To reduce the noise generated by motors and equipment, the following measures are usually used:
1.1. NOISE PROTECTION FOR MOTORS AND EQUIPMENT
To prevent noise in air-conditioning systems when different methods are used for each noise source. In this section we present noise protection measures commonly used in air-conditioning systems.
MEASURES AND SOUND AND ANTI-NOISE MEASURES IN THE REGISTRATION
Source of noise when coming out the vents.
Source of noise from outside: According to the air flow, according to the construction structure, the air duct, or the gap into the room.
Source of aerodynamic noise from the air flow of the air conditioning and ventilation systems.
Noise sources are caused by motors and equipment inside the room.
Noise in the air-conditioned space is generated by many different sources, namely:
Voice is one of the amenities of air-conditioned space. Noise that affects the health of people causes mental stress fatigue, affects the level of concentration on work, and in some cases can affect the quality of certain special products.
SOUND AND NOISE COMBINATION IN AIR CONDITIONING.
a. Choose devices (FCU, AHU, fans, air-conditioners) with low noise level for indoor installation. This is the first job that the designer needs attention. Noise levels of most air conditioners are available in the catalogs from manufacturers. However, it is necessary to check and adjust before installation.
b. Install machine assemblies and devices in a separate room isolated from the work area. This solution usually applies to large assemblies, such as AHUs, large-capacity chiller units, often installed in separate rooms, which can be soundproof or depending on specific projects. .
c. Regularly maintenance of equipment: lubrication of moving parts, bearings, alignment of drive belts of equipment, dynamic balance of rotating mechanisms, etc.
d. Local soundproofing of equipment: In case of force majeure, when installing a machine assembly, equipment with high noise levels in these cases, it is necessary to conduct local soundproofing of such equipment. For example, in the case of AHU, FCU and large-capacity ventilation fans installed immediately on the false ceiling of the room. In this case, the noise potential of the equipment is quite large, in many cases exceeding the permissible level, in order to overcome such encapsulation of devices (see photo 1a and b).
Photo 1: Sound absorption of devices installed on a false ceiling
1.2. Noise protection created by aerodynamic airflow
The air flow has a high speed when moving on the pipeline, especially through the auxiliary equipment of the wind pipe such as regulating valves, bypasses, turning, expansion or contraction, etc. Significant noise out. To overcome the aerodynamic noise of a moving air stream, it is necessary to:
a. Choose the speed of air flow on the pipe reasonable. That speed is given in specialized documents, depending on the performance of the office and is related to noise levels. The rooms that require low speed are the bedrooms, the types of recording studios, the radio rooms, the studio studios, etc. In contrast, in factories, restaurants, supermarkets, it is possible to choose the higher speed.
b. Design and installation of piping equipment (numbs, quails, forks, tapers, etc.) must comply with technical standards. Those standards are strictly regulated in DW / 142 and SMACNA pipe design standards documents. For branched pipe sections, the designs are not suddenly cheap, they must bend according to the prescribed standards, or use wind blades (example in photo 2).
1.3. Noise protection from transmissions from outside sources into the room
a. Noise source travels through the air stream into the room
One of the noise sources for rooms is the transmission of sound from external sources of noise, emanating from equipment, machines, fans and air-conditioning clusters of air moving in air channels into the room. To eliminate noise from these sources, people use measures:
Install aamm targets on suction and push lines. There are many different types of sound boxes, but the most common are rectangular boxes, clear boxes and sheet-type sound boxes (Photo 3a, b and c).
Thermal insulation inside pipes: One of the most effective and commonly used solutions today is thermal insulation inside pipes. These insulation layers are tightly sandwiched under layers of cloth and iron mesh, which has a very good ammonia absorption effect. However, because the internal insulation is difficult and the cost is quite large, people often cover only a few meters of equipment (AHU, FCU and fans) next to the suction and push sides. The insulated pipe sections are essentially the sound absorption boxes.
Keep the equipment away from the building: It is obvious that if the equipment is located right next to the works with very short wind duct, the sound absorption is also very difficult and does not achieve as expected. In many cases it is imperative to extend the ductwork and place the equipment away from the site.
Photo 3: Sound absorption geometry
b. Noise sources transmitted according to the construction structure
Sound transmission according to construction structure is quite complicated and relatively difficult to handle, these tasks belong to the construction structure designer. Transmission by construction structure includes: Transmission by walls and or floors. .
For noise sources transmitted through walls, most masonry walls meet soundproofing under normal conditions, for special rooms requiring small noise levels such as recording studios, radio rooms and studio studios of radio and television stations need to conduct soundproofing inside these rooms.
For noise sources due to vibrations transmitted according to the construction structure, it is common to transmit vibrations of machine assemblies and equipment on inertial platforms and damping springs (Photo 4a, b).
The inertia platform must have a mass consistent with the vibrations generated by the assembly, in general the larger the mass, the better. It is also possible to use measures to separate completely the construction structure of the working room from rooms with high noise potential, avoiding impacts on walls and floors.
c. Source of noise due to transmission materials by wind pipes, water pipes into the room
Fans and pumps of air-conditioning systems during work usually produce vibrations. These vibrations can produce sound and travel through water pipes and especially wind pipes.
In order to eliminate the vibration transmitted from the machinery assemblies and equipment along the pipeline, keeping the assemblies, pumps, fans with pipes, people install soft cloth sections, specialized hoses or rubber pipes. In addition, when connecting to output devices such as blowing and suction vents, rubber hose sections are also used. In addition, when connected to ddaadu devices such as the blowing mouth and the suction mouth, people also use soft pipes. (pictures 1 and 4)
Photo 4: Reducing the cluster of machines located on the upper floor
d. Noise is transmitted from outside into openings into the room
External noise sources transmitted according to the air environment through the openings in the room are the sources of noise which are the most difficult to determine, prevent and prevent randomly. For these noise sources, people take the following measures:
For normal working rooms, external noise sources are not frequent and continuous sound, no special measures are required, because air-conditioned rooms with minimum tightness can be overcome.
For rooms where special noise is low or near sources of frequent, continuous and high intensity noise, it is necessary to seal the room with soundproof bugs inside and use rubber mats , sucking to seal doors, windows.
1.4. Anti-noise airflow output vents
When the speed of entry into the suction mouth and large protruding mouths can create additional sounds that affect the building.
In order to reduce the noise generated at the blowing and suction nozzles need to:
Choose mouth and suction mouth with low noise level. The dispersion type blow mouths often have low oodn level.
Reduce the rate of airflow in or out or increase the size of the air inlets.
II. STUDYING THE SOUND DEPLOYMENT CHARACTERISTICS OF SOUND BOXES
In this section we present the results of research on the influence of a number of factors on the sound absorption capacity of the sound box which is often used for installation on supply and recovery pipes, to take measures. legal when designing the sound box.
The formula for calculating the attenuation through the rectangular sound box is referenced by the documents [1,2].
2.1 Sound absorption treatment box
a. Effect of negative material layer thickness
To study the effect of the thickness of the sound absorption layer, we proceeded to calculate the level of attenuation of the sound box with the parameters: height h = 500; width w = 1000mm; sound-absorbing material is glass wool with ρ = 160 kg / m3; at different thicknesses δ = 25 ÷ 250mm.
The result of calculating the stable reduction through 01m of sound box length at different thickness of glass wool layer is shown in Photo 5. From the research results, the following remarks can be drawn
As the thickness of the negative material layer increases, the level of sound absorption increases rapidly, exponentially.
The higher the audio frequency, the greater the noise reduction
Sound absorption of the sound box of the sound box is quite good, for example, the sound box has a size of about 1.0 meters to reduce the noise level by about 30 dB at 125 Hz, just thick glass wool layer is about 50mm is satisfactory.
Photo 5: Noise reduction level depends on the sound insulation thickness
b.Effect of density of sound-absorbing material layer
To study the effect of material density on noise reduction efficiency, we performed calculations for sound absorption boxes with the following parameters: 100mm cotton layer thickness; transverse section dimensions of boxes 800x1000m; audio frequencies: 63 Hz, 125 Hz and 250 Hz; The density of cotton in 48 ÷ 192 kg / m3.
Photo 6: Noise reduction level depends on material density
The calculation results are shown in Figure 6, from which can draw the following conclusions:
As the density of the material layers increases, the degree of sound absorption increases very quickly exponentially.
The higher the sound frequency, the greater the reduction in noise level
c. Effect of distance between sound absorption plates
Geometric parameters of the sound box have a great influence on their sound absorption ability. In the geometric parameters, the gap between the sound absorption plates of the box according to height (height) is a very important parameter.
To study the effect of height on noise reduction ability, we performed calculations for sound absorption box with the following parameters: 100mm cotton layer thickness; horizontal dimensions of the box 1000mm; audio frequencies: 63 Hz, 125 Hz and 250 Hz; h height varies within h <1000mm. The calculation results are shown in Photo 7. From the calculation results show:
Photo 7: The level of noise reduction depends on the distance of the sound absorption plate
As the distance between the box's sound plates increases, the level of noise reduction decreases, at all audio frequencies. Therefore, it is not advisable to design a sound box with a large gap between the sound absorption plates, in case of large pipe size and inlet air flow, a plate type box should be used to reduce this distance between plates.
It is recommended that sound absorption boxes with a spacing above 500 mm should not be designed.
At low audio frequencies the level of attenuation efficiency is negligible, but at large sound frequencies the effect of sound absorption is quite rapid when increasing the distance between panels.
2.2 Corrugated box
The level of attenuation of the soundboard in the form of plate has been calculated and included in many specialized technical documents . sound absorbing material is glass wool with ρ = 15 ÷ 20 kg / m3; in different thicknesses δ = 100, 200, 400 and 800 mm; At different frequency bands shows (Photo 8):
Photo 8: Noise reduction level depends on the thickness of the sound insulation layer at different frequencies of the box
In the low frequency bands, the sound box is large size (thickness of large soundproof panels and large gaps), has better sound suppression capability. In contrast, the number of small-size boxes is more negative.
For a device any suppression capability depends on the sound frequency and reaches the optimum value at certain frequency values. For a box with a thickness of 100mm panels, a distance of 100mm eliminates the sounds at the best 1000Hz frequency.
COMMENTS AND CONCLUSIONS
From the summarized results and studies mentioned above, we draw conclusions:
1. Noise levels in air-conditioned spaces are generated for many reasons, in order to reduce noise and sound absorption, a number of different measures must be used to achieve the best noise reduction effect.
2. The sound absorption capacity of the sound box mounted on the suction and push pipes of the air conditioning system depends on many factors.