NPR : News

Filed Under:

What Does Sound Look Like?

When light passes between areas of different air density, it bends. You've probably noticed the way distant pavement seems to shimmer on a hot day, or the way stars appear to twinkle. You're seeing light that has been distorted as it passes through varying air densities, which are in turn created by varying temperatures and pressures.

In the mid-19th century, German physicist August Toepler invented a photography technique called Schlieren Flow Visualization to visually capture these changes in density. The setup is a bit hard to explain in words (watch the video above for a full explanation) but it allows scientists and engineers to see things that are normally invisible: the rising heat from a candle, the turbulence around an airplane wing, the plume of a sneeze.

It can also be used to see sound. Sound, after all, is just another change in air density — a traveling compression wave. A speaker pushes on the surrounding air, creating a wave that travels outward until it encounters the ear drum.

As a result, Schlieren Flow Visualization can be used to see sound as well. Sound waves travel extremely fast (761.2 miles per hour) so a high-speed camera is needed to see the waves' passage.

Michael Hargather, a professor of mechanical engineering at New Mexico Tech, uses just this approach to study the shock wave caused by explosives. He's made Schlieren videos of everything from firecrackers to car bombs, but he's also filmed more benign shock waves — a book landing on a table, the end of a towel being snapped, and a man clapping. You can see them all in the video.

Copyright 2014 NPR. To see more, visit


Kentucky Boy Solves Rubik's Cube In Record Time

A 14-year-old boy from Lexington, Ky., has solved a Rubik's Cube in less than five seconds. As Guinness World Records declares this a record-breaker, we hear how NPR covered a Rubik's Cube competition in 1981.
WAMU 88.5

Recipes Get A Makeover With GIFs, Science, And Emojis

On Food Wednesday, we explore the new ways recipes are being presented, with everything from GIFs to scientific method.

WAMU 88.5

International Military Action In Syria

French President Hollande meets with President Obama in Washington to seek additional U.S. support in the fight against ISIS in Syria, and NATO holds an emergency meeting over the downed Russian fighter jet: An update on international military strategy in Syria.


Used Rocket Is A New Breakthrough For Blue Origin's Space Plan

Blue Origin, the space company founded by Amazon billionaire Jeff Bezos, has sent a craft past the edge of space and then landed its rocket safely – and vertically — in Texas.

Leave a Comment

Help keep the conversation civil. Please refer to our Terms of Use and Code of Conduct before posting your comments.