PWM: Integrated Air Control
As processors are being clocked at ever-increasing rates and using more power to accomplish this, the effort required to cool them is likewise rising. In the case of conventional fan coolers, this has led to an increase in the level of noise in the system. For that reason, in its specifications for the LGA775 systems, Intel has taken the opportunity to implement a way to make systems quieter under typical processor load.
To use the feature that Intel calls "Acoustic Fan Speed Control", the motherboard must be set up for it, and have a controller supporting pulse width modulation (PWM). The only thing visible to the user is the motherboard's new four-pin fan connector. The processor cooler does not absolutely need to have a thermal diode, but Intel recommends one in its thermal design guide. Intel's reference cooler is equipped with such a diode.
The graph illustrates the regulated range of the cooler.
A sample implementation from Intel's thermal design guide.