Acoustic Levitation in ULTRA SLOW MOTION - Smarter Every Day 134

Hey, it's me Destin. Welcome back to Smarter Every Day. I am in Chicago. Anthony picked me up in his awesome Mustang and told me to come to this building because we're gonna film acoustic levitation.

What is this?

• This is an acoustic levitator. It's single axis, which means that we only control things on one axis here. So we have two horns or transducers.

• Any time I see an oscilloscope I get excited. So am I gonna hear it on the camera?

• You'll... so the main frequency we're using again is 22kHz, which is higher than human hearing. What you'll be hearing are sub-harmonic around 17-18kHz.

• But I can just put a low pass filter on the audio and I can...

• I'm guessing that you cut it out, yeah.

• OK.

• Turn it up... And on the oscilloscope, you'll see the two waves from each of the two horns. [high pitched erratic noise]

• Well, let's stinking levitate something, dude.

• Yeah, so that's just a Styrofoam ball. And the spoon has all those slots, and so sound passes right through it.

• No way. And by sound, you mean pressure?

• Yeah, the sound waves, which are pressure waves, I mean. But you can see the wavelength spacing basically between these.

• That's stinking awesome.

You may have seen sound drawn like this, but it's a bit too simplified. A real sound wave actually looks like this. The bright parts are the higher pressure, and the darker parts are the low pressure. These ripples move along at the speed of sound. When you have two identical sound waves and you line them up just right, they no longer act like waves moving across the room. If you have a wave coming from this side and another wave coming from the other side, they become what's called a standing wave. You get a spot right here where there's a pressure flipping on and off, and a spot right here where the pressure isn't changing at all; there's no oscillation.

If you flip this thing upright, and then you put something tiny right inside that little pocket, you can use that air pressure to bump something up against the force of gravity. So it ends up looking like this ping pong paddle. The particle wants to drop, but it keeps getting knocked back up by the high pressure wave below it. And that is acoustic levitation.

So if it works with foam, it should work with water, right? Anthony uses a syringe to balance little drops of water on top of that oscillating wave. Just like the ping pong ball, you can see it bouncing up and down. Look at the one on the bottom, bouncing up and down trying to find its spot.

Another cool thing is the fact that if I make the water drops too big, they always seem to explode. Why would it do that? It happens just a little too fast to see with the naked eye. I flew home wishing I had some way of bringing together the acoustic levitation machine and a high-speed camera.

Ten months went by. I had no way of doing this, but then I got a call from Ford. They wanted to know if I could do any sound experiments on American Idol out in Hollywood. We have two hours with the most amazing setup I've ever worked with, an ultrasonic levitation device and two really fast high-speed cameras. We're gonna do 20,000 frames per second with the V2511, and we're gonna do 7,500 frames per second with the V711.

I didn't want to just make a TV segment. I actually wanted to hang out with these guys and teach them something new about sound.

• Hi, I'm Jax.

• I'm Rayvon.

• I'm Clark.

• I'm Tyanna.

• I'm Nick.

• Alright, so we're not gonna pretend like these guys don't know what's up because we've already done a segment, and they're actually... You know how this works now, right?

• Yeah. [all agree]

• OK. Come over here and let's show everybody how it works.

• It looks to me like just an amp.

• OK.

• And when I turn up the amplitude, sound waves through the horns will come down against each other and will make a column where you can actually suspend liquid or things of physical...

• [laughter] Yes, that's right, that's right. (Destin) Did he get it right?

• I think he did.

There's an equation that dictates how big a raindrop can be, OK. So the pressure that's on the inside of the drop is determined by two times the surface tension divided by the radius. If the drop gets too big, what happens?

• It explodes.

• It explodes.

We're recording high-speed video of these drops that we're levitating, and then we're ramping up the amplitude, which is the same as changing the pressure, right? So once we do that, the pressure gets so great that the surface tension can't hold the drop together anymore, and then it just ruptures. It overcomes that internal pressure, and you get an explosion.

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Alright, I hope you enjoyed this episode of Smarter Every Day. I'm not obligated to say anything about any product, but Ford was awesome. Seriously, they let me do awesome science on national TV, and they made all that happen. So thank you very much Ford, I appreciate that. Also, I did not have to get a Ford Focus at the airport, but I wanted to check it out, and I'm happy with it. So, there you go.

Please support Ford. They supported Smarter Every Day. I don't have to say that. I hope you believe me. Also, please consider subscribing if you learned something or checking us out at Smarter Every Day on Patreon. I'm Destin, you're getting Smarter Every Day. Have a good one.

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I'm with you. OK, teach me something else.

• So, one kind of cool thing, if I put my hand under, I take them all out. My hand blocks the sound.

(Destin) What could you possibly be doing that's more important than acoustic levitation?

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