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 levels are determined sound pressure 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
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. 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 silencers are made in the form of a straight section of a metal duct, round or rectangular section, lined from the inside with sound-absorbing material, are used with a cross-sectional area of \u200b\u200bair 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 on the basis of acoustic calculation 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 louvred 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°). The supply chamber is equipped with a radial fan VTS4 75-4 with the following parameters: capacity 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.
| No. pp | Determined quantities | Conditional consignment - values | U measurement | Formula (source) | Values in octane bands, Hz | |
| Permissible noise level in the room | dB | |||||
| Octane sound power level of fan aerodynamic noise | dB | 80,4 | 77,4 | |||
| 2.1. | Fan Noise Criteria | dB | ||||
| 2.2. | Fan pressure | Pa | 315,1 | 315,1 | ||
| 2.3. | Second fan performance | Q | m 3 / s | L/3600 | 0,6 | 0,6 |
| 2.4. | Correction for fan operation mode | dB | ||||
| 2.5. | Correction taking into account the distribution of sound power in octane bands | dB | ||||
| 2.6. | Correction for duct connection | dB |
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.
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, the overall performance, as well as the type of building and its functional purpose. The calculation of noise from ventilation is designed to select the mechanisms of operation and the materials used, under which it will not go beyond the normative values, and is included in the design of ventilation systems as one of the points.
Ventilation systems consist of separate elements, each of which is a source of unpleasant sounds:
- For a fan, this can be a blade or a motor. The blade makes noise due to a sharp pressure drop on one side and the other. Engine - due to breakdown or improper installation. Refrigeration units make noise for the same reasons, plus improper compressor operation.
- 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 duct changes. Problems are solved by reducing the speed of gas movement.
- Building construction. Side noise from vibrations of fans and other installations transmitted to building elements. The solution is carried out by installing special supports or gaskets to dampen vibrations. A good example is an air conditioner in an apartment: if the outdoor unit is not fixed at all points, or the installers forgot to put protective pads, then its operation can cause acoustic discomfort to the owners of the installation or their neighbors.
Transfer Methods
There are three sound propagation paths, and in order to calculate the sound load, you need to know exactly how 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, which has air conditioners and refrigeration units located at the back of the building. Sound waves propagate in all directions to nearby houses.
- Hydraulic: Noise source - liquid pipes. Sound waves are transmitted over long distances throughout a building. It is caused by a change in the size of the pipeline section and a malfunction of the compressor.
- Vibrating: source - building structures. Caused by improper installation of fans or other parts of the system. It is transmitted throughout the building and beyond.
Some specialists use scientific research from other countries in their calculations. For example, there is a formula published in a German magazine: it calculates the sound generation by the walls of an air duct, depending on the speed of the air flow.
Measuring method
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 propagation sound waves. The measurement is carried out using three filters that allow you to cut off unwanted 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 several points. For example, in the immediate vicinity of the noise source, then at a certain distance, then at the most distant point.
Norms and rules
The rules for calculating noise from ventilation operation and the algorithms for performing calculations are specified in SNiP 23-03-2003 "Protection from noise"; GOST 12.1.023-80 “System of labor 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 the standard values are given for intermittent mechanical ventilation and open windows. If closed windows and a forced air exchange system capable of providing the design multiplicity are taken into account, then other parameters are used as norms. The maximum noise level around the building is increased to the limit, which allows maintaining the normative parameters inside the building.
Sound load requirements for residential and public buildings depend on their category:
- A is the best condition.
- B - comfortable environment.
- B is the noise level at the limit limit.
Acoustic calculation
It is used by designers to determine noise reduction. The main task of acoustic calculation is to calculate the active spectrum of sound loads at all points determined in advance, and compare the obtained value with the normative, maximum allowable. 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 technical documentation.
Settlement points:
- direct installation site of the equipment;
- adjoining premises;
- all rooms where the ventilation system operates, including basements;
- rooms for transit applications of air channels;
- places of inlet supply or exhaust outlet.
Acoustic calculation is performed according to two main formulas, the choice of which depends on the location of the point.
- 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 directionality and other parameters. Formula 1 for determining octave sound pressure levels from one or more fans looks like this:
where L Pi is the sound power in each octave;
∆L pomi - 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 mi:
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 calculation point, m 2;
B is the constant value of the acoustic constant in the room, m 2 .
- The settlement point is taken outside the building in the surrounding area. The sound from operation propagates through the walls of the ventilation shafts, grilles and the fan housing. It is conditionally assumed that the noise source is a point one (the distance from the fan to the calculated position is an order of magnitude greater than the size of the apparatus). Then the octave noise pressure level is calculated by formula 3:
where L Pocti - octave power of the noise source, dB;
∆L Pneti - loss of sound power during its propagation through the duct, dB;
∆L ni - sound radiation directivity index, dB;
r - length of the segment from the fan to the calculation point, m;
W is the angle of sound emission in space;
b a - reduction of noise intensity in the atmosphere, dB/km.
If several sources of noise act on one point, 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 go the other way, removing all unnecessary data. The difference between the largest and the least intense source is calculated, and the resulting value is compared with the standard parameter and added to the level of the largest.
Reduced sound load from fan operation
There is a set of measures that allow leveling the noise factors from the operation of the fan that are unpleasant to the human ear:
- Choice 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 a large power margin. There is a wide range of fans with silencers on the market, they protect well from unpleasant sounds and vibrations.
- Choice of installation location. Powerful ventilation equipment 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 a panel house, then the residents on the top floor will immediately feel discomfort. Therefore, only roof fans are used in such cases.
- Selection of the speed of air movement through the channels. Designers proceed from acoustic calculation. For example, for a classic air duct 300×900 mm, it is no more than 10 m/s.
- Vibration isolation, sound isolation and shielding. Vibration isolation involves the installation of special supports that dampen vibrations. Soundproofing is carried out by pasting the cases with a special material. Shielding involves cutting off a sound source from a building or room using a shield.
The 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 complex task requiring skills and experience in this area.
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 the regions bordering on it. The technical support service will answer all questions by phone numbers listed 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 conditions of the environment.
The growth of industrial and housing construction, the rapid development of various types of transport, the increasing use of sanitary and engineering equipment in residential and public buildings, household appliances have led to the fact that noise levels in residential areas of the city have become comparable to noise levels in production.
The noise regime of large cities is formed mainly by road and rail transport, which makes up 60-70% of all noise.
The increase in air traffic, the emergence of new powerful aircraft and helicopters, as well as railway transport, open metro lines and shallow metro have a noticeable impact on the noise level.
At the same time, in some large cities, where measures are being taken to improve the noise situation, noise levels are decreasing.
There are acoustic and non-acoustic noises, what is the difference between them?
Acoustic noise is defined as a combination of sounds of different strength and frequency, resulting from the oscillatory motion of particles in elastic media (solid, liquid, gaseous).
Non-acoustic noise - Radio-electronic noise - random fluctuations of currents and voltages in radio-electronic devices, arise as a result of uneven emission of electrons in electrovacuum devices (shot noise, flicker noise), uneven processes of 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. (cosmic noise).
Acoustic calculation, noise level calculation.
In the process of construction and operation of various facilities, noise control problems 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.
Normalization of noise characteristics is understood as the establishment of restrictions on the values of these characteristics, under which the noise affecting people should not exceed the permissible levels regulated by the current sanitary norms and rules.
The objectives of the acoustic calculation are:
Identification of noise sources;
Determination of their noise characteristics;
Determination of the degree of influence of noise sources on normalized 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 need in any building (whether residential or administrative), acoustic calculation should be carried out for rooms of this type. So, if the noise level is not calculated, it may turn out that the room has a very low level of sound absorption, and this greatly complicates the process of communication between people in it.
Therefore, before installing a ventilation system in a room, it is necessary to carry out an acoustic calculation. If it turns out that the room is characterized by poor acoustic properties, it is necessary to propose a series of measures to improve the acoustic situation in the room. Therefore, acoustic calculations are also performed for the installation of household air conditioners.
Acoustic calculation is most often carried out for objects that have complex acoustics or have high requirements for sound quality.
Sound sensations arise in the hearing organs when they are exposed to sound waves in the range from 16 Hz to 22 thousand Hz. Sound propagates in air at a speed of 344 m/s in 3 seconds. 1 km.
The value of the hearing threshold depends on the frequency of perceived sounds and is equal to 10-12 W/m 2 at frequencies close to 1000 Hz. The upper limit is the pain threshold, which is less dependent on frequency and lies within 130 - 140 dB (at a frequency of 1000 Hz, intensity 10 W / m 2, sound pressure).
The ratio of intensity level and frequency determines the sensation of sound volume, i.e. sounds that have different frequencies and intensities can be assessed by a person as equally loud.
When perceiving sound signals against a certain acoustic background, the effect of signal masking can be observed.
The masking effect can be detrimental to acoustic indicators and can be used to improve the acoustic environment, i.e. in the case of masking a high-frequency tone with a low-frequency one, which is less harmful to humans.
The procedure for performing acoustic calculation.
To perform an acoustic calculation, the following data will be required:
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;
Noise spectrum from the source;
Characteristics of the barrier;
Distance data from the center of the noise source to the acoustic calculation point.
In the calculation, the sources of noise and their characteristic properties are first determined. Next, on the object under study, points are selected at which calculations will be carried out. At selected points of the object, a preliminary sound pressure level is calculated. Based on the results obtained, a calculation is performed to reduce noise to the required standards. Having received all the necessary data, a project is carried out to develop measures that will reduce the noise level.
Properly 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;
* the 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 noise suppressors in ventilation and air conditioning systems.
Carrying out 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 area.
noise acoustic calculation measurement
In the simplest definition, the main task of acoustic calculation is the assessment of the noise level generated by the noise source at a given design point with the established quality of the acoustic impact.
The acoustic calculation process consists of the following main steps:
1. Collection of 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);
Geometric parameters of the room in which the noise sources are located;
Analysis of the weakened elements of the enclosing structures, through which the noise will penetrate into the environment;
Geometric and soundproof parameters of weakened elements of enclosing structures;
Analysis of nearby objects with the established quality of acoustic impact, determination of permissible sound levels for each object;
Analysis of distances from external noise sources to normalized objects;
Analysis of possible shielding elements on the path of sound wave propagation (buildings, green spaces, etc.);
Analysis of weakened elements of enclosing structures (windows, doors, etc.), through which noise will penetrate into normalized premises, identification of their soundproofing ability.
2. Acoustic calculation is carried out on the basis of current guidelines and recommendations. Basically, these are “Methods of calculation, standards”.
At each calculated point, it is necessary to sum up 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 the equivalent value of the 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 step in the acoustic calculation can be the design of the necessary noise protection measures that will reduce the acoustic impact at the calculated 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 attestation of workplaces;
Carrying out work on instrumental measurement of noise levels within the framework of the project;
Carrying out work on instrumental measurement of noise levels as part of technical reports when approving the boundaries of the SPZ;
Implementation of any instrumental measurements of noise exposure.
Conducting instrumental measurements of noise levels is carried out by a specialized mobile laboratory using modern equipment.
Timing of acoustic calculation. Terms of performance of work depend on volume of calculations and measurements. If it is necessary to make an acoustic calculation for projects of residential buildings or administrative facilities, then they are performed on average 1 - 3 weeks. Acoustic calculation for large or unique objects (theaters, organ halls) takes more time, based on the provided source materials. In addition, the number of studied noise sources, as well as external factors, largely affect the period of operation.
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