The fan noise spectrum, is a composite of blade by frequency and broadband composition of air dynamic noise superposition. Blade through frequency determined by the type:
Fn = Nbn / 60 hz
Type:
B -- -- -- -- -- leaf number
N -- -- -- -- -- fan RPM, revolutions per minute
N -- -- -- -- -- harmonics Ordinal Numbers: n = 1 (fundamental frequency), n = 2 (the second harmonics)
Fan noise sound level, not only with the structure of the fan, but also with its working status (determined by the total pressure and air volume). Different series, different models of fan, the sound level is not the same. The same fan, under different conditions, the sound level are also different. Fan work at peak efficiency, low sound level often take quantity.
In order to better characterize the noise performance of A fan, appeared A grade than the concept. Than A levels is refers to the fan in the unit flow (1 m/min) and the total pressure (1 mm water column) produced by A sound level.. The same fan under different conditions than A levels are different. At the highest efficiency point, than A levels in low quantity. Different series of the fan under the rated conditions than A sound level of high and low and the quality of the products of this series of fan noise level. So at home and abroad used more than A sound level as an indicator of fan noise limit. The same series of different models of fan, than its equivalent sound level A. The higher the fan machining precision and better aerodynamic performance, less than A levels.
Generally speaking, the former to the blade centrifugal fan, the than A levels lower than 2 dba; After the plate type centrifugal fan blades, less than 30 dba wing type centrifugal fan blades, below 25 dba; Axial flow ventilator, below 38 bba. Under different conditions, the fan noise sound level determined by the type:

LA = LSA + 10 LGQP dBA
Type:
La - fan inlet or the outlet of A levels, the dBA;
Lsa - fan inlet (or the outlet) than A levels, the dBA;
Q - air volume, 3 m/min;
P - fan pressure, full of mm water column.
Control the fan noise commonly used method is in the inlet and outlet of the fan installation impedance muffler. There is a requirement for higher noise reduction for occasions, silencing sound box can be used, and between the unit and the foundation settlement shock absorbers. Take the above method, the general can obtain obvious effect of noise reduction.
Domestic now has a lot of noise control equipment factory, can provide all kinds of fan noise elimination acoustic noise, sound box and shock absorbers. Fan noise problem, technically, in our country has basically can get effective control, and low noise fan has also begun to appear.
But, for a fan muffler with special requirements, such as waterproof, moistureproof, high temperature resistant or dust, etc., there are a lot of research work worth doing