We hear sounds as pressure waves transmitted through the air. Our ears are designed to pick up on this, but our eyes aren't. What our eyes can do, however, is detect changes in the density of air based on how it messes with light passing through it, which is analogous to a change in pressure. Using a fancy camera, researchers at DLR are able to make maps of these density changes, and by hovering a helicopter above a neutral background (the face of a cliff), they can generate a visual image of the sound that the helicopter is making. As it turns out, the reason that a helicopter is as noisy as it is is those vortices created by the tips of the rotor blade. Each rotor blade is constantly running into the vortex left by the rotor blade in front of it, which is where that annoying racket comes from.
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