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Assembly recommendations for small fans

The method of assembling small fans is of significant importance for the value of the emitted noise. In order to achieve the minimum values of noise after the installation of fans, it is necessary to take into account the catalogue values of noise and adhere to the following instructions.

Levels of acoustic pressure
indicated in the catalogue are the values corresponding to the measurement in an open acoustic field (anechoic acoustic chamber) at a specified distance. However, such conditions do not correspond absolutely to the acoustic properties of such areas as, for example, bathrooms or toilets. For this reason, the resulting measured value differs in such areas from the catalogue value depending on the reflection from the walls, the distance of the microphone from the source of the noise, the quality of installation, the connection to air ducts, etc. The measured value is always higher.

Connection to air ducts
must be insulated from the duct so that the fan is connected to the air duct in a flexible manner and vibrations cannot be transferred to the air duct. Primarily the economical designs of small axial-flow and radial-flow fans are equipped with cage induction motors that produce a significant vibration component with the third overtone of the network frequency. If the fan is connected rigidly to the air duct, vibrations are transferred to the air duct and the noise level increases. It is expedient to install branches from the vertical leads with the "damping T pieces connected under a 45° angle". The above-indicated connection minimizes the noise reflection from the fan exhaust toward the direction of suction. A connection that is absolutely unsuitable is a short rigid connection to a T-piece installed on the riser.

Fan installation on a wall
must be carried out with care. It is always necessary to perform the installation on masonry walls with the largest possible mass rather than an installation on lightweight structures with a tendency to resonate (plasterboard walls, wooden panels, lightweight ceilings, etc.). It is generally known that the transfer of vibrations can be reduced by using the largest possible mass or conversely, with flexible materials. For this reason, it is suitable to use pads of elastic materials (such as neoprene, etc.) under the assembly points of fans during their installation. Vibrations that spread through building structures create nodes and antinodes (maxima and minima) that can be measured in the neighbouring rooms and are qualified as a defect during the final acceptance. It is necessary to pay attention to these problems at the level of the construction design, because subsequent construction modifications are much more expensive.

Noise in rooms adjacent to
bathrooms and toilets is a frequent parameter that is documented during the final acceptance proceedings. This value is a function of the sound transmission of building structures that separate and insulate the neighbouring rooms from the acoustic power of the source (in this case, the fan). According to a frequent but erroneous view, the fan should meet ‘hygienic regulations’ that specify the binding values of acoustic load in the adjacent rooms during the day and at night. It is necessary to realize that the above-indicated regulation does not define the acoustic parameters of fans. It is always necessary to design a building structure so that it ensures the required damping of the installed sources of noise (for example, fans). In some cases it is impossible to find a fan that meets the requirements of an unsuitable building structure. In such cases, it is necessary to find a position for the fan outside the building (on the roof, etc.).
 

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