1, "220V AC Cooling Fan" system impedance
Range between a housing inlet and outlet into the total system impedance from 60 to 80%, in addition to the greater air flow, the noise is relatively higher. System impedance is higher, the greater airflow required for cooling, so in order to minimize the noise, the system impedance must be minimized.
2, "220V AC Cooling Fan" disrupt airflow
Spoiler along obstruct airflow path encountered caused generate noise. Therefore any obstacles, particularly in the critical range of air inlet and outlet must be avoided in order to reduce noise.
3, "220V AC Cooling Fan" fan speed and size
Due to the high speed fan noise greater than the low speed fan, so you should try and use low-speed fans as possible. And a large size, low speed fan, generally, quieter than the small size, high speed of the fan at the same air volume delivery.