The three patent applications, all titled "Centrifugal blower with asymmetric blade spacing" and numbered sequentially by the U.S. Patent and Trademark Office (1, 2, 3) cover separate fan designs that feature asymmetrically aligned fan blades, two with 31 blades and one with 61 blades.
Apple first introduced its asymmetric fan design in June with the 15-inch MacBook Pro with Retina display and a subsequent teardown revealed that the laptop uses a 31-blade unit.
Typical fans incorporate a prime number of blades that are spaced at angles equidistant to each other, an industry standard aimed at reducing unwanted sound. At issue is the blade pass frequency (BPF) which produces harmonics from the pressure wave formed at the tip of each blade. The most noticeable source of noise is the pole pass frequency (PPF) tone, or the "vibration and resulting pressure waves created by the poles in the motor of the fan."
Source: Graph of sound pressure levels in a traditional fan versus on with asymmetric fan blades.
Apple's design calls for variably-angled blades that controls the spectral distribution of tones created by the fan. First-hand tests have found the new design to not necessarily quiet fan noise as much as create a less grating sound.
From the patent:
Dispersing the energy of a tone over a number of discrete frequencies can make the tone seem less noisy to the listener by reducing the perception on the tonal BPF [blade pass frequency]. Spacing fan blades unevenly, while maintaining impeller balance, is one method of controlling pure-tone effects.
According to the invention, the rearrangemnt of the fan blade angles cancels some of the noise usually heard in conventional portable computers but allows for the unit to still be balanced as the center of mass is located at the shaft of the impeller. The modified design also allows for the fan system to be smaller, thus permitting a thinner laptop as seen with the Retina MacBook Pros.