These fans are for replacing old, loud fans or AC fans that lack the quality and control features of DC Fans. These are top-quality, DC (low-voltage) fans with a special magnetic levitation system for quiet and long lasting operation. It is not commonly known that ball bearing and sleeve bearing fans are not intended for vertical mounting which is how most fans in audio video cooling are mounted. The reason is that the fan spindles rest on the bearings quickly wearing them out and creating noise. The Magnetic Levitation system floats the spindle so there is no direct contact between the spindle and the bearings. This accounts for the long lasting, quiet operation and, being low voltage, there are more control options for the fans as well.The most obvious application for these fans are for use in rack manufacturer's faceplates and fan tops that utilize 4.7 inch or 120mm fans.