The Rosewill RCX-Z2 heatsink is designed for socket 775 Intel Pentium D and Core 2 Duo processors. Pulse Width Modulation compatible fans are noted by their 4-pin power connectors, and what PWM does is enable the BIOS to directly control fan speed. The impeller speed increases or decreases relative to the processor's temperature at any given moment, in this case between 500RPM and 3400RPM.