It goes without saying that helicopters are really loud. What's trickier is explaining why, and finding a way to make the explanation visible to the naked eye.

But researchers from the German Aerospace Center (DLR) are the first to provide a visual explanation of the cause for all that noise.

They're now in a better position to make helicopters quieter and more comfortable, good news for civilian and military pilots and passengers.

To answer the question of why helicopters are so noisy, researchers set up camp in a limestone quarry and pushed skilled pilots to pull off risky maneuvers just 30 feet above the ground.

The DLR explained that most noise comes from the helicopter's main rotor. As it spins, air pressure drops above each blade, and rises below it (pushing the helicopter up). To even out the pressure difference, air flows around the blade, creating a concentrated vortex. What the vortex meets the next blade, you get a vibration — sound.

The clever bit is how the German researchers found a way to capture that effect on camera.

They took advantage of a common phenomenon: the way air seems to shimmer on hot days (due to changes in air density, which refract the light). Against the right background, that shimmering makes the air vortices created by helicopter blades visible.

In 2013, the DLR researchers successfully visualized those rotor blade vortices by flying in front of a rocky background in the Alps, and over fields and meadows.

Then they took things to the next level, with a BO 105 helicopter flying in the limestone quarry near Hanover. After setting up ten cameras at various angles, the team asked the pilots to rock the helicopter just 30 feet above the ground, in front of an exposed rock face.

It paid off: The experiment generated the first 3D images of an in-flight helicopter's rotor blade vortices.

The DLR says it plans to continue this kind of testing, and the results could help make helicopters quieter and more comfortable.

Here's a composite of the vortices seen from different angles:

And some photos from the experiment in the quarry:





[An earlier version of this article was written by Alex Davies.]