For convenience, the spectrum of audible frequencies is divided into octave bands.

The octave band is characterized by a lower limit frequency f n, an upper limit frequency f b, and a geometric mean frequency f avg.geom. =

Based on frequency, noise is divided into:

1. Low frequency, when the maximum sound pressure falls in the frequency range up to 300 Hz.

2. Mid-frequency, when the maximum sound pressure is from 300 to 800 Hz.

3.High frequency, when the maximum sound pressure is more than 800Hz.

Noise is also divided into constant - when the pressure level P changes by ≤5 dBA over an 8-hour working day, and non-constant - by ≥ 5 dBA

According to the spectral composition Noise: - broadband with a continuous spectrum more than an octave wide. – Tonal, when the spectrum is dominated by pronounced discrete tones.

Standardization is carried out: 1) according to the maximum noise spectrum, that is, according to the level P in octave frequency bands. 2) at the equivalent level in dBA.

The effect of noise on the human body. Noise level measuring device

Increased noise levels primarily affect the cardiovascular, central nervous systems, and visual analyzers.

Long-term work in the field of noise leads to hearing loss, which manifests itself in partial hearing loss.

Noise measuring instruments - sound level meters - usually consist of a sensor (microphone), an amplifier, frequency filters (frequency analyzer), a recording device (recorder or tape recorder) and an indicator showing the level of the measured value in dB. Sound level meters are equipped with frequency correction blocks with switches A, B, C, D and time characteristics with switches F (fast) - fast, S (slow) - slow, I (pik) - impulse. The F scale is used when measuring constant noise, S - oscillating and intermittent noise, I - pulsed.

Based on accuracy, sound level meters are divided into four classes 0, 1, 2 and 3. Class 0 sound level meters are used as exemplary measuring instruments; Class 1 devices - for laboratory and field measurements; 2 - for technical measurements; 3 - for approximate measurements. Each class of instruments corresponds to a frequency measurement range: sound level meters of classes 0 and 1 are designed for the frequency range from 20 Hz to 18 kHz, class 2 - from 20 Hz to 8 kHz, class 3 - from 31.5 Hz to 8 kHz.

Integrating sound level meters are used to measure the equivalent noise level when averaging over a long period of time.

Instruments for measuring noise are built on the basis of frequency analyzers, consisting of a set of bandpass filters and instruments that show the sound pressure level in a certain frequency band.

Depending on the type of frequency characteristics of the filters, analyzers are divided into octave, third-octave and narrowband.

Frequency response of the filter K( f) =U out / U in represents the dependence of the signal transmission coefficient from the filter input U input to its output U output from signal frequency f.

Frequency response typical octave bandpass filter shown in Fig. 3.6. A bandpass filter is characterized by its bandwidth B= f 2 - f 1, i.e. frequency region between two frequencies f 1 and f 2, at which the frequency response K( f) has a value (attenuation) of no more than 3 dB.

Infrasound. Effect on humans. Methods of protection.

Sounds with a frequency less than<20 Гц – инфразвук и >20000 Hz – ultrasound. Infrasound occurs during the operation of compressors, ventilation, air conditioning and other cases. In addition, infrasound is accompanied by natural phenomena - earthquakes, tsunamis, etc. Infrasound is characterized by a long wavelength and the ability to travel long distances, skirting obstacles. It has the greatest impact on the entire human body, leading to a decrease in visual and hearing acuity, disruption of the vestibular apparatus, headaches, and causes a feeling of fear and anxiety.

The long wavelength allows infrasound to travel over long distances and cannot be stopped by building premises

Control measures must be applied to the source of its formation:

Increasing the number of rotating shafts.

Increasing the rigidity of the oscillatory system.

Elimination of low frequency vibration.

Protective measures (infrasound - less than 16 Hz)

1. Decrease in. sound at its source.

2. Personal protective equipment.

Basic concepts and definitions. Auditory perception as a means of obtaining information is the second most important (after visual) psychophysiological process for a person.

Noise- any sound that is undesirable for a person. Sound waves excite vibrations of particles in the sound medium, resulting in changes in atmospheric pressure.

Sound pressure– the difference between the instantaneous pressure value at a point in the medium and the static pressure at the same point, i.e. pressure in an undisturbed environment: P = P mg – P st .

Sound pressure is an alternating quantity. At moments of condensation (compression or compaction) of the particles of the medium, it is positive; at moments of rarefaction it is negative.

The hearing organs perceive not instantaneous, but root-mean-square sound pressure:

Pressure averaging time: T o = 30 – 100 ms.

When a sound wave propagates, it occurs energy transfer.

The average energy flux at a point in the medium per unit time per unit surface normal to the direction of wave propagation is called sound intensity (sound intensity) at this point.

Intensity, W/m 2, is related to sound pressure by the dependence

Where ρ×с– specific acoustic resistance.

The values ​​of sound pressure and sound intensity that one has to deal with in noise control practice can vary widely: in pressure - up to 10 8 times, in intensity - up to 10 16 times. It is somewhat inconvenient to operate with such numbers.

In addition, the auditory analyzer obeys the basic psychophysical law (Weber-Fechner):

Where E– intensity of sensations; I– intensity of the stimulus; WITH And TO– some constant quantities.

Therefore, they were introduced logarithmic quantities sound pressure level and sound intensity.

Sound pressure level, dB:

Where R o= 2×10 -5 Pa – threshold sound pressure; R– root mean square sound pressure.

Sound intensity level, dB:

Where I– effective sound intensity; I o= 10 -12 W/m 2 – sound intensity corresponding to the threshold of audibility (at a frequency of 1000 Hz).

The value of the intensity level is used when obtaining formulas for acoustic calculations, and the sound pressure level is used to measure noise and assess its impact on a person, since the hearing organ is sensitive not to intensity, but to root-mean-square pressure.

Intensity Imax and sound pressure value Pmax, corresponding to the pain threshold: Imax= 10 2 W/m, Pmax= 2×10 2 Pa.

Frequency spectrum of noise– dependence of the intensity level (sound pressure level) on frequency: L = L(ƒ). The entire audible frequency range is divided into 9 octave bands. Octave band, or octave – this is the frequency range for which the condition is satisfied

The following types of spectra are distinguished:

- discrete (ruled)– spectrum, the sinusoidal components of which are separated from each other by frequency (Fig. 6.1);

KHOREV Anatoly Anatolyevich, Doctor of Technical Sciences, Professor

TECHNICAL CHANNELS FOR LEAKAGE OF ACOUSTIC (SPEECH) INFORMATION

General characteristics of the speech signal

Acoustic information is usually understood as information whose carriers are acoustic signals. If the source of information is human speech, acoustic information is called speech.

The primary sources of acoustic signals are mechanical oscillatory systems, for example, human speech organs, and the secondary sources are various types of transducers, for example, loudspeakers.

Acoustic signals are longitudinal mechanical waves. They are emitted by a source - an oscillating body - and propagate in solids, liquids and gases in the form of acoustic vibrations (waves), that is, oscillatory movements of particles of the medium under the influence of various disturbances. The space in which acoustic vibrations propagate is called acoustic field, direction of propagation of acoustic vibrations - acoustic beam, and the surface connecting all adjacent points of the field with the same phase of oscillation of particles of the medium - wave front. In the general case, the wave front has a complex shape, but in practice, depending on the specific problem being solved, we usually limit ourselves to considering three types of fronts: flat, spherical and cylindrical.

The characteristics of the acoustic field are divided into linear and energy.

The linear characteristics of the acoustic field are:

Acoustic pressure p (Pa) - the difference between the instantaneous value of pressure p am at a point in the medium when an acoustic wave passes through it and the static pressure p ac at the same point (1 Pa = 1 N/m 2): p = p am – p ac ; (1)

Displacement u (m) - deviation of particles of the medium from its static position under the influence of a passing acoustic wave;

Oscillation speed n (m/s) - the speed of movement of medium particles under the influence of a passing acoustic wave: n = du/dt, (2), where u is the displacement of medium particles, m; t - time, s;

Specific acoustic resistance z (kg/m 2 s) - the ratio of sound pressure p to the vibration speed of medium particles n: z = p/n.(3)

The energy characteristics of the acoustic field are:

Intensity of acoustic vibrations I (W/m 2) - the amount of energy passing per second through a unit surface area perpendicular to the direction of wave propagation;

Energy density e (J/m 3) - the amount of energy of acoustic vibrations located in a unit volume. The energy density is related to the intensity of acoustic vibrations I by the relation:
e = I/v sound (4), where v sound is the speed of sound.

In gaseous media, the speed of sound depends on the density of the medium r (the density of air depends on its temperature) and static atmospheric pressure p ac.

For an air temperature of 15 - 20 ° C and a pressure of 101325 Pa (760 mm Hg), the speed of sound is v sound = 340 - 343 m/s.

For oscillations with a period T, the sound wavelength l, that is, the distance between adjacent wave fronts with the same phase (for example, between the maxima or minima of oscillations), and the oscillation frequency f are calculated using the formulas:

l = v sv T; (5)
f = 1/T. (6)

The frequencies of acoustic vibrations in the range of 20 - 20,000 Hz are called sound (they can be perceived by the human ear), below 20 Hz - infrasonic, and above 20,000 Hz - ultrasonic.

In acoustics, the levels of acoustic field characteristics are taken to be values ​​proportional to the logarithms of the relative values ​​(relative to the zero value) of these characteristics.

The conventional (normalized) value of the zero intensity level of acoustic vibrations is taken to be an intensity equal to I 0 = 10 -12 W/m 2 , while the relative intensity level will be equal to:

L I = 10log(I/I 0), dB. (7)

The level of acoustic pressure for air is determined relative to the acoustic pressure corresponding to the zero value of the intensity level for specific acoustic resistance equal to z = 400 kg/(m 2 s):

L p = 20lg(p/p 0), dB, (8)

where p 0 = 2 10 -5 Pa is the conditional value of the zero acoustic pressure level.

The values ​​p 0 and I 0 approximately correspond to the threshold of auditory perception (audibility).

The unit of relative level is the decibel (dB). An increase in level by 1 dB corresponds to an increase in sound pressure by 12%, and sound intensity by 26%.

The acoustic field in open space in the presence of a single power source is characterized by the intensity of acoustic vibrations, calculated by the formula:

(9)
where P W is the power of the radiation source, W;
c is the coefficient taking into account the influence of the near acoustic field (for open space c » 1);
r is the distance from the source to the calculated point, m;
G is the directivity coefficient of the radiation source;
W is the spatial angle of radiation (for radiation into a dihedral angle W = p, for radiation into a half-space W = 2p, for radiation into space W = 4p), rad.

Theoretically, it is quite difficult to calculate the intensity level of acoustic vibrations from real objects. Therefore, most often the intensity level of acoustic vibrations is measured in a certain direction at a certain distance from the object r0, and then recalculated to any other distance r in the same direction using the formula:

, dB, (10)

where r 0 is the distance at which the level of intensity of acoustic vibrations was measured (in most cases, r 0 = 1 m).

The measured intensity level of acoustic vibrations at a distance r 0 .

At r 0 = 1 m for open space, the intensity level of acoustic vibrations at a distance r from the source will be equal to:

, dB. (eleven)

When propagating an acoustic signal in premises, it is necessary to take into account their attenuation when passing through enclosing structures:

DB, (12)
where Z ok is the attenuation coefficient of the acoustic signal in the enclosing structure (sound insulation coefficient), dB.

Depending on the shape of acoustic vibrations, there are simple (tonal) And complex signals. A tonal signal is a signal caused by an oscillation that occurs according to a sinusoidal law. A complex signal includes a whole spectrum of harmonic components. The speech signal is a complex acoustic signal.

Speech can be characterized by three groups of characteristics:

The semantic or semantic side of speech characterizes the meaning of those concepts that are conveyed with its help;

Phonetic characteristics of speech are data characterizing speech from the point of view of its sound composition. The main phonetic characteristic of the sound composition is the frequency of occurrence of various sounds and their combinations in speech;

Physical characteristics - quantities and dependencies that characterize speech as an acoustic signal.

In addition to the fact that speech sounds, when combined into certain phonetic combinations, form some semantic elements, they also differ in purely physical parameters: power, sound pressure, frequency spectrum, sound duration.

The frequency spectrum of speech sounds contains a large number of harmonic components, the amplitudes of which decrease with increasing frequency. The height of the fundamental tone (first harmonic) of this series characterizes the type of voice of the speaker: bass, baritone, tenor, alto, contralto, soprano, but in most cases it plays almost no role in distinguishing speech sounds from each other.

There are forty-one speech sounds (phonemes) in the Russian language. In terms of spectral composition, speech sounds differ from each other in the number of formants and their location in the frequency spectrum. Consequently, the intelligibility of transmitted speech depends, first of all, on which part of the formants reached the listener’s ear without distortion and which part was distorted, or for one reason or another was not heard at all.

A formant can be characterized either by the frequency band it occupies, or by the average frequency corresponding to the maximum amplitude or energy of the components in the formant band, and the average level of this energy.

Most speech sounds have one or two formants, which is due to the participation in the formation of these sounds of the main resonators of the vocal apparatus - the pharyngeal cavity and nasopharynx.

A maximum of 6 amplified frequency regions were observed in individual sounds. However, not all of them are formants. Some of them have no significance for sound recognition, although they carry quite significant energy.

One or two frequency regions are formant. The exclusion of any of these areas from the transmission causes distortion of the transmitted sound, that is, either its transformation into another sound, or even the loss of the characteristics of the sound of human speech.

The formants of speech sounds are located in a wide range of frequencies from approximately 150 to 8600 Hz. The last limit is exceeded only by the components of the formant band of sound F, which can lie in the region up to 12,000 Hz. However, the overwhelming majority of speech sound formants lie in the range from 300 to 3400 Hz, which allows us to consider this frequency band to be quite sufficient to ensure good intelligibility of transmitted speech. The formants are located not only close to each other, but even overlapping.

Different types of speech correspond to typical integral levels of speech signals, measured at a distance of 1 m from the speech source (speaking person, sound-reproducing device): l s = 64 dB - quiet speech; L s = 70 dB - medium volume speech; l s = 76 dB - loud speech; l s = 84 dB - very loud speech, amplified by technical means.

Typically, speech signal levels are measured in octave or third-octave bands of the speech frequency range. The characteristics of the octave and third-octave bands of the speech frequency range and the numerical values ​​of the typical levels of the speech signal in them l s.i depending on their integral level l s are presented in Table. 1 and table. 2.

Table 1. Typical speech signal levels in octave bands of the speech frequency range L s.i

Table 2. Typical speech signal levels in one-third octave bands of the speech frequency range L s.i

The first and seventh octave bands are uninformative, therefore, most often, to assess the capabilities of acoustic reconnaissance means, speech signal levels are measured only in five (2 - 6) octave bands.

The spectral composition of speech largely depends on the gender, age and individual characteristics of the speaker. For different people, the deviation of signal levels measured in octave bands from typical levels can be as much as 6 dB.

The interception of speech information by means of acoustic reconnaissance is carried out against the background of natural noise (Table 3). The process of speech perception in noise is accompanied by losses of the constituent elements of the speech message. The intelligibility of a speech message is characterized by the number of correctly accepted words, reflecting the qualitative area of ​​intelligibility, which is expressed in terms of the details of the certificate of the intercepted conversation compiled by the “enemy” (the person intercepting the information).

Table 3. Average integrated acoustic noise level

To quantify the quality of intercepted speech information, the indicator most often used is verbal speech intelligibility. W, which refers to the relative number (in percentage) of correctly understood words.

The analysis showed the possibility of ranking the comprehensibility of intercepted speech information. For practical reasons, a certain scale for assessing the quality of an intercepted conversation can be established:

1. Intercepted speech information contains a number of correctly understood words sufficient to compile a detailed report on the content of the intercepted conversation.

2. Intercepted speech information contains a number of correctly understood words, sufficient only to compile a brief summary, reflecting the subject, problem, purpose and general meaning of the intercepted conversation.

3. Intercepted speech information contains individual correctly understood words that make it possible to establish the subject of the conversation.

4. When listening to the soundtrack of an intercepted conversation, it is impossible to determine the subject of the conversation.

Practical experience shows that drawing up a detailed report on the content of an intercepted conversation is impossible when verbal intelligibility is less than 60–70%, and a brief summary is impossible when verbal intelligibility is less than 40–60%. When verbal intelligibility is less than 20 - 40%, it is significantly difficult to establish even the subject of an ongoing conversation, and when verbal intelligibility is less than 10 - 20%, this is practically impossible even when using modern noise reduction methods.

Classification of technical channels for leakage of acoustic (speech) information

To discuss restricted access information (meetings, discussions, conferences, negotiations, etc.), special rooms are used (offices, assembly halls, conference rooms, etc.), which are called dedicated premises (VP). To prevent the interception of information from these premises, as a rule, special means of protection are used, therefore dedicated premises are in some cases called protected premises (ZP).

In dedicated premises, as well as at the facilities of technical means of transmission, processing, storage and display of information (TSPI), auxiliary technical means and systems (VTSS).

Dedicated premises are located within controlled area (CR), which is understood as a space (territory, building, part of a building) in which the uncontrolled presence of employees and visitors of the organization, as well as vehicles, is excluded. The border of the controlled zone may be the perimeter of the organization’s protected territory or the enclosing structures of a protected building or a protected part of a building if it is located in an unprotected area. In some cases, the boundary of the controlled area may be the enclosing structures (walls, floor, ceiling) of the allocated room. In some cases, for the period of a closed event, a controlled zone may be temporarily established larger than the protected territory of the enterprise. In this case, organizational, operational and technical measures must be taken that exclude or significantly complicate the possibility of intercepting information in this zone.

Under technical channel for leakage of acoustic (speech) information (TKU AI) understand the totality of the reconnaissance object (dedicated premises), the technical means of acoustic (speech) reconnaissance (TS AR), with the help of which speech information is intercepted, and the physical environment in which the information signal is propagated.

Depending on the physical nature of the occurrence of information signals and the environment of their propagation, technical leakage channels of acoustic (speech) information can be divided into direct acoustic (air), vibroacoustic (vibration), acousto-optical (laser), acoustoelectric and acoustoelectromagnetic (parametric).

Literature

1. Acoustics: Handbook/Ed. M.A. Sapozhkova. 2nd ed., revised. and additional M.: Radio and communication, 1989. 336 p.
2. GOST R 51275-99. Data protection. Information object. Factors influencing information. General provisions. (Adopted and put into effect by Resolution of the State Standard of Russia dated May 12, 1999 No. 160).
3. Zheleznyak, V.K., Makarov Yu.K., Khorev A.A. Some methodological approaches to assessing the effectiveness of speech information protection // Special equipment, 2000, No. 4, p. 39 – 45.
4. Pokrovsky N.B. Calculation and measurement of speech intelligibility. M.: State. Publishing house of literature on communications and radio, 1962. 392 p.
5. Handbook of radio-electronic devices, in 2 volumes. T. 2/Varlamov R.G., Dodik S.D., Ivanov-Tsiganov A.I. and others/Ed. D.P. Linda. M.: Energy, 1978. 328 p.
6. Technical acoustics of transport vehicles/ Sub. Ed. N.I. Ivanova. St. Petersburg: Politekhnika, 1992. 365 p.

Examples of frequency intervals of sound created by the human vocal apparatus and perceived by the human hearing aid are given in Table 4.

Examples of frequency ranges of some musical instruments are given in Table 5. They cover not only the audio range, but also the ultrasonic range.

Animals, birds and insects create and perceive sound in different frequency ranges than humans (Table 6).

In music, each sinusoidal sound wave is called in a simple tone, or tone. Pitch depends on frequency: the higher the frequency, the higher the tone. Main tone complex musical sound is called the tone corresponding lowest frequency in its spectrum. Tones corresponding to other frequencies are called overtones. If overtones multiples frequency of the fundamental tone, then the overtones are called harmonic. The overtone with the lowest frequency is called the first harmonic, the one with the next one is called the second, etc.

Musical sounds with the same fundamental tone may differ timbre. Timbre depends on the composition of overtones, their frequencies and amplitudes, the nature of their rise at the beginning of the sound and decline at the end.

Sound speed

For sound in various media, general formulas (1), (2), (3), (4) are valid:

If the wave propagates in gases, then

. (2)

If an elastic wave propagates in a liquid, then

, (3)

Where K – module of all-round compression of the liquid. Its value for different liquids is given in reference books, the unit of measurement is pascal:

.

If an elastic wave propagates in solids, then the velocity of the longitudinal wave

, (4)

and the shear wave speed

, (5)

Where E – tensile or compressive deformation modulus (Young’s modulus), G – shear deformation modulus. Their values ​​for different materials are given in reference books, the unit of measurement is pascal:

,

.

It should be noted that formula (1) or (2) is applicable in the case of dry atmospheric air and, taking into account the numerical values ​​of Poisson’s ratio, molar mass and universal gas constant, can be written as:

.

However, real atmospheric air always has humidity, which affects the speed of sound. This is due to the fact that Poisson's ratio  depends on the ratio of the partial pressure of water vapor ( p steam) to atmospheric pressure ( p). In humid air, the speed of sound is determined by the formula:

. (1*)

From the last equation it can be seen that the speed of sound in humid air is slightly greater than in dry air.

Numerical estimates of the speed of sound, taking into account the influence of temperature and humidity of atmospheric air, can be carried out using the approximate formula:

These estimates show that when sound propagates along the horizontal direction ( 0 x) with an increase in temperature by 1 0 C the speed of sound increases by 0.6 m/s. Under the influence of water vapor with a partial pressure of no more than 10 Pa the speed of sound increases by less than 0.5 m/s. But in general, at the maximum possible partial pressure of water vapor at the Earth’s surface, the speed of sound increases by no more than 1 m/s.

Wavelength

Knowing the speed and period of the wave, you can find another characteristic - wavelength according to the formula:

. (26)

This value is measured in meters:

.

Physical meaning of wavelength: wavelength is equal to the distance that the wave travels with speed  in a time equal to the period of oscillation. Consequently, particles of the medium, between which there is a distance , oscillate with the same phase. So, wavelength is the minimum distance along the beam between particles that oscillate in phase(Fig. 9).

Sound pressure

In the absence of sound, the atmosphere (air) is an undisturbed medium and has static atmospheric pressure (
).

When sound waves propagate, additional variable pressure is added to this static pressure due to condensations and rarefaction of air. In the case of plane waves we can write:

Where p sound, max– sound pressure amplitude,  - cyclic frequency of sound, k – wave number. Consequently, the atmospheric pressure at a fixed point at a given time becomes equal to the sum of these pressures:

Sound pressure is a variable pressure equal to the difference between the instantaneous actual atmospheric pressure at a given point during the passage of a sound wave and the static atmospheric pressure in the absence of sound:

Sound pressure changes its value and sign during the oscillation period.

Sound pressure is almost always much less than atmospheric

It becomes large and comparable to atmospheric pressure when shock waves occur during powerful explosions or during the passage of a jet aircraft.

The sound pressure units are as follows:

- pascal in SI
,

- bar in GHS
,

- millimeter of mercury ,

- atmosphere .

In practice, instruments do not measure the instantaneous value of sound pressure, but the so-called efficient (or current ) sound pressure . It is equal the square root of the average value of the square of the instantaneous sound pressure at a given point in space at a given time

(44)

and therefore is also called root mean square sound pressure . Substituting expression (39) into formula (40), we obtain:

. (45)

Sound impedance

Sound (acoustic) resistance called amplitude ratio sound pressure and vibrational velocity of particles of the medium:

. (46)

Physical meaning of sound resistance: it is numerically equal to the sound pressure causing vibrations of particles of the medium at a unit speed:

SI unit of measurement of sound impedance – pascal second per meter:

.

In the case of a plane wave particle oscillation speed equal to

.

Then formula (46) will take the form:

. (46*)

There is also another definition of sound resistance, as the product of the density of a medium and the speed of sound in this medium:

. (47)

Then it's physical meaning is that it is numerically equal to the density of the medium in which the elastic wave propagates at unit speed:

.

In addition to acoustic resistance, acoustics uses the concept mechanical resistance (R m). Mechanical resistance is the ratio of the amplitudes of the periodic force and the oscillatory velocity of the particles of the medium:

, (48)

Where S– surface area of ​​the sound emitter. Mechanical resistance is measured in newton seconds per meter:

.

Energy and power of sound

A sound wave is characterized by the same energy quantities as an elastic wave.

Each volume of air in which sound waves propagate has energy that is the sum of the kinetic energy of oscillating particles and the potential energy of elastic deformation of the medium (see formula (29)).

The sound intensity is usually calledthe power of sound . It is equal

. (49)

That's why physical meaning of sound power is similar to the meaning of energy flux density: numerically equal to the average value of energy that is transferred by a wave per unit time through the transverse surface of a unit area.

The unit of sound intensity is watt per square meter:

.

The sound intensity is proportional to the square of the effective sound pressure and inversely proportional to the sound (acoustic) pressure:

, (50)

or, taking into account expressions (45),

, (51)

Where R ak – acoustic resistance.

Sound can also be characterized by sound power. Sound power is the total amount of sound energy emitted by a source over a specified period of time through a closed surface surrounding the sound source:

, (52)

or, taking into account formula (49),

. (52*)

Sound power, like any other, is measured in watts:

.

Subjective characteristics of sound. Spectral sensitivity of sound. Perception of sound by the human ear*.

Subjective sound characteristics

The subjective characteristics of sound are determined by the ability of the human hearing organs to perceive sound vibrations. Perception is individual.

Sound level

and difference in sound intensity levels

It has been noticed that the human ear registers changes in sound intensity according to a logarithmic law. This means that it is not the absolute value of the sound intensity that is important, but its logarithmic value. Size lg(I) , equal to the decimal logarithm of sound strength (intensity) is called logarithmic level sound strength .

Size L, equal to the difference of logarithmic levels is called level difference sound strength

,

. (53)

Unit of measurement of sound intensity level and level difference – white:

,
.

One white - This difference in sound intensity levels on a decimal logarithm scale if the sound intensity has increased tenfold :

.

hundredfold an increase in sound intensity corresponds to two whites

thousandfold the increase is equal to three whites

The minimum difference in sound intensity levels that our ear can perceive is equal to one decibel:

.

Therefore, in practice, instead of formula (53), the formula is used:

. (54)

Comment:

If the sound level is determined not by decimal, but by natural logarithm

,

then the unit of measurement is neper:

.

One neper is the difference in sound intensity levels on a scale of natural logarithms, if the ratio of sound intensity is equal to 10 :

.

Relationship between white and neper:

The perceived sound has lower and upper limits, i.e. minimum and maximum intensity:

.

The minimum value of sound intensity (sound strength) perceived by the human ear is calledhearing threshold: .

Sound intensity below the threshold of audibility

is not perceived by humans.

With respect to the hearing threshold, the difference in sound intensity levels is determined by the formulas:

, (55)

or
(56)

If the sound intensity is equal to the hearing threshold, then

This value L 0 called zero (or threshold ) volume level .

Example: meaning of the expression " The sound level in the speakers is one hundred decibels".

Means: Relative to the hearing threshold, the difference in sound intensity levels is equal to
.

Let's compare with formula (56):
.

Hence,

On the other side,
.

That's why
,

As a result, the absolute value of the sound intensity is:

.

Maximum the intensity of sound that the human ear perceives is called pain threshold :

The sound intensity is above the pain threshold

is not perceived by humans, but causes pain in the ears.

The difference between the levels of pain threshold and hearing threshold is called dynamic range of hearing and is equal to

. (57)

If sound is emitted by two or more sound sources with sound intensity levels L 1, L 2, ..., L i, ..., L N, then their total sound level is determined by the formula:

(58)

Volume level

and volume difference

In accordance with expression (51), the sound intensity is proportional to the square of the sound pressure amplitude:

.

Size lg (p sound, max 2 ) , equal to the decimal logarithm of the square of the sound pressure amplitude is called volume level .

Volume difference name the quantity L p , equal to the difference

. (59)

The unit of measurement for volume level and volume difference is white, and dB:

,
.

Hence,

. (61)

(62)

Minimum sound pressure (p 0 ) are calledthreshold pressure . Relative to the threshold pressure, the difference in volume levels (at a standard frequency 1000 Hz) is equal to

(63)

(64)

Spectral sensitivity of the ear

The sensitivity of human hearing is not the same for different frequency ranges. Therefore there is spectral sensitivity ear: sounds of the same intensity (strength) I, but of different frequencies  The human ear perceives differently.

N The spectral sensitivity is clearly depicted using sensitivity curves – graphs of sound intensity dependences I( ), sound intensity levelL I ( ) and sound pressurep( ) on sound frequency presented in logarithmic scale (Fig. 13).

The upper curve corresponds to mechanical effects on human hearing, bordering on painful perception of the intensity of sounds of the corresponding frequency. The lower curve corresponds to the hearing threshold at the indicated frequencies. It can be seen that sensitivity varies selectively depending on the frequency of sound ranging from the threshold of audibility to the threshold of pain sound. For each frequency there are certain values ​​of the hearing threshold I 0 and pain threshold I B .

1. For sound frequency 100 Hz the hearing threshold, its level and minimum sound pressure are

,
,
,

and the pain threshold, its level and maximum sound pressure -

,
,
;

at this frequency is equal to

2. Sound frequency 1000 Hz in physiological acoustics it is taken as standard frequency . The hearing threshold at a standard frequency is called standard hearing threshold . The standard hearing threshold, its level and minimum sound pressure are respectively equal

,
,
.

For sounds with standard frequency pain threshold , its level and maximum sound pressure have the following values:

,
,
.

Dynamic hearing range for standard frequency is

Examples of differences in sound intensity levels of a standard frequency are given in table. 7.

Table 7.

At 140 dB severe pain is felt when 150 dB ear damage occurs. In general, it is desirable that the operating volume range covering all frequencies should not exceed 100 - 110 dB.

3. To hear a sound frequency 10 kHz you will need a sound source that provides the hearing threshold, its level and minimum sound pressure:

,
,
,

Ears at this sound frequency will begin to hurt at the values ​​of the pain threshold, its level and maximum sound pressure

,
,
.

Dynamic range of hearing for such a frequency is

Comment: Equal intervals of loudness level (sound pressure) correspond to different levels of sound intensity (intensity). Therefore, to characterize loudness levels, a unit is introduced - background.Background – volume difference two sounds given frequency, for which sounds with frequency 1000 Hz, having the same loudness, differ in intensity by 10 dB. Backgrounds are counted from zero, equal to the intensity of the hearing threshold. For sound waves with frequency 1000 Hz level volume sound matches the level of its intensity.

More detailed sensitivity curves I( ) And L I ( ) are given in Fig. 14.

Noise characteristics and effects

Industrial noise is characterized by a spectrum, which consists of sound waves of different frequencies.

When studying noise, the typically audible range of 16 Hz - 20 kHz is divided into frequency bands and the sound pressure, intensity or sound power per band is determined.

As a rule, the noise spectrum is characterized by the levels of these quantities, distributed over octave frequency bands.

A frequency band whose upper limit is twice as large as the lower limit, i.e. f2 = 2 f1, called an octave.

For a more detailed study of noise, third-octave frequency bands are sometimes used, for which

f2 = 21/3 f1 = 1.26 f1.

The main parameters characterizing a sound wave are:

• · sound pressure psv, Pa;
• · sound intensity I, W/m2.
• · sound wavelength l, m;
• · wave propagation speed s, m/s;
• · oscillation frequency f, Hz.

An octave or third-octave band is usually specified by the geometric mean frequency:

The manifestations of the harmful effects of noise on the human body are very diverse.

Long-term exposure to intense noise (above 80 dBA) on a person’s hearing leads to partial or complete loss of hearing. Depending on the duration and intensity of noise exposure, a greater or lesser decrease in the sensitivity of the hearing organs occurs, expressed as a temporary shift in the hearing threshold, which disappears after the end of noise exposure, and with a long duration and (or) intensity of noise, irreversible hearing loss (hearing loss) occurs, characterized by permanent changing the hearing threshold.

There are the following degrees of hearing loss:

I degree (mild hearing loss) - hearing loss in the region of speech frequencies is 10 - 20 dB, at a frequency of 4000 Hz - 20 - 60 dB;

II degree (moderate hearing loss) - hearing loss in the area of ​​​​speech frequencies is 21 - 30 dB, at a frequency of 4000 Hz - 20 - 65 dB;

III degree (significant hearing loss) - hearing loss in the region of speech frequencies is 31 dB or more, at a frequency of 4000 Hz - 20 - 78 dB.

The effect of noise on the human body is not limited to the effect on the organ of hearing. Through the fibers of the auditory nerves, noise irritation is transmitted to the central and autonomic nervous systems, and through them it affects the internal organs, leading to significant changes in the functional state of the body, affecting the mental state of a person, causing a feeling of anxiety and irritation. A person exposed to intense (more than 80 dB) noise spends on average 10 - 20% more physical and neuropsychic effort to maintain the output achieved at a sound level below 70 dB(A). An increase of 10 - 15% in the overall incidence of workers in noisy industries has been established. The effect on the autonomic nervous system is manifested even at low sound levels (40 - 70 dB(A). Of the autonomic reactions, the most pronounced is a violation of peripheral circulation due to narrowing of the capillaries of the skin and mucous membranes, as well as an increase in blood pressure (at sound levels above 85 dBA).

The impact of noise on the central nervous system causes an increase in the latent (hidden) period of the visual motor reaction, leads to disruption of the mobility of nervous processes, changes in electroencephalographic parameters, disrupts the bioelectric activity of the brain with the manifestation of general functional changes in the body (even with noise of 50 - 60 dBA), significantly changes the biopotentials of the brain, their dynamics, causes biochemical changes in the structures of the brain.

With impulsive and irregular noise, the degree of noise exposure increases.

Changes in the functional state of the central and autonomic nervous systems occur much earlier and at lower noise levels than a decrease in auditory sensitivity.

Currently, “noise disease” is characterized by a complex of symptoms:

• -decreased hearing sensitivity;
• -changes in digestive function, expressed in decreased acidity;
• -cardiovascular failure;
• - neuroendocrine disorders.

Those working in conditions of prolonged noise exposure experience irritability, headaches, dizziness, memory loss, increased fatigue, decreased appetite, ear pain, etc. Exposure to noise can cause negative changes in a person’s emotional state, including stressful ones. All this reduces a person’s performance and productivity, quality and safety of work. It has been established that in work that requires increased attention, when the sound level increases from 70 to 90 dBA, labor productivity decreases by 20%.

Ultrasounds (above 20,000 Hz) also cause hearing damage, although the human ear does not respond to them. Powerful ultrasound affects nerve cells in the brain and spinal cord, causing a burning sensation in the external auditory canal and a feeling of nausea.

No less dangerous are the infrasound effects of acoustic vibrations (less than 20 Hz). At sufficient intensity, infrasounds can affect the vestibular system, reducing auditory sensitivity and increasing fatigue and irritability, and lead to loss of coordination. A special role is played by infrafrequency oscillations with a frequency of 7 Hz. As a result of their coincidence with the natural frequency of the alpha rhythm of the brain, not only hearing impairment is observed, but internal bleeding may also occur. Infrasounds (6 - 8 Hz) can lead to cardiac and circulatory problems.