Description:
The norms and rules in force in the country prescribe that the projects should provide for measures to protect against noise of equipment used for human life support. Such equipment includes ventilation and air conditioning systems.
V.P. Gusev, doctor tech. sciences, head. laboratory of noise protection of ventilation and engineering-technological equipment (NIISF)
The norms and rules in force in the country prescribe that the projects should provide for measures to protect against noise of equipment used for human life support. Such equipment includes ventilation and air conditioning systems.
The basis for the design of sound attenuation ventilation systems and air conditioning is an 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 air, structural ventilation noise at design 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 calculated points is carried out, taking into account the effectiveness of these measures.
The materials below do not pretend to be a complete presentation of the methodology for acoustic calculation of ventilation systems (installations). They contain information that clarifies, supplements or in a new way reveals various aspects of this technique using an example acoustic calculation fan as the main source of noise in the ventilation system. The materials will be used in the preparation of a set of rules for the calculation and design of noise suppression ventilation units to the new SNiP.
The initial data for the 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, 1,000, 2,000, 4,000, 8,000 Hz. For approximate calculations, the corrected sound power levels of noise sources in dBA are sometimes used.
Design 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 are in transit; in the area of the intake or exhaust device, or only intake air for recirculation.
In general, the sound pressure levels in the room depend on the sound power of the source and the directivity factor of the noise emission, the number of noise sources, the location of the design point relative to the source and the enclosing building structures, on the size and acoustic quality of the room.
The octave sound pressure levels generated by the fan (s) at the installation site (in the ventilation chamber) are:
where Фi is the directivity factor of the noise source (dimensionless);
S is the area of an imaginary sphere or its part surrounding the source and passing through the calculated point, m 2;
B is the acoustic constant of the room, m 2.
The octave levels of airborne noise penetrating through the fence into the insulated room adjacent to the room where the fan is installed are determined by the soundproofing ability of the fences in a noisy room and the acoustic qualities of the protected room, which is expressed by the formula:
| (3) |
where L w is the octave sound pressure level in a room with a noise source, dB;
R - insulation from airborne noise by the enclosing structure through which noise penetrates, dB;
S is the area of the enclosing structure, m 2;
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 noise from the fan spreads through the air duct (air duct), partially attenuates in its elements and through the air distribution and air intake grilles penetrates into the served room. The 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:
| (4) |
where L Pi is the sound power level in the i-th octave radiated 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 with i - the same as in formula (1) - formula (2).
Attenuation in the network (in the air channel) D L P network - the sum of attenuation in its elements, sequentially located along the course of sound waves. The energy theory of sound propagation through pipes assumes that these elements do not affect each other. In fact, the sequence of shaped elements and straight sections form a single wave system, in which the principle of independence of damping in the general case cannot be justified on 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 ducts made of sheet material is due to losses due to deformation of the walls and radiation of sound outward. The decrease in the sound power level D L P per 1 m of the length of straight sections of metal air ducts, depending on the frequency, can be judged from the data in Fig. one.
As you can see, in rectangular ducts, the attenuation (decrease in USM) decreases with increasing sound frequency, and round section increases. In the presence of thermal insulation on metal air ducts, shown in Fig. 1, the values should be approximately doubled.
The concept of attenuation (decrease) of the level of the sound energy flow cannot be equated with the concept of change in the level of sound pressure in the air duct. 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 the sound pressure level. In a narrowing channel, despite the attenuation of the total energy flux, the sound pressure level can increase due to an increase in the sound energy density. Conversely, in an expanding duct, 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 levels of sound pressure in the initial and final sections of the channel section along the course of sound waves;
F 1 and F 2 - cross-sectional areas, respectively, at the beginning and end of the channel section.
Attenuation at bends (in bends, bends) with smooth walls, the cross-section of which is less than the wavelength, is determined by the reactance of the additional mass type and the appearance of higher-order modes. The kinetic energy of the flow at the turn without changing the channel cross-section increases due to the resulting non-uniformity of the velocity field. The square rotation acts like a low pass filter. Cornering noise reduction 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 equal to zero and the incident plane sound wave is theoretically completely reflected by turning the channel. The maximum noise reduction occurs when the turning depth is approximately half the wavelength. The value of the theoretical modulus of the coefficient of sound transmission through rectangular bends can be judged from Fig. 2.
In real structures, according to the data of works, the maximum attenuation is 8-10 dB, when half the wavelength fits in the channel width. With increasing frequency, the attenuation decreases to 3-6 dB in the range of wavelengths close in magnitude to the doubled channel width. Then it smoothly increases again at high frequencies, reaching 8-13 dB. In fig. 3 shows the curves of noise attenuation at channel turns for plane waves (curve 1) and for random, diffuse sound incidence (curve 2). These curves are obtained on the basis of theoretical and experimental data. The presence of a maximum noise reduction at a = l / 2 can be used to reduce noise with low-frequency discrete components by adjusting the channel sizes at bends to the frequency of interest.
Noise reduction on bends less than 90 ° is roughly proportional to the steering angle. For example, the noise reduction in a 45 ° corner is equal to half the noise reduction in a 90 ° corner. Noise reduction is not taken into account when cornering less than 45 °. 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 branchings, the transverse dimensions of which are less than half the wavelength of the sound wave, the physical causes of attenuation are similar to those of attenuation in the elbows and branches. This attenuation is determined as follows (Fig. 5).
Based on the equation of continuity 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 decrease in the sound power level with a branch cross-sectional area
 |
(11) |
 |
(12) |
(13) |
With a sudden change in the cross-section of a channel with transverse dimensions less than half-wavelengths (Fig. 6 a), the decrease in the sound power level can be determined in the same way as in the case of branching.
The calculation formula for such a change in the channel cross-section has the form
 |
(14) |
where m is the ratio larger area section of the channel to the smaller one.
The decrease in sound power levels when the channel sizes are larger than the half-wavelength of non-planar waves with a sudden narrowing of the channel is
If the channel expands or gradually narrows (Fig. 6 b and 6 d), then the decrease in the sound power level is zero, since the reflection of waves with a length less than the channel dimensions does not occur.
V simple elements ventilation systems take the following reduction values at all frequencies: heaters and air coolers 1.5 dB, central air conditioners 10 dB, mesh filters 0 dB, the place where the fan is adjacent to the air duct network is 2 dB.
Reflection of sound from the end of the duct occurs if the transverse dimension of the duct is less than the length of the sound wave (Fig. 7).
If a plane wave propagates, then there is no reflection in the large duct, and we can assume that there are no reflection losses. However, if the opening connects a large room and an open space, then only diffuse sound waves, directed towards the opening, enter the opening, the energy of which is equal to a quarter of the energy of the diffuse field. Therefore, in this case, the sound intensity level is attenuated by 6 dB.
Directional characteristics of sound emission by air distribution grilles are shown in Fig. eight.
When a noise source is located in space (for example, on a column in a large room) S = 4p r 2 (radiation into the full sphere); in the middle part of the wall, floors S = 2p r 2 (radiation into the hemisphere); in the dihedral angle (radiation in 1/4 of the sphere) S = p r 2; in a triangular corner S = p r 2/2.
The attenuation of the noise level in the room is determined by the formula (2). The design point is selected at the place of permanent residence of people closest to the noise source, at a distance of 1.5 m from the floor. If the noise at the design point is generated by several gratings, then the acoustic calculation is performed taking into account their total impact.
When the source of noise is a section of a transit air duct passing through a room, the octave levels of sound power of the noise emitted by it, determined by the approximate formula, serve as the initial data for the calculation using formula (1):
(16) |
where L pi is the sound power level of the source in the i-th octave frequency band, dB;
D L 'Pseti - attenuation in the network between the source and the considered transit section, dB;
R Ti - sound insulation of the structure of the transit section of the air duct, dB;
S T is the surface area of the transit section that goes into the room, m 2;
F T - area cross section section of the duct, m 2.
Formula (16) does not take into account the increase in the density of sound energy in the duct due to reflections; the conditions for the incidence and passage of sound through the duct structure are significantly different from the passage of diffuse sound through the enclosures of the room.
Fan noise propagates through the duct and is radiated into the surrounding space through a grill or shaft, directly through the walls of the fan casing or an open branch pipe when the fan is installed outside the building.
When the distance from the fan to the design point is much larger than its size, the noise source can be considered a point source.
In this case, the octave sound pressure levels at the calculated points are determined by the formula
(17) |
where L Pokti - octave sound power level of the noise source, dB;
D L Pnetsi is the total decrease in the sound power level along the path of sound propagation in the duct in the considered octave band, dB;
D L ni - directivity index of sound radiation, dB;
r is the distance from the noise source to the design point, m;
W is the spatial angle of sound radiation;
b a - attenuation of sound in the atmosphere, dB / km.
If there is a row of several fans, grilles or other extended noise source of limited dimensions, then the third term in formula (17) is taken equal to 15 lgr.
Structure-borne noise in rooms adjacent to ventilation chambers results from the transfer of dynamic forces from the fan to the ceiling. The octave sound pressure level in the adjacent insulated room is determined by the formula
For fans located in a technical room outside the overlap 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 - total wave impedance of vibration isolator elements on which the refrigeration machine, N s / m;
Z lane - the input impedance of the floor - the bearing slab, in the absence of a floor on an elastic foundation, the floor slab - if available, N s / m;
S is the conditional overlap area of the technical room above the insulated room, m 2;
S = S 1 for S 1> S u / 4; S = S u / 4; at S 1 ≤ S u / 4, or if the technical room is not located above the insulated room, but has one common wall with it;
S 1 - the area of the technical room above the insulated room, m 2;
S u - area of the insulated room, m 2;
S in - the total area of the technical room, m 2;
R - own insulation of airborne noise by overlap, dB.
The required reduction in octave sound pressure levels is calculated separately for each noise source (fan, fittings, fittings), but this takes into account the number of noise sources of the same type in the sound power spectrum and the sound pressure levels generated by each of them at the design point. In general, the required noise reduction for each source should be such that the total levels in all octave bands from all noise sources do not exceed the permissible sound pressure levels.
In the presence of one noise source, the required octave sound pressure level reduction is determined by the formula
where n is the total number of noise sources taken into account.
The total number of noise sources n when determining D L tri the required octave sound pressure level reduction in the urban area 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 tri for design points in a room protected from the noise of the ventilation system, the total number of noise sources should include:
When calculating the required fan noise reduction - 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 fittings is not taken into account;
When calculating the required reduction of noise generated by fittings and air distribution devices of the considered branch, - the number of fittings and chokes, the noise levels of which 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 by 10 dB less than the permissible one, with their number not exceeding 3 and 15 dB less than the permissible one with their number not exceeding 10.
As you can see, acoustic calculation is not an easy task. The required accuracy of its solution is provided by acoustics specialists. The efficiency of noise suppression and the cost of its implementation depend on the accuracy of the performed acoustic calculation. If the value of the calculated required noise reduction is underestimated, then the measures will not be effective enough. In this case, it will be necessary to eliminate the shortcomings at the operating facility, which is inevitably associated with significant material costs. If the required noise reduction is overestimated, unjustified costs are incorporated directly into the project. So, just by installing mufflers, the length of which is 300-500 mm longer than the required one, additional costs for medium and large objects can be 100-400 thousand rubles or more.
1. SNiP II-12-77. Noise protection. Moscow: 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, AVOK, no. 2004. No. 4.
4. Guidelines for the calculation and design of sound attenuation of ventilation units. Moscow: Stroyizdat, 1982.
5. Yudin E. Ya., Terekhin A.S. The fight against the noise of mine ventilation units. Moscow: Nedra, 1985.
6. Reducing noise in buildings and residential areas. Ed. G. L. Osipova, E. Ya. Yudina. Moscow: Stroyizdat, 1987.
7. Khoroshev S. A., Petrov Yu. I., Egorov P. F. Fight against fan noise. M .: Energoizdat, 1981.
Depending on the method of air movement, ventilation is natural and forced.
The parameters of the air entering the intake openings and openings of local suction of technological and other devices that are located in the working 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. Consequently, the ventilation must provide an air flow rate of 90 cubic meters / hour. In the summer, it is necessary to provide for the installation of an air conditioner in order to avoid excess temperature in the room for the 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.
The power (more precisely, the cooling power) of the air conditioner is its main characteristic, it depends on what volume of the room it is designed for. For approximate calculations, 1 kW per 10 m 2 is taken with a ceiling height of 2.8 - 3 m (in accordance with SNiP 2.04.05-86 "Heating, ventilation and air conditioning").
A simplified technique was used to calculate the heat flow in a given room:
where: Q - Heat flows
S - Room area
h - Room height
q - Factor equal to 30-40 W / m 3 (in this case 35 W / m 3)
For a room of 15 m 2 and a height of 3 m, the heat fluxes will be:
Q = 15 3 35 = 1575 W
In addition, heat generation from office equipment and people should be taken into account, it is considered (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 a copier of 0.3 kW, by adding these values to the total heat gains, the required cooling power can be obtained.
Q add = (HS oper) + (C S comp) + (PS print) (4.9)
where: Q add - The sum of additional heat fluxes
C - Computer heat dissipation
H - Heat dissipation of the operator
D - Printer heat dissipation
S comp - Number of workstations
S print - Number of printers
S operas - Number of operators
Additional heat fluxes of the room will be:
Q add1 = (0.1 2) + (0.3 2) + (0.3 1) = 1.1 (kW)
The total amount of heat gains is equal to:
Q total1 = 1575 + 1100 = 2675 (W)
In accordance with these calculations, it is necessary to select the appropriate capacity and number of air conditioners.
For the room for which the calculation is being made, air conditioners with a nominal power of 3.0 kW should be used.
One of the unfavorable factors of the working environment at the ITC is high level noise generated by printing devices, air conditioning equipment, cooling fans in the computers themselves.
To decide whether it is necessary and advisable to reduce noise, 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 calculation results obtained are compared with the permissible noise level for a given workplace. If the calculation results are higher than the permissible noise level, then special measures are required to reduce the noise. These include: lining the walls and ceiling of the hall with sound-absorbing materials, reducing noise at the source, correct layout of equipment and rational organization of the operator's workplace.
Sound pressure levels of noise sources acting on 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: hard drive in the system unit, fan (s) of PC cooling systems, monitor, keyboard, printer and scanner.
Substituting the values of the sound pressure level for each type of equipment in the formula (4.4), we get:
L = 10 lg (104 + 104.5 + 101.7 + 101 + 104.5 + 104.2) = 49.5 dB
The resulting 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 consider that it is unlikely that such peripheral devices as a scanner and a printer will be used at the same time, then this figure will be even lower. In addition, the direct presence of the operator is not necessary when the printer is operating. the printer is equipped with an automatic sheet feeding mechanism.
Engineering and construction journal, N 5, 2010
Category: Technology
Doctor of Technical Sciences, Professor I.I.Bogolepov
GOU Saint Petersburg State Polytechnic University
and GOU St. Petersburg State Marine Technical University;
Master A.A. Gladkikh,
GOU Saint Petersburg State Polytechnic University
Ventilation and air conditioning system (SVKV) - essential system for modern buildings and structures. However, in addition to the necessary high-quality air, the system transports noise to the premises. It comes from the fan and other sources, spreads through the duct and is radiated into the ventilated room. Noise is incompatible with normal sleep, learning, creative work, high-performance work, good rest, treatment, and quality information. V building codes and the rules of Russia have such a situation. The method of acoustic calculation of UHCW of buildings used in the old SNiP II-12-77 "Protection against noise" is outdated and therefore did not enter the new SNiP 23-03-2003 "Protection against noise". So, old method is outdated, and there is no new generally accepted one yet. The following is a simple approximate method for acoustic calculation of UHCW in modern buildings, developed using the best production experience, in particular, on sea vessels.
The proposed acoustic calculation is based on the theory of long sound propagation lines in an acoustically narrow pipe and on the theory of sound in rooms with a practically diffuse sound field. It is performed in order to assess the sound pressure levels (hereinafter referred to as SPL) and their compliance with the current standards of permissible noise. It provides for the determination of the SPL from the SVKV due to the operation of the fan (hereinafter referred to as the "machine") for the following typical groups of rooms:
1) in the room where the machine is located;
2) in rooms through which air ducts pass in transit;
3) in rooms served by the system.
Initial data and requirements
It is proposed to perform calculation, design and control of protection of people from noise for the most important for human perception octave frequency bands, namely: 125 Hz, 500 Hz and 2000 Hz. The octave frequency band of 500 Hz is a geometric mean in the range of noise-normalized octave frequency bands of 31.5 Hz - 8000 Hz. For constant noise, the calculation provides for the determination of SPL in octave frequency bands from the sound power levels (SPL) in the system. The values of SPL and SPL are related by the general ratio = - 10, where - SPL relative to the threshold value 2 · 10 N / m; - UZM relative to the threshold value of 10 W; - the area of propagation of the front of sound waves, m.
The SPL should be determined at the design points of the premises normalized for noise by the formula = +, where is the SPL of the noise source. The value that takes into account the effect of the room on the noise in it is calculated by the formula:
where is the coefficient taking into account the influence of the near field; - spatial angle of radiation of the noise source, rad .; - radiation directivity factor, taken according to experimental data (in the first approximation it is equal to one); - distance from the center of the noise emitter to the design point in m; = - acoustic constant of the room, m; - the average coefficient of sound absorption of the internal surfaces of the room; - the total area of these surfaces, m; - coefficient that takes into account the violation of the diffuse sound field in the room.
The indicated values, calculated points and norms of permissible noise are regulated for premises of various buildings by SNiPom 23-03-2003 "Protection against noise". If the calculated SPL values exceed the permissible noise norm in at least one of the three frequency bands, then it is necessary to design measures and means of noise reduction.
The initial data for acoustic calculation and design of UHCW are:
- layout diagrams used in the structure of the structure; dimensions of machines, air ducts, control valves, elbows, tees and air distributors;
- the speed of air movement in the mains and branches - according to the technical specifications and aerodynamic calculation;
- Drawings of the general arrangement of the premises serviced by the SVKV - according to the data of the construction project of the structure;
- noise characteristics of machines, control valves and air distributors SVKV - according to the technical documentation for these products.
The noise characteristics of the machine are the following levels of UZM of airborne noise in octave frequency bands in dB: - UZM of noise propagating from the machine into the suction duct; - USM of noise propagating from the machine into the discharge air duct; - USM of noise emitted by the body of the machine into the surrounding space. All machine noise characteristics are currently determined based on acoustic measurements in accordance with applicable national or international standards and others. regulatory documents.
The noise characteristics of mufflers, air ducts, adjustable fittings and air distributors are presented by the UZM of airborne noise in octave frequency bands in dB:
- USM of noise generated by the elements of the system when the air flow passes through them (noise generation); - UZM noise, scattered or absorbed in the elements of the system when passing through them a stream of sound energy (noise reduction).
The efficiency of generation and noise reduction by UHCW elements is determined on the basis of acoustic measurements. We emphasize that the values of and must be indicated in the corresponding technical documentation.
At the same time, due attention is paid to the accuracy and reliability of the acoustic calculation, which are included in the error of the result by the values and.
Calculation for the premises where the machine is installed
Let in room 1, where the machine is installed, there is a fan, the sound power level of which radiated into the suction, discharge pipe and through the machine body are values in dB, and. Suppose that a noise muffler with a muffling efficiency in dB () is installed on the discharge side of the fan. Workplace located at a distance from the car. The wall separating room 1 and room 2 is located at a distance from the car. Sound absorption constant of the room 1: =.
For room 1, the calculation involves the solution of three problems.
1st task... Compliance with the norm of permissible noise.
If the suction and discharge nozzles are removed from the machine room, then the SPL calculation in the room where it is located is made according to the following formulas.
The octave SPL at the design point of the room is determined in dB by the formula:
where is the USM of the noise emitted by the machine body, taking into account the accuracy and reliability using. The value indicated above is determined by the formula:
If the premises are located n noise sources, the SPL from each of which are equal at the design point, then the total SPL from all of them is determined by the formula:
As a result of the acoustic calculation and design of the UHCS for room 1, where the machine is installed, it must be ensured that the permissible noise standards are met at the design points.
2nd task. The calculation of the UZM value in the discharge duct from room 1 to room 2 (the room through which the air duct passes through), namely the value in dB, is made according to the formula
3rd task. The calculation of the UZM value emitted by the wall with soundproofing area from room 1 to room 2, namely the value in dB, is performed according to the formula
Thus, the result of the calculation in room 1 is the fulfillment of the noise standards in this room and the receipt of the initial data for the calculation in room 2.
Calculation for rooms through which the duct passes in transit
For room 2 (for rooms through which the air duct passes in transit), the calculation provides for the solution of the following five problems.
1st task. Calculation of the sound power emitted by the walls of the duct into room 2, namely the determination of the value in dB by the formula:
In this formula: - see above the 2nd problem for room 1;
= 1.12 - equivalent cross-sectional diameter of the duct with a cross-sectional area;
- room length 2.
Sound insulation of the walls of a cylindrical duct in dB is calculated by the formula:
where is the dynamic modulus of elasticity of the duct wall material, N / m;
- inner diameter of the duct in m;
- duct wall thickness in m;
Sound insulation of walls of rectangular ducts is calculated according to the following formula in DB:
where = is the mass per unit surface of the duct wall (product of the material density in kg / m and the wall thickness in m);
- geometric mean frequency of octave bands in Hz.
2nd task. The calculation of SPL at the design point of room 2, located at a distance from the first noise source (air duct), is performed according to the formula, dB:
3rd task. Calculation of SPL at the design point of room 2 from the second noise source (SPL emitted by the wall of room 1 to room 2, - value in dB) is performed according to the formula, dB:
4th task. Compliance with the norm of permissible noise.
The calculation is carried out according to the formula in dB:
As a result of the acoustic calculation and design of the UHCW for room 2, through which the air duct passes in transit, it must be ensured that the permissible noise standards are met at the design points. This is the first result.
5th task. Calculation of the UZM value in the discharge duct from room 2 to room 3 (room served by the system), namely, the value in dB by the formula:
The amount of losses due to radiation of sound power of noise by the walls of air ducts on straight sections of air ducts of unit length in dB / m is presented in Table 2. The second result of the calculation in room 2 is to obtain the initial data for the acoustic calculation of the ventilation system in room 3.
Calculation for rooms served by the system
In rooms 3, serviced by SVKV (for which the system is ultimately intended), design points and norms of permissible noise are adopted in accordance with SNiP 23-03-2003 "Noise protection" and technical specifications.
For room 3, the calculation involves the solution of two problems.
1st task. The calculation of the sound power emitted by the air duct through the air outlet into room 3, namely the determination of the value in dB, is proposed to be performed as follows.
Particular task
1
for low speed system with air speed v<< 10 м/с и = 0 и трех типовых помещений (см. ниже
пример акустического расчета) решается с помощью формулы в дБ:
Here
() - losses in the noise muffler in room 3;
() - losses in the tee in room 3 (see the formula below);
- losses due to reflection from the end of the duct (see table 1).
General task 1 consists of solving for many of three typical rooms using the following dB formula:
Here - UZM of noise propagating from the machine into the discharge air duct in dB, taking into account the accuracy and reliability of the value (taken according to the technical documentation for the machine);
- USM of noise generated by the air flow in all elements of the system in dB (taken according to the data of the technical documentation for these elements);
- USM of noise absorbed and dissipated when the sound energy flow passes through all elements of the system in dB (taken according to the data of technical documentation for these elements);
- the value that takes into account the reflection of sound energy from the end outlet of the air duct in dB is taken from Table 1 (this value is equal to zero, if it already includes);
- a value equal to 5 dB for low-speed UHCW (air speed in mains is less than 15 m / s), equal to 10 dB for medium-speed UHCW (air speed in mains is less than 20 m / s) and equal to 15 dB for high-speed UHCW (speed in highways is less 25 m / s).
Table 1. Value in dB. Octave stripes
Ventilation systems make noise and vibrations. 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 ventilation noise 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 ventilation system project, as one of the points.
Ventilation systems consist of separate elements, each of which is a source of unpleasant sounds:
There are three paths for sound propagation, and in order to calculate the sound load, you need to know exactly how it is transmitted in all three ways:
Some experts use scientific research from other countries in their calculations. For example, there is a formula published in a German journal: with its help, the generation of sound by the walls of the duct is calculated, depending on the speed of the air flow.
It is often required to measure the permissible noise level or vibration intensity in already installed, operating ventilation systems. The classical method of measurement involves the use of a special device "sound level meter": it determines the strength of the propagation of sound waves. Measurement is carried out using three filters that allow you to cut off unnecessary sounds outside the studied area. The first filter measures the sound, the intensity of which 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 multiple points. For example, in the immediate vicinity of a noise source, then at a certain distance, then at the most distant point.
The rules for calculating noise from ventilation and algorithms for performing calculations are specified in SNiP 23-03-2003 "Protection against noise"; GOST 12.1.023-80 “Occupational safety standards system (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 the guideline values are given for intermittent mechanical ventilation and open windows. If closed windows and a forced air exchange system capable of providing the design frequency are taken into account, then other parameters are used as norms. The maximum noise level around the building is increased to a limit that allows maintaining the normative parameters inside the building.
Sound load requirements for residential and public buildings depend on their category:
It is used by designers to determine noise absorption. The main task of the acoustic calculation is to calculate the active spectrum of sound loads at all points determined in advance, and the resulting value is compared with the normative, maximum permissible. If necessary, reduce to established standards.
The calculation is carried out according to the noise characteristics of the ventilation equipment, they must be indicated in the technical documentation.
Calculation points:
The acoustic calculation is carried out according to two basic formulas, the choice of which depends on the location of the point.
where L Pi is the sound power in each octave;
∆L for i - a decrease in the intensity of the noise load associated with the multidirectional movement of sound waves and power losses from propagation in the air;
According to formula 2, ∆L is determined by i:
where Фi is the dimensionless factor of the wave propagation vector;
S is the area of a sphere or hemisphere that captures the fan and the point of calculation, m 2;
B - constant value of the acoustic constant in the room, m 2.
where L Pokti - octave power of the noise source, dB;
∆L Pnetsi - loss of sound power during its propagation through the duct, dB;
∆L ni - directivity index of sound radiation, dB;
r is the length of the segment from the fan to the point of calculation, m;
W is the angle of sound radiation in space;
b a - reduction of noise intensity in the atmosphere, dB / km.
If several noise sources act on one point, for example, a fan and an air conditioner, then the calculation methodology changes slightly. You can't just take and add all the sources, so experienced designers take a different path, removing all unnecessary data. The difference between the largest and the smallest source in terms of intensity is calculated, and the resulting value is compared with the standard parameter and added to the level of the largest.
There is a set of measures to neutralize the noise factors from the fan operation, which are unpleasant to the human ear:
Calculation of noise from ventilation systems involves finding such technical solutions when the operation of the equipment will not interfere with people. This is a challenging task that requires skills and experience in this area.
The company "Mega.ru" has been dealing with ventilation and creating optimal microclimate conditions for a long time. Our experts solve problems of any complexity. We work in Moscow and neighboring regions. Technical support service will answer all questions by phone numbers indicated on the page. Remote collaboration is possible. Contact us!
Acoustic calculations
Among the problems of improving the environment, the fight against noise is one of the most urgent. In large cities, noise is one of the main physical factors that shape the living environment.
The growth of industrial and residential construction, the rapid development of various types of transport, the increasing use of plumbing and engineering equipment in residential and public buildings, have led to the fact that the noise levels in residential areas of the city have become comparable to the noise levels at work.
The noise regime of large cities is formed mainly by road and rail transport, which makes up 60-70% of all noise.
An increase in the intensity of air traffic, the emergence of new powerful aircraft and helicopters, as well as railway transport, open metro and shallow metro lines have a noticeable effect on the noise level.
At the same time, in some large cities, where measures are being taken to improve the noise environment, a decrease in noise levels is observed.
There are acoustic and non-acoustic noises, what is the difference between them?
Acoustic noise is defined as a set of sounds of different strength and frequency, arising from the vibrational motion of particles in elastic media (solid, liquid, gaseous).
Non-acoustic noise - Radio-electronic noise - random fluctuations of currents and voltages in electronic devices, arise as a result of uneven emission of electrons in vacuum devices (shot noise, flicker noise), irregularities in the generation and recombination of charge carriers (conduction electrons and holes) in semiconductor devices, thermal motion of current carriers in conductors (thermal noise), thermal radiation of the Earth and the Earth's atmosphere, as well as planets, the Sun, stars, the interstellar medium, etc. (space noise).
Acoustic calculation, noise level calculation.
In the process of construction and operation of various objects, the problems of noise control are an integral part of labor protection and protection of public health. Machines, vehicles, mechanisms and other equipment can act as sources. Noise, its magnitude of impact and vibration on a person depends on the level of sound pressure, frequency characteristics.
The standardization of noise characteristics is understood as the establishment of restrictions on the values of these characteristics, at which the noise affecting people should not exceed the permissible levels regulated by the current sanitary norms and rules.
The objectives of the acoustic design are:
Identification of sources of noise;
Determination of their noise characteristics;
Determination of the degree of influence of noise sources on standardized objects;
Calculation and construction of individual zones of acoustic discomfort of noise sources;
Development of special noise protection measures that provide the required acoustic comfort.
The installation of ventilation and air conditioning systems is already considered a natural requirement in any building (be it residential or administrative), the acoustic calculation should also be performed for premises of this type. So, if the calculation of the noise level is not carried out, it may turn out that the level of sound absorption in the room is very low, and this greatly complicates the process of communication between people in it.
Therefore, before installing ventilation systems in the room, it is imperative to carry out an acoustic calculation. If it turns out that the room is characterized by poor acoustic properties, it is necessary to propose to carry out a number of measures to improve the acoustic environment in the room. Therefore, acoustic calculations are performed for the installation of household air conditioners.
The acoustic calculation is most often carried out for objects that have complex acoustics or have increased requirements for sound quality.
Sound sensations arise in the organs of hearing when they are exposed to sound waves in the range from 16 Hz to 22 thousand Hz. The sound propagates in the air at a speed of 344 m / s, in 3 seconds. 1 km.
The value of the hearing threshold depends on the frequency of the perceived sounds and is equal to 10-12 W / m2 at frequencies close to 1000 Hz. The upper limit is the pain threshold, which is less dependent on frequency and lies in the range of 130 - 140 dB (at a frequency of 1000 Hz in intensity 10 W / m2, in sound pressure).
The ratio of the level of intensity and frequency determines the perception of the loudness of the sound, i.e. sounds of different frequency and intensity can be assessed by a person as equally loud.
When sound signals are perceived against a certain acoustic background, the signal masking effect can be observed.
The masking effect can negatively affect acoustic indicators and can be used to improve the acoustic environment, i.e. in the case of masking the high-frequency tone with a low-frequency one, which is less harmful to humans.
The procedure for performing an acoustic calculation.
To perform the acoustic calculation, the following data is required:
The dimensions of the room for which the calculation of the noise level will be carried out;
The main characteristics of the premises and its properties;
Source noise spectrum;
Description of the obstacle;
Distance data from the center of the noise source to the point of acoustic calculation.
When calculating, for a start, the sources of noise and their characteristic properties are determined. Further, on the object under study, points are selected at which calculations will be carried out. At the selected points of the object, a preliminary sound pressure level is calculated. Based on the results obtained, a calculation is made to reduce the noise to the required standards. Having received all the necessary data, a project is being carried out to develop measures, thanks to which the noise level will be reduced.
A correctly performed acoustic calculation is the key to excellent acoustics and comfort in a room of any size and design.
Based on the performed acoustic calculation, the following measures can be proposed to reduce the noise level:
* installation of soundproof structures;
* use of seals in windows, doors, gates;
* the use of structures and screens that absorb sound;
* implementation of planning and development of the residential area in accordance with SNiP;
* the use of silencers in ventilation and air conditioning systems.
Acoustic calculation.
Work on the calculation of noise levels, assessment of acoustic (noise) impact, as well as the design of specialized noise protection measures should be carried out by a specialized organization with a relevant field.
noise acoustic calculation measurement
In the simplest definition, the main task of acoustic calculation is to estimate the level of noise generated by a noise source at a given design point with a specified quality of acoustic impact.
The acoustic calculation process consists of the following main stages:
1. Collecting the necessary initial data:
The nature of noise sources, their mode of operation;
Acoustic characteristics of noise sources (in the range of geometric mean frequencies 63-8000 Hz);
Geometrical parameters of the room in which the noise sources are located;
Analysis of weakened elements of the enclosing structure, through which noise will penetrate into the environment;
Geometric and soundproofing parameters of weakened elements of enclosing structures;
Analysis of nearby objects with the established quality of acoustic impact, definitions of permissible sound levels for each object;
Analysis of distances from external noise sources to standardized objects;
Analysis of possible shielding elements on the path of sound wave propagation (buildings, green spaces, etc.);
Analysis of weakened elements of enclosing structures (window openings, doors, etc.), through which noise will penetrate into the standardized premises, identifying their sound insulation capacity.
2. The acoustic calculation is carried out on the basis of the current guidelines and recommendations. Basically, these are "Calculation methods, standards".
At each calculated point, it is necessary to summarize all available noise sources.
The result of the acoustic calculation are certain values (dB) in octave bands with geometric mean frequencies of 63-8000 Hz and an equivalent sound level (dBA) at the calculated point.
3. Analysis of the calculation results.
The analysis of the results obtained is carried out by comparing the values obtained at the calculated point with the established Sanitary Standards.
If necessary, the next stage of the acoustic calculation can be the design of the necessary noise protection measures that will reduce the acoustic impact at the design points to an acceptable level.
Carrying out instrumental measurements.
In addition to acoustic calculations, it is possible to calculate instrumental measurements of noise levels of any complexity, including:
Measurement of noise impact of existing ventilation and air conditioning systems for office buildings, private apartments, etc .;
Carrying out measurements of noise levels for certification of workplaces;
Carrying out work on instrumental measurement of noise levels within the project;
Carrying out works on instrumental measurement of noise levels within the framework of technical reports when approving the boundaries of the SPZ;
Carrying out any instrumental measurements of noise exposure.
Instrumental measurements of noise levels are carried out by a specialized mobile laboratory using modern equipment.
The timing of the acoustic calculation. The timing of the work depends on the volume of calculations and measurements. If it is necessary to make an acoustic calculation for projects of residential buildings or administrative buildings, then they are carried out on average 1 - 3 weeks. Acoustic design for large or unique objects (theaters, organ halls) takes more time based on the source materials provided. In addition, the number of noise sources investigated, as well as external factors, greatly affect the operating life.
