B - room constant in the considered octave band in m 2, taken in accordance with paragraphs. 3.4 or 3.5;

AZ-i - an amendment that takes into account the distribution of sound power levels of ceiling lamps and gratings in octave bands, taken according to Table. 6, in dB;

D - correction for the location of the noise source; when the source is located in the working area (not higher than 2 m from the floor), A = 3 dB; if the source is above this zone, A *■ 0;

0.7 - safety factor;

F, B - the designations are the same as in paragraph 2.9, formula (5).

Note. The determination of the allowable air speed is carried out only for one frequency, which is equal to VNIIGS 250 Shch for ceiling lamps, 500 Hz for disk ceiling lamps, and 2000 Hz for anemostats and gratings.

2.14. In order to reduce the sound power level of the noise generated by bends and tees of air ducts, areas of a sharp change in the cross-sectional area, etc., it is necessary to limit the speed of air movement in the main air ducts of public buildings and auxiliary buildings of industrial enterprises to 5-6 m / s, and on branches up to 2-4 m/sec. For industrial buildings, these speeds can be respectively doubled, if this is permissible according to technological and other requirements.

3. CALCULATION OF OCTAVE SOUND PRESSURE LEVELS AT CALCULATED POINTS

3.1. Octave levels of sound pressure at permanent workplaces or in rooms (at design points) should not exceed the established norms.

(Notes: 1. If the regulatory requirements for sound pressure levels are different during the day, then the acoustic calculation of the installations should be made for the lowest permissible sound pressure levels.

2. Sound pressure levels at permanent workplaces or in rooms (at design points) depend on the sound power and the location of noise sources and the sound-absorbing qualities of the room in question.

3.2. When determining the octave levels of sound pressure, the calculation should be made for permanent workplaces or settlement points in rooms closest to noise sources (heating and ventilation units, air distribution or air intake devices, air or air curtains, etc.). In the adjacent territory, the design points should be taken as the points closest to noise sources (fans located openly on the territory, exhaust or air intake shafts, exhaust devices of ventilation installations, etc.), for which sound pressure levels are normalized.

a - noise sources (autonomous air conditioner and ceiling) and the calculated point are in the same room; b - noise sources (fan and installation elements) and the calculated point are located in different rooms; c - source of noise - the fan is located in the room, the calculated point is on the arrival side of the territory; 1 - autonomous air conditioner; 2 - calculated point; 3 - noise-generating ceiling; 4 - vibration-isolated fan; 5 - flexible insert; in - the central muffler; 7 - sudden narrowing of the duct section; 8 - branching of the duct; 9 - rectangular turn with guide vanes; 10 - smooth turn of the air duct; 11 - rectangular turn of the duct; 12 - lattice; /

3.3. Octave/Sound pressure levels at design points should be determined as follows.

Case 1. The noise source (noise-generating grille, ceiling lamp, autonomous air conditioner, etc.) is located in the room under consideration (Fig. 3). Octave sound pressure levels generated at the calculated point by one noise source should be determined by the formula

L-L, + I0! g (-£-+--i-l (8)

Oct \ 4 I g g W t )

N o t e. For ordinary rooms that do not have special requirements for acoustics, according to the formula

L \u003d Lp - 10 lg H w -4- D - (- 6, (9)

where Lp okt is the octave sound power level of the noise source (determined according to Section 2) in dB\

B w - room constant with a noise source in the considered octave band (determined according to paragraphs 3.4 or 3.5) in g 2;

D - correction for the location of the noise source If the noise source is located in the working area, then for all frequencies D \u003d 3 dB; if above the working area, - D=0;

Ф - radiation directivity factor of the noise source (determined from the curves in Fig. 4), dimensionless; d - distance from the geometric center of the noise source to the calculated point in g.

The graphical solution of equation (8) is shown in fig. 5.

Case 2. The calculated points are located in a room isolated from noise. Noise from a fan or unit element propagates through the air ducts and is radiated into the room through the air distribution or air inlet device (grille). Octave sound pressure levels generated at design points should be determined by the formula

L \u003d L P -DL p + 101g (-% + -V (10)

Note. For ordinary rooms, for which there are no special requirements for acoustics, - according to the formula

L - L p -A Lp -10 lgiJ H ~b A -f- 6, (11)

where L p in is the octave level of the sound power of the fan or installation element radiated into the duct in the considered octave band in dB (determined in accordance with paragraphs 2.5 or 2.10);

AL r in - the total reduction in the level (loss) of the sound power of the noise of the fan or electric

installation time in the octave band under consideration along the sound propagation path in dB (determined in accordance with clause 4.1); D - correction for the location of the noise source; if the air distribution or air intake device is located in the working area, A \u003d 3 dB, if it is higher, - D \u003d 0; Ф and - directivity factor of the installation element (hole, grate, etc.) emitting noise into the isolated room, dimensionless (determined from the graphs in Fig. 4); rn is the distance from the installation element emitting noise into the isolated room to the calculated point in m

B and - the constant of the room isolated from noise in the considered octave band in m 2 (determined according to paragraphs 3.4 or 3.5).

Case 3. The calculated points are located on the territory adjacent to the building. Fan noise propagates through the duct and is radiated to the atmosphere through the grate or shaft (Fig. 6). Octave levels of sound pressure generated at design points should be determined by the formula

I = L p -AL p -201gr a -i^- + A-8, (12)

where r a is the distance from the installation element (grid, hole) emitting noise into the atmosphere to the design point in m \ p a - sound attenuation in the atmosphere, taken according to Table. 7 in dB/km

A is the correction in dB, taking into account the location of the calculated point relative to the axis of the installation element emitting noise (for all frequencies, it is taken according to Fig. 6).

1 - ventilation shaft; 2 - louvre

The remaining quantities are the same as in formulas (10)

3.4. The room constant B should be determined from the graphs in fig. 7 or according to table. 9, using the table. 8 to determine the characteristics of the room.

3.5. For rooms with special requirements for acoustics (unique

halls, etc.), the constant of the room should be determined in accordance with the instructions for acoustic calculation for these rooms.

3.6. If the design point receives noise from several noise sources (for example, supply and recirculation grilles, an autonomous air conditioner, etc.), then for the considered design point, according to the corresponding formulas in clause 3.2, the octave sound pressure levels generated by each of the noise sources separately should be determined , and the total level in

These "Instructions on the acoustic calculation of ventilation units" were developed by the Research Institute of Building Physics of the USSR State Construction Committee together with the institutes Santekhproekt of the USSR State Construction Committee and Giproniiaviaprom of Minaviaprom.

The instructions were developed in development of the requirements of the chapter SNiP I-G.7-62 “Heating, ventilation and air conditioning. Design Standards” and “Sanitary Design Standards for Industrial Enterprises” (SN 245-63), which establish the need to reduce the noise of ventilation, air conditioning and air heating installations for buildings and structures for various purposes when it exceeds the sound pressure levels allowed by the standards.

Editors: A. No. 1. Koshkin (Gosstroy of the USSR), Doctor of Engineering. sciences, prof. E. Ya. Yudin and candidates of tech. Sciences E. A. Leskov and G. L. Osipov (Research Institute of Building Physics), Ph.D. tech. Sciences I. D. Rassadi

The Guidelines set out the general principles of acoustic calculations for mechanically driven ventilation, air conditioning and air heating installations. Methods for reducing sound pressure levels at permanent workplaces and in rooms (at design points) to the values ​​established by the norms are considered.

at (Giproniiaviaprom) and eng. | g. A. Katsnelson / (GPI Santekhproekt)

1. General Provisions............ - . . , 3

2. Noise sources of installations and their noise characteristics 5

3. Calculation of octave levels of sound pressure in the calculated

points................. 13

4. Reducing the levels (losses) of the sound power of noise in

various elements of air ducts ........ 23

5. Determining the required reduction in sound pressure levels. . . *. ............... 28

6. Measures to reduce sound pressure levels. 31

Appendix. Examples of acoustic calculation of ventilation, air conditioning and air heating installations with mechanical stimulation...... 39

Plan I quarter. 1970, No. 3

3.7. If the calculated points are located in a room through which a “noisy” duct passes, and noise enters the room through the walls of the duct, then the octave sound pressure levels should be determined by the formula

L - L p -AL p + 101g --R B - 101gB „-J-3, (13)

where Lp 9 is the octave level of the sound power of the noise source radiated into the duct, in dB (determined in accordance with paragraphs 2 5 and 2.10);

ALp b is the total reduction in sound power levels (losses) along the sound propagation path from the noise source (fan, throttle, etc.) to the beginning of the considered section of the duct that emits noise into the room, in dB (determined in accordance with Section 4);

State Committee of the Council of Ministers of the USSR for Construction Affairs (Gosstroy of the USSR)

1. GENERAL PROVISIONS

1.1. These Guidelines are developed in development of the requirements of the chapter SNiP I-G.7-62 “Heating, ventilation and air conditioning. Design Standards” and “Sanitary Design Standards for Industrial Enterprises” (SN 245-63), which established the need to reduce the noise of mechanically driven ventilation, air conditioning and air heating installations to sound pressure levels acceptable by the standards.

1.2. The requirements of these Guidelines apply to acoustic calculations of airborne (aerodynamic) noise generated during the operation of the installations listed in clause 1.1.

Note. These Guidelines do not consider calculations of vibration isolation of fans and electric motors (isolation of shocks and sound vibrations transmitted to building structures), as well as calculations of sound insulation of enclosing structures of ventilation chambers.

1.3. The method for calculating airborne (aerodynamic) noise is based on determining the sound pressure levels of noise generated during the operation of the installations specified in clause 1.1 at permanent workplaces or in rooms (at design points), determining the need to reduce these noise levels and measures to reduce sound levels pressure to the values ​​allowed by the standards.

Notes: 1. Acoustic calculation should be included in the design of mechanically driven ventilation, air conditioning and air heating installations for buildings and structures for various purposes.

Acoustic calculation should be done only for rooms with normalized noise levels.

2. Air (aerodynamic) fan noise and noise generated by air flow in air ducts have broadband spectra.

3. In these Guidelines, under the noise should be understood any kind of sounds that interfere with the perception of useful sounds or break the silence, as well as sounds that have a harmful or irritating effect on the human body.

1.4. When acoustically calculating a central ventilation, air conditioning and hot air heating installation, the shortest duct run should be considered. If the central unit serves several rooms, for which the normative noise requirements are different, then an additional calculation should be made for the duct branch serving the room with the lowest noise level.

Separate calculations should be made for autonomous heating and ventilation units, autonomous air conditioners, units of air or air curtains, local exhausts, units of air shower installations, which are closest to the calculated points or have the highest performance and sound power.

Separately, it is necessary to carry out an acoustic calculation of the branches of the air ducts that exit into the atmosphere (suction and exhaust of air by installations).

If there are throttling devices (diaphragms, throttle valves, dampers), air distribution and air intake devices (grilles, shades, anemostats, etc.) between the fan and the serviced room, sudden changes in the cross section of air ducts, turns and tees, acoustic calculation of these devices should be made and plant elements.

1.5. Acoustic calculation should be made for each of the eight octave bands of the auditory range (for which noise levels are normalized) with the geometric mean frequencies of the octave bands 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.

Notes: 1. For central air heating, ventilation and air conditioning systems in the presence of an extensive network of air ducts, it is allowed to calculate only for frequencies of 125 and 250 Hz.

2. All intermediate acoustic calculations are performed with an accuracy of 0.5 dB. The final result is rounded to the nearest whole number of decibels.

1.6. Required measures to reduce noise generated by ventilation, air conditioning and air heating installations, if necessary, should be determined for each source separately.

2. SOURCES OF NOISE IN INSTALLATIONS AND THEIR NOISE CHARACTERISTICS

2.1. Acoustic calculations to determine the sound pressure level of air (aerodynamic) noise should be made taking into account the noise generated by:

a) a fan

b) when the air flow moves in the elements of the installations (diaphragms, chokes, dampers, turns of air ducts, tees, grilles, shades, etc.).

In addition, the noise transmitted through the ventilation ducts from one room to another should be taken into account.

2.2. Noise characteristics (octave sound power levels) of noise sources (fans, heating units, room air conditioners, throttling, air distribution and air intake devices, etc.) should be taken from the passports for this equipment or from catalog data

In the absence of noise characteristics, they should be determined experimentally on the instructions of the customer or by calculation, guided by the data given in these Guidelines.

2.3. The total sound power level of the fan noise should be determined by the formula

L p =Z+251g#+l01gQ-K (1)

where 1^P is the total sound power level of vein noise

tilator in dB re 10“ 12 W;

L-noise criterion, depending on the type and design of the fan, in dB; should be taken according to the table. one;

I is the total pressure created by the fan, in kg / m 2;

Q - fan performance in m^/s;

5 - correction for the fan operation mode in dB.

Table 1

Notes: 1. The value of 6 when the deviation of the fan operation mode is not more than 20% of the maximum efficiency mode should be taken equal to 2 dB. In the fan operation mode with maximum efficiency 6=0.

2. To facilitate the calculations in fig. 1 shows a graph for determining the value of 251gtf+101gQ.

3. The value obtained by formula (1) characterizes the sound power radiated by an open inlet or outlet pipe of the fan in one direction into the free atmosphere or into the room in the presence of a smooth air supply to the inlet pipe.

4. When the air supply to the inlet pipe is not smooth or the throttle is installed in the inlet pipe to the values ​​specified in

tab. 1, should be added for axial fans 8 dB, for centrifugal fans 4 dB

2.4. The octave sound power levels of fan noise emitted by an open inlet or outlet of the fan L p a, into the free atmosphere or into the room, should be determined by the formula

(2)

where is the total sound power level of the fan in dB;

ALi - correction that takes into account the distribution of the sound power of the fan in octave bands in dB, taken depending on the type of fan and the number of revolutions according to table. 2.

table 2

Amendments ALu taking into account the distribution of the sound power of the fan in octave bands, in dB

Notes: 1. Given in Table. 2 data without brackets are valid when the fan speed is in the range of 700-1400 rpm.

2. At a fan speed of 1410-2800 rpm, the entire spectrum should be shifted down an octave, and at a speed of 350-690 rpm, an octave up, taking for the extreme octaves the values ​​indicated in brackets for frequencies of 32 and 16000 Hz.

3. When the fan speed is more than 2800 rpm, the entire spectrum should be shifted two octaves down.

2.5. Octave sound power levels of fan noise radiated into the ventilation network should be determined by the formula

Lp - L p ■- A L-± -|~ L i-2,

where AL 2 is the correction that takes into account the effect of connecting the fan to the duct network in dB, determined from the table. 3.

2.6. The total sound power level of the noise radiated by the fan through the walls of the casing (housing) into the ventilation chamber room should be determined by formula (1), provided that the value of the noise criterion L is taken from Table. 1 as its average value for the suction and discharge sides.

The octave levels of the sound power of the noise emitted by the fan into the room of the ventilation chamber should be determined by the formula (2) and Table. 2.

2.7. If several fans operate simultaneously in the ventilation chamber, then for each octave band it is necessary to determine the total level

sound power of the noise emitted by all fans.

The total noise sound power level L cyu during operation of n identical fans should be determined by the formula

£sum = Z.J + 10 Ign, (4)

where Li is the sound power level of the noise of one fan in dB-, n is the number of identical fans.

Table 4.

2.8. Octave sound power levels of noise radiated into the room by autonomous air conditioners, heating and ventilation units, air shower units (without air duct networks) with axial fans should be determined by formula (2) and Table. 2 with a 3dB up-correction.

For autonomous units with centrifugal fans, the octave sound power levels of the noise emitted by the suction and discharge pipes of the fan should be determined by formula (2) and table. 2, and the total noise level - according to table. 4.

Note. When air is taken in by installations outside, it is not necessary to take a higher correction.

2.9. The total sound power level of noise generated by throttling, air distribution and air intake devices (throttle valves.

Noise sources in ventilation systems are a running fan, an electric motor, air distributors, and air intake devices.

According to the nature of occurrence, aerodynamic and mechanical noise are distinguished. Aerodynamic noise is caused by pressure pulsations during rotation of the fan wheel with blades, as well as due to intense flow turbulence. Mechanical noise occurs as a result of vibration of the walls of the fan casing, in bearings, in the transmission.

The fan is characterized by the existence of three independent ways of noise propagation: through the suction ducts, through the discharge ducts, through the walls of the casing into the surrounding space. In supply systems, the most dangerous is the propagation of noise in the direction of discharge, in exhaust systems - in the direction of suction. The sound pressure levels in these directions, measured in accordance with the standards, are indicated in the passport data and catalogs of ventilation equipment.

To reduce noise and vibration, a number of preventive measures are taken: careful balancing of the fan impeller; the use of fans with a lower number of revolutions (with blades curved back and maximum efficiency); fastening of fan units on vibration bases; connection of fans to air ducts using flexible connectors; ensuring acceptable air speeds in air ducts, air distribution and air intake devices.

If the above measures are not enough, special silencers are used to reduce noise in ventilated rooms.

Silencers are tubular, plate and chamber type.

Tubular mufflers are made in the form of a straight section of a metal air duct with a round or rectangular cross section, lined with sound-absorbing material from the inside, and are used with a cross-sectional area of ​​​​air ducts up to 0.25 m 2.

For large sections, plate silencers are used, the main element of which is a sound-absorbing plate - a metal box perforated on the sides, filled with sound-absorbing material. The plates are installed in a rectangular casing.

Silencers are usually installed in supply mechanical ventilation systems of public buildings on the discharge side, in exhaust systems - on the suction side. The need to install silencers is determined based on the acoustic calculation of the ventilation system. The meaning of acoustic calculation:

1) the permissible sound pressure level for a given room is established;

2) the sound power level of the fan is determined;

3) a decrease in the sound pressure level in the ventilation network is determined (on straight sections of air ducts, in tees, etc.);

4) the sound pressure level is determined at the design point of the room closest to the fan on the discharge side for the supply system and on the suction side for the exhaust system;

5) the sound pressure level at the design point of the room is compared with the permissible level;

6) in case of excess, a silencer of the required design and length is selected, the aerodynamic resistance of the silencer is determined.

SNiP establishes permissible sound pressure levels, dB, for various rooms at geometric mean frequencies: 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz. The fan noise is most intense in low octave bands (up to 300 Hz), therefore, in the course project, acoustic calculation is performed in octave bands of 125, 250 Hz.

In the course project, it is necessary to make an acoustic calculation of the supply ventilation system of the longevity center and select a silencer. The nearest room from the fan discharge side is an observation room (on duty) with a size of 3.7x4.1x3 (h) m, a volume of 45.5 m 3, the air enters through a louvered grille of the P150 type with a size of 150x150 mm. The air outlet velocity does not exceed 3 m/s. The air from the grate exits parallel to the ceiling (angle Θ = 0°). A radial fan VTS4 75-4 is installed in the supply chamber with the following parameters: productivity L = 2170 m 3 /h, developed pressure P = 315.1 Pa, rotational speed n = = 1390 rpm. Fan wheel diameter D=0.9 ·D nom.

The scheme of the calculated branch of the air ducts is shown in fig. 13.1a

1) Set the permissible sound pressure level for this room.

2) We determine the octane level of the sound power of aerodynamic noise emitted into the ventilation network from the discharge side, dB, according to the formula:

Since we perform the calculation for two octane bands, it is convenient to use the table. The results of calculating the octave level of the sound power of aerodynamic noise emitted into the ventilation network from the discharge side are entered in Table. 13.1.

3) Determine the reduction in sound power in the elements of the ventilation network, dB:

where is the sum of the reductions in the sound pressure level in various elements of the duct network before entering the design room.

3.1. Reducing the sound power level in sections of a metal duct with a circular cross section:

The value of the reduction in the sound power level in metal circular ducts is taken according to

3.2. Reducing the sound power level in smooth turns of air ducts, determined by . With a smooth turn with a width of 125-500 mm - 0 dB.

3.3. Reduction of octane levels of sound power in the branch, dB:

where m n is the ratio of the cross-sectional areas of the air ducts;

Sectional area of ​​the branch duct, m 2 ;

Sectional area of ​​the duct in front of the branch, m 2 ;

The total cross-sectional area of ​​the branch ducts, m 2 .

Branching nodes for the ventilation system (Fig. 13.1a) are shown in Figures 13.1, 13.2,13.3,13.4

Node 1 Fig 13.1.

Calculation for 125 Hz and 250 Hz bands.

For a tee - turn (node ​​1):

Node 2 Fig 13.2.

For tee - turn (node ​​2):

Node 3 Fig 13.3.

For a tee - turn (node ​​3):

Node 4 Fig 13.4.

For a tee - turn (node ​​4):

3.4. Loss of sound power as a result of sound reflection from the supply grille P150 for a frequency of 125 Hz - 15 dB, 250 Hz - 9 dB.

Total reduction of the sound power level in the ventilation network up to the design room

In the 125Hz octane band:

In the 250 Hz octane band:

4) We determine the octane levels of sound pressure at the design point of the room. With a room volume of up to 120 m 3 and with the location of the calculated point at least 2 m from the grating, the room average octane sound pressure level in the room, dB, can be determined:

B - room constant, m 2.

The room constant in the octane frequency bands should be determined by the formula

Since the octave sound power level at the design point of the room is less than the permissible one (for the geometric mean frequency 125 48.5<69; для среднегеометрической частоты 250 53,6< 63) ,то шумоглушитель устанавливать не стоит.

Acoustic calculation produced for each of the eight octave bands of the auditory range (for which noise levels are normalized) with geometric mean frequencies of 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz.

For central ventilation and air conditioning systems with branched air duct networks, it is allowed to carry out acoustic calculation only for frequencies of 125 and 250 Hz. All calculations are performed with an accuracy of 0.5 Hz and the final result is rounded to the nearest whole number of decibels.

When the fan operates in efficiency modes greater than or equal to 0.9, the maximum efficiency 6 = 0. If the fan operation mode deviates by no more than 20% of the maximum efficiency, 6 = 2 dB is taken, and with a deviation of more than 20% - 4 dB.

It is recommended to reduce the level of sound power generated in the air ducts, to take the following maximum air speeds: in the main air ducts of public buildings and auxiliary premises of industrial buildings 5-6 m/s, and in branches - 2-4 m/s. For industrial buildings, these speeds can be increased by a factor of 2.

For ventilation systems with an extensive network of air ducts, acoustic calculation is done only for the branch to the nearest room (at the same permissible noise levels), at different noise levels - for the branch with the lowest permissible level. Acoustic calculation for air intake and exhaust shafts is done separately.

For centralized ventilation and air conditioning systems with an extensive network of air ducts, the calculation can only be done for frequencies of 125 and 250 Hz.

When noise enters the room from several sources (from supply and exhaust grilles, from units, local air conditioners, etc.), several design points are selected at workplaces closest to the noise sources. For these points, octave sound pressure levels are determined from each noise source separately.

With different regulatory requirements for sound pressure levels during the day, acoustic calculation is performed at the lowest permissible levels.

In the total number of noise sources m, sources that create octave levels 10 and 15 dB lower than the standard ones at the design point are not taken into account, with their number no more than 3 and 10, respectively. Choking devices for fans are also not taken into account.

Several supply or exhaust grilles from one fan evenly distributed throughout the room can be considered as one source of noise when noise from one fan penetrates through them.

When several sources of the same sound power are located in the room, the sound pressure levels at the selected design point are determined by the formula

The basis for the design of sound attenuation of ventilation and air conditioning systems is acoustic calculation - a mandatory application to the ventilation project of any object. The main tasks of such a calculation are: determination of the octave spectrum of airborne, structural ventilation noise at the calculated points and its required reduction by comparing this spectrum with the permissible spectrum according to hygienic standards. After the selection of construction and acoustic measures to ensure the required noise reduction, a verification calculation of the expected sound pressure levels at the same design points is carried out, taking into account the effectiveness of these measures.

The initial data for acoustic calculation are the noise characteristics of the equipment - sound power levels (SPL) in octave bands with geometric mean frequencies of 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz. Corrected sound power levels of noise sources in dBA can be used for indicative calculations.

The calculated points are located in human habitats, in particular, at the place where the fan is installed (in the ventilation chamber); in rooms or in areas adjacent to the installation site of the fan; in rooms served by a ventilation system; in rooms where air ducts pass in transit; in the area of ​​​​the air intake or exhaust device, or only the air intake for recirculation.

The calculated point is in the room where the fan is installed

In general, sound pressure levels in a room depend on the sound power of the source and the directivity factor of noise emission, the number of noise sources, the location of the design point relative to the source and the enclosing building structures, and the size and acoustic qualities of the room.

The octave sound pressure levels generated by the fan (fans) at the installation site (in the ventilation chamber) are equal to:

where Фi is the directivity factor of the noise source (dimensionless);

S is the area of ​​an imaginary sphere or part thereof surrounding the source and passing through the calculated point, m 2 ;

B is the acoustic constant of the room, m 2 .

Settlement points are located on the territory adjacent to the building

Fan noise propagates through the air duct and is radiated into the surrounding space through a grill or shaft, directly through the walls of the fan housing or an open pipe when the fan is installed outside the building.

When the distance from the fan to the calculated point is much larger than its dimensions, the noise source can be considered as a point source.

In this case, the octave sound pressure levels at the calculated points are determined by the formula

where L Pocti is the octave level of the sound power of the noise source, dB;

∆L Pneti - total reduction of the sound power level along the path of sound propagation in the duct in the considered octave band, dB;

∆L ni - sound radiation directivity indicator, dB;

r - distance from the noise source to the calculated point, m;

W - spatial angle of sound emission;

b a - sound attenuation in the atmosphere, dB/km.