The frequency spectrum of fan noise is a composite spectrum, which is the superposition of blade passing frequency and broadband aerodynamic noise components. The blade passing frequency is determined by the following formula:
fn=Nbn/60 Hz
Where:
B-----Number of blades
N-----the number of revolutions of the ventilator, rpm
n----- Homophony sequence number: n=1 (fundamental frequency), n=2 (second homophony)
The noise level of the fan is not only related to the structure of the fan, but also to its working state (determined by the total pressure and air volume). Different series and different models of fans have different sound levels. The same fan has different sound levels under different working conditions. When the fan works at the highest efficiency point, the sound level often takes a low value.
In order to characterize the noise performance of the wind turbine, the concept of “Better than A” has appeared. The sound level ratio A refers to the sound level A produced by the ventilator at unit flow rate (1 m/min) and unit total pressure (1 mm water column). . The ratio A sound level of the same fan under different working conditions is different. At the highest efficiency point, it is lower than the A sound level. The ratio A sound level of different series of fans under rated conditions characterizes the noise level of the series of fans and the quality of the products. Therefore, the ratio A sound level is often used as the limit index of fan noise at home and abroad. The sound level of fans of different models in the same series is roughly similar to that of A. The higher the machining accuracy of the fan, the better the aerodynamic performance, and the lower the sound level than A.
Generally speaking, the forward blade centrifugal fan has a sound level lower than 2dBA than A; the rearward plate blade centrifugal fan is lower than 30dBA and the airfoil blade centrifugal fan is lower than 25dBA; the axial flow fan is lower than 38bBA . Under different working conditions, the sound level of the fan noise is determined by the following formula:
LA=LSA+10LgQP dBA
Where:
La----A sound level at the air inlet (or outlet) of the fan, dBA;
Lsa-the sound level of the fan air inlet (or air outlet) than A, dBA;
Q----air volume, m3/min;
P----The total pressure of the fan, mm water column.
The common method of controlling fan noise is to install resistive mufflers at the inlet and outlet of the fan. For occasions with higher requirements for noise reduction, a silencing sound insulation box can be used, and a shock absorber can be placed between the unit and the foundation. By adopting the above methods, a significant noise reduction effect can generally be obtained.
There are many noise control equipment factories in the country, which can provide various types of fan mufflers, sound insulation boxes and shock absorbers. Technically speaking, the noise problem of fans has basically been effectively controlled in our country, and low-noise fans have begun to appear.
However, for fan mufflers with special requirements, such as waterproof, moisture-proof, high-temperature resistance or dust-proof, there is still a lot of research work worth carrying out
