The way in which small fans are mounted has, among other things, a significant effect on the value of the noise emitted. In order to achieve the minimum noise values after the fans have been installed, the catalogue noise data must be taken into account and the following guidelines must be observed.
Sound pressure levels
given in the catalogue are the values corresponding to measurements in a free sound field (an anechoic chamber) at a defined distance. Such conditions do not correspond in the slightest to the acoustic properties of rooms such as bathrooms and toilets. The resulting measured value in such rooms therefore differs from the catalogue value depending on the reflection of the walls, the distance of the microphone from the noise source, the quality of the mounting, the connection to the ducts, etc. The measured value is always higher.
The connection to the air ducts
must be made in an isolated manner so that the fan is connected to the air duct in a flexible manner and vibrations cannot be transmitted to the air duct. In particular, economical designs of small axial and radial fans are equipped with short-threaded asynchronous motors which produce a significant component of vibration with the third harmonic of the mains frequency. When rigidly connected to air ducts, vibration will be transmitted to the duct and noise will increase. It is preferable to make taps from the riser duct with 'damping T-pieces with 45° connections'. This connection minimises the reflection of noise from the fan discharge towards the intake. A short fixed connection to a T fitting mounted on the riser is totally inappropriate.
Mounting the fan on the wall
must be done judiciously. Preference should always be given to mounting on masonry walls with as much weight as possible, rather than on lightweight structures prone to resonance (plasterboard, wood panels, lightweight ceilings, etc.). It is generally acknowledged that vibration transmission can be reduced by using as much mass as possible or, conversely, by being more compliant. It is therefore also advisable to use pads made of flexible material (such as neoprene, etc.) under the mounting points when mounting fans. The vibrations that propagate through the building structures create nodes and oscillations (maxima and minima) that can then be measured in adjacent rooms and which qualify as a fault on acceptance. This needs to be addressed at the building design level, as later building modifications are much more costly.
Noise in rooms adjacent
to bathrooms and toilets is a common parameter that is documented during acceptance procedures. This value is a function of the transmissivity of the building structures separating and isolating the adjacent rooms from the sound power of the exciter (here the fan). A common misconception is that the fan has to comply with "hygiene regulations" which set binding values for the noise load in adjacent rooms during day and night hours. It is important to note that this regulation does not define the noise parameters of fans in any way. It is always necessary to design the building structure to ensure the required attenuation of installed noise sources (e.g. fans). Finding a fan that will accommodate an inappropriate building design is in some cases impossible, in which case it is necessary to choose to locate the fan outside the building (on the roof, etc.).