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How to Control Fan Noise Technically

The frequency spectrum of fan noise is a composite spectrum, which is the superposition of blade pass frequency and broadband aerodynamic noise components. The blade passing frequency is determined by the following formula:

fn = Nbn / 60

In the formula:

B ----- Number of leaves

N ----- Number of revolutions of fan, revolutions / minute

n ----- Homophonic sequence number: n = 1 (fundamental frequency), n = 2 (second harmonic)

The sound level of fan noise is not only related to the structural form 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 is operating at the highest efficiency point, the sound level is often taken as a low value.


In order to better characterize the noise performance of the fan, the concept of Class A has emerged. The specific A sound level refers to the A sound level produced by the fan at a unit flow rate (1 m / min) and a unit full pressure (1 mm water column). . The sound level A of the same fan is different under different working conditions. At the highest efficiency point, it is lower than the sound level of A. The relative A sound level of different series of fans under rated conditions indicates the noise level of this series of fans and the quality of the product. Therefore, at present, more than A sound level is used as the limit indicator of fan noise. Different models of fans in the same series have similar sound levels than A. The higher the processing accuracy of the fan, the better the aerodynamic performance, and the lower the sound level than A.


Generally speaking, forward blade centrifugal fans have a sound level lower than 2dBA than A; backward plate blade centrifugal fans, lower than 30dBA wing blade centrifugal fans, lower than 25dBA; axial flow fans, lower than 38bBA . Under different operating conditions, the sound level of the fan noise is determined by the following formula:

LA = LSA + 10LgQP dBA


La ---- A sound level of the air inlet (or air outlet) of the fan, dBA;

Lsa ---- A sound level of the air inlet (or air outlet) of the fan, dBA;

Q ---- Air volume, m3 / min;

P ---- full pressure of fan, mm water column.


A common method of controlling fan noise is to install resistive mufflers at the inlet and outlet of the fan. For the occasions with higher noise reduction requirements, a sound insulation box can be used, and a shock absorber is placed between the unit and the foundation. By adopting the above methods, a noticeable noise reduction effect is generally obtained.

There are many noise control equipment factories in China, which can provide various types of fan mufflers, mufflers, and shock absorbers. Technically speaking, the noise problem of fans has basically been effectively controlled in China, and low-noise fans have begun to appear.

However, for fan mufflers with special requirements, such as water resistance, moisture resistance, high temperature resistance or dust resistance, there are still many research works worthy of development.




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