DC Cooling Fan noise is how come?

"DC Cooling Fan" noise and friction, air flow related. Fan speed, the greater the amount of wind noise caused by the greater will be another fan's own shock factor can not be ignored. Of course, fans of high-quality shock itself will be very small, but the front two who is difficult to overcome. To solve the "DC Cooling Fan" this problem, we can try to use larger size fan. Should be under the same amount of wind, the large fan working noise at low speed is less than a small fan operating noise at high speed. Another factor is that we tend to ignore the "DC Cooling Fan" bearings. Since the fan rotates at high speed friction between the shaft and the bearing to the collision, it is also a major source of fan noise.