@deckingman Intel's specification for 4-wire PWM fans specified a nominal pwm frequency of 25kHz with an acceptable operational range 21 kHz to 28 kHz. Many fans simply won't behave correctly with a PWM frequency outside that range. It's the duty cycle of that pulse train that controls the speed of the fan.
Oh... For 3 or 2 wire fans, it's the actual motor driving current that's modulated to control speed. The frequency can also play a part there as well.