Ventilation systems are noisy and vibrate. The intensity and area of ​​sound propagation depends on the location of the main units, the length of the air ducts, overall performance, as well as the type of building and its functional purpose. The calculation of noise from ventilation is intended to select the operating mechanisms and materials used, in which it will not go beyond the standard values, and is included in the design of ventilation systems as one of the points.

Ventilation systems consist of individual elements, each of which is a source of unpleasant sounds:

• For a fan, this may be a blade or a motor. The blade is noisy due to sharp drop pressure from one side and the other. Engine - due to breakdown or improper installation. Cooling units make noise for the same reasons, with the addition of incorrect operation compressor.
• Air ducts. There are two reasons: the first is vortex formations from the air hitting the walls. We talked about this in more detail in the article. The second is a hum in places where the cross-section of the air duct changes. Problems are solved by reducing the gas velocity.
• Building structures. Incidental noise from vibrations of fans and other installations transmitted to building elements. The solution is achieved by installing special supports or gaskets to dampen vibration. A good example- air conditioning in the apartment: if external unit is not secured at all points, or the installers forgot to install protective gaskets, then its operation may cause acoustic discomfort to the owners of the installation or their neighbors.

Transfer methods

There are three ways sound travels, and in order to calculate the sound load, you need to know how exactly it is transmitted in all three ways:

• Airborne: noise from operating installations. Distributed both inside and outside the building. The main source of stress for people. For example, a large store, air conditioners and refrigeration units which are located at the rear of the building. Sound waves travel in all directions to nearby houses.
• Hydraulic: the source of noise is pipes with liquid. Sound waves are transmitted over long distances throughout the building. Caused by a change in the size of the pipeline cross-section and a malfunction of the compressor.
• Vibration: source - building structures. Caused by improper installation of fans or other system parts. It is transmitted throughout the building and beyond.

Some experts use scientific research from other countries in their calculations. For example, there is a formula published in a German magazine: it is used to calculate the generation of sound by the walls of the air duct, depending on the speed of the air flow.

Measuring method

It is often necessary to measure the permissible noise level or vibration intensity in already installed, operating ventilation systems. Classic way measurement involves the use of a special device “sound level meter”: it determines the strength of propagation of sound waves. Measurement is carried out using three filters that allow you to cut off unnecessary sounds outside the boundaries of the studied area. The first filter measures sound whose intensity does not exceed 50 dB. The second is from 50 to 85 dB. The third is over 80 dB.

Vibrations are measured in Hertz (Hz) for several points. For example, in the immediate vicinity of the noise source, then at a certain distance, after that - at the most distant point.

Rules and regulations

Rules for calculating noise from ventilation and algorithms for performing calculations are specified in SNiP 23-03-2003 “Protection from noise”; GOST 12.1.023-80 “System of occupational safety standards (SSBT). Noise. Methods for establishing the values ​​of noise characteristics of stationary machines.”

When determining the sound load near buildings, it must be remembered that standard values given for interval operating mechanical ventilation And open windows. If we take into account closed windows And coercive system air exchange, capable of providing the design multiplicity, then other parameters are used as standards. The maximum noise level around the building increases to a limit that allows maintaining standard parameters indoors.

Requirements for the sound load level for the core and public buildings depend on their category:

1. A – the best conditions.
2. B - comfortable environment.
3. B – noise level at the limit limit.

Acoustic calculation

Used by designers to determine noise absorption. The main task of acoustic calculation is to calculate the active spectrum of sound loads at all points determined in advance, and compare the resulting value with the standard, maximum permissible. If necessary, reduce to established standards.

The calculation is carried out based on the noise characteristics of ventilation equipment; they must be indicated in technical documentation.

Calculation points:

• direct installation location of the equipment;
• neighboring premises;
• all rooms where the ventilation system operates, including basements;
• air duct transit rooms;
• places of inlet inlet or exhaust outlet.

Acoustic calculations are performed using two basic formulas, the choice of which depends on the location of the point.

1. The calculation point is taken inside the building, in the immediate vicinity of the fan. Sound pressure depends on the power and number of fans, wave direction and other parameters. Formula 1 for determining octave levels sound pressure from one or more fans looks like this:

where L Pi is the sound power in each octave;
∆L pomi - reduction in the intensity of the noise load associated with multidirectional movement of sound waves and power losses from propagation in the air;

According to formula 2, ∆L is determined:

where Фi is the dimensionless factor of the wave propagation vector;
S is the area of ​​the sphere or hemisphere that covers the fan and the calculation point, m 2 ;
B is the constant value of the acoustic constant in the room, m2.

1. The calculation point is taken outside the building in the nearby area. The sound from operation spreads through the walls of the ventilation shafts, grilles and fan housing. It is conventionally assumed that the source of noise is a point source (the distance from the fan to the calculated position is an order of magnitude greater than the size of the device). Then the octave noise pressure level is calculated using formula 3:

where L Pocti is the octave power of the noise source, dB;
∆L Pneti - loss of sound power as it propagates through the air duct, dB;
∆L ni - sound radiation directivity indicator, dB;
r is the length of the segment from the fan to the calculation point, m;
W is the angle of sound radiation in space;
b a - reduction in noise intensity in the atmosphere, dB/km.

If one point is affected by several noise sources, for example, a fan and an air conditioner, then the calculation method changes slightly. You can’t just take and add up all the sources, so experienced designers take a different path, removing all unnecessary data. The difference between the largest and smallest source in intensity is calculated, and the resulting value is compared with the standard parameter and added to the level of the largest.

Reducing the sound load from fan operation

There is a set of measures that make it possible to level out the noise factors caused by the fan that are unpleasant to the human ear:

• Selection of equipment. A professional designer, unlike an amateur, always pays attention to the noise from the system and selects fans that provide standard microclimate parameters, but, at the same time, without large stock by power. Available on the market wide range fans with silencers, they provide good protection from unpleasant sounds and vibrations.
• Selecting an installation location. Powerful ventilation equipment it is mounted only outside the serviced premises: it can be a roof or a special chamber. For example, if you put a fan in the attic in panel house, then the residents on top floor will immediately feel discomfort. Therefore, in such cases only roof fans are used.
• Selection of air speed through channels. Designers proceed from acoustic calculations. For example, for a classic 300×900 mm air duct it is no more than 10 m/s.
• Vibration insulation, sound insulation and shielding. Vibration isolation involves installing special supports that dampen vibrations. Sound insulation is carried out by pasting the housings special material. Shielding involves cutting off the sound source from a building or room using a shield.

Calculation of noise from ventilation systems involves finding such technical solutions when the operation of the equipment will not interfere with people. This difficult task, requiring skills and experience in this field.

The Mega.ru company has long been involved in the issues of ventilation and creation optimal conditions microclimate. Our specialists solve problems of any complexity. We work in Moscow and its neighboring regions. Service technical support will answer all questions by phone numbers listed on the page. Remote collaboration is possible. Contact us!

Ventilation calculation

Depending on the method of air movement, ventilation can be natural or forced.

Parameters of air entering the intake openings and openings of local suctions of technological and other devices that are located in work area, should be taken in accordance with GOST 12.1.005-76. With a room size of 3 by 5 meters and a height of 3 meters, its volume is 45 cubic meters. Therefore, ventilation should provide an air flow of 90 cubic meters per hour. IN summer time It is necessary to provide for the installation of an air conditioner in order to avoid exceeding the temperature in the room for stable operation of the equipment. It is necessary to pay due attention to the amount of dust in the air, as this directly affects the reliability and service life of the computer.

Power ( more precisely power cooling) of an air conditioner is its main characteristic; it determines the volume of the room it is designed for. For approximate calculations, take 1 kW per 10 m 2 with a ceiling height of 2.8 - 3 m (in accordance with SNiP 2.04.05-86 "Heating, ventilation and air conditioning").

To calculate the heat inflows of a given room, a simplified method was used:

where:Q - Heat inflow

S - Room area

h - Room height

q - Coefficient equal to 30-40 W/m 3 (in this case 35 W/m 3)

For a room of 15 m2 and a height of 3 m, the heat gain will be:

Q=15·3·35=1575 W

In addition, the heat emission from office equipment and people should be taken into account; it is believed (in accordance with SNiP 2.04.05-86 “Heating, ventilation and air conditioning”) that in a calm state a person emits 0.1 kW of heat, a computer or copy machine 0.3 kW, By adding these values ​​to the total heat inflows, you can get required power cooling.

Q additional =(H·S opera)+(С·S comp)+(P·S print) (4.9)

where: Q additional - Sum of additional heat inflows

C - Computer heat dissipation

H - Operator Heat Dissipation

D - Printer Heat Dissipation

S comp - Number of workstations

S print - Number of printers

S operators - Number of operators

Additional heat inflows in the room will be:

Q additional1 =(0.1 2)+(0.3 2)+(0.3 1)=1.1(kW)

The total sum of heat inflows is equal to:

Q total1 =1575+1100=2675 (W)

In accordance with these calculations, it is necessary to select the appropriate power and number of air conditioners.

For the room for which the calculation is being carried out, air conditioners with a rated power of 3.0 kW should be used.

Noise level calculation

One of the unfavorable factors of the production environment in the ICC is high level noise generated by printing devices, air conditioning equipment, fans of cooling systems in the computers themselves.

To address questions about the need and feasibility of noise reduction, it is necessary to know the noise levels at the operator’s workplace.

The noise level arising from several incoherent sources operating simultaneously is calculated based on the principle of energy summation of emissions from individual sources:

L = 10 lg (Li n), (4.10)

where Li is the sound pressure level of the i-th noise source;

n is the number of noise sources.

The obtained calculation results are compared with the permissible noise level for a given workplace. If the calculation results are higher permissible value noise level, special noise reduction measures are required. These include: cladding the walls and ceiling of the hall sound-absorbing materials, noise reduction at the source, correct layout equipment and rational organization of the operator’s workplace.

The sound pressure levels of noise sources affecting the operator at his workplace are presented in table. 4.6.

Table 4.6 - Sound pressure levels of various sources

Typically, the operator's workplace is equipped with the following equipment: a hard drive in system unit, PC cooling fan(s), monitor, keyboard, printer and scanner.

Substituting the sound pressure level values ​​for each type of equipment into formula (4.4), we obtain:

L=10 lg(104+104.5+101.7+101+104.5+104.2)=49.5 dB

The obtained value does not exceed the permissible noise level for the operator’s workplace, equal to 65 dB (GOST 12.1.003-83). And if we take into account that it is unlikely that peripheral devices such as a scanner and printer will be used simultaneously, then this figure will be even lower. In addition, when the printer is operating, the direct presence of the operator is not necessary, because The printer is equipped with an automatic sheet feed mechanism.

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 systems and air conditioning systems with extensive networks of air ducts, it is allowed to carry out acoustic calculations only for frequencies of 125 and 250 Hz. All calculations are performed with an accuracy of 0.5 Hz and rounding the final result to a whole number of decibels.

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

To reduce the level of sound power generated in air ducts, it is recommended 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 doubled.

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

For centralized ventilation and air conditioning systems with an extensive network of air ducts, calculations can only be made 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 the workplaces closest to the noise sources. For these points, octave sound pressure levels from each noise source are determined separately.

When regulatory requirements for sound pressure levels vary throughout the day, acoustic calculations are performed at the lowest permissible levels.

In the total number of noise sources m, sources are not taken into account that create octave levels at the design point that are 10 and 15 dB below the standard ones, when their number is no more than 3 and 10, respectively. Throttling 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 a room, the sound pressure levels at the selected design point are determined by the formula

Description:

The rules and regulations in force in the country stipulate that projects must include measures to protect equipment used for human life support from noise. Such equipment includes ventilation and air conditioning systems.

Acoustic calculation as a basis for designing a low-noise ventilation (air conditioning) system

V. P. Gusev, Doctor of Technical Sciences sciences, head laboratory for noise protection of ventilation and engineering-technological equipment (NIISF)

The rules and regulations in force in the country stipulate that projects must include measures to protect equipment used for human life support from noise. Such equipment includes ventilation and air conditioning systems.

The basis for designing sound attenuation of ventilation and air conditioning systems is acoustic calculation - a mandatory application to the ventilation project of any facility. The main tasks of such a calculation are: determination of the octave spectrum of airborne, structural ventilation noise at design points and its required reduction by comparing this spectrum with the permissible spectrum according to hygienic standards. After selecting 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 materials given below do not claim to be a complete presentation of the methodology for acoustic calculation of ventilation systems (installations). They contain information that clarifies, complements or reveals in a new way various aspects of this technique using the example of the acoustic calculation of a fan as the main source of noise in a ventilation system. The materials will be used in the preparation of a set of rules for the calculation and design of noise attenuation ventilation units to the new SNiP.

The initial data for acoustic calculations are the noise characteristics of the equipment - sound power levels (SPL) in octave bands with geometric mean frequencies 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000 Hz. For approximate calculations, adjusted sound power levels of noise sources in dBA are sometimes used.

Calculation points are located in human habitats, in particular, at the installation site of the fan (in the ventilation chamber); in rooms or areas adjacent to the fan installation site; in rooms served by a ventilation system; in rooms where air ducts pass through in transit; in the area of ​​the device for receiving or exhausting air, or only receiving air for recirculation.

The design 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 directional factor of noise emission, the number of noise sources, the location of the design point relative to the source and enclosing building structures, the size and acoustic qualities of the room.

The octave sound pressure levels created by the fan(s) at the installation location (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 of it surrounding the source and passing through the calculated point, m2;

B is the acoustic constant of the room, m2.

The design point is located in the room adjacent to the room where the fan is installed

Octave levels airborne noise penetrating through the fence into the insulated room adjacent to the room where the fan is installed are determined by the sound insulating ability of the fences of the noisy room and the acoustic qualities of the protected room, which is expressed by the formula:

where L w is the octave sound pressure level in the room with the noise source, dB;

R - insulation from airborne noise by the enclosing structure through which noise penetrates, dB;

S - area of ​​the enclosing structure, m2;

B u - acoustic constant of the insulated room, m 2;

k is a coefficient that takes into account the violation of the diffuseness of the sound field in the room.

The design point is located in the room served by the system

The noise from the fan spreads through the air duct (air channel), is partially attenuated in its elements and penetrates into the serviced room through the air distribution and air intake grilles. Octave sound pressure levels in a room depend on the amount of noise reduction in the air duct and the acoustic qualities of that room:

where L Pi is the sound power level in the i-th octave emitted by the fan into the air duct;

D L networki - attenuation in the air channel (in the network) between the noise source and the room;

D L pomi - the same as in formula (1) - formula (2).

Attenuation in the network (in the air channel) D L P of the network is the sum of attenuation in its elements sequentially located along the sound waves. The energy theory of sound propagation through pipes assumes that these elements do not influence each other. In fact, the sequence of shaped elements and straight sections form a single wave system, in which the principle of independence of attenuation in the general case cannot be justified in pure sinusoidal tones. At the same time, in octave (wide) frequency bands, standing waves created by individual sinusoidal components cancel each other out, and therefore an energy approach that does not take into account the wave pattern in air ducts and considers the flow of sound energy can be considered justified.

Attenuation in straight sections of air ducts made of sheet material is caused by losses due to wall deformation and sound radiation outward. The reduction in sound power level D L P per 1 m length of straight sections of metal air ducts depending on frequency can be judged from the data in Fig. 1.

As you can see, in the air ducts rectangular section attenuation (decrease in USM) decreases with increasing sound frequency, and the circular cross-section increases. If there is thermal insulation on metal air ducts, shown in Fig. 1 values ​​should be increased approximately twice.

The concept of attenuation (decrease) in the level of sound energy flow cannot be identified with the concept of a change in the sound pressure level in the air channel. As a sound wave moves through a channel, the total amount of energy it carries decreases, but this is not necessarily associated with a decrease in sound pressure level. In a narrowing channel, despite the attenuation of the overall energy flow, the sound pressure level can increase due to an increase in the density of sound energy. In an expanding duct, on the other hand, the energy density (and sound pressure level) can decrease faster than the total sound power. The sound attenuation in a section with a variable cross-section is equal to:

(5)

where L 1 and L 2 are the average sound pressure levels in the initial and final sections of the channel section along the sound waves;

F 1 and F 2 are the cross-sectional areas at the beginning and end of the channel section, respectively.

Attenuation at turns (in elbows, bends) with smooth walls, the cross section of which is less than the wavelength, is determined by reactance such as additional mass and the occurrence of higher order modes. The kinetic energy of the flow at a turn without changing the channel cross-section increases due to the resulting unevenness of the velocity field. Square rotation acts like a low pass filter. The amount of noise reduction when turning in the plane wave range is given by an exact theoretical solution:

(6)

where K is the modulus of the sound transmission coefficient.

For a ≥ l /2, the value of K is zero and the incident plane sound wave is theoretically completely reflected by the rotation of the channel. Maximum noise reduction occurs when the turning depth is approximately half the wavelength. The value of the theoretical modulus of the sound transmission coefficient through rectangular turns can be judged from Fig. 2.

In real designs, according to the work, the maximum attenuation is 8-10 dB, when half the wavelength fits into the channel width. With increasing frequency, the attenuation decreases to 3-6 dB in the region of wavelengths close in magnitude to twice the channel width. Then it smoothly increases again at high frequencies, reaching 8-13 dB. In Fig. Figure 3 shows noise attenuation curves at channel turns for plane waves (curve 1) and for a random, diffuse sound incidence (curve 2). These curves are obtained based on theoretical and experimental data. The presence of a noise reduction maximum at a = l /2 can be used to reduce noise with low-frequency discrete components by adjusting the channel sizes at turns to the frequency of interest.

Noise reduction on turns less than 90° is approximately proportional to the turning angle. For example, the reduction in noise level at a 45° turn is equal to half the reduction at a 90° turn. On turns with angles less than 45°, noise reduction is not taken into account. For smooth turns and straight bends of air ducts with guide vanes, the noise reduction (sound power level) can be determined using the curves in Fig. 4.

In channel branches, the transverse dimensions of which are less than half the sound wavelength, the physical causes of attenuation are similar to the causes of attenuation in elbows and bends. This attenuation is determined as follows (Fig. 5).

Based on the continuity equation of the medium:

From the condition of pressure continuity (r p + r 0 = r pr) and equation (7), the transmitted sound power can be represented by the expression

and the reduction in sound power level with the cross-sectional area of ​​the branch

	(11)
	(12)
	(13)

If there is a sudden change in the cross-section of a channel with transverse dimensions less than half-wavelengths (Fig. 6 a), a decrease in the sound power level can be determined in the same way as with branching.

The calculation formula for such a change in the channel cross-section has the form

(14)

where m is the ratio of the larger cross-sectional area of ​​the channel to the smaller one.

The reduction in sound power levels when channel sizes are larger than the half-wavelength of out-of-plane waves due to a sudden narrowing of the channel is

If the channel expands or smoothly narrows (Fig. 6 b and 6 d), then the decrease in the sound power level is zero, since reflection of waves with a length shorter than the size of the channel does not occur.

In simple elements of ventilation systems, the following reduction values ​​are accepted at all frequencies: heaters and air coolers 1.5 dB, central air conditioners 10 dB, mesh filters 0 dB, the place where the fan adjoins the air duct network 2 dB.

Sound reflection from the end of the air duct occurs if the transverse size of the air duct is less than the sound wavelength (Fig. 7).

If a plane wave propagates, then there is no reflection in a large duct, and we can assume that there are no reflection losses. However, if an opening connects a large room and an open space, then only diffuse sound waves directed towards the opening, the energy of which is equal to a quarter of the energy of the diffuse field, enter the opening. Therefore, in this case, the sound intensity level is weakened by 6 dB.

The directional characteristics of sound radiation from air distribution grilles are shown in Fig. 8.

When the noise source is located in space (for example, on a column in a large room) S = 4p r 2 (radiation into a full sphere); in the middle part of the wall, ceiling S = 2p r 2 (radiation into the hemisphere); in a dihedral angle (radiation into 1/4 sphere) S = p r 2 ; in a trihedral angle S = p r 2 /2.

The attenuation of the noise level in the room is determined by formula (2). The design point is selected in the place of permanent residence of people, closest to the noise source, at a distance of 1.5 m from the floor. If noise at the design point is created by several gratings, then the acoustic calculation is made taking into account their total impact.

When the source of noise is a section of a transit air duct passing through a room, the initial data for calculation using formula (1) are the octave levels of sound power of the noise emitted by it, determined by the approximate formula:

where L pi is the sound power level of the source in the i-th octave frequency band, dB;

D L’ Рnetii - attenuation in the network between the source and the transit section under consideration, dB;

R Ti - sound insulation of the structure of the transit section of the air duct, dB;

S T - surface area of ​​the transit section opening into the room, m 2 ;

F T - cross-sectional area of ​​the air duct section, m 2.

Formula (16) does not take into account the increase in sound energy density in the air duct due to reflections; The conditions for the incidence and transmission of sound through the duct structure are significantly different from the transmission of diffuse sound through the enclosures of the room.

Calculation points are located in the area adjacent to the building

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

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

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

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

D L Pneti - total reduction in sound power level along the sound propagation path in the air duct in the octave band under consideration, dB;

D L ni - sound radiation directivity indicator, dB;

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

W is the spatial angle of sound radiation;

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

If there is a row of several fans, grilles or other extended noise source of limited size, then the third term in formula (17) is taken equal to 15 lgr.

Structure-borne noise calculation

Structural noise in rooms adjacent to ventilation chambers arises as a result of the transfer of dynamic forces from the fan to the ceiling. The octave sound pressure level in an adjacent insulated room is determined by the formula

For fans located in a technical room outside the ceiling above the insulated room:

(20)

where L Pi is the octave sound power level of airborne noise emitted by the fan into the ventilation chamber, dB;

Z c is the total wave resistance of the vibration isolator elements on which the refrigeration machine is installed, N s/m;

Z per - input impedance of the floor - load-bearing slab, in the absence of a floor on an elastic foundation, floor slab - if present, N s/m;

S is the conventional floor area of ​​the technical room above the insulated room, m 2 ;

S = S 1 for S 1 > S u /4; S = S u /4; when S 1 ≤ S u /4, or if the technical room is not located above the insulated room, but has one wall in common with it;

S 1 - area of ​​the technical room above the insulated room, m 2 ;

S u - area of ​​the insulated room, m 2 ;

S in - total area technical room, m 2;

R - own airborne noise insulation by the ceiling, dB.

Determining the required noise reduction

The required reduction in octave sound pressure levels is calculated separately for each noise source (fan, shaped elements, fittings), but the number of noise sources of the same type in the sound power spectrum and the magnitude of the sound pressure levels created by each of them at the design point are taken into account. In general, the required noise reduction for each source should be such that the total levels in all octave frequency bands from all noise sources do not exceed the permissible sound pressure levels.

In the presence of one noise source, the required reduction in octave sound pressure levels is determined by the formula

where n is the total number of noise sources taken into account.

When determining D L three of the required reduction in octave sound pressure levels in urban areas, the total number of noise sources n should include all noise sources that create sound pressure levels at the design point that differ by less than 10 dB.

When determining D L three for design points in a room protected from noise from the ventilation system, the total number of noise sources should include:

When calculating the required reduction in fan noise - the number of systems serving the room; noise generated by air distribution devices and fittings is not taken into account;

When calculating the required noise reduction generated by the air distribution devices of the considered ventilation system, - the number of ventilation systems serving the room; the noise of the fan, air distribution devices and shaped elements is not taken into account;

When calculating the required noise reduction generated by shaped elements and air distribution devices of the branch in question, - the number of shaped elements and chokes whose noise levels differ from one another by less than 10 dB; The noise of the fan and grilles is not taken into account.

At the same time, the total number of noise sources taken into account does not take into account noise sources that create a sound pressure level at the design point that is 10 dB less than permissible when their number is no more than 3 and 15 dB less than permissible when their number is no more than 10.

As you can see, the acoustic calculation is not simple task. Acoustics specialists provide the necessary accuracy of its solution. The effectiveness of noise reduction and the cost of its implementation depend on the accuracy of the acoustic calculation performed. If the calculated required noise reduction is underestimated, the measures will not be effective enough. In this case, it will be necessary to eliminate deficiencies at the existing facility, which is inevitably associated with significant material costs. If the required noise reduction is too high, unjustified costs are built directly into the project. Thus, only due to the installation of mufflers, the length of which is 300-500 mm longer than required, additional costs at medium and large facilities can amount to 100-400 thousand rubles or more.

Literature

1. SNiP II-12-77. Noise protection. M.: Stroyizdat, 1978.

2. SNiP 23-03-2003. Noise protection. Gosstroy of Russia, 2004.

3. Gusev V.P. Acoustic requirements and design rules for low-noise ventilation systems // ABOK. 2004. No. 4.

4. Guidelines for calculation and design of noise attenuation of ventilation units. M.: Stroyizdat, 1982.

5. Yudin E. Ya., Terekhin A. S. Combating noise from mine ventilation units. M.: Nedra, 1985.

6. Reducing noise in buildings and residential areas. Ed. G. L. Osipova, E. Ya. Yudina. M.: Stroyizdat, 1987.

7. Khoroshev S. A., Petrov Yu. I., Egorov P. F. Combating fan noise. M.: Energoizdat, 1981.