Acoustic Standing Waves and the Levitation of Small Objects

Acoustic levitation meets schlieren imaging: By reflecting a sound wave back onto itself, one can secure a standing wave if the distance between the source of the sound and the reflector is equal to an integral number of half wavelengths. In this demonstration we use 28 kHz ultrasound whose wavelength in air is 1.2 cm. The objects are small Styrofoam spheres, roughly 4 mm in diameter and 1 mg of mass.

Images employing schlieren optics are very sensitive to changes in the density of air, and these changes refract light into the camera. Note that the little spheres settle down where there are bright bands of light. The bright bands of light in the schlieren images are known to be the result of either increasing or decreasing air pressure with respect to vertical position—in other words, the pressure nodes.

For more information on our schlieren optics set-up, see

Although 28 kHz is beyond the range of human hearing, ear protection should be worn whenever attempting this experiment to avoid damaging vibrations in parts of the ear. The sound you hear in this video is not ultrasound but rather a subharmonic and is not dangerous to your ears.

[notyou]

why do they sit in the high pressure zones? i would have thought they would sit in the low pressure ones

[salinaajaykumar]

It's really a great job to show a sound wave in real life

[cas]

That Schlieren optics shot was beautiful. That strange, ghostly glow around the objects and the mirror looking like a window that looks out into space. Wow.

[Sheen]

I never thought i would get to literally see a sound wave! Thank you for this!

[observantowl]

I was in the hospital one time many years ago and my roommate snored very loud and one night his snoring hit a resonant frequency and a picture on the wall started vibrating. I'm not kidding either.

[MrDhartz]

I don't get it. The title says "Ultrasonic Levitation" and the video demonstrates ultrasonic levitation, but still 264 people disliked the video. What were they expecting?!

[Rascals]

I didn't hear anything but my dog is deaf now.

[Dunken_Donut]

This guy: "Even though I can't hear it I'm still going to use ear protection."

Our generation: "Even though I'm bleeding from my ears I'm gonna turn up that bass."

[ahmedlotfy]

the last ball to fall reminded me of tom and jerry when tom realizes he is in the air.

[rankmedia]

My alien says this is so 6 thousand years old

[sirtinley-knot]

"Scientist dangles his balls in the air with close up"

[vinceb]

Crystal clear explanation, demonstration and overall presentation. Doesn't get much better than this :)

[TheQwixilverBandit]

awesome to see someone doing this with schlieren imaging, I haven't seen anyone else on youtube doing that before, and it's really cool to be able to actually 'see' what's happening!

[sakshamsingh]

Probably the most useful video I ever saw on youtube.... Amazing...

[Reth_Hard]

Am I the only one who is more interested by this weird magical mirror than those levitating balls?

[Unmannedair]

That's freaking badass! What a great idea to use a schlieren projector. I've known about this phenomenon for years but it never occurred to me to visualize it like this. Absolutely awesome! So happy. 😁

[SoDamnMetal]

Don't let him fool ya with all the science jargon, this guy is an actual wizard!

[vikrant]

3:03 it looks like a portal to another world where thunder and lightning is going on !

[zachreyhelmberger]

Wow! I LOVE the Schlieren optics idea for visualizing the effect!!

[RobotN]

experiments with the Schlieren method are very revealing !!! awesome !!!