"The Akasa AK-960E heatsink is designed for socket 775 Intel Pentium 4/D and Core 2 Duo processors. As with most heatsinks that are vying to push the stock thermal solution out of the PC, the AK-960E comes equipped with a Pulse Width Modulation
controlled fan that can scale between 500-3000RPM.
In other words, when your PC is running on idle, the fan will slow down to reduce noise. When the processor is knee deep in a stream of number crunching, the fan spins up to meet the heat. This approach is soon to be the standard for all heatsinks, whether they be for CPU, GPU or chipset thermal solutions. The 4-pin PWM fan on Akasa's AK-960E is controlled by the BIOS on compatible
motherboards. When it's installed on motherboards that don't support PWM, the
fan defaults to full speed. At full tilt the heatsink produces a little more
than 56 dBA noise."