. If we imagine that at every point in the environment there are miniature sensors of devices that measure these quantities, then their simultaneous readings will give us an instant picture of the state of the environment. A series of such instantaneous pictures following each other will give a change in the state of the environment over time. Since wave motion is periodic in both time and space, then, knowing the speed of propagation of a sound wave and observing the change in the above characteristics at one point of an isotropic medium with low attenuation, we can find them for the entire space occupied by the medium in which sound waves propagate The space filled with a medium in a state of sound motion is called sound field.

Lecture 6 NOISE PROTECTION

Among the basic human senses, hearing and vision play the most important role - they allow a person to master sound and visual information fields.

Even a cursory analysis of the human-machine-environment system gives reason to consider the problem of noise pollution of the environment as one of the priority problems of human interaction with the environment, especially at the local level (workshop, site).

Long-term exposure to noise can lead to hearing loss and, in some cases, to deafness. Noise pollution in the workplace has an adverse effect on workers: attention decreases, energy consumption increases with the same physical activity, the speed of mental reactions slows down, etc. As a result, labor productivity and the quality of work performed decrease.

Knowledge of the physical laws of the process of noise radiation and propagation will allow making decisions aimed at reducing its negative impact on humans.

Sound. Basic characteristics of the sound field. Sound propagation

Concept sound , as a rule, is associated with the auditory sensations of a person with normal hearing. Auditory sensations are caused by vibrations of an elastic medium, which are mechanical vibrations propagating in a gaseous, liquid or solid medium and affecting the human hearing organs. In this case, vibrations of the environment are perceived as sound only in a certain frequency range (16 Hz - 20 kHz) and at sound pressures exceeding the human hearing threshold.

The frequencies of vibrations of the medium lying below and above the range of audibility are called respectively infrasonic And ultrasonic . They are not related to a person’s auditory sensations and are perceived as physical influences of the environment.

Sound vibrations of particles of an elastic medium are complex in nature and can be represented as a function of time a = a(t)(Fig. 1, A).

Rice. 1. Vibrations of air particles.

The simplest process is described by a sinusoid (Fig. 1, b)

,

Where a max- amplitude of oscillations;

w = 2 p f - angular frequency;

f- oscillation frequency.

Harmonic vibrations with amplitude a max and frequency f are called tone.

Depending on the method of excitation of vibrations, there are:

A plane sound wave created by a flat oscillating surface;

A cylindrical sound wave created by the radially oscillating side surface of the cylinder;

A spherical sound wave created by a point source of vibration such as a pulsating ball.

The main parameters characterizing a sound wave are:

Sound pressure p sv, Pa;

Sound intensity I, W/m2.

Sound wavelength l, m;

Wave propagation speed s, m/s;

Oscillation frequency f, Hz.

If oscillations are excited in a continuous medium, they diverge in all directions. A clear example is the vibrations of waves on water. From a physical point of view, the propagation of vibrations consists of the transfer of momentum from one molecule to another. Thanks to elastic intermolecular bonds, the movement of each of them repeats the movement of the previous one. The transfer of impulse requires a certain amount of time, as a result of which the movement of molecules at observation points occurs with a delay in relation to the movement of molecules in the zone of excitation of vibrations. Thus, vibrations propagate at a certain speed. Sound wave speed With is a physical property of the environment.

Sound vibrations in the air lead to its compression and rarefaction. In areas of compression, air pressure increases, and in areas of rarefaction it decreases. The difference between the pressure existing in a disturbed medium p Wed at the moment, and atmospheric pressure p atm, called sound pressure (Fig. 2). In acoustics, this parameter is the main one through which all others are determined.

p sv = p Wed - p atm.

Rice. 2. Sound pressure

The medium in which sound propagates has specific acoustic resistance Z A, which is measured in Pa*s/m (or in kg/(m 2 *s) and is the ratio of sound pressure p sound to the vibrational velocity of particles of the medium u:

z A = p sound /u =r*With,

Where With - sound speed , m; r - density of the medium, kg/m3.

For different environments values ZA are different.

A sound wave is a carrier of energy in the direction of its movement. The amount of energy transferred by a sound wave in one second through a section with an area of ​​1 m 2 perpendicular to the direction of movement is called sound intensity . Sound intensity is determined by the ratio of sound pressure to the acoustic resistance of the medium W/m2:

For a spherical wave from a sound source with power W, W sound intensity on the surface of a sphere of radius r is equal to:

I= W / (4p r 2),

that is, intensity spherical wave decreases with increasing distance from the sound source. When plane wave sound intensity does not depend on distance.

6.1.1 . Acoustic field and its characteristics

The surface of a body that vibrates is an emitter (source) of sound energy, which creates an acoustic field.

Acoustic field called the region of an elastic medium, which is a means of transmitting acoustic waves. The acoustic field is characterized by:

- sound pressure p sv, Pa;

- acoustic resistance Z A, Pa*s/m.

The energy characteristics of the acoustic field are:

- intensity I, W/m2;

- sound power W, W is the amount of energy passing per unit time through the surface surrounding the sound source.

An important role in the formation of the acoustic field is played by characteristic of directionality of sound emission F , i.e. angular spatial distribution of sound pressure generated around the source.

All of these quantities are interrelated and depend on the properties of the medium in which sound propagates. If the acoustic field is not limited to the surface and extends almost to infinity, then such a field is called a free acoustic field. In a confined space (for example, indoors), the propagation of sound waves depends on the geometry and acoustic properties of the surfaces located in the path of the waves.

The process of forming a sound field in a room is associated with the phenomena reverberation And diffusion.

If a sound source begins to operate in the room, then at the first moment of time we have only direct sound. When the wave reaches the sound-reflecting barrier, the field pattern changes due to the appearance of reflected waves. If an object whose dimensions are small compared to the length of the sound wave is placed in the sound field, then practically no distortion of the sound field is observed. For effective reflection it is necessary that the dimensions of the reflecting barrier be greater than or equal to the length of the sound wave.

A sound field in which a large number of reflected waves appear in different directions, as a result of which the specific density of sound energy is the same throughout the field, is called diffuse field.

After the source stops emitting sound, the acoustic intensity of the sound field decreases to zero level over an infinite time. In practice, a sound is considered to be completely attenuated when its intensity drops to 10 6 times the level existing at the moment it is turned off. Any sound field as an element of a vibrating medium has its own sound attenuation characteristic - reverberation(“after-sound”).

Z The sound field manifests itself in the form of kinetic energy of oscillating material bodies, sound waves in media with an elastic structure (solids, liquids and gases). The process of propagation of vibrations in an elastic medium is called wave. The direction of propagation of the sound wave is called sound beam, and the surface connecting all adjacent points of the field with the same phase of oscillation of the particles of the medium is wave front. In solids, vibrations can propagate in both the longitudinal and transverse directions. They only spread in the air longitudinal waves.

Free sound field called a field in which the direct sound wave predominates, and reflected waves are absent or negligibly small.

Diffuse sound field- this is a field in which at each point the density of sound energy is the same and in all directions of which identical flows of energy propagate per unit of time.

Sound waves are characterized by the following basic parameters.

Wavelength- equal to the ratio of the speed of sound (340 m/s in air) to the frequency of sound vibrations. Thus, the wavelength in air can vary from 1.7 cm (for f= 20000 Hz) up to 21 m (for f= 16 Hz).

Sound pressure- is defined as the difference between the instantaneous pressure of the sound field at a given point and the statistical (atmospheric) pressure. Sound pressure is measured in Pascals (Pa), Pa = N/m2. Physical analogues – electrical voltage, current.

Sound intensity– the average amount of sound energy passing per unit time through a unit surface perpendicular to the direction of wave propagation. Intensity is measured in units of W/m2 and represents the active component of the power of sound vibrations. The physical analogue is electrical power.

In acoustics, measurement results are usually displayed in the form of relative logarithmic units. To evaluate the auditory sensation, a unit called Bel (B) is used. Since Bel is a fairly large unit, a smaller value was introduced - decibel (dB) equal to 0.1 B.

Sound pressure and sound intensity are expressed in relative acoustic levels:

,

Zero values ​​of acoustic levels correspond to generally accepted and W/m 2 with harmonic sound vibration with a frequency of 1000 Hz. The given values ​​correspond approximately to the minimum values ​​causing auditory sensations (absolute hearing threshold).

Conditions for measuring microphone characteristics. Acoustic measurements have a number of specific features. Thus, the measurement of some characteristics of electroacoustic equipment must be carried out in free field conditions, i.e. when there are no reflected waves.

In ordinary rooms this condition cannot be met, and taking measurements outdoors is difficult and not always possible. First, outdoors it is difficult to avoid reflections from surfaces such as the ground. Secondly, measurements in this case depend on atmospheric conditions (wind, etc.) and can lead to large errors, not to mention a number of other inconveniences. Thirdly, in the open air it is difficult to avoid the influence of extraneous (industrial, etc.) noise.

Therefore, to carry out measurements in a free field, special sound-attenuated chambers are used, in which reflected waves are practically absent.

Measuring microphone characteristics in an anechoic chamber. To measure the sensitivity of a free-field microphone, one would first measure the sound pressure at the point where the microphone under test would be placed, and then place it at that point. But since there is practically no interference in the chamber, and the distance of the microphone from the loudspeaker is taken equal to 1 - 1.5 m (or more) with an emitter diameter of no more than 25 cm, the measuring microphone can be placed close to the microphone under test. The diagram of the measuring setup is shown in Fig. 4. Sensitivity is determined over the entire nominal frequency range. By setting the required pressure using a sound pressure meter (sound meter), measure the voltage developed by the microphone under test and determine its axial sensitivity.

E O.C. = U M /P( mV/Pa)

Sensitivity is determined either by open circuit voltage or by voltage at rated load. As a rule, the internal resistance module of a microphone at a frequency of 1000 Hz is taken as the rated load.

Fig.4. Functional diagram of microphone sensitivity measurement:

1 - tone or white noise generator; 2 - octave filter (one-third octave); 3 - amplifier; 4 - anechoic chamber; 5 – acoustic emitter; 6 - microphone under test; 7 - measuring microphone; 8 - millivoltmeter; 9 - millivoltmeter, graduated in pascals or decibels (sound level meter).

Sensitivity level is defined as the sensitivity, expressed in decibels, relative to a value equal to 1.

Standard sensitivity level (in decibels) is defined as the ratio of the voltage developed at the nominal load resistance at a sound pressure of 1 Pa to the voltage corresponding to power = 1 mW and is calculated using the formula:

where is the voltage (V) developed by the microphone at the nominal load resistance (Ohm) at a sound pressure of 1 Pa.

Frequency response microphone sensitivity is the dependence of microphone sensitivity on frequency at constant values ​​of sound pressure and microphone supply current. The frequency response is measured by smoothly changing the frequency of the generator. Based on the obtained frequency response, its unevenness in the nominal and operating frequency ranges is determined.

Directional characteristics The microphone is removed according to the same scheme (Fig. 4), and depending on the task, either at several frequencies, using a tone generator, or for a noise signal in one-third octave bands, or for a given frequency band, using a corresponding bandpass filter instead of one-third octave filters.

To measure the directional characteristics, the microphone under test is mounted on a rotating disk with a dial. The disk is rotated manually or automatically, synchronously with the recording table. The characteristic is taken in one plane passing through the working axis of the microphone, if it is a body of rotation around its axis. For other microphone shapes, the characteristic is taken for given planes passing through the working axis. The rotation angle is measured between the working axis and the direction towards the sound source. The directivity characteristic is normalized relative to the axial sensitivity.

A sound field is a region of space in which sound waves propagate, that is, acoustic vibrations of particles of an elastic medium (solid, liquid or gaseous) filling this region occur. The concept of a sound field is usually used for areas whose dimensions are on the order of or greater than the sound wavelength.

On the energy side, the sound field is characterized by the density of sound energy (the energy of the oscillatory process per unit volume) and sound intensity.

The surface of a body that vibrates is an emitter (source) of sound energy, which creates an acoustic field.

Acoustic field called the region of an elastic medium, which is a means of transmitting acoustic waves. The acoustic field is characterized by:

· sound pressure p sv, Pa;

· acoustic resistance z A, Pa*s/m.

The energy characteristics of the acoustic field are:

· intensity I, W/m2;

· sound power W, W is the amount of energy passing per unit time through the surface surrounding the sound source.

An important role in the formation of the acoustic field is played by characteristic of directionality of sound emission F, i.e. angular spatial distribution of sound pressure generated around the source.

All of these quantities are interrelated and depend on the properties of the medium in which sound propagates.

If the acoustic field is not limited to the surface and extends almost to infinity, then such a field is called a free acoustic field.

In a confined space (for example, indoors), the propagation of sound waves depends on the geometry and acoustic properties of the surfaces located in the path of the waves.

The process of forming a sound field in a room is associated with the phenomena reverberation And diffusion.

If a sound source begins to operate in the room, then at the first moment of time we have only direct sound. When the wave reaches the sound-reflecting barrier, the field pattern changes due to the appearance of reflected waves. If an object whose dimensions are small compared to the length of the sound wave is placed in the sound field, then practically no distortion of the sound field is observed. For effective reflection it is necessary that the dimensions of the reflecting barrier be greater than or equal to the length of the sound wave.

A sound field in which a large number of reflected waves appear in different directions, as a result of which the specific density of sound energy is the same throughout the field, is called diffuse field.

After the source stops emitting sound, the acoustic intensity of the sound field decreases to zero level over an infinite time. In practice, a sound is considered to be completely attenuated when its intensity drops to 10 6 times the level existing at the moment it is turned off. Any sound field as an element of a vibrating medium has its own sound attenuation characteristic - reverberation(“after-sound”).