If you’ve ever been to an airshow (or watched the opening of the film Apocalypse Now), you’ll know about BPF, short for blade pass frequency.
If you count the number of whops per second in the whop-whop-whop sound that a single-rotor helicopter makes as it passes overhead, you’ll know its BPF.
Unsurprisingly, the higher the BPF, the higher the pitch of the sound, because sound rises with frequency – an octave for every doubling in frequency, as it happens.
You’ll also know, all other things being equal, that the higher the BPF, the louder the noise a fan makes.
In fact, the noise from a fan apparently increases as the fifth power of the BPF, which is why your bedroom fan goes from a mild buzz on setting I, through an annoying drone on II, to industrial-grade, sleep-busting noise pollution on level III.
This means, in theory, that in a controlled setting (such in the neighbourhood of the average PC or laptop), you can figure out the speed of one or more of a computer’s cooling fans by recording the ambient sound, isolating the fan noise, and estimating its frequency.
Furthermore, many computers have one or more fans built in, and the speed of those fans can often be controlled programmatically.