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  • I'm going to show you the sound of a clap ...

  • and I don't mean some digital depiction of a clap. I mean that when this man's

  • hands come together you're going to see something that is normally invisible.

  • You're going to see the actual sound wave leaving his hands and

  • traveling outward at 761 miles per hour --

  • the speed of sound!

  • And here it is again.

  • How is this possible?

  • Well, I'll start the explanation not with sound but with the heat

  • from a lighter. There's a puff of butane, sparks fly,

  • and the fuel ignites. But that shape billowing up from the flame isn't smoke --

  • That's normal air that has been expanded by heat.

  • We're able to see the density change thanks to a technique called

  • Schlieren Flow Visualization. Here's how it works.

  • You start off with a light, shining through a single slit.

  • If you reflect that light of a parabolic mirror all the rays become parallel

  • and then you can use another parabolic mirror to refocus the light down to a single focal point

  • and then in through the lens of a camera to make a picture.

  • Now here's the trick -- you place some sort of barrier right at the focal point.

  • Now you add something that will distort the air -- like a candle.

  • The candle will block light rays making a silhouette, and the flame will make light.

  • But rising heat will change the density of the air above the lighter

  • and that will bend the light rays. The bent light ray won't pass through the focal point.

  • it will be blocked by the barrier ... and the picture will be darker.

  • This technique can be used to see anything that distorts the air.

  • The heat from a hair straightener for example.

  • Even the heat coming off a human hand.

  • Epidemiologists used to study sneezes and coughs.

  • Engineers use it to study aerodynamic flow.

  • And sound? Well -- that's just another change in air density.

  • A traveling compression wave. So Schlieren Visualization

  • along with a high speed camera can be used to see it as well.

  • Here's a book landing on a table.

  • The end of a towel being snapped.

  • A firecracker.

  • An AK-47.

  • And of course, a clap.

I'm going to show you the sound of a clap ...

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B1 中級

聲音是什麼樣子的? (What Does Sound Look Like?)

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    VoiceTube 發佈於 2021 年 01 月 14 日
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