Ligtenberg’s group built the Pro’s fan system — three axial fans
in the front, with a blower in the back. Since most off-the-shelf
fans would be too loud, Apple designs them internally.
“Years ago, we started redistributing the blades ,” he says.
“They’re still dynamically balanced, but they’re actually
randomized in terms of their BPF [blade pass frequency]. So you
don’t get huge harmonics that tend to be super annoying.”
Noise is a major factor in the design of modern machinery. In this
case: “That [solution is] borrowed almost entirely from automobile
tires,” Ligtenberg says. “There’s a bit of math behind it, but you
can create broadband noise instead of total noise with that
Obsessing over the fan noise — reducing it where possible, and making what noise there is as pleasing to the ears as possible — is Apple at its best.