Noise in ventilation systems and air conditioning

Designed to ensure maximum comfort equipment ventilation and air conditioning may be the cause of constant discomfort, if his work will be accompanied by a high noise level. Bring this level up to certain standards, standards can become a serious problem, especially if there have been shortcomings in the design stage. It is at this stage of work appropriate to lay the foundation for meeting all requirements of SNIP functioning of assemblies and mechanisms, rather than later, in hindsight, with great effort and material costs, "calming" has already installed systems. What equipment is more powerful, more difficult to stay within the defined standards of conditions. With the equipment low and medium power in most cases there is no problem, but today, customers of such systems often are not only industry but also the builders of social, administrative and residential buildings. And their demands to limit the noise can be much tougher to those specified by state regulations. The phenomenon of Ventilation and Air Conditioning is a source of elastic vibrations. And as you know, the sound is none other than the propagation of longitudinal waves in an elastic medium. When a chaotic superposition of sound waves are perceived as noise. Among the parameters that characterize the noise, along with the frequency of key players are the volume (determined by the energy transferred by the sound wave) and force (proportional to the amplitude of oscillations and the area of ??the body, causing sound vibrations). The peculiarity of the sound (noise) is that the increase in the intensity fluctuations in the geometric progression of human hearing is perceived in the arithmetic. In particular, and therefore to measure the volume of convenient size, which is a function of the logarithm of the power of sound near the source and the base level corresponding to the threshold of audibility - decibels. The human ear perceives the sound frequency from 18 to 20,000 Hz. What is below this boundary - infrasound, above - an ultrasound. By itself, infrasound is not audible, but he is able to provoke the vibration of the walls of airways, and it will be accompanied by an already visible to the human ear noise. At different frequencies sound affects unequally. Ear is most sensitive to frequencies from 1 to 4 thousand Hz, and the "peak" - the minimum value threshold of audibility, at appropriate intervals of 3 to 4 thousand Hz. At him the same account and the minimum value of the pain threshold - about 110 dB. But it is not correct to think that only by achieving this magnitude can cause harm to the sound of the human body. Prolonged exposure to even very strong (50-80 dB) noise causes various functional disorders - from easily avoidable failures in the nervous system (sleep disturbance and depression) to a persistent decrease in hearing, and even myocardial infarction. Although should not rush to the other extreme, viewing any noise as the absolute evil. Many of the noise of natural origin - the murmur of the brook, the rustling of the forest or the sound of the surf have a strong positive stain. Noise to 30 dB is harmless to humans and is a natural sound background. And he created by man-made environment, small noise can even be beneficial by suppressing other more unpleasant. Problem The main sources of noise in air conditioning and ventilation systems are fans. They single-handedly "responsible" for the noise produced by fan coil units (30 -50 dB) and air-cooled condenser (40-50 dB) and give up half of the sound waves produced by refrigeration units (35-50 dB) and rooftops (70-80 dB ). In addition to fan notable "contribution" to the noisy accompaniment systems make working compressors. Noise accompanied by leakage of high-speed air. Its source often hydraulic systems. And on them without appreciable loss of power fluctuations can "travel" to a very considerable distance from direct sources. Although in general the power of sound is inversely proportional to the square of the distance from a point source propagating in a corridor or tunnel, its intensity varies slightly. Summarizes the noise pressure level for air-conditioning OborudovanieUroven noise, dB (A) Ducted air conditioning, slim model, more powerful models 35-45 and 70 Floor-ceiling indoor units of split-sistem35-40 Wall-mounted indoor units of split-sistem26-40 Window air-conditioners in 1940 -52 30-50 Fan Coils Heat fans, power 2-30 kVt32-75 Rooftops Up to 80 cause of the noise in hydraulic systems can be abrupt changes in pipe diameter (and therefore those should be avoided), the sound of valves (to be collected hydraulic valves with reduced levels of noise ), planted in the design of high speed fluid in the pipes (the rate should be as low as possible, of course, without prejudice to the normal functioning of the unit). Motor and rotating in the bearings (especially worn) shaft are sources of noise in pumps. In addition to the noise pressure level of interest is the identification of frequencies in the range of which is its maximum. Preference frequencies for different types of equipment vary. So, fan coil units, usually "stacked" in frequency up to 1 thousand Hz, condensing units with air cooling - up to 2 thousand Therefore, when considering the equipment for ventilation and air conditioning, usually the most interesting is the range to 4 thousand . Hz (although the frequency of noise in air ducts can reach more than 6,000). However, the design of HVAC systems is useful to know the full range of noise frequencies. That is how the technical documentation contains information in the tables noise characteristics of the equipment, it is so given the permissible noise levels for ventilation and air conditioning in the SNIP. Decision Measures to reduce noise is addressed as a direct source of noise, and to its distribution channels. Calculations made at the design stage, allow you to select best suited for particular conditions kind of equipment. If necessary, this equipment can be a special shumoponizhayuschem performance. Quiet operation sought to isolate the sources of noise, with noise absorbing pads or anti-vibration materials, using fan with reduced speed (decrease in the rate of 4% allows you to remove the 1 dB). For better noise reduction should be applied not only insulating materials (they are usually well-reflected sound energy, which can lead to a resonance effect), and materials and absorbent. Due to their structure (due to the porous structure), they do not reflect a "take" excess sound energy, turning it into heat. A good effect is the imposition of sound-absorbing material to soundproof (of course, that such a separation to a certain degree relative). Remove unnecessary 12/10 dB helps protective device of acoustic barrier. For this purpose, a panel of steel sheet and absorbent pads. Of great importance is the correct location of equipment. So, do not install inside mount flights of stairs next to the windows and doors - the noise level while only increase. It is undesirable to have them close to walls or in corners of rooms. Reduce vibration, and consequently, the noise in hydraulic systems facilitates the use of elastic (springs, rubber and polymer strip) mounting pipes to the walls. Particular attention is paid to air ducts. They represent not only the communications for the transportation of air, but are channels for the distribution of noise, and sometimes themselves become a secondary source it. It is well known enough effective measures that contribute to noise reduction are: placing anti-vibration pads between the duct and fan nozzle, the use of expansion nozzles (a sharp change in channel cross section is almost always fraught with the advent of noise), the presence of contributing to reducing air flow turbulence, straight section just behind the place of connection duct to the fan, coaxial connection to the air distributor. Good effect makes the inner surface of duct sound-absorbing material. Effective means of reducing noise in ventilation systems are mufflers. Most often they are placed between the fan and main duct. When the device in the inlet air intake system near the window openings to reduce the noise muffler is placed just behind The air valve. The most effective noise suppressors at frequencies from 500 Hz to 4 thousand. With the low-frequency noise (those that are the cause of so-called. Hum phenomena, often occurring in the ducts), they cope much worse. The flow rate should be limited because the muffler itself can provoke the emergence of a new noise due to a sharp deceleration of air at its collision with a sound-absorbing material. The most commonly used tube and plate silencers. Plate is a box made of thin sheet metal, the flow area is divided into plates lined sound-absorbing material. In this capacity, commonly used mineral wool, felt, fiber, etc., treated to make them protivoabrazivnyh properties. Tubular muffler is a nested two pipes (round or rectangular, depending on the shape of the duct). The size of the inner tube coincides with the dimensions of the duct to which it is installed. The space between them filled with sound-absorbing material. With low noise mufflers are used with an air layer. In this case, at certain frequencies, they are not inferior to normal. Improved performance muffler has a central plate - the presence of a plate of noise-dampening material increases the effective surface area. Even higher efficiency in cell silencer. It airflow is divided into several running parallel to the cylindrical channels (honeycomb). Mufflers are available, consisting of several elastic elements, the resonant frequency of which forms a row. If you receive a sound signal, at least some of them begin to vibrate at a frequency coinciding with the frequency of a sound signal, thereby providing noise reduction.