You hear that? No? I'm surprised. It's the fan noise coming out of my laptop, and it's awful. A company called RotoSub claims that it can make computer fans nearly silent by using the fan blades themselves as an active noise cancellation system.
While RotoSub's tech isn't able to minimize the noise of moving air, it can completely cancel out any mechanical noises made by the fan itself. The way this works is half physics and half black magic, as the fan somehow slightly modulates the angle of attack of its own blades to generate a sound that has "the same phase structure as the noise, the same position, and radiation pattern," except inverse. Essentially, it works the same way that noise canceling headphones do, except inside a fan. And it seems to really work:
While this poor little demo is just of a teeny 80 mm CPU cooling fan, the fundamental technology inside it can be scaled up to any size you want, from air conditioners to ceiling fans all the way up to aircraft turbine engines, and apparently, an unnamed aircraft engine manufacturer has already expressed some interest. For its part, RotoSub isn't actually interested in producing any hardware themselves, but they'll happily license out its technology to anyone who is.